PHILOSOPHICAL MAGAZINE A 2000 VOL 80 NO 3 659plusmn 672

TiN coatings in CMgplusmn Al composites microstructurenanochemistry and function

ARMIN FELDHOFF ECKHARD P IPPEL and JOiumlRG WOLTERSDORF

Max-Planck-Institut fuEgrave r Mikrostrukturphysik Weinberg 2 D-06120 HalleGermany

[Received 7 December 1998 and accepted in revised form 20 May 1999]

ABSTRACTThe characteristic features of TiN interlayers were investigated in three

di erent materials reg rstly TiN-coated C-reg bres of the high-tensile-strength typein pure Mg secondly TiN-coated reg bres in a Mg alloy containing 5 wt Al(AM50) thirdly for comparison the latter alloy reinforced with the uncoatedreg bres The results were obtained using high-voltage electron microscopy high-resolution electron microscopy energy-reg ltered transmission electron microscopyand scanning transmission electron microscopy in combination with energy-dispersive X-ray spectroscopy and electron-energy-loss spectroscopy includinganalyses of near-edge reg ne structures The chemically vapour-deposited coatingwas proven to consist partly of a nanocrystalline titanium carbonitride (TiCxNy)rather than of titanium nitride (TiN) It varies in thickness mainly between 10 and30 nm In the composite with the pure Mg matrix the interfacial structure ischaracterized by an almost stringent separation of matrix and reg bre by thecoating A graphitic ribbon 10plusmn 15 nm thick was only rarely observed at thecoatingmatrix interface Thus the coating almost fully prevents the Ctransport across the reg brematrix interface Accordingly changing the matrix tothe Mgplusmn Al alloy did not initiate a distinct formation of aluminium carbides Onthe other hand combining the same matrix with the uncoated reg bre resulted in thesubstantial formation of many Al2MgC2 carbides of di erent sizes at the reg brematrix interface which embrittle the composite as described in our former workTherefore an appropriate titanium nitride interlayer enables the applicability ofMg matrices with high contents of Al which is of particular interest for themanufacturing of hybrid components

1 INTRODUCTION

As is well known the properties of metal matrix composites (MMCs) are a ectedmainly by those of the reg brematrix interface region (Feldho et al 1997 HaEgrave hnel etal 1997) Therefore it is crucial for the optimization of these materials especially forthe full use of the outstanding properties of the reg bres to study the evolution ofinterface structures and their inmacr uence on the mechanical properties

In previous work (OEgrave ttinger et al 1995 Feldho et al 1997 1999 HaEgrave hnel et al1997 Feldho 1998) we investigated the possibilities of interface tailoring inthe initially non-reactive system (C reg breMg matrix) by adding various amountsof Al to the matrix and by using carbon reg bres of di erent surface microstructures

Philosophical Magazine A ISSN 0141plusmn 8610 printISSN 1460-6992 online 2000 Taylor amp Francis Ltdhttpwwwtandfcoukjournalstf01418610html

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We demonstrated that appropriate interface properties can be achieved with smallamounts of Al (approximately 2 wt) and carbon reg bres of the high-tensile-strength type via the moderate formation of the ternary carbide Al2MgC2 inthe reg brematrix interlayer thus adjusting the interfacial bonding

Quite a di erent way to generate an appropriate interlayer is the direct reg brecoating prior to the composite processing The coating should meet the followingconditions reg rstly to provide an adequate reg brematrix adhesion and secondly to actas a di usion barrier The latter results from the observation that the reg bre strength isdegraded if signireg cant amounts of Mg (5plusmn 10 at) penetrate into the volume of theturbostratic-textured carbon reg bres (Picouet et al 1990 Viala et al 1991 OEgrave ttinger1996 Feldho 1998) Moreover a potential application of CMg composites with astrength of 1plusmn 2 GPa is the partial reinforcement of highly loaded parts in mechanicalcomponents (hybrids) In parts of the components where the stresses are lower thestrength of Mg alloys with 5plusmn 9 wt Al (approximately 250 MPa at room tempera-ture) su ces which is also markedly higher than that of pure Mg (150plusmn 180 MPa)However in contact with carbon reg bres Mgplusmn Al alloys with such large amounts of Alcause extensive formation of Al2MgC2 leading to embrittlement of the composite(OEgrave ttinger et al 1995 Feldho et al 1997 1999 HaEgrave hnel et al 1997 Feldho 1998)Coating the reg bre surface to slow down the deleterious mass transfer across theinterface is a promising way to suppress carbide formation as well as reg bre degrada-tions and to provide a mechanical protection of the reg bres

Interstitial compounds seem to be suitable coating materials for this purpose asthey are known to consist of close-packed metal atom sublattices with their inter-stices (octahedral andor tetrahedral) being reg lled with smaller non-metal atoms (HC N B etc) so that they generally show a low permeability (Andrievski 1997) Ingeneral interstitial compounds are hard and brittle and therefore the coating shouldnot exceed a critical thickness of about 1 of the reg bre diameter (df ˆ 7 mm) to avoidnotch e ects which could arise from microcracks in the coating and would lower thereg bre strength (Shorshorov et al 1979) As it is necessary to achieve appropriate reg lmson each reg lament in the multireg lamentous carbon reg bre yarn chemical vapour deposi-tion (CVD) seems to be the appropriate technique

As the two interfaces (reg brecoating and coatingmatrix) simultaneously have toserve as a suitable interconnection between reg bre and matrix the number of possibleinterstitial compounds to be used for the system (C reg bre)coating(Mg matrix) isrestricted In addition to provide near-net-shape components a liquid-phase sinter-ing of the composite material is favourable Thus it is important to point out that agood wetting of the metal on the coating correlates positively with its tendency toform bonds with the constituent elements of the coating (Naidich 1981 Delannay etal 1987) Accordingly the wettability of metal-like compounds by metals is gener-ally better than that of covalent compounds as a high percentage of delocalizedelectrons in the solid phase favours the electron exchange which is necessary forthe formation of stable chemical bonds with the liquid-phase metal (Naidich 1981)

TiN was chosen as the coating material in particular as it is of the cubic rock-saltstructure which is the simplest structure of interstitial compounds Thus isotropicgrowth during CVD could be expected which would lead to a homogeneous cover-ing of the reg bre even in thin reg lms (ie some ten nanometres in thickness)

The paper is organized as follows First microstructural and nanochemicalstudies of the reg brematrix interregion in CMg composites or CMgplusmn Al alloycomposites with coated and uncoated reg bres will be described performed by applying

660 A Feldho et al

high-voltage electron microscopy (HVEM) high-resolution electron microscopy(HREM) energy-reg ltered transmission electron microscopy (EFTEM) and scanningtransmission electron microscopy (STEM) in combination with energy-dispersiveX-ray spectroscopy (EDXS) and electron-energy-loss spectrometry (EELS) withparticular emphasis on energy-loss near-edge structures (ELNES) Then the resultsgained on the di erent materials will be treated together Finally an evaluation ofthe coating properties for the manufacturing of composites or hybrid componentswill follow

2 EXPERIMENTAL DETAILS

To prepare the composites at the University of Erlangen (Wurm et al 1997Wurm 1998) the C-reg bre yarn Tenax 5331HTA (T5331HTA) (TohoAkzo) of thehigh-tensile-strength type (6000 single reg laments tensile strength frac14f ˆ 3950 MPaYoungrsquos modulus Mf ˆ 238 MPa) was thermally desized and passed through aCVD counter-current reactor using a mixture of TiCl4 N2 and H2 as the precursorgas The parameters of the CVD process were chosen so as to yield an invariance inthe reg bre properties compared with the as-received state and a complete coveringof all the reg laments in the yarn with TiN (Wurm et al 1997 Wurm 1998)Unidirectionally wound C-reg bre pre-forms were inreg ltrated with matrices of com-mercial purity Mg (cp-Mg) and of a Mgplusmn Al alloy (AM50) (5 wt Alplusmn 04 wt Mn(Polmear 1981)) via a gas-pressure melt inreg ltration process (OEgrave ttinger and Singer1993) at 7208C and 10 MPa employing a 5 min reg breplusmn melt contact to obtainMMCs with a reg bre content of approximately 58 vol The following materialswere investigated reg rstly T5331HTATiNcp-Mg secondly T5331HTATiNAM50thirdly T5331HTAAM50 Mechanical testing data can be found in the studies byWurm et al (1997) and Wurm (1998)

For transmission electron microscopy (TEM) investigations cross-sections of thereg brematrix interface area were prepared by cutting thin (500 mm) slices of 3 mmdiameter planar grinding and double-mould dimpling to a thickness of about 20 mmTo prevent galvanic attack on the Mg a water-free lubricant was used Final thin-ning was performed by ion milling (ArDagger 5 kV) down to electron transparency(Feldho 1998)

HVEM investigations were carried out using a JEOL 1000-06 microscope oper-ating at 1 MV For the HREM STEM EDXS EFTEM and EELS investigations acombined (scanning) transmission electron microscope of the type Philips CM 20FEG operating at 200 kV was used which was equipped with a light element X-raydetector (Voyager II Tracor) and a post-column electron energy reg lter (GatanImaging Filter GIF 200 model 667) as well as with a digital scanning module(Gatan Digiscan) To minimize contamination e ects a cooling specimen holder(Gatan model 668) was used during the STEM procedures For image processingand electron-energy-loss spectrum treatment the Gatan Digital Micrograph andELP software run on a Power Macintosh 720075 were employed

The thermally assisted reg eld emission gun enables an energy resolution of theelectron-energy-loss spectra of 08plusmn 1 eV which were usually acquired at a dispersionof 05 eV per channel This allows us to obtain information on the chemical bondingstate of the elements of interest by analysing the ELNES of the relevant ionizationedges which can be attributed to transitions of core-shell electrons into unoccupiedstates above the Fermi level (Brydson et al 1991 Rez 1992)

T iN coatings in CMgplusmn Al composites 661

To achieve high lateral resolution in the EDXS and EELS procedures onlysample regions which were suitable for HREM have been analysed by these meth-ods Additionally to exclude multiple-scattering e ects on core-loss spectra inEELS here the sample thickness has been counter-checked by the peak intensityratios of the zero loss to volume plasmons in corresponding low-loss spectra

To gain information about the elemental distribution in the reg brematrix interfaceregion EFTEM with the three-window method (Krivanek et al 1992 Hofer et al1995) was used A slit (20plusmn 30 eV wide) is inserted in the intermediate spectral plane ofthe energy reg lter to select electrons which have lost a specireg c amount of energy byinelastic interaction with the specimen With these electrons an image is projected onto a slow-scan change-coupled device camera One image is taken with the energywindow slightly above an ionization edge of the element of interest and two imagesare taken with the energy windows below the ionization edge (for background extra-polation) to calculate one reg nal image in which the bright contrast represents thespatial distribution of the considered element

3 R ESULTS

In the bright-reg eld image of T5331HTATiNcp-Mg in reg gure 1 (a) the coatingappears dark as a continuous band with its thickness varying here from 30 to 100 nmBy combining STEM and EDXS an almost homogeneous distribution of approxi-mately 1plusmn 2 at Mg was detected in the volume of the carbon reg bre in this composite(reg gure 1 (b))

The interfacial structure in this material is mainly characterized by an almoststringent separation of matrix and reg bre by the coating as is shown in the bright-reg eldimage in reg gure 2 (a) and the corresponding EFTEM images in reg gures 2 (b) plusmn (d) takenat the ionization edges Ti-L23 (456 eV) (reg gure 2 (b)) C-K (284 eV) (reg gure 2 (c)) andMg-K (1305 eV) (reg gure 2 (d)) with the reg bre on the left and the matrix on the right

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Figure 1 T5331HTATiNcp-Mg (a) bright-reg eld image showing the coating on three reg breswith dark contrast to be continuous (b) energy-dispersive X-ray spectrum of a carbon reg bre

The elemental distribution of Ti (reg gure 2 (b)) reveals the continuous covering of thereg bre with the reg lm here 10plusmn 20 nm in thickness The distribution of C (reg gure 2 (c))represents mainly the reg bre and that of Mg (reg gure 2 (d)) the matrix A slight penetra-tion of Mg through the coating towards the reg bre is indicated by some brightcontrast features in reg gure 2 (d) left of the coating The HREM image in reg gure 3

T iN coatings in CMgplusmn Al composites 663

Figure 2 T5331HTATiNcp-Mg (a) bright-reg eld image (b) (c) EFTEM at the ionizationedges

Figure 3 HREM image of T5331HTATiNcp-Mg showing the (10 -10dagger lattice fringes of theMg matrix ending on the coating (left reg bre right matrix)

shows the turbostratic C of the reg bre (left) and the (10 -10) lattice fringes of the Mgmatrix (right) ending at the coating (middle part) The polycrystalline nature of thecoating with grain sizes in the range of the reg lm thickness is clearly revealed

In reg gure 4 (a) the integrity of the coating between reg bre (left) and matrix (right) isshown by the elemental distribution of titanium Only rarely did T5331HTATiNcp-Mg exhibit C on the matrix side of the coating as revealed by the EFTEM imagein reg gure 4 (b) This carbon forms a 10 nm thick graphitic ribbon with its basal planesin parallel orientation to the reg bre surface as shown in the corresponding HREMimage in reg gure 5

In reg gure 6 (a) a three-dimensional plot of electron-energy-loss spectra is givenwhich were recorded across the reg brematrix interregion at 30 equidistant pointsalong the white marker in the STEM bright-reg eld image (reg gure 6 (b)) with thepoint-to-point distance amounting to about 13 nm In the energy interval between250 and 750eV there appear the C-K N-K Ti-L23 and O-K ionization edges and

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Figure 4 EFTEM at the ionization edges of the reg brematrix interregion in T5331HTATiNcp-Mg (a) Ti-L23 (b) C-K

Figure 5 HREM image of T5331HTATiNcp-Mg showing a graphitic ribbon on the matrixside of the coating (left reg bre right matrix)

their associated reg ne structures In the region of the reg bre (spectra in the backgroundreg gure 6 (a)) only the C-K ionization edge at 284 eV arises from the exponentiallydecreasing background At the reg brecoating interface a Ti-L23 ionization edgeappears and the C-K ELNES changes Within a gradient of approximately 5 nma N-K ionization edge arises and the C-K edge disappears The Ti-L23 ELNES isvery clear above the background owing to the presence of sharp white lines whichoccur at the L23 ionization edges of the transition metals and at the M45 ionizationedges of the rare-earth elements because the Fermi energy is located in the narrow3d and 4f valence band respectively (Rez 1992) At the coatingmatrix interface O ispresent indicated by an O-K ionization edge (see reg gure 6 (a)) Towards the matrixagain C is evident

In the region of the coating the C-K and N-K edges show similar ELNESproreg les More clearly the C-K ELNES of spectrum 9 (at 12 nm) and the N-KELNES of spectrum 16 (at 213 nm) are compared in reg gure 7 Both ELNES proreg lesare essentially the same as those of stoichiometric TiC and TiN (Craven 1995Craven and Garvie 1995 Hosoi et al 1986) as they both show two narrow peaksat about 45 and 14 eV above their onset energy of about 284 eV (C-K) and 398eV(N-K) respectively A third broader peak is present in both cases at about 34 eVabove the respective onset energy

To obtain the [C][Ti] and [N][Ti] ratios and their sum (permilCŠ Dagger permilNŠ)[Ti] repre-sented in reg gure 8 the electron-energy-loss spectra of the near-reg bre coating region

T iN coatings in CMgplusmn Al composites 665

Figure 6 T5331HTATiNcp-Mg (a) three-dimensional plot of electron-energy-loss spectrataken at 30 equidistant points across the reg brematrix interregion arranged from reg bre(back) to matrix (front) (b) STEM bright-reg eld image showing the analysis distance asa white line

666 A Feldho et al

Figure 7 T5331HTATiNcp-Mg comparison of the C-K ELNES (top) and the N-KELNES (bottom) of two electron-energy-loss spectra taken from the coating

Figure 8 T5331HTATiNcp-Mg graph of [C][Ti] and [N][Ti] ratios and their sum(permilCŠ Dagger permilNŠ=permilTiŠdagger across the coating obtained by the quantireg cation of the electron-energy-loss spectra presented in reg gure 6

shown in reg gure 6 (a) were quantireg ed as standardless using hydrogenic scatteringcross-sections The carbon content ([C][Ti]) decreases from the reg bre (left) to thecoating with the nitrogen content ([N][Ti]) increasing The sum of both (permilCŠ Dagger permilNŠ)[Ti] remains constant across the coating but is always smaller than unity at about08

Changing the matrix to the Mg-Al alloy did not initiate a distinct formation ofcarbides (reg gure 9 (a)) For T5331HTATiNAM50 approximately the same micro-structure as with the pure Mg matrix was observed Only a few carbide precipitateshave locally formed at the reg brematrix interface as can be seen in reg gure 9 (a) at thereg bre on the top left Combining EDXS with STEM (reg gure 9 (b)) revealed approxi-mately 1plusmn 2 at Mg to be almost homogeneously distributed in the volume of thecarbon reg bre but no Al had penetrated into the body of the reg bre

However combining the same Mgplusmn Al matrix with the uncoated reg bre resulted inthe substantial formation of many carbidic plates of di erent size at the reg brematrixinterface (reg gure 10 (a)) In T5331HTAAM50 the plate-shaped precipitatesextend up to 15 mm into the matrix (reg gure 10 (a)) Using HREM and selected-area di raction they were identireg ed to be the ternary carbide Al2MgC2 as shownby its (0002) lattice fringe spacing of 062nm Often planar defects had formedparallel to the (0001) habit planes of the carbidic platelets (see also Feldho et al(1999)) In this composite EDXS in combination with STEM (reg gure 10 (b)) revealedapproximately 4plusmn 6 at Mg almost homogeneously distributed in the volume of thecarbon reg bre

Finally it should be mentioned that occasionally larger isolated carbide platescan be observed with dimensions above 1 mm which may pierce the TiCxNy reg lm andreach into the reg bre surface This may occur even in parts of the composite where thereg bre coating is relatively thick as shown in reg gure 11 for T5331HTATiNAM50with the coating thickness being 60plusmn 100 nm

T iN coatings in CMgplusmn Al composites 667

Figure 9 T5331HTATiNAM50 (a) bright-reg eld image showing the reg brematrix interfacialregion to be mainly free of carbide precipitates (b) energy-dispersive X-ray spectrumof a carbon reg bre

4 D ISCUSSION

The titanium nitride coating is polycrystalline with a grain size in the range 5plusmn10 nm and covering the reg bre surfaces almost completely The thickness of the coatingcan in some cases reach more than 100 nm but mainly varies between 10 and 30 nm

Concerning the chemical composition of the coating a strong correspondencebetween the C-K and the N-K ELNES features (see reg gure 7) was observed This is

668 A Feldho et al

Figure 10 T5331HTATiNAM50 (a) HVEM bright-reg eld image showing many platelets ofAl2MgC2 in the reg brematrix interregion (b) energy-dispersive X-ray spectrum of acarbon reg bre

Figure 11 T5331HTATiNAM50 HVEM bright-reg eld image showing a carbide precipitatepiercing the coating on the reg bre

caused by a strong relationship between the crystal chemistry of TiC and TiN whichare isotypic (Hosoi et al 1986) Both compounds exhibit the rock-salt structure asthe Ti atoms form a fcc lattice with the octahedral interstices being completely reg lledwith C and N atoms respectively The lattice parameters of TiC (a ˆ 0432 nm) andTiN (a ˆ 0423 nm) di er by only about 9 pm (ie 21) If not all octahedralinterstices of the Ti sublattice are occupied substoichiometric carbides TiCx(x lt 1) and nitrides TiNy (y lt 1dagger result The occupation of some octahedral inter-stices with C and others with N leads to the formation of titanium carbonitridesTiCxNy (x Dagger y 4 1) substitutional solid solutions with a wide solubility for C and N(Jonsson 1996) The lattice parameters of these ternary phases lie between those ofTiC and TiN Changes in the stoichiometry hardly a ect the C-K and N-K ELNESsof these carbonitrides Principally they lead to slight changes in the peak intensitieswhile the peak energies as well as the onset energy remain constant within the energyresolution (Pmacr uEgrave ger et al 1982)

Taking into consideration the observed reg ne-structure details at the C-K and N-Kionization edges (reg gure 7) together with the quantireg cation proreg le of electron-energy-loss spectra (reg gure 8) allows the following conclusion to be drawn The EELSanalyses shown in reg gures 6plusmn 8 prove that the coating consists of a substoichiometrictitanium carbonitride (TiCxNy) with a high C content at the reg brecoating interfacegradually becoming richer in N on going towards the middle of thereg lm Nevertheless the vacancy concentration in the non-metal sublattice is almostconstant (x Dagger y ordm 08dagger (see reg gure 8) It is pointed out that TiCxNy with evenhigher vacancy concentrations (x Dagger y lt 07) have been observed (Em andTashmetov 1996)

The compositional changes within the coating hint to a reaction of the reactorgas mixture (TiCl4 N2 and H2) with C from the reg bre during the initial state of CVDThus the term reactive chemical vapour deposition (RCVD) which wasproposed by Vincent et al (1992) seems to be suitable here and the chemicalprocesses during deposition are much more complex than previously assumed bythe reaction TiCl4 Dagger 1

2 N2 Dagger H2 TiN Dagger 4 HCl used by Wurm et al (1997) andWurm (1998)

In reg gure 6 (a) the electron-energy-loss spectrum 20 (at 266 nm) at the coatingmatrix interface shows almost no C or N but clearly Ti and O Probably theoccurrence of O correlates to that of Mg the ionization edges of which (Mg-L23

at 49 eV Mg-K at 1305 eV) are outside the investigated energy interval An under-standing of the bonding mechanism between the Mg matrix and the TiCxNy coatingrequires the comprehensive analysis of ELNES details at the coatingmatrix interfaceas it may be associated with phase formation on the nanoscopic scale (Tiplusmn Mg inter-metallics Tiplusmn Mg spinels ternary nitrides or quarternary carbonitrides of the systemTiplusmn Mgplusmn Cplusmn N) Accordingly the ionization edges of Mg at about 49 eV (Mg-L23) and1305 eV (Mg-K) have to be analysed whereas an overlap of the Mg-L23 ELNESwith the Ti-M23 and Ti-M1 ELNES has to be taken into consideration as their onsetenergies are at about 35 or 60 eV respectively These analyses are beyond the scope ofthis paper They are of particular interest as Wurm et al (1997) and Wurm (1998)found for the T5331HTATiNcp-Mg composite a relatively high tensile strengthperpendicular to the reg bre axis (approximately 25 MPa) which hints at good adhe-sion of the Mg matrix on the TiN-coated carbon reg bres Nevertheless here we couldprove that the adhesion does not correlate with the formation of larger brittlephases

T iN coatings in CMgplusmn Al composites 669

The microstructure of T5331HTATiNAM50 strongly resembles that ofT5331HTATiNcp-Mg with its reg brematrix interfaces being mainly free of carbides(see reg gures 9 (a) and 1 (a)) On the other hand the same Mgplusmn Al alloy in contact withthe uncoated carbon reg bre resulted in the substantial formation of Al2MgC2 (reg gure10 (a)) thus indicating a strong di usion barrier e ect of the titanium nitride coat-ing This is conreg rmed by a lowered mass transport in the reverse directionthat is from matrix to reg bre due to the TiCxNy interfacial reg lm as only1plusmn 2 at Mg penetrated into the coated reg bre (reg gures 1 (b) and 9 (b)) compared with4plusmn 6 at Mg in case of the uncoated reg bre (reg gure 10 (b)) Thus carbide formationand reg bre degradation are simultaneously limited

Nevertheless a graphitic interlayer has in certain samples been detected at thecoatingmatrix interface in T5331HTATiNcp-Mg indicating that locally C wastransported from the reg bre to the matrix As this phenomenon is not very pro-nounced it is concluded that the C transport across the reg brematrix interfacedoes not occur through TiCxNy grains even if they exhibit signireg cant vacancyconcentrations in the non-metal sublattice Geometrical considerations on thethree-dimensional structure of the coating lead to the conclusion that there existlocally reg ssures in the coating (the reg lm thickness is in the range of the grain dia-meters) Thus locally liquid Mg penetrates through reg ssures in the reg lm towards thereg bre (reg gure 2 (d)) In successive steps C from the reg bre dissolves and di uses in theliquid Mg through the coating and precipitates as graphitic ribbons at the coatingmatrix interface The thermodynamics of such an isothermal dissolutionplusmn di usionplusmnprecipitation process have been described by Fitzer and Kegel (1968) and theyshowed that it is driven by a di erence in the chemical potential of C in the reg breand in the graphitic ribbon

Accordingly the formation of some large Al2MgC2 plates in T5331HTATiNAM50 is not full excluded and these plates may even pierce the reg bre coating (reg gures9 (a) and 11) At these locations notching may occur which can initiate reg bre crack-ing Consequently the strength distribution function of the reg bres (Danzer 1994) isa ected by the introduction of this new class of defects and the probability of reg brefracture at low loads is enhanced The formation of many large carbide plates evenleads to an embrittlement of CMgplusmn Al composites (Feldho et al 1997 HaEgrave hnel et al1997 Feldho 1998) As there are only a few of these large carbide plates inT5331HTATiNAM50 that is the e ect is very limited here accordingly thestrength of this composite was found to be only slightly lower than that ofT5331HTATiNcp-Mg (Wurm et al 1997 Wurm 1997)

Altogether the amount of carbide formation in T5331HTATiNAM50 (reg gure9 (a)) is signireg cantly lower than in T5331HTAAM50 (reg gure 10 (a)) with theuncoated carbon reg bres This is because the TiCxNy reg lm serves well as an inhibitorof deleterious carbide formation also when using Mg alloys with large amounts ofAl as matrices Thus the use of Mgplusmn Al matrices for the manufacture of hybridcomponents is enabled by the di usion retarding e ect of the chemically vapour-deposited TiCxNy coating

5 CONCLUSIONS

CVD coating on the carbon reg bres has been shown to be partly substoichiometrictitanium carbonitride (TiCxNy) rather than titanium nitride (TiN) Even though thebonding mechanism at the coatingmatrix interface is still not clear it has been

670 A Feldho et al

shown that the adhesion of Mg on the TiCxNy coating is not accompanied by theformation of extensive brittle phases

A titanium carbonitride reg lm homogeneously covering the reg bre surfaces ham-pers the transport processes across the reg brematrix interface The penetration of Mginto the carbon reg bre is signireg cantly lowered and so is the formation of Al2MgC2with deleterious notch e ects Thus the TiCxNy coating enables the use of highcontents of Al in the matrix alloy without the negative e ects of the carbide pre-cipitates on the mechanical properties of the composite

ACKNOWLEDGEMENTS

The authors would like to thank Professor R F Singer and Dr D Wurm(University of Erlangenplusmn NuEgrave rnberg) for providing the MMC samples

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struct 2 159CRAVEN A J 1995 J Microsc 180 250CRAVEN A J and G ARVIE L A J 1995 Microsc Microanal Microstruct 6 89D ANZER R 1994 The Encyclopedia of Advanced Materials Vol 1 edited by D Bloor R J

Brook M C Flemings S Mahajan and R W Cahn (Oxford Pergamon)pp 385plusmn 398

D ELANNAY F FROYEN L and D ERUYTTERE A 1987 J Mater Sci 22 1EM V T and TASHMETOV M YU 1996 Phys Stat sol (b) 198 571F ELDHOFF A 1998 PhD Thesis Martin Luther University of Halle-Wittenberg

(Aachen-Maastricht Shaker)F ELDHOFF A P IP PEL E and WOLTERSDORF J 1997 J Microsc 185 122 1999 Phil

Mag A 79 1263F ITZER E and K EGEL B 1968 Carbon 6 433H AiumlHNEL A PIP PEL E F ELDHOFF A SCHNEIDER R and WOLTERSDORF J 1997 Mater

Sci Engng 237 173H OFER F WARBICHLER P and GROGGER W 1995 Ultramicroscopy 59 15H OSOI J O IKAW A T and BANDO Y 1986 J Electron Microsc 35 129JONSSON S 1996 Z Metall 87 713K RIVANEKOLG RUBBENSA JDELLBYN and M EYERCE1992 Microsc Microanal

Microstruct 3 187N AIDICH J V 1981 Prog Surf Membr Sci 14 353Oiuml TTING ER O 1996 PhD Thesis Friedrich Alexander University of Erlangen-NuEgrave rnberg

(DuEgrave sseldorf VDI)Oiuml TTING ER O G RAU C W INTER R SINGER R F F ELDHOFF A PIPPEL E and

WOLTERSDORF J 1995 Proceedings of the Tenth International Conference onComposite Materials Vol VI Whistler Canada (Cambridge Woodhead) pp 447plusmn454

Oiuml TTING ER O and SINGER R F 1993 Z Metall 84 827PFLUiuml GER J F INK J CRECELIUS G BOHNEN K P and W INTER H 1982 Solid St

Commun 44 489PICOUET L ABIVEN H VIALA J C and BOUIX J 1990 Advanced Composite Materials

Proceedings of the First Frenchplusmn Japanese Seminar on Composite Materials edited by CBathias and M Uemura (Paris SIRPE) pp 121plusmn 131

POLMEAR I J 1981 L ight AlloysETH Metallurgy of the L ight Alloys (London Edward Arnold)R EZ P1992 Transmission Electron Energy L oss Spectrometry in Materials Science edited by

M M Disko C C Ahn and B Fultz (Warrendale Pennsylvania Minerals Metalsand Materials Society) pp 107plusmn 129

SHORSHOROV M K U STINOV L M Z IRLIN A M and OLEFIRENKO V I 1979 JMater Sci 14 1850

T iN coatings in CMgplusmn Al composites 671

high-voltage electron microscopy (HVEM) high-resolution electron microscopy(HREM) energy-reg ltered transmission electron microscopy (EFTEM) and scanningtransmission electron microscopy (STEM) in combination with energy-dispersiveX-ray spectroscopy (EDXS) and electron-energy-loss spectrometry (EELS) withparticular emphasis on energy-loss near-edge structures (ELNES) Then the resultsgained on the di erent materials will be treated together Finally an evaluation ofthe coating properties for the manufacturing of composites or hybrid componentswill follow

2 EXPERIMENTAL DETAILS

To prepare the composites at the University of Erlangen (Wurm et al 1997Wurm 1998) the C-reg bre yarn Tenax 5331HTA (T5331HTA) (TohoAkzo) of thehigh-tensile-strength type (6000 single reg laments tensile strength frac14f ˆ 3950 MPaYoungrsquos modulus Mf ˆ 238 MPa) was thermally desized and passed through aCVD counter-current reactor using a mixture of TiCl4 N2 and H2 as the precursorgas The parameters of the CVD process were chosen so as to yield an invariance inthe reg bre properties compared with the as-received state and a complete coveringof all the reg laments in the yarn with TiN (Wurm et al 1997 Wurm 1998)Unidirectionally wound C-reg bre pre-forms were inreg ltrated with matrices of com-mercial purity Mg (cp-Mg) and of a Mgplusmn Al alloy (AM50) (5 wt Alplusmn 04 wt Mn(Polmear 1981)) via a gas-pressure melt inreg ltration process (OEgrave ttinger and Singer1993) at 7208C and 10 MPa employing a 5 min reg breplusmn melt contact to obtainMMCs with a reg bre content of approximately 58 vol The following materialswere investigated reg rstly T5331HTATiNcp-Mg secondly T5331HTATiNAM50thirdly T5331HTAAM50 Mechanical testing data can be found in the studies byWurm et al (1997) and Wurm (1998)

For transmission electron microscopy (TEM) investigations cross-sections of thereg brematrix interface area were prepared by cutting thin (500 mm) slices of 3 mmdiameter planar grinding and double-mould dimpling to a thickness of about 20 mmTo prevent galvanic attack on the Mg a water-free lubricant was used Final thin-ning was performed by ion milling (ArDagger 5 kV) down to electron transparency(Feldho 1998)

HVEM investigations were carried out using a JEOL 1000-06 microscope oper-ating at 1 MV For the HREM STEM EDXS EFTEM and EELS investigations acombined (scanning) transmission electron microscope of the type Philips CM 20FEG operating at 200 kV was used which was equipped with a light element X-raydetector (Voyager II Tracor) and a post-column electron energy reg lter (GatanImaging Filter GIF 200 model 667) as well as with a digital scanning module(Gatan Digiscan) To minimize contamination e ects a cooling specimen holder(Gatan model 668) was used during the STEM procedures For image processingand electron-energy-loss spectrum treatment the Gatan Digital Micrograph andELP software run on a Power Macintosh 720075 were employed

The thermally assisted reg eld emission gun enables an energy resolution of theelectron-energy-loss spectra of 08plusmn 1 eV which were usually acquired at a dispersionof 05 eV per channel This allows us to obtain information on the chemical bondingstate of the elements of interest by analysing the ELNES of the relevant ionizationedges which can be attributed to transitions of core-shell electrons into unoccupiedstates above the Fermi level (Brydson et al 1991 Rez 1992)

T iN coatings in CMgplusmn Al composites 661

To achieve high lateral resolution in the EDXS and EELS procedures onlysample regions which were suitable for HREM have been analysed by these meth-ods Additionally to exclude multiple-scattering e ects on core-loss spectra inEELS here the sample thickness has been counter-checked by the peak intensityratios of the zero loss to volume plasmons in corresponding low-loss spectra

To gain information about the elemental distribution in the reg brematrix interfaceregion EFTEM with the three-window method (Krivanek et al 1992 Hofer et al1995) was used A slit (20plusmn 30 eV wide) is inserted in the intermediate spectral plane ofthe energy reg lter to select electrons which have lost a specireg c amount of energy byinelastic interaction with the specimen With these electrons an image is projected onto a slow-scan change-coupled device camera One image is taken with the energywindow slightly above an ionization edge of the element of interest and two imagesare taken with the energy windows below the ionization edge (for background extra-polation) to calculate one reg nal image in which the bright contrast represents thespatial distribution of the considered element

3 R ESULTS

In the bright-reg eld image of T5331HTATiNcp-Mg in reg gure 1 (a) the coatingappears dark as a continuous band with its thickness varying here from 30 to 100 nmBy combining STEM and EDXS an almost homogeneous distribution of approxi-mately 1plusmn 2 at Mg was detected in the volume of the carbon reg bre in this composite(reg gure 1 (b))

The interfacial structure in this material is mainly characterized by an almoststringent separation of matrix and reg bre by the coating as is shown in the bright-reg eldimage in reg gure 2 (a) and the corresponding EFTEM images in reg gures 2 (b) plusmn (d) takenat the ionization edges Ti-L23 (456 eV) (reg gure 2 (b)) C-K (284 eV) (reg gure 2 (c)) andMg-K (1305 eV) (reg gure 2 (d)) with the reg bre on the left and the matrix on the right

662 A Feldho et al

Figure 1 T5331HTATiNcp-Mg (a) bright-reg eld image showing the coating on three reg breswith dark contrast to be continuous (b) energy-dispersive X-ray spectrum of a carbon reg bre

The elemental distribution of Ti (reg gure 2 (b)) reveals the continuous covering of thereg bre with the reg lm here 10plusmn 20 nm in thickness The distribution of C (reg gure 2 (c))represents mainly the reg bre and that of Mg (reg gure 2 (d)) the matrix A slight penetra-tion of Mg through the coating towards the reg bre is indicated by some brightcontrast features in reg gure 2 (d) left of the coating The HREM image in reg gure 3

T iN coatings in CMgplusmn Al composites 663

Figure 2 T5331HTATiNcp-Mg (a) bright-reg eld image (b) (c) EFTEM at the ionizationedges

Figure 3 HREM image of T5331HTATiNcp-Mg showing the (10 -10dagger lattice fringes of theMg matrix ending on the coating (left reg bre right matrix)

shows the turbostratic C of the reg bre (left) and the (10 -10) lattice fringes of the Mgmatrix (right) ending at the coating (middle part) The polycrystalline nature of thecoating with grain sizes in the range of the reg lm thickness is clearly revealed

In reg gure 4 (a) the integrity of the coating between reg bre (left) and matrix (right) isshown by the elemental distribution of titanium Only rarely did T5331HTATiNcp-Mg exhibit C on the matrix side of the coating as revealed by the EFTEM imagein reg gure 4 (b) This carbon forms a 10 nm thick graphitic ribbon with its basal planesin parallel orientation to the reg bre surface as shown in the corresponding HREMimage in reg gure 5

In reg gure 6 (a) a three-dimensional plot of electron-energy-loss spectra is givenwhich were recorded across the reg brematrix interregion at 30 equidistant pointsalong the white marker in the STEM bright-reg eld image (reg gure 6 (b)) with thepoint-to-point distance amounting to about 13 nm In the energy interval between250 and 750eV there appear the C-K N-K Ti-L23 and O-K ionization edges and

664 A Feldho et al

Figure 4 EFTEM at the ionization edges of the reg brematrix interregion in T5331HTATiNcp-Mg (a) Ti-L23 (b) C-K

Figure 5 HREM image of T5331HTATiNcp-Mg showing a graphitic ribbon on the matrixside of the coating (left reg bre right matrix)

their associated reg ne structures In the region of the reg bre (spectra in the backgroundreg gure 6 (a)) only the C-K ionization edge at 284 eV arises from the exponentiallydecreasing background At the reg brecoating interface a Ti-L23 ionization edgeappears and the C-K ELNES changes Within a gradient of approximately 5 nma N-K ionization edge arises and the C-K edge disappears The Ti-L23 ELNES isvery clear above the background owing to the presence of sharp white lines whichoccur at the L23 ionization edges of the transition metals and at the M45 ionizationedges of the rare-earth elements because the Fermi energy is located in the narrow3d and 4f valence band respectively (Rez 1992) At the coatingmatrix interface O ispresent indicated by an O-K ionization edge (see reg gure 6 (a)) Towards the matrixagain C is evident

In the region of the coating the C-K and N-K edges show similar ELNESproreg les More clearly the C-K ELNES of spectrum 9 (at 12 nm) and the N-KELNES of spectrum 16 (at 213 nm) are compared in reg gure 7 Both ELNES proreg lesare essentially the same as those of stoichiometric TiC and TiN (Craven 1995Craven and Garvie 1995 Hosoi et al 1986) as they both show two narrow peaksat about 45 and 14 eV above their onset energy of about 284 eV (C-K) and 398eV(N-K) respectively A third broader peak is present in both cases at about 34 eVabove the respective onset energy

To obtain the [C][Ti] and [N][Ti] ratios and their sum (permilCŠ Dagger permilNŠ)[Ti] repre-sented in reg gure 8 the electron-energy-loss spectra of the near-reg bre coating region

T iN coatings in CMgplusmn Al composites 665

Figure 6 T5331HTATiNcp-Mg (a) three-dimensional plot of electron-energy-loss spectrataken at 30 equidistant points across the reg brematrix interregion arranged from reg bre(back) to matrix (front) (b) STEM bright-reg eld image showing the analysis distance asa white line

666 A Feldho et al

Figure 7 T5331HTATiNcp-Mg comparison of the C-K ELNES (top) and the N-KELNES (bottom) of two electron-energy-loss spectra taken from the coating

Figure 8 T5331HTATiNcp-Mg graph of [C][Ti] and [N][Ti] ratios and their sum(permilCŠ Dagger permilNŠ=permilTiŠdagger across the coating obtained by the quantireg cation of the electron-energy-loss spectra presented in reg gure 6

shown in reg gure 6 (a) were quantireg ed as standardless using hydrogenic scatteringcross-sections The carbon content ([C][Ti]) decreases from the reg bre (left) to thecoating with the nitrogen content ([N][Ti]) increasing The sum of both (permilCŠ Dagger permilNŠ)[Ti] remains constant across the coating but is always smaller than unity at about08

Changing the matrix to the Mg-Al alloy did not initiate a distinct formation ofcarbides (reg gure 9 (a)) For T5331HTATiNAM50 approximately the same micro-structure as with the pure Mg matrix was observed Only a few carbide precipitateshave locally formed at the reg brematrix interface as can be seen in reg gure 9 (a) at thereg bre on the top left Combining EDXS with STEM (reg gure 9 (b)) revealed approxi-mately 1plusmn 2 at Mg to be almost homogeneously distributed in the volume of thecarbon reg bre but no Al had penetrated into the body of the reg bre

However combining the same Mgplusmn Al matrix with the uncoated reg bre resulted inthe substantial formation of many carbidic plates of di erent size at the reg brematrixinterface (reg gure 10 (a)) In T5331HTAAM50 the plate-shaped precipitatesextend up to 15 mm into the matrix (reg gure 10 (a)) Using HREM and selected-area di raction they were identireg ed to be the ternary carbide Al2MgC2 as shownby its (0002) lattice fringe spacing of 062nm Often planar defects had formedparallel to the (0001) habit planes of the carbidic platelets (see also Feldho et al(1999)) In this composite EDXS in combination with STEM (reg gure 10 (b)) revealedapproximately 4plusmn 6 at Mg almost homogeneously distributed in the volume of thecarbon reg bre

Finally it should be mentioned that occasionally larger isolated carbide platescan be observed with dimensions above 1 mm which may pierce the TiCxNy reg lm andreach into the reg bre surface This may occur even in parts of the composite where thereg bre coating is relatively thick as shown in reg gure 11 for T5331HTATiNAM50with the coating thickness being 60plusmn 100 nm

T iN coatings in CMgplusmn Al composites 667

Figure 9 T5331HTATiNAM50 (a) bright-reg eld image showing the reg brematrix interfacialregion to be mainly free of carbide precipitates (b) energy-dispersive X-ray spectrumof a carbon reg bre

4 D ISCUSSION

The titanium nitride coating is polycrystalline with a grain size in the range 5plusmn10 nm and covering the reg bre surfaces almost completely The thickness of the coatingcan in some cases reach more than 100 nm but mainly varies between 10 and 30 nm

Concerning the chemical composition of the coating a strong correspondencebetween the C-K and the N-K ELNES features (see reg gure 7) was observed This is

668 A Feldho et al

Figure 10 T5331HTATiNAM50 (a) HVEM bright-reg eld image showing many platelets ofAl2MgC2 in the reg brematrix interregion (b) energy-dispersive X-ray spectrum of acarbon reg bre

Figure 11 T5331HTATiNAM50 HVEM bright-reg eld image showing a carbide precipitatepiercing the coating on the reg bre

caused by a strong relationship between the crystal chemistry of TiC and TiN whichare isotypic (Hosoi et al 1986) Both compounds exhibit the rock-salt structure asthe Ti atoms form a fcc lattice with the octahedral interstices being completely reg lledwith C and N atoms respectively The lattice parameters of TiC (a ˆ 0432 nm) andTiN (a ˆ 0423 nm) di er by only about 9 pm (ie 21) If not all octahedralinterstices of the Ti sublattice are occupied substoichiometric carbides TiCx(x lt 1) and nitrides TiNy (y lt 1dagger result The occupation of some octahedral inter-stices with C and others with N leads to the formation of titanium carbonitridesTiCxNy (x Dagger y 4 1) substitutional solid solutions with a wide solubility for C and N(Jonsson 1996) The lattice parameters of these ternary phases lie between those ofTiC and TiN Changes in the stoichiometry hardly a ect the C-K and N-K ELNESsof these carbonitrides Principally they lead to slight changes in the peak intensitieswhile the peak energies as well as the onset energy remain constant within the energyresolution (Pmacr uEgrave ger et al 1982)

Taking into consideration the observed reg ne-structure details at the C-K and N-Kionization edges (reg gure 7) together with the quantireg cation proreg le of electron-energy-loss spectra (reg gure 8) allows the following conclusion to be drawn The EELSanalyses shown in reg gures 6plusmn 8 prove that the coating consists of a substoichiometrictitanium carbonitride (TiCxNy) with a high C content at the reg brecoating interfacegradually becoming richer in N on going towards the middle of thereg lm Nevertheless the vacancy concentration in the non-metal sublattice is almostconstant (x Dagger y ordm 08dagger (see reg gure 8) It is pointed out that TiCxNy with evenhigher vacancy concentrations (x Dagger y lt 07) have been observed (Em andTashmetov 1996)

The compositional changes within the coating hint to a reaction of the reactorgas mixture (TiCl4 N2 and H2) with C from the reg bre during the initial state of CVDThus the term reactive chemical vapour deposition (RCVD) which wasproposed by Vincent et al (1992) seems to be suitable here and the chemicalprocesses during deposition are much more complex than previously assumed bythe reaction TiCl4 Dagger 1

2 N2 Dagger H2 TiN Dagger 4 HCl used by Wurm et al (1997) andWurm (1998)

In reg gure 6 (a) the electron-energy-loss spectrum 20 (at 266 nm) at the coatingmatrix interface shows almost no C or N but clearly Ti and O Probably theoccurrence of O correlates to that of Mg the ionization edges of which (Mg-L23

at 49 eV Mg-K at 1305 eV) are outside the investigated energy interval An under-standing of the bonding mechanism between the Mg matrix and the TiCxNy coatingrequires the comprehensive analysis of ELNES details at the coatingmatrix interfaceas it may be associated with phase formation on the nanoscopic scale (Tiplusmn Mg inter-metallics Tiplusmn Mg spinels ternary nitrides or quarternary carbonitrides of the systemTiplusmn Mgplusmn Cplusmn N) Accordingly the ionization edges of Mg at about 49 eV (Mg-L23) and1305 eV (Mg-K) have to be analysed whereas an overlap of the Mg-L23 ELNESwith the Ti-M23 and Ti-M1 ELNES has to be taken into consideration as their onsetenergies are at about 35 or 60 eV respectively These analyses are beyond the scope ofthis paper They are of particular interest as Wurm et al (1997) and Wurm (1998)found for the T5331HTATiNcp-Mg composite a relatively high tensile strengthperpendicular to the reg bre axis (approximately 25 MPa) which hints at good adhe-sion of the Mg matrix on the TiN-coated carbon reg bres Nevertheless here we couldprove that the adhesion does not correlate with the formation of larger brittlephases

T iN coatings in CMgplusmn Al composites 669

The microstructure of T5331HTATiNAM50 strongly resembles that ofT5331HTATiNcp-Mg with its reg brematrix interfaces being mainly free of carbides(see reg gures 9 (a) and 1 (a)) On the other hand the same Mgplusmn Al alloy in contact withthe uncoated carbon reg bre resulted in the substantial formation of Al2MgC2 (reg gure10 (a)) thus indicating a strong di usion barrier e ect of the titanium nitride coat-ing This is conreg rmed by a lowered mass transport in the reverse directionthat is from matrix to reg bre due to the TiCxNy interfacial reg lm as only1plusmn 2 at Mg penetrated into the coated reg bre (reg gures 1 (b) and 9 (b)) compared with4plusmn 6 at Mg in case of the uncoated reg bre (reg gure 10 (b)) Thus carbide formationand reg bre degradation are simultaneously limited

Nevertheless a graphitic interlayer has in certain samples been detected at thecoatingmatrix interface in T5331HTATiNcp-Mg indicating that locally C wastransported from the reg bre to the matrix As this phenomenon is not very pro-nounced it is concluded that the C transport across the reg brematrix interfacedoes not occur through TiCxNy grains even if they exhibit signireg cant vacancyconcentrations in the non-metal sublattice Geometrical considerations on thethree-dimensional structure of the coating lead to the conclusion that there existlocally reg ssures in the coating (the reg lm thickness is in the range of the grain dia-meters) Thus locally liquid Mg penetrates through reg ssures in the reg lm towards thereg bre (reg gure 2 (d)) In successive steps C from the reg bre dissolves and di uses in theliquid Mg through the coating and precipitates as graphitic ribbons at the coatingmatrix interface The thermodynamics of such an isothermal dissolutionplusmn di usionplusmnprecipitation process have been described by Fitzer and Kegel (1968) and theyshowed that it is driven by a di erence in the chemical potential of C in the reg breand in the graphitic ribbon

Accordingly the formation of some large Al2MgC2 plates in T5331HTATiNAM50 is not full excluded and these plates may even pierce the reg bre coating (reg gures9 (a) and 11) At these locations notching may occur which can initiate reg bre crack-ing Consequently the strength distribution function of the reg bres (Danzer 1994) isa ected by the introduction of this new class of defects and the probability of reg brefracture at low loads is enhanced The formation of many large carbide plates evenleads to an embrittlement of CMgplusmn Al composites (Feldho et al 1997 HaEgrave hnel et al1997 Feldho 1998) As there are only a few of these large carbide plates inT5331HTATiNAM50 that is the e ect is very limited here accordingly thestrength of this composite was found to be only slightly lower than that ofT5331HTATiNcp-Mg (Wurm et al 1997 Wurm 1997)

Altogether the amount of carbide formation in T5331HTATiNAM50 (reg gure9 (a)) is signireg cantly lower than in T5331HTAAM50 (reg gure 10 (a)) with theuncoated carbon reg bres This is because the TiCxNy reg lm serves well as an inhibitorof deleterious carbide formation also when using Mg alloys with large amounts ofAl as matrices Thus the use of Mgplusmn Al matrices for the manufacture of hybridcomponents is enabled by the di usion retarding e ect of the chemically vapour-deposited TiCxNy coating

5 CONCLUSIONS

CVD coating on the carbon reg bres has been shown to be partly substoichiometrictitanium carbonitride (TiCxNy) rather than titanium nitride (TiN) Even though thebonding mechanism at the coatingmatrix interface is still not clear it has been

670 A Feldho et al

shown that the adhesion of Mg on the TiCxNy coating is not accompanied by theformation of extensive brittle phases

A titanium carbonitride reg lm homogeneously covering the reg bre surfaces ham-pers the transport processes across the reg brematrix interface The penetration of Mginto the carbon reg bre is signireg cantly lowered and so is the formation of Al2MgC2with deleterious notch e ects Thus the TiCxNy coating enables the use of highcontents of Al in the matrix alloy without the negative e ects of the carbide pre-cipitates on the mechanical properties of the composite

ACKNOWLEDGEMENTS

The authors would like to thank Professor R F Singer and Dr D Wurm(University of Erlangenplusmn NuEgrave rnberg) for providing the MMC samples

REFERENCESANDRIEVSKI R A 1997 J Mater Sci 32 4463BRYDSON R SAUER H ENGEL W and ZEITLER E 1991 Microsc Microanal Micro-

struct 2 159CRAVEN A J 1995 J Microsc 180 250CRAVEN A J and G ARVIE L A J 1995 Microsc Microanal Microstruct 6 89D ANZER R 1994 The Encyclopedia of Advanced Materials Vol 1 edited by D Bloor R J

Brook M C Flemings S Mahajan and R W Cahn (Oxford Pergamon)pp 385plusmn 398

D ELANNAY F FROYEN L and D ERUYTTERE A 1987 J Mater Sci 22 1EM V T and TASHMETOV M YU 1996 Phys Stat sol (b) 198 571F ELDHOFF A 1998 PhD Thesis Martin Luther University of Halle-Wittenberg

(Aachen-Maastricht Shaker)F ELDHOFF A P IP PEL E and WOLTERSDORF J 1997 J Microsc 185 122 1999 Phil

Mag A 79 1263F ITZER E and K EGEL B 1968 Carbon 6 433H AiumlHNEL A PIP PEL E F ELDHOFF A SCHNEIDER R and WOLTERSDORF J 1997 Mater

Sci Engng 237 173H OFER F WARBICHLER P and GROGGER W 1995 Ultramicroscopy 59 15H OSOI J O IKAW A T and BANDO Y 1986 J Electron Microsc 35 129JONSSON S 1996 Z Metall 87 713K RIVANEKOLG RUBBENSA JDELLBYN and M EYERCE1992 Microsc Microanal

Microstruct 3 187N AIDICH J V 1981 Prog Surf Membr Sci 14 353Oiuml TTING ER O 1996 PhD Thesis Friedrich Alexander University of Erlangen-NuEgrave rnberg

(DuEgrave sseldorf VDI)Oiuml TTING ER O G RAU C W INTER R SINGER R F F ELDHOFF A PIPPEL E and

WOLTERSDORF J 1995 Proceedings of the Tenth International Conference onComposite Materials Vol VI Whistler Canada (Cambridge Woodhead) pp 447plusmn454

Oiuml TTING ER O and SINGER R F 1993 Z Metall 84 827PFLUiuml GER J F INK J CRECELIUS G BOHNEN K P and W INTER H 1982 Solid St

Commun 44 489PICOUET L ABIVEN H VIALA J C and BOUIX J 1990 Advanced Composite Materials

Proceedings of the First Frenchplusmn Japanese Seminar on Composite Materials edited by CBathias and M Uemura (Paris SIRPE) pp 121plusmn 131

POLMEAR I J 1981 L ight AlloysETH Metallurgy of the L ight Alloys (London Edward Arnold)R EZ P1992 Transmission Electron Energy L oss Spectrometry in Materials Science edited by

M M Disko C C Ahn and B Fultz (Warrendale Pennsylvania Minerals Metalsand Materials Society) pp 107plusmn 129

SHORSHOROV M K U STINOV L M Z IRLIN A M and OLEFIRENKO V I 1979 JMater Sci 14 1850

T iN coatings in CMgplusmn Al composites 671

To achieve high lateral resolution in the EDXS and EELS procedures onlysample regions which were suitable for HREM have been analysed by these meth-ods Additionally to exclude multiple-scattering e ects on core-loss spectra inEELS here the sample thickness has been counter-checked by the peak intensityratios of the zero loss to volume plasmons in corresponding low-loss spectra

To gain information about the elemental distribution in the reg brematrix interfaceregion EFTEM with the three-window method (Krivanek et al 1992 Hofer et al1995) was used A slit (20plusmn 30 eV wide) is inserted in the intermediate spectral plane ofthe energy reg lter to select electrons which have lost a specireg c amount of energy byinelastic interaction with the specimen With these electrons an image is projected onto a slow-scan change-coupled device camera One image is taken with the energywindow slightly above an ionization edge of the element of interest and two imagesare taken with the energy windows below the ionization edge (for background extra-polation) to calculate one reg nal image in which the bright contrast represents thespatial distribution of the considered element

3 R ESULTS

In the bright-reg eld image of T5331HTATiNcp-Mg in reg gure 1 (a) the coatingappears dark as a continuous band with its thickness varying here from 30 to 100 nmBy combining STEM and EDXS an almost homogeneous distribution of approxi-mately 1plusmn 2 at Mg was detected in the volume of the carbon reg bre in this composite(reg gure 1 (b))

The interfacial structure in this material is mainly characterized by an almoststringent separation of matrix and reg bre by the coating as is shown in the bright-reg eldimage in reg gure 2 (a) and the corresponding EFTEM images in reg gures 2 (b) plusmn (d) takenat the ionization edges Ti-L23 (456 eV) (reg gure 2 (b)) C-K (284 eV) (reg gure 2 (c)) andMg-K (1305 eV) (reg gure 2 (d)) with the reg bre on the left and the matrix on the right

662 A Feldho et al

Figure 1 T5331HTATiNcp-Mg (a) bright-reg eld image showing the coating on three reg breswith dark contrast to be continuous (b) energy-dispersive X-ray spectrum of a carbon reg bre

The elemental distribution of Ti (reg gure 2 (b)) reveals the continuous covering of thereg bre with the reg lm here 10plusmn 20 nm in thickness The distribution of C (reg gure 2 (c))represents mainly the reg bre and that of Mg (reg gure 2 (d)) the matrix A slight penetra-tion of Mg through the coating towards the reg bre is indicated by some brightcontrast features in reg gure 2 (d) left of the coating The HREM image in reg gure 3

T iN coatings in CMgplusmn Al composites 663

Figure 2 T5331HTATiNcp-Mg (a) bright-reg eld image (b) (c) EFTEM at the ionizationedges

Figure 3 HREM image of T5331HTATiNcp-Mg showing the (10 -10dagger lattice fringes of theMg matrix ending on the coating (left reg bre right matrix)

shows the turbostratic C of the reg bre (left) and the (10 -10) lattice fringes of the Mgmatrix (right) ending at the coating (middle part) The polycrystalline nature of thecoating with grain sizes in the range of the reg lm thickness is clearly revealed

In reg gure 4 (a) the integrity of the coating between reg bre (left) and matrix (right) isshown by the elemental distribution of titanium Only rarely did T5331HTATiNcp-Mg exhibit C on the matrix side of the coating as revealed by the EFTEM imagein reg gure 4 (b) This carbon forms a 10 nm thick graphitic ribbon with its basal planesin parallel orientation to the reg bre surface as shown in the corresponding HREMimage in reg gure 5

In reg gure 6 (a) a three-dimensional plot of electron-energy-loss spectra is givenwhich were recorded across the reg brematrix interregion at 30 equidistant pointsalong the white marker in the STEM bright-reg eld image (reg gure 6 (b)) with thepoint-to-point distance amounting to about 13 nm In the energy interval between250 and 750eV there appear the C-K N-K Ti-L23 and O-K ionization edges and

664 A Feldho et al

Figure 4 EFTEM at the ionization edges of the reg brematrix interregion in T5331HTATiNcp-Mg (a) Ti-L23 (b) C-K

Figure 5 HREM image of T5331HTATiNcp-Mg showing a graphitic ribbon on the matrixside of the coating (left reg bre right matrix)

their associated reg ne structures In the region of the reg bre (spectra in the backgroundreg gure 6 (a)) only the C-K ionization edge at 284 eV arises from the exponentiallydecreasing background At the reg brecoating interface a Ti-L23 ionization edgeappears and the C-K ELNES changes Within a gradient of approximately 5 nma N-K ionization edge arises and the C-K edge disappears The Ti-L23 ELNES isvery clear above the background owing to the presence of sharp white lines whichoccur at the L23 ionization edges of the transition metals and at the M45 ionizationedges of the rare-earth elements because the Fermi energy is located in the narrow3d and 4f valence band respectively (Rez 1992) At the coatingmatrix interface O ispresent indicated by an O-K ionization edge (see reg gure 6 (a)) Towards the matrixagain C is evident

In the region of the coating the C-K and N-K edges show similar ELNESproreg les More clearly the C-K ELNES of spectrum 9 (at 12 nm) and the N-KELNES of spectrum 16 (at 213 nm) are compared in reg gure 7 Both ELNES proreg lesare essentially the same as those of stoichiometric TiC and TiN (Craven 1995Craven and Garvie 1995 Hosoi et al 1986) as they both show two narrow peaksat about 45 and 14 eV above their onset energy of about 284 eV (C-K) and 398eV(N-K) respectively A third broader peak is present in both cases at about 34 eVabove the respective onset energy

To obtain the [C][Ti] and [N][Ti] ratios and their sum (permilCŠ Dagger permilNŠ)[Ti] repre-sented in reg gure 8 the electron-energy-loss spectra of the near-reg bre coating region

T iN coatings in CMgplusmn Al composites 665

Figure 6 T5331HTATiNcp-Mg (a) three-dimensional plot of electron-energy-loss spectrataken at 30 equidistant points across the reg brematrix interregion arranged from reg bre(back) to matrix (front) (b) STEM bright-reg eld image showing the analysis distance asa white line

666 A Feldho et al

Figure 7 T5331HTATiNcp-Mg comparison of the C-K ELNES (top) and the N-KELNES (bottom) of two electron-energy-loss spectra taken from the coating

Figure 8 T5331HTATiNcp-Mg graph of [C][Ti] and [N][Ti] ratios and their sum(permilCŠ Dagger permilNŠ=permilTiŠdagger across the coating obtained by the quantireg cation of the electron-energy-loss spectra presented in reg gure 6

shown in reg gure 6 (a) were quantireg ed as standardless using hydrogenic scatteringcross-sections The carbon content ([C][Ti]) decreases from the reg bre (left) to thecoating with the nitrogen content ([N][Ti]) increasing The sum of both (permilCŠ Dagger permilNŠ)[Ti] remains constant across the coating but is always smaller than unity at about08

Changing the matrix to the Mg-Al alloy did not initiate a distinct formation ofcarbides (reg gure 9 (a)) For T5331HTATiNAM50 approximately the same micro-structure as with the pure Mg matrix was observed Only a few carbide precipitateshave locally formed at the reg brematrix interface as can be seen in reg gure 9 (a) at thereg bre on the top left Combining EDXS with STEM (reg gure 9 (b)) revealed approxi-mately 1plusmn 2 at Mg to be almost homogeneously distributed in the volume of thecarbon reg bre but no Al had penetrated into the body of the reg bre

However combining the same Mgplusmn Al matrix with the uncoated reg bre resulted inthe substantial formation of many carbidic plates of di erent size at the reg brematrixinterface (reg gure 10 (a)) In T5331HTAAM50 the plate-shaped precipitatesextend up to 15 mm into the matrix (reg gure 10 (a)) Using HREM and selected-area di raction they were identireg ed to be the ternary carbide Al2MgC2 as shownby its (0002) lattice fringe spacing of 062nm Often planar defects had formedparallel to the (0001) habit planes of the carbidic platelets (see also Feldho et al(1999)) In this composite EDXS in combination with STEM (reg gure 10 (b)) revealedapproximately 4plusmn 6 at Mg almost homogeneously distributed in the volume of thecarbon reg bre

Finally it should be mentioned that occasionally larger isolated carbide platescan be observed with dimensions above 1 mm which may pierce the TiCxNy reg lm andreach into the reg bre surface This may occur even in parts of the composite where thereg bre coating is relatively thick as shown in reg gure 11 for T5331HTATiNAM50with the coating thickness being 60plusmn 100 nm

T iN coatings in CMgplusmn Al composites 667

Figure 9 T5331HTATiNAM50 (a) bright-reg eld image showing the reg brematrix interfacialregion to be mainly free of carbide precipitates (b) energy-dispersive X-ray spectrumof a carbon reg bre

4 D ISCUSSION

The titanium nitride coating is polycrystalline with a grain size in the range 5plusmn10 nm and covering the reg bre surfaces almost completely The thickness of the coatingcan in some cases reach more than 100 nm but mainly varies between 10 and 30 nm

Concerning the chemical composition of the coating a strong correspondencebetween the C-K and the N-K ELNES features (see reg gure 7) was observed This is

668 A Feldho et al

Figure 10 T5331HTATiNAM50 (a) HVEM bright-reg eld image showing many platelets ofAl2MgC2 in the reg brematrix interregion (b) energy-dispersive X-ray spectrum of acarbon reg bre

Figure 11 T5331HTATiNAM50 HVEM bright-reg eld image showing a carbide precipitatepiercing the coating on the reg bre

caused by a strong relationship between the crystal chemistry of TiC and TiN whichare isotypic (Hosoi et al 1986) Both compounds exhibit the rock-salt structure asthe Ti atoms form a fcc lattice with the octahedral interstices being completely reg lledwith C and N atoms respectively The lattice parameters of TiC (a ˆ 0432 nm) andTiN (a ˆ 0423 nm) di er by only about 9 pm (ie 21) If not all octahedralinterstices of the Ti sublattice are occupied substoichiometric carbides TiCx(x lt 1) and nitrides TiNy (y lt 1dagger result The occupation of some octahedral inter-stices with C and others with N leads to the formation of titanium carbonitridesTiCxNy (x Dagger y 4 1) substitutional solid solutions with a wide solubility for C and N(Jonsson 1996) The lattice parameters of these ternary phases lie between those ofTiC and TiN Changes in the stoichiometry hardly a ect the C-K and N-K ELNESsof these carbonitrides Principally they lead to slight changes in the peak intensitieswhile the peak energies as well as the onset energy remain constant within the energyresolution (Pmacr uEgrave ger et al 1982)

Taking into consideration the observed reg ne-structure details at the C-K and N-Kionization edges (reg gure 7) together with the quantireg cation proreg le of electron-energy-loss spectra (reg gure 8) allows the following conclusion to be drawn The EELSanalyses shown in reg gures 6plusmn 8 prove that the coating consists of a substoichiometrictitanium carbonitride (TiCxNy) with a high C content at the reg brecoating interfacegradually becoming richer in N on going towards the middle of thereg lm Nevertheless the vacancy concentration in the non-metal sublattice is almostconstant (x Dagger y ordm 08dagger (see reg gure 8) It is pointed out that TiCxNy with evenhigher vacancy concentrations (x Dagger y lt 07) have been observed (Em andTashmetov 1996)

The compositional changes within the coating hint to a reaction of the reactorgas mixture (TiCl4 N2 and H2) with C from the reg bre during the initial state of CVDThus the term reactive chemical vapour deposition (RCVD) which wasproposed by Vincent et al (1992) seems to be suitable here and the chemicalprocesses during deposition are much more complex than previously assumed bythe reaction TiCl4 Dagger 1

2 N2 Dagger H2 TiN Dagger 4 HCl used by Wurm et al (1997) andWurm (1998)

In reg gure 6 (a) the electron-energy-loss spectrum 20 (at 266 nm) at the coatingmatrix interface shows almost no C or N but clearly Ti and O Probably theoccurrence of O correlates to that of Mg the ionization edges of which (Mg-L23

at 49 eV Mg-K at 1305 eV) are outside the investigated energy interval An under-standing of the bonding mechanism between the Mg matrix and the TiCxNy coatingrequires the comprehensive analysis of ELNES details at the coatingmatrix interfaceas it may be associated with phase formation on the nanoscopic scale (Tiplusmn Mg inter-metallics Tiplusmn Mg spinels ternary nitrides or quarternary carbonitrides of the systemTiplusmn Mgplusmn Cplusmn N) Accordingly the ionization edges of Mg at about 49 eV (Mg-L23) and1305 eV (Mg-K) have to be analysed whereas an overlap of the Mg-L23 ELNESwith the Ti-M23 and Ti-M1 ELNES has to be taken into consideration as their onsetenergies are at about 35 or 60 eV respectively These analyses are beyond the scope ofthis paper They are of particular interest as Wurm et al (1997) and Wurm (1998)found for the T5331HTATiNcp-Mg composite a relatively high tensile strengthperpendicular to the reg bre axis (approximately 25 MPa) which hints at good adhe-sion of the Mg matrix on the TiN-coated carbon reg bres Nevertheless here we couldprove that the adhesion does not correlate with the formation of larger brittlephases

T iN coatings in CMgplusmn Al composites 669

The microstructure of T5331HTATiNAM50 strongly resembles that ofT5331HTATiNcp-Mg with its reg brematrix interfaces being mainly free of carbides(see reg gures 9 (a) and 1 (a)) On the other hand the same Mgplusmn Al alloy in contact withthe uncoated carbon reg bre resulted in the substantial formation of Al2MgC2 (reg gure10 (a)) thus indicating a strong di usion barrier e ect of the titanium nitride coat-ing This is conreg rmed by a lowered mass transport in the reverse directionthat is from matrix to reg bre due to the TiCxNy interfacial reg lm as only1plusmn 2 at Mg penetrated into the coated reg bre (reg gures 1 (b) and 9 (b)) compared with4plusmn 6 at Mg in case of the uncoated reg bre (reg gure 10 (b)) Thus carbide formationand reg bre degradation are simultaneously limited

Nevertheless a graphitic interlayer has in certain samples been detected at thecoatingmatrix interface in T5331HTATiNcp-Mg indicating that locally C wastransported from the reg bre to the matrix As this phenomenon is not very pro-nounced it is concluded that the C transport across the reg brematrix interfacedoes not occur through TiCxNy grains even if they exhibit signireg cant vacancyconcentrations in the non-metal sublattice Geometrical considerations on thethree-dimensional structure of the coating lead to the conclusion that there existlocally reg ssures in the coating (the reg lm thickness is in the range of the grain dia-meters) Thus locally liquid Mg penetrates through reg ssures in the reg lm towards thereg bre (reg gure 2 (d)) In successive steps C from the reg bre dissolves and di uses in theliquid Mg through the coating and precipitates as graphitic ribbons at the coatingmatrix interface The thermodynamics of such an isothermal dissolutionplusmn di usionplusmnprecipitation process have been described by Fitzer and Kegel (1968) and theyshowed that it is driven by a di erence in the chemical potential of C in the reg breand in the graphitic ribbon

Accordingly the formation of some large Al2MgC2 plates in T5331HTATiNAM50 is not full excluded and these plates may even pierce the reg bre coating (reg gures9 (a) and 11) At these locations notching may occur which can initiate reg bre crack-ing Consequently the strength distribution function of the reg bres (Danzer 1994) isa ected by the introduction of this new class of defects and the probability of reg brefracture at low loads is enhanced The formation of many large carbide plates evenleads to an embrittlement of CMgplusmn Al composites (Feldho et al 1997 HaEgrave hnel et al1997 Feldho 1998) As there are only a few of these large carbide plates inT5331HTATiNAM50 that is the e ect is very limited here accordingly thestrength of this composite was found to be only slightly lower than that ofT5331HTATiNcp-Mg (Wurm et al 1997 Wurm 1997)

Altogether the amount of carbide formation in T5331HTATiNAM50 (reg gure9 (a)) is signireg cantly lower than in T5331HTAAM50 (reg gure 10 (a)) with theuncoated carbon reg bres This is because the TiCxNy reg lm serves well as an inhibitorof deleterious carbide formation also when using Mg alloys with large amounts ofAl as matrices Thus the use of Mgplusmn Al matrices for the manufacture of hybridcomponents is enabled by the di usion retarding e ect of the chemically vapour-deposited TiCxNy coating

5 CONCLUSIONS

CVD coating on the carbon reg bres has been shown to be partly substoichiometrictitanium carbonitride (TiCxNy) rather than titanium nitride (TiN) Even though thebonding mechanism at the coatingmatrix interface is still not clear it has been

670 A Feldho et al

shown that the adhesion of Mg on the TiCxNy coating is not accompanied by theformation of extensive brittle phases

A titanium carbonitride reg lm homogeneously covering the reg bre surfaces ham-pers the transport processes across the reg brematrix interface The penetration of Mginto the carbon reg bre is signireg cantly lowered and so is the formation of Al2MgC2with deleterious notch e ects Thus the TiCxNy coating enables the use of highcontents of Al in the matrix alloy without the negative e ects of the carbide pre-cipitates on the mechanical properties of the composite

ACKNOWLEDGEMENTS

The authors would like to thank Professor R F Singer and Dr D Wurm(University of Erlangenplusmn NuEgrave rnberg) for providing the MMC samples

REFERENCESANDRIEVSKI R A 1997 J Mater Sci 32 4463BRYDSON R SAUER H ENGEL W and ZEITLER E 1991 Microsc Microanal Micro-

struct 2 159CRAVEN A J 1995 J Microsc 180 250CRAVEN A J and G ARVIE L A J 1995 Microsc Microanal Microstruct 6 89D ANZER R 1994 The Encyclopedia of Advanced Materials Vol 1 edited by D Bloor R J

Brook M C Flemings S Mahajan and R W Cahn (Oxford Pergamon)pp 385plusmn 398

D ELANNAY F FROYEN L and D ERUYTTERE A 1987 J Mater Sci 22 1EM V T and TASHMETOV M YU 1996 Phys Stat sol (b) 198 571F ELDHOFF A 1998 PhD Thesis Martin Luther University of Halle-Wittenberg

(Aachen-Maastricht Shaker)F ELDHOFF A P IP PEL E and WOLTERSDORF J 1997 J Microsc 185 122 1999 Phil

Mag A 79 1263F ITZER E and K EGEL B 1968 Carbon 6 433H AiumlHNEL A PIP PEL E F ELDHOFF A SCHNEIDER R and WOLTERSDORF J 1997 Mater

Sci Engng 237 173H OFER F WARBICHLER P and GROGGER W 1995 Ultramicroscopy 59 15H OSOI J O IKAW A T and BANDO Y 1986 J Electron Microsc 35 129JONSSON S 1996 Z Metall 87 713K RIVANEKOLG RUBBENSA JDELLBYN and M EYERCE1992 Microsc Microanal

Microstruct 3 187N AIDICH J V 1981 Prog Surf Membr Sci 14 353Oiuml TTING ER O 1996 PhD Thesis Friedrich Alexander University of Erlangen-NuEgrave rnberg

(DuEgrave sseldorf VDI)Oiuml TTING ER O G RAU C W INTER R SINGER R F F ELDHOFF A PIPPEL E and

WOLTERSDORF J 1995 Proceedings of the Tenth International Conference onComposite Materials Vol VI Whistler Canada (Cambridge Woodhead) pp 447plusmn454

Oiuml TTING ER O and SINGER R F 1993 Z Metall 84 827PFLUiuml GER J F INK J CRECELIUS G BOHNEN K P and W INTER H 1982 Solid St

Commun 44 489PICOUET L ABIVEN H VIALA J C and BOUIX J 1990 Advanced Composite Materials

Proceedings of the First Frenchplusmn Japanese Seminar on Composite Materials edited by CBathias and M Uemura (Paris SIRPE) pp 121plusmn 131

POLMEAR I J 1981 L ight AlloysETH Metallurgy of the L ight Alloys (London Edward Arnold)R EZ P1992 Transmission Electron Energy L oss Spectrometry in Materials Science edited by

M M Disko C C Ahn and B Fultz (Warrendale Pennsylvania Minerals Metalsand Materials Society) pp 107plusmn 129

SHORSHOROV M K U STINOV L M Z IRLIN A M and OLEFIRENKO V I 1979 JMater Sci 14 1850

T iN coatings in CMgplusmn Al composites 671

The elemental distribution of Ti (reg gure 2 (b)) reveals the continuous covering of thereg bre with the reg lm here 10plusmn 20 nm in thickness The distribution of C (reg gure 2 (c))represents mainly the reg bre and that of Mg (reg gure 2 (d)) the matrix A slight penetra-tion of Mg through the coating towards the reg bre is indicated by some brightcontrast features in reg gure 2 (d) left of the coating The HREM image in reg gure 3

T iN coatings in CMgplusmn Al composites 663

Figure 2 T5331HTATiNcp-Mg (a) bright-reg eld image (b) (c) EFTEM at the ionizationedges

Figure 3 HREM image of T5331HTATiNcp-Mg showing the (10 -10dagger lattice fringes of theMg matrix ending on the coating (left reg bre right matrix)

shows the turbostratic C of the reg bre (left) and the (10 -10) lattice fringes of the Mgmatrix (right) ending at the coating (middle part) The polycrystalline nature of thecoating with grain sizes in the range of the reg lm thickness is clearly revealed

In reg gure 4 (a) the integrity of the coating between reg bre (left) and matrix (right) isshown by the elemental distribution of titanium Only rarely did T5331HTATiNcp-Mg exhibit C on the matrix side of the coating as revealed by the EFTEM imagein reg gure 4 (b) This carbon forms a 10 nm thick graphitic ribbon with its basal planesin parallel orientation to the reg bre surface as shown in the corresponding HREMimage in reg gure 5

In reg gure 6 (a) a three-dimensional plot of electron-energy-loss spectra is givenwhich were recorded across the reg brematrix interregion at 30 equidistant pointsalong the white marker in the STEM bright-reg eld image (reg gure 6 (b)) with thepoint-to-point distance amounting to about 13 nm In the energy interval between250 and 750eV there appear the C-K N-K Ti-L23 and O-K ionization edges and

664 A Feldho et al

Figure 4 EFTEM at the ionization edges of the reg brematrix interregion in T5331HTATiNcp-Mg (a) Ti-L23 (b) C-K

Figure 5 HREM image of T5331HTATiNcp-Mg showing a graphitic ribbon on the matrixside of the coating (left reg bre right matrix)

their associated reg ne structures In the region of the reg bre (spectra in the backgroundreg gure 6 (a)) only the C-K ionization edge at 284 eV arises from the exponentiallydecreasing background At the reg brecoating interface a Ti-L23 ionization edgeappears and the C-K ELNES changes Within a gradient of approximately 5 nma N-K ionization edge arises and the C-K edge disappears The Ti-L23 ELNES isvery clear above the background owing to the presence of sharp white lines whichoccur at the L23 ionization edges of the transition metals and at the M45 ionizationedges of the rare-earth elements because the Fermi energy is located in the narrow3d and 4f valence band respectively (Rez 1992) At the coatingmatrix interface O ispresent indicated by an O-K ionization edge (see reg gure 6 (a)) Towards the matrixagain C is evident

In the region of the coating the C-K and N-K edges show similar ELNESproreg les More clearly the C-K ELNES of spectrum 9 (at 12 nm) and the N-KELNES of spectrum 16 (at 213 nm) are compared in reg gure 7 Both ELNES proreg lesare essentially the same as those of stoichiometric TiC and TiN (Craven 1995Craven and Garvie 1995 Hosoi et al 1986) as they both show two narrow peaksat about 45 and 14 eV above their onset energy of about 284 eV (C-K) and 398eV(N-K) respectively A third broader peak is present in both cases at about 34 eVabove the respective onset energy

To obtain the [C][Ti] and [N][Ti] ratios and their sum (permilCŠ Dagger permilNŠ)[Ti] repre-sented in reg gure 8 the electron-energy-loss spectra of the near-reg bre coating region

T iN coatings in CMgplusmn Al composites 665

Figure 6 T5331HTATiNcp-Mg (a) three-dimensional plot of electron-energy-loss spectrataken at 30 equidistant points across the reg brematrix interregion arranged from reg bre(back) to matrix (front) (b) STEM bright-reg eld image showing the analysis distance asa white line

666 A Feldho et al

Figure 7 T5331HTATiNcp-Mg comparison of the C-K ELNES (top) and the N-KELNES (bottom) of two electron-energy-loss spectra taken from the coating

Figure 8 T5331HTATiNcp-Mg graph of [C][Ti] and [N][Ti] ratios and their sum(permilCŠ Dagger permilNŠ=permilTiŠdagger across the coating obtained by the quantireg cation of the electron-energy-loss spectra presented in reg gure 6

shown in reg gure 6 (a) were quantireg ed as standardless using hydrogenic scatteringcross-sections The carbon content ([C][Ti]) decreases from the reg bre (left) to thecoating with the nitrogen content ([N][Ti]) increasing The sum of both (permilCŠ Dagger permilNŠ)[Ti] remains constant across the coating but is always smaller than unity at about08

Changing the matrix to the Mg-Al alloy did not initiate a distinct formation ofcarbides (reg gure 9 (a)) For T5331HTATiNAM50 approximately the same micro-structure as with the pure Mg matrix was observed Only a few carbide precipitateshave locally formed at the reg brematrix interface as can be seen in reg gure 9 (a) at thereg bre on the top left Combining EDXS with STEM (reg gure 9 (b)) revealed approxi-mately 1plusmn 2 at Mg to be almost homogeneously distributed in the volume of thecarbon reg bre but no Al had penetrated into the body of the reg bre

However combining the same Mgplusmn Al matrix with the uncoated reg bre resulted inthe substantial formation of many carbidic plates of di erent size at the reg brematrixinterface (reg gure 10 (a)) In T5331HTAAM50 the plate-shaped precipitatesextend up to 15 mm into the matrix (reg gure 10 (a)) Using HREM and selected-area di raction they were identireg ed to be the ternary carbide Al2MgC2 as shownby its (0002) lattice fringe spacing of 062nm Often planar defects had formedparallel to the (0001) habit planes of the carbidic platelets (see also Feldho et al(1999)) In this composite EDXS in combination with STEM (reg gure 10 (b)) revealedapproximately 4plusmn 6 at Mg almost homogeneously distributed in the volume of thecarbon reg bre

Finally it should be mentioned that occasionally larger isolated carbide platescan be observed with dimensions above 1 mm which may pierce the TiCxNy reg lm andreach into the reg bre surface This may occur even in parts of the composite where thereg bre coating is relatively thick as shown in reg gure 11 for T5331HTATiNAM50with the coating thickness being 60plusmn 100 nm

T iN coatings in CMgplusmn Al composites 667

Figure 9 T5331HTATiNAM50 (a) bright-reg eld image showing the reg brematrix interfacialregion to be mainly free of carbide precipitates (b) energy-dispersive X-ray spectrumof a carbon reg bre

4 D ISCUSSION

The titanium nitride coating is polycrystalline with a grain size in the range 5plusmn10 nm and covering the reg bre surfaces almost completely The thickness of the coatingcan in some cases reach more than 100 nm but mainly varies between 10 and 30 nm

Concerning the chemical composition of the coating a strong correspondencebetween the C-K and the N-K ELNES features (see reg gure 7) was observed This is

668 A Feldho et al

Figure 10 T5331HTATiNAM50 (a) HVEM bright-reg eld image showing many platelets ofAl2MgC2 in the reg brematrix interregion (b) energy-dispersive X-ray spectrum of acarbon reg bre

Figure 11 T5331HTATiNAM50 HVEM bright-reg eld image showing a carbide precipitatepiercing the coating on the reg bre

caused by a strong relationship between the crystal chemistry of TiC and TiN whichare isotypic (Hosoi et al 1986) Both compounds exhibit the rock-salt structure asthe Ti atoms form a fcc lattice with the octahedral interstices being completely reg lledwith C and N atoms respectively The lattice parameters of TiC (a ˆ 0432 nm) andTiN (a ˆ 0423 nm) di er by only about 9 pm (ie 21) If not all octahedralinterstices of the Ti sublattice are occupied substoichiometric carbides TiCx(x lt 1) and nitrides TiNy (y lt 1dagger result The occupation of some octahedral inter-stices with C and others with N leads to the formation of titanium carbonitridesTiCxNy (x Dagger y 4 1) substitutional solid solutions with a wide solubility for C and N(Jonsson 1996) The lattice parameters of these ternary phases lie between those ofTiC and TiN Changes in the stoichiometry hardly a ect the C-K and N-K ELNESsof these carbonitrides Principally they lead to slight changes in the peak intensitieswhile the peak energies as well as the onset energy remain constant within the energyresolution (Pmacr uEgrave ger et al 1982)

Taking into consideration the observed reg ne-structure details at the C-K and N-Kionization edges (reg gure 7) together with the quantireg cation proreg le of electron-energy-loss spectra (reg gure 8) allows the following conclusion to be drawn The EELSanalyses shown in reg gures 6plusmn 8 prove that the coating consists of a substoichiometrictitanium carbonitride (TiCxNy) with a high C content at the reg brecoating interfacegradually becoming richer in N on going towards the middle of thereg lm Nevertheless the vacancy concentration in the non-metal sublattice is almostconstant (x Dagger y ordm 08dagger (see reg gure 8) It is pointed out that TiCxNy with evenhigher vacancy concentrations (x Dagger y lt 07) have been observed (Em andTashmetov 1996)

The compositional changes within the coating hint to a reaction of the reactorgas mixture (TiCl4 N2 and H2) with C from the reg bre during the initial state of CVDThus the term reactive chemical vapour deposition (RCVD) which wasproposed by Vincent et al (1992) seems to be suitable here and the chemicalprocesses during deposition are much more complex than previously assumed bythe reaction TiCl4 Dagger 1

2 N2 Dagger H2 TiN Dagger 4 HCl used by Wurm et al (1997) andWurm (1998)

In reg gure 6 (a) the electron-energy-loss spectrum 20 (at 266 nm) at the coatingmatrix interface shows almost no C or N but clearly Ti and O Probably theoccurrence of O correlates to that of Mg the ionization edges of which (Mg-L23

at 49 eV Mg-K at 1305 eV) are outside the investigated energy interval An under-standing of the bonding mechanism between the Mg matrix and the TiCxNy coatingrequires the comprehensive analysis of ELNES details at the coatingmatrix interfaceas it may be associated with phase formation on the nanoscopic scale (Tiplusmn Mg inter-metallics Tiplusmn Mg spinels ternary nitrides or quarternary carbonitrides of the systemTiplusmn Mgplusmn Cplusmn N) Accordingly the ionization edges of Mg at about 49 eV (Mg-L23) and1305 eV (Mg-K) have to be analysed whereas an overlap of the Mg-L23 ELNESwith the Ti-M23 and Ti-M1 ELNES has to be taken into consideration as their onsetenergies are at about 35 or 60 eV respectively These analyses are beyond the scope ofthis paper They are of particular interest as Wurm et al (1997) and Wurm (1998)found for the T5331HTATiNcp-Mg composite a relatively high tensile strengthperpendicular to the reg bre axis (approximately 25 MPa) which hints at good adhe-sion of the Mg matrix on the TiN-coated carbon reg bres Nevertheless here we couldprove that the adhesion does not correlate with the formation of larger brittlephases

T iN coatings in CMgplusmn Al composites 669

The microstructure of T5331HTATiNAM50 strongly resembles that ofT5331HTATiNcp-Mg with its reg brematrix interfaces being mainly free of carbides(see reg gures 9 (a) and 1 (a)) On the other hand the same Mgplusmn Al alloy in contact withthe uncoated carbon reg bre resulted in the substantial formation of Al2MgC2 (reg gure10 (a)) thus indicating a strong di usion barrier e ect of the titanium nitride coat-ing This is conreg rmed by a lowered mass transport in the reverse directionthat is from matrix to reg bre due to the TiCxNy interfacial reg lm as only1plusmn 2 at Mg penetrated into the coated reg bre (reg gures 1 (b) and 9 (b)) compared with4plusmn 6 at Mg in case of the uncoated reg bre (reg gure 10 (b)) Thus carbide formationand reg bre degradation are simultaneously limited

Nevertheless a graphitic interlayer has in certain samples been detected at thecoatingmatrix interface in T5331HTATiNcp-Mg indicating that locally C wastransported from the reg bre to the matrix As this phenomenon is not very pro-nounced it is concluded that the C transport across the reg brematrix interfacedoes not occur through TiCxNy grains even if they exhibit signireg cant vacancyconcentrations in the non-metal sublattice Geometrical considerations on thethree-dimensional structure of the coating lead to the conclusion that there existlocally reg ssures in the coating (the reg lm thickness is in the range of the grain dia-meters) Thus locally liquid Mg penetrates through reg ssures in the reg lm towards thereg bre (reg gure 2 (d)) In successive steps C from the reg bre dissolves and di uses in theliquid Mg through the coating and precipitates as graphitic ribbons at the coatingmatrix interface The thermodynamics of such an isothermal dissolutionplusmn di usionplusmnprecipitation process have been described by Fitzer and Kegel (1968) and theyshowed that it is driven by a di erence in the chemical potential of C in the reg breand in the graphitic ribbon

Accordingly the formation of some large Al2MgC2 plates in T5331HTATiNAM50 is not full excluded and these plates may even pierce the reg bre coating (reg gures9 (a) and 11) At these locations notching may occur which can initiate reg bre crack-ing Consequently the strength distribution function of the reg bres (Danzer 1994) isa ected by the introduction of this new class of defects and the probability of reg brefracture at low loads is enhanced The formation of many large carbide plates evenleads to an embrittlement of CMgplusmn Al composites (Feldho et al 1997 HaEgrave hnel et al1997 Feldho 1998) As there are only a few of these large carbide plates inT5331HTATiNAM50 that is the e ect is very limited here accordingly thestrength of this composite was found to be only slightly lower than that ofT5331HTATiNcp-Mg (Wurm et al 1997 Wurm 1997)

Altogether the amount of carbide formation in T5331HTATiNAM50 (reg gure9 (a)) is signireg cantly lower than in T5331HTAAM50 (reg gure 10 (a)) with theuncoated carbon reg bres This is because the TiCxNy reg lm serves well as an inhibitorof deleterious carbide formation also when using Mg alloys with large amounts ofAl as matrices Thus the use of Mgplusmn Al matrices for the manufacture of hybridcomponents is enabled by the di usion retarding e ect of the chemically vapour-deposited TiCxNy coating

5 CONCLUSIONS

CVD coating on the carbon reg bres has been shown to be partly substoichiometrictitanium carbonitride (TiCxNy) rather than titanium nitride (TiN) Even though thebonding mechanism at the coatingmatrix interface is still not clear it has been

670 A Feldho et al

shown that the adhesion of Mg on the TiCxNy coating is not accompanied by theformation of extensive brittle phases

A titanium carbonitride reg lm homogeneously covering the reg bre surfaces ham-pers the transport processes across the reg brematrix interface The penetration of Mginto the carbon reg bre is signireg cantly lowered and so is the formation of Al2MgC2with deleterious notch e ects Thus the TiCxNy coating enables the use of highcontents of Al in the matrix alloy without the negative e ects of the carbide pre-cipitates on the mechanical properties of the composite

ACKNOWLEDGEMENTS

The authors would like to thank Professor R F Singer and Dr D Wurm(University of Erlangenplusmn NuEgrave rnberg) for providing the MMC samples

REFERENCESANDRIEVSKI R A 1997 J Mater Sci 32 4463BRYDSON R SAUER H ENGEL W and ZEITLER E 1991 Microsc Microanal Micro-

struct 2 159CRAVEN A J 1995 J Microsc 180 250CRAVEN A J and G ARVIE L A J 1995 Microsc Microanal Microstruct 6 89D ANZER R 1994 The Encyclopedia of Advanced Materials Vol 1 edited by D Bloor R J

Brook M C Flemings S Mahajan and R W Cahn (Oxford Pergamon)pp 385plusmn 398

D ELANNAY F FROYEN L and D ERUYTTERE A 1987 J Mater Sci 22 1EM V T and TASHMETOV M YU 1996 Phys Stat sol (b) 198 571F ELDHOFF A 1998 PhD Thesis Martin Luther University of Halle-Wittenberg

(Aachen-Maastricht Shaker)F ELDHOFF A P IP PEL E and WOLTERSDORF J 1997 J Microsc 185 122 1999 Phil

Mag A 79 1263F ITZER E and K EGEL B 1968 Carbon 6 433H AiumlHNEL A PIP PEL E F ELDHOFF A SCHNEIDER R and WOLTERSDORF J 1997 Mater

Sci Engng 237 173H OFER F WARBICHLER P and GROGGER W 1995 Ultramicroscopy 59 15H OSOI J O IKAW A T and BANDO Y 1986 J Electron Microsc 35 129JONSSON S 1996 Z Metall 87 713K RIVANEKOLG RUBBENSA JDELLBYN and M EYERCE1992 Microsc Microanal

Microstruct 3 187N AIDICH J V 1981 Prog Surf Membr Sci 14 353Oiuml TTING ER O 1996 PhD Thesis Friedrich Alexander University of Erlangen-NuEgrave rnberg

(DuEgrave sseldorf VDI)Oiuml TTING ER O G RAU C W INTER R SINGER R F F ELDHOFF A PIPPEL E and

WOLTERSDORF J 1995 Proceedings of the Tenth International Conference onComposite Materials Vol VI Whistler Canada (Cambridge Woodhead) pp 447plusmn454

Oiuml TTING ER O and SINGER R F 1993 Z Metall 84 827PFLUiuml GER J F INK J CRECELIUS G BOHNEN K P and W INTER H 1982 Solid St

Commun 44 489PICOUET L ABIVEN H VIALA J C and BOUIX J 1990 Advanced Composite Materials

Proceedings of the First Frenchplusmn Japanese Seminar on Composite Materials edited by CBathias and M Uemura (Paris SIRPE) pp 121plusmn 131

POLMEAR I J 1981 L ight AlloysETH Metallurgy of the L ight Alloys (London Edward Arnold)R EZ P1992 Transmission Electron Energy L oss Spectrometry in Materials Science edited by

M M Disko C C Ahn and B Fultz (Warrendale Pennsylvania Minerals Metalsand Materials Society) pp 107plusmn 129

SHORSHOROV M K U STINOV L M Z IRLIN A M and OLEFIRENKO V I 1979 JMater Sci 14 1850

T iN coatings in CMgplusmn Al composites 671

shows the turbostratic C of the reg bre (left) and the (10 -10) lattice fringes of the Mgmatrix (right) ending at the coating (middle part) The polycrystalline nature of thecoating with grain sizes in the range of the reg lm thickness is clearly revealed

In reg gure 4 (a) the integrity of the coating between reg bre (left) and matrix (right) isshown by the elemental distribution of titanium Only rarely did T5331HTATiNcp-Mg exhibit C on the matrix side of the coating as revealed by the EFTEM imagein reg gure 4 (b) This carbon forms a 10 nm thick graphitic ribbon with its basal planesin parallel orientation to the reg bre surface as shown in the corresponding HREMimage in reg gure 5

In reg gure 6 (a) a three-dimensional plot of electron-energy-loss spectra is givenwhich were recorded across the reg brematrix interregion at 30 equidistant pointsalong the white marker in the STEM bright-reg eld image (reg gure 6 (b)) with thepoint-to-point distance amounting to about 13 nm In the energy interval between250 and 750eV there appear the C-K N-K Ti-L23 and O-K ionization edges and

664 A Feldho et al

Figure 4 EFTEM at the ionization edges of the reg brematrix interregion in T5331HTATiNcp-Mg (a) Ti-L23 (b) C-K

Figure 5 HREM image of T5331HTATiNcp-Mg showing a graphitic ribbon on the matrixside of the coating (left reg bre right matrix)

their associated reg ne structures In the region of the reg bre (spectra in the backgroundreg gure 6 (a)) only the C-K ionization edge at 284 eV arises from the exponentiallydecreasing background At the reg brecoating interface a Ti-L23 ionization edgeappears and the C-K ELNES changes Within a gradient of approximately 5 nma N-K ionization edge arises and the C-K edge disappears The Ti-L23 ELNES isvery clear above the background owing to the presence of sharp white lines whichoccur at the L23 ionization edges of the transition metals and at the M45 ionizationedges of the rare-earth elements because the Fermi energy is located in the narrow3d and 4f valence band respectively (Rez 1992) At the coatingmatrix interface O ispresent indicated by an O-K ionization edge (see reg gure 6 (a)) Towards the matrixagain C is evident

In the region of the coating the C-K and N-K edges show similar ELNESproreg les More clearly the C-K ELNES of spectrum 9 (at 12 nm) and the N-KELNES of spectrum 16 (at 213 nm) are compared in reg gure 7 Both ELNES proreg lesare essentially the same as those of stoichiometric TiC and TiN (Craven 1995Craven and Garvie 1995 Hosoi et al 1986) as they both show two narrow peaksat about 45 and 14 eV above their onset energy of about 284 eV (C-K) and 398eV(N-K) respectively A third broader peak is present in both cases at about 34 eVabove the respective onset energy

To obtain the [C][Ti] and [N][Ti] ratios and their sum (permilCŠ Dagger permilNŠ)[Ti] repre-sented in reg gure 8 the electron-energy-loss spectra of the near-reg bre coating region

T iN coatings in CMgplusmn Al composites 665

Figure 6 T5331HTATiNcp-Mg (a) three-dimensional plot of electron-energy-loss spectrataken at 30 equidistant points across the reg brematrix interregion arranged from reg bre(back) to matrix (front) (b) STEM bright-reg eld image showing the analysis distance asa white line

666 A Feldho et al

Figure 7 T5331HTATiNcp-Mg comparison of the C-K ELNES (top) and the N-KELNES (bottom) of two electron-energy-loss spectra taken from the coating

Figure 8 T5331HTATiNcp-Mg graph of [C][Ti] and [N][Ti] ratios and their sum(permilCŠ Dagger permilNŠ=permilTiŠdagger across the coating obtained by the quantireg cation of the electron-energy-loss spectra presented in reg gure 6

shown in reg gure 6 (a) were quantireg ed as standardless using hydrogenic scatteringcross-sections The carbon content ([C][Ti]) decreases from the reg bre (left) to thecoating with the nitrogen content ([N][Ti]) increasing The sum of both (permilCŠ Dagger permilNŠ)[Ti] remains constant across the coating but is always smaller than unity at about08

Changing the matrix to the Mg-Al alloy did not initiate a distinct formation ofcarbides (reg gure 9 (a)) For T5331HTATiNAM50 approximately the same micro-structure as with the pure Mg matrix was observed Only a few carbide precipitateshave locally formed at the reg brematrix interface as can be seen in reg gure 9 (a) at thereg bre on the top left Combining EDXS with STEM (reg gure 9 (b)) revealed approxi-mately 1plusmn 2 at Mg to be almost homogeneously distributed in the volume of thecarbon reg bre but no Al had penetrated into the body of the reg bre

However combining the same Mgplusmn Al matrix with the uncoated reg bre resulted inthe substantial formation of many carbidic plates of di erent size at the reg brematrixinterface (reg gure 10 (a)) In T5331HTAAM50 the plate-shaped precipitatesextend up to 15 mm into the matrix (reg gure 10 (a)) Using HREM and selected-area di raction they were identireg ed to be the ternary carbide Al2MgC2 as shownby its (0002) lattice fringe spacing of 062nm Often planar defects had formedparallel to the (0001) habit planes of the carbidic platelets (see also Feldho et al(1999)) In this composite EDXS in combination with STEM (reg gure 10 (b)) revealedapproximately 4plusmn 6 at Mg almost homogeneously distributed in the volume of thecarbon reg bre

Finally it should be mentioned that occasionally larger isolated carbide platescan be observed with dimensions above 1 mm which may pierce the TiCxNy reg lm andreach into the reg bre surface This may occur even in parts of the composite where thereg bre coating is relatively thick as shown in reg gure 11 for T5331HTATiNAM50with the coating thickness being 60plusmn 100 nm

T iN coatings in CMgplusmn Al composites 667

Figure 9 T5331HTATiNAM50 (a) bright-reg eld image showing the reg brematrix interfacialregion to be mainly free of carbide precipitates (b) energy-dispersive X-ray spectrumof a carbon reg bre

4 D ISCUSSION

The titanium nitride coating is polycrystalline with a grain size in the range 5plusmn10 nm and covering the reg bre surfaces almost completely The thickness of the coatingcan in some cases reach more than 100 nm but mainly varies between 10 and 30 nm

Concerning the chemical composition of the coating a strong correspondencebetween the C-K and the N-K ELNES features (see reg gure 7) was observed This is

668 A Feldho et al

Figure 10 T5331HTATiNAM50 (a) HVEM bright-reg eld image showing many platelets ofAl2MgC2 in the reg brematrix interregion (b) energy-dispersive X-ray spectrum of acarbon reg bre

Figure 11 T5331HTATiNAM50 HVEM bright-reg eld image showing a carbide precipitatepiercing the coating on the reg bre

caused by a strong relationship between the crystal chemistry of TiC and TiN whichare isotypic (Hosoi et al 1986) Both compounds exhibit the rock-salt structure asthe Ti atoms form a fcc lattice with the octahedral interstices being completely reg lledwith C and N atoms respectively The lattice parameters of TiC (a ˆ 0432 nm) andTiN (a ˆ 0423 nm) di er by only about 9 pm (ie 21) If not all octahedralinterstices of the Ti sublattice are occupied substoichiometric carbides TiCx(x lt 1) and nitrides TiNy (y lt 1dagger result The occupation of some octahedral inter-stices with C and others with N leads to the formation of titanium carbonitridesTiCxNy (x Dagger y 4 1) substitutional solid solutions with a wide solubility for C and N(Jonsson 1996) The lattice parameters of these ternary phases lie between those ofTiC and TiN Changes in the stoichiometry hardly a ect the C-K and N-K ELNESsof these carbonitrides Principally they lead to slight changes in the peak intensitieswhile the peak energies as well as the onset energy remain constant within the energyresolution (Pmacr uEgrave ger et al 1982)

Taking into consideration the observed reg ne-structure details at the C-K and N-Kionization edges (reg gure 7) together with the quantireg cation proreg le of electron-energy-loss spectra (reg gure 8) allows the following conclusion to be drawn The EELSanalyses shown in reg gures 6plusmn 8 prove that the coating consists of a substoichiometrictitanium carbonitride (TiCxNy) with a high C content at the reg brecoating interfacegradually becoming richer in N on going towards the middle of thereg lm Nevertheless the vacancy concentration in the non-metal sublattice is almostconstant (x Dagger y ordm 08dagger (see reg gure 8) It is pointed out that TiCxNy with evenhigher vacancy concentrations (x Dagger y lt 07) have been observed (Em andTashmetov 1996)

The compositional changes within the coating hint to a reaction of the reactorgas mixture (TiCl4 N2 and H2) with C from the reg bre during the initial state of CVDThus the term reactive chemical vapour deposition (RCVD) which wasproposed by Vincent et al (1992) seems to be suitable here and the chemicalprocesses during deposition are much more complex than previously assumed bythe reaction TiCl4 Dagger 1

2 N2 Dagger H2 TiN Dagger 4 HCl used by Wurm et al (1997) andWurm (1998)

In reg gure 6 (a) the electron-energy-loss spectrum 20 (at 266 nm) at the coatingmatrix interface shows almost no C or N but clearly Ti and O Probably theoccurrence of O correlates to that of Mg the ionization edges of which (Mg-L23

at 49 eV Mg-K at 1305 eV) are outside the investigated energy interval An under-standing of the bonding mechanism between the Mg matrix and the TiCxNy coatingrequires the comprehensive analysis of ELNES details at the coatingmatrix interfaceas it may be associated with phase formation on the nanoscopic scale (Tiplusmn Mg inter-metallics Tiplusmn Mg spinels ternary nitrides or quarternary carbonitrides of the systemTiplusmn Mgplusmn Cplusmn N) Accordingly the ionization edges of Mg at about 49 eV (Mg-L23) and1305 eV (Mg-K) have to be analysed whereas an overlap of the Mg-L23 ELNESwith the Ti-M23 and Ti-M1 ELNES has to be taken into consideration as their onsetenergies are at about 35 or 60 eV respectively These analyses are beyond the scope ofthis paper They are of particular interest as Wurm et al (1997) and Wurm (1998)found for the T5331HTATiNcp-Mg composite a relatively high tensile strengthperpendicular to the reg bre axis (approximately 25 MPa) which hints at good adhe-sion of the Mg matrix on the TiN-coated carbon reg bres Nevertheless here we couldprove that the adhesion does not correlate with the formation of larger brittlephases

T iN coatings in CMgplusmn Al composites 669

The microstructure of T5331HTATiNAM50 strongly resembles that ofT5331HTATiNcp-Mg with its reg brematrix interfaces being mainly free of carbides(see reg gures 9 (a) and 1 (a)) On the other hand the same Mgplusmn Al alloy in contact withthe uncoated carbon reg bre resulted in the substantial formation of Al2MgC2 (reg gure10 (a)) thus indicating a strong di usion barrier e ect of the titanium nitride coat-ing This is conreg rmed by a lowered mass transport in the reverse directionthat is from matrix to reg bre due to the TiCxNy interfacial reg lm as only1plusmn 2 at Mg penetrated into the coated reg bre (reg gures 1 (b) and 9 (b)) compared with4plusmn 6 at Mg in case of the uncoated reg bre (reg gure 10 (b)) Thus carbide formationand reg bre degradation are simultaneously limited

Nevertheless a graphitic interlayer has in certain samples been detected at thecoatingmatrix interface in T5331HTATiNcp-Mg indicating that locally C wastransported from the reg bre to the matrix As this phenomenon is not very pro-nounced it is concluded that the C transport across the reg brematrix interfacedoes not occur through TiCxNy grains even if they exhibit signireg cant vacancyconcentrations in the non-metal sublattice Geometrical considerations on thethree-dimensional structure of the coating lead to the conclusion that there existlocally reg ssures in the coating (the reg lm thickness is in the range of the grain dia-meters) Thus locally liquid Mg penetrates through reg ssures in the reg lm towards thereg bre (reg gure 2 (d)) In successive steps C from the reg bre dissolves and di uses in theliquid Mg through the coating and precipitates as graphitic ribbons at the coatingmatrix interface The thermodynamics of such an isothermal dissolutionplusmn di usionplusmnprecipitation process have been described by Fitzer and Kegel (1968) and theyshowed that it is driven by a di erence in the chemical potential of C in the reg breand in the graphitic ribbon

Accordingly the formation of some large Al2MgC2 plates in T5331HTATiNAM50 is not full excluded and these plates may even pierce the reg bre coating (reg gures9 (a) and 11) At these locations notching may occur which can initiate reg bre crack-ing Consequently the strength distribution function of the reg bres (Danzer 1994) isa ected by the introduction of this new class of defects and the probability of reg brefracture at low loads is enhanced The formation of many large carbide plates evenleads to an embrittlement of CMgplusmn Al composites (Feldho et al 1997 HaEgrave hnel et al1997 Feldho 1998) As there are only a few of these large carbide plates inT5331HTATiNAM50 that is the e ect is very limited here accordingly thestrength of this composite was found to be only slightly lower than that ofT5331HTATiNcp-Mg (Wurm et al 1997 Wurm 1997)

Altogether the amount of carbide formation in T5331HTATiNAM50 (reg gure9 (a)) is signireg cantly lower than in T5331HTAAM50 (reg gure 10 (a)) with theuncoated carbon reg bres This is because the TiCxNy reg lm serves well as an inhibitorof deleterious carbide formation also when using Mg alloys with large amounts ofAl as matrices Thus the use of Mgplusmn Al matrices for the manufacture of hybridcomponents is enabled by the di usion retarding e ect of the chemically vapour-deposited TiCxNy coating

5 CONCLUSIONS

CVD coating on the carbon reg bres has been shown to be partly substoichiometrictitanium carbonitride (TiCxNy) rather than titanium nitride (TiN) Even though thebonding mechanism at the coatingmatrix interface is still not clear it has been

670 A Feldho et al

shown that the adhesion of Mg on the TiCxNy coating is not accompanied by theformation of extensive brittle phases

A titanium carbonitride reg lm homogeneously covering the reg bre surfaces ham-pers the transport processes across the reg brematrix interface The penetration of Mginto the carbon reg bre is signireg cantly lowered and so is the formation of Al2MgC2with deleterious notch e ects Thus the TiCxNy coating enables the use of highcontents of Al in the matrix alloy without the negative e ects of the carbide pre-cipitates on the mechanical properties of the composite

ACKNOWLEDGEMENTS

The authors would like to thank Professor R F Singer and Dr D Wurm(University of Erlangenplusmn NuEgrave rnberg) for providing the MMC samples

REFERENCESANDRIEVSKI R A 1997 J Mater Sci 32 4463BRYDSON R SAUER H ENGEL W and ZEITLER E 1991 Microsc Microanal Micro-

struct 2 159CRAVEN A J 1995 J Microsc 180 250CRAVEN A J and G ARVIE L A J 1995 Microsc Microanal Microstruct 6 89D ANZER R 1994 The Encyclopedia of Advanced Materials Vol 1 edited by D Bloor R J

Brook M C Flemings S Mahajan and R W Cahn (Oxford Pergamon)pp 385plusmn 398

D ELANNAY F FROYEN L and D ERUYTTERE A 1987 J Mater Sci 22 1EM V T and TASHMETOV M YU 1996 Phys Stat sol (b) 198 571F ELDHOFF A 1998 PhD Thesis Martin Luther University of Halle-Wittenberg

(Aachen-Maastricht Shaker)F ELDHOFF A P IP PEL E and WOLTERSDORF J 1997 J Microsc 185 122 1999 Phil

Mag A 79 1263F ITZER E and K EGEL B 1968 Carbon 6 433H AiumlHNEL A PIP PEL E F ELDHOFF A SCHNEIDER R and WOLTERSDORF J 1997 Mater

Sci Engng 237 173H OFER F WARBICHLER P and GROGGER W 1995 Ultramicroscopy 59 15H OSOI J O IKAW A T and BANDO Y 1986 J Electron Microsc 35 129JONSSON S 1996 Z Metall 87 713K RIVANEKOLG RUBBENSA JDELLBYN and M EYERCE1992 Microsc Microanal

Microstruct 3 187N AIDICH J V 1981 Prog Surf Membr Sci 14 353Oiuml TTING ER O 1996 PhD Thesis Friedrich Alexander University of Erlangen-NuEgrave rnberg

(DuEgrave sseldorf VDI)Oiuml TTING ER O G RAU C W INTER R SINGER R F F ELDHOFF A PIPPEL E and

WOLTERSDORF J 1995 Proceedings of the Tenth International Conference onComposite Materials Vol VI Whistler Canada (Cambridge Woodhead) pp 447plusmn454

Oiuml TTING ER O and SINGER R F 1993 Z Metall 84 827PFLUiuml GER J F INK J CRECELIUS G BOHNEN K P and W INTER H 1982 Solid St

Commun 44 489PICOUET L ABIVEN H VIALA J C and BOUIX J 1990 Advanced Composite Materials

Proceedings of the First Frenchplusmn Japanese Seminar on Composite Materials edited by CBathias and M Uemura (Paris SIRPE) pp 121plusmn 131

POLMEAR I J 1981 L ight AlloysETH Metallurgy of the L ight Alloys (London Edward Arnold)R EZ P1992 Transmission Electron Energy L oss Spectrometry in Materials Science edited by

M M Disko C C Ahn and B Fultz (Warrendale Pennsylvania Minerals Metalsand Materials Society) pp 107plusmn 129

SHORSHOROV M K U STINOV L M Z IRLIN A M and OLEFIRENKO V I 1979 JMater Sci 14 1850

T iN coatings in CMgplusmn Al composites 671

their associated reg ne structures In the region of the reg bre (spectra in the backgroundreg gure 6 (a)) only the C-K ionization edge at 284 eV arises from the exponentiallydecreasing background At the reg brecoating interface a Ti-L23 ionization edgeappears and the C-K ELNES changes Within a gradient of approximately 5 nma N-K ionization edge arises and the C-K edge disappears The Ti-L23 ELNES isvery clear above the background owing to the presence of sharp white lines whichoccur at the L23 ionization edges of the transition metals and at the M45 ionizationedges of the rare-earth elements because the Fermi energy is located in the narrow3d and 4f valence band respectively (Rez 1992) At the coatingmatrix interface O ispresent indicated by an O-K ionization edge (see reg gure 6 (a)) Towards the matrixagain C is evident

In the region of the coating the C-K and N-K edges show similar ELNESproreg les More clearly the C-K ELNES of spectrum 9 (at 12 nm) and the N-KELNES of spectrum 16 (at 213 nm) are compared in reg gure 7 Both ELNES proreg lesare essentially the same as those of stoichiometric TiC and TiN (Craven 1995Craven and Garvie 1995 Hosoi et al 1986) as they both show two narrow peaksat about 45 and 14 eV above their onset energy of about 284 eV (C-K) and 398eV(N-K) respectively A third broader peak is present in both cases at about 34 eVabove the respective onset energy

To obtain the [C][Ti] and [N][Ti] ratios and their sum (permilCŠ Dagger permilNŠ)[Ti] repre-sented in reg gure 8 the electron-energy-loss spectra of the near-reg bre coating region

T iN coatings in CMgplusmn Al composites 665

Figure 6 T5331HTATiNcp-Mg (a) three-dimensional plot of electron-energy-loss spectrataken at 30 equidistant points across the reg brematrix interregion arranged from reg bre(back) to matrix (front) (b) STEM bright-reg eld image showing the analysis distance asa white line

666 A Feldho et al

Figure 7 T5331HTATiNcp-Mg comparison of the C-K ELNES (top) and the N-KELNES (bottom) of two electron-energy-loss spectra taken from the coating

Figure 8 T5331HTATiNcp-Mg graph of [C][Ti] and [N][Ti] ratios and their sum(permilCŠ Dagger permilNŠ=permilTiŠdagger across the coating obtained by the quantireg cation of the electron-energy-loss spectra presented in reg gure 6

shown in reg gure 6 (a) were quantireg ed as standardless using hydrogenic scatteringcross-sections The carbon content ([C][Ti]) decreases from the reg bre (left) to thecoating with the nitrogen content ([N][Ti]) increasing The sum of both (permilCŠ Dagger permilNŠ)[Ti] remains constant across the coating but is always smaller than unity at about08

Changing the matrix to the Mg-Al alloy did not initiate a distinct formation ofcarbides (reg gure 9 (a)) For T5331HTATiNAM50 approximately the same micro-structure as with the pure Mg matrix was observed Only a few carbide precipitateshave locally formed at the reg brematrix interface as can be seen in reg gure 9 (a) at thereg bre on the top left Combining EDXS with STEM (reg gure 9 (b)) revealed approxi-mately 1plusmn 2 at Mg to be almost homogeneously distributed in the volume of thecarbon reg bre but no Al had penetrated into the body of the reg bre

However combining the same Mgplusmn Al matrix with the uncoated reg bre resulted inthe substantial formation of many carbidic plates of di erent size at the reg brematrixinterface (reg gure 10 (a)) In T5331HTAAM50 the plate-shaped precipitatesextend up to 15 mm into the matrix (reg gure 10 (a)) Using HREM and selected-area di raction they were identireg ed to be the ternary carbide Al2MgC2 as shownby its (0002) lattice fringe spacing of 062nm Often planar defects had formedparallel to the (0001) habit planes of the carbidic platelets (see also Feldho et al(1999)) In this composite EDXS in combination with STEM (reg gure 10 (b)) revealedapproximately 4plusmn 6 at Mg almost homogeneously distributed in the volume of thecarbon reg bre

Finally it should be mentioned that occasionally larger isolated carbide platescan be observed with dimensions above 1 mm which may pierce the TiCxNy reg lm andreach into the reg bre surface This may occur even in parts of the composite where thereg bre coating is relatively thick as shown in reg gure 11 for T5331HTATiNAM50with the coating thickness being 60plusmn 100 nm

T iN coatings in CMgplusmn Al composites 667

Figure 9 T5331HTATiNAM50 (a) bright-reg eld image showing the reg brematrix interfacialregion to be mainly free of carbide precipitates (b) energy-dispersive X-ray spectrumof a carbon reg bre

4 D ISCUSSION

The titanium nitride coating is polycrystalline with a grain size in the range 5plusmn10 nm and covering the reg bre surfaces almost completely The thickness of the coatingcan in some cases reach more than 100 nm but mainly varies between 10 and 30 nm

Concerning the chemical composition of the coating a strong correspondencebetween the C-K and the N-K ELNES features (see reg gure 7) was observed This is

668 A Feldho et al

Figure 10 T5331HTATiNAM50 (a) HVEM bright-reg eld image showing many platelets ofAl2MgC2 in the reg brematrix interregion (b) energy-dispersive X-ray spectrum of acarbon reg bre

Figure 11 T5331HTATiNAM50 HVEM bright-reg eld image showing a carbide precipitatepiercing the coating on the reg bre

caused by a strong relationship between the crystal chemistry of TiC and TiN whichare isotypic (Hosoi et al 1986) Both compounds exhibit the rock-salt structure asthe Ti atoms form a fcc lattice with the octahedral interstices being completely reg lledwith C and N atoms respectively The lattice parameters of TiC (a ˆ 0432 nm) andTiN (a ˆ 0423 nm) di er by only about 9 pm (ie 21) If not all octahedralinterstices of the Ti sublattice are occupied substoichiometric carbides TiCx(x lt 1) and nitrides TiNy (y lt 1dagger result The occupation of some octahedral inter-stices with C and others with N leads to the formation of titanium carbonitridesTiCxNy (x Dagger y 4 1) substitutional solid solutions with a wide solubility for C and N(Jonsson 1996) The lattice parameters of these ternary phases lie between those ofTiC and TiN Changes in the stoichiometry hardly a ect the C-K and N-K ELNESsof these carbonitrides Principally they lead to slight changes in the peak intensitieswhile the peak energies as well as the onset energy remain constant within the energyresolution (Pmacr uEgrave ger et al 1982)

Taking into consideration the observed reg ne-structure details at the C-K and N-Kionization edges (reg gure 7) together with the quantireg cation proreg le of electron-energy-loss spectra (reg gure 8) allows the following conclusion to be drawn The EELSanalyses shown in reg gures 6plusmn 8 prove that the coating consists of a substoichiometrictitanium carbonitride (TiCxNy) with a high C content at the reg brecoating interfacegradually becoming richer in N on going towards the middle of thereg lm Nevertheless the vacancy concentration in the non-metal sublattice is almostconstant (x Dagger y ordm 08dagger (see reg gure 8) It is pointed out that TiCxNy with evenhigher vacancy concentrations (x Dagger y lt 07) have been observed (Em andTashmetov 1996)

The compositional changes within the coating hint to a reaction of the reactorgas mixture (TiCl4 N2 and H2) with C from the reg bre during the initial state of CVDThus the term reactive chemical vapour deposition (RCVD) which wasproposed by Vincent et al (1992) seems to be suitable here and the chemicalprocesses during deposition are much more complex than previously assumed bythe reaction TiCl4 Dagger 1

2 N2 Dagger H2 TiN Dagger 4 HCl used by Wurm et al (1997) andWurm (1998)

In reg gure 6 (a) the electron-energy-loss spectrum 20 (at 266 nm) at the coatingmatrix interface shows almost no C or N but clearly Ti and O Probably theoccurrence of O correlates to that of Mg the ionization edges of which (Mg-L23

at 49 eV Mg-K at 1305 eV) are outside the investigated energy interval An under-standing of the bonding mechanism between the Mg matrix and the TiCxNy coatingrequires the comprehensive analysis of ELNES details at the coatingmatrix interfaceas it may be associated with phase formation on the nanoscopic scale (Tiplusmn Mg inter-metallics Tiplusmn Mg spinels ternary nitrides or quarternary carbonitrides of the systemTiplusmn Mgplusmn Cplusmn N) Accordingly the ionization edges of Mg at about 49 eV (Mg-L23) and1305 eV (Mg-K) have to be analysed whereas an overlap of the Mg-L23 ELNESwith the Ti-M23 and Ti-M1 ELNES has to be taken into consideration as their onsetenergies are at about 35 or 60 eV respectively These analyses are beyond the scope ofthis paper They are of particular interest as Wurm et al (1997) and Wurm (1998)found for the T5331HTATiNcp-Mg composite a relatively high tensile strengthperpendicular to the reg bre axis (approximately 25 MPa) which hints at good adhe-sion of the Mg matrix on the TiN-coated carbon reg bres Nevertheless here we couldprove that the adhesion does not correlate with the formation of larger brittlephases

T iN coatings in CMgplusmn Al composites 669

The microstructure of T5331HTATiNAM50 strongly resembles that ofT5331HTATiNcp-Mg with its reg brematrix interfaces being mainly free of carbides(see reg gures 9 (a) and 1 (a)) On the other hand the same Mgplusmn Al alloy in contact withthe uncoated carbon reg bre resulted in the substantial formation of Al2MgC2 (reg gure10 (a)) thus indicating a strong di usion barrier e ect of the titanium nitride coat-ing This is conreg rmed by a lowered mass transport in the reverse directionthat is from matrix to reg bre due to the TiCxNy interfacial reg lm as only1plusmn 2 at Mg penetrated into the coated reg bre (reg gures 1 (b) and 9 (b)) compared with4plusmn 6 at Mg in case of the uncoated reg bre (reg gure 10 (b)) Thus carbide formationand reg bre degradation are simultaneously limited

Nevertheless a graphitic interlayer has in certain samples been detected at thecoatingmatrix interface in T5331HTATiNcp-Mg indicating that locally C wastransported from the reg bre to the matrix As this phenomenon is not very pro-nounced it is concluded that the C transport across the reg brematrix interfacedoes not occur through TiCxNy grains even if they exhibit signireg cant vacancyconcentrations in the non-metal sublattice Geometrical considerations on thethree-dimensional structure of the coating lead to the conclusion that there existlocally reg ssures in the coating (the reg lm thickness is in the range of the grain dia-meters) Thus locally liquid Mg penetrates through reg ssures in the reg lm towards thereg bre (reg gure 2 (d)) In successive steps C from the reg bre dissolves and di uses in theliquid Mg through the coating and precipitates as graphitic ribbons at the coatingmatrix interface The thermodynamics of such an isothermal dissolutionplusmn di usionplusmnprecipitation process have been described by Fitzer and Kegel (1968) and theyshowed that it is driven by a di erence in the chemical potential of C in the reg breand in the graphitic ribbon

Accordingly the formation of some large Al2MgC2 plates in T5331HTATiNAM50 is not full excluded and these plates may even pierce the reg bre coating (reg gures9 (a) and 11) At these locations notching may occur which can initiate reg bre crack-ing Consequently the strength distribution function of the reg bres (Danzer 1994) isa ected by the introduction of this new class of defects and the probability of reg brefracture at low loads is enhanced The formation of many large carbide plates evenleads to an embrittlement of CMgplusmn Al composites (Feldho et al 1997 HaEgrave hnel et al1997 Feldho 1998) As there are only a few of these large carbide plates inT5331HTATiNAM50 that is the e ect is very limited here accordingly thestrength of this composite was found to be only slightly lower than that ofT5331HTATiNcp-Mg (Wurm et al 1997 Wurm 1997)

Altogether the amount of carbide formation in T5331HTATiNAM50 (reg gure9 (a)) is signireg cantly lower than in T5331HTAAM50 (reg gure 10 (a)) with theuncoated carbon reg bres This is because the TiCxNy reg lm serves well as an inhibitorof deleterious carbide formation also when using Mg alloys with large amounts ofAl as matrices Thus the use of Mgplusmn Al matrices for the manufacture of hybridcomponents is enabled by the di usion retarding e ect of the chemically vapour-deposited TiCxNy coating

5 CONCLUSIONS

CVD coating on the carbon reg bres has been shown to be partly substoichiometrictitanium carbonitride (TiCxNy) rather than titanium nitride (TiN) Even though thebonding mechanism at the coatingmatrix interface is still not clear it has been

670 A Feldho et al

shown that the adhesion of Mg on the TiCxNy coating is not accompanied by theformation of extensive brittle phases

A titanium carbonitride reg lm homogeneously covering the reg bre surfaces ham-pers the transport processes across the reg brematrix interface The penetration of Mginto the carbon reg bre is signireg cantly lowered and so is the formation of Al2MgC2with deleterious notch e ects Thus the TiCxNy coating enables the use of highcontents of Al in the matrix alloy without the negative e ects of the carbide pre-cipitates on the mechanical properties of the composite

ACKNOWLEDGEMENTS

The authors would like to thank Professor R F Singer and Dr D Wurm(University of Erlangenplusmn NuEgrave rnberg) for providing the MMC samples

REFERENCESANDRIEVSKI R A 1997 J Mater Sci 32 4463BRYDSON R SAUER H ENGEL W and ZEITLER E 1991 Microsc Microanal Micro-

struct 2 159CRAVEN A J 1995 J Microsc 180 250CRAVEN A J and G ARVIE L A J 1995 Microsc Microanal Microstruct 6 89D ANZER R 1994 The Encyclopedia of Advanced Materials Vol 1 edited by D Bloor R J

Brook M C Flemings S Mahajan and R W Cahn (Oxford Pergamon)pp 385plusmn 398

D ELANNAY F FROYEN L and D ERUYTTERE A 1987 J Mater Sci 22 1EM V T and TASHMETOV M YU 1996 Phys Stat sol (b) 198 571F ELDHOFF A 1998 PhD Thesis Martin Luther University of Halle-Wittenberg

(Aachen-Maastricht Shaker)F ELDHOFF A P IP PEL E and WOLTERSDORF J 1997 J Microsc 185 122 1999 Phil

Mag A 79 1263F ITZER E and K EGEL B 1968 Carbon 6 433H AiumlHNEL A PIP PEL E F ELDHOFF A SCHNEIDER R and WOLTERSDORF J 1997 Mater

Sci Engng 237 173H OFER F WARBICHLER P and GROGGER W 1995 Ultramicroscopy 59 15H OSOI J O IKAW A T and BANDO Y 1986 J Electron Microsc 35 129JONSSON S 1996 Z Metall 87 713K RIVANEKOLG RUBBENSA JDELLBYN and M EYERCE1992 Microsc Microanal

Microstruct 3 187N AIDICH J V 1981 Prog Surf Membr Sci 14 353Oiuml TTING ER O 1996 PhD Thesis Friedrich Alexander University of Erlangen-NuEgrave rnberg

(DuEgrave sseldorf VDI)Oiuml TTING ER O G RAU C W INTER R SINGER R F F ELDHOFF A PIPPEL E and

WOLTERSDORF J 1995 Proceedings of the Tenth International Conference onComposite Materials Vol VI Whistler Canada (Cambridge Woodhead) pp 447plusmn454

Oiuml TTING ER O and SINGER R F 1993 Z Metall 84 827PFLUiuml GER J F INK J CRECELIUS G BOHNEN K P and W INTER H 1982 Solid St

Commun 44 489PICOUET L ABIVEN H VIALA J C and BOUIX J 1990 Advanced Composite Materials

Proceedings of the First Frenchplusmn Japanese Seminar on Composite Materials edited by CBathias and M Uemura (Paris SIRPE) pp 121plusmn 131

POLMEAR I J 1981 L ight AlloysETH Metallurgy of the L ight Alloys (London Edward Arnold)R EZ P1992 Transmission Electron Energy L oss Spectrometry in Materials Science edited by

M M Disko C C Ahn and B Fultz (Warrendale Pennsylvania Minerals Metalsand Materials Society) pp 107plusmn 129

SHORSHOROV M K U STINOV L M Z IRLIN A M and OLEFIRENKO V I 1979 JMater Sci 14 1850

T iN coatings in CMgplusmn Al composites 671

666 A Feldho et al

Figure 7 T5331HTATiNcp-Mg comparison of the C-K ELNES (top) and the N-KELNES (bottom) of two electron-energy-loss spectra taken from the coating

Figure 8 T5331HTATiNcp-Mg graph of [C][Ti] and [N][Ti] ratios and their sum(permilCŠ Dagger permilNŠ=permilTiŠdagger across the coating obtained by the quantireg cation of the electron-energy-loss spectra presented in reg gure 6

shown in reg gure 6 (a) were quantireg ed as standardless using hydrogenic scatteringcross-sections The carbon content ([C][Ti]) decreases from the reg bre (left) to thecoating with the nitrogen content ([N][Ti]) increasing The sum of both (permilCŠ Dagger permilNŠ)[Ti] remains constant across the coating but is always smaller than unity at about08

Changing the matrix to the Mg-Al alloy did not initiate a distinct formation ofcarbides (reg gure 9 (a)) For T5331HTATiNAM50 approximately the same micro-structure as with the pure Mg matrix was observed Only a few carbide precipitateshave locally formed at the reg brematrix interface as can be seen in reg gure 9 (a) at thereg bre on the top left Combining EDXS with STEM (reg gure 9 (b)) revealed approxi-mately 1plusmn 2 at Mg to be almost homogeneously distributed in the volume of thecarbon reg bre but no Al had penetrated into the body of the reg bre

However combining the same Mgplusmn Al matrix with the uncoated reg bre resulted inthe substantial formation of many carbidic plates of di erent size at the reg brematrixinterface (reg gure 10 (a)) In T5331HTAAM50 the plate-shaped precipitatesextend up to 15 mm into the matrix (reg gure 10 (a)) Using HREM and selected-area di raction they were identireg ed to be the ternary carbide Al2MgC2 as shownby its (0002) lattice fringe spacing of 062nm Often planar defects had formedparallel to the (0001) habit planes of the carbidic platelets (see also Feldho et al(1999)) In this composite EDXS in combination with STEM (reg gure 10 (b)) revealedapproximately 4plusmn 6 at Mg almost homogeneously distributed in the volume of thecarbon reg bre

Finally it should be mentioned that occasionally larger isolated carbide platescan be observed with dimensions above 1 mm which may pierce the TiCxNy reg lm andreach into the reg bre surface This may occur even in parts of the composite where thereg bre coating is relatively thick as shown in reg gure 11 for T5331HTATiNAM50with the coating thickness being 60plusmn 100 nm

T iN coatings in CMgplusmn Al composites 667

Figure 9 T5331HTATiNAM50 (a) bright-reg eld image showing the reg brematrix interfacialregion to be mainly free of carbide precipitates (b) energy-dispersive X-ray spectrumof a carbon reg bre

4 D ISCUSSION

The titanium nitride coating is polycrystalline with a grain size in the range 5plusmn10 nm and covering the reg bre surfaces almost completely The thickness of the coatingcan in some cases reach more than 100 nm but mainly varies between 10 and 30 nm

Concerning the chemical composition of the coating a strong correspondencebetween the C-K and the N-K ELNES features (see reg gure 7) was observed This is

668 A Feldho et al

Figure 10 T5331HTATiNAM50 (a) HVEM bright-reg eld image showing many platelets ofAl2MgC2 in the reg brematrix interregion (b) energy-dispersive X-ray spectrum of acarbon reg bre

Figure 11 T5331HTATiNAM50 HVEM bright-reg eld image showing a carbide precipitatepiercing the coating on the reg bre

caused by a strong relationship between the crystal chemistry of TiC and TiN whichare isotypic (Hosoi et al 1986) Both compounds exhibit the rock-salt structure asthe Ti atoms form a fcc lattice with the octahedral interstices being completely reg lledwith C and N atoms respectively The lattice parameters of TiC (a ˆ 0432 nm) andTiN (a ˆ 0423 nm) di er by only about 9 pm (ie 21) If not all octahedralinterstices of the Ti sublattice are occupied substoichiometric carbides TiCx(x lt 1) and nitrides TiNy (y lt 1dagger result The occupation of some octahedral inter-stices with C and others with N leads to the formation of titanium carbonitridesTiCxNy (x Dagger y 4 1) substitutional solid solutions with a wide solubility for C and N(Jonsson 1996) The lattice parameters of these ternary phases lie between those ofTiC and TiN Changes in the stoichiometry hardly a ect the C-K and N-K ELNESsof these carbonitrides Principally they lead to slight changes in the peak intensitieswhile the peak energies as well as the onset energy remain constant within the energyresolution (Pmacr uEgrave ger et al 1982)

Taking into consideration the observed reg ne-structure details at the C-K and N-Kionization edges (reg gure 7) together with the quantireg cation proreg le of electron-energy-loss spectra (reg gure 8) allows the following conclusion to be drawn The EELSanalyses shown in reg gures 6plusmn 8 prove that the coating consists of a substoichiometrictitanium carbonitride (TiCxNy) with a high C content at the reg brecoating interfacegradually becoming richer in N on going towards the middle of thereg lm Nevertheless the vacancy concentration in the non-metal sublattice is almostconstant (x Dagger y ordm 08dagger (see reg gure 8) It is pointed out that TiCxNy with evenhigher vacancy concentrations (x Dagger y lt 07) have been observed (Em andTashmetov 1996)

The compositional changes within the coating hint to a reaction of the reactorgas mixture (TiCl4 N2 and H2) with C from the reg bre during the initial state of CVDThus the term reactive chemical vapour deposition (RCVD) which wasproposed by Vincent et al (1992) seems to be suitable here and the chemicalprocesses during deposition are much more complex than previously assumed bythe reaction TiCl4 Dagger 1

2 N2 Dagger H2 TiN Dagger 4 HCl used by Wurm et al (1997) andWurm (1998)

In reg gure 6 (a) the electron-energy-loss spectrum 20 (at 266 nm) at the coatingmatrix interface shows almost no C or N but clearly Ti and O Probably theoccurrence of O correlates to that of Mg the ionization edges of which (Mg-L23

at 49 eV Mg-K at 1305 eV) are outside the investigated energy interval An under-standing of the bonding mechanism between the Mg matrix and the TiCxNy coatingrequires the comprehensive analysis of ELNES details at the coatingmatrix interfaceas it may be associated with phase formation on the nanoscopic scale (Tiplusmn Mg inter-metallics Tiplusmn Mg spinels ternary nitrides or quarternary carbonitrides of the systemTiplusmn Mgplusmn Cplusmn N) Accordingly the ionization edges of Mg at about 49 eV (Mg-L23) and1305 eV (Mg-K) have to be analysed whereas an overlap of the Mg-L23 ELNESwith the Ti-M23 and Ti-M1 ELNES has to be taken into consideration as their onsetenergies are at about 35 or 60 eV respectively These analyses are beyond the scope ofthis paper They are of particular interest as Wurm et al (1997) and Wurm (1998)found for the T5331HTATiNcp-Mg composite a relatively high tensile strengthperpendicular to the reg bre axis (approximately 25 MPa) which hints at good adhe-sion of the Mg matrix on the TiN-coated carbon reg bres Nevertheless here we couldprove that the adhesion does not correlate with the formation of larger brittlephases

T iN coatings in CMgplusmn Al composites 669

The microstructure of T5331HTATiNAM50 strongly resembles that ofT5331HTATiNcp-Mg with its reg brematrix interfaces being mainly free of carbides(see reg gures 9 (a) and 1 (a)) On the other hand the same Mgplusmn Al alloy in contact withthe uncoated carbon reg bre resulted in the substantial formation of Al2MgC2 (reg gure10 (a)) thus indicating a strong di usion barrier e ect of the titanium nitride coat-ing This is conreg rmed by a lowered mass transport in the reverse directionthat is from matrix to reg bre due to the TiCxNy interfacial reg lm as only1plusmn 2 at Mg penetrated into the coated reg bre (reg gures 1 (b) and 9 (b)) compared with4plusmn 6 at Mg in case of the uncoated reg bre (reg gure 10 (b)) Thus carbide formationand reg bre degradation are simultaneously limited

Nevertheless a graphitic interlayer has in certain samples been detected at thecoatingmatrix interface in T5331HTATiNcp-Mg indicating that locally C wastransported from the reg bre to the matrix As this phenomenon is not very pro-nounced it is concluded that the C transport across the reg brematrix interfacedoes not occur through TiCxNy grains even if they exhibit signireg cant vacancyconcentrations in the non-metal sublattice Geometrical considerations on thethree-dimensional structure of the coating lead to the conclusion that there existlocally reg ssures in the coating (the reg lm thickness is in the range of the grain dia-meters) Thus locally liquid Mg penetrates through reg ssures in the reg lm towards thereg bre (reg gure 2 (d)) In successive steps C from the reg bre dissolves and di uses in theliquid Mg through the coating and precipitates as graphitic ribbons at the coatingmatrix interface The thermodynamics of such an isothermal dissolutionplusmn di usionplusmnprecipitation process have been described by Fitzer and Kegel (1968) and theyshowed that it is driven by a di erence in the chemical potential of C in the reg breand in the graphitic ribbon

Accordingly the formation of some large Al2MgC2 plates in T5331HTATiNAM50 is not full excluded and these plates may even pierce the reg bre coating (reg gures9 (a) and 11) At these locations notching may occur which can initiate reg bre crack-ing Consequently the strength distribution function of the reg bres (Danzer 1994) isa ected by the introduction of this new class of defects and the probability of reg brefracture at low loads is enhanced The formation of many large carbide plates evenleads to an embrittlement of CMgplusmn Al composites (Feldho et al 1997 HaEgrave hnel et al1997 Feldho 1998) As there are only a few of these large carbide plates inT5331HTATiNAM50 that is the e ect is very limited here accordingly thestrength of this composite was found to be only slightly lower than that ofT5331HTATiNcp-Mg (Wurm et al 1997 Wurm 1997)

Altogether the amount of carbide formation in T5331HTATiNAM50 (reg gure9 (a)) is signireg cantly lower than in T5331HTAAM50 (reg gure 10 (a)) with theuncoated carbon reg bres This is because the TiCxNy reg lm serves well as an inhibitorof deleterious carbide formation also when using Mg alloys with large amounts ofAl as matrices Thus the use of Mgplusmn Al matrices for the manufacture of hybridcomponents is enabled by the di usion retarding e ect of the chemically vapour-deposited TiCxNy coating

5 CONCLUSIONS

CVD coating on the carbon reg bres has been shown to be partly substoichiometrictitanium carbonitride (TiCxNy) rather than titanium nitride (TiN) Even though thebonding mechanism at the coatingmatrix interface is still not clear it has been

670 A Feldho et al

shown that the adhesion of Mg on the TiCxNy coating is not accompanied by theformation of extensive brittle phases

A titanium carbonitride reg lm homogeneously covering the reg bre surfaces ham-pers the transport processes across the reg brematrix interface The penetration of Mginto the carbon reg bre is signireg cantly lowered and so is the formation of Al2MgC2with deleterious notch e ects Thus the TiCxNy coating enables the use of highcontents of Al in the matrix alloy without the negative e ects of the carbide pre-cipitates on the mechanical properties of the composite

ACKNOWLEDGEMENTS

The authors would like to thank Professor R F Singer and Dr D Wurm(University of Erlangenplusmn NuEgrave rnberg) for providing the MMC samples

REFERENCESANDRIEVSKI R A 1997 J Mater Sci 32 4463BRYDSON R SAUER H ENGEL W and ZEITLER E 1991 Microsc Microanal Micro-

struct 2 159CRAVEN A J 1995 J Microsc 180 250CRAVEN A J and G ARVIE L A J 1995 Microsc Microanal Microstruct 6 89D ANZER R 1994 The Encyclopedia of Advanced Materials Vol 1 edited by D Bloor R J

Brook M C Flemings S Mahajan and R W Cahn (Oxford Pergamon)pp 385plusmn 398

D ELANNAY F FROYEN L and D ERUYTTERE A 1987 J Mater Sci 22 1EM V T and TASHMETOV M YU 1996 Phys Stat sol (b) 198 571F ELDHOFF A 1998 PhD Thesis Martin Luther University of Halle-Wittenberg

(Aachen-Maastricht Shaker)F ELDHOFF A P IP PEL E and WOLTERSDORF J 1997 J Microsc 185 122 1999 Phil

Mag A 79 1263F ITZER E and K EGEL B 1968 Carbon 6 433H AiumlHNEL A PIP PEL E F ELDHOFF A SCHNEIDER R and WOLTERSDORF J 1997 Mater

Sci Engng 237 173H OFER F WARBICHLER P and GROGGER W 1995 Ultramicroscopy 59 15H OSOI J O IKAW A T and BANDO Y 1986 J Electron Microsc 35 129JONSSON S 1996 Z Metall 87 713K RIVANEKOLG RUBBENSA JDELLBYN and M EYERCE1992 Microsc Microanal

Microstruct 3 187N AIDICH J V 1981 Prog Surf Membr Sci 14 353Oiuml TTING ER O 1996 PhD Thesis Friedrich Alexander University of Erlangen-NuEgrave rnberg

(DuEgrave sseldorf VDI)Oiuml TTING ER O G RAU C W INTER R SINGER R F F ELDHOFF A PIPPEL E and

WOLTERSDORF J 1995 Proceedings of the Tenth International Conference onComposite Materials Vol VI Whistler Canada (Cambridge Woodhead) pp 447plusmn454

Oiuml TTING ER O and SINGER R F 1993 Z Metall 84 827PFLUiuml GER J F INK J CRECELIUS G BOHNEN K P and W INTER H 1982 Solid St

Commun 44 489PICOUET L ABIVEN H VIALA J C and BOUIX J 1990 Advanced Composite Materials

Proceedings of the First Frenchplusmn Japanese Seminar on Composite Materials edited by CBathias and M Uemura (Paris SIRPE) pp 121plusmn 131

POLMEAR I J 1981 L ight AlloysETH Metallurgy of the L ight Alloys (London Edward Arnold)R EZ P1992 Transmission Electron Energy L oss Spectrometry in Materials Science edited by

M M Disko C C Ahn and B Fultz (Warrendale Pennsylvania Minerals Metalsand Materials Society) pp 107plusmn 129

SHORSHOROV M K U STINOV L M Z IRLIN A M and OLEFIRENKO V I 1979 JMater Sci 14 1850

T iN coatings in CMgplusmn Al composites 671

shown in reg gure 6 (a) were quantireg ed as standardless using hydrogenic scatteringcross-sections The carbon content ([C][Ti]) decreases from the reg bre (left) to thecoating with the nitrogen content ([N][Ti]) increasing The sum of both (permilCŠ Dagger permilNŠ)[Ti] remains constant across the coating but is always smaller than unity at about08

Changing the matrix to the Mg-Al alloy did not initiate a distinct formation ofcarbides (reg gure 9 (a)) For T5331HTATiNAM50 approximately the same micro-structure as with the pure Mg matrix was observed Only a few carbide precipitateshave locally formed at the reg brematrix interface as can be seen in reg gure 9 (a) at thereg bre on the top left Combining EDXS with STEM (reg gure 9 (b)) revealed approxi-mately 1plusmn 2 at Mg to be almost homogeneously distributed in the volume of thecarbon reg bre but no Al had penetrated into the body of the reg bre

However combining the same Mgplusmn Al matrix with the uncoated reg bre resulted inthe substantial formation of many carbidic plates of di erent size at the reg brematrixinterface (reg gure 10 (a)) In T5331HTAAM50 the plate-shaped precipitatesextend up to 15 mm into the matrix (reg gure 10 (a)) Using HREM and selected-area di raction they were identireg ed to be the ternary carbide Al2MgC2 as shownby its (0002) lattice fringe spacing of 062nm Often planar defects had formedparallel to the (0001) habit planes of the carbidic platelets (see also Feldho et al(1999)) In this composite EDXS in combination with STEM (reg gure 10 (b)) revealedapproximately 4plusmn 6 at Mg almost homogeneously distributed in the volume of thecarbon reg bre

Finally it should be mentioned that occasionally larger isolated carbide platescan be observed with dimensions above 1 mm which may pierce the TiCxNy reg lm andreach into the reg bre surface This may occur even in parts of the composite where thereg bre coating is relatively thick as shown in reg gure 11 for T5331HTATiNAM50with the coating thickness being 60plusmn 100 nm

T iN coatings in CMgplusmn Al composites 667

Figure 9 T5331HTATiNAM50 (a) bright-reg eld image showing the reg brematrix interfacialregion to be mainly free of carbide precipitates (b) energy-dispersive X-ray spectrumof a carbon reg bre

4 D ISCUSSION

The titanium nitride coating is polycrystalline with a grain size in the range 5plusmn10 nm and covering the reg bre surfaces almost completely The thickness of the coatingcan in some cases reach more than 100 nm but mainly varies between 10 and 30 nm

Concerning the chemical composition of the coating a strong correspondencebetween the C-K and the N-K ELNES features (see reg gure 7) was observed This is

668 A Feldho et al

Figure 10 T5331HTATiNAM50 (a) HVEM bright-reg eld image showing many platelets ofAl2MgC2 in the reg brematrix interregion (b) energy-dispersive X-ray spectrum of acarbon reg bre

Figure 11 T5331HTATiNAM50 HVEM bright-reg eld image showing a carbide precipitatepiercing the coating on the reg bre

caused by a strong relationship between the crystal chemistry of TiC and TiN whichare isotypic (Hosoi et al 1986) Both compounds exhibit the rock-salt structure asthe Ti atoms form a fcc lattice with the octahedral interstices being completely reg lledwith C and N atoms respectively The lattice parameters of TiC (a ˆ 0432 nm) andTiN (a ˆ 0423 nm) di er by only about 9 pm (ie 21) If not all octahedralinterstices of the Ti sublattice are occupied substoichiometric carbides TiCx(x lt 1) and nitrides TiNy (y lt 1dagger result The occupation of some octahedral inter-stices with C and others with N leads to the formation of titanium carbonitridesTiCxNy (x Dagger y 4 1) substitutional solid solutions with a wide solubility for C and N(Jonsson 1996) The lattice parameters of these ternary phases lie between those ofTiC and TiN Changes in the stoichiometry hardly a ect the C-K and N-K ELNESsof these carbonitrides Principally they lead to slight changes in the peak intensitieswhile the peak energies as well as the onset energy remain constant within the energyresolution (Pmacr uEgrave ger et al 1982)

Taking into consideration the observed reg ne-structure details at the C-K and N-Kionization edges (reg gure 7) together with the quantireg cation proreg le of electron-energy-loss spectra (reg gure 8) allows the following conclusion to be drawn The EELSanalyses shown in reg gures 6plusmn 8 prove that the coating consists of a substoichiometrictitanium carbonitride (TiCxNy) with a high C content at the reg brecoating interfacegradually becoming richer in N on going towards the middle of thereg lm Nevertheless the vacancy concentration in the non-metal sublattice is almostconstant (x Dagger y ordm 08dagger (see reg gure 8) It is pointed out that TiCxNy with evenhigher vacancy concentrations (x Dagger y lt 07) have been observed (Em andTashmetov 1996)

The compositional changes within the coating hint to a reaction of the reactorgas mixture (TiCl4 N2 and H2) with C from the reg bre during the initial state of CVDThus the term reactive chemical vapour deposition (RCVD) which wasproposed by Vincent et al (1992) seems to be suitable here and the chemicalprocesses during deposition are much more complex than previously assumed bythe reaction TiCl4 Dagger 1

2 N2 Dagger H2 TiN Dagger 4 HCl used by Wurm et al (1997) andWurm (1998)

In reg gure 6 (a) the electron-energy-loss spectrum 20 (at 266 nm) at the coatingmatrix interface shows almost no C or N but clearly Ti and O Probably theoccurrence of O correlates to that of Mg the ionization edges of which (Mg-L23

at 49 eV Mg-K at 1305 eV) are outside the investigated energy interval An under-standing of the bonding mechanism between the Mg matrix and the TiCxNy coatingrequires the comprehensive analysis of ELNES details at the coatingmatrix interfaceas it may be associated with phase formation on the nanoscopic scale (Tiplusmn Mg inter-metallics Tiplusmn Mg spinels ternary nitrides or quarternary carbonitrides of the systemTiplusmn Mgplusmn Cplusmn N) Accordingly the ionization edges of Mg at about 49 eV (Mg-L23) and1305 eV (Mg-K) have to be analysed whereas an overlap of the Mg-L23 ELNESwith the Ti-M23 and Ti-M1 ELNES has to be taken into consideration as their onsetenergies are at about 35 or 60 eV respectively These analyses are beyond the scope ofthis paper They are of particular interest as Wurm et al (1997) and Wurm (1998)found for the T5331HTATiNcp-Mg composite a relatively high tensile strengthperpendicular to the reg bre axis (approximately 25 MPa) which hints at good adhe-sion of the Mg matrix on the TiN-coated carbon reg bres Nevertheless here we couldprove that the adhesion does not correlate with the formation of larger brittlephases

T iN coatings in CMgplusmn Al composites 669

The microstructure of T5331HTATiNAM50 strongly resembles that ofT5331HTATiNcp-Mg with its reg brematrix interfaces being mainly free of carbides(see reg gures 9 (a) and 1 (a)) On the other hand the same Mgplusmn Al alloy in contact withthe uncoated carbon reg bre resulted in the substantial formation of Al2MgC2 (reg gure10 (a)) thus indicating a strong di usion barrier e ect of the titanium nitride coat-ing This is conreg rmed by a lowered mass transport in the reverse directionthat is from matrix to reg bre due to the TiCxNy interfacial reg lm as only1plusmn 2 at Mg penetrated into the coated reg bre (reg gures 1 (b) and 9 (b)) compared with4plusmn 6 at Mg in case of the uncoated reg bre (reg gure 10 (b)) Thus carbide formationand reg bre degradation are simultaneously limited

Nevertheless a graphitic interlayer has in certain samples been detected at thecoatingmatrix interface in T5331HTATiNcp-Mg indicating that locally C wastransported from the reg bre to the matrix As this phenomenon is not very pro-nounced it is concluded that the C transport across the reg brematrix interfacedoes not occur through TiCxNy grains even if they exhibit signireg cant vacancyconcentrations in the non-metal sublattice Geometrical considerations on thethree-dimensional structure of the coating lead to the conclusion that there existlocally reg ssures in the coating (the reg lm thickness is in the range of the grain dia-meters) Thus locally liquid Mg penetrates through reg ssures in the reg lm towards thereg bre (reg gure 2 (d)) In successive steps C from the reg bre dissolves and di uses in theliquid Mg through the coating and precipitates as graphitic ribbons at the coatingmatrix interface The thermodynamics of such an isothermal dissolutionplusmn di usionplusmnprecipitation process have been described by Fitzer and Kegel (1968) and theyshowed that it is driven by a di erence in the chemical potential of C in the reg breand in the graphitic ribbon

Accordingly the formation of some large Al2MgC2 plates in T5331HTATiNAM50 is not full excluded and these plates may even pierce the reg bre coating (reg gures9 (a) and 11) At these locations notching may occur which can initiate reg bre crack-ing Consequently the strength distribution function of the reg bres (Danzer 1994) isa ected by the introduction of this new class of defects and the probability of reg brefracture at low loads is enhanced The formation of many large carbide plates evenleads to an embrittlement of CMgplusmn Al composites (Feldho et al 1997 HaEgrave hnel et al1997 Feldho 1998) As there are only a few of these large carbide plates inT5331HTATiNAM50 that is the e ect is very limited here accordingly thestrength of this composite was found to be only slightly lower than that ofT5331HTATiNcp-Mg (Wurm et al 1997 Wurm 1997)

Altogether the amount of carbide formation in T5331HTATiNAM50 (reg gure9 (a)) is signireg cantly lower than in T5331HTAAM50 (reg gure 10 (a)) with theuncoated carbon reg bres This is because the TiCxNy reg lm serves well as an inhibitorof deleterious carbide formation also when using Mg alloys with large amounts ofAl as matrices Thus the use of Mgplusmn Al matrices for the manufacture of hybridcomponents is enabled by the di usion retarding e ect of the chemically vapour-deposited TiCxNy coating

5 CONCLUSIONS

CVD coating on the carbon reg bres has been shown to be partly substoichiometrictitanium carbonitride (TiCxNy) rather than titanium nitride (TiN) Even though thebonding mechanism at the coatingmatrix interface is still not clear it has been

670 A Feldho et al

shown that the adhesion of Mg on the TiCxNy coating is not accompanied by theformation of extensive brittle phases

A titanium carbonitride reg lm homogeneously covering the reg bre surfaces ham-pers the transport processes across the reg brematrix interface The penetration of Mginto the carbon reg bre is signireg cantly lowered and so is the formation of Al2MgC2with deleterious notch e ects Thus the TiCxNy coating enables the use of highcontents of Al in the matrix alloy without the negative e ects of the carbide pre-cipitates on the mechanical properties of the composite

ACKNOWLEDGEMENTS

The authors would like to thank Professor R F Singer and Dr D Wurm(University of Erlangenplusmn NuEgrave rnberg) for providing the MMC samples

REFERENCESANDRIEVSKI R A 1997 J Mater Sci 32 4463BRYDSON R SAUER H ENGEL W and ZEITLER E 1991 Microsc Microanal Micro-

struct 2 159CRAVEN A J 1995 J Microsc 180 250CRAVEN A J and G ARVIE L A J 1995 Microsc Microanal Microstruct 6 89D ANZER R 1994 The Encyclopedia of Advanced Materials Vol 1 edited by D Bloor R J

Brook M C Flemings S Mahajan and R W Cahn (Oxford Pergamon)pp 385plusmn 398

D ELANNAY F FROYEN L and D ERUYTTERE A 1987 J Mater Sci 22 1EM V T and TASHMETOV M YU 1996 Phys Stat sol (b) 198 571F ELDHOFF A 1998 PhD Thesis Martin Luther University of Halle-Wittenberg

(Aachen-Maastricht Shaker)F ELDHOFF A P IP PEL E and WOLTERSDORF J 1997 J Microsc 185 122 1999 Phil

Mag A 79 1263F ITZER E and K EGEL B 1968 Carbon 6 433H AiumlHNEL A PIP PEL E F ELDHOFF A SCHNEIDER R and WOLTERSDORF J 1997 Mater

Sci Engng 237 173H OFER F WARBICHLER P and GROGGER W 1995 Ultramicroscopy 59 15H OSOI J O IKAW A T and BANDO Y 1986 J Electron Microsc 35 129JONSSON S 1996 Z Metall 87 713K RIVANEKOLG RUBBENSA JDELLBYN and M EYERCE1992 Microsc Microanal

Microstruct 3 187N AIDICH J V 1981 Prog Surf Membr Sci 14 353Oiuml TTING ER O 1996 PhD Thesis Friedrich Alexander University of Erlangen-NuEgrave rnberg

(DuEgrave sseldorf VDI)Oiuml TTING ER O G RAU C W INTER R SINGER R F F ELDHOFF A PIPPEL E and

WOLTERSDORF J 1995 Proceedings of the Tenth International Conference onComposite Materials Vol VI Whistler Canada (Cambridge Woodhead) pp 447plusmn454

Oiuml TTING ER O and SINGER R F 1993 Z Metall 84 827PFLUiuml GER J F INK J CRECELIUS G BOHNEN K P and W INTER H 1982 Solid St

Commun 44 489PICOUET L ABIVEN H VIALA J C and BOUIX J 1990 Advanced Composite Materials

Proceedings of the First Frenchplusmn Japanese Seminar on Composite Materials edited by CBathias and M Uemura (Paris SIRPE) pp 121plusmn 131

POLMEAR I J 1981 L ight AlloysETH Metallurgy of the L ight Alloys (London Edward Arnold)R EZ P1992 Transmission Electron Energy L oss Spectrometry in Materials Science edited by

M M Disko C C Ahn and B Fultz (Warrendale Pennsylvania Minerals Metalsand Materials Society) pp 107plusmn 129

SHORSHOROV M K U STINOV L M Z IRLIN A M and OLEFIRENKO V I 1979 JMater Sci 14 1850

T iN coatings in CMgplusmn Al composites 671

4 D ISCUSSION

The titanium nitride coating is polycrystalline with a grain size in the range 5plusmn10 nm and covering the reg bre surfaces almost completely The thickness of the coatingcan in some cases reach more than 100 nm but mainly varies between 10 and 30 nm

Concerning the chemical composition of the coating a strong correspondencebetween the C-K and the N-K ELNES features (see reg gure 7) was observed This is

668 A Feldho et al

Figure 10 T5331HTATiNAM50 (a) HVEM bright-reg eld image showing many platelets ofAl2MgC2 in the reg brematrix interregion (b) energy-dispersive X-ray spectrum of acarbon reg bre

Figure 11 T5331HTATiNAM50 HVEM bright-reg eld image showing a carbide precipitatepiercing the coating on the reg bre

caused by a strong relationship between the crystal chemistry of TiC and TiN whichare isotypic (Hosoi et al 1986) Both compounds exhibit the rock-salt structure asthe Ti atoms form a fcc lattice with the octahedral interstices being completely reg lledwith C and N atoms respectively The lattice parameters of TiC (a ˆ 0432 nm) andTiN (a ˆ 0423 nm) di er by only about 9 pm (ie 21) If not all octahedralinterstices of the Ti sublattice are occupied substoichiometric carbides TiCx(x lt 1) and nitrides TiNy (y lt 1dagger result The occupation of some octahedral inter-stices with C and others with N leads to the formation of titanium carbonitridesTiCxNy (x Dagger y 4 1) substitutional solid solutions with a wide solubility for C and N(Jonsson 1996) The lattice parameters of these ternary phases lie between those ofTiC and TiN Changes in the stoichiometry hardly a ect the C-K and N-K ELNESsof these carbonitrides Principally they lead to slight changes in the peak intensitieswhile the peak energies as well as the onset energy remain constant within the energyresolution (Pmacr uEgrave ger et al 1982)

Taking into consideration the observed reg ne-structure details at the C-K and N-Kionization edges (reg gure 7) together with the quantireg cation proreg le of electron-energy-loss spectra (reg gure 8) allows the following conclusion to be drawn The EELSanalyses shown in reg gures 6plusmn 8 prove that the coating consists of a substoichiometrictitanium carbonitride (TiCxNy) with a high C content at the reg brecoating interfacegradually becoming richer in N on going towards the middle of thereg lm Nevertheless the vacancy concentration in the non-metal sublattice is almostconstant (x Dagger y ordm 08dagger (see reg gure 8) It is pointed out that TiCxNy with evenhigher vacancy concentrations (x Dagger y lt 07) have been observed (Em andTashmetov 1996)

The compositional changes within the coating hint to a reaction of the reactorgas mixture (TiCl4 N2 and H2) with C from the reg bre during the initial state of CVDThus the term reactive chemical vapour deposition (RCVD) which wasproposed by Vincent et al (1992) seems to be suitable here and the chemicalprocesses during deposition are much more complex than previously assumed bythe reaction TiCl4 Dagger 1

2 N2 Dagger H2 TiN Dagger 4 HCl used by Wurm et al (1997) andWurm (1998)

In reg gure 6 (a) the electron-energy-loss spectrum 20 (at 266 nm) at the coatingmatrix interface shows almost no C or N but clearly Ti and O Probably theoccurrence of O correlates to that of Mg the ionization edges of which (Mg-L23

at 49 eV Mg-K at 1305 eV) are outside the investigated energy interval An under-standing of the bonding mechanism between the Mg matrix and the TiCxNy coatingrequires the comprehensive analysis of ELNES details at the coatingmatrix interfaceas it may be associated with phase formation on the nanoscopic scale (Tiplusmn Mg inter-metallics Tiplusmn Mg spinels ternary nitrides or quarternary carbonitrides of the systemTiplusmn Mgplusmn Cplusmn N) Accordingly the ionization edges of Mg at about 49 eV (Mg-L23) and1305 eV (Mg-K) have to be analysed whereas an overlap of the Mg-L23 ELNESwith the Ti-M23 and Ti-M1 ELNES has to be taken into consideration as their onsetenergies are at about 35 or 60 eV respectively These analyses are beyond the scope ofthis paper They are of particular interest as Wurm et al (1997) and Wurm (1998)found for the T5331HTATiNcp-Mg composite a relatively high tensile strengthperpendicular to the reg bre axis (approximately 25 MPa) which hints at good adhe-sion of the Mg matrix on the TiN-coated carbon reg bres Nevertheless here we couldprove that the adhesion does not correlate with the formation of larger brittlephases

T iN coatings in CMgplusmn Al composites 669

The microstructure of T5331HTATiNAM50 strongly resembles that ofT5331HTATiNcp-Mg with its reg brematrix interfaces being mainly free of carbides(see reg gures 9 (a) and 1 (a)) On the other hand the same Mgplusmn Al alloy in contact withthe uncoated carbon reg bre resulted in the substantial formation of Al2MgC2 (reg gure10 (a)) thus indicating a strong di usion barrier e ect of the titanium nitride coat-ing This is conreg rmed by a lowered mass transport in the reverse directionthat is from matrix to reg bre due to the TiCxNy interfacial reg lm as only1plusmn 2 at Mg penetrated into the coated reg bre (reg gures 1 (b) and 9 (b)) compared with4plusmn 6 at Mg in case of the uncoated reg bre (reg gure 10 (b)) Thus carbide formationand reg bre degradation are simultaneously limited

Nevertheless a graphitic interlayer has in certain samples been detected at thecoatingmatrix interface in T5331HTATiNcp-Mg indicating that locally C wastransported from the reg bre to the matrix As this phenomenon is not very pro-nounced it is concluded that the C transport across the reg brematrix interfacedoes not occur through TiCxNy grains even if they exhibit signireg cant vacancyconcentrations in the non-metal sublattice Geometrical considerations on thethree-dimensional structure of the coating lead to the conclusion that there existlocally reg ssures in the coating (the reg lm thickness is in the range of the grain dia-meters) Thus locally liquid Mg penetrates through reg ssures in the reg lm towards thereg bre (reg gure 2 (d)) In successive steps C from the reg bre dissolves and di uses in theliquid Mg through the coating and precipitates as graphitic ribbons at the coatingmatrix interface The thermodynamics of such an isothermal dissolutionplusmn di usionplusmnprecipitation process have been described by Fitzer and Kegel (1968) and theyshowed that it is driven by a di erence in the chemical potential of C in the reg breand in the graphitic ribbon

Accordingly the formation of some large Al2MgC2 plates in T5331HTATiNAM50 is not full excluded and these plates may even pierce the reg bre coating (reg gures9 (a) and 11) At these locations notching may occur which can initiate reg bre crack-ing Consequently the strength distribution function of the reg bres (Danzer 1994) isa ected by the introduction of this new class of defects and the probability of reg brefracture at low loads is enhanced The formation of many large carbide plates evenleads to an embrittlement of CMgplusmn Al composites (Feldho et al 1997 HaEgrave hnel et al1997 Feldho 1998) As there are only a few of these large carbide plates inT5331HTATiNAM50 that is the e ect is very limited here accordingly thestrength of this composite was found to be only slightly lower than that ofT5331HTATiNcp-Mg (Wurm et al 1997 Wurm 1997)

Altogether the amount of carbide formation in T5331HTATiNAM50 (reg gure9 (a)) is signireg cantly lower than in T5331HTAAM50 (reg gure 10 (a)) with theuncoated carbon reg bres This is because the TiCxNy reg lm serves well as an inhibitorof deleterious carbide formation also when using Mg alloys with large amounts ofAl as matrices Thus the use of Mgplusmn Al matrices for the manufacture of hybridcomponents is enabled by the di usion retarding e ect of the chemically vapour-deposited TiCxNy coating

5 CONCLUSIONS

CVD coating on the carbon reg bres has been shown to be partly substoichiometrictitanium carbonitride (TiCxNy) rather than titanium nitride (TiN) Even though thebonding mechanism at the coatingmatrix interface is still not clear it has been

670 A Feldho et al

shown that the adhesion of Mg on the TiCxNy coating is not accompanied by theformation of extensive brittle phases

A titanium carbonitride reg lm homogeneously covering the reg bre surfaces ham-pers the transport processes across the reg brematrix interface The penetration of Mginto the carbon reg bre is signireg cantly lowered and so is the formation of Al2MgC2with deleterious notch e ects Thus the TiCxNy coating enables the use of highcontents of Al in the matrix alloy without the negative e ects of the carbide pre-cipitates on the mechanical properties of the composite

ACKNOWLEDGEMENTS

The authors would like to thank Professor R F Singer and Dr D Wurm(University of Erlangenplusmn NuEgrave rnberg) for providing the MMC samples

REFERENCESANDRIEVSKI R A 1997 J Mater Sci 32 4463BRYDSON R SAUER H ENGEL W and ZEITLER E 1991 Microsc Microanal Micro-

struct 2 159CRAVEN A J 1995 J Microsc 180 250CRAVEN A J and G ARVIE L A J 1995 Microsc Microanal Microstruct 6 89D ANZER R 1994 The Encyclopedia of Advanced Materials Vol 1 edited by D Bloor R J

Brook M C Flemings S Mahajan and R W Cahn (Oxford Pergamon)pp 385plusmn 398

D ELANNAY F FROYEN L and D ERUYTTERE A 1987 J Mater Sci 22 1EM V T and TASHMETOV M YU 1996 Phys Stat sol (b) 198 571F ELDHOFF A 1998 PhD Thesis Martin Luther University of Halle-Wittenberg

(Aachen-Maastricht Shaker)F ELDHOFF A P IP PEL E and WOLTERSDORF J 1997 J Microsc 185 122 1999 Phil

Mag A 79 1263F ITZER E and K EGEL B 1968 Carbon 6 433H AiumlHNEL A PIP PEL E F ELDHOFF A SCHNEIDER R and WOLTERSDORF J 1997 Mater

Sci Engng 237 173H OFER F WARBICHLER P and GROGGER W 1995 Ultramicroscopy 59 15H OSOI J O IKAW A T and BANDO Y 1986 J Electron Microsc 35 129JONSSON S 1996 Z Metall 87 713K RIVANEKOLG RUBBENSA JDELLBYN and M EYERCE1992 Microsc Microanal

Microstruct 3 187N AIDICH J V 1981 Prog Surf Membr Sci 14 353Oiuml TTING ER O 1996 PhD Thesis Friedrich Alexander University of Erlangen-NuEgrave rnberg

(DuEgrave sseldorf VDI)Oiuml TTING ER O G RAU C W INTER R SINGER R F F ELDHOFF A PIPPEL E and

WOLTERSDORF J 1995 Proceedings of the Tenth International Conference onComposite Materials Vol VI Whistler Canada (Cambridge Woodhead) pp 447plusmn454

Oiuml TTING ER O and SINGER R F 1993 Z Metall 84 827PFLUiuml GER J F INK J CRECELIUS G BOHNEN K P and W INTER H 1982 Solid St

Commun 44 489PICOUET L ABIVEN H VIALA J C and BOUIX J 1990 Advanced Composite Materials

Proceedings of the First Frenchplusmn Japanese Seminar on Composite Materials edited by CBathias and M Uemura (Paris SIRPE) pp 121plusmn 131

POLMEAR I J 1981 L ight AlloysETH Metallurgy of the L ight Alloys (London Edward Arnold)R EZ P1992 Transmission Electron Energy L oss Spectrometry in Materials Science edited by

M M Disko C C Ahn and B Fultz (Warrendale Pennsylvania Minerals Metalsand Materials Society) pp 107plusmn 129

SHORSHOROV M K U STINOV L M Z IRLIN A M and OLEFIRENKO V I 1979 JMater Sci 14 1850

T iN coatings in CMgplusmn Al composites 671

caused by a strong relationship between the crystal chemistry of TiC and TiN whichare isotypic (Hosoi et al 1986) Both compounds exhibit the rock-salt structure asthe Ti atoms form a fcc lattice with the octahedral interstices being completely reg lledwith C and N atoms respectively The lattice parameters of TiC (a ˆ 0432 nm) andTiN (a ˆ 0423 nm) di er by only about 9 pm (ie 21) If not all octahedralinterstices of the Ti sublattice are occupied substoichiometric carbides TiCx(x lt 1) and nitrides TiNy (y lt 1dagger result The occupation of some octahedral inter-stices with C and others with N leads to the formation of titanium carbonitridesTiCxNy (x Dagger y 4 1) substitutional solid solutions with a wide solubility for C and N(Jonsson 1996) The lattice parameters of these ternary phases lie between those ofTiC and TiN Changes in the stoichiometry hardly a ect the C-K and N-K ELNESsof these carbonitrides Principally they lead to slight changes in the peak intensitieswhile the peak energies as well as the onset energy remain constant within the energyresolution (Pmacr uEgrave ger et al 1982)

Taking into consideration the observed reg ne-structure details at the C-K and N-Kionization edges (reg gure 7) together with the quantireg cation proreg le of electron-energy-loss spectra (reg gure 8) allows the following conclusion to be drawn The EELSanalyses shown in reg gures 6plusmn 8 prove that the coating consists of a substoichiometrictitanium carbonitride (TiCxNy) with a high C content at the reg brecoating interfacegradually becoming richer in N on going towards the middle of thereg lm Nevertheless the vacancy concentration in the non-metal sublattice is almostconstant (x Dagger y ordm 08dagger (see reg gure 8) It is pointed out that TiCxNy with evenhigher vacancy concentrations (x Dagger y lt 07) have been observed (Em andTashmetov 1996)

The compositional changes within the coating hint to a reaction of the reactorgas mixture (TiCl4 N2 and H2) with C from the reg bre during the initial state of CVDThus the term reactive chemical vapour deposition (RCVD) which wasproposed by Vincent et al (1992) seems to be suitable here and the chemicalprocesses during deposition are much more complex than previously assumed bythe reaction TiCl4 Dagger 1

2 N2 Dagger H2 TiN Dagger 4 HCl used by Wurm et al (1997) andWurm (1998)

In reg gure 6 (a) the electron-energy-loss spectrum 20 (at 266 nm) at the coatingmatrix interface shows almost no C or N but clearly Ti and O Probably theoccurrence of O correlates to that of Mg the ionization edges of which (Mg-L23

at 49 eV Mg-K at 1305 eV) are outside the investigated energy interval An under-standing of the bonding mechanism between the Mg matrix and the TiCxNy coatingrequires the comprehensive analysis of ELNES details at the coatingmatrix interfaceas it may be associated with phase formation on the nanoscopic scale (Tiplusmn Mg inter-metallics Tiplusmn Mg spinels ternary nitrides or quarternary carbonitrides of the systemTiplusmn Mgplusmn Cplusmn N) Accordingly the ionization edges of Mg at about 49 eV (Mg-L23) and1305 eV (Mg-K) have to be analysed whereas an overlap of the Mg-L23 ELNESwith the Ti-M23 and Ti-M1 ELNES has to be taken into consideration as their onsetenergies are at about 35 or 60 eV respectively These analyses are beyond the scope ofthis paper They are of particular interest as Wurm et al (1997) and Wurm (1998)found for the T5331HTATiNcp-Mg composite a relatively high tensile strengthperpendicular to the reg bre axis (approximately 25 MPa) which hints at good adhe-sion of the Mg matrix on the TiN-coated carbon reg bres Nevertheless here we couldprove that the adhesion does not correlate with the formation of larger brittlephases

T iN coatings in CMgplusmn Al composites 669

The microstructure of T5331HTATiNAM50 strongly resembles that ofT5331HTATiNcp-Mg with its reg brematrix interfaces being mainly free of carbides(see reg gures 9 (a) and 1 (a)) On the other hand the same Mgplusmn Al alloy in contact withthe uncoated carbon reg bre resulted in the substantial formation of Al2MgC2 (reg gure10 (a)) thus indicating a strong di usion barrier e ect of the titanium nitride coat-ing This is conreg rmed by a lowered mass transport in the reverse directionthat is from matrix to reg bre due to the TiCxNy interfacial reg lm as only1plusmn 2 at Mg penetrated into the coated reg bre (reg gures 1 (b) and 9 (b)) compared with4plusmn 6 at Mg in case of the uncoated reg bre (reg gure 10 (b)) Thus carbide formationand reg bre degradation are simultaneously limited

Nevertheless a graphitic interlayer has in certain samples been detected at thecoatingmatrix interface in T5331HTATiNcp-Mg indicating that locally C wastransported from the reg bre to the matrix As this phenomenon is not very pro-nounced it is concluded that the C transport across the reg brematrix interfacedoes not occur through TiCxNy grains even if they exhibit signireg cant vacancyconcentrations in the non-metal sublattice Geometrical considerations on thethree-dimensional structure of the coating lead to the conclusion that there existlocally reg ssures in the coating (the reg lm thickness is in the range of the grain dia-meters) Thus locally liquid Mg penetrates through reg ssures in the reg lm towards thereg bre (reg gure 2 (d)) In successive steps C from the reg bre dissolves and di uses in theliquid Mg through the coating and precipitates as graphitic ribbons at the coatingmatrix interface The thermodynamics of such an isothermal dissolutionplusmn di usionplusmnprecipitation process have been described by Fitzer and Kegel (1968) and theyshowed that it is driven by a di erence in the chemical potential of C in the reg breand in the graphitic ribbon

Accordingly the formation of some large Al2MgC2 plates in T5331HTATiNAM50 is not full excluded and these plates may even pierce the reg bre coating (reg gures9 (a) and 11) At these locations notching may occur which can initiate reg bre crack-ing Consequently the strength distribution function of the reg bres (Danzer 1994) isa ected by the introduction of this new class of defects and the probability of reg brefracture at low loads is enhanced The formation of many large carbide plates evenleads to an embrittlement of CMgplusmn Al composites (Feldho et al 1997 HaEgrave hnel et al1997 Feldho 1998) As there are only a few of these large carbide plates inT5331HTATiNAM50 that is the e ect is very limited here accordingly thestrength of this composite was found to be only slightly lower than that ofT5331HTATiNcp-Mg (Wurm et al 1997 Wurm 1997)

Altogether the amount of carbide formation in T5331HTATiNAM50 (reg gure9 (a)) is signireg cantly lower than in T5331HTAAM50 (reg gure 10 (a)) with theuncoated carbon reg bres This is because the TiCxNy reg lm serves well as an inhibitorof deleterious carbide formation also when using Mg alloys with large amounts ofAl as matrices Thus the use of Mgplusmn Al matrices for the manufacture of hybridcomponents is enabled by the di usion retarding e ect of the chemically vapour-deposited TiCxNy coating

5 CONCLUSIONS

CVD coating on the carbon reg bres has been shown to be partly substoichiometrictitanium carbonitride (TiCxNy) rather than titanium nitride (TiN) Even though thebonding mechanism at the coatingmatrix interface is still not clear it has been

670 A Feldho et al

shown that the adhesion of Mg on the TiCxNy coating is not accompanied by theformation of extensive brittle phases

A titanium carbonitride reg lm homogeneously covering the reg bre surfaces ham-pers the transport processes across the reg brematrix interface The penetration of Mginto the carbon reg bre is signireg cantly lowered and so is the formation of Al2MgC2with deleterious notch e ects Thus the TiCxNy coating enables the use of highcontents of Al in the matrix alloy without the negative e ects of the carbide pre-cipitates on the mechanical properties of the composite

ACKNOWLEDGEMENTS

The authors would like to thank Professor R F Singer and Dr D Wurm(University of Erlangenplusmn NuEgrave rnberg) for providing the MMC samples

REFERENCESANDRIEVSKI R A 1997 J Mater Sci 32 4463BRYDSON R SAUER H ENGEL W and ZEITLER E 1991 Microsc Microanal Micro-

struct 2 159CRAVEN A J 1995 J Microsc 180 250CRAVEN A J and G ARVIE L A J 1995 Microsc Microanal Microstruct 6 89D ANZER R 1994 The Encyclopedia of Advanced Materials Vol 1 edited by D Bloor R J

Brook M C Flemings S Mahajan and R W Cahn (Oxford Pergamon)pp 385plusmn 398

D ELANNAY F FROYEN L and D ERUYTTERE A 1987 J Mater Sci 22 1EM V T and TASHMETOV M YU 1996 Phys Stat sol (b) 198 571F ELDHOFF A 1998 PhD Thesis Martin Luther University of Halle-Wittenberg

(Aachen-Maastricht Shaker)F ELDHOFF A P IP PEL E and WOLTERSDORF J 1997 J Microsc 185 122 1999 Phil

Mag A 79 1263F ITZER E and K EGEL B 1968 Carbon 6 433H AiumlHNEL A PIP PEL E F ELDHOFF A SCHNEIDER R and WOLTERSDORF J 1997 Mater

Sci Engng 237 173H OFER F WARBICHLER P and GROGGER W 1995 Ultramicroscopy 59 15H OSOI J O IKAW A T and BANDO Y 1986 J Electron Microsc 35 129JONSSON S 1996 Z Metall 87 713K RIVANEKOLG RUBBENSA JDELLBYN and M EYERCE1992 Microsc Microanal

Microstruct 3 187N AIDICH J V 1981 Prog Surf Membr Sci 14 353Oiuml TTING ER O 1996 PhD Thesis Friedrich Alexander University of Erlangen-NuEgrave rnberg

(DuEgrave sseldorf VDI)Oiuml TTING ER O G RAU C W INTER R SINGER R F F ELDHOFF A PIPPEL E and

WOLTERSDORF J 1995 Proceedings of the Tenth International Conference onComposite Materials Vol VI Whistler Canada (Cambridge Woodhead) pp 447plusmn454

Oiuml TTING ER O and SINGER R F 1993 Z Metall 84 827PFLUiuml GER J F INK J CRECELIUS G BOHNEN K P and W INTER H 1982 Solid St

Commun 44 489PICOUET L ABIVEN H VIALA J C and BOUIX J 1990 Advanced Composite Materials

Proceedings of the First Frenchplusmn Japanese Seminar on Composite Materials edited by CBathias and M Uemura (Paris SIRPE) pp 121plusmn 131

POLMEAR I J 1981 L ight AlloysETH Metallurgy of the L ight Alloys (London Edward Arnold)R EZ P1992 Transmission Electron Energy L oss Spectrometry in Materials Science edited by

M M Disko C C Ahn and B Fultz (Warrendale Pennsylvania Minerals Metalsand Materials Society) pp 107plusmn 129

SHORSHOROV M K U STINOV L M Z IRLIN A M and OLEFIRENKO V I 1979 JMater Sci 14 1850

T iN coatings in CMgplusmn Al composites 671

The microstructure of T5331HTATiNAM50 strongly resembles that ofT5331HTATiNcp-Mg with its reg brematrix interfaces being mainly free of carbides(see reg gures 9 (a) and 1 (a)) On the other hand the same Mgplusmn Al alloy in contact withthe uncoated carbon reg bre resulted in the substantial formation of Al2MgC2 (reg gure10 (a)) thus indicating a strong di usion barrier e ect of the titanium nitride coat-ing This is conreg rmed by a lowered mass transport in the reverse directionthat is from matrix to reg bre due to the TiCxNy interfacial reg lm as only1plusmn 2 at Mg penetrated into the coated reg bre (reg gures 1 (b) and 9 (b)) compared with4plusmn 6 at Mg in case of the uncoated reg bre (reg gure 10 (b)) Thus carbide formationand reg bre degradation are simultaneously limited

Nevertheless a graphitic interlayer has in certain samples been detected at thecoatingmatrix interface in T5331HTATiNcp-Mg indicating that locally C wastransported from the reg bre to the matrix As this phenomenon is not very pro-nounced it is concluded that the C transport across the reg brematrix interfacedoes not occur through TiCxNy grains even if they exhibit signireg cant vacancyconcentrations in the non-metal sublattice Geometrical considerations on thethree-dimensional structure of the coating lead to the conclusion that there existlocally reg ssures in the coating (the reg lm thickness is in the range of the grain dia-meters) Thus locally liquid Mg penetrates through reg ssures in the reg lm towards thereg bre (reg gure 2 (d)) In successive steps C from the reg bre dissolves and di uses in theliquid Mg through the coating and precipitates as graphitic ribbons at the coatingmatrix interface The thermodynamics of such an isothermal dissolutionplusmn di usionplusmnprecipitation process have been described by Fitzer and Kegel (1968) and theyshowed that it is driven by a di erence in the chemical potential of C in the reg breand in the graphitic ribbon

Accordingly the formation of some large Al2MgC2 plates in T5331HTATiNAM50 is not full excluded and these plates may even pierce the reg bre coating (reg gures9 (a) and 11) At these locations notching may occur which can initiate reg bre crack-ing Consequently the strength distribution function of the reg bres (Danzer 1994) isa ected by the introduction of this new class of defects and the probability of reg brefracture at low loads is enhanced The formation of many large carbide plates evenleads to an embrittlement of CMgplusmn Al composites (Feldho et al 1997 HaEgrave hnel et al1997 Feldho 1998) As there are only a few of these large carbide plates inT5331HTATiNAM50 that is the e ect is very limited here accordingly thestrength of this composite was found to be only slightly lower than that ofT5331HTATiNcp-Mg (Wurm et al 1997 Wurm 1997)

Altogether the amount of carbide formation in T5331HTATiNAM50 (reg gure9 (a)) is signireg cantly lower than in T5331HTAAM50 (reg gure 10 (a)) with theuncoated carbon reg bres This is because the TiCxNy reg lm serves well as an inhibitorof deleterious carbide formation also when using Mg alloys with large amounts ofAl as matrices Thus the use of Mgplusmn Al matrices for the manufacture of hybridcomponents is enabled by the di usion retarding e ect of the chemically vapour-deposited TiCxNy coating

5 CONCLUSIONS

CVD coating on the carbon reg bres has been shown to be partly substoichiometrictitanium carbonitride (TiCxNy) rather than titanium nitride (TiN) Even though thebonding mechanism at the coatingmatrix interface is still not clear it has been

670 A Feldho et al

shown that the adhesion of Mg on the TiCxNy coating is not accompanied by theformation of extensive brittle phases

A titanium carbonitride reg lm homogeneously covering the reg bre surfaces ham-pers the transport processes across the reg brematrix interface The penetration of Mginto the carbon reg bre is signireg cantly lowered and so is the formation of Al2MgC2with deleterious notch e ects Thus the TiCxNy coating enables the use of highcontents of Al in the matrix alloy without the negative e ects of the carbide pre-cipitates on the mechanical properties of the composite

ACKNOWLEDGEMENTS

The authors would like to thank Professor R F Singer and Dr D Wurm(University of Erlangenplusmn NuEgrave rnberg) for providing the MMC samples

REFERENCESANDRIEVSKI R A 1997 J Mater Sci 32 4463BRYDSON R SAUER H ENGEL W and ZEITLER E 1991 Microsc Microanal Micro-

struct 2 159CRAVEN A J 1995 J Microsc 180 250CRAVEN A J and G ARVIE L A J 1995 Microsc Microanal Microstruct 6 89D ANZER R 1994 The Encyclopedia of Advanced Materials Vol 1 edited by D Bloor R J

Brook M C Flemings S Mahajan and R W Cahn (Oxford Pergamon)pp 385plusmn 398

D ELANNAY F FROYEN L and D ERUYTTERE A 1987 J Mater Sci 22 1EM V T and TASHMETOV M YU 1996 Phys Stat sol (b) 198 571F ELDHOFF A 1998 PhD Thesis Martin Luther University of Halle-Wittenberg

(Aachen-Maastricht Shaker)F ELDHOFF A P IP PEL E and WOLTERSDORF J 1997 J Microsc 185 122 1999 Phil

Mag A 79 1263F ITZER E and K EGEL B 1968 Carbon 6 433H AiumlHNEL A PIP PEL E F ELDHOFF A SCHNEIDER R and WOLTERSDORF J 1997 Mater

Sci Engng 237 173H OFER F WARBICHLER P and GROGGER W 1995 Ultramicroscopy 59 15H OSOI J O IKAW A T and BANDO Y 1986 J Electron Microsc 35 129JONSSON S 1996 Z Metall 87 713K RIVANEKOLG RUBBENSA JDELLBYN and M EYERCE1992 Microsc Microanal

Microstruct 3 187N AIDICH J V 1981 Prog Surf Membr Sci 14 353Oiuml TTING ER O 1996 PhD Thesis Friedrich Alexander University of Erlangen-NuEgrave rnberg

(DuEgrave sseldorf VDI)Oiuml TTING ER O G RAU C W INTER R SINGER R F F ELDHOFF A PIPPEL E and

WOLTERSDORF J 1995 Proceedings of the Tenth International Conference onComposite Materials Vol VI Whistler Canada (Cambridge Woodhead) pp 447plusmn454

Oiuml TTING ER O and SINGER R F 1993 Z Metall 84 827PFLUiuml GER J F INK J CRECELIUS G BOHNEN K P and W INTER H 1982 Solid St

Commun 44 489PICOUET L ABIVEN H VIALA J C and BOUIX J 1990 Advanced Composite Materials

Proceedings of the First Frenchplusmn Japanese Seminar on Composite Materials edited by CBathias and M Uemura (Paris SIRPE) pp 121plusmn 131

POLMEAR I J 1981 L ight AlloysETH Metallurgy of the L ight Alloys (London Edward Arnold)R EZ P1992 Transmission Electron Energy L oss Spectrometry in Materials Science edited by

M M Disko C C Ahn and B Fultz (Warrendale Pennsylvania Minerals Metalsand Materials Society) pp 107plusmn 129

SHORSHOROV M K U STINOV L M Z IRLIN A M and OLEFIRENKO V I 1979 JMater Sci 14 1850

T iN coatings in CMgplusmn Al composites 671

shown that the adhesion of Mg on the TiCxNy coating is not accompanied by theformation of extensive brittle phases

A titanium carbonitride reg lm homogeneously covering the reg bre surfaces ham-pers the transport processes across the reg brematrix interface The penetration of Mginto the carbon reg bre is signireg cantly lowered and so is the formation of Al2MgC2with deleterious notch e ects Thus the TiCxNy coating enables the use of highcontents of Al in the matrix alloy without the negative e ects of the carbide pre-cipitates on the mechanical properties of the composite

ACKNOWLEDGEMENTS

The authors would like to thank Professor R F Singer and Dr D Wurm(University of Erlangenplusmn NuEgrave rnberg) for providing the MMC samples

REFERENCESANDRIEVSKI R A 1997 J Mater Sci 32 4463BRYDSON R SAUER H ENGEL W and ZEITLER E 1991 Microsc Microanal Micro-

struct 2 159CRAVEN A J 1995 J Microsc 180 250CRAVEN A J and G ARVIE L A J 1995 Microsc Microanal Microstruct 6 89D ANZER R 1994 The Encyclopedia of Advanced Materials Vol 1 edited by D Bloor R J

Brook M C Flemings S Mahajan and R W Cahn (Oxford Pergamon)pp 385plusmn 398

D ELANNAY F FROYEN L and D ERUYTTERE A 1987 J Mater Sci 22 1EM V T and TASHMETOV M YU 1996 Phys Stat sol (b) 198 571F ELDHOFF A 1998 PhD Thesis Martin Luther University of Halle-Wittenberg

(Aachen-Maastricht Shaker)F ELDHOFF A P IP PEL E and WOLTERSDORF J 1997 J Microsc 185 122 1999 Phil

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