Thin Solid Films 416 (2002) 31–37 0040-6090/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0040-6090 Ž 02 . 00628-4 The effect of heat treatment on the microstructure of electroless Ni–P coatings containing SiC particles C.K. Chen*, H.M. Feng, H.C. Lin, M.H. Hon Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan, ROC Received 3 August 2001; received in revised form 14 June 2002; accepted 4 July 2002 Abstract Electroless Ni–P coatings containing SiC particles were co-deposited on SKD61 tool steel substrate. The effect of heat treatment on the microstructure of Ni–P–SiC composite coatings was investigated by X-ray diffraction and transmission electron microscopy. The presence of SiC particles did not affect the microstructure of the Ni–P alloy matrix when annealing temperature was below 400 8C. However, by increasing annealing temperature to 450 8C, SiC particles decomposed and reacted with nickel to form g- Ni Si and b -Ni Si phases with a consequent free carbon precipitation. The structure of carbon was crystalline graphite with 5 2 1 3 (0002) preferred orientation and tended to aggregate while the amount of free carbon increased. On further annealing at 500 8C, phosphorus was incorporated into the Ni Si lattice, forming a Ni (Si ,P ) solid solution. 5 2 5 1yx x 2 2002 Elsevier Science B.V. All rights reserved. Keywords: Ni; SiC; Composite electroless plating; Heat treatment 1. Introduction Current trends of coating techniques involve compos- ite coatings, such as multilayer or multiphase coatings, which are expected to have tailor-made properties for some specific applications. Recent progress in electroless plating is the co-deposition of solid particles into coat- ings, although electroless Ni–P coatings have been widely used in industry during the past 20 years for wear and corrosion protection. Consequently, functional composite coatings with highly specific characteristics can easily be produced by choosing suitable particulate materials. These solid particles can be hard materials (such as SiC, Al O and diamond) w1–3x to enhance 2 3 the hardness andyor wear resistance of the deposits, or can be dry lubricants (such as MoS , PTFE and graph- 2 ite) w4–6x to impart lubricity and reduce the coefficient of friction. Among the particulate materials used for reinforce- ment, SiC is the most frequently studied and applied. Broszeit w7x found that mechanical properties, such as *Corresponding author. Tel.: q886-6-2380208; fax: q886-6- 2380208. E-mail address: [email protected](C.K. Chen). hardness, strength and elastic modulus can be increased with increasing content of SiC particles in the composite coating. Xinmin and Zongang w8x suggested that the SiC particles can increase the hardness of a composite coating and improve the resistance to abrasion, but a hard and stable matrix is necessary to support them. Unfortunately, the high temperature application of Ni– SiC composite coatings is limited by the thermal decom- position of SiC particles in the nickel matrix at approximately 500 8C w7x. Pan and Baptista w9x estab- lished that the nickel silicide, which is thermodynami- cally more stable than SiC, makes SiC unstable. The chemical instability of SiC in presence of nickel at high temperature results in uncontrollable mechanical prop- erties of the material and limits the application of Ni– P–SiC composite coatings. Microscopically, what really happens to the coating matrix and the particles is not well understood and needs further investigation. In this paper, an attempt was made to incorporate SiC particles into a Ni–P alloy matrix by electroless plating. The purpose of this work was to study the effect of heat treatment on microstructural changes of electroless Ni– P–SiC composite coatings by X-ray diffractometry (XRD) and transmission electron microscopy (TEM).
Transcript
Thin Solid Films 416(2002) 31–37
0040-6090/02/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0040-6090Ž02.00628-4
The effect of heat treatment on the microstructure of electroless Ni–Pcoatings containing SiC particles
C.K. Chen*, H.M. Feng, H.C. Lin, M.H. Hon
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan, ROC
Received 3 August 2001; received in revised form 14 June 2002; accepted 4 July 2002
Abstract
Electroless Ni–P coatings containing SiC particles were co-deposited on SKD61 tool steel substrate. The effect of heat treatmenton the microstructure of Ni–P–SiCcomposite coatings was investigated by X-ray diffraction and transmission electron microscopy.The presence of SiC particles did not affect the microstructure of the Ni–P alloy matrix when annealing temperature was below400 8C. However, by increasing annealing temperature to 4508C, SiC particles decomposed and reacted with nickel to formg-Ni Si and b -Ni Si phases with a consequent free carbon precipitation. The structure of carbon was crystalline graphite with5 2 1 3
(0 0 0 2) preferred orientation and tended to aggregate while the amount of free carbon increased. On further annealing at 5008C, phosphorus was incorporated into the Ni Si lattice, forming a Ni(Si , P ) solid solution.5 2 5 1yx x 2
Current trends of coating techniques involve compos-ite coatings, such as multilayer or multiphase coatings,which are expected to have tailor-made properties forsome specific applications. Recent progress in electrolessplating is the co-deposition of solid particles into coat-ings, although electroless Ni–P coatings have beenwidely used in industry during the past 20 years forwear and corrosion protection. Consequently, functionalcomposite coatings with highly specific characteristicscan easily be produced by choosing suitable particulatematerials. These solid particles can be hard materials(such as SiC, Al O and diamond) w1–3x to enhance2 3
the hardness andyor wear resistance of the deposits, orcan be dry lubricants(such as MoS , PTFE and graph-2
ite) w4–6x to impart lubricity and reduce the coefficientof friction.Among the particulate materials used for reinforce-
ment, SiC is the most frequently studied and applied.Broszeit w7x found that mechanical properties, such as
hardness, strength and elastic modulus can be increasedwith increasing content of SiC particles in the compositecoating. Xinmin and Zongangw8x suggested that theSiC particles can increase the hardness of a compositecoating and improve the resistance to abrasion, but ahard and stable matrix is necessary to support them.Unfortunately, the high temperature application of Ni–SiC composite coatings is limited by the thermal decom-position of SiC particles in the nickel matrix atapproximately 5008C w7x. Pan and Baptistaw9x estab-lished that the nickel silicide, which is thermodynami-cally more stable than SiC, makes SiC unstable. Thechemical instability of SiC in presence of nickel at hightemperature results in uncontrollable mechanical prop-erties of the material and limits the application of Ni–P–SiC composite coatings. Microscopically, what reallyhappens to the coating matrix and the particles is notwell understood and needs further investigation.In this paper, an attempt was made to incorporate SiC
particles into a Ni–P alloy matrix by electroless plating.The purpose of this work was to study the effect of heattreatment on microstructural changes of electroless Ni–P–SiC composite coatings by X-ray diffractometry(XRD) and transmission electron microscopy(TEM).
32 C.K. Chen et al. / Thin Solid Films 416 (2002) 31–37
Fig. 2. SEM micrographs of as-deposited coatings,(a) Ni–P; (b) Ni–P–SiC(100 gyl SiC suspension).
2. Experimental
Tool steel JIS SKD61 specimens with a thickness of2 mm and a diameter of 1.5 mm were used as thesubstrate material. The specimens were ground and thensurface polished with 1mm alumina powder. Beforeelectroless plating, the specimens were degreased andultrasonically cleaned in a dilute hydrochloric acidsolution. The Ni–P plating and Ni–P–SiC compositeplating were carried out in a beaker heated by a
thermostatically controlled bath. The substrates werevertically positioned as illustrated in Fig. 1 in the platingbath which was a commercial electroless nickel solution(Nickora, product of Schering Company). The b-SiCpowder with an average particle size of 0.84mm wasadded to the bath to produce Ni–P–SiC compositecoating. Three concentrations of SiC particles: 0, 10 and100 gyl were used to obtain coatings with different SiCcontents. To keep the particles in suspension, the solu-tions were mechanically agitated. The bath was keptconstantly at pH 5.0 and a temperature of 908C. Inorder to investigate the microstructural stability, thespecimens were heat-treated in a vacuum(26.6 Pa)chamber at 350, 400, 450 and 5008C for 1 h prior tofurnace cooling.The composition of the deposited films was deter-
mined with a glow discharge optical spectrometer(GDOS; Model LECD GDS-750 QDP). The cross-section morphology was observed by scanning electronmicroscopy(SEM; Model JOEL JSM-5200). Film struc-ture was analyzed by XRD(Model Rigaku DyMax-IV)with a CuKa X-ray source. Additional structure char-acterization was performed by energy-dispersive spec-troscopy (EDS) and selected-area electron diffraction(SAD) mode of a TEM(Model JEOL JEM-3010) at300 kV.
3. Results and discussion
The cross-sectional SEM micrographs of the depositedNi–P and composite coatings in Fig. 2 show that theSiC particles are uniformly distributed in the entire Ni–P film matrix. The thicknesses of the Ni–P and Ni–P–SiC coatings are approximately 15 and 20mm,respectively. The composition of the deposited Ni–Pand composite coatings measured by GDOS in Fig. 3indicates that SiC concentration in the composite coatingincreases with increasing SiC concentration in the plat-ing bath. Phosphorus concentration in these coatingsfalls in the range between 6.2 and 6.8 wt.%. Backovic´et al. w10x indicated that the as-deposited Ni–P alloyforms a metallic glass when the phosphorus contentexceeds 6 wt.%. According to the low temperature phasediagram of Ni–Pw11x, the structure of as-deposited Ni–P coating consists of microcrystallineb-phase and amor-phousg-phase for a phosphorus content between 4.5and 11 wt.%. The XRD patterns of the as-depositedNi–P in Fig. 4a show a single broad, diffuse peak whichis identified as an amorphous phase. The XRD patternsof the as-deposited Ni–P–SiC composite coatings aresimilar to that of Ni–P with the exception of a peak at2us35.68 corresponding to SiC(1 1 1) which becomesstronger with increasing SiC concentration in the speci-men, hence the plating bath as determined by GDOSanalysis. For the as-deposited films, it can be deducedthat both the Ni–P matrix and SiC particles in the
33C.K. Chen et al. / Thin Solid Films 416 (2002) 31–37
Fig. 3. Composition of films deposited with different SiC concentra-tions in the plating bath.
composite coating conserve their original structures. Thepresence of added SiC particles did not change thestructure of the Ni–P matrix.The XRD patterns of the heat-treated Ni–P and
composite coatings at various temperatures of 350, 400,450 and 5008C for 1 h are shown in Fig. 4b and c,respectively. Duncanw11x indicated that theb-phaseconverts toa-nickel at 250–2908C, whilst g-phaseconverts to Ni P anda-nickel at 310–3308C. In this3
study, the XRD analysis confirms the above results byconfirming the transformation from an amorphous struc-ture to crystalline nickel and Ni P at 3508C. With a3
further increase in temperature, the peak intensities ofnickel and Ni P increase without any other phase being3
detected, which is also consistent with Refs.w11,12x.Comparing with Ni–P coatings, the structure of the Ni–P matrix in the Ni–P–SiC composite coating waschanged from amorphous to crystalline at 3508C, asshown in Fig. 4c. The amount of nickel and Ni P phases3
increased further at 4008C. When the temperature wasincreased to 4508C, the diffraction peaks of nickel andSiC phases decreased and the nickel peak shifted to ahigher angle. Furthermore, a new peak at approximately2us47.28 was observed indicating that phase transfor-mation occurred, which became more obvious at 5008Cannealing. Moreover, it can be observed that the newpeak slightly shifted to a higher angle, but the peakattributed to SiC eventually disappeared, as shown inFig. 4d. The above observation from XRD resultsimplies that nickel reacted with SiC to form the newphase, when the annealing temperature exceeded 4508C, which is somewhat lower than Broszeit’sw7x find-ings by thermal differential analysis that chemical reac-
tion between nickel and SiC took place at 5808C forthe Ni–P–SiC composite coating. The new phasesresulting from this chemical reaction appeared by opticalmicrograph observation to be Ni Si and carbon. Pan and3
Baptista w9x also indicated that SiC was not stable inthe presence of nickel in the temperature range 1127–1727 8C and the resulting reaction produced nickelsilicide and free carbon.According to the International Center for Diffraction
Data(ICDD) card filew13x and the Ni–Si phase diagramw14x, the structure ofb -Ni Si phase is face-centered-1 3
cubic with the lattice constant slightly smaller than thatof nickel. It may therefore be presumed that the reactionbetween nickel and SiC producedb -Ni Si phase, and1 3
resulted in a peak shifting to a higher angle. The extrapeak at approximately 2us47.28 might be correspond-ing to g-Ni Si (3 0 0). Canali et al.w15x proposed that5 2
the compound formation in a Ni–Si system is driventowards the phases that are richer with the remainingelement. In this study, the amount of Si was far lowerthan that of Ni. In other words, when silicon wasconsumed the compound formation was driven towardsg-Ni Si , and subsequentlyb -Ni Si. As a result, the5 2 1 3
peak at approximately 2us47.28 shifted to a higherangle, while the annealing temperature was increased toabove 4508C. It is then strongly suggested that theproducts were not onlyg-Ni Si andb -Ni Si, but also5 2 1 3
another phase. Conclusive identification requires workwith TEM, because there was only one peak in the XRDpatterns obtained.TEM bright field image of a Ni–P–7.0 wt.% SiC
composite coating annealed at 4508C for 1 h is exhibitedin Fig. 5a. From the EDS spectra(in Fig. 5b and c) andSAD patterns(not shown here), it is clear that thephases in regions A and B are Ni P and Ni, respectively.3
Also, it was found that nickel particles precipitated in acontinuous matrix of Ni P, as previously reported3
w16,17x. The composition in region C consists of nickeland silicon as indicated in Fig. 5d. The correspondingSAD patterns, as illustrated in Fig. 6 show that the twoadjacent big spots correspond to a superlattice. The bigspots of these SAD patterns correspond to the NaCl
type crystal structure with Bw0 1 1x¯ ¯w x w xB 1 1 1 , B 1 1 2 ,and Bw0 0 1x zone axes, respectively, which are similarto that of nickel. These superlattice spots confirm asixfold symmetry in Fig. 6a, a twofold symmetry in Fig.6b and c, and a fourfold symmetry in Fig. 6d. That isexactly a pattern of superlattice of cubic structure, i.e.the relationship between superlattice and NaCl typereflections is cube–cube orientation. In other words, thiscompound possesses a NaCl structure with orderedphase, which should be the Ni Si with ordered L13 2
structure, as determined by XRD analysis. It is assumedthat nickel reacted with SiC to produce the Ni Si(L1 )3 2
phase, whose amount increased with increasing anneal-
34 C.K. Chen et al. / Thin Solid Films 416 (2002) 31–37
Fig. 4. XRD patterns of(a) as-deposited Ni–P coatings with different SiC contents;(b) Ni–P coatings annealed at different temperatures for 1h; (c) Ni–P–7.0wt.% SiC composite coatings annealed at different temperatures for 1 h; and(d) Ni–P coatings with different SiC contentsannealed at 5008C for 1 h.
ing temperature, as well as SiC content in the Ni–P–SiC composite coating. Therefore, the peak in the XRDpattern would be a superimposed one contributed bynickel and Ni Si phase, and will gradually shift to a3
higher angle as temperature and SiC content increase.TEM bright field image of Ni–P–3.7 wt.% SiC
composite coating annealed at 5008C for 1 h, Fig. 7a,shows that there is a long and narrow strip(A). EDSspectrum of the composition in region A as exhibited inFig. 7b shows exclusively carbon. The correspondingSAD pattern in Fig. 7c indicates a crystalline graphitewith (0 0 0 2) preferred orientation. Crystalline graphitecould also be found in the Ni–Si reaction area(B inFig. 7a). The phases in the Ni–Si reaction area consistof nickel silicide and graphite. Gulpen et al.w18x pointed¨out that when nickel silicide forms, carbon appears as a
separate phase(graphite) since it cannot dissolve in anysilicide. Comparing Fig. 5a and Fig. 7a, it appears thatthe graphite precipitates in the Ni–Si reaction area whenthe amount of free carbon is low, whereas, it tends toaggregate with increasing amount of free carbon.Fig. 8a shows the TEM bright field image from
another arbitrary area of Ni–P–3.7wt.% SiC compositecoating annealed at 5008C for 1 h and Fig. 8b showsthe corresponding SAD pattern which is close to that ofg-Ni Si with zone axis. From EDS spectrum,¯w xB 0 1 1 05 2
Fig. 8c, it appears that this area consists of nickel,silicon and a small amount of phosphorus. Thus, thephase might consist nickel, silicon and phosphorus,presumably which is incorporated into the Ni Si lattice5 2
to form a Ni (Si , P ) solid solution, with the value5 1yx x 2
of x between 0.04 and 0.26. Because the ionic radius of
35C.K. Chen et al. / Thin Solid Films 416 (2002) 31–37
Fig. 5. (a) TEM bright field image of Ni–P–7.0wt.% SiC composite coatings annealed at 4508C for 1 h, and EDS spectra of regions:(b) A;(c) B and(d) C.
Fig. 6. SAD patterns of region C(in Fig. 5a) along various zone axes for(a) (b) (c) w0 1 1x and(d) w0 0 1x.¯ ¯w x w x1 1 1 ; 1 1 2 ;
36 C.K. Chen et al. / Thin Solid Films 416 (2002) 31–37
Fig. 7. (a) TEM bright field image of Ni–P–3.7wt.% SiC compositecoating annealed at 5008C for 1 h; (b) EDS spectrum and(c) SADpattern of region A.
Fig. 8. (a) TEM bright field image of Ni–P–3.7wt.% SiC compositecoating annealed at 5008C for 1 h; (b) EDS spectrum and(c) SADpattern of(a).
P (0.212 nm) is smaller than that of Si (0.271 nm)3y 4y
w19x a substitution of phosphorus atom into silicon latticeleads to a reduction in Ni Si lattice constant, with a5 2
shift of the Ni Si (3 0 0) peak in XRD pattern to a5 2
higher angle, as indicated in Fig. 4c. This evidence forphosphorus substitution in the Ni Si lattice agrees with5 2
the results of XRD analysis of the sample annealed at500 8C for 1 h.Results on the heat-treated Ni–P–SiCcomposite coat-
ing were not so clearly reported before. Also, phasesdetermined in this study may be helpful to tribologicalapplications. Graphite, as an example, is expected tohave a great influence on the friction coefficient.
4. Summary
SiC particles were successfully incorporated and uni-
formly distributed in a Ni–P alloy matrix by the elec-troless composite plating method. The SiC content inthe composite coating increased with increasing SiCpowder concentration in the plating bath. Heat treatmentchanged the structure of composite coatings. The struc-ture of Ni–P–SiCcomposite coatings was similar tothat of Ni–P coatings in both the as-deposited and theannealed below 4008C states. During annealing above450 8C, the nickel reacted with SiC to produceg-Ni Si , Ni Si (L1 ) and graphite precipitate. On further5 2 3 2
annealing at 5008C, the phosphorus was incorporatedinto the Ni Si lattice, forming a Ni(Si , P ) solid5 2 5 1yx x 2
solution.
Acknowledgments
The authors wish to thank the National Science
37C.K. Chen et al. / Thin Solid Films 416 (2002) 31–37
Council of Taiwan, ROC for financial support undercontract ‘NSC89-2218-E-006-017’.
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