Design and performance of AlTiN and TiAlCrN PVD coatings for machining of hard tocut materials
G.S. Fox-Rabinovich a,⁎, A.I. Kovalev b, M.H. Aguirre c, B.D. Beake d, K. Yamamoto e, S.C. Veldhuis a,J.L. Endrino f, D.L. Wainstein b, A.Y. Rashkovskiy b
a Department of Mechanical Engineering, McMaster University, McMaster University 1280 Main St. W., Hamilton, Ont., Canada L8S 4L7b Surface Phenomena Research Group, CNIICHERMET, 9/23, 2nd Baumanskaya str., Moscow 105005, Russian Federationc EMPA, Solid State Chemistry and Catalysis, CH-8600, Dübendorf, Switzerlandd Micro Materials Limited, Willow House, Yale Business Village, Ellice Way, Wrexham LL13 7YL, United Kingdome Materials Research Laboratory, Kobe Steel Ltd., 1-5-5 Takatsuka-dai, Nishi-ku, Kobe 651-2271, Hyogo 651-2271, Japanf Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, E-28049 Madrid, Spain
Article history:Received 6 April 2009Accepted in revised form 17 August 2009Available online 26 August 2009
Keywords:Hard coatingHard to cut materialsMachining
Machining of hard to cut materials such as hardened steels and high temperature strong aerospace materialsis a challenge of modern manufacturing. Two categories of the aluminum-rich TiAlN-based Physical VaporDeposited (PVD) coatings, namely AlTiN and TiAlCrN, are commonly used for this area of application. Acomparative investigation of the structural characteristics, various micro-mechanical properties, oxidationresistance and service properties of the both coatings has been performed.Crystal structure has been studied using High Resolution Transmission Electron Microscopy (HR TEM).Electronic structure has been investigated using X-ray Photoelectron Spectroscopy (XPS). Micro-mechanicalproperties (microhardness, plasticity index, impact fatigue fracture resistance) have been evaluated using aMicro Materials Nano-Test System. Short-term oxidation resistance has been studied at 900 °C in air. The toollife of the coating was studied during ball nose end milling of hardened H 13 tool steel as well as end millingof aerospace alloys such as Ni-based superalloy (Waspalloy) and Ti alloy (TiAl6V4).It was shown that the set of characteristics that control wear performance strongly depend on specificapplications. For machining of hardened tool steels, when heavy loads/high temperatures control wearbehavior, the coating has to possess a well-known combination of high hot hardness and improved oxidationresistance at elevated temperatures. To achieve these properties, crystal structure for TiAlN-based coatingsshould be mainly B1, and elemental composition of the coating should ensure formation of strong inter-atomic bonds such as Al–Cr metal-covalent bonds in the TiAlCrN coating. Nano-crystalline structure withgrain size of around 10–30 nm enhances necessary properties of the coating.In contrast, for machining of aerospace alloys, when elevated load/temperature combined with intensiveadhesive interaction with workpiece material results in unstable attrition wear with deep surface damage,the coating should possess a different set of characteristics. Crystal structure for TiAlN-based coatings isbasically B1; but due to a high amount of aluminum, the AlTiN coating contains AlN domains. The coating hasa very fine-grained nano-crystalline structure (grains sized around 5 nm). Electron structure of energy levelsindicates formation of metallic bonds. This results in plasticity increase at the cost of hot hardness reduction.The surface is able to dissipate energy by means of plastic deformation (instead of crack formation) and inthis way, surface damage is reduced.
High performancemachining of hard to cutmaterials is a challengeof modern manufacturing. There are two major categories of mostwidely used hard to cut materials: 1) hardened tool steels (such as H13) for dies and molds fabrication as well as 2) high temperature
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09 Published by Elsevier B.V. All rig
strong aerospace materials such as Ni-based super-alloys and Ti-based alloys [1]. A general feature of the machining process of hard tocut materials is generation of elevated stress/temperatures on thecutting tool surface resulting in severe operating conditions. However,specific features of the cutting processes for these two categories ofmachined materials (hardened tool steels and high temperatureaerospace materials) differ significantly due to the intensity ofadhesive interaction at the tool/workpiece interface during cutting.For hardened tool steels heavy loads and very high temperatures(1000 °C and above) control wear performance; in contrast, for
aerospace materials an intensive adhesive interaction with theformation of strong adhesive bonds at the workpiece/tool interfaceand further deep surface damage is a major factor in the tribologicalperformance of cutting tools [1].
Solid carbide tooling with advanced PVD coating has beensuccessfully used for these applications. One class of coatings thathave shown promising tool life under these conditions is the Al-richTiAlN family of coatings [2]. The amount of Al in these coatings is ashigh as 67 at.% and above of metal content [2]. Based on XRD and TEMdata, crystal structure of the coatings is mostly cubic B1 [3–6]. It hasbeen shown that by introducing Cr to the Ti–Al–N system, cubic B1structure could be stabilized at a higher Al content up to 73 at.% [7].
Twobasic categories of nano-structuredhardcoatings arewidelyusedin practice for machining of hard to cutmaterials. They are (Al0.67Ti0.33)Nand (Al0.7Cr0.2 Ti0.1)N coatings with composition and deposition condi-tions optimized in previous research [4,6,7], hereafter referred to as AlTiNand TiAlCrN respectively. Newly developed hard coatings such as nano-composite [8], nano-crystalline [3,4], nano-multilayered [9–11] arecommonly based either on AlTiN or TiAlCrN coatings [8–11]. However,the characteristics of these ‘basic’ coatings differ significantly [12–14].Thus it is timely to investigatewhich characteristics of the ‘basic’ coatingsare better suited for specific tribological applications.
The traditional approach to hard coating development based onthe idea that high hardness combined with high oxidation resistanceis essential to improve the tool life of a novel coating [15]. Howeverthis approach is not universal for a wide variety of applications relatedto machining of hard to cut materials. Thus, the question arises forwhat specific application this approach is valid and for which it has tobe modified.
To answer this question the detailed studies of the structure andproperties for the ‘basic’ AlTiN and TiAlCrN coating have to be donesystematically, including comprehensive investigations of electronicstructure, crystal structure, micro-mechanical properties, oxidationresistance as well as service characteristics of these coatings underspecific operation conditions. Based on these investigations somemetallurgical design principles for hard PVD coating for machining ofhard to cut materials have been outlined.
2. Experimental
The (Ti0.1Al0.7Cr0.2)N coatings were deposited using Ti0.1Al0.7Cr0.2targets fabricated by powder metallurgical process on the mirrorpolished cemented carbide WC–Co substrates in a R&D-type hybridPVD coater (Kobe Steel Ltd.) using a plasma-enhanced [7] arc source.Samples were heated up to about 500 °C and cleaned through Ar ionetching process. Ar–N2 mixture gas was fed to the chamber at apressure of 2.7 Pa with a N2 partial pressure of 1.3 Pa. The arc sourcewas operated at 100 A for a 100 mm diameter×16 mm thick target. Asingle target was used for coating deposition. Other depositionparameters are as follows: bias voltage: 100 V; substrate rotation:5 rpm.
The (Al0.67Ti0.33)N hard coating was deposited using Al0.67Ti0.33fabricated by a powder metallurgical process on WC–Co using anOerlikon Balzers' Rapid Coating System (RCS) in cathodic arc ion-platingmode. During the deposition, the chamberwas back filled witha pure reactive nitrogen atmosphere and the pressure was in therange of 1 to 4 Pa. The substrates had 3-fold rotation and as they wereheated to a temperature of approximately 600 °C, an average voltagebias of −100 V was also applied to them. The RCS machine isequipped with 6 cathodic arc sources positioned at two differentheights. The applied arc current varied from 100 to 150A during thedeposition.
The parameters employed in each of these two coatings duringdeposition have been optimized to achieve better possible tool life[7,12,13,16]. The thickness of the both coatings was around 3 µm forthe film characterization and cutting test work.
Cross-sectional TEM observation was employed in combinationwith FIB (focused ion beam) thinning for investigation of the AlTiNand TiAlCrN hard coatings on the cemented carbideWC/Co substrates.Low and high-resolution transmission electron microscopy (HRTEM),and selected area electron diffraction (SAED) were performed in aPhilips CM30 microscope at an acceleration voltage of 300 kV. Thephase composition and electronic structure of the coatings werestudied by means of photoelectron spectroscopy (XPS). The typicalconcentration detection limits are 0.1–1.0% of monolayer withaccuracy of 2.0–3.0% [17]. The samples were prepared using astandard procedure. First, the samples surface was cleaned by highpurity acetone before loading the samples into the spectrometer. Thenthe samples were cleaned in the spectrometer preparation chamber byAr+ ion etching at the following conditions: initial vacuum 2×10−8Pa,oil free; Ar pressure 1×10−4Pa, accelerating voltage 8 kV, samplecurrent 100×10−6A, duration 300 s. Spectra were acquired andprocessed by the ESCALAB MK2 (VG) electron spectrometer andUNIFIT 2008 software package. An X-ray tube with monochromaticAl Kα radiation (hν=1486.6 eV) was used as the signal excitationsource. Full width at half maximum while scanning the line Au 4p5/2and 4p7/2 was 1.2 eV.
Photoelectron spectra were referenced to the C 1s line of theadventitious carbon set at 285.0 eV. Analysis conditions were chosenin such a way as to ensure the best energy resolution with a goodsignal/noise ratio. The results were severely affected by the selectionof the background subtraction method (Shirley-type) and the curvefitting procedure for the doublet lines.
Themicro-mechanical characteristics of the coatingsweremeasuredon WC–Co using a Micro Materials NanoTest system. Nanoindentationwas performed in load controlled mode with a Berkovich diamondindenter calibrated for load, displacement, frame compliance andindenter shape according to an ISO14577-4 procedure. The areafunction for the indenter was determined by indentations to 0.5–500 mN into a fused silica reference sample. For the nanoindentationinto the coatings, the peak load was 40 mN and 40 indentations wereperformed for each coating. This load was chosen to minimize anyinfluence of surface roughness on the data whilst ensuring that theindentation contact depth was under 1/10 film thickness so that acoating-only (load-invariant) hardness could bemeasured in combina-tion with coating-dominated elastic modulus [18]. Nanoindentationwas performed at room temperature and elevated temperature. Nano-impact testing [16] was performed at room temperature with aNanoTest fitted with a cube corner indenter as an impact probe. Theindenter was accelerated from 12 m above the coating surface with150 mN coil force to produce an impact every 4 s for total test durationof 600 s. The coatings' nano-impact fatigue fracture resistance wasassessed by the final measured impact depth [19,20] and confirmed bymicroscopic analysis of impact craters.
A Siemens D500 diffractometer with a CuKα tube was used toperform XRD. Cutting tool life was studied under different conditions.Cutting data is presented in Table 1. At least three cutting tests wereperformed for each kind of coatings under corresponding operations.The scatter of the tool life measurements was approximately 10%.
3. Results and discussion
3.1. Electronic structure investigations (XPS)
Fig. 1 shows high-resolution XPS core-level Al 2p, Ti 2p, Cr 2p andvalence band spectra of AlTiN (a–c) and TiAlCrN (d–f) coatings. The Al2p peaks (Fig. 1a, d) de-convolution indicated that for AlTiN coating(Fig. 1a) the peak is comprised of two components, and for TiAlCrNcoating (Fig. 1d) the peak contains three components. The character-istic peak at the binding energy of 74.2 eV, corresponds to AlN nano-precipitates [2], whilst the peak at a binding energy of 73.1–73.2 eVcorresponds to Al–Ti bonds in complex nitride. The intensity of Al 2p
Table 1Cutting data for the experiments performed.
component associated with AlN phase is lower for the TiAlCrN coatingcompared to AlTiN. The Ti 2p photoelectron spectrum (Fig. 1b)confirms that TiN phase forms in AlTiN coating during deposition. TheTi 2p photoelectron line for the AlCrTiN coating had the same shapeand structure, and it was omitted. This data is in good correlation withTEM and SAED (Figs. 2 and 3). Photoelectron spectra of the TiAlCrNcoating have many interesting features. The Al 2p (Fig. 1d) and Cr 2ppeaks (Fig. 1e) centered at 72.4 and 576.0 eV show that Al and Cr arepresent not only in a nitride form. The binding energies of these
Fig. 1. High-resolution XPS core-level Al 2p, Ti 2p, Cr 2p and val
components are different in comparison to elemental states. The Cratoms are donors, and Al atoms are acceptors. This is attributed to theformation of Al–Cr bonds in chromium aluminide, which wasnucleated during coating deposition in the form of three-dimensionalnano-islands. The deposited atoms can move on the surface and formdimers, or attach to islands, move as a dimer then form second layerson the top of the islands. During high energetic growth, some of theatoms can re-evaporate and leave the substrate [21,22]. The analysis ofintensities for Al and Cr bonded XPS components reveals its
ence band spectra of AlTiN (a-c) and TiAlCrN (d-f) coatings.
Fig. 2. TEM cross-section with selected area electron diffraction (SAED) patterns of the coated samples: a-c) AlTiN; b-d) TiAlCrN coatings.
composition as CrAl7. The interaction of chromium with aluminumleaves less aluminum available for Al–N phase formation and thiscould explain why amount of AlN phase in TiAlCrN coating is lowerthan in AlTiN coating (Fig. 1a and d).
The fine structure of the valence band is different for thesecoatings (Fig. 1c, f) as well. The XPS spectrum of valence banddemonstrates concentrations of electrons that provide various typesof chemical bonds. The electrons with lower binding energy (BE) areforming softer, more plastic metallic bonds, and the electrons withhigher BE make stronger covalent bonds. The distance from Fermilevel determines the level of metallic properties. As it is shown inFig. 1c, high density of the states (DOS) at Fermi level corresponds toTi 3d and Al 3p states indicates the formation of metallic bonds andresults in lower hardness and higher plasticity of the AlTiN coating(Table 2). In contrast, the TiAlCrN coating has low DOS of Ti 3d atFermi level and hybridization of Al 3p, 3s and Cr 3d, 4s states (Fig. 1f).Chromium addition decreases DOS at Fermi level and increases DOSredistribution of N 2p electrons at 7.0 eV and Ti 4s electrons at 9.0 eV.The higher room temperature and hot hardness and lower plasticityof TiAlCrN coating (Table 2) could be partially explained by lowelectron concentration at Fermi level, increasing DOS at the levelswith high binding energy and formation of metal-covalent bonds inthis coating.
Based on the data obtained we can assume that the aluminum rich(Al0.67Ti0.33)N coating has strong metallic bonds. Investigation ofvarious properties presented below confirms this hypothesis(Table 2). Addition of Cr creates strong Al–Cr metallic bonds in(Ti0.1Al0.7Cr0.2)N that probably control the hot hardness and oxidationresistance improvement at high temperatures.
3.2. Structural studies
Previous XRD and TEM [7,13] crystal structure studies show thatboth coatings mainly have TiN-based FCC crystal structure. They aremainly a solid solution of Al (for AlTiN coating) or Al and Cr (forTiAlCrN coating) in TiN phase [7,13].
Fig. 2 shows the bright field TEM image and diffraction patterns ofthe coated samples. TEM analysis of Fig. 2a revealed a densehomogenous nano-grain for (Al0.67Ti0.33)N while a columnar graingrowth is presented in Fig. 2b for (Al0.7Cr0.2 Ti0.1)N coating.
AlTiN coating has a microstructure with very fine grains (Fig. 2a).The grain size measured in AlTiN is 5–10 nm or even less in diameter.In TiAlCrN coating grains are still within nano-scale level and within arange of 20 nm and above (Fig. 2b).
The diffraction ring pattern for AlTiN system (Fig. 2c) ischaracteristic of a nano-crystalline material that is presenting acontinuous ring without sharp diffraction spots due to the small grainsize and theirmultiplicity of different orientations. The ring diffractionpattern of AlTiN also present some spread intensity or diffusescattering that could be attributed to the presence of AlN and TiNphases with slightly different a parameters or structure compared tothe solid solutions. The indexation of the ring patterns are shown inFig. 2c for the AlTiN coating.
The diffraction pattern of the TiAlCrN coating (Fig. 2d) shows amore crystalline material compared to the AlTiN coating. A minoramount of h-AlN, which could not be detected by XRD [13,14] hasbeen found in both coatings. However, amount of this phase seems tobe less in the TiAlCrN coating than in the AlTiN coating because ofweak first ring intensity displayed in the first coating (Fig. 2c)
Fig. 3. High resolution TEM micrographs of AlTiN and TiAlCrN coatings: a) The HRTEM micrograph of AlTiN coating shows regions with dislocations, the dark/white contrast arisesbecause of the strain that dislocations produce. The FFT show the three first rings diffraction that are similar to described in Figure 2: 1° ring corresponds to AlNhex and the 2° and 3° toAlTiN/TiN cubic phase, b) The TiAlCrN HRTEM shows larger grains where it is possible to calculate FFT, the spots in the calculated diffraction are indexed in the cubic structure witha=0.42 nm; c) HRTEM showing a larger grain of TiAlCrN. The FFT corresponds to [011]c zone axis and it was calculated from the square region in the micrograph.
compared to the second one (Fig. 2d). This data corresponds to XPS(Fig. 1a, d). The indexation of AlTiN and TiAlCrN were made based onthe previously collected data [13].
Fig. 3a–b and c presents HRTEM images of AlTiN and TiAlCrNcoatings. The HRTEM image of AlTiN coating shows regions withdislocations marked with black arrows (in Fig. 3a); the dark/whitecontrast arises because of the strains produced by this defect. It isworth noting the short range of crystallinity presented by the nano-grains. The Fast Fourier Transform (FFT) patterns inset present thatthe three first rings are similar to the ones described in Fig. 2c withstrong intensity in the position of AlNhex . HRTEM images of TiAlCrNcoatings (Fig. 3b and c) show grains with long-range order and highcrystallinity. The FFT patterns calculated from big grains (see Fig. 3band c; regions marked with 1, 2 and 3) can be indexed in the cubicstructure with a≈0.42 nm and correspond to the [011]c zone axis.There is also strain represented by the black/white contrast, becauseof the dislocations and the grain boundaries.
3.3. Physical and micro-mechanical properties studies
Table 2 presents data on micro-mechanical properties of thecoatings at room and elevated temperatures.
Table 2Micro-mechanical and physical properties for AlTiN and TiAlCrN coatings at RT and elevate
Nano-crystalline structure of the AlTiN coating corresponds tolower hardness values, especially at high temperatures [13]. One of thegreat advantages of AlTiN coatings is their high plasticity (Table 2);twice as high impact fatigue fracture resistance (Table 2; Fig. 4a) andsignificantly diminished probability of crack formation (Fig. 4b) ascompared to TiAlCrN coating (Table 2, Fig. 4c–d). On the other hand,the oxidation resistance of the coating is lower than TiAlCrN (Table 2).
Hardness, especially hot hardness in the TiAlCrN coatings, issignificantly higher than that of the AlTiN coating but impact fatiguefracture resistance is much lower and probability of crack formation issignificantly higher than that of the AlTiN coating (Table 2; Fig. 4c–d).On the other hand, addition of Cr to the AlTiN-based coating results indramatic improvement in oxidation resistance due to the changes inelectronic structure as described above (Fig. 1).
3.4. Service properties
3.4.1. Machining of hardened steelsFig. 5a presents comparative tool life data during ball nose end
milling of hardened tool steel H 13 (HRC 50–52). The data presentedshow that TiAlCrN coating outperform AlTiN coating by 27% (withrespect of length of cut) under these conditions.
Fig. 4. Impact depth with time during nano-impact fatigue test and optical images of the tested area for a-b) AlTiN and c-d) TiAlCrN coatings on cemented carbide substrates.
3.4.2. Machining of aerospace materialsFig. 5b exhibits comparative tool life of AlTiN and TiAlCrN coatings
under end milling conditions of aerospace materials such as TiAl6V4
alloy and Ni-based superalloy. Under these conditions, the trend intool life is opposite. AlTiN coating supersede s TiAlCrN by 12 and 70%(with respect of length of cut) for TiAl6V4 and Ni-based super-alloyrespectively.
4. PVD coating design optimization for specific applications
Machining of hardened steels, especially under dry (coolant free)conditions, is related to heavy loads/temperature conditions withsimultaneous intensive oxidation attack from the environment (oxygenfrom air). To sustain under these severe operating conditions a coatingshould combine a high hot hardness and oxidation resistance [15].Hence TiAlCrN,whichpossesses the necessary combination of structuralcharacteristics and related properties (Table 2) is better suited for thisapplication (Fig. 5a).
In contrast, the operating conditions are strongly different duringmachining of aerospace materials [1,23]. Modern aerospace materials,such as Ti-based alloys and especially Ni-based super alloys, are favoredfor their exceptional thermal resistance and ability to retainmechanicalproperties at elevated temperatures. They are classified as difficult-to-machine materials due to their high shear strength, work hardeningtendency, and strong tendency to weld and to form built-up edges.There is much similarity in the machining of these two categories ofaerospace materials [23,25]. Nickel and titanium-based alloys easilycreate strong adhesive bonds [26]. When adhesive bonds are formed,high shear strength and ductility of the alloys at elevated temperaturesprevent the welds from being sheared or torn easily during friction[24,26]. Attrition wear with intensive surface damage [21,23–26] easilydevelops as a result of this interaction.
For this specific application, a hard PVD coating should possess thefollowing generic characteristics: resistance to elevated load/temperatureoperating conditions, reduction of adhesive interaction with workpiecematerial, and reduction of surface damage intensity. Both of the coatings
Fig. 5. Tool life of AlTiN and TiAlCrN coating under different machining conditions:a) ball nose end milling of the hardened H 13 (hardness 50-52 HRC); end milling ofaerospace materials: b) Nickel based super-alloy (Waspalloy) and c) TiAl6V4 alloy.
studied are capable of forming protective alumina tribo-films and in thisway adhesive interaction at the tool/workpiece interface is stronglyreduced [25,26]. However, once occurred, the welds' shearing involvesintensive surface damage. An AlTiN coating that has higher plasticity andhigher impact fatigue fracture resistance (Table 2) has lower wearintensity under conditions of attrition wear (Fig. 5b). Based on the dataobtained it is possible to outline some principles of hard PVD coatingdesign for specific applications:
1. Heavy load/high temperature conditions: coating for themachining ofhardened steels. This coating has to possess a well-known combina-tion of basic characteristics, i.e. high hot hardness and improvedoxidation resistance. To achieve these properties, crystal structure forTiAlN-based coating should be mainly B1; elemental composition ofthe coating shouldensure formationof strong inter-atomicbonds suchas Al–Cr metal-covalent bonds in the TiAlCrN coating.
2. Intensive adhesive interaction with workpiece material; elevatedload/temperature conditions: coating for machining of aerospace
materials. Crystal structure for TiAlN-based coating is basically B1; butdue to a high amount of aluminum, the AlTiN coating contains AlNdomains (AlN metastable cubic and traces of AlN hexagonal). Thecoating has very fine-grained nano-crystalline structure (grains sizedaround 5 nm). Electron structure of energy levels indicates formationof metallic bonds. This results in plasticity increase for the cost of hothardness reduction (Table 2). The surface is able to dissipate energy bymeans of plastic deformation (instead of crack formation, Fig. 4d) andin thisway, surface damage is reduced. This is extremely important forunstable conditions of attrition wear with deep surface damage.
5. Conclusions
A comprehensive investigation of structural characteristics (crys-tal structure, electronic structure) and physical–mechanical proper-ties (microhardness, plasticity index, oxidation resistance at elevatedtemperatures) has been performed for two categories of aluminum-rich hard nano-crystalline PVD (Al0.67Ti0.33)N and (Al0.7Cr0.2Ti0.1)N)coatings. It has been shown that the set of characteristics that controlwear performance and consequently the principles of metallurgicaldesign for the hard coatings strongly depend on specific applications.
For machining of hardened tool steels, when heavy loads/hightemperatures control wear behavior, the coating should possess atraditional combination of hot hardness and oxidation resistance atelevated temperatures. The necessary characteristics are achievedbecause incorporating Cr in meta-stable Al-rich c-Ti1− xAlxN matrixincreases the solubility limit of AlN phase in the complex nitride. It isaccompanied by decreasing the number of Ti 3d–Al 3p bonds andhybridization of Al 3p, 3s and Cr 3d, 4s states, that indicates formationof metal-covalent bonds. For this application, the best-suited coatingis the one with a nano-crystalline structure with grain size of around10–30 nm.
For machining of aerospace materials when elevated load/temperature conditions and mainly adhesive interaction with theworkpiece material control wear performance, the coating shouldhave a completely different set of characteristics. Crystal structure isstill mainly B1, which is solid solution of Al in TiN fcc structure, butdue to high amount of aluminum, the AlTiN coating contains AlNdomains. The coating has very fine-grained nano-crystalline structure(around 5 nm). Electron structure of energy levels reveals theformation of metallic bonds. This structure results in the increase ofthe plasticity of coating at the cost of hot hardness reduction,especially at elevated temperatures. Therefore hardness of coatingsbegins to be a marginal property for this application, where the majorproperties are the plasticity and impact fatigue fracture resistance. Asurface with these characteristics is able to dissipate energy by meansof plastic deformation and in this way surface damage and wear rateare reduced. With this set of characteristics, the AlTiN coating serveswith a high efficiency as a protective layer against severe tribologicalimpact under unstable conditions of attrition wear, which is typicalfor the machining of hard to cut aerospace alloys.
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