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Impact wear and abrasion resistance of CrN, AlCrN and AlTiN PVD coatings
Surface & Coatings Technology 215 (2013) 170–177
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Impact wear and abrasion resistance of CrN, AlCrN and AlTiN PVD coatings J.L. Mo a, b, M.H. Zhu a, A. Leyland b, A. Matthews b,⁎ a b
Tribology Research Institute, Traction Power State Key Laboratory, Southwest Jiaotong University, Chengdu 610031, China Department of Materials Science and Engineering, The University of Sheffield, Sheffield S1 3JD, UK
a r t i c l e
i n f o
Available online 6 November 2012 Keywords: Abrasive wear Impact wear PVD CrN AlCrN AlTiN
a b s t r a c t The properties of CrN, AlCrN and AlTiN coatings deposited on cemented carbide substrates by a multiple-arc Physical Vapour Deposition (PVD) technique were evaluated by cyclic impact wear and micro-scale abrasion testing. In the impact wear test, a 6 mm diameter tungsten carbide ball was used as the impacting body and the impact frequency (f) was set at 10 Hz. In the micro-scale abrasion test, a micro-blasted 25 mm diameter hardened steel ball was used as the counterface and a suspension of SiC particles (mean size of 4–5 μm) in distilled water as the abrasive slurry. After these wear tests, the wear craters were studied by stylus profilometry, SEM and EDX, to investigate wear behaviour. It is shown that the CrN coating suffered much more severe impact deformation as compared to the two ternary coatings, and exhibited a non-linear increase of the maximum wear depth with increasing number of impact cycles. The impact wear mechanisms of the CrN coating were mainly plastic deformation and micro-delamination. The AlTiN coating exhibited the worst impact wear resistance among the three coatings, mainly due to adhesive wear; in contrast, the AlCrN coating exhibited a lower tendency for the coating to pick-up the ball counterface material, and accordingly demonstrated good impact wear resistance. The AlCrN coating exhibited both the best impact wear performance and the best abrasion resistance amongst the three coatings. The CrN coating exhibited the worst abrasive wear resistance due to its comparatively low hardness. The abrasive wear mechanisms of the CrN coating were a combination of plastic deformation, fine micro-cracking and micro-spallation. The AlTiN coating suffered more severe abrasive wear compared to the AlCrN coating, although both coatings had similar hardnesses. © 2012 Elsevier B.V. All rights reserved.
1. Introduction It is well known that transition metal nitride hard coatings prepared by plasma assisted Physical Vapour Deposition (PVD) techniques can be widely applied to improve the performance and lifetime of industrial tools and machine parts [1,2]. Amongst them, CrN coatings have been shown to have attractive properties, such as a high oxidation temperature and excellent corrosion resistance under severe environmental conditions [3]. To further improve the general performance of CrN coatings, alloying with another metal to form multi-component coatings has been undertaken [4,5]. It is well established that CrN with added aluminium shows a significant increase in hardness, wear resistance and high temperature oxidation resistance, and that the oxidation rate of CrAlN coatings decreases with increasing aluminium content [6–13]. Compared to TiAlN-based coatings, CrAlN-based coatings have a greater potential to improve oxidation and wear resistance [6,12]. Therefore, many recent studies have been dedicated to Al-rich AlCrN PVD coatings, which are of particular interest for tools and machine-components, and are a promising candidate for other protective coating applications in tribology [10–13].
⁎ Corresponding author. Tel.: +44 1142225466; fax: +44 1142225943. E-mail address: A.Matthews@sheffield.ac.uk (A. Matthews). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.08.077
In many industrial applications, PVD hard coatings are used to increase the performance and lifetime of components exposed to repetitive dynamic loading and to abrasion. Reproducible and well-characterised methods are therefore needed to assess the resistance of PVD hard coatings to these types of wear [14–17]. An impact test for PVD thin films was first proposed by Knotek et al. [14] in 1992. During the impact test the specimen is cyclically loaded by a hard ball that repetitively impacts on the specimen surface. The loading geometry is usually a ball-on-flat system which permits application of a high load with simple geometry to induce a Hertzian contact stress, simulating a wide range of tribological systems. In recent years, the impact test has been used successfully to investigate PVD coating properties under dynamic loading and evaluate the local fatigue strength of several coating/substrate composite systems [18–24]. The micro-scale abrasion test was also first described by Kassman et al. [25] in 1991 as a means to measure the abrasion resistance of thin hard coatings. In the last two decades, micro-scale abrasion tests using the ball-cratering configuration have been widely used to characterise the wear behaviour of thin hard coatings. In this test a ball is rotated against a specimen in the presence of a slurry of fine abrasive particles [26–31]. Sliding wear properties of CrN, AlCrN and AlTiN coatings have been studied in our previous work [12,32]. There was a need to perform impact wear and micro-scale abrasion tests of these coatings to evaluate wear caused by dynamic repetitive loading and abrasion, which
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simulates industrial applications where dynamic loads and abrasion are the main cause of coating degradation. Although some studies have been conducted on the impact wear and abrasion properties of CrN, CrAlN and TiAlN coatings [18–21,26,33–35], limited work has been conducted to investigate such properties for high aluminium content (i.e. AlCrN and AlTiN) coatings which are now increasingly used on commercial cutting tools [10,36]. The present work was therefore undertaken to investigate the impact wear and abrasion properties of such Al-rich coatings, with a focus on their tribological response in impact wear and micro-scale abrasion.
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in diameter, was used as the impacting body. Each sample was subjected to a range of impact cycles of 103, 5 ×103, 104, 2× 104 and 5× 104 cycles at maximum normal loads of 150 N and 300 N. The maximum (initial) Hertzian contact stress was calculated to be ~6.6 GPa for the AlCrN and CrN coatings and ~5.9 GPa for the AlTiN coating at a normal load of 150 N, and ~8.4 GPa for the AlCrN and CrN coatings and ~7.4 GPa for the AlTiN coating at a normal load of 300 N. The tests were conducted under atmospheric conditions with relative humidity of around (50–60% RH) and at room temperature of (20–25 °C). The surfaces of the ball and coating were cleaned with acetone before testing, and were always changed for each test.
2. Experimental procedure 2.3. Micro-scale abrasion test 2.1. Sample preparation and characterization CrN, AlCrN and AlTiN coatings were deposited by a multiple arc vapour deposition technique. Cemented carbide (90 wt.% WC+Cr3C2 +VC and 10 wt.% Co, K40UF from Konrad Friedrichs Ltd.) with microhardness of HV30 1610±40 kg/mm2 was used as the substrate material. All of the substrates were polished to a surface roughness of approximately 0.04 μm Ra and then cleaned and dried before the coating deposition. Pure Cr targets in a reactive nitrogen atmosphere were used to obtain the CrN coatings, while customized Al70Cr30 and Al67Ti33 (at.%) targets were used to obtain the AlCrN and AlTiN coatings, respectively. The temperature of the specimens during deposition was held at approximately 450 °C for the CrN coating, 500 °C for the AlCrN coating and 600 °C for the AlTiN coating. The substrate DC-bias voltage was in the range of −50 V to −150 V. All the three coatings have cubic structures as determined by XRD studies (not shown). The properties of the coatings are shown in Table 1. The surface roughness of the coating was measured by using stylus profilometry. Four measurements (at random position and orientation) were recorded to calculate an average Ra value. A peak load of 30 mN was adopted for the nano-indentation tests to avoid significant influence on measured hardness from the substrate, by ensuring that the resulting indentation depth was within 10% of the total coating thickness; 20 indentations per sample were performed to obtain average values. 2.2. Impact wear test The impact wear tests were performed on a proprietary pneumaticallyactuated cyclic impact tester [15,26], in which the specimen surface is repetitively stressed at a defined contact point by impacts from a hard sphere. The piezoelectric force transducer, which outputs the resultant impact force, was mounted below the sample holder. The impact head assembly holding a hard sphere was cyclically moved upwards and downwards by a two-way air valve and piston. The measured impact force was continuously monitored and the maximum value was the same for the duration of the test. A schematic view of the impact test geometry is shown in Fig. 1(a). The impact frequency (f) and the initial ball to sample distance (d) were set at 10 Hz and 15 mm, respectively. A tungsten carbide ball (grade 25 from Spheric Engineering Ltd.), 6 mm
The abrasion tests were performed on a Plint TE66 micro-scale abrasion tester [28]. A schematic of the tester is shown in Fig. 1(b). A set of 10 wear craters was produced for each sample, corresponding to a normal load (Fn) of 0.2 N with 5 different numbers of ball revolutions (N) of 10, 20, 50, 100 and 200. The corresponding sliding distance increased from 0.785 m (10 revolutions) to 15.7 m (200 revolutions). The ball counterface was a micro-blasted 25 mm diameter hardened steel sphere (SAE 52100, 61±2 HRC, Ra = 2.5±0.3 μm) and the abrasive slurry was a suspension of SiC particles (F1200-C6, mean size 4–5 μm) in distilled water (proportion 80 g to 100 ml distilled water). The ball rotational speed was set to provide a linear velocity of 0.1 m/s in all tests. After the impact and abrasion tests, the wear craters were studied using stylus profilometry (Veeco Dektak 150), Scanning Electron Microscopy (SEM; JEOL JSM 6400) and Energy Dispersive X-ray (EDX) spectroscopy, and the impact and abrasion wear behaviour of the coatings were considered in relation to their mechanical properties. 3. Results and discussion 3.1. Impact wear properties The wear crater cross-sectional areas were measured using stylus profilometry. However, it is inaccurate to use this to estimate the wear volume of the coating, considering that the arc-shape of the wear scar profile was somewhat irregular. Therefore, the maximum wear depth of the coatings was also measured in this work. The maximum depths of the wear craters of the CrN, AlCrN and AlTiN coatings, corresponding to increasing numbers of impact cycles, are shown in Fig. 2. The CrN coating showed similar wear depth at low numbers of impact cycles, compared to the AlCrN coating. However, a rapid increase in wear depth was observed for the CrN coating at higher numbers of impact cycles (after 5×103), particularly at the higher normal load of 300 N. The maximum wear depth of the CrN coating was found to increase non-linearly with an increasing number of impact cycles. The AlCrN coating exhibited the best impact wear resistance while the AlTiN coating the worst among the three coatings. The maximum wear depth of the AlCrN coating was found to increase linearly with increasing numbers of impact cycles; by contrast, there was no clear linear relationship for the AlTiN coating.
Table 1 Properties of CrN, AlCrN and AlTiN coatings. Coatings
CrN AlCrN AlTiN
Coating thickness, μm
6 ± 0.2 3 ± 0.2 3 ± 0.2
Surface roughness (Ra), μm
0.25 ± 0.15 0.13 ± 0.02 0.15 ± 0.04
Elemental atomic ratios estimated from EDX analyses
Measured mechanical properties
Cr/N
Ti/N
Al/N
Nanoindentation hardness, (H), GPa
Elastic modulus (E), GPa
H/E ratio
H3/E2 ratio
1.00 0.33 –
– – 0.38
– 0.65 0.55
23.4 ± 5.5 32.5 ± 8.3 31.8 ± 6.6
473.2 ± 90 474.6 ± 80 359.9 ± 40
0.05 0.07 0.09
0.06 0.15 0.25
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Fig. 1. Schematic of the impact wear test (a) and micro-scale abrasion test (b).
Observation of the worn surface morphologies indicates that the CrN coating underwent significant plastic deformation under impact, as shown in Fig. 3. Features of plastic deformation and micro-delamination can be observed in the middle of the wear scar after 104 impact cycles (Fig. 3(a)). With the number of impact cycles increasing to 5×104, the middle area of the wear scar was polished by the repeated cyclic loading, but the morphology in the intermediate zone showed the accumulation of plastic deformation (Fig. 3(b, c, d)). No transfer of the ball material was found inside the wear scar of the coating, as indicated by the EDX analysis. Therefore, the impact wear mechanisms of the CrN coating were mainly plastic deformation and micro-delamination. Under normal loads of 150 N and 300 N, the worn surfaces of the AlCrN coating showed a smooth morphology, on which no significant pick-up and transfer of the ball material was observed. There was no significant change for the wear morphologies of the AlCrN coating with increasing impact cycles, except that some very minor transfer of ball material was observed on the worn surface after 5 ×104 impact cycles, as shown in Fig. 4. Therefore, the wear debris hardly adhered to the worn surface of the AlCrN coating which avoided adhesive wear and the detachment or delamination of coating by the cyclic loading. This explains the good impact wear resistance of the AlCrN coating. The wear of the AlCrN coating can be described as a gradual process of reduction in coating thickness by repeated impact loading. The AlTiN coating provided different impact wear behaviours to the AlCrN coating, exhibiting mainly adhesive wear. Large areas of pick-up and ball material transfer were observed inside the wear scar of the
AlTiN coating at lower numbers of impact cycles (103 and 5 × 103). The morphology of the transfer layer indicates that the ball counterface suffered progressive surface degradation, experiencing continuous detachment of particles from its surface (Fig. 5a, b). EDX spectra of the transfer layer and the underlying worn surface show a significant presence of oxygen in the transfer layer, due to tribo-oxidation occurring during the impact test; however, no presence of elements from the ball material (or of oxygen) can be detected in the underlying worn surface (Fig. 5c). Eventually, these transfer layers became detached after a higher number of impact cycles. The exfoliation of these transfer layers leads to the introduction of loose particles to the contact surface, which tends to accelerate coating wear. Back-Scattered Electron (BSE) imaging of wear scars shows that the AlTiN coating is worn through to the substrate after only 2 × 104 impact cycles (Fig. 6a). No significant pick-up of the ball material can be observed inside the wear scar at this stage; however, some wear debris was scattered on the worn surface and around the edge of the wear scar. The bright area inside the wear scar is indicative of substrate material exposure, suggesting that the coating has been removed in this zone. Observation of this zone at higher magnification reveals that the degradation mechanism of the AlTiN coating can be described as a gradual process of reduction in coating thickness by repeated impact loading (Fig. 6b). Observation of the worn surface morphology indicates that the CrN coating underwent much more severe impact deformation compared to the AlCrN coating. The main difference in impact wear behaviour between CrN and AlCrN was the level of plastic deformation, which can
Fig. 2. Maximum wear depths of CrN, AlCrN and AlTiN coatings under normal loads of 150 N (a) and 300 N (b): N = 103, 5 × 103, 104, 2 × 104 and 5 × 104 cycles.
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Fig. 3. SEM images of the CrN coating after 104 (a) and 5 × 104 (b, c, and d) impact cycles: Fn = 300 N.
explain the completely non-linear increase of the maximum wear depth of the CrN coating with increasing number of impact cycles. In this work, the impact wear depth of the coating was found to increase linearly with increasing numbers of impact cycles, as long as no
significant plastic deformation occurred for the coating. The AlCrN coating showed that a high aluminium content provides significant increases in hardness and impact wear resistance over CrN. The main difference in impact wear behaviour between the AlCrN and AlTiN
Fig. 4. SEM images of the AlCrN coating after 104 (a and b) and 5 × 104 (b and c) impact cycles: Fn = 300 N.
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Fig. 5. SEM images (a and b) and EDX spectra (c) of transferred counterface material and the undamaged AlTiN coating (each sampled as shown in b) after 5×103 impact cycles: Fn =300 N.
coatings was the different levels of adhesive wear. The AlCrN coating exhibited much better impact wear resistance than the AlTiN coating, which can be mainly attributed to the fact that the AlCrN coating had less ‘tribochemical’ interaction with the ball counterface. This was consistent with the sliding wear testing results of the two coatings reported in our previous work [12], in which the AlCrN coating showed a lower tendency of the coating to pick-up ball material or wear debris and
accordingly exhibited better sliding wear resistance as compared to the AlTiN coating. Overall, these differences can probably be attributed to two main factors. The first is related to the fact that Ti in the AlTiN coating had a stronger affinity to ball counterface elements. Secondly, the AlTiN coating had a higher friction coefficient, as a consequence of the greater ‘tribochemical’ interaction between the AlTiN coating and the ball counterface. It is interesting to notice that there was no visible
Fig. 6. SEM-BSE (a) and SEM-SE (b) images of the AlTiN coating after 2 × 104 impact cycles: Fn = 300 N.
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Fig. 7. Maximum wear depth of the CrN, AlCrN and AlTiN coatings after different ball revolutions.
crack formation for all the three coatings during the impact tests, which can be attributed to their good toughness as well as the high stiffness and load support provided by the cemented carbide substrates. 3.2. Abrasive wear properties The maximum wear crater depths of the CrN, AlCrN and AlTiN coatings with increasing number of ball revolutions are shown in Fig. 7. The AlCrN coating was found to have the best abrasive wear resistance among the three coatings. The CrN coating became worn through during 50–100 ball revolutions and exhibited much worse abrasive wear resistance as compared to the two Al-containing coatings. The hardness of the SiC abrasive media is 2100–2600 HV; therefore, both the AlCrN and AlTiN coatings are much harder than the SiC particles whereas the CrN coating is slightly softer. A well-known parameter in abrasive wear testing is the ratio between the hardness of the substrate (Hs)
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and of the abradant (Ha). Particulate abrasion typically occurs when the ratio Hs/Ha is less than 1.3. In this situation, the abrasion will lead to much greater wear rates [34]. In this work, the CrN coating had a low Hs/Ha ratio of about 0.72 and thus a low abrasion resistance. However, it is interesting that the AlCrN coating exhibited much better anti-abrasive wear properties than the AlTiN coating, although both coatings had comparable Hs/Ha ratios in the range of 1.22–1.54. It is known that the types of damage produced in micro-abrasion testing can be classified into two principal categories, depending on the predominant topography of the scar surface. Abrasive wear scars tend to exhibit either unidirectional parallel grooves or a multiple indented surface topography with no noticeable directionality, which can be identified as being caused by either two- or three-body abrasion, respectively. In this work a low normal load of 0.2 N was adopted, to ensure that three-body rolling was the dominant wear mechanism, to provide repeatable results [27,28,31]. The CrN coating exhibited much more severe abrasive wear compared to that of the other two coatings, which was characterised by brittle fracture. The wear mechanisms of the CrN coating were a combination of plastic deformation, fine micro-cracking and micro-spallation, as shown in Fig. 8. Because the CrN coating was softer than the SiC abradant particles, this seems to encourage dragging of the particles into the contact and then scoring of the coating by a micro-cutting action. Fine micro-cracking and micro-spalling of the coating occurred within just the first 10 ball revolutions (Fig. 8a and b), which continued with increasing numbers of ball revolutions until the coating was finally delaminated and removed by the cutting of the abrasive particles (Fig. 8c and d). Fig. 9(a) shows SEM micrographs of the AlCrN coating corresponding to increasing numbers of ball revolutions. The surface morphology shows that the SiC particles did not embed into the AlCrN coating surface in the first 10 ball revolutions; they rolled between the two counter surfaces and produced a very slightly deformed and multiple indented worn surface with no evident surface directionality. With increasing number of ball revolutions, the abrasive wear became visible on the worn surface, as traces of the microscopic defects (e.g. macro-droplets, pores and pinholes), besides some very slight grooving. Considering
Fig. 8. Abrasive wear morphologies of the CrN coating after 10 (a and b) and 100 (c and d) ball revolutions.
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that the AlCrN coating is much harder than the SiC particles, from the gradual occurrence of shallow grooves it can be deduced that some of the abrasive particles were embedded in the surface of the steel ball counterface and acted as fixed indenters, which consequently produced the damage observed. Moreover, the size of these grooves was clearly comparable to the mean size of the abrasive particles used. Therefore, the abrasive wear of the AlCrN coating can be described as gradual
polishing and micro-abrasion by the abrasive particles; no heavy plastic deformation or micro-cracking occurred. Compared to the situation with the AlCrN coating, the wear of the AlTiN coating was mainly characterised by a combination of multiple indentions and plastic deformation, which increased with increasing number of ball revolutions. Finally the coating material became worn through after a certain number of ball revolutions, exposing the substrate material, as shown in
Fig. 9. Abrasive wear morphologies and EDX spectra of the AlCrN (a) and AlTiN (b) coatings after different ball revolutions.
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Fig. 9(b). Also, it is worth noting that microscopic coating defects did not act as the original wear site during the abrasive wear process for both AlCrN and AlTiN coatings. EDX analysis conducted on the worn surfaces shows that no material transfer or tribo-oxidation can be detected for either the AlCrN or the AlTiN coatings during the abrasive wear (Fig. 9). For both coatings, no significant change can be observed between the EDX spectra of the worn surfaces and the as-deposited coating. In this work, the coating hardness was found to play a key factor in the abrasion resistance when the coating had a comparable or relatively lower hardness than the abrasive particles, in accordance with the expectation that the ability of the abrasive particles to “scratch” the coating surface decreased with increasing coating hardness. As expected, the AlCrN coating with a higher hardness provided a significant increase in abrasion resistance compared to CrN. However, it is noted that the AlCrN coating also exhibited much better abrasion resistance than the AlTiN coating, although both coatings had similar hardnesses and the latter even had a lower measured elastic modulus. It is reported that a coating designed to resist micro-abrasive wear must indeed have high hardness to resist scratching and ploughing but should also have a low elastic modulus to resist plastic deformation during contact against a counterface. A material with a lower Young's modulus can also elastically deform to distribute the contact strain over a larger coating volume, thereby reducing the maximum contact stress [31,37]. It can also be shown that resistance to plastic deformation is dependent on the ratio H3/E2, and the contact loads required to induce plastic deformation in materials with high hardness and low elastic modulus (i.e. a high H3/E2 ratio) are higher [38]. Furthermore, a material with high H3/E2 value can better resist the initiation of cracking damage [39]. Based on these factors, the AlTiN coating should exhibit better abrasion resistance than the AlCrN coating. However, the fact is that the AlCrN coating with a higher elastic modulus had much better abrasion resistance than the AlTiN coating with a lower elastic modulus – although it should be noted that the (stiff) cemented carbide substrate may significantly influence the measured coating elastic modulus values. Comparing the abrasive wear behaviours of the AlCrN and AlTiN coatings, it was found that the AlTiN coating suffers more severe multiple indentation. AlTiN coatings have been reported to have a higher friction coefficient compared to AlCrN coatings in both reciprocating sliding and ball-on-disc wear tests in our previous work [12]. The more severe wear of the AlTiN coating from the abrasive particles can possibly be caused by the fact that the AlTiN coating had a higher friction coefficient (possibly due to tribochemical interactions in the sliding contact), which would cause more abrasive particles to be dragged into the contact. 4. Conclusions CrN, AlCrN and AlTiN coatings were deposited by a multiple-arc plasma-assisted PVD technique and mechanical properties were studied. Impact wear and micro-scale abrasion tests were performed to investigate the impact and abrasion resistance of the three coatings. There was no visible crack formation for all three coatings during the impact tests. The AlCrN coating exhibited both good impact wear performance and good abrasion resistance. The impact wear resistance of the AlCrN coating can be mainly attributed to the low tendency of the coating to pick-up ball material. In contrast, the impact wear of the AlTiN coating was caused mainly by adhesive interaction with the ball counterface, and the coating accordingly demonstrated worse impact wear resistance. The differences between AlCrN and AlTiN in tendency to pick-up ball material can be mainly attributed to the elemental composition of the coatings, i.e. Ti and Cr showed different affinities to ball counterface elements and accordingly resulted in different friction coefficients. The CrN coating was found to undergo much more severe plastic deformation under impact as compared to both the AlCrN and AlTiN coatings. The impact wear mechanisms of the CrN coating were mainly plastic deformation and micro-delamination. The CrN coating exhibited the worst abrasive wear resistance among the three coatings due to its
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comparatively low hardness (lower than that of the SiC abradant). Plastic deformation, fine micro-cracking and micro-spallation processes played a substantial role in the abrasive wear of CrN. The AlTiN coating exhibited a lower abrasion resistance than the AlCrN coating, despite the fact that both coatings had similar hardnesses. This can be attributed to the AlTiN coating with higher friction coefficient suffering a more severe multiple indentation from the abrasive particles.
Acknowledgements The authors would like to thank Prof. Z.R. Zhou, Southwest Jiaotong University, for helpful discussions and B. Lei, Zigong Cemented Carbide (Chengdu tool department) Corp. Ltd., for providing coated carbide samples. The authors are grateful for the financial support of the National Scientific Foundation of China (No. 51005191), of the Research Fund for the Doctoral Program of Higher Education of China (No. 20100184120003) and for a scholarship to J.L. Mo, under the State Scholarship Fund of the China Scholarship Council (CSC), to pursue study in the University of Sheffield as an Academic Visitor.
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Impact wear and abrasion resistance of CrN, AlCrN and AlTiN PVD coatings J.L. Mo a, b, M.H. Zhu a, A. Leyland b, A. Matthews b,⁎ a b
Tribology Research Institute, Traction Power State Key Laboratory, Southwest Jiaotong University, Chengdu 610031, China Department of Materials Science and Engineering, The University of Sheffield, Sheffield S1 3JD, UK
a r t i c l e
i n f o
Available online 6 November 2012 Keywords: Abrasive wear Impact wear PVD CrN AlCrN AlTiN
a b s t r a c t The properties of CrN, AlCrN and AlTiN coatings deposited on cemented carbide substrates by a multiple-arc Physical Vapour Deposition (PVD) technique were evaluated by cyclic impact wear and micro-scale abrasion testing. In the impact wear test, a 6 mm diameter tungsten carbide ball was used as the impacting body and the impact frequency (f) was set at 10 Hz. In the micro-scale abrasion test, a micro-blasted 25 mm diameter hardened steel ball was used as the counterface and a suspension of SiC particles (mean size of 4–5 μm) in distilled water as the abrasive slurry. After these wear tests, the wear craters were studied by stylus profilometry, SEM and EDX, to investigate wear behaviour. It is shown that the CrN coating suffered much more severe impact deformation as compared to the two ternary coatings, and exhibited a non-linear increase of the maximum wear depth with increasing number of impact cycles. The impact wear mechanisms of the CrN coating were mainly plastic deformation and micro-delamination. The AlTiN coating exhibited the worst impact wear resistance among the three coatings, mainly due to adhesive wear; in contrast, the AlCrN coating exhibited a lower tendency for the coating to pick-up the ball counterface material, and accordingly demonstrated good impact wear resistance. The AlCrN coating exhibited both the best impact wear performance and the best abrasion resistance amongst the three coatings. The CrN coating exhibited the worst abrasive wear resistance due to its comparatively low hardness. The abrasive wear mechanisms of the CrN coating were a combination of plastic deformation, fine micro-cracking and micro-spallation. The AlTiN coating suffered more severe abrasive wear compared to the AlCrN coating, although both coatings had similar hardnesses. © 2012 Elsevier B.V. All rights reserved.
1. Introduction It is well known that transition metal nitride hard coatings prepared by plasma assisted Physical Vapour Deposition (PVD) techniques can be widely applied to improve the performance and lifetime of industrial tools and machine parts [1,2]. Amongst them, CrN coatings have been shown to have attractive properties, such as a high oxidation temperature and excellent corrosion resistance under severe environmental conditions [3]. To further improve the general performance of CrN coatings, alloying with another metal to form multi-component coatings has been undertaken [4,5]. It is well established that CrN with added aluminium shows a significant increase in hardness, wear resistance and high temperature oxidation resistance, and that the oxidation rate of CrAlN coatings decreases with increasing aluminium content [6–13]. Compared to TiAlN-based coatings, CrAlN-based coatings have a greater potential to improve oxidation and wear resistance [6,12]. Therefore, many recent studies have been dedicated to Al-rich AlCrN PVD coatings, which are of particular interest for tools and machine-components, and are a promising candidate for other protective coating applications in tribology [10–13].
⁎ Corresponding author. Tel.: +44 1142225466; fax: +44 1142225943. E-mail address: A.Matthews@sheffield.ac.uk (A. Matthews). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.08.077
In many industrial applications, PVD hard coatings are used to increase the performance and lifetime of components exposed to repetitive dynamic loading and to abrasion. Reproducible and well-characterised methods are therefore needed to assess the resistance of PVD hard coatings to these types of wear [14–17]. An impact test for PVD thin films was first proposed by Knotek et al. [14] in 1992. During the impact test the specimen is cyclically loaded by a hard ball that repetitively impacts on the specimen surface. The loading geometry is usually a ball-on-flat system which permits application of a high load with simple geometry to induce a Hertzian contact stress, simulating a wide range of tribological systems. In recent years, the impact test has been used successfully to investigate PVD coating properties under dynamic loading and evaluate the local fatigue strength of several coating/substrate composite systems [18–24]. The micro-scale abrasion test was also first described by Kassman et al. [25] in 1991 as a means to measure the abrasion resistance of thin hard coatings. In the last two decades, micro-scale abrasion tests using the ball-cratering configuration have been widely used to characterise the wear behaviour of thin hard coatings. In this test a ball is rotated against a specimen in the presence of a slurry of fine abrasive particles [26–31]. Sliding wear properties of CrN, AlCrN and AlTiN coatings have been studied in our previous work [12,32]. There was a need to perform impact wear and micro-scale abrasion tests of these coatings to evaluate wear caused by dynamic repetitive loading and abrasion, which
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simulates industrial applications where dynamic loads and abrasion are the main cause of coating degradation. Although some studies have been conducted on the impact wear and abrasion properties of CrN, CrAlN and TiAlN coatings [18–21,26,33–35], limited work has been conducted to investigate such properties for high aluminium content (i.e. AlCrN and AlTiN) coatings which are now increasingly used on commercial cutting tools [10,36]. The present work was therefore undertaken to investigate the impact wear and abrasion properties of such Al-rich coatings, with a focus on their tribological response in impact wear and micro-scale abrasion.
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in diameter, was used as the impacting body. Each sample was subjected to a range of impact cycles of 103, 5 ×103, 104, 2× 104 and 5× 104 cycles at maximum normal loads of 150 N and 300 N. The maximum (initial) Hertzian contact stress was calculated to be ~6.6 GPa for the AlCrN and CrN coatings and ~5.9 GPa for the AlTiN coating at a normal load of 150 N, and ~8.4 GPa for the AlCrN and CrN coatings and ~7.4 GPa for the AlTiN coating at a normal load of 300 N. The tests were conducted under atmospheric conditions with relative humidity of around (50–60% RH) and at room temperature of (20–25 °C). The surfaces of the ball and coating were cleaned with acetone before testing, and were always changed for each test.
2. Experimental procedure 2.3. Micro-scale abrasion test 2.1. Sample preparation and characterization CrN, AlCrN and AlTiN coatings were deposited by a multiple arc vapour deposition technique. Cemented carbide (90 wt.% WC+Cr3C2 +VC and 10 wt.% Co, K40UF from Konrad Friedrichs Ltd.) with microhardness of HV30 1610±40 kg/mm2 was used as the substrate material. All of the substrates were polished to a surface roughness of approximately 0.04 μm Ra and then cleaned and dried before the coating deposition. Pure Cr targets in a reactive nitrogen atmosphere were used to obtain the CrN coatings, while customized Al70Cr30 and Al67Ti33 (at.%) targets were used to obtain the AlCrN and AlTiN coatings, respectively. The temperature of the specimens during deposition was held at approximately 450 °C for the CrN coating, 500 °C for the AlCrN coating and 600 °C for the AlTiN coating. The substrate DC-bias voltage was in the range of −50 V to −150 V. All the three coatings have cubic structures as determined by XRD studies (not shown). The properties of the coatings are shown in Table 1. The surface roughness of the coating was measured by using stylus profilometry. Four measurements (at random position and orientation) were recorded to calculate an average Ra value. A peak load of 30 mN was adopted for the nano-indentation tests to avoid significant influence on measured hardness from the substrate, by ensuring that the resulting indentation depth was within 10% of the total coating thickness; 20 indentations per sample were performed to obtain average values. 2.2. Impact wear test The impact wear tests were performed on a proprietary pneumaticallyactuated cyclic impact tester [15,26], in which the specimen surface is repetitively stressed at a defined contact point by impacts from a hard sphere. The piezoelectric force transducer, which outputs the resultant impact force, was mounted below the sample holder. The impact head assembly holding a hard sphere was cyclically moved upwards and downwards by a two-way air valve and piston. The measured impact force was continuously monitored and the maximum value was the same for the duration of the test. A schematic view of the impact test geometry is shown in Fig. 1(a). The impact frequency (f) and the initial ball to sample distance (d) were set at 10 Hz and 15 mm, respectively. A tungsten carbide ball (grade 25 from Spheric Engineering Ltd.), 6 mm
The abrasion tests were performed on a Plint TE66 micro-scale abrasion tester [28]. A schematic of the tester is shown in Fig. 1(b). A set of 10 wear craters was produced for each sample, corresponding to a normal load (Fn) of 0.2 N with 5 different numbers of ball revolutions (N) of 10, 20, 50, 100 and 200. The corresponding sliding distance increased from 0.785 m (10 revolutions) to 15.7 m (200 revolutions). The ball counterface was a micro-blasted 25 mm diameter hardened steel sphere (SAE 52100, 61±2 HRC, Ra = 2.5±0.3 μm) and the abrasive slurry was a suspension of SiC particles (F1200-C6, mean size 4–5 μm) in distilled water (proportion 80 g to 100 ml distilled water). The ball rotational speed was set to provide a linear velocity of 0.1 m/s in all tests. After the impact and abrasion tests, the wear craters were studied using stylus profilometry (Veeco Dektak 150), Scanning Electron Microscopy (SEM; JEOL JSM 6400) and Energy Dispersive X-ray (EDX) spectroscopy, and the impact and abrasion wear behaviour of the coatings were considered in relation to their mechanical properties. 3. Results and discussion 3.1. Impact wear properties The wear crater cross-sectional areas were measured using stylus profilometry. However, it is inaccurate to use this to estimate the wear volume of the coating, considering that the arc-shape of the wear scar profile was somewhat irregular. Therefore, the maximum wear depth of the coatings was also measured in this work. The maximum depths of the wear craters of the CrN, AlCrN and AlTiN coatings, corresponding to increasing numbers of impact cycles, are shown in Fig. 2. The CrN coating showed similar wear depth at low numbers of impact cycles, compared to the AlCrN coating. However, a rapid increase in wear depth was observed for the CrN coating at higher numbers of impact cycles (after 5×103), particularly at the higher normal load of 300 N. The maximum wear depth of the CrN coating was found to increase non-linearly with an increasing number of impact cycles. The AlCrN coating exhibited the best impact wear resistance while the AlTiN coating the worst among the three coatings. The maximum wear depth of the AlCrN coating was found to increase linearly with increasing numbers of impact cycles; by contrast, there was no clear linear relationship for the AlTiN coating.
Table 1 Properties of CrN, AlCrN and AlTiN coatings. Coatings
CrN AlCrN AlTiN
Coating thickness, μm
6 ± 0.2 3 ± 0.2 3 ± 0.2
Surface roughness (Ra), μm
0.25 ± 0.15 0.13 ± 0.02 0.15 ± 0.04
Elemental atomic ratios estimated from EDX analyses
Measured mechanical properties
Cr/N
Ti/N
Al/N
Nanoindentation hardness, (H), GPa
Elastic modulus (E), GPa
H/E ratio
H3/E2 ratio
1.00 0.33 –
– – 0.38
– 0.65 0.55
23.4 ± 5.5 32.5 ± 8.3 31.8 ± 6.6
473.2 ± 90 474.6 ± 80 359.9 ± 40
0.05 0.07 0.09
0.06 0.15 0.25
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Fig. 1. Schematic of the impact wear test (a) and micro-scale abrasion test (b).
Observation of the worn surface morphologies indicates that the CrN coating underwent significant plastic deformation under impact, as shown in Fig. 3. Features of plastic deformation and micro-delamination can be observed in the middle of the wear scar after 104 impact cycles (Fig. 3(a)). With the number of impact cycles increasing to 5×104, the middle area of the wear scar was polished by the repeated cyclic loading, but the morphology in the intermediate zone showed the accumulation of plastic deformation (Fig. 3(b, c, d)). No transfer of the ball material was found inside the wear scar of the coating, as indicated by the EDX analysis. Therefore, the impact wear mechanisms of the CrN coating were mainly plastic deformation and micro-delamination. Under normal loads of 150 N and 300 N, the worn surfaces of the AlCrN coating showed a smooth morphology, on which no significant pick-up and transfer of the ball material was observed. There was no significant change for the wear morphologies of the AlCrN coating with increasing impact cycles, except that some very minor transfer of ball material was observed on the worn surface after 5 ×104 impact cycles, as shown in Fig. 4. Therefore, the wear debris hardly adhered to the worn surface of the AlCrN coating which avoided adhesive wear and the detachment or delamination of coating by the cyclic loading. This explains the good impact wear resistance of the AlCrN coating. The wear of the AlCrN coating can be described as a gradual process of reduction in coating thickness by repeated impact loading. The AlTiN coating provided different impact wear behaviours to the AlCrN coating, exhibiting mainly adhesive wear. Large areas of pick-up and ball material transfer were observed inside the wear scar of the
AlTiN coating at lower numbers of impact cycles (103 and 5 × 103). The morphology of the transfer layer indicates that the ball counterface suffered progressive surface degradation, experiencing continuous detachment of particles from its surface (Fig. 5a, b). EDX spectra of the transfer layer and the underlying worn surface show a significant presence of oxygen in the transfer layer, due to tribo-oxidation occurring during the impact test; however, no presence of elements from the ball material (or of oxygen) can be detected in the underlying worn surface (Fig. 5c). Eventually, these transfer layers became detached after a higher number of impact cycles. The exfoliation of these transfer layers leads to the introduction of loose particles to the contact surface, which tends to accelerate coating wear. Back-Scattered Electron (BSE) imaging of wear scars shows that the AlTiN coating is worn through to the substrate after only 2 × 104 impact cycles (Fig. 6a). No significant pick-up of the ball material can be observed inside the wear scar at this stage; however, some wear debris was scattered on the worn surface and around the edge of the wear scar. The bright area inside the wear scar is indicative of substrate material exposure, suggesting that the coating has been removed in this zone. Observation of this zone at higher magnification reveals that the degradation mechanism of the AlTiN coating can be described as a gradual process of reduction in coating thickness by repeated impact loading (Fig. 6b). Observation of the worn surface morphology indicates that the CrN coating underwent much more severe impact deformation compared to the AlCrN coating. The main difference in impact wear behaviour between CrN and AlCrN was the level of plastic deformation, which can
Fig. 2. Maximum wear depths of CrN, AlCrN and AlTiN coatings under normal loads of 150 N (a) and 300 N (b): N = 103, 5 × 103, 104, 2 × 104 and 5 × 104 cycles.
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Fig. 3. SEM images of the CrN coating after 104 (a) and 5 × 104 (b, c, and d) impact cycles: Fn = 300 N.
explain the completely non-linear increase of the maximum wear depth of the CrN coating with increasing number of impact cycles. In this work, the impact wear depth of the coating was found to increase linearly with increasing numbers of impact cycles, as long as no
significant plastic deformation occurred for the coating. The AlCrN coating showed that a high aluminium content provides significant increases in hardness and impact wear resistance over CrN. The main difference in impact wear behaviour between the AlCrN and AlTiN
Fig. 4. SEM images of the AlCrN coating after 104 (a and b) and 5 × 104 (b and c) impact cycles: Fn = 300 N.
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Fig. 5. SEM images (a and b) and EDX spectra (c) of transferred counterface material and the undamaged AlTiN coating (each sampled as shown in b) after 5×103 impact cycles: Fn =300 N.
coatings was the different levels of adhesive wear. The AlCrN coating exhibited much better impact wear resistance than the AlTiN coating, which can be mainly attributed to the fact that the AlCrN coating had less ‘tribochemical’ interaction with the ball counterface. This was consistent with the sliding wear testing results of the two coatings reported in our previous work [12], in which the AlCrN coating showed a lower tendency of the coating to pick-up ball material or wear debris and
accordingly exhibited better sliding wear resistance as compared to the AlTiN coating. Overall, these differences can probably be attributed to two main factors. The first is related to the fact that Ti in the AlTiN coating had a stronger affinity to ball counterface elements. Secondly, the AlTiN coating had a higher friction coefficient, as a consequence of the greater ‘tribochemical’ interaction between the AlTiN coating and the ball counterface. It is interesting to notice that there was no visible
Fig. 6. SEM-BSE (a) and SEM-SE (b) images of the AlTiN coating after 2 × 104 impact cycles: Fn = 300 N.
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Fig. 7. Maximum wear depth of the CrN, AlCrN and AlTiN coatings after different ball revolutions.
crack formation for all the three coatings during the impact tests, which can be attributed to their good toughness as well as the high stiffness and load support provided by the cemented carbide substrates. 3.2. Abrasive wear properties The maximum wear crater depths of the CrN, AlCrN and AlTiN coatings with increasing number of ball revolutions are shown in Fig. 7. The AlCrN coating was found to have the best abrasive wear resistance among the three coatings. The CrN coating became worn through during 50–100 ball revolutions and exhibited much worse abrasive wear resistance as compared to the two Al-containing coatings. The hardness of the SiC abrasive media is 2100–2600 HV; therefore, both the AlCrN and AlTiN coatings are much harder than the SiC particles whereas the CrN coating is slightly softer. A well-known parameter in abrasive wear testing is the ratio between the hardness of the substrate (Hs)
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and of the abradant (Ha). Particulate abrasion typically occurs when the ratio Hs/Ha is less than 1.3. In this situation, the abrasion will lead to much greater wear rates [34]. In this work, the CrN coating had a low Hs/Ha ratio of about 0.72 and thus a low abrasion resistance. However, it is interesting that the AlCrN coating exhibited much better anti-abrasive wear properties than the AlTiN coating, although both coatings had comparable Hs/Ha ratios in the range of 1.22–1.54. It is known that the types of damage produced in micro-abrasion testing can be classified into two principal categories, depending on the predominant topography of the scar surface. Abrasive wear scars tend to exhibit either unidirectional parallel grooves or a multiple indented surface topography with no noticeable directionality, which can be identified as being caused by either two- or three-body abrasion, respectively. In this work a low normal load of 0.2 N was adopted, to ensure that three-body rolling was the dominant wear mechanism, to provide repeatable results [27,28,31]. The CrN coating exhibited much more severe abrasive wear compared to that of the other two coatings, which was characterised by brittle fracture. The wear mechanisms of the CrN coating were a combination of plastic deformation, fine micro-cracking and micro-spallation, as shown in Fig. 8. Because the CrN coating was softer than the SiC abradant particles, this seems to encourage dragging of the particles into the contact and then scoring of the coating by a micro-cutting action. Fine micro-cracking and micro-spalling of the coating occurred within just the first 10 ball revolutions (Fig. 8a and b), which continued with increasing numbers of ball revolutions until the coating was finally delaminated and removed by the cutting of the abrasive particles (Fig. 8c and d). Fig. 9(a) shows SEM micrographs of the AlCrN coating corresponding to increasing numbers of ball revolutions. The surface morphology shows that the SiC particles did not embed into the AlCrN coating surface in the first 10 ball revolutions; they rolled between the two counter surfaces and produced a very slightly deformed and multiple indented worn surface with no evident surface directionality. With increasing number of ball revolutions, the abrasive wear became visible on the worn surface, as traces of the microscopic defects (e.g. macro-droplets, pores and pinholes), besides some very slight grooving. Considering
Fig. 8. Abrasive wear morphologies of the CrN coating after 10 (a and b) and 100 (c and d) ball revolutions.
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that the AlCrN coating is much harder than the SiC particles, from the gradual occurrence of shallow grooves it can be deduced that some of the abrasive particles were embedded in the surface of the steel ball counterface and acted as fixed indenters, which consequently produced the damage observed. Moreover, the size of these grooves was clearly comparable to the mean size of the abrasive particles used. Therefore, the abrasive wear of the AlCrN coating can be described as gradual
polishing and micro-abrasion by the abrasive particles; no heavy plastic deformation or micro-cracking occurred. Compared to the situation with the AlCrN coating, the wear of the AlTiN coating was mainly characterised by a combination of multiple indentions and plastic deformation, which increased with increasing number of ball revolutions. Finally the coating material became worn through after a certain number of ball revolutions, exposing the substrate material, as shown in
Fig. 9. Abrasive wear morphologies and EDX spectra of the AlCrN (a) and AlTiN (b) coatings after different ball revolutions.
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Fig. 9(b). Also, it is worth noting that microscopic coating defects did not act as the original wear site during the abrasive wear process for both AlCrN and AlTiN coatings. EDX analysis conducted on the worn surfaces shows that no material transfer or tribo-oxidation can be detected for either the AlCrN or the AlTiN coatings during the abrasive wear (Fig. 9). For both coatings, no significant change can be observed between the EDX spectra of the worn surfaces and the as-deposited coating. In this work, the coating hardness was found to play a key factor in the abrasion resistance when the coating had a comparable or relatively lower hardness than the abrasive particles, in accordance with the expectation that the ability of the abrasive particles to “scratch” the coating surface decreased with increasing coating hardness. As expected, the AlCrN coating with a higher hardness provided a significant increase in abrasion resistance compared to CrN. However, it is noted that the AlCrN coating also exhibited much better abrasion resistance than the AlTiN coating, although both coatings had similar hardnesses and the latter even had a lower measured elastic modulus. It is reported that a coating designed to resist micro-abrasive wear must indeed have high hardness to resist scratching and ploughing but should also have a low elastic modulus to resist plastic deformation during contact against a counterface. A material with a lower Young's modulus can also elastically deform to distribute the contact strain over a larger coating volume, thereby reducing the maximum contact stress [31,37]. It can also be shown that resistance to plastic deformation is dependent on the ratio H3/E2, and the contact loads required to induce plastic deformation in materials with high hardness and low elastic modulus (i.e. a high H3/E2 ratio) are higher [38]. Furthermore, a material with high H3/E2 value can better resist the initiation of cracking damage [39]. Based on these factors, the AlTiN coating should exhibit better abrasion resistance than the AlCrN coating. However, the fact is that the AlCrN coating with a higher elastic modulus had much better abrasion resistance than the AlTiN coating with a lower elastic modulus – although it should be noted that the (stiff) cemented carbide substrate may significantly influence the measured coating elastic modulus values. Comparing the abrasive wear behaviours of the AlCrN and AlTiN coatings, it was found that the AlTiN coating suffers more severe multiple indentation. AlTiN coatings have been reported to have a higher friction coefficient compared to AlCrN coatings in both reciprocating sliding and ball-on-disc wear tests in our previous work [12]. The more severe wear of the AlTiN coating from the abrasive particles can possibly be caused by the fact that the AlTiN coating had a higher friction coefficient (possibly due to tribochemical interactions in the sliding contact), which would cause more abrasive particles to be dragged into the contact. 4. Conclusions CrN, AlCrN and AlTiN coatings were deposited by a multiple-arc plasma-assisted PVD technique and mechanical properties were studied. Impact wear and micro-scale abrasion tests were performed to investigate the impact and abrasion resistance of the three coatings. There was no visible crack formation for all three coatings during the impact tests. The AlCrN coating exhibited both good impact wear performance and good abrasion resistance. The impact wear resistance of the AlCrN coating can be mainly attributed to the low tendency of the coating to pick-up ball material. In contrast, the impact wear of the AlTiN coating was caused mainly by adhesive interaction with the ball counterface, and the coating accordingly demonstrated worse impact wear resistance. The differences between AlCrN and AlTiN in tendency to pick-up ball material can be mainly attributed to the elemental composition of the coatings, i.e. Ti and Cr showed different affinities to ball counterface elements and accordingly resulted in different friction coefficients. The CrN coating was found to undergo much more severe plastic deformation under impact as compared to both the AlCrN and AlTiN coatings. The impact wear mechanisms of the CrN coating were mainly plastic deformation and micro-delamination. The CrN coating exhibited the worst abrasive wear resistance among the three coatings due to its
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comparatively low hardness (lower than that of the SiC abradant). Plastic deformation, fine micro-cracking and micro-spallation processes played a substantial role in the abrasive wear of CrN. The AlTiN coating exhibited a lower abrasion resistance than the AlCrN coating, despite the fact that both coatings had similar hardnesses. This can be attributed to the AlTiN coating with higher friction coefficient suffering a more severe multiple indentation from the abrasive particles.
Acknowledgements The authors would like to thank Prof. Z.R. Zhou, Southwest Jiaotong University, for helpful discussions and B. Lei, Zigong Cemented Carbide (Chengdu tool department) Corp. Ltd., for providing coated carbide samples. The authors are grateful for the financial support of the National Scientific Foundation of China (No. 51005191), of the Research Fund for the Doctoral Program of Higher Education of China (No. 20100184120003) and for a scholarship to J.L. Mo, under the State Scholarship Fund of the China Scholarship Council (CSC), to pursue study in the University of Sheffield as an Academic Visitor.
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