Tribo-layer formation during sliding wear of TiN coatings

Tribo-layer formation during sliding wear of TiN coatings

Wear 245 (2000) 223–229 Tribo-layer formation during sliding wear of TiN coatings S. Wilson, A.T. Alpas∗ Mechanical and Materials Engineering Program...

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Wear 245 (2000) 223–229

Tribo-layer formation during sliding wear of TiN coatings S. Wilson, A.T. Alpas∗ Mechanical and Materials Engineering Program, University of Windsor, Windsor, Ont., Canada N9B 3P4 Received 25 November 1999

Abstract The effects of atmospheric humidity and microstructure of counterface materials on the formation of tribo-layers on TiN coatings have been investigated. Pin-on-disc sliding wear experiments were conducted on physical vapour deposition (PVD) TiN-coated high speed steel (HSS) discs against HSS, mild steel and A356 Al–15% SiC pin materials under conditions of low and high relative humidity (RH). The TiN coatings undergo rapid wear by tribochemical oxidation and polishing at low sliding speeds and contact loads. This effect is reversed when contact loads and sliding speeds are raised and tribochemical wear is diminished. Increasing the humidity raises TiN wear rates and tribochemical wear seen at low loads and speeds, extending these phenomena to higher loads and sliding speeds. Hard abrasive inclusions or particles (e.g. SiC) in the counterface material prevent the formation of stable tribo-layers on the TiN surface and accelerate tribochemical wear effects by microabrasion of the TiN, particularly at high RHs. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Tribo-layer; Sliding wear; TiN coatings; Wear maps; Wear mechanisms

1. Introduction The wear of hard coatings such as TiN can change dramatically with adjustments of parameters such as contact load, sliding speed, contact geometry and humidity. At very low sliding speeds and loads, the dominant wear mechanism for TiN in dry sliding against steels is by polishing to a smooth mirror finish and the formation of TiO2 and FeTiO3 powder debris by an oxidation reaction [1]. When the sliding wear conditions are made more severe, mechanical wear mechanisms such as abrasion and microcutting become dominant [2], although there have been few systematic studies on the relative contributions of chemical and mechanical wear. The use of wear and surface temperature maps provides effective tools for studying the influence of parameters such as contact load, sliding speed and frictional heating on wear mechanisms in metals [3–5], ceramics [6], composites [7,8] and coated surfaces [5,9,10]. The demarcation of wear mechanisms on each map is established using not only wear rate information, but also worn surface morphology and wear debris or surface tribo-layer compositions. In particular, the maps provide a useful format for studying tribological phenomena such as tribo-layer formation and stability in different environments and for different material sliding combinations. ∗ Corresponding author. Tel.: +1-519-253-4232; fax: +1-519-973-7062 E-mail address: [email protected] (A.T. Alpas).

This study investigates the effects of pin material and atmospheric humidity on the formation of tribo-layers on TiN-coated M2 high speed steel (HSS) discs in pin-on-disc wear experiments. These effects are summarized in a wear mechanism diagram. The effect of hard abrasive phases in the pin material on tribo-layer stability at high humidities is considered. 2. Experimental 2.1. Materials Titanium nitride was deposited onto AISI M2 HSS (0.80 C, 4.0 Cr, 5.0 Mo, 6.0 W, 2.0 V, balance Fe (in wt.%)) coupons 25.4 mm in diameter and 3 mm thick by an electric arc physical vapour deposition (PVD) process. The substrates had been ground to an average surface roughness of R a = 0.35 ± 0.3 ␮m and then PVD coated with TiN to a coating thickness of 2.5–3.0 ␮m. With the application of the TiN layer, the surface roughness of the ground AISI M2 steel surface is reduced to give an average R a = 0.29 ± 0.08 ␮m. The Knoop diamond indentation hardness of the HSS substrate was 1058 ± 53 kg mm−2 (50 g load). The Knoop hardness of the TiN coating was measured on flat surface asperities parallel to the grinding direction using a 50 g load to give 2320 ± 270 kg mm−2 . AISI M2 HSS pins 5.0 mm in diameter and of the same composition and hardness as the substrate were used as the slider material for wear tests.

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Wear tests were also performed with pins made from cold drawn AISI 1020 mild steel (HV1 kg = 300 ± 23 kg mm−2 ) and an aluminium metal matrix composite A356 Al–15% SiC (Duralcan F3.15S) (HV1 kg = 117 ± 16 kg mm−2 ). 2.2. Wear tests Sliding tests were performed on the TiN coupon specimens using a pin-on-disc sliding wear apparatus [11]. The apparatus is comprised of a variable speed rotating shaft arrangement to which a stainless steel TiN specimen holder is attached. A vertical loading arrangement, attached to the pin specimen, is lowered onto the rotating TiN coupons to produce a circular wear track 16.0 mm in average diameter. The pin contact geometry used with the coating was a flat-on-flat configuration during testing. Each of the experiments was performed at a set load and sliding speed and run up to a constant sliding distance of 12 ×103 m. Both pin and TiN disc were ultrasonically cleaned in acetone, dried in an oven at 100◦ C for 20 min and weighed on a four-point balance prior to wear testing. After each test, specimens were cleaned of loose debris using compressed air and weighed to determine mass loss (or gain). Wear rates of both TiN coating and pins were obtained by dividing mass loss/gain by the total sliding distance of 12 × 103 m. Wear experiments under high-humidity conditions were performed by fitting a polyethylene shroud around the pin-on-disc arrangement. Water-saturated cotton wool was added to the enclosure to create a humid microclimate and a hygroscope probe was used to measure the percentage relative humidity (% RH).

3. Results and discussion 3.1. TiN against HSS wear mechanism map A recently developed wear mechanism map [9], representing pin-on-disc dry sliding wear of TiN coatings on HSS (AISI M2 HSS) disc substrates against AISI M2 HSS pins, is shown in Fig. 1. Wear rates are given numerically at different sliding speed and load combinations and the map has been demarcated into four regimes using wear rate, surface damage and wear mechanism information (Fig. 1). 3.1.1. Regime I Rapid removal of TiN and exposure of the HSS substrate occurs by tribochemical polishing to a characteristic mirror finish at low sliding speeds and loads, exposing the steel substrate (Fig. 1a). TiN reacts mostly with oxygen and iron from the pin material to produce fine TiO2 and FeTiO3 powder debris [1] which is visible as a white powder to the naked eye but has a dark-coloured granular appearance under backscattered electron (BSE) scanning electron conditions (Fig. 1a). The debris has been smeared over the exposed HSS substrate

(light grey with white carbides) and EDS (15 keV) analysis (Fig. 2a) reveals that it is rich in Ti and O with Fe, V, W, Mo and Cr contributions from the HSS. The darker appearance of the debris relative to the HSS substrate under BSE conditions indicates that it is comparatively richer in the lower atomic number species, namely Ti and O. 3.1.2. Regime II Slight increases in either load or sliding speed inhibit the tribochemical reaction and introduce a transition to significantly lower TiN wear rates. Order of magnitude increases in HSS pin wear result from enhanced abrasion against the TiN surface asperities, which are now less prone to tribochemical degradation (Fig. 1b and c). Increased transfer of Fe-rich oxidized HSS pin material (Fig. 2b) to the TiN results and can contribute to negative wear rates or mass gains (Fig. 1). An EDS spectrum (15 keV) of the as-deposited and unworn TiN coating is shown in Fig. 2c for comparison. Apart from Ti and N peaks, there are no signals from the HSS substrate at this accelerating voltage. The transferred material is white in appearance under BSE imaging (Fig. 1b and c), indicating that it is comprised of the higher atomic number oxidized Fe, W, Mo, V, Cr from the HSS pin with minimal presence of Ti. 3.1.3. Regime III The onset of rapid TiN wear arises when measurements by thermocouple probe of the HSS pin indicate that the surface sliding temperature, Tb , reaches a temperature range of 150–200◦ C [9], and is similar to previous pin-on-disc TiN versus HSS wear studies [11] where coating damage accelerated above a critical Tb of about 160◦ C, accompanied by increased ductile deformation of the TiN (Fig. 1d). Patches of oxidized HSS pin material (Fig. 2d) approximately 20–50 ␮m in diameter have transferred to the disc surface and are concentrated in regions of the exposed HSS substrate. 3.1.4. Regime IV Whilst increased friction heating effects appear to introduce rapid ductile wear of the coating in Regime III, the effect is reversed at higher speeds in Regime IV by severe plastic deformation and ductile fracture of the HSS pin. The softening of the HSS pin material diminishes its ability to remove the coating and mild wear of TiN ensues. This is accompanied by extensive transfer of heavily deformed Fe from the HSS pin to the TiN surface, which is now predominantly metallic in nature (Fig. 2e). The onset of severe wear in the HSS pins correlates approximately with a Tb of 300◦ C [9]. This phenomenon is also typical of the sliding wear behaviour of other metals that display severe wear at critical sliding surface temperatures, e.g. 6061 Al and A356 Al alloys at about 125◦ C, when sliding against SAE 52100 bearing steel [7,8,12,13] and SAE 52100 steel sliding against itself at approximately 200◦ C [12].

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Fig. 1. Wear map showing wear rates and mechanisms for TiN in dry sliding against HSS pin material. The TiN wear rates have units of 10−8 g m−1 . Experiments were conducted using the pin-on-disc geometry, at room temperature in air (RH=13%). (a–e) SEM micrographs (backscattered electron contrast (BSE)) of worn TiN surface morphologies from Regimes I–IV (see text for explanations): the arrow indicating TiN disc sliding direction relative to HSS pin applies to all micrographs in a–e; the continuous longitudinal features perpendicular to the sliding direction are the polishing marks left over from the original grinding process.

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Fig. 2. (a) EDS analysis of transferred wear debris (dark granular plastically smeared region in Fig. 1a) on the surface of TiN worn (against HSS) under tribochemical polishing conditions (Regime I, Fig. 1): the debris is rich in Ti and O, indicating oxidation of the TiN, with Fe, V, W, Cr and Mo contributions from the HSS. (b) EDS analysis of Fe- and O-rich debris (oxidized HSS) transferred to the coating under mild (Regime II, Fig. 1) conditions and compacted (white/light grey, granular regions in Fig. 1b and c) between worn TiN asperities (dark grey flat regions, Fig. 1b and c). (c) EDS analysis of as-deposited, unworn TiN coating. (d) EDS analysis of patches of oxidized HSS debris (Fig. 1d, light grey, and approximately 20–30 ␮m in size) that have been transferred to the worn coating surface under rapid TiN wear conditions (Regime III, Fig. 1). (e) EDS analysis of debris that has been transferred to the TiN coating during severe wear of the HSS pin specimen (Fig. 1e): the transferred material is predominantly metallic HSS with little evidence of oxidation.

3.2. Effect of humidity on tribochemical reactions in Regime I wear The TiN coating is considerably harder than the HSS pin material, so it is somewhat counter-intuitive to see rapid TiN removal at low contact loads and speeds in Regime I. The reverse effect would be expected, as observed in Regime II, where the softer HSS pin is worn preferentially. Fig. 3a compares wear rates for the TiN coating and HSS pin in laboratory air (13–15% RH, 25◦ C) and high humidity (91–93% RH, 25◦ C) conditions at 20 N and 0.42 m s−1 . At 13–15%

RH, damage to the TiN coating is minimal (Fig. 3b) and is typical of Regime II wear where low TiN wear rates prevail, accompanied by high HSS pin wear arising from abrasion against the harder TiN surface asperities. When the humidity is increased to 91–93% RH, tribochemical wear dominates, producing a mirror polished wear track and fine Ti-rich oxide debris that is white in colour to the naked eye. A higher TiN wear rate ensues and the coating is rapidly worn through to the HSS substrate (Fig. 3c). Therefore, Regime I extends to higher loads and speeds when the RH is increased. Consequently, a marked reduction in HSS wear arises from

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Fig. 3. (a) Effect of relative humidity (RH) on TiN coating wear against HSS pins: a transition from low TiN wear and high HSS pin wear (Regime II, 13–15% RH) to rapid tribochemical degradation and TiN coating removal (Regime I) arises under high humidity conditions (91–93% RH). (b) SEM (backscattered electron contrast) of the worn surface of TiN coating (against HSS) under Regime II wear (20 N, 0.42 m s−1 , 13–15% RH): damage to the TiN coating is minimal and is accompanied by oxidized HSS pin transfer (white patches) arising from abrasion against the TiN surface asperities. (c) SEM (backscattered electron contrast) of the worn surface of TiN coating (against HSS) produced by introducing highly humid conditions (Regime I wear, 20 N, 0.42 m s−1 , 91–93% RH): as a result of tribochemical wear, TiN is polished to a very smooth surface finish, and in the middle of the wear track, the coating is worn through to the HSS substrate (white regions).

reduced abrasion against the TiN, which has been tribochemically polished and removed (Fig. 3c). In summary, increasing humidity assists TiN oxidation and wear and this is a phenomenon common to the wear of many ceramics in aqueous environments or high ambient humidities where there is moisture adsorbed on the surfaces in sliding contact [6,14,15]. Water can have an ambiguous effect on ceramic wear [14] by either contributing towards high wear by accelerating crack growth and microfracture, or reducing wear by the formation of extremely smooth ‘solid–lubricant tribo-layers’ formed by tribochemical reactions. For example, nitride ceramics such as Si3 N4 undergo a hydrolysis reaction with water and oxygen to produce a complex silicon hydrox-

ide by-product [11,14–16]. Under very mild wear conditions, a thin Si(OH)x :SiO2 tribo-layer film is created on the surfaces in contact, producing extremely smooth surfaces, which can act as hydrodynamic lubricating layers. A similar process is observed for wear of Al2 O3 in water to produce an aluminium hydroxide rich tribo-layer [14]. Whilst polishing of TiN to a smooth finish is observed for M2 HSS versus TiN under humid conditions (Fig. 3c), wear is substantial and this is most likely accelerated by the presence of very hard W, V, Cr and Mo carbides in the AISI M2 HSS pin. The abrasive action of carbides is likely to contribute towards microabrasion of the TiN and removal of the products of tribochemical wear from the contact surfaces. The effect of abrasion by hard second phase particles in the

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Fig. 4. (a) Wear rates of TiN coatings and pins for sliding contact against AISI 1020 mild steel and A356 Al–15% SiC pin materials under low (15% RH) and high (96% RH) humidity. (b) SEM (backscattered electron contrast) showing Regime II-type wear on TiN coating after wear against a mild steel pin: there is minimal damage to the coating accompanied by transfer of steel oxides (white regions) to the TiN. (c) SEM (backscattered electron contrast) of a TiN wear track after wear against mild steel at high humidity (5 N, 0.25 m s−1 , 96% RH): damage to the coating surface is superficial (only a small polishing effect on TiN asperities can be seen and steel substrate is not exposed) by comparison with that inflicted by the other pin materials (e.g. compare with Fig. 3b); thin stable tribo-layer patches are present in the wear track; these have an atomic number contrast similar to the underlying TiN coating, indicating that they are comprised mostly of Ti; some regions of the patches have higher atomic number white inclusions in them, indicating that steel pin material has been mixed into the tribo-layer, albeit on a small scale. (d) EDS analysis of the tribo-layers, shown in Fig. 4c, reveals that they contain Ti, O and Fe — indicative of tribochemical oxidation of TiN and some steel pin transfer. (e) Worn A356 Al–15% SiC pin surface showing SiC particles (dark angular constituents) standing proud of the surface against which wear debris has accumulated (5 N, 0.25 m s−1 , 96% RH): the particles are load-bearing as indicated by their smooth polished finish, and thus, act as the abrasive agent in contact with the TiN.

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pin material on the stability of tribo-layers is discussed in Section 3.3. 3.3. Effect of counterface material on the stability of tribo-layers in Regime I wear When the HSS steel counterface pin material is changed to a softer material such as mild steel (HV1 kg = 300 ± 23 kg mm−2 ), under low-humidity (15% RH) conditions at low contact loads and speeds (Fig. 4a), the TiN wear mechanism reverts back to Regime II-type wear, i.e. minimal damage to the coating, high steel wear and transfer of steel oxides to the TiN (Fig. 4b). Increasing the humidity increases the TiN wear to the extent that tribochemical polishing of the wear track occurs and mild steel wear is lowered. However, damage to the coating surface is superficial in comparison with that inflicted by the other pin materials (Fig. 4c). Thin tribo-layer patches are present in the wear track. These have low atomic number contrast under BSE imaging and EDS analysis (Fig. 4d) reveals that they are rich in Ti, Fe and O — indicative of tribochemical oxidation of TiN. Therefore, the absence of any hard abrasives in the mild steel contributes towards the formation of a smooth stable tribo-layer, e.g. Ti(OH)x :TiO2 , similar to that seen for Si3 N4 wear at high humidities [14]. When a pin material having a bulk hardness lower than mild steel is used, but which contains hard abrasive particles, the pin, i.e. A356 Al–15% SiC with a bulk hardness, HV1 kg = 117 ± 16 kg mm−2 , produces rapid Regime I-type tribochemical wear of TiN is maintained at low and high humidities (Fig. 4a) with accelerated wear at 96% RH. The worn pin surface reveals SiC particles standing proud of the surface against which wear debris has accumulated (Fig. 4e). The particles are load-bearing as indicated by their smooth polished finish, and thus, act as the major abrasive agent in contact with the TiN. The bulk hardness of the pin material is therefore not as important to the wear process in Regime I, given that the hard SiC particles act as microabrasives against the TiN, leading to greater microabrasion or microcutting component as opposed to pure tribochemical wear and stable tribo-layer formation. Again, as in the case of M2 HSS wear against TiN, there is no evidence for the formation of a smooth tribo-layer due to the microabrasive effect.

4. Conclusions 1. TiN coatings are prone to rapid degradation by tribochemical wear at low sliding speeds and contact loads. The TiN is worn away to a characteristic mirror finish by an AISI M2 steel counterface that is softer than the coating. 2. Tribochemical wear of TiN against AISI M2 steel is diminished when contact loads and sliding speeds are raised under a constant humidity level. Increased transfer of Fe-rich HSS pin material to the TiN results and can

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contribute to negative TiN wear rates or mass gains. The transferred material causing the mass gain consists of oxidized Fe, W, Mo, V, Cr from the HSS pin with minimal presence of Ti. Further increase in load and/or velocity can cause severe wear of AISI M2 steel and TiN due to frictional heating of contact surfaces. TiN surfaces exhibit heavily deformed steel layers predominantly metallic in nature. 3. The oxidation and hydrolysis associated with tribochemical wear of TiN against AISI M2 steel is accelerated under conditions of high RH. Coating wear rates are increased and tribochemical wear seen at low loads and speeds is extended to higher loads and sliding speeds. 4. When TiN is worn against mild steel, smooth, stable tribo-layer patches are formed in the wear track. These contain oxidation products of TiN and some material transferred from the steel pin. These layers reduce the rate of tribochemical wear of TiN. 5. The presence of hard abrasive particles (e.g. SiC) in the counterface accelerate tribochemical wear effects by microabrasion of the TiN, particularly at high RHs. Acknowledgements Financial support from NSERC Canada and a PreCompetitive Industrial Research Consortium on Hard Coatings under the auspices of Materials and Manufacturing Ontario (MMO) comprised of Balzers Tool Coating Inc., Exactatherm Ltd., and Multi-Arc Inc. is gratefully acknowledged. The authors would like to thank Mr. P. Lidster of Exactatherm, Mr. J. Taylor of Multi-Arc Inc. and Mr. F. Teeter of Balzers Tool Coating Inc. for provision of M2 HSS and TiN samples. References [1] I.L. Singer, S. Fayeulle, P.D. Ehni, Wear 149 (1991) 375–394. [2] K.-H. Habig, G. Meier zu Kocker, J. Phys. D: Appl. Phys. 25 (1992) A307–A312. [3] N.C. Welsh, Philos. Trans. R. Soc. A 257 (1965) 31–50. [4] S.C. Lim, M.F. Ashby, Acta Metall. 35 (1987) 1–24. [5] H. Kato, T.S. Eyre, B. Ralph, Acta Metall. Mater. 42 (1994) 1703– 1713. [6] S.M. Hsu, D.S. Lim, Y.S. Wang, R.G. Munro, Lubr. Eng. 47 (1991) 49. [7] J. Zhang, A.T. Alpas, Acta Mater. 45 (1997) 513–528. [8] S. Wilson, A.T. Alpas, Wear 212 (1997) 41–49. [9] S. Wilson, A.T. Alpas, Wear mechanism maps for TiN coated high speed steel, Surf. Coatings Technol. 120–121 (1999) 519–527. [10] K. Kato, Surf. Coatings Technol. 76/77 (1995) 469–474. [11] S. Wilson, A.T. Alpas, Surf. Coatings Technol. 108/109 (1998) 369– 376. [12] Y. Wang, T. Lei, M. Yan, C. Gao, J. Phys. D: Appl. Phys. 25 (1992) A165–A169. [13] S. Wilson, A.T. Alpas, Wear 225–227 (1999) 440–449. [14] J.K. Lancaster, Y.A.-H. Mashal, A.G. Atkins, J. Phys. D: Appl. Phys. 25 (1992) A205–A211. [15] S.R. Hah, T.E. Fischer, J. Electrochem. Soc. 145 (1998) 1708–1714. [16] Y.G. Gogotsi, M. Yoshimura, MRS Bull., Volume 19, No. 10 (1994) 39–45.