An investigation on sliding wear behavior of PVD coatings

An investigation on sliding wear behavior of PVD coatings

Tribology International 43 (2010) 2196–2202 Contents lists available at ScienceDirect Tribology International journal homepage:

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Tribology International 43 (2010) 2196–2202

Contents lists available at ScienceDirect

Tribology International journal homepage:

An investigation on sliding wear behavior of PVD coatings ~ M.Y.P. Costa n, M.O.H. Cioffi, H.J.C. Voorwald, V.A. Guimaraes ´, UNESP – Univ Estadual Paulista, Materials and Technology Department, Fatigue and Aeronautic Materials Research Group, Av. Ariberto Faculdade de Engenharia de Guaratingueta ´, Cep: 12516 410, S.P. Brazil Pereira da Cunha, 333, Guaratingueta

a r t i c l e in f o

a b s t r a c t

Article history: Received 10 February 2010 Received in revised form 5 July 2010 Accepted 5 July 2010 Available online 15 July 2010

Ti–6Al–4V alloy rubbing against aluminum–bronze 630 was evaluated in this work. High velocity oxygen fuel (HVOF) WC–10%Co–4%Cr thermal sprayed and TiN, CrN and DLC physical vapor deposition (PVD) coatings were applied to increase titanium substrate wear resistance. Pin-on-disk tests were performed with a normal force of 5 N and at a speed of 0.5 m/s, with a quantitative comparison between the five conditions studied. Results showed higher wear resistance for Ti–6Al–4V alloy DLC coated and aluminum–bronze 630 tribological pair and that the presence of graphite carbon structure acting as solid lubricant was the main wear preventing mechanism. & 2010 Elsevier Ltd. All rights reserved.

Keywords: Sliding wear Coating Titanium alloy Aluminum bronze

1. Introduction Titanium alloys have been used in aircraft components, such as landing gears, due to their high strength-to-weight ratio, stiffness and corrosion resistance [1,2]. Considering the titanium hexagonal close-packed crystalline structure in addition to the low shear resistance, a specific surface treatment is required in mechanical components subjected to friction [3,4]. High velocity oxygen fuel (HVOF) and physical vapor deposition (PVD) are environmental friendly technologies developed to protect against corrosion and wear, often used as a substitute of an electroplating processes [5–8]. The HVOF process is an improvement of a thermal spray one. The main advance is to deposit particles without overheating, in order to avoid carbides decomposition and to increase the resistance of the coatings to wear. This process presents twice the plasma thermal spray velocity and is producing denser coatings with less oxide content [9–11]. The resistance to wear of thermal coatings has been evaluated recently in several papers. Wank et al. [12] studied six different carbide compositions and hard chromium coating, concluding that the tribological system will determine the wear performance. In dry abrasive wear, the WC–10%Co–4%Cr presented higher wear resistance than hard chromium and other HVOF coatings compositions. On the other hand, the resistance to wear hard chromium coating was the greatest during oscillating wear tests [12].


Corresponding author. Tel.: +55 1231232865; fax: + 55 1231232852. E-mail address: [email protected] (M.Y.P. Costa).

0301-679X/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2010.07.002

The powder size distribution also modifies HVOF wear behavior. Picas et al. [13] investigated powder size distribution of three different CrC–NiCr powder agglomerates. It was found that the HVOF coatings deposited with use of the lowest feedstock powder size presented the best wear resistance, with superior performance in comparison to hard chrome coating. This behavior was related to reduction of particles pull out, which is involved in the abrasive wear mechanism [13]. Likewise, a number of investigations of tribological properties of the systems composed of PVD coatings increased recently due to their attractive mechanical properties as low environmental impact, low friction coefficient, low wear rate and relatively high hardness up to 2000 HV [14,15]. The PVD coatings are classified into two main groups: one composed of traditional wear resistant coatings from the nitrides and carbides of titanium and chromium (as, f.ex., TiN, TiCN, TiAlN and CrN), which are suitable to avoid galling problems and the other one composed solid lubricating coatings, characteristic by of a low coefficient of friction. These coatings are most frequently based on carbon or MoS2 [8,16]. Studies on PVD coatings provide a useful information on mechanical properties of different substrates and how they can be modified by coatings’ deposition. Sundaram [17] observed a superior wear and fatigue resistance of DLC coated AISI 4340 steel in comparison with a chromium coated one. In addition, an excellent adhesion of DLC film to the steel surface was achieved. Nolan et al. [18] investigated the sliding wear behavior of TiN thin film deposited by PVD and plasma nitriding process on Ti– 6Al–4V alloy substrate. It turned out that TiN film provided lower resistance to sliding wear under applied normal load as a result of a greater rigidity of the underlying substrate. The increase in the substrate hardness provided by plasma nitriding process

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enhanced sliding wear behavior of the Ti6Al4V alloy under all normal loads studied in the work. Although properties of various PVD coatings have been intensively researched, the PVD coatings represent an aspect in a given tribological system, which include surface treatment and all wear parts and mechanical components involved. Therefore, the whole tribological system must be studied thoroughly before choosing an alternative technology [8,16]. In the present paper, the friction and wear behavior of Ti–6Al– 4V/aluminum bronze 630 rubbing pairs were investigated. The aluminum bronze alloy was chosen because it is the main substrate for bearings in landing gears. The main purpose is to evaluate the influence of the WC-10%Co–4%Cr HVOF coating as well as of the TiN, CrN and DLC PVD coatings on the Ti6Al4V alloy onto the performance of Ti–6Al–4V/aluminum bronze 630 friction couple.


was evaluated by SEM. Several specimens blasted with aluminum oxide were observed with use of the SEM technique as well. 2.3. PVD coatings Three different coatings were applied by Bodycote Brasimet Co. with PVD process. TiN and CrN coatings were deposited with a cathodic arc configuration for 7 h, with deposition temperature of 450 1C, coating thickness observed was about 4 mm and surface roughness obtained was Ra ¼1.0670.04 and 1.0870.15 mm, respectively. WC:H (DLC) coatings were applied with magnetron sputtering system for 10 h, with deposition temperature of 180 1C. Previously to the deposition of DLC coating with 2.4 mm thickness, a chromium interlayer with 2 mm was applied to increase adhesion between substrate/coating. Final surface roughness was Ra ¼0.5770.13 mm. Coatings microhardness were provided by company with the following values: TiN: 2500 Hv, CrN: 2000 Hv and DLC: 1200 Hv.

2. Experimental procedures 2.4. Wear tests 2.1. Materials The substrate used was Ti–6Al–4V alloy with the following chemical composition 88.76% Ti, 6.29% Al, 4.95% V obtained by energy dispersive X-ray spectroscopy (EDS). The alloy was characteristic of a metallurgical duplex structure with a 30% volume fraction of equiaxial primary a phase and a 70% fraction of (a + b) eutectic of a lamellar microstructure. The surface of the titanium alloy specimens was ground to a roughness of Ra ¼0.9870.11 mm, cut-off 0.8 mm. Main mechanical properties were as follows: ultimate tensile strength of 1270 MPa, microhardness 380728 HV0.3 for primary a and 354 74 HV0.3 for lamellar a + b structure, in the annealed condition. The counterbody was machined from an aluminum bronze 630 bar with the following chemical composition: 9.48 wt%Al, 69.63 wt%Cu, 0.0037 wt%Ag, 6.93 wt%Fe, 4.72 wt%Ni, 0.019 wt%Co and 1.5 wt%Mg, with ultimate tensile strength of 517 MPa and microhardness 312722 HV0.3 tungsten carbide coating.

2.2. WC–10 wt%Co–4 wt%Cr coating

Dry sliding wear tests were performed on Ti–6Al–4V/aluminum–bronze 630 tribological pair with and without coatings on the titanium substrate, using a pin-on-disk equipment according to ASTM G99 Standard [20]. The pin and disk specimens were prepared according to Fig. 1. Five tribological pairs were evaluated :


Ti–6Al–4V/ aluminum–bronze 630; Ti–6Al–4V with WC–10%Co–4%Cr/aluminum–bronze 630; Ti–6Al–4V with DLC/aluminum–bronze 630; Ti–6Al–4V with TiN/aluminum–bronze 630; Ti–6Al–4V with CrN/aluminum–bronze 630.

Tribological characterization was conducted with at normal force of 5 N and a speed of 0.5 m/s, and results of the five conditions were compared quantitatively in the same condition. Wear volume was measured after a total distance of 3 km by specimens measuring loss. Specific wear rates values were obtained according to [21] k¼

The WC–10 wt%Co–4 wt%Cr coating was deposited by means of HVOF spray method from WC powder with 10 wt%Co and 4 wt%Cr with use of a Praxair Surface Technologies system. The average thickness of the coating was equal to 150 mm, the roughness of the coating Ra was 1.6270.35 mm and the microhardness was 1485 7120 HV0.3. Prior to the tungsten carbide thermal spray coating process, specimens were blasted with aluminum oxide mesh 60 to enhance adhesion. Praxair performed the bond strength tests, according to the ASTM C633 Standard [19]. Minimum adhesion strength of 68.9 MPa between the coating and the substrate has been measured.The following process parameters were employed:


process: TAFA JP 5000; oxygen pressure: 938–1007 kPa; fuel: kerosene; fuel pressure: 786–855 kPa; powder supply pressure: 21–41 kPa; spraing distance: 300 mm; maximum substrate temperature during spraying: 170 1C.

After deposition the specimen with the WC–10%Co–4%Cr thermal spray coating was at first polished and its morphology



where k is the specific wear rate [m3/Nm], V is the wear volume [m3], W is the normal load [N] and L is the sliding distance [m]. In order to elucidate the wear process and consequently the fundamental mechanisms, the wear tracks on the disks were examined by scanning electron microscopy model LEO 1450VP equipped with an energy-dispersive spectroscopy system (EDS).

3. Results 3.1. Ti–6Al–4V and aluminum–bronze 630 wear behavior Fig. 2 presents material loss during wear tests. The wear volume of the disk from theTi–6Al–4V alloy was higher in comparison to that of the aluminum–bronze 630, despite the higher mechanical resistance of titanium alloys in comparison with the pin material. On landing gears, aluminum–bronze-630 is used due to the low adhesive strength between copper and iron and because wear mechanism in this tribological pair occurs mainly in bearings and sleeves [21]. Aluminum–bronze 630 and Ti–6Al–4V alloy presented a specific wear rate of 1.41  10  13 and 2.01  10  13, respectively, classifying the tribological couple as one of low wear resistance [21].


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Fig. 1. Wear test specimens.

3.2. Ti–6Al–4V alloy WC–10%Co–4%Cr thermal spray coated and aluminum–bronze 630 wearing behavior

Fig. 2. Volume loss for aluminum-bronze 630 and Ti–6Al–4V alloy.

Wearing track in Ti–6Al–4V alloy disk is presented in Fig. 3 in two distinct regions. Bright areas where material transference occurs are indicated in Fig. 3b and d. EDS analysis showed the composition from both bright and dark areas. Dark area composition was 92.41%Ti, 4.10%Al and 3.49%V (wt%) and bright area composition was 19.38%Ti, 5.78%Al, 0.83%V, 2.58%O, 0.70%Mn, 3.71%Fe, 4.01%Ni and 62.99%Cu (wt%). Results showed the adhesive mechanism where transference from pin, weaker material, into Ti–6Al–4V disk occurred. However, Fig. 3a and c confirm that material was also pulled out from the disk, which was the main wear mechanism and the reason of the higher wear volume loss of the Ti–6Al–4V alloy disk during wear tests. Titanium alloys have a natural oxide film with 5 nm thickness [22]. During dry sliding wear, this film is easily removed by spalling or microfragmentation and does not protect the subsurface layers [23]. This layer is formed and continuously removed during wear test in contact with the atmosphere [4,22,23].

Fig. 4 presents material loss during sliding wear between Ti– 6Al–4V alloy WC–10%Co–4%Cr thermal spray coated disk and aluminum–bronze 630 pin. Results indicate a severe pin wear, with volume loss 32 times higher when compared to the disk. Hardness difference between both materials and hard WC particles were the main reason for the pin abrasive wear mechanism [24,25]. Wearing track of Ti–6Al–4V alloy WC–10%Co–4%Cr thermal spray coated is shown in Fig. 5. For 5 N load, there is no evidence of deformation or grooves due to pin contact, as it is observed in Fig. 5a. During scanning electron microscopy (SEM) analysis, a marker was used as a technique to locate the region where the copper was visually observed, aiming to find wearing track. Considering the coating polishing before the wear test, pin adherence on coating is evidenced by backscattered electron image, as indicated in Fig. 5b. The bright and dark area composition were, respectively, 68.72%W, 11.92%C, 14.29%Co, 5.07%Cr (wt%) and 10.59%C, 1.91%Cr, 3.99%Co, 17.32%W, 10.76%Al, 46.01%Cu, 1.23%Mg, 4.29%Ni, 3.74%Fe, 0.17%Ag, obtained by EDS analysis. The backscattered electron image confirms the adhesive mechanism of pin transference to disk by contrast based on the atomic number, as indicated in Fig. 5c. The distinct microscopic composition variations are Tungsten (74—atomic number), dark area, and Copper (29), bright area. The coating integrity is confirmed by the fact that plastic deformation or microfragmentation in WC–10%Co–4%Cr in Fig. 5c was not observed. Besides, Fig. 5d indicates no morphological difference between coating before and after the wear test [24,25]. The predominant tribological wear mechanism was the pin abrasion, with few evidence of adhesive wear. It is an important remark that both mechanisms never act alone [21].

3.3. Ti–6Al–4V alloy PVD coated and aluminum–bronze 630 wear behavior Fig. 6 shows material loss during sliding wear with Ti–6Al–4V alloy DLC/TiN/CrN coated by PVD rubbing against

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Fig. 3. Wear track on the Ti–6Al–4V alloy: (a) 200  —secondary electrons image (SE)–region 1; (b) 200  —backscattered electrons image (BSE)—region 1; (c) 200  –SE– region 2; and (d) 200  –BSE–region 2.

Fig. 4. Volume loss for aluminum–bronze 630 and Ti–6Al–4V alloy WC–10%Co– 4%Cr thermal spray coated.

aluminum–bronze 630. Pin abrasive wear was observed when counter bodies were TiN and CrN coatings. The tribological pair DLC and pin aluminum–bronze presented wear resistance in both sliding materials. The worn surfaces of PVD coatings are shown in Fig. 7. Wear track of TiN and CrN coatings are presented in Fig. 7a and c,

respectively, with similar wear mechanisms. When high hardness coatings are rubbing against low hardness counterpart with low contact load, it is possible to observe the transfer of pin material to disk and coating minimum damage [26]. Regions with strong evidence of adhesive wear mechanism are shown in Fig. 7b and d, indicated by arrows. The wear adhesive mechanism is confirmed by EDS analysis in bright region of CrN wearing track, 74.91%Cu, 5.13%Ni, 6.43%Al, 2.02Cr, 0.67%Ti, 0.82%Mn, 3.96%Fe and 6.07%O (wt%). The dark composition was 1.69%N, 95.9%Cr, 1.83%Ti and 0.56%Cu., showing that material transference from pin, the weakest material, into CrN coating occurs. The absence of any hard abrasives in the tribological pair contributes towards tribochemical oxidation, indicated by the oxygen presence in the bright area. The higher concentration of Cu in the disk then previous pairs analyzed and chromium existence in regions where pin adhere to the coating, are evidences that confirm the adhesive wear mechanism [21,26]. TiN coating wear track also presents a higher copper concentration, 76.31 wt%, when compared with HVOF and base material disks. In addition, 3.93%O and 2.64%Ti (wt%) observed in the bright region confirm the similar wear mechanism between CrN and TiN rubbing against aluminum–bronze-630. A distinct wear behavior in DLC coatings can be demonstrated by both mass loss graphic in Fig. 6 and image analysis in Fig. 7e and f. Low mass loss in the tribological pair was achieved and backscattering images showed that the pin track did not change the polishing disk direction, as indicated in Fig. 7f. The hydrogen presence in coating composition (WC:H), which allow graphite


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Fig. 5. Wear track on the Ti–6Al–4V alloy WC–10%Co–4%Cr thermal spray coated: (a) 100  –SE; (b) 100  –BSE; (c) 500  –BSE–detail in (b); and (d) surface polished before wear test.

materials good wear resistance materials would exhibit specific wear rate at about 10  16 [m3/Nm] and materials particularly not wear resistance with rates at about 10  14 [m3/Nm] or higher. Results indicated that aluminum–bronze 630 presents the best wear resistance when rubbing against Ti–6Al–4V alloy DLC coated.

4. Conclusions

Fig. 6. Volume loss for aluminum–bronze 630 and Ti–6Al–4V alloy PVD coated.

formation acting as solid lubrication, was the mechanism responsible for the DLC high wear resistance and low friction [27,28]. Specific wear rate for all tribological pairs are presented in Table 1. Wear resistance was established according to Stachowiak and Batchelor [21] who considered that

Experimental results showed that Ti–6Al–4V alloy rubbing against aluminum–bronze 630 was not good enough tribological couple. Pulling out of titanium atoms by means of oxide film spalling or microfragmentation were the main mechanisms of titanium wear. All the proposed coatings protected theTi–6Al–4V alloy against wear. HVOF thermal spray coating did not present deformation or microfragmentation after the wear test. Severe abrasive wear of the pin from the aluminum bronze 630 and adhesive wear of the WC–10%Co–4%Cr HVOF coating was observed. TiN and CrN PVD coatings exhibit tribochemical oxidation with strong adhesive wear mechanism, which increase the mass loss of the pin. The carbon-based DLC acting as solid lubricant, is responsible for a high resistance of the DLC coated Ti–6Al–4V alloy against aluminum–bronze 630. Considering all coatings conditions investigated in this work, the DLC coated Ti–6Al–4V alloy rubbing against

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Fig. 7. Wear track on the Ti–6Al–4V alloy PVD coated (a)TiN–200X–SE; (b) TiN–200X–BSE; (c) CrN–200X–SE; (d) CrN–200X–BSE; (e) DLC–200X–SE; (f) DLC–200X–BSE.

Table 1 Specific wear rate for Ti–6Al–4V and aluminum–bronze 630 for base material and all coating conditions. Specific wear rate [m3/N m]

Base material X pin HVOF X pin TiN X pin CrN X pin DLC X pin



1.41  10  13 4.08  10  13 2.71  10  13 2.87  10  13 3.66  10  16

2.01  10  13 1.25  10  14 2.96  10  16 1.04  10  15 7.04  10  15

aluminum-bronze 630 was the best alternative characteristic of low specific wear rate of the both materials composing the investigated tribological couple.

Acknowledgements The authors are grateful to the research supporters CAPES, FAPESP through the projects numbers 2006/03570-9 and 2006/ 02121-6 and CNPq through the projects numbers 304155/2006-4, 470074/2006-0, 427570/2006-4 and 300233/2006-0. We would like to extend our thanks to Bodycote Brasimet Co. for PVD depositions.

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