Improving tribological properties of Ti6Al4V alloy with duplex surface treatment

Improving tribological properties of Ti6Al4V alloy with duplex surface treatment

Surface & Coatings Technology 205 (2010) 320–324 Contents lists available at ScienceDirect Surface & Coatings Technology j o u r n a l h o m e p a g...

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Surface & Coatings Technology 205 (2010) 320–324

Contents lists available at ScienceDirect

Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t

Improving tribological properties of Ti6Al4V alloy with duplex surface treatment A.F. Yetim a,⁎, A. Celik b, A. Alsaran b a b

Ataturk University, Engineering Faculty, Metallurgical and Materials Engineering Department, 25240-Erzurum, Turkey Ataturk University, Engineering Faculty, Mechanical Engineering Department, 25240-Erzurum, Turkey

a r t i c l e

i n f o

Article history: Received 16 March 2010 Accepted in revised form 22 June 2010 Available online 30 June 2010 Keywords: Ti-DLC Ti6Al4V Wear Nitriding CFUBMS

a b s t r a c t Ti-doped diamond like carbon films were deposited on both untreated and plasma nitrided Ti6Al4V alloy using Closed Field Unbalanced Magnetron Sputtering (CFUMBS) method and their tribological properties were evaluated by conducting sliding wear conditions. The influence of the nitrided layer on tribological behavior of Ti-DLC films was studied by means of XRD, SEM, scratch tester, microhardness tester and pin-ondisc tribotester. The microhardness results pointed out that the duplex treatment dramatically increased the surface hardness and reduced the plastic deformation of the alloy. Wear tests showed that Ti-DLC coatings on both untreated and nitrided surfaces caused a reduction in the coefficient of friction. The reason of the reduction in the coefficient of friction was found to be the formation of transfer film between the sliding surfaces. Wear rates demonstrated that wear resistance of duplex treated (Ti-DLC coating after nitriding) Ti6Al4V alloy was significantly improved. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Titanium alloys are widely used for various industrial fields especially biomedical and aerospace engineering because they have several beneficial properties such as high strength to weight ratio, low density, good fatigue properties, high corrosion resistance and biocompatibility [1]. In contrast to these advantages, they have relatively poor tribological properties (i.e high and unstable coefficient of friction, low abrasive and adhesive wear resistance). A considerable amount of effort has been made to overcome these problems and improve the service life of components by applying several surface treatments such as plasma and laser nitriding, plasma spray coating [2–5]. The past two decades have seen many developments in the field of surface engineering in order to achieve longer service life, and duplex surface treatment has been developed as a novel method. Duplex surface treatment involves the sequential application of two or more established surface treatments to produce a surface with combined advantages which are unobtainable through any individual surface treatment [6]. PVD treatment of pre-nitrided samples is a commonly used duplex treatment [7,8]. As well known, plasma nitriding is an effective surface modification technique to enhance hardness, fatigue and wear resistance of steels [9,10]. Recently, plasma nitriding has also applied to the metals used as biomaterials such as 316L stainless steel and Ti6Al4V alloy. It was reported that the nitriding parameters were effective on the surface properties of bio-metals [11–14]. It is

known that process parameters have an influence on modified layer properties of nitrided materials. It was observed that even if plasma nitriding increased hardness and wear resistance of biomaterials as process time and temperature increased, it may also cause to decrease their corrosion resistance and/or fatigue life. Therefore, nitriding parameters limit improvement of tribological properties of biomaterials. So, it is necessary to improve wear resistance without reduction of corrosion and biocompatibility properties of the materials. For that reason, to make a proper coating with corrosion resistance provides optimum conditions on the nitrided surface at which the parameters giving the best results in terms of wear. DLC films are the most suitable coatings for bio-medical applications because these films have extraordinary properties such as low coefficient of friction, good wear resistance and hardness, excellent corrosion resistance and biocompatibility [15–18]. Although DLC films have numerous advantages, their adhesion properties are inadequate and when they are deposited on a soft substrate, like Ti6Al4V alloy, plastic deformation will occur with high loads which it will lead to catastrophic premature failure because soft material may not be able to provide adequate support for the hard DLC film [14,19]. Therefore, it was assumed that hardening of the surface of the soft material before DLC coating will be effective for tribological properties. For this aim, Ti6Al4V alloy was coated with Ti-doped DLC thin film after plasma nitriding treatment and its mechanical and tribological properties were investigated. 2. Experimental details

⁎ Corresponding author. Tel.: + 90 442 2314860; fax: + 90 442 2360957. E-mail address: [email protected] (A.F. Yetim). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.06.048

Ti6Al4V (Grade5) specimens with a surface area of 200 mm2 substrate were cut from cylindrical bars. The specimens were

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grinded by 220–1200 mesh emery-papers, and then polished with alumina powder with 1 μm grain size. After cleaning with alcohol, the specimens were placed into the plasma nitriding unit which was cold wall reactor. Then, plasma nitriding chamber pressure was evacuated to 2.5 Pa. The temperature was monitored by a thermocouple connected to the specimen through the cathode. Prior to the process, the specimens were subjected to cleaning by hydrogen sputtering for 15 min under a voltage of 500 V and a pressure of 5 × 102 Pa to remove surface contaminations. Afterwards, plasma nitriding was performed at 750 °C for 2 h at a gas mixture of 25%Ar + 75%N2. An auxiliary heating was not used in the nitridation equipment and mean residence time of gas mixture was process time plus 45 min. The Ti-DLC film was carried out using Closed Field Unbalanced Magnetron Sputtering (CFUBMS) system manufactured by Teer Coatings Ltd. Before DLC deposition, Ti interlayer was deposited at 6 A current, 250 V bias and 0.4 Pa pressure for 5 min to improve adhesion between the film and the substrate. The thickness of Ti interlayer was about 100–150 nm. Then, Ti-DLC thin film was deposited at 6 A current, 100 V and 2.5 μs duty time for 45 min on the substrates. Three pure carbon and titanium targets were used as cathode. After the applied surface treatments, Rigaku X-Ray diffractometer operated at 30 kV and 30 mA with CuKα radiation was used for XRD analysis. The morphology of the coated and nitrided layers, scratched and worn surfaces was examined using a scanning electron microscope (SEM) Jeol 6400. The surface roughnesses of untreated and treated samples were measured by a profilometer Mitutuyo. Adhesion tests were performed by using CSM Revetester with a 200 μm tip radius Rockwell-C diamond indenter, under loading rate of a 100 N/min, a sliding speed of 10 mm/min and scratch length of 5–8 mm at standard atmosphere conditions (RH %45–55). The wear tests were carried out on Teer POD-2 pin-on-disc tester, using a 5 mm diameter WC-Co ball as the pin. The friction force was monitored continuously by using a force transducer. The unlubricated wear tests with a sliding distance of 141 m were carried out at room temperature (≈ 18 °C) and a relative humidity of about 50%, a sliding speed of 0.078 ms− 1, normal load of 10 N and a wear track diameter of 10 mm. To calculate the wear volume, the profiles were recorded before and after the wear tests by a profilometer Mitutuyo. Then, the wear volume was calculated from superimposed profiles.


columnar structure on the top of the sample, diffusion layer beneath the Ti-DLC film and finally, α-Ti region enriched with nitrogen. Columnar structure of DLC films and granular nature of nitride phases increased the surface roughness of both DLC film coated and plasma nitrided samples. While mean surface roughness (Ra) of the untreated Ti6Al4V alloy was between 0.06 and 0.07 μm, it increased up to 0.3, 0.08, 0.26 μm after plasma nitriding, Ti-DLC film coating and duplex treatment, respectively. The XRD patterns of untreated and treated Ti6Al4V alloys are given in Fig. 2. It is obvious that the untreated sample has the

3. Results and discussion 3.1. Characterization of the plasma nitrided layer and Ti-DLC film Visual observations showed that while metallic grey appearance of untreated Ti6Al4V alloy changed into golden in color indicating titanium nitride phase after the nitriding treatment, TiDLC films were opaque and dark black in color. Fig. 1 shows the cross-section SEM micrographs of the plasma nitrided, Ti-DLC coated and duplex treated samples. The cross-section morphology of plasma nitrided Ti6Al4V alloy consisted of compound and diffusion layers (Fig. 1a). While the compound layer had a thickness of about 2–3 μm, the diffusion layer had a thickness of about 100 μm and it contained ε-Ti2N, δ-TiN and α-Ti enriched with nitrogen phases. Fig. 1b displays the cross-sectional SEM micrograph of Ti-DLC thin film deposited on the silicon wafer substrate. It can be seen that Ti-DLC film thickness is about 2 μm and its morphology exhibits uniform, free of porosity, dense, but columnar structure. The PVD film is dense and does not exhibit droplet defects typical for unfiltered cathodic arc films. Fig. 1c shows the cross-section morphology of duplex treated Ti6Al4V alloy. As seen in Fig. 1c, it was observed the Ti-DLC film with

Fig. 1. Cross-section SEM micrographs of (a) plasma nitrided, (b) Ti-DLC coated and (c) duplex treated samples.


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Fig. 2. XRD results of plasma nitrided, Ti-DLC coated and duplex treated Ti6Al4V alloy.

hexagonal α-Ti and the cubic β-Ti phases because Ti6Al4V alloy is an alpha-beta titanium alloy. After nitriding treatment, mainly δTiN, minor ε-Ti2N and α-Ti phases were seen in the modified layer. In addition, the diffraction peaks of α-Ti phase shift towards the lower diffraction angles due to enlargement of the α-Ti lattice with incorporated nitrogen atoms [12]. On the other hand, Ti-DLC coated sample showed TiC peaks together with α-Ti from substrate. In the case of duplex treatment, composite layers exhibited δ-TiN, ε-Ti2N peaks from nitrided layer and TiC peaks from Ti-DLC coated layer. After the scratch tests, the first critical load values (LC1) of TiDLC films deposited on both untreated and plasma nitrided Ti6Al4V alloy were measured as 25 and 32 N, respectively. Also, the highest critical load (LC2) was measured as 42 and 48 N for TiDLC coated and duplex treated Ti6Al4V alloy, respectively. Critical load values proved that plasma nitriding treatment before DLC coating improved the adhesion properties of thin film. It can be said that the critical load for coating detachment increases as the substrate hardness increases and the size of the scratch track at a given load is reduced as previously suggested [20]. Fig. 3a shows typical scratch lines for Ti-DLC coated films on soft substrates. It was observed that the first damage as conformal cracks formed inside the scratch line. Flaking and buckling failures in the edge of the scratch were seen as the applied load increased. When loading reached up to the LC2, Ti-DLC film delaminated from surface and substrate was appeared. In the case of DLC coating on hard substrate formed after nitriding, chipping failures observed in the edge of scratch line (Fig. 3b). When the widths of traces were compared, it was observed that scratch line of duplex treated samples was narrower than Ti-DLC coated samples even if the normal load was lower.

Ti-DLC coated samples. The formation of this graphitized layer between mating bodies acted as a lubricant and this was the reason for low friction coefficient values after Ti-DLC coating. The low shear at the sliding interfaces can be attributed to micrographitization as has been previously suggested [19,21,22].

3.2. Tribological properties Friction test results of the untreated, plasma nitrided, Ti-DLC coated and duplex treated Ti6Al4V alloy are given in Fig. 4. While average friction coefficient of the untreated sample was 0.45, this value increased to 0.49 with increasing surface roughness after nitriding. The lowest friction coefficient value was 0.21 and it was obtained from the Ti-DLC coated samples due to its graphitic structure. Although the duplex treated samples also showed low friction coefficient (0.25), plasma nitriding which is the first step of duplex treatment caused higher friction coefficient rather than

Fig. 3. Morphologies of scratch tracks (a) Ti-DLC coated and (b) duplex treated samples.

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Fig. 4. Friction test results of the untreated, plasma nitrided, Ti-DLC coated and duplex treated Ti6Al4V alloy.

Fig. 5 illustrates the relation between the wear rate and surface hardness values of untreated and surface treated Ti6Al4V alloys. All surface treatments applied in this study improved the surface hardness and the wear resistance of Ti6Al4V alloy. While the surface hardness value of untreated sample was 380 HV0.01, this increased to 1500 HV0.01, 1250 HV0.01 and 2600 HV0.01 after Ti-DLC coating, plasma nitriding and duplex treatment, respectively. As seen in this graph, wear rate of samples decreased as surface hardness increased. In the duplex treatment, plasma nitriding before Ti-DLC coating produced a graded hardened case which served as an excellent supporting layer for DLC coating [23]. The wear resistance of the alloy significantly improved with the application of Ti-DLC film on nitrided Ti6Al4V alloy because the load bearing capacity of coating system increased. Fig. 6 shows the SEM images of wear tracks. The worn surface of the untreated Ti6Al4V alloy showed that the alloy underwent high plastic deformation and adhesive wear [24]. Extensive shear deformation occurred during the wear test caused by the pile up of material throughout wear scar. In addition to, plate-like wear debris obviously indicated adhesive wear (Fig. 6a). In fact, improvement on wear resistance after surface treatments is related to changing of surface chemistry and getting a harder surface. A micro-abrasive type wear mechanism on the wear track was characterized for the plasma nitrided samples because the thin and hard compound layer broke off during sliding. The increase of surface hardness with nitriding reduced the penetration depth of the pin to the surface and accordingly, amount of plastic deformation decreased. Therefore,

Fig. 5. Relation wear rate and surface hardness values of untreated and surface treated Ti6Al4V alloys.

nitrided samples demonstrated no severe deformation and narrower wear tracks than untreated sample (Fig. 6b). SEM images displayed that adhesive wear was the effective mechanism for both Ti-DLC coated and duplex treated samples (Fig. 6c and d). Transfer film formed between the pin and the surface on both Ti-DLC coated and duplex treated samples and wear occurred within this film. In Fig. 6c, it was seen that although wear tack was superficial, lateral cracks and spallations formed along with track because hard and thin film cracked on the soft substrate which had not enough load bearing capacity. The narrowest wear track was obtained in the duplex treated samples (Fig. 6d). The Ti-DLC film deposited on nitrided surface showed locally flaking failures. In addition to, it was observed that TiDLC film delaminated at some regions and nitrided surface was appeared. 4. Conclusions Ti6Al4V alloy had been treated with different surface treatments in order to improve its tribological properties and the following conclusions have been derived from the above results and discussions; • While a compound layer of 2–3 μm and a diffusion layer of 100 μm beneath the compound layer occurred after plasma nitriding, Ti-DLC film thickness was about 2–3 μm. Diffusion layer produced a graded hardened case and thus, it improved the load bearing capacity of the alloy. • All surface treatments increased the surface hardness of the alloy 4– 7 times. The highest hardness value was obtained from duplex treated samples. • Scratch test results showed that duplex treatment modestly increased the adhesion properties of Ti-DLC film. • Ti-DLC film coating on both untreated and plasma nitrided surface considerably reduced friction coefficient. The reduction of the friction coefficient was ascribed to the formation of graphitized transfer layer. • Although each surface treatment applied to the surface in this study improved the wear resistance to the Ti6Al4V alloy, wear performance of duplex treatment was significantly higher that the individual performances of both Ti-DLC film coating and plasma nitriding because duplex system had combined benefits from both plasma nitriding and DLC coating. Superior wear performance of duplex treatment was attributed to higher load bearing capacity and reduced subsurface deformation.


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Fig. 6. SEM images of wear tracks (a) untreated, (b) plasma nitrided, (c) Ti-DLC coated and (d) duplex treated samples.

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