Tribological performance of titanium nitride coatings: A comparative study on TiN-coated stainless steel and titanium alloy

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Tribological performance of titanium nitride coatings: A comparative study on TiN-coated stainless steel and titanium alloy

T

⁎

Magdalena Łępickaa, , Małgorzata Grądzka-Dahlkea, Daniel Pieniakb, Kamil Pasierbiewiczc, Kamila Kryńskaa, Andrzej Niewczasb a

Department of Materials and Production Engineering, Faculty of Mechanical Engineering, Bialystok University of Technology, Wiejska 45C, 15-351 Bialystok, Poland Department of Mechanics and Mechanical Engineering, Faculty of Transport and Computer Science, University of Economics and Innovation, Projektowa 4, 20-209 Lublin, Poland c Department of Materials Science, Faculty of Mechanical Engineering, Lublin University of Technology, Nadbystrzycka 36, 20-618 Lublin, Poland b

A R T I C LE I N FO

A B S T R A C T

Keywords: Hard coating Dry friction Wear-resistant coating Titanium nitride

Though TiN coatings are considered a reliable anti-wear solution, the recent studies show that the anti-wear capacity of the TiN-coated metallic materials can be limited by various factors. Therefore, in our study, the CAEPVD TiN film was deposited on two popular metallic materials: 316LVM stainless steel and Ti6Al4V alloy. The surface roughness, mechanical properties, adhesion as well as anti-wear performance in tribological pair with Ø6 mm WC-Co ball of both bare and surface modified specimens were evaluated. According to the findings, the elastic modulus mismatch between the coating and the substrate is one of the key factors that determine the operational durability of the TiN-coated alloys. Moreover, the well-established H/E and H3/E2 quotients should be treated with caution when the wear performance of a coated material is assessed.

1. Introduction Introduced to the global market in the 1970s, thin films based on the transition metal nitrides are one of the first commercially applied anti-wear coatings [1]. Their undiminished popularity results from the exceptional properties of the nitride-based films: high hardness, chemical stability, good corrosion performance, high melting point and good electrical conductivity [2]. To this day, one of the most popular and the most easily available anti-wear films are the yellow-gold TiN coatings, which were invented shortly after the TiC films [3]. The titanium nitride that crystallizes in the B1 system [2] is a versatile material with nitrogen content varying from 37.5% to 50% [4]. Nowadays, titanium nitride is used primarily as a coating material in order to extend the service life of cutting tools made of high speed steels or sintered carbides. Moreover, compared to the non-modified cutting tools, the use of titanium nitride films allows to increase the cutting speed and feed during machining [5]. A combination of high hardness with good plasticity and sufficient corrosion performance in physiological fluids made the titanium nitride coatings the number one choice in wear-related medical applications, e.g. orthopaedic implants [6–9]. Moreover, TiN can also be used in stamping tools [3], bearings [3] or simply as a decorative material [3,10]. In the recent years, a tendency to accelerate the R&D processes of ⁎

the newly developed products by using mathematical models to predict the materials’ behaviour in the actual working conditions has been emerging. For example, predicting the wear performance of the tribological pair while using the well-established relationships between the mechanical properties of the substrate and the coating was proposed. As it was shown in the literature, when the hardness H and elastic modulus E of the coating are known, it is possible to determine its susceptibility to elastic strain to failure, as expressed by the H/E quotient [11], as well as its resistance to plastic deformation defined by the H3/E2 ratio [12]. In general, in most engineering applications, the higher are the H/E and H3/E2 coefficients, the greater the wear resistance the coating should represent [11,12]. As demonstrated in the research by Musil and Jirout [13], the high value of H3/E2 coefficient is primarily attributed to fracture toughness of the thin films. Low elastic modulus of the coating is particularly advantageous when it is possible to match it with the underlying substrate. A small Young's modulus of the material facilitates distribution of stresses at the interface between the coating and the substrate, thereby reducing the coefficient of wear K [14], which is determined by the friction track length and the volume of the worn material. However, adjusting stiffness of the components is difficult to achieve when a hard ceramic coating is used. It is believed that the Young's modulus of a typical antiwear coating is usually three to four times higher than E of the

Corresponding author. E-mail address: [email protected] (M. Łępicka).

https://doi.org/10.1016/j.wear.2019.01.029 Received 23 October 2018; Received in revised form 6 January 2019; Accepted 7 January 2019 Available online 08 January 2019 0043-1648/ © 2019 Elsevier B.V. All rights reserved.

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deposited on a hip implant head was reported. In the literature there have also been given examples of localized flaking and, as a result, increased surface roughness and wear of TiN coated implants made of Ti6Al4V [26,27]. In addition, some cases of wear processes intensification after deposition of TiN on other engineering metallic materials are also known. When titanium nitride is deposited on TiAl intermetallics [28], mild steels [29], low-alloy steels [30] or aluminium 1000 series [29], their wear performance strongly depends on the wear test parameters, e.g. applied load. Based on the provided literature review, it can be seen that the wear performance of TiN-coated metallic alloys strongly depends both on the substrate material and the friction conditions. Therefore, the aim of our study was to experimentally test applicability of Hcoating/Ecoating, Hcoating3/Ecoating2 as well as Ecoating/Esubstrate quotients in predicting wear of TiN coated metallic substrates. For comparison purposes, two wellknown metallic materials which differ significantly in terms of their mechanical properties – two-phase α + β Ti6Al4V titanium alloy and 316LVM austenitic stainless steel – were selected as the model materials to be tribologically tested. In order to ensure impartial friction conditions for both tested substrates, WC-Co sintered carbide was selected as the counter material, as well as all friction tests were conducted in dry conditions. As no electrolyte was used as a lubricant, we were able to monitor also the durability of the examined film and to identify the specific distance at which total failure of the coating was observed.

substrate, e.g. steel [11]. For this reason, according to some researchers, the parameter that more accurately describes wear resistance of the surface-modified materials is the quotient Ecoating/Esubstrate. As shown in the study conducted by Huang et al. [15] on the TiN- or TiAlN-coated copper, high speed steel or tungsten carbide substrates, the greater is the mismatch of Young's modulus between the substrate and the coating (i.e. Ecoating/Esubstrate differs from 1), the greater is the observed wear. Moreover, according to the authors [15], the value of Ecoating/Esubstrate ratio remarkably influences friction coefficient (COF) of the tribological pair. The WC-Co substrate, which is characterized by the highest loadbearing capacity among tested substrates, exhibited the lowest COF for both TiN and TiAlN coatings [15]. The influence of Ecoating/Esubstrate coefficient on wear of hard ceramic coatings was examined also by Avelar-Batista et al. [16]. Among the coatings: (Ti,Al)N, TiN and CrN applied on AISI H13 steel, the greatest wear resistance was reported for (Ti,Al)N coated steel due to the best match of Young's moduli between the substrate and the coating (Ecoating/Esubstrate = 1.60). TiN films are often used to enhance the wear capacity of two popular metallic materials: the corrosion resistant austenitic stainless steel 316L and the alpha-beta titanium alloy Ti6Al4V. In the literature there are given multiple examples of successful surface modification of those materials with the TiN films. In the wear tests conducted in dry sliding conditions in the ball-on-disc configuration, where the counter-sample was the WC-Co ball [17], compared to the non-coated material, the wear resistance of the TiN-modified 316 L steel improved approximately 6.5-fold. According to Zhang et al. [18], the PVD deposited titanium nitride coatings improve the wear performance of 316L stainless steel in the tribological pair with Si3N4 as well. Very enthusiastic reports on the tribological performance of TiN coated 316L were published also by Saravanan et al. [19] and Zuo et al. [20]. According to Saravanan et al. [19], if the 316L austenitic stainless steel is coated with TiN, under the load of 2 N compared to bare material its wear is reduced 6-fold, while under the load of 6 N wear of the tribological pair is even 7 times smaller. Moreover, in comparison to the bare steel, after titanium nitride deposition the average coefficient of friction (COF) is reduced more than 7 times. What is more, according to numerous reports, deposition of TiN films on Ti6Al4V titanium alloy provides enhancement of its wear performance as well. If the Ti6Al4V alloy is surface modified with PIII (plasma immersion ion implantation) TiN, in tribological pair with SiC not only substantial reduction of COF can be noticed, but the wear performance of the frictional pair improves as well [21]. Moreover, the results from the tribological study in which the CVD TiN was deposited on the Ti6Al4V surface, are not less enthusiastic. According to the authors [22], in the bovine serum lubricated conditions the TiN film provides the best anti-wear performance among all tested coatings (diamond-like-carbon, amorphous carbon and TiN). As presented in the research paper by Wang et al. [22], in the tribological contact with zirconia ball, after deposition of the DLC film wear of Ti alloy was reduced 2.5-fold, while after TiN deposition – it decreased 10-fold. Furthermore, the study conducted by Varidaj and Kamaraj [23] showed that the PVD TiN helps to improve the fretting wear performance of two Ti-based alloys – Ti6Al4V and Ti6Al7Nb. Nevertheless, though TiN is considered a versatile and a multipurpose coating, some cases of wear intensification of metallic substrates after TiN deposition are also known. For example, though TiN films are used in medical applications in order to enhance the tribological performance of metallic materials, numerous examples of adverse effect of the titanium nitride coatings on wear performance of implant alloys have been reported. In the study conducted by Williams et al. [24] two tribological pairs working in bovine serum lubricated conditions were compared: (a) CoCrMo – CoCrMo and (b) TiN CoCrMo – TiN CoCrMo. It was noticed that in case of TiN – TiN contact, the wear of tribological pair was 4 times greater than that of the non-modified pair. What is more, in the research by Fisher et al. [25] conducted on a hip friction simulator, localized flaking and chipping of the TiN coating

2. Experimental details 2.1. Materials As noted in the Introduction section, TiN coating was deposited on two metallic materials that differ significantly in terms of their mechanical properties: 316LVM stainless steel and Ti6Al4V titanium alloy. Moreover, for the wear test purposes, ceramic balls made of WC-Co were selected as the counter material. In order to ensure uniformity of the films and repeatability of the PVD process, the commercial anti-wear solution was selected for the analysis purposes. This has been done to obtain credible analysis on the impact of the mechanical properties of substrate and coating on wear performance of the analysed film. Before the thin film deposition, the substrates were carefully cleaned: degreased in an alkaline ultrasonic bath, rinsed with de-ionized water, and dried using a vacuum drying system. TiN coatings were deposited by the cathodic arc evaporation (CAE-PVD) method from Ti target in an argon and nitrogen mixture. Before deposition, the base pressure in the deposition chamber was 10−3 Pa. The process was conducted at a temperature of ca. 350 °C, under a working pressure in the range from 0.1 to 0.2 Pa. 2.2. Film characterization The roughness of both non-modified and TiN-coated specimens was examined using the Hommel-Etamic T1000 (Jenoptic, Germany) roughness meter. The measurements were taken in accordance to the ISO 4288:2011 [31] standard. Each measurement was replicated 10 times for each type of a tested sample. The mechanical properties (hardness, Young's modulus) of substrates and coating were studied using the Ultra Nanoindentation Tester (CSM Instruments, Switzerland) equipped with a Berkovich indenter (0.8 mN load). The hardness and elastic modulus were calculated from the load-displacement (L-D) curves according to the Oliver Pharr method [32]. The theoretical Poisson ratio values used in calculations were as follows: (a) 316LVM: 0.30, (b) Ti6Al4V: 0.31, and (c) TiN: 0.20 [33]. The loading and unloading times were 150 s. In order to assess the tendency to creep of the examined materials, hold time of the peak force was set to 5 s. In each sample group, nanoindentation measurements were replicated 20 times. The adhesion of the coating to both substrate materials was examined using the Micro Scratch Tester (Anton Paar GmbH, Germany) 69

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The wear rate of the examined materials was calculated using the following formula:

W=

V mm3 [ ] PL Nm

(1) 3

where V is the volumetric wear [mm ], P is the applied load [N] and L is the total sliding distance [m]. 2.4. Statistical analysis For all the presented numerical data, a post-hoc two-tailed Student's t-test was conducted assuming unequal variances between the analysed groups (α = 0.05). Moreover, statistical power analysis (Pwr = 1-β, where β is the probability of the type II error) of the collected data was done using the free G*Power software [35]. The statistical power threshold was set at 0.90. Therefore, the probability of rejecting the H0 hypothesis that the sample means are equal given that the statistically significant difference was really there equalled 90%. Fig. 1. Schematic representation of the ball-on-disc wear test system.

3. Results according to the ASTM C1624–05(2015) [34] standard. In the scratch testing, a diamond Rockwell C indenter (120° conical tip, 100 µm radius) was pulled over the surface of the TiN coated specimens with normal load that was progressively increasing from 1 to 30 N. The loading rate was 11.6 N/min and the stylus progressive speed was 2 mm/min, while the total scratch length was 5 mm. For each sample group, scratch test was replicated 10 times. Optical (OM, MCT, Anton Paar GmbH, Germany) and scanning electron (SEM, Phenom G2 Pro, Phenom-World B.V., Holland) microscopy examination was carried out for the coatings after each scratch testing in order to determine the cracking pattern of the film and the critical cracking loads (Lc). The Lc1 load corresponded to the cohesive failure (occurrence of the first cracks on the surface), Lc2 – the first symptoms of adhesive failure (spalling or chipping), Lc3 – total exposure of the substrate material.

3.1. Surface characterization Fig. 2 presents results of the roughness measurements conducted on both bare and TiN-modified samples. For each of the given parameters: Ra, Rt, Rz, 10 measurements were taken, based on which the average values and the standard deviations were calculated. According to the presented data, for both 316LVM stainless steel and Ti6Al4V ELI implant alloy a difference in surface roughness after TiN film deposition can be seen. There is a statistically significant difference in Ra, Rt and Rz parameters between the non-modified specimens and the TiN-coated ones. In case of both 316LVM steel and Ti6Al4V ELI alloy, the greatest difference between the bare and coated specimens can be seen for the Rt parameter, which denotes the total height of the roughness profile – that is, the difference in height between the highest peaks and the deepest valleys within the evaluation length. The change in roughness corresponds to change in surface morphology after TiN deposition. According to the other authors, TiN coating obtained by the cathodic arc evaporation method can have some surface defects which affect the surface roughness of the material [36].

2.3. Wear test The tribological test was conducted on a CSM Intruments (Switzerland) microtribometer in a ball-on-disc configuration (Fig. 1). The disc was working in a rotating mode, while the Ø6 mm WC-Co ball was fixed. The sliding velocity was 10 cm/s and the diameter of the wear track equalled 6 mm. Sliding conditions were selected with regard to the information presented in the book “Coatings Tribology: Properties, Mechanisms, Techniques and Applications in Surface Engineering” by Holmberg and Matthews [10]. Though AISI 52100 bearing steel is often used as the counter-material in wear tests, in our preliminary studies we observed severe adhesive wear in the tribological pair bearing steel vs. Ti alloys. Therefore, selection of the WC-Co ball as the counter-material resulted both from our own observations and literature review. In wear tests, a total of 24 discs was used (6 discs per each group). Therefore, 6 replications of tribological tests for each tested pair were done. Prior to the tribological tests, all of the specimens were carefully cleaned with acetone, isopropanol and dried. The test was conducted under dry sliding conditions in room temperature (21 °C) and ambient humidity. For all the samples, the normal load of 10 N was used. In order to determine the resistance of the TiN coatings to being worn out, the electrical resistance and coefficient of friction of the tribological pair was controlled during the whole test. To detect all the subtle changes in the registered signals, the frequency of data acquisition was set to 10 Hz. After the wear tests, the morphology of the wear tracks was assessed by SEM (Phenom G2 Pro and Phenom XL, Phenom-World B.V., Holland) and the basic elemental analysis of the wear track was done using the built-in EDS detector. The cross-section of the wear track was measured by a contact profilometer (Veeco Instruments Inc., USA).

3.2. Coating thickness In order to evaluate thickness of the TiN coating deposited on 316LVM stainless steel and Ti6Al4V ELI titanium alloy, metallographically prepared cross-sections of the surface modified samples were used. Measurements were taken using the scanning electron microscope. As it can be seen in Fig. 3, the analysed TiN coating was a monolayer of uniform thickness. According to data presented in Table 1, there was no statistically significant difference in TiN thickness between the titanium alloy and the samples made of stainless steel. 3.3. Nanoindentation Fig. 4 shows the load-displacement (L-D) curves obtained in the nanoindentation tests. As it can be seen, during the pause of 5 s both substrate materials exhibited tendency to creep, as the displacement of the diamond indenter was growing under the constant maximum load. On the other hand, the TiN coating presented no creep. It is a valuable information e.g. for the self-locking screw joints designers, as the nonrotational loosening effect of the screws is often caused by creep of the mating materials [37]. As shown in Fig. 4, under the load of 0.8 mN the maximum penetration depth for bare 316LVM stainless steel achieved value of 55 nm. To overcome problems associated with the small permanent indent 70

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Fig. 2. The surface roughness parameters measured for bare and TiN-modified 316LVM and Ti6Al4V, number of test replications n = 10.

the contact area A, the unloading stiffness S (S = dF/dh), the dimensionless parameter β which is a constant dependent on the indenter geometry, and the reduced elastic modulus Er, as shown in Eq. (3):

2 S = β Er A π

(3)

The reduced elastic modulus Er is defined by (4):

1−νi2 1 1−ν 2 = − Er E Ei

When calculating reduced elastic modulus, the occurrence of elastic displacements in both diamond indenter and the examined material is taken into account. Therefore, the reduced elastic modulus is calculated based on the Poisson ratio ν and the sample elastic modulus E, as well as on the Poisson ratio of the diamond indenter νi = 0.07 [32,38] and its elastic modulus Ei = 1140 GPa [32,38]. According to the results presented in Table 2, the nanohardness and elastic modulus of the deposited TiN coatings are few times greater than those of bare alloys. Fig. 5 presents the results of the statistical analysis conducted on the nanoindentation data collected for all considered materials. According to the provided information, there is a statistically significant difference in hardness and elastic modulus between the noncoated samples and their coated equivalents. What is more, no statistically significant difference was observed between the TiN-coated specimens. Due to that, it can be assumed that in the measurements that we took, the effect of substrate on the measured mechanical properties of the coating was negligible. What is more, a statistically significant difference is observed also in hardness and Young's modulus of both substrates – 316LVM stainless steel and Ti6Al4V ELI titanium alloy (Fig. 5). According to the data presented in Table 2, 316LVM steel is characterized by the higher Young's modulus than Ti6Al4V ELI alloy. On the other hand, Ti6Al4V alloy is harder than bare stainless steel. Nevertheless, findings obtained in the hardness measurements should be interpreted with caution. In contrary to the traditional hardness measurement techniques, e.g. Vickers hardness measurements, where hardness of the material is calculated i.a. from the dimensions of the permanent indent impression, in the Oliver-Pharr nanohardness measurements, hardness of the material is calculated based not only on the applied maximum normal load

Fig. 3. Cross-section of the TiN-modified Ti6Al4V sample. Table 1 TiN coating thickness measurements (mean ± SD); number of test replications n = 8. TiN coating thickness [μm] 316LVM stainless steel 1.48 ± 0.03

Ti6Al4V alloy 1.46 ± 0.03

Fig. 4. The load–displacement curves of bare substrates and TiN coated specimens obtained in the nanoindentation measurements.

Table 2 The mechanical properties of tested materials determined in nanoindentation tests (mean ± SD): Wtotal – total work done, Welastic – elastic work done, number of test replications n = 20.

impression, in the nanoindentation measurements, hardness of the material is calculated using the assumed contact area A. The contact area A is calculated from the value hc, which is the contact depth with an indenter under the maximum load Fmax [32]. Thus, the hardness H is calculated from the Eq. (2):

H=

Fmax A (hc )

(4)

(2)

According to Oliver and Pharr [32], there is a relationship between 71

Material

H [GPa]

E [GPa]

316LVM Ti6Al4V TiN 316LVM TiN Ti6Al4V

7.5 9.6 27.4 27.0

231.7 137.0 417.9 418.9

± ± ± ±

0.7 1.1 4.0 3.7

± ± ± ±

21.4 13.5 65.0 66.6

Wtotal [pJ]

Welastic [pJ]

20.9 21.0 8.8 9.0

5.0 8.7 6.0 6.0

± ± ± ±

1.4 1.7 0.8 0.9

± ± ± ±

0.4 0.7 0.3 0.8

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Fig. 5. Results from the t-Student (α = 0.05; number of test replications n = 20) analysis done for the hardness and Young's modulus measurements: a) hardness, b) elastic modulus. Inside the table cells the statistical power of the ttest is given.

Fig. 6. Cracking pattern (a–d) of the TiN-coated 316LVM steel.

Fmax, but also on the hc value (contact depth with an indenter under the maximum load) – in other words, elastic deformation of the material is also taken into account. As a result, Ti6Al4V alloy, which is characterized by a statistically significant higher rate of the elastic deformation than the stainless steel (Table 2), can also be characterized by the higher hardness.

seen in Fig. 6c, though at the beginning spalls are relatively small and randomly distributed over the scratch channel, they have a tendency to group in bigger agglomerates. As the normal load was increasing, destruction of the coating progressed, as evidenced by the extensive growth of the spalling regions (Fig. 6d). With intensification of the coating spalling process, the amount of hard debris originating from the coating was growing, as shown in Fig. 6d. Some of the hard particles were trapped between the indenter and the examined material surface, causing ploughing of the material (Fig. 6d). In case of both 316LVM steel and Ti6Al4V titanium alloy, ploughing damage led to total exposure of the substrate and removal of the coating from the surface of the metallic substrate. Values of the critical loads registered during the scratch tests are presented in Table 3. As it can be seen in Fig. 7, for the cohesive failure, no statistically significant difference in Lc1 load was observed for both types of the analysed samples. Nevertheless, a difference in the scratch resistance of the TiN coated materials is observed for the Lc2 critical load (Fig. 7). The resistance to adhesive failure (Lc2) of the TiN coating was greater for the samples in which 316LVM stainless steel was used as a substrate material. On the other hand, though in the t-test statistical difference in the total failure of the coating (Lc3) was observed between the TiN coated Ti6Al4V alloy and stainless steel, we failed to reject the H0 hypothesis that the sample means are equal due to low statistical power of the test (0.66). Therefore, it cannot be claimed that in scratch testing the substrate material influences the resistance of the coating to total failure.

3.4. Adhesion strength Adhesion strength of the TiN coating deposited on Ti6Al4V alloy and 316LVM stainless steel was investigated using the micro-scratch technique in the PLST (progressive loading) mode with Fmax = 30 N. The critical normal loads (Lc) were determined while observing the scratch channels by OM. SEM examinations of the scratch channels were conducted as well in order to determine the main deformation mechanisms of the coating that take place during the scratch testing. The results obtained during the scratch test resemble findings presented in our previous paper, where the TiN-coated ground 316LVM stainless steel was analysed [39]. For both types of TiN coated specimens, four main cracking deformation regimes were registered. It should be emphasized that though both substrates are characterized by different mechanical properties, the differences between the critical failure loads of the coating were noted, but the main deformation regimes remained the same. The cohesive failure of the TiN coating started from its parallel cracking (Fig. 6a). Immediately after the first parallel cracks, the angular cracks appeared in the film (Fig. 6b). The observed cracks were the actual response of the coated system to the intensifying normal load – as the load was progressing, the parallel and angular cracks were getting thicker and longer. In the meanwhile, in the back of the moving diamond stylus, tensile stresses were intensifying. As a result, third regime of deformation started – the ductile semi-circular through-thickness cracking (Fig. 6b). Though the failure area remained localized and did not propagate beyond the scratch groove (Fig. 6b), the number of the semi-circular cracks was growing rapidly as soon as the first crack appeared. As it can be seen in Fig. 6b, formation of the first semi-circular cracks leads to immediate adhesive failure of the coating, which corresponds to the critical load Lc2. Spalling of the coating is an effect of superimposition of the compressive stresses in front of the intender which is pulled over the examined sample surface and the tensile stresses that are generated in the rear of the stylus. According to Kataria [40], spalling is caused by the cross-influence of both cohesive and adhesive damaging action of the coating. As it can be

3.5. Wear behaviour of bare and TiN coated materials In this study, a CSM Instruments microtribometer was used to evaluate the wear behaviour of TiN coated 316LVM stainless steel and Ti6Al4V ELI titanium alloy. During the examinations, values of the Table 3 Critical loads registered during the scratch testing: Lc1 – first symptoms of cohesive failure (angular or parallel cracking), Lc2 – beginning of adhesive failure (buckling, chipping, spalling, etc.), Lc3 – total failure of the coating or massive exposure of the substrate (mean ± SD), number of test replications n = 10.

72

Material

Lc1 [N]

Lc2 [N]

Lc3 [N]

TiN 316LVM TiN Ti6Al4V

2.3 ± 0.8 2.0 ± 0.7

6.9 ± 1.5 4.6 ± 1.3

17.1 ± 1.7 19.3 ± 2.1

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ball is the adhesive wear. As a result of the applied normal load, asperities that were initially present at the surface of the examined sample and the counter-specimen became plastically deformed and soon after that – micro-welding of both materials took place. While relative displacement between the ball and the disc was constantly present, the micro-welds were subsequently fractured and delamination of the softer material – in this case Ti alloy – occurred (Fig. 10d). During constant friction, some of the wear products were removed from the contact zone (Fig. 10c), but in most cases – wear debris rapidly oxidized in the friction zone (Fig. 10b). As a result of formation of the oxide-rich tribolayers, intermittent decrease in the fricitonal forces was observed (Fig. 8). Nevertheless, in contact with the hard WC-Co ball, tribofilms were rapidly damaged and subsequent exhibition of the bare substrate material occurred. Again, high reactivity of bare Ti alloy led to adhesive welding between the examined material and the counter-specimen, which resulted in the subsequent fracture of the micro-welds. This wear mechanism is reflected also in fluctuations of the coefficient of friction (Fig. 8). The observed wear mechanism leads to catastrophic failure of the examined material in a relatively short time. Because the friction distance differed substantially between 316LVM stainless steel and Ti gr. 5 titanium alloy, linear wear of both materials cannot be compared. Due to that, wear rate coefficient K (Eq. (1), Section 2.3) was introduced. It turned out that the wear rate of bare Ti6Al4V alloy was approximately 4 times greater for the non-modified 316LVM steel than for the Ti alloy. Tribological examinations were conducted also for the samples that were surface modified with the titanium nitride coating. The dynamics of COF registered for bare and TiN coated 316LVM stainless steel are presented in Fig. 11. For TiN coated steel, stable increase in COF values from 0.6 to 0.7 in the first 700–800 m of the wear test is observed, followed by its sudden decrease. After the first 1000 m of friction, dynamics of the µ fluctuations were similar to the ones observed for the uncoated steel – changes in COF value were observed, but the average value remained stable up to the end of observations. Attention is paid particularly to the significant reduction in COF value after TiN coating deposition – as presented in Fig. 12, the average COF value calculated from the batch of the examined samples was approx. 25% lower than for the bare steel. The aforementioned reduction in coefficient of friction after approx. 800 m of friction (Fig. 11) can be the proof of the coating failure and the substrate exposure. This fact is confirmed by the sudden change in contact resistance of the tribological pair (examined material vs. WC-Co ball – Fig. 13). After the coating failure, the reduced electrical resistance is observed during few hundred revolutions due to subsequent exposure of the metallic substrate. Then, short-time transients in the contact resistance are registered. Rapid increase in contact resistance is observed, which is immediately followed by its reduction. This phenomenon corresponds to the natural passivation of the stainless steel in ambient conditions, growth of oxygen-rich tribofilm in the contact area of the wear track, its following failure and once more, exposure of the bare substrate. This is reflected in rapid changes in COF values (Fig. 13). At the same time, oxygen-rich tribofilms caused not only increase in value of the contact resistance, but also decrease in friction forces. Damage of the tribofilm promoted adhesion between the substrate and the counter specimen, what resulted in rapid increase in COF value accompanied by the reduction of the contact resistance. Formation of the tribofilm in the contact zone between the TiN coated 316LVM and WC-Co ball is confirmed also by the microscopic observations (Fig. 14). The EDX analysis of the wear track proved the high level of oxygen in the tribofilm – the atomic concentration of oxygen was almost equal to the iron content (39.2% vs. 42.3%). What is more, small amount of tungsten originating from the counter-specimen has been also detected, mostly in the regions characterized by the higher oxygen content. Despite that, wear of the counter-specimen was negligible. Nevertheless, it should be emphasized that though TiN coating was

Fig. 7. Results from the t-Student (α = 0.05; number of test replications n = 20) analysis done for the critical loads. Inside the cells the statistical power of the t-test is given.

coefficient of friction and the contact resistance were registered at the frequency of 10 Hz, whereas the volumetric wear of the examined materials was assessed after the wear tests. For 316LVM stainless steel, the aimed wear distance equalled 2500 m, what corresponds to 132,189 revolutions in the proposed ball-on-disc configuration. In wear tests of the Ti6Al4V alloy, the distance of friction was reduced due to intensive wear of the examined material that was revealed in the pilot study. Therefore, for comparison purposes, the wear rate K was calculated according to Eq. (1). In Eq. (1), the relationship between the volumetric wear, normal force and total sliding distance is used. Fig. 8 presents the fluctuations of the coefficient of friction (COF) registered for both bare 316LVM stainless steel and Ti6Al4V titanium alloy during 2500 m of friction. It can be noticed that in case of 316LVM stainless steel, the COF value stabilizes at a high level after approximately 100 m of friction. The COF value is fluctuating, but its upper limit is ca. 0.92. The observed fluctuations in COF might be caused by the mixture of lubricating effect of the transfer layers, adhesive wear of the substrate and its abrasive wear. Signs of abrasive wear (arrows), which has been caused by the presence of high amounts of oxidized wear debris in the friction zone, can be seen in Fig. 9. In the wear channel abrasive grooves, agglomerates of the oxidized wear debris as well as tribofilms that were spontaneously formed as a result of crossinteraction between the substrate and the wear products, are present. Compared to 316LVM steel, wear of bare Ti6Al4V alloy proceeded differently. Despite the lower values of coefficient of friction (Fig. 8), severe wear of the titanium alloy was registered. After achieving only half of the planned friction distance, the width of the friction channel reached 3.5 mm (Fig. 10a), while for the 316LVM steel the width of the friction channel was 2 mm after 2500 m of friction. Due to that, in order to avoid wear seizure of the tribological pair Ti alloy – WC-Co ball, the total distance of friction was limited to 1250 m. In case of Ti6Al4V alloy, radially arranged agglomerates of wear debris are present in the wear track (Fig. 10a). Micrographs taken on the worn surface (Fig. 10b) show that the main wear mechanism that occurs during dry friciton in the tribological pair Ti gr. 5 disc vs. WC-Co

Fig. 8. COF values registered in dry conditions for bare substrate materials under normal load of 10 N. 73

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Fig. 9. SEM micrographs of wear track obtained for 316LVM after 2500 m of friction under the load of 10 N.

areas, cohesive cracking of the coating occurs. The inclination of the cracks with respect to the wear track is dependent on the superimposition of those stresses. On the worn surface, tribofilms as well as channels that were formed as a result of ploughing are present. As presented in the work by Guo et al. [44], brittle cracking of the film can induce cracking of the ductile substrate as well at a considerably low strain level. Nevertheless, in our study only cohesive cracking of the tribofilm took place, while the substrate material remained undamaged. Ploughing wear was caused by the wear debris that was trapped between the ball and the disc, as shown in Fig. 17c. However, the most severe damage was done by the agglomerates of the detached parts of a hard TiN coating (Fig. 17d). A particle that originated from the titanium nitride coating with a diameter of several micrometers can be seen e.g. in Fig. 17c. A similar wear mechanism of TiN coated Ti6Al4V was presented also in the work by Wan et al. [21]. However, it should be emphasized that the presence of the tribofilm in the friction zone was beneficial not only for the 316LVM steel, but also for the Ti6Al4V titanium alloy. Compared to bare titanium alloy, TiN coated material is characterized by more than 2 times smaller wear coefficient K (Fig. 18). As presented in Fig. 18, compared to bare metallic materials, surface modification with TiN of both 316LVM stainless steel and Ti6Al4V alloy results in significant reduction of wear. Nevertheless, both substrate materials undergo different wear mechanisms in dry sliding conditions. As a result, wear coefficient of the bare Ti alloy is more than 4 times greater than that of bare 316LVM steel.

worn out from the substrate due to friction, the tribofilm that has been formed maintained its protective function. In order to assess wear of the analysed materials, wear tracks were analysed using the contact profilometry technique. In the Fig. 15a wear tracks of the TiN-coated and bare 316LVM obtained after 2500 m of friction are presented. Compared to the non-coated steel substrate, worn profile registered for the TiN coated 316LVM is much shallower. For bare steel, the depth of the worn profile was approx. 180 µm, while for the titanium nitride modified steel – it equalled only 6 µm. For the TiN coated steel, furrows that were caused by the abrasive wear of substrate due to presence of hard particles of the coating in the contact zone are visible only in higher magnifications (Fig. 15b). Compared to the stainless steel, wear resistance of the TiN coating deposited on a titanium substrate was significantly lower. In Fig. 16, comparison in the operational durability of the TiN coating on both substrates – stainless steel and titanium alloy, measured by the friction distance or the revolutions number to the substrate exposure, has been presented. There is a statistically significant difference in the operational durability of the TiN coating on both aforementioned substrates. For TiN deposited on Ti6Al4V ELI, the mean friction distance to the total substrate exposure is approx. 9 times lower than that of the 316LVM steel (Fig. 16). Micrographs obtained after wear tests for TiN coated Ti6Al4V titanium alloy after 263.7 m of friction are presented in Fig. 17. A complete exposure of the substrate can be seen, as well as high amount of the wear products agglomerated alongside the wear track is present. High amount of wear products present even after a short wear distance is caused by the natural tendency to oxidation of titanium and its alloys. A naturally formed oxide passive layer is usually only few nanometers thick [41,42] and comprises of TiO2 [41], Ti2O3 and TiO [43]. Since titanium is characterized by the high reactivity with the oxygen-rich environment, immediate oxidation of both bare substrate and the wear debris occurs. The subsequent wear of the oxidized tribofilm is the reason of presence of high amounts of the wear debris in the proximity to the wear track. Right by the edge of the wear track, numerous cracks of the hard TiN coating are present (Fig. 17e). Due to displacement of the ball with reference to the examined disc, various types of stresses are generated in both coating and substrate. In the areas adjacent to the contact surface, the rotary motion of the sample causes coating deformation mechanisms well known from the scratch tests – cracking of the coating due to the tensile stresses [39]. In the back of the counter-specimen, tensile stresses that are parallel to the wear track are generated. On the other hand, in front of the ball, at the interface between the examined disc and the counter-specimen, bending stresses occur. Their presence causes the co-occurrence of the tensile stresses perpendicular to the wear track. As a result of stresses co-occurrence in the overlapping

4. Discussion As presented in the study, no statistically significant difference was observed in the mechanical properties of the TiN coating deposited on two metallic materials – 316LVM stainless steel and Ti6Al4V alloy. Due to that, it can be assumed that the quality of the TiN coating obtained on both substrates was comparable. Nevertheless, though it might seem that titanium nitride should be characterized by greater adhesion to the Ti substrate due to similarity of the elemental composition of both materials [45], and therefore – greater resistance to wear could be expected, in tribological tests the TiN coated Ti alloy was outperformed by the 316LVM stainless steel. Because of that, it can be assumed that the wear performance and adhesion strength of the titanium nitride coatings to the metallic alloys might greatly depend not only on the surface topography and applied surface preparation routine, but also on the mechanical and chemical properties of the substrate and the film itself. There have been numerous studies that focused on the relationship between the mechanical properties of the substrate and the coating and 74

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Fig. 10. SEM micrographs of wear track obtained for Ti6Al4V alloy after 1250 m of friction under the load of 10 N.

mechanical properties of the substrate material as well. In the work by Huang et al. [15], where the wear performance of TiN and TiAlN coatings was discussed, it was stated that for a typical ceramic coating, its wear performance increases with the increase of hardness end elastic modulus of the substrate material. However, our findings show that for the TiN-coated systems, this statement is not entirely true. In our research, as in the paper by Huang et al. [15], the nanoindentation technique was used to assess the mechanical properties of the substrate and the coating. It is well-known that the results obtained in the nanoindentation measurements depend on numerous intrinsic and extrinsic factors, e.g. the applied load. However, it has to be noted that in our case, as discussed in the Results section, the Ti alloy exhibited

the resulting wear performance. In numerous research papers, it has been shown that the greater are the values of the Hcoating/Ecoating (strain to failure) and the Hcoating3/Ecoating2 (plasticity index) ratios, the better ought be the wear performance of the coating [11,12]. Nevertheless, as shown in our study, the Hcoating/Ecoating and Hcoating3/Ecoating2 ratios fail when the wear performance of one coating deposited on different substrates is assessed. In our case, the values of both Hcoating/Ecoating and Hcoating3/Ecoating2 ratios were the same for the coating deposited on two different substrates, while the tribological performance of TiN differed significantly. Moreover, in some papers it has been noticed that the anti-wear capacity of the ceramic coatings depends not only on the strain to failure ratio or the plasticity index pf the film, but on the 75

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Fig. 11. COF values registered in dry conditions for bare and TiN-modified 316LVM stainless steel under normal load of 10 N.

Fig. 13. COF vs. contact resistance at the friction distance of 2500 m – TiN 316LVM.

higher hardness than the 316LVM steel, while the wear performance of the TiN coated Ti6Al4V was more than two times lower than that of the TiN-modified 316LVM steel. This indicates that in dry sliding conditions, not only the mechanical properties of the substrate and coating should be taken into account, but the chemical reactivity of the coating material and the substrate as well. In the work by Saravanan et al. [19], it was shown that the surface morphology of the wear track, as well as the tribological performance of TiN coated 316LVM steel depend on the friction conditions applied in the wear test. With the increase of the test load, the abrasive wear mechanisms intensify. In our research, after 2500 m of dry friction the 316LVM substrate is covered in oxygen-rich tribofilm. According to Saravanan et al. [19], at greater distances of dry friction the tribofilm is formed due to plastic deformation of the substrate material as well as due to the temperature rise in the contact region. It is highly likely that the oxidation of both 316LVM stainless steel and Ti6Al4V alloy was intensified by the temperature rise in the friction contact. This reveals that in further analyses, temperature measurements in the contact region should also be considered. Moreover, as suggested by the other authors, selection of the counter-sample material also greatly influences the main wear patterns observed in the tribological pair. For example, as presented in Fig. 9, the prevalent wear mechanism observed in the tribological pair consisting of WC-Co – bare 316LVM stainless steel was the mixture of adhesive and abrasive wear. Those results are in agreement with the works by Dong et al. [46], Kayali [17] and Kayali et al. [47]. Moreover, after coating stainless steel with TiN the wear pattern remained the same – a co-occurrence of both adhesive and abrasive wear was observed (Figs. 14 and 15). Nevertheless, though the wear mechanism didn’t change substantially, TiN film enhanced overall wear performance of the coated steel (Fig. 18). On the other hand, in contact with WC-Co, Ti6Al4V alloy undergoes severe adhesive wear, as it was

presented in Fig. 10. Similar findings were presented e.g. in the work by Cassar et al. [48]. However, after coating with TiN, the prevalent wear pattern in tribological pair with WC-Co changes from severe adhesive wear to a mixture of adhesive wear and ploughing, as presented in Fig. 17. Ploughing wear of the substrate material is caused by the hard particles of the damaged film that were entrapped in the WC-Co ball. Furthermore, apart from the earlier discussed Hcoating/Ecoating and Hcoating3/Ecoating2 quotients, the elastic recovery of the coating Ie is also recognized as a good indicator of the expected performance of the coating under normal load [13]. While 1 – Ie = Ip, where Ip is the % of the plastic response in the total deformation of the coating, when the hc and hp values are known, using the relationship (5):

Ip =

hp hc

*100%

(5)

it is possible to determine the percentage share of the plastic deformation in the total deformation of the tested material. As it might be expected, for hard ceramic coatings, the Ip increases as the plasticity index decreases [13]. According to Musil and Jirout [13], an increased tendency to crack under normal load is typical for coatings characterized by the Ip ≥ 50%. In our case, the Ip value was 56.6%. This might explain the low resistance to cohesive cracking (Lc1 load) of the coating registered during the scratch tests (Table 3) and therefore, high susceptibility to brittle cracking of the film. Moreover, according to our results, in scratch tests there was no statistically significant difference in the Lc1 value for the TiN coating deposited on both substrates (Fig. 7). Due to that, it can be assumed that the resistance to cohesive cracking of a thin film depends mainly on the intrinsic properties of the coating, while the mechanical properties of the substrate material are negligible. The influence of intrinsic properties of the TiN films on their fracture toughness was thoroughly

Fig. 12. The average COF values registered during the wear studies, number of test replications n = 6. 76

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Fig. 14. SEM micrographs of wear track obtained for TiN coated 316LVM steel after 2500 m of friction under the load of 10 N.

[49], the greatest influence of TiN film thickness on the residual stress levels is observed if the coating is thinner than 1 µm due to the “atomic shot peening” effect which occurs at the beginning of PVD deposition processes. Therefore, further work in establishing the effect of the residual stress levels of the thin films and their resulting anti-wear performance is advised. Furthermore, in scratch tests, a statistically significant difference between both coated substrates was observed for the Lc2 (adhesive failure) critical load. An interesting relationship between the Lc2 value and the wear performance of both coated systems was noticed – the greater the resistance to adhesive cracking (Lc2) of the coating assessed by the scratch test, the better its wear resistance in dry sliding conditions. However, one should remember that the tests were done on a small sample (2 types of the substrate materials) and therefore, no rPearson correlation coefficient can be calculated. For this reason, it could be interesting to repeat the research using a greater number of tested substrate materials and to determine the correlation coefficient between the Lc2 value measured in the scratch tests and the wear resistance of the TiN coated systems. In the recent years, some interesting improvements of the CAE-PVD TiN deposition method have been proposed by the other authors. For example, an extensive work on the relationship between the deposition parameters and the properties of CAE-PVD TiN films has been presented in the works by Sobol et al. [50,51]. Moreover, TiN-rich films can be obtained on titanium alloys by the plasma nitriding methods [52]. Nevertheless, in our study, in order to ensure repeatability of the deposition process with regard to the aim of the research, a well-established commercial TiN film that has been extensively used for many years in various industrial applications has been selected for the analysis purposes. Therefore, one should be aware of the limitations of our work that are set by the presented approach. Despite this, our example indicates that though in many cases assessment of the anti-wear properties of the coatings using the mechanical properties of the substrate and the thin film might provide good results, it should always be treated with caution. If, like in our case, the coating is being worn out during the test and the intensity of wear is determined by the physicochemical properties of the examined materials, e.g. their susceptibility to interact with ambient air, segregation of the wear products or the intermittent lubricating effect of the wear debris, the discussed coefficients should not be treated as the main determinants of the anti-wear performance of the coated systems [53].

Fig. 15. Cross-sections of worn profiles for bare and TiN modified 316LVM stainless steel.

Fig. 16. Operational durability of the TiN coatings deposited on 316LVM stainless steel and Ti6Al4V titanium alloy; number of test replications n = 6.

discussed in the work by Zhang et al. [49]. As stated by the authors, the residual stress level of the film and its resulting mechanical properties are strongly influenced by the coating thickness. In our case, no statistically significant difference in coating thickness deposited on two substrates was observed. Nevertheless, as presented by Zhang et al.

5. Conclusions In this study, titanium nitride coatings were deposited on two model metallic materials: 316LVM stainless steel and Ti6Al4V titanium alloy. 77

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Fig. 17. OM (a) and SEM (b)–(e) micrographs of wear track obtained for TiN coated Ti6Al4V alloy after 263.7 m of friction in dry conditions.

Fig. 18. Wear rates K of the examined materials, number of test replications n = 6. 78

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The surface roughness, mechanical properties, adhesion as well as antiwear performance in tribological pair with WC-Co ball of both bare and surface modified specimens were evaluated. Under the given set of data obtained in tribological tests conducted in ball-on-disc sliding conditions against a WC-Co ball, the following conclusions on the effect of metallic substrates on the tribological performance of TiN coatings can be drawn:

[12] [13] [14]

[15]

1. The anti-wear performance of CAE-PVD deposited TiN coatings highly depends on the mechanical properties of the underlying substrate material. As presented in the study, it seems that the elastic modulus mismatch between the substrate material and the coating is one of the most important factors that determine the tribological performance of titanium nitride films. Moreover, it has been shown that though it is meaningful to assess the mechanical properties of both substrate and coating while examining wear, many other factors, e.g. chemical reactivity of the tested materials, should also be taken into account. 2. Though in case of both substrate materials, 316LVM stainless steel and Ti6Al4V titanium alloy, the TiN coating was worn out during the wear test, the tribofilms that were formed on the surface of the substrate maintained their protective function – for both substrate materials, an improvement in wear coefficient K was observed. Moreover, even if the coating was worn out during the tribological test, the coefficient of friction remained reduced until the end of the measurement. Therefore, it can be stated that the tribofilms that were formed during dry friction reduced the friction forces in the tested systems. 3. In wear tests conducted in dry conditions, a particular attention should be paid to the chemical reactivity the coating and the substrate material. As provided in the findings, if the titanium nitride coating is being worn out from the substrate surface during the wear test conducted in ambient conditions, the oxidation processes of the wear debris and the wear track take place. Though some of the oxide-rich transfer layers promote the anti-wear capacity of the metallic alloys, if those are worn out, in case of 316LVM stainless steel the abrasive wear is dominant, while for the Ti6Al4V alloy – the adhesive wear takes place.

[16]

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[21]

[22]

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[25]

[26] [27] [28] [29] [30] [31]

Acknowledgements

[32]

The work was supported by the National Science Centre (Poland) within the PRELUDIUM 13 grant proposal, project no. UMO2017/25/N/ST8/02270.

[33]

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