Tribological performance of novel nickel-based composite coatings with lubricant particles

The present study is focused on the evaluation of the tribological performance of novel Ni/hBN and Ni/WS2 composite coatings electrodeposited from an additive-free Watts bath with the assistance of ultrasound. Lubricated and non-lubricated scratch tests were performed on both novel composite coatings and on standard Ni deposits used as a benchmark coating to have an initial idea of the effect of the presence of particles within the Ni matrix. Under lubricated conditions, the performance of the Ni/hBN composite coating was very similar to the benchmark Ni coating, whereas the Ni/WS2 behaved quite differently, as the latter did not only show a lower coefficient of friction, but also prevented the occurrence of stick-slip motion that was clearly observed in the other coatings. Under non-lubricated conditions, whereas the tribological performance of the Ni/hBN composite coating was again very similar to that of the benchmark Ni coating, the Ni/WS2 composite coatings again showed a remarkable enhancement, as the incorporation of the WS2 particles into the Ni coating not only resulted in a lower coefficient of friction, but also in the prevention of coating failure.


Introduction
Electrodeposited Ni-based composites coatings [1] have received a wide attention from the research community in recent years due to the enhanced tribological performance that such coatings may exhibit in terms of coefficient of friction (CoF) and wear resistance [2][3][4][5][6]. Nevertheless, most of the studies available in the literature deal with the performance of Ni-based composite coatings with hard particles, whereas the incorporation of inherently soft and lubricant particles such as hBN or WS 2 into electrodeposited Ni-based coatings has been studied to a much lesser extent [7,8], despite of the enhancement of both CoF and wear resistance that could be expected when lubricant particles [9] are incorporated into Ni coatings.
In a previous work, the authors developed novel Ni/WS 2 and Ni/hBN composite coatings (Figs. 1 and 2) electrodeposited from an additive-free Watts bath with the aid of ultrasound [10]. The Ni/WS 2 composite coatings contained 1% by weight of WS 2 particles and, compared to standard pure Ni electrodeposited from the same bath [11], exhibited an apparently nano-size crystalline structure which resulted in higher hardness. Regarding the Ni/hBN composite coatings, the hBN content was around 0.2% by weight, and the structure of the coating was similar to that of the benchmark Ni coating (fragmented structured with noticeable columnar crystals), resulting in a slightly lower hardness due to the softening effect of the presence of the hBN particles. The tribological performance of these recently developed composite coatings has been initially evaluated in this study to shed more light on the properties of these electrodeposited Ni-based composite coatings containing lubricant particles  Ni coatings used as benchmark. Adapted from Ref. [10], with permission from Elsevier.

Materials and methods
The tribological analysis was performed on the Ni/hBN and Ni/WS 2 composite coatings and the standard pure Ni deposit in order to have an initial idea of the tribological performance of the novel composite coatings. A UMT Scratch Tester (Bruker) was used for this analysis for which a multiple scratch method was developed. Although this method might appear controversial to the reader, scratch tests are a reliable tool for friction measurements at low sliding speeds [12], and multiple scratch tests have been an effective method to evaluate the tribological performance of electrodeposited coatings by capturing the lubricious effects of novel features in the structure of triblogical coatings and eliminating the effect of the surface roughness of the coatings [13].
Scratch tests were performed under two different conditions: (1) Lubricated scratch tests where lubrication was achieved via a single drop of SAE 10 oil immediately before the tests started in order to study the performance of the electrodeposited Ni-based coatings under "mixed-film"/"boundary" lubrication where the sliding contacting bodies are supported on a combination of asperity-asperity contact points and fluid regions between asperities [14].
(2) Non-lubricated scratch tests where no lubricant was used in order to study the performance of the electrodeposited Ni-based coatings under dry "boundary" lubrication where asperity-asperity contact points fully support the load [15].
The parameters of the scratch tests were:  Sliding distance: 10 mm;  Sliding speed: 10 mm/min;  Load: 60 N;  Ball diameter: 6.3 mm;  Ball material: JIS SUJ2 -ISO 100Cr6;  Temperature: 20 °C ;  Number of scratches: 10 (starting and ending on the same respective start/end points). The maximum contact pressure at the start of the tests, assuming a Hertz contact resulting in a circular contact area [16], was estimated around 2.3 GPa for the applied load (60 N). High-resolution images of the ball and the trace left on the coatings after the tests were obtained with a Keyence optical microscope.  Figure 3 displays CoF curves obtained during the lubricated scratch tests performed on the novel Ni/hBN and Ni/WS 2 composite coatings and the standard Ni coating used as a benchmark. For both standard pure Ni and Ni/hBN coatings (Figs. 3(a) and 3(b), respectively), the lowest CoF values were measured during the first scratch, progressively increasing with each new scratch until a relatively steady CoF around 0.3 was achieved. From scratches 2-3, the signal measured apparently showed a significant amount of "noise". A closer look at the CoF curves revealed a very distinctive repetition of cycles alternating a "concave down" increase followed by a sudden drop of CoF. Such repeated feature observed in the CoF curves corresponding to scratches 2 to 10 constitutes a very strong indication of "stick-slip" motion [17]. In stick-slip motion, the test ball virtually stops sliding (maximum observed in each cycle) and then experiences rapid sliding which progressively slows down until it reaches the point where very slow, almost imperceptible sliding motion occurs again [14]. For the Ni/WS 2 composite coatings, the lowest CoF values were again observed during scratch 1. In this case though, CoF values slowly increased with each new scratch until a steady CoF value around 0.24 was achieved in scratch 6, being up to 20% lower than the CoF values measured for the other Ni-based coatings. In addition, CoF curves did not indicate the occurrence of stick-slick motion in this case, as opposed to what was observed for the other Ni-based coatings tested. In all cases, no signs of critical coating failure were seen in the recorded acoustic emission (AE) signal (AE signal | https://mc03.manuscriptcentral.com/friction registered during the scratch tests is available online as Electronic Supplementary Material (ESM)). Figure 4 displays the surface of the test balls used in the scratch test performed on the different coatings after completion of the scratch test, while Fig. 5 displays the wear track left on the tested samples. In all cases, a small "wear lump" formed as a result of material transfer from the coating to the ball during the scratch test, and accumulation of debris forming ridges on both sides of the wear track was observed. Nevertheless, a few differences were observed in the test ball and the wear track corresponding to the tests performed  respectively) and the standard Ni deposit used as a benchmark (Figs. 4(a) and 5(a), respectively). For the Ni/WS 2 composite coating, the contact area of the wear lump attached to the test ball presented a smooth black surface suggesting the formation of a thin tribofilm during the tests. Also, a smaller amount of worn-out material accumulated on the sides of the wear track of the Ni/WS 2 composite, which exhibited a smoother appearance with low wear at the end of the wear track. The latter can be easily seen in Fig. 6, which displays the cross-sectional depth profile measured at the middle of each wear track in order to compare the depth of each wear track and the debris accumulated on the sides of the track. It is also worth pointing out that, taking into account the thickness of the coatings (6 μm in all cases), the final scratches would partially take place over the Pb-bronze substrate, especially in both Ni deposits and the Ni/hBN composite coating, where only a small portion of the test ball would be in contact with the Pb-bronze substrate in the Ni/WS 2 composite coating.  trend with no clear average CoF value. Scratches 3 and 4 yielded CoF curves showing an intermediate behaviour between scratches 1 and 2, and the system seemed to settle down from scratch 5 onwards. The AE signal was also recorded during the non-lubricated scratch test (available online as ESM) showing in this case very clear signal peaks indicating critical failure of the coatings on different scratches. Regarding this, a very strong signal peak was recorded during scratch 3. It was, at this latter point, where significant coating failure occurred, reflected in the fact that the system seemed to start settling down from that point onwards.

Non-lubricated tests
Once the system had settled down, CoF values remained between 0.2 and 0.3. For the Ni/hBN composite coating ( Fig. 7(b)), low CoF values depicting a smooth curve were again measured during scratch 1. CoF values measured from scratch 2 onwards did not fluctuate as much as in the case of the benchmark Ni coatings, and CoF values decreased during the second half of the scratches, possibly due to the action of the hBN particles, which could have more influence in this case due to the absence of a liquid lubricant than in the lubricated scratch tests. The CoF values measured during scratch 5 seemed to follow the same trend of previous scratches until a significant drop in CoF was observed after 10-15 seconds, when a first AE signal peak was observed (available online as ESM) due to coating failure. After that point, CoF values went down during the rest of the scratches and the tribosystem settled down and CoF values around 0.2-0.25 were measured from scratch 6 onwards. For the Ni/WS 2 composite coating, the CoF curves obtained during the different scratches were very similar, ranging between 0.15 and 0.3 during the whole experiment. As opposed to what was observed for the other Ni-based coatings evaluated under non-lubricated conditions, no signs of coating failure were seen in the recorded AE signal (available online as ESM).    surface of the wear lump attached to the test ball. These distinctive asperities matched the deep scratches observed on the surface of the wear tracks left on these samples coated with said coatings (Figs. 9(a) and 9(b), respectively). The wear tracks were now much rougher and irregular and presented a greater amount of debris on the sides of the tracks forming greater ridges than the wear tracks left in samples coated with the same coatings after the lubricated scratch test. In addition, a significant number of flat flakes were clearly noticed around the ridges and over the wear tracks and the test balls for both coatings. For the Ni/WS 2 composite coating, a reasonably large wear lump was observed in the test ball used during the non-lubricated scratch test (Fig. 8(c)), although in this case the amount of material transferred from the ball to the coating was not as significant as in the non-lubricated scratch tests performed on samples coated with the other Ni-based coatings here presented. A smaller amount of worn-out material forming ridges on both sides of the wear track was observed in the sample coated with the Ni/WS 2 composite coating ( Fig. 9(c)). Little accumulation was also noticed at the end of the wear track, as opposed to the larger amount of debris that was observed on the other Ni-based coatings tested under the same non-lubricated conditions. Although the surface of the wear track left after the non-lubricated scratch test was not as smooth as the one left after the lubricated test (some scratches were clearly seen in this case), it was far more rounded than the wear track left in the samples coated with the other Ni-based coatings. In addition, just a few flakes were noticed around the wear track and in the test ball.
Significant differences were also observed when comparing the cross-sectional depth profile measured at the middle of each wear track left after non-lubricated scratch tests performed on the Ni-based coatings tested (Fig. 10). Whereas very irregular wear tracks with very deep scratches (up to 25 μm depth) were produced in the benchmark Ni and the Ni/hBN composite coatings, a reasonably smoother wear track with far less deeper and irregular scratches (up to 11 μm deep) was always left on the samples coated with the Ni/WS 2 composite coating. The cause for the depth achieved in the benchmark Ni and Ni/hBN composite coatings was the abrasive action of the hard Ni asperities formed on the large wear lumps formed covering the contact surface of the test balls. As a result, the test ball ended up sliding mainly over the Pb-bronze substrate in the samples coated with both the benchmark Ni and  Ni/hBN composite coatings, and hence the decrease in CoF values down to 0.2-0.3 once the coating had failed and the tribosystem had settled down. In the case of the Ni/WS 2 composite coatings though, the ball was still partially sliding over the coating after 10 scratches with relatively constant CoF or around 0.22.

Discussion
Under lubricated conditions (Fig. 11), the standard pure Ni deposits used as the benchmark coating material and the Ni/hBN composite coatings exhibited very similar behaviour, not only in terms of the CoF values measured, but also in terms of the stick-slip motion developed during the course of the tests. In all the tests performed on these coatings, stick-slip motion routinely started during the second half of scratch 2 or early on during scratch 3. Stick-slip motion would occur as a result of the combination of Ni-Ni sliding contact in the presence of the lubricant. Ni-Ni sliding contact took place due to the formation and growth of the wear lump on the ball by the repeated sliding action of the ball on the tested sample (lower CoF values and no signs of stick-slip motion were observed during the first scratch for the Ni/hBN composite coating and the Ni deposits as no material had been transferred from the coating to the ball yet). High friction is expected in those situations where Ni-Ni sliding contact occurs, whereas low friction is expected when a lubricant film forms between the sliding surface and the counter-part, and hence the stick-slip phenomenon observed for these coatings. This would be partly due to the generally poor tribological performance of Ni, especially once there has been a transfer of material from the coating to the test ball, but also due to the harsh mixed-film/ boundary lubrication regime expected in a tribosystem such as the one studied (test ball/lubricant/Ni-based coating/high hertzian-contact pressure/slow sliding) where the load is borne partly by the lubricant film and partly by the asperities present in the surface of the test ball and the wear track formed in the coating [18]. The features observed in the test ball (formation of a wear lump on its surface) and the surface of the tested samples (fairly regular wear track with ridges formed on both sides) would suggest that ploughinglike abrasive wear occurred during the test, where repeated sliding and accumulation of plastic flow at the surface of the coating would result in the formation and growth of the wear lump observed in the test ball [19].
The novel Ni/WS 2 composite coatings exhibited a very different behaviour under lubricated conditions (Fig. 11), not only in terms of CoF (up to 20% lower), but also in terms of sliding motion, as smooth sliding occurred with no signs of the stick-slip phenomenon previously reported for the other Ni-based coatings tested. As a consequence of the smooth sliding, the wear track left in the Ni/WS 2 composites coatings after the scratch tests was not only shallower than the wear tracks left in the other Ni-based coatings evaluated, but also presented a smoother surface with a smaller amount of debris accumulated on the sides of the track. Again, ploughing-like abrasive wear would be the wear mechanism in this case. The causes for this beneficial presence of the WS 2 particles in lubricated tribosystems are diverse: (i) rolling friction [20], (ii) WS 2 particles acting as spacers, preventing the contact between the asperities in both metal surfaces [20], (iii) third body material transfer (i.e., gradual transference of layers of WS 2 onto the metal surfaces during repeated sliding) [21], and (iv) the gradual formation of a thin WS 2 film after continuous and repeated sliding [22]. In the latter referenced study, a thin black WS 2 film gradually formed on the metal surface, very similar to the black film observed on the contact surface of the test ball used for the scratch test performed on the Ni/WS 2 composite coating (Fig. 5(c)). The potential formation of a thin WS 2 film would protect the contact surface and restrict the rise of the friction force, decreasing the influence of asperity contact and improving the tribological performance of the whole system under mixed-filmed lubrication [23].
Under non-lubricated conditions, high CoF values were observed for the benchmark Ni and the Ni/hBN composite coatings after scratch 1 (Fig. 12). These materials would eventually fail after a few scratches: the Ni coatings routinely failed during scratch 3, while the Ni/hBN composite coatings generally failed during scratches 4 or 5. In both cases, coating failure was preceded by the highest average CoF value, which would be measured in the scratch prior to the one where coating failure was actually noticed. Different phenomena would contribute to the occurrence of coating failure such as changes in the area of contact and surface temperature [17], which would be expected where very similar/compatible materials are in contact with an absence of lubrication as in non-lubricated Ni-Ni contact resulting from the formation of a wear lump in the test ball after continuous sliding. Regarding the wear mechanism, the matching between the asperities formed on the wear lumps and the surface of the wear tracks suggest that these hard abrasive asperities were formed by work hardening, phase transitions and third-body formation at the contact interface during repeated and continuous sliding [19], indicating the occurrence of ploughing-like abrasive wear during the non-lubricated scratch test. A different matter is the presence of flat flakes around the ridges and over the wear tracks and test balls. These flakes would originate from deformed surface material caused by heavy sliding contact [24]: (1) As Ni-Ni contact occurs under non-lubricated conditions, strong "adhesive effect" between the "softer" areas of the surface of the wear lump in the test ball and the surface of the wear track might be expected due to high material compatibility.
(2) Tangential shear under compression when strong "adhesive effect" occurs generates "slips" along slip planes in the contact region, resulting in the formation of flake-like shear tongues [19].
Some of these flakes would adhere to the surface of the wear lump during continuous sliding, and hence the larger wear lumps observed in the scratch tests performed in the absence of lubrication. In addition, large amounts of accumulated, plastically-deformed debris were left at the end of the wear tracks generated on the samples coated with both benchmark Ni coatings and the novel Ni/hBN composite coatings, growing after each scratch due to partial removal of the debris attached to the wear lump in the ball and the accumulation of flakes in that area. These latter features are characteristic of adhesive wear [19], which would gain much more importance in the "dry" scratch tests due to the absence of a liquid lubricant. Overall, combined abrasive-adhesive wear would be the wear mechanism during the non-lubricated scratch tests, as opposed to the more abrasive wear observed in the lubricated tests. Combined abrasive-adhesive wear with lack of lubrication would therefore be responsible for the cohesive coating failure and the severe wear observed, as no evidence of delamination due to adhesive failure was observed between the coating and the substrate.
The Ni/WS 2 composite coatings again exhibited a very different behaviour under non-lubricated conditions, not only due to the reasonably smooth and constant CoF values measured during the consecutive scratches (Fig. 12), but also due to the fact that the Ni/WS 2 composite coatings did not fail during any of the non-lubricated scratch tests performed on these coatings. The main reason for this would again be the presence of the WS 2 particles. Nevertheless, no thin WS 2 film apparently formed on either the wear track or the test ball used. In addition, asperities still formed on the surface of the wear lump in the test ball, resulting in the appearance of scratches in the wear track. This leads to the assumption that, in this case, WS 2 particles would act as a solid lubricant lowering CoF by rolling friction mechanisms as suggested by Rapoport et al. [20]. Rolling friction mechanisms provided by WS 2 nanoparticles were also suggested by García-Lecina et al. [8] after their non-lubricated tribological experiments (ball-on-disc tests conducted under very low loads of 1 N) performed on ultrasoundassisted electrodeposited Ni composite coatings with inorganic-like fullerene WS 2 (IF-WS 2 ) particles, where some of the IF-WS 2 particles incorporated into the coating would be released, serving as spacers and preventing asperity contact between the mating surfaces. A different friction mechanism, also under non-lubricated conditions, was suggested by Sivandipoor and Ashrafizadeh for their electroless deposited Ni-P/WS 2 composite coatings (pin-on-disc tests also conducted under very low loads of 1 N), as they reported the formation of a WS 2 -rich film forming discrete regions along the surface of the wear track left on the tested coatings [25]. No clear evidence of such films was noticed in the non-lubricated tests performed on the Ni/WS 2 composite coatings developed during the present research project. Nevertheless, the potential presence and contribution of these WS 2 -rich regions on the surface of the wear track must not be discarded, and more detailed studies are on their way in order to fully understand the friction mechanisms that may take place when WS 2 particles are incorporated into Ni coatings used as tribologically active layers in non-lubricated tribological systems. The latter is of particular importance, as all of the more fundamental research dealing with the friction mechanisms in systems with the presence of WS 2 particles is mainly focused on the use of WS 2 particles in lubricated systems [20][21][22][23][24][25][26].
Overall, the incorporation of WS 2 particles into electrodeposited Ni coatings remarkably enhances their tribological performance either under lubricated or non-lubricated conditions. Regarding the lower CoF and absence of stick-slip motion observed during lubricated tests, such improvement would be a consequence of the lubricious nature of the WS 2 particles. Regarding the apparently higher wear resistance of the Ni/WS 2 composites, roughly indicated by the shallower wear tracks measured for these coatings under both lubricated and non-lubricated conditions, it is worth discussing the work of Sivandipoor and Ashrafizadeh [25]. Their work is interesting because it deals with an electroless Ni-P coating with amorphous structure as deposited, implying that no grain refinement could ever influence the tribological performance of such coatings. In their case, the incorporation of WS 2 particles into said coatings resulted in a significant reduction of hardness due to the soft nature of WS 2 in comparison with Ni. Although their Ni-P/WS 2 composite coatings presented a lower CoF than their bechmark Ni-P coatings due to the presence of WS 2 particles within the coating, they also exhibited higher weight loss during wear tests due to the lower hardness. Grain size refinement in crystalline Ni deposits, which is often achieved by adding grain-refining additives (e.g., saccharine) into Ni plating baths, has a remarkable effect on the hardness and wear resistance of electrodeposited Ni coatings [27,28], but generally little effect on the CoF [27. Therefore, whereas lower CoF and absence of stick-slip motion during lubricated tests in the Ni/WS 2 composite coatings here presented can directly be attributed to the presence of the WS 2 particles within the Ni matrix, the apparent higher wear resistance can be related to the significantly smaller grain size and consequent higher hardness of the composite coating. It must be pointed out that this remarkable grain refinement and consequent increase in hardness were achieved by incorporating the WS 2 particles into the coating [10]. Regarding

Conclusions
Whereas novel Ni/hBN composite coatings show a tribological performance similar to that of a pure Ni deposit, novel Ni/WS 2 composite coatings exhibited a remarkable enhancement of the tribological performance under both lubricated and non-lubricated conditions. Under lubricated conditions, a thin tribofilm grew on the surface of the wear lump formed on the test ball resulting in lower coefficient of friction and prevention of stick-slip motion. Under non-lubricated conditions, reasonably smooth sliding was achieved due to the potential release and presence of WS 2 particles in the contact, resulting in lower coefficient of friction of the coating, and prevention of coating failure.