Three sputtered nanocomposite coatings, MoN-Cu, VN-Cu, and MoVN-Cu, were prepared as potential tribocatalytically reactive materials (Fig. 1). The surface morphology analysis of all three coatings shows a relatively uniform texture of the surfaces (Fig. 1a-c). The incorporated Cu slightly varied among three coatings with VN-Cu having the highest amount of 5.9 wt.% (Fig. 1b) corresponding to the lowest hardness of 18.3 GPa of the film (Fig. 1g). The XRD analysis reveals MoN-Cu, VN-Cu, and MoVN-Cu have Mo2N, VN, and MoVN as the dominant phases, respectively. Also, traces of metallic (110) Mo and (200) V phase at 40.1° and 62.9° have been detected in MoN-Cu and VN-Cu, correspondingly (Fig. 1d-f). The grain sizes of the coatings calculated by Debye-Scherrer’s equation were 7–8 nm for MoN-Cu, 12–13 nm for VN-Cu, and 7–8 nm for MoVN-Cu. The nano-indentation results of the coatings indicate MoVN-Cu having the highest hardness and elastic modulus of 30.3 GPa, and 346 GPa, respectively.
To determine which coating among the three candidates has the best tribological performance, a set of tribology tests was conducted in decane at 1 N load at 50°C, followed by the analysis of the wear tracks (Fig. 2). The results were compared to steel with 55–60 HRC hardness that was used as a coating substrate material. Among all samples in this study, MoN-Cu nanocomposite demonstrated the lowest coefficient of friction. The optical micrographs revealed that all three sputtered nanocomposite coatings have much smaller wear than the uncoated steel. Surprisingly, the stylus profiler analysis demonstrated that the use of the MoN-Cu coating did not only drastically suppress the wear but even resulted in protective tribofilm material build-up as was indicated by positive cross-section profile changes of the wear track. This film formation for MoN-Cu has been more vivid than for two other samples, VN-Cu and MoVN-Cu. The results suggest that MoN-Cu tends to form a protective layer on the contact area that protects the surface from mass loss during tribology (Fig. 2a-e).
The EDS elemental composition and Raman 2D mapping were performed to analyze the nature of the formed during sliding tribofilms (Fig. 3). The blue dashed lines on the optical micrographs indicate the areas for which EDS and Raman analyses were performed. EDS mapping results clearly show the carbon nature of the formed films. Notably, the presence of carbon in the wear tracks is more pronounced for MoN-Cu and VN-Cu samples. 2D Raman analysis indicates the characteristic carbon D (at ~ 1340 cm− 1) and G peaks (at ~ 1560 cm− 1) suggesting the Diamond-Like-Carbon (DLC) structure, similarly to the previously observed tribocatalytic formation of DLC from oils [17]. Raman spectroscopy reveals the relatively high intensity of DLC formation in the MoN-Cu wear track which is supported by higher uniformity of the G-band intensity in the wear track (Fig. 3g). These results suggest that though the hardness is usually considered a positive contributor to the wear reduction of the coating [8], it still should be low enough to release the catalytic centers to the environment. Specifically, the MoVN-Cu has the highest hardness among other candidates, 30.3 GPa (Fig. 1g), but the lowest Cu content shows the minimum C formation on the wear track (Fig. 3f). The softest among the three candidates (with the highest Cu content), VN-Cu coating, shows some lowering in friction and more distinguished signs of carbon film formation. Meanwhile, the middle case, MoN-Cu film with ~ 2.3 wt% of Cu, demonstrates the highest promise to lowering the friction and surviving the overall wear of the surfaces by facilitating higher carbon formation as indicated from the characteristic Raman peaks intensity.
Since MoN-Cu had the lowest COF (Fig. 2a) and the most promising film formation among other candidates (Fig. 3), it was selected to further understand how load, temperature, and different alkane solutions influence the DLC film formation of the coatings. The results from the tribology tests performed at various loads within the range of 0.25-1 N and at 25 and 50°C upon immersion in decane, dodecane, and hexadecane are illustrated in Fig. 4a-h. At low temperature and contact pressure, the COF behavior is not steady and results in higher friction values (Figs. 4b and 4d). As the temperature and load increase, the COF becomes steadier (Fig. 4h). Though the observed average friction changes are not as dramatic, the wear evaluation (Figs. 4i and 4j) suggests that the increase in the operating temperature and applied load lead to lowering of the wear track width as a result of the protective tribofilm formation.
The summary of the wear track width changes for each sample after tribology tests at 25 and 50°C is represented in Figs. 4i and 4j, respectively. Direct comparison between 25 and 50°C runs suggests that raising temperature reduces the wear track with, i.e., at 0.25 N in hexadecane, the wear track width was 160.8 µm at 25°C and reduced to 82.3 µm as temperature increased to 50°C. Stylus profilometry results indicate that the nature of the hydrocarbon is also important for protective film formation. Interestingly, decane and hexadecane form the carbon film in the wear tracks more readily than dodecane; it is consistent with the film formation across the whole wear track (Fig. 4k-p). This formation is further accelerated at 50°C, which is noticeable by the larger volume of carbon built-up in the wear track (Fig. 4n-p).
To have a better knowledge of how alkanes impact the DLC film formation on the MoN-Cu, a series of 2D Raman maps were collected for the wear tracks (Fig. 5). The characteristic DLC phase D and G-bands were observed for the wear tracks formed in all three candidate hydrocarbons. However, the intensity of the G-band for decane and hexadecane was higher than for dodecane (Fig. 5d-f). Also, the distribution of the DLC in the dodecane wear track is less uniform (Fig. 5e). EDS mapping of the wear track reveals the same conclusion, much less carbon is formed from dodecane (Fig. 5j-l)
The observed results suggest that in all the cases, the MoN-Cu surface shows very promising tribocatalytic performance. Similarly to previous studies [17], this tribocatalytic performance is expected to originate from the copper presence in the coating with the possible contribution of Mo [23] released during sliding. The contact pressure and temperature supported by local asperity heating events [24] are assisting in the tribocatalytic process by providing enough energy for alkane chain dehydrogenation and dissociation [25] in presence of the catalyst leading to the release of carbon and formation of the DLC film.
The formed DLC film is expected to transfer on the counterbody to ensure easier shearing and better surface protection from the wear. Indeed, further characterization performed on the alumina counter-body surfaces after the tribology tests revealed the presence of the DLC debris on the counterbody (Fig. 6). The 2D-Raman mapping of the alumina surface after the run in decane at 1N load and 50°C confirms the DLC tribofilm transfer during sliding (Fig. 6a-c). This transferred film facilities shearing between the nanocomposite surface and Al2O3 counter-body which is translated into the lower COF and smaller wear track width (Fig. 2a).
Though all three alkanes show the signs of DLC formation, the rate of the film formation among them differs with hexadecane showing the higher signs of the material build-up, as suggested by profilometry (Fig. 4p) and Raman intensity (Fig. 5f) results, and better coverage of the film in the wear track (Figs. 5c and 5f). Notably, the contact angle measurement results for three alkanes suggest very good wetting of all three alkanes with a slightly higher contact angle for dodecane than for decane and hexadecane (Fig. 6d-f). This suggests that decane, dodecane, and hexadecane have a high potential for lubricating the surfaces during sliding and the observed discrepancies in the tribocatalytic activity have a different origin.
The long-chain hydrocarbons are generally considered to be more reactive than short-chain hydrocarbons [26]. Previous studies performed on various alkanes inside the diamond anvil cell exposed to high temperature and high-pressure conditions indicated higher yields of carbon deposition for longer chain alkanes [27, 28] that is in agreement with observed better tribocatalytic activity for hexadecane. It should be, however, noted that during the tribosliding, the local temperature increase and pressure distribution are affected by the lubricant presence that can be pushed outside of the sliding contact by the contact pressures [29] thus leading to boundary lubrication regime of solid/solid contact interface [30, 31]. Though the presence of lubricant helps to reduce the wear, as the longer chain hydrocarbons show higher viscosity and thus result in the formation of a thicker lubrication film, less heating in the contact is expected. The competition of the contributions of two interconnected mechanisms, lowering in local heating for longer chain alkanes and reduction in the activation energy for longer chain alkane decomposition, are, therefore, expected to lead to the observed reduced formation of carbon film in dodecane relative to decane and hexadecane.
Formation of the DLC tribofilm plays a pivotal role in observed friction and wear reduction of the coatings in comparison to steel counterparts. When the tribofilm is eventually worn away, the copper clusters in the coating are re-exposed to the hydrocarbons causing the tribocatalysis to restart and develop new layers of tribofilm protecting the surface. The whole process is continuous and self-regulating.
MoN-Cu, MoVN-Cu, and VN-Cu nanocomposite coatings were evaluated for their tribocatalytic potential. These nanocomposites provide relatively high hardness values with the maximum hardness of 30.3 GPa for MoVN-Cu. The tribological behavior of the sputtered nanocomposites coatings tested against alumina counterparts in decane at 1 N of load and 50°C reveals that all three coatings demonstrate large improvement in comparison to uncoated steel substrate with MoN-Cu having better friction and wear behavior than two other candidates. All three nanocomposite coatings show near-zero wear volume with signs of material built up in the wear track. The wear track characterization reveals the formation of diamond-like carbon film inside the wear track that helps in the tribological performance of the coatings.
Tribology tests on the MoN-Cu sample in the range of 0.25-1 N load and at 25 and 50°C in decane, dodecane, and hexadecane indicate that the COF behavior becomes steadier and leads to smaller wear track width at the elevated temperature and higher applied load regime.
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Among three selected alkanes, dodecane shows the lowest tribofilm formation tendency. Meanwhile, in the case of decane and hexadecane, the formed tribofilms have a more uniform structure.
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The EDS mapping shows C-rich film formation in the wear track and Raman spectroscopy analysis detects the DLC nature of the tribofilm. Material transfer between the nanocomposite surface and counter-body facilities easy shearing action.
The difference in the observed hydrocarbon lubrication and tribofilm formation facilitation is attributed to the lower carbon yield from the long chain hydrocarbons and lower wetting of dodecane on the coating surface.