# Influence of Carbon: Metal Ratio on Tribological Behavior of Mo-W-C Coating
###### Abstract
These findings indicate that the amount of metal content in a carbon-based coating significantly controls the coating composition and microstructure; hence, it influences the tribological properties. Recently a new-generation nanostructured Mo and W doped graphitic carbon-based coating (Mo-W-C) has been developed by using a combined unbalanced magnetron sputtering (UBMS) and high-power impulse magnetron sputtering (HIPIMS) technology [21], and several studies have established the uniqueness of simultaneous doping of Mo and W in terms of achieving high thermal stability up to \(\sim\)800 \({}^{\circ}\)C [22] and outstanding tribological properties at both ambient and elevated temperature conditions during boundary lubricated sliding [23]. The tribo-chemical wear mechanism has been identified as the key to achieve these excellent tribological properties at ambient temperature, and it is then rapidly accelerated with an increase in the test temperature. The coating wear rate has been observed as almost negligible against the metallurgically inert counterpart, such as alumina at the elevated test temperature when compared to the steel [23]. These low-friction and high-temperature wear-resistant characteristics of graphitic Mo-W-C coating make it suitable for an alternative to the commercially available multilayered metal-doped DLC coatings in car-engine applications, in contrast to any other GLC coatings. In a typical commercial four-target PVD coating machine, any single metal/non-metal-doped DLC coating is often deposited with minimum of three graphite targets in order to compensate for the significantly low sputtering rate of carbon. When there is more than one dopant in a DLC coating, a multi-target (six or eight targets) PVD coating machine is usually required to deposit such ternary or quaternary carbon-based coatings. This is also applicable when the dopants have a significantly higher sputtering rate than carbon; for example, the individual sputtering yields of Mo and W are \(\sim\)2-8 times higher than graphite at any accelerating voltage and at different incident angles in an argon atmosphere [24]. In order to eliminate these limitations, a compound metallic target has been designed, where a certain number Mo discs (of 40 mm diameter) were incorporated into a WC base (190 mm \(\times\) 600 mm), resulting in a judiciously preselected Mo:WC ratio of 30:70. This compound metallic target has been used as a single sputtering source of both the doping elements, and it enables Mo-W-C coating to be deposited in a four-target coating machine rather than a multi-target machine. A variation in operating power of this compound target straightaway deposits the graphitic Mo-W-C coatings with different metallic content (including both Mo and W, hence termed 'Mo+W'), and this subsequently leads to different carbon: metal ratios, i.e., C/(Mo+W), in the deposited coatings. The higher the metallic content found in a coating is, the lower the C/(Mo+W) ratio is. The novelty of graphitic Mo-W-C coating is its capability to retain its excellent tribological behavior at both ambient and elevated temperatures when deposited with a precise Mo+W content [23]. However, it has not been looked into if a variation in Mo+W content influences the coating microstructure and thereby tribological properties accordingly. In this work, Mo-W-C coatings were deposited with three different Mo+W contents leading to three different C/(Mo+W) ratios. The influence of C/(Mo+W) ratio on coating microstructure and followed by tribological properties at both ambient temperature and at 200 \({}^{\circ}\)C (typical for engine application) are demonstrated in this work.