# Influence of Carbon: Metal Ratio on Tribological Behavior of Mo-W-C Coating
###### Abstract  
Mo-W-C coatings with three different C/(Mo+W) ratios (5:1, 2.8:1 and 2.2:1) were deposited by using combined unbalanced magnetron sputtering (UBMS) and high-power impulse magnetron sputtering (HIPIMS) technology. The influence of the C/(Mo+W) ratio on coating microstructure and related tribological properties at ambient temperature and at 200 \({}^{\circ}\)C were studied in lubricated condition (up to 7500 m and 1800 m of sliding distances, respectively). Results showed that a decrease in the C/(Mo+W) ratio could be correlated with an increase in coating thickness, adhesion strength, hardness and elastic modulus values, and a decrease in the degree of graphitization. At ambient temperature, outstanding tribological properties (very low friction and negligible wear) were observed irrespective of the C/(Mo+W) ratio. At 200 \({}^{\circ}\)C, low C/(Mo+W) ratios (2.8:1 and 2.2:1) were found particularly beneficial to achieve excellent tribological properties. The keys to significant friction reduction at 200 \({}^{\circ}\)C were (i) in situ formation of MoS2 and WS2 due to tribo-chemical reactions and (ii) presence of amorphous carbon debris particles in the protective tribolayer. With an increase in sliding distance, the tribolayer gradually lowered the friction coefficient by protecting both the coating and counterpart from severe wear. On the other hand, a high C/(Mo+W) ratio (5:1) led to low friction but noticeable abrasive wear at 200 \({}^{\circ}\)C.  
C/(Mo+W) ratio; microstructure; sliding friction; abrasive wear; tribo-chemical wear; Raman analysis Article  
Influence of Carbon: Metal Ratio on Tribological Behavior of Mo-W-C Coating  
1][PERSON]  
1][PERSON] Influence of Carbon: Metal Ratio on Tribological Behavior of Mo-W-C Coating. 2021 11 10189 1]C.  
contact stress (2.41 GPa), the magnetron sputtered GLC coatings showed \(\mu\)-0.06-0.2 when sliding against steel ball at room temperature, which was further reduced to \(\mu\)-0.04-0.09 with an increase in temperature to 90 \({}^{\circ}\)C. However, GLC coatings could not outperform DLC coatings particularly in terms of providing better wear resistance even when deposited via the same magnetron sputtering process in the same coating deposition unit [17]. Apart from the enhanced tribological properties, the addition of either Mo or W in DLC coating also increased the thermal stability up to \(\sim\)500 \({}^{\circ}\)C [18]. With an increase in Mo content in reactive RF sputtered Mo-DLC coatings, Mo-containing amorphous carbon phase was gradually changed to Mo-C phase but the tribological behavior was observed independent of it. The lowest \(\mu\) (\(<\)0.1) was found at 11% Mo content during dry sliding (against WC ball under 2 N normal load), whereas relatively higher \(\mu\) (\(>\)0.15) was observed when Mo content was either increased or decreased from 11% [12]. On the other hand, combined ion beam and DC magnetron sputtered multilayer W-DLC coating exhibited a linear increase in sp\({}^{2}\) content, elastic modulus and coating hardness with an increase in W content from 3.08% to 27.7%. During reciprocating ball-on-disc sliding tests in boundary lubricated conditions (against steel ball under 20 N normal load), the friction coefficients were observed as being independent of W content, in contrast to the wear rates. The best anti-wear properties were observed at 10.73% W content, due to its high hardness-to-Young's-modulus (H/E) ratio [19]. A different behavior was observed for magnetron-sputtered W-GLC coatings. With an increase in W content up to 10.25%, the adhesion strength, hardness, H/E ratio, toughness and wear rate were linearly increased, while the friction coefficient (\(\mu\) \(<\) 0.03) stayed the same during dry sliding at room temperature. An opposite trend in coating behavior was noticed with a further increase in W content up to 17.2% [20].