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 °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 °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 °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 °C.

Microstructural, Electrical, and Tribomechanical Properties of Mo-W-C Nanocomposite Films

“…Mo-W-C coatings studied by Mandal [ 20 ] exhibited the best mechanical properties at the C/(Mo + W) of about 2, which was close to the carbon-to-metal ratios observed in present films with higher hardness. In addition, the calculated and experimental hardness values of β-Mo 2 C were only 11.38 GPa and 14.8 GPa, respectively [ 6 ].…”

“…Hen enhancement of hardness was likely to be attributed to the nanocomposite nature deposited Mo-W-C films, i.e., a combination of hard carbide nanocrystallite different strictures embedded into an amorphous carbon matrix [63]. Mo-W-C coatings studied by Mandal [20] exhibited the best mechanical properties at the C/(Mo + W) of about 2, which was close to the carbon-to-metal ratios observed in present films with higher hardness. In addition, the calculated and experimental hardness values of β-Mo 2 C were only 11.38 GPa and 14.8 GPa, respectively [6].…”

“…From Equation ( 25), it follows that the static susceptibility should increase with temperature due to the increase in the number of tunnelling per unit time per unit volume, Equation (20).…”

“…Moreover, Page et al [ 19 ] demonstrated that nanostructured Mo 2 C and WC with a large surface area can be used for catalytic processes as substitute materials for platinum group metal heterogeneous catalysts. In one of the recent works, Mo-W-C coatings with three different C/(Mo + W) ratios (5:1, 2.8:1, 2.2:1) were synthesised using unbalanced magnetron sputtering combined with high-power impulse magnetron sputtering [ 20 ]. The research results showed that coatings with low values of the carbon-to-metal elements ratio had the best tribological properties during lubricated sliding.…”