Wear Mechanism of Mo-W Doped Carbon-Based Coating during Boundary Lubricated Sliding

Abstract

The high temperature tribological applications of state-of-the-art diamond-like-carbon (DLC) coatings in automotive industry are often compromised due to their poor adhesion strength and low thermal stability. A molybdenum and tungsten doped carbon-based coating (Mo−W−C) is developed in order to overcome these limitations and to enhance tribological performance during boundary lubricated sliding at ambient and elevated temperature. The coating was deposited utilising HIPIMS technology. Mo−W−C coating showed lowest mean friction coefficient (µ=0.033) compared to a number of commercially available state-of-the-art DLC coatings when pin-on-disc experiments were carried out at ambient temperature. Similarly at 200°C, a significant reduction in friction coefficient was observed for Mo−W−C coating with increase in sliding distance unlike DLC coating. Raman spectroscopy revealed importance of combined Mo and W doping and tribochemically reactive wear mechanism of Mo−W−C coating during sliding. The significant decrease in friction and wear rate was attributed to the presence of graphitic carbon particles (from coating) and 'in-situ' formed metal sulphides (WS2 and MoS2, where metals from coating and sulphur from oil) in transfer layer.