Clarification of high wear resistance mechanism of ta-CNx coating under poly alpha-olefin (PAO) lubrication

Tetrahedral amorphous carbon nitride (ta-CNx) is suitable for tribological application due to its excellent mechanical and adhesion properties. In this study, ta-CNx coatings with N/C ratios (0-11%) are deposited by ion beam assisted filtered arc deposition (IBA-FAD), and the tribological properties are investigated with a ball-on-disk tribo-tester. The results show that nitrogen doping decreases the fraction of sp3 bonding, hardness, Young's modulus and roughness. In oil ta-CNx presents slightly decreasing friction coefficient and improving wear resistance with the N/C ratio, indicating softer ta-CNx with better wear resistance. The lower surface roughness after friction of high nitrogen content ta-CNx makes it more likely to transit to mixed lubrication, resulting in lower contact load that can explain the better wear resistance.