Tribological Performance of DLC Coatings Under Starved Lubrication

Abstract Diamond-like carbon (DLC) coatings, known for their low friction and wear resistance in certain conditions, offer a promising alternative to reduce or eliminate lubrication with no pronounced detriment to tribological performance. As an approach to minimise lubricant use in gear, thus enhancing its energy efficiency by reducing lubricant drag and making the application more environmentally friendly, this study investigated the tribological performance of DLC coatings under unlubricated and increasingly severe starved lubrication conditions. Uncoated, hydrogen-free and hydrogenated amorphous carbon (a-C and a-C:H, respectively)-coated steel samples were tested to evaluate the potential of DLC coatings as a solid lubricant and their ability to improve friction and load-carrying capacity compared to uncoated steel under starved lubrication. The experimental results demonstrated that both types of DLC coatings significantly outperformed uncoated steel under all test conditions, maintaining lower friction and better wear resistance. Under starved lubrication, DLC-coated samples exhibited stable, low friction and minimal wear, with no significant correlation between lubricant quantity and wear rate. In unlubricated conditions, DLC coatings showed increased wear at elevated temperatures (60 °C), but performed well at ambient temperature and humidity; a-C proved particularly effective with regard to wear in ambient conditions. These findings suggest that DLC coatings offer a viable and sustainable solution to optimise gear efficiency by reducing the amount of lubricant needed, while still providing superior tribological performance compared to uncoated steel, ensuring low friction and high load-carrying capacity.