Green oil additive g-C3N4: a feasible strategy to enhance the tribological properties of DLC film

Abstract

The decrease of both friction and wear is very crucial for reducing oil consumption and the associated environmental pollution. Accordingly, the search continues for novel materials, coatings, lubricants, and systems constituted by the aforementioned ones that can potentially reduce friction and wear. In this paper, a solid–liquid composite lubrication system was constituted by combining g-C3N4 nanosheets as the lubricating oil additives with Ti-DLC film, to achieve a green lubricant providing extreme anti-wear and friction reduction functionality. Tribological properties of the composite lubrication system with different concentrations of g-C3N4 were evaluated using a ball-on-disc sliding friction tester at different normal loads. The results show that both the friction coefficient and wear rate first decreases and then increases with the increase of the g-C3N4 concentration. The lubricant with 1 wt% g-C3N4 exhibits the optimal lubricant property with the friction coefficient of 0.076, 0.078 and 0.082, reducing by nearly 24%, 20%, and 12% compared with base oil for the normal loads of 50 N, 100 N, and 200 N, respectively. However, further increase in the concentration of g-C3N4 results in significant increase in the friction and wear of the tracks. The wear rates show no marked variation with the increasing load, indicating that g-C3N4 nanosheets possess a wide load adaptation range in wear-resistance. As environmentally friendly oil additives, g-C3N4 nanosheets can effectively improve the tribological property of the composite lubrication system. Based on the characterizations of both the Ti-DLC film and wear scars by SEM, XPS, non-contact 3D surface profiler and EDS, the synergistic lubrication mechanism of the g-C3N4 nanosheets and Ti-DLC film is also proposed.