Solid-Oil Synergistic Lubrication Constructed by Graphite-Like Amorphous Carbon Films and Different Silicone Oils

Silicone oils have been widely applied for the lubrication of mechanical devices due to their high chemical stability, good thermal resistance and superior viscosity-temperature properties. However, the lubrication performance of the silicone oils is usually non-ideal for the metal such as stainless steel tribopairs under the boundary lubrication regime because of the high friction coefficient and severe wear. Recently, the solid-oil synergistic lubrication strategy of which thin solid film and coating materials commonly were used has been increasingly employed to develop superior lubrication technologies. Compared with the single oil lubrication and solid lubrication, solid-oil synergistic lubrication has a shorter run-in period, higher stability and lower wear resistance. Amorphous carbon coatings have attracted widely attention in the field of solid lubrication materials due to their high hardness, low friction, superior wear resistance and outstanding chemical inertness. Here, a solid-oil synergistic lubrication system was constructed by using the graphite-like carbon (GLC), a classic amorphous carbon with high fraction of sp² carbon bonds, and silicone oils. The solid-oil lubrication behaviors of GLC films combining different groups of terminated silicone oils under boundary lubrication were comparatively studied to survey the effects of terminated group of silicon oils. The results showed that the graphite-like carbon films exhibited obvious solid-oil synergistic lubrication effects with silicone oils. And the performance of such solid-oil synergistic lubrication system was influenced by terminated group of silicon oils. Meanwhile, the effect of carbon bonds structure on the solid-oil synergistic lubrication behaviors were scrutinized by using different GLC films with and without hydrogen ion implantation which was a common technology of inducing the change of carbon hybridization of amorphous carbon films. The friction coefficient and wear rate of the solid-oil synergistic lubrication system were as low as 0.02 and 5.3×10⁻⁸ mm³/(N·m) when silicon oil with the polar group of carbinol (hydroxy) was used. It was also found that silicone oils terminated by polar groups could be combined with non-modified graphite-like amorphous carbon to produce better lubrication performance compared with silicone oils terminated by non-polar groups. This might be attributed to the existence of rich sp² hybrid carbon bonds in original GLC films, and the π electrons of sp² carbon bonds could generate dipoles with the polar groups of silicone oil to form more stable thin oil films at the friction interfaces. It also should be pointed out that the hydrogen ion implantation did not be beneficial for the reduction of friction coefficient of GLC films when the silicone oils with polar groups were used since the sp² carbon decrease with the increase of hydrogen implantation. However, the implanted GLC films showed higher wear resistance, which could stem from the improvement of mechanical properties of the modified films.