DLC-based solid–liquid synergetic lubricating coatings for improving tribological behavior of boundary lubricated surfaces under high vacuum condition

In the present study, five kinds of liquid lubricants, including MACs, IL, Zdol, PAO and silicon oil, were homogeneously spun on DLC coatings and consequently the DLC-based solid–liquid lubricating dual-layer coatings were successfully fabricated. The tribological behaviors of the DLC/steel combinations and steel/steel combinations lubricated with above five liquid lubricants were comparatively investigated using a high-vacuum tribometer simulated for space environments. The SEM, 3D surface profiler, Raman spectrum and EDS were employed to analyze the worn surfaces of the friction pairs. The analysis results showed that three kinds of solid–liquid lubricating coatings including MACs, IL, Zdol exhibited excellent tribological properties, in which DLC/steel combinations lubricated with these three liquid lubricants exhibited smoother running-in and much lower steady-state friction than steel/steel combinations, and the wear rates of DLC/steel combinations were in the range of 1–3 orders of magnitude lower than that of steel/steel combinations. But for PAO and silicon oil, they demonstrated the poorest tribological behaviors with high friction coefficients and wear rates. Such significant improvement in tribological performance of DLC-based solid–liquid lubricating dual-layer coatings under high vacuum can be attributed to the synergy lubrication mechanism by the combination of the solid lubrication effect of DLC film and the boundary lubrication of the liquid lubricants. Moreover, the differences among different liquid lubricants in tribological properties of DLC-based solid–liquid lubricating dual-layer coatings under high vacuum were possible due to the differences in viscosity and components of the five kinds of liquid lubricants.