Bio-inspired surface engineering and tribology of MoS2 overcoated cBN–TiN composite coating

A hybrid deposition technique, i.e., electrostatic spray coating (ESC) followed by chemical vapor infiltration (CVI), was used to synthesize cubic boron nitride and titanium nitride (cBN–TiN) hard composite coatings. The as-prepared coatings have a surface texture similar to that of lotus leaf. The “valleys” embossed inside the biomimetic surface structure served as nano-/micro-reservoirs of solid lubricant particles of MoS2, sized from nano- to submicron in diameter. MoS2 particles were applied to coating surfaces by a tumbling method. The tumbled solid lubricant particles covered all surface structures, leading to a smoother coating surface (or reduced average surface roughness Ra). Tribological tests were carried out for the coatings both before and after tumbling. Sliding results suggest that the tumbled cBN–TiN coating has a significantly lower coefficient of friction. Effectiveness of the tumbled lubricant layer decreased with increasing testing temperature, as indicated by sliding results at elevated temperatures. Worn coating surfaces and pin scars were characterized using surface profilometer, scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), focus ion beam, Raman spectroscopy and scanning transmission electron microscopy (STEM). The tumbled MoS2 layer is responsible for friction reduction during dry sliding at different temperature conditions. Sliding induced reorientation of the MoS2 platelets is correlated with the lubrication mechanism. Oxidation caused degradation of MoS2 at higher temperatures, which is consistent with the frictional behavior at elevated temperatures. It is suggested that the biomimetic surface structure is effective for entrapping application-specific solid lubricant particles.