Formation mechanism of the lubrication film on the plasma sprayed NiCoCrAlY-Cr2O3-AgMo coating at high temperatures

A self-lubricating NiCoCrAlY-Cr2O3-AgMo composite coating was deposited by atmospheric plasma spray technology. Phase composition, microstructure and thermal behavior of this coating were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Thermogravimetric Analysis (TGA) respectively. Tribological tests of this coating were conducted at 500 °C with different normal loads and sliding velocities. The results showed that the AgMo-containing composite coating exhibited lower friction coefficients around 0.3 at 10 N normal loads due to the formation of Ag2MoO4. However, Ag2MoO4 could not be formed under lower normal load at the critical temperature of 500 °C. Further investigation on the long-term wear behaviors of the coating revealed that the excellent friction reduction and wear resistance performance of the AgMo-containing composite coating at elevated temperatures could be ascribed to a liquid Ag2MoO4 film spreading on the upper surface and a dense oxide protective layer underneath.