Microstructural and tribological characterization of NiAl matrix self-lubricating composite coatings by atmospheric plasma spraying

The NiAl-Mo-Ag and NiAl-Cr₂O₃-Mo-Ag composite coatings were fabricated by atmospheric plasma spraying with the substrate material of Inconel 718. The effect of heat treatment on the microstructure, adhesive strength, microhardness and tribological properties of NiAl-Cr₂O₃-Mo-Ag composite coating was investigated. The heat treatment temperatures were chosen as 400, 500 and 600 °C. The composition and microstructure of composite coatings were analyzed by X-ray diffraction (XRD), scanning election microscopy (SEM) and transmission electron microscopy (TEM). The results show that the addition of Cr₂O₃could effectively reduce the wear rate of NiAl-Mo-Ag composite coating. The adhesive strength, microhardness and tribological properties of NiAl-Cr₂O₃-Mo-Ag composite coating were improved by heat treatment. After 500 °C heat treatment, the microstructure of composite coating became more homogeneous and the coating had the highest adhesive strength, microhardness and the best tribological properties. The sliding process could promote the silver, nickel and molybdenum occur tribo-chemical reaction to form silver molybdates and nickel molybdates lubricating films which were responsible for the good tribological properties of composite coatings at elevated temperatures.