Microstructure, mechanical and tribological properties of plasma-sprayed NiCrAlY-Mo-Ag coatings from conventional and nanostructured powders

The conventional and nanostructured NiCrAlY-Mo-Ag composite coatings were prepared by atmospheric plasma spraying so as to make the hot parts achieve and maintain excellent tribological properties at elevated temperature of up to 900 °C. The microstructure, mechanical and tribological properties of these two composite coatings were investigated and compared. The X-ray diffraction (XRD), micro-Raman, scanning election microscopy (SEM) and transmission electron microscopy (TEM) were used to analyze the composition and microstructure of the composite coatings. Results show that the nanostructured composite coating presented higher microhardness and adhesive strength than conventional composite coating. Meanwhile, the nanostructured composite coating had less defects, lower porosity, more compact microstructure and smaller grains as compared to the corresponding conventional coating which was useful to improve the performance of the composite coating. The nanostructured composite coating possessed better tribological properties as compared with the conventional composite coating. The sliding process could promote the tribo-chemical reaction between silver, nickel and molybdenum to form silver molybdates and nickel molybdates lubricating films on the worn surface at high temperature. The silver molybdates and nickel molybdates could also enhance tribological properties of composite coatings at high temperature.