Influence of heat treatments on the microstructure as well as mechanical and tribological properties of NiCrAlY-Mo-Ag coatings

The influence of heat treatments on the microstructure, microhardness, adhesive strength and tribological properties from 25 °C to 900 °C of NiCrAlY-Mo-Ag composite coatings were investigated in this paper. The composite coatings were prepared by atmospheric plasma spraying and heat treated at 400 °C, 500 °C and 600 °C, respectively. The results shown that the microstructure of the coating became more homogeneous and more precipitated phase of Y₂O₃ arised after 500 °C heat treatment. The precipitated phase dispersively distributed in the matrix which could work as the oxide dispersive strengthening phase in the composite coating. Meanwhile, the heat treatment could also improve the micohardness of the composite coatings. The adhesive strength between composite coating and substrate enhanced by heat treatment and that coating heat treated at 500 °C registered the highest adhesive strength of 49.7 MPa. The composite coating heat-treated at 500 °C possessed the excellent lubricating properties at all test temperatures. The tribo-chemical reaction between silver and molybdenum occurred and the silver molybdate lubricating film formed on the worn surface at 700 °C and 900 °C. At 900 °C, the diffraction peaks of NiO were identified on the worn surface. The silver molybdate and NiO played the important lubricating role in the rubbing process for the NiCrAlY-Mo-Ag composite coatings at high temperature.