Effect of heat treatment on structure and property evolutions of atmospheric plasma sprayed NiCrBSi coatings

To investigate the evolutions of structures and properties of NiCrBSi coatings on cylinder liner of engines during its service, atmospheric plasma sprayed (APS) NiCrBSi coatings were heat treated at 300, 500, or 700 °C in this work. The effect of heat treatment on microstructure, phase composition, microhardness and tribological performance has been investigated. It was identified that as-sprayed coatings mainly consisted of γ-(Ni, Fe) phase while CrB and Ni3B phases precipitated from the coatings during the heat treatments at 500 and 700 °C. In addition, inter-splat oxidation of the coating during the heat treatment occurred. Moreover, the crystallinity of the coating was significantly enhanced with increasing the heat treatment temperature. The heat treatment improved obviously the microhardness whereas it did not exert a pronounced effect on the friction coefficients of the coatings. In spite of the enhanced microhardness, the heat treatment at 700 °C increased the wear rate comparatively. It was revealed that inter-splat debonding of the coating surface layer involved in the friction dominated the wear resistance of the coating. Inter-splat oxidation and reduction of coating toughness due to heat treatment can account for the increased wear rate.