Microstructure and tribological properties of ZrB2-enhanced NiCrBSi coatings prepared by high-velocity oxy-fuel spraying

This paper presented a novel study of the preparation of ZrB2-enhanced NiCrBSi coatings and its microstructure and tribological properties. NiCrBSi-ZrB2 composite powder and coatings with ZrB2 contents of 20 %, 40 %, and 50 % were prepared by spray-drying and high-velocity oxygen-fuel (HVOF) spraying, respectively. The microstructure and high-temperature wear performance at 700.C were compared with those of an NiCrBSi coating. The results showed that the NiCrBSi and ZrB2 powder exhibited good wetting properties in the solid-liquid sintering zone and the CrB phase was formed in the liquid phase contact zone in composite coatings with ZrB2 contents of 20 % and 40 %, while the CrB phase disappeared in the 50 %ZrB2 coating due to the pinning effect caused by excessive ZrB2. The microhardness of composite coatings tended to increase, then decrease, and the 40%ZrB2 coating reached a maximum hardness of 874 HV. The NiCrBSi coating presented oxidative wear and adhesive wear. Although the fracture toughness of composite coatings was lower than that of the NiCrBSi coating, the wear resistance in the composite coating was significantly enhanced due to the evenly distributed ZrB2 and CrB phases. The 20%ZrB2 coating with low fracture toughness and the 50 %ZrB2 coating with hardness mismatch caused by the absence of CrB produced severe abrasive wear. The hardness gradient with the matrix, ZrB2 and CrB phases reduced abrasive wear caused by peeling in 40%ZrB2 coating, and the ceramic phases acted as skeletons reducing the deformation of the load contact area. The formation of Cr2O3 during the friction process further improved the wear resistance. The wear mechanism of 40%ZrB2 coating underwent light adhesive wear, mild abrasive wear and oxidative wear, and the coating exhibited the best wear resistance with the lowest wear rate of 1.16 x 10(-1)3 m(3)/N.m (a decreased of 56 %).