Mechanisms of High Hardness in Cr₃C₂-NiCr Cermet Coatings Formed by Vacuum Plasma Spraying

Hardness and microstructure of Cr₃C₂-NiCr coating formed by vacuum plasma spraying process (VPS coating) were investigated in compare with that formed by High Velocity Oxygen Fuel Spraying process (HVOF coating). The results are as follows. (1) The hardness of VPS coatings in as sprayed condition was HV1243 ± 80, which was much higher than that of HVOF coatings, HV958±44, and never went down under HV1100 even after heat treatment at 1273K for 3.6ks. (2) VPS coating presents dense lameller structures composed of Cr₂C₃, Cr₇C₃ and γ-NiCr phase, while HVOF coating presents lameller structures with many fine gaps, composed of Cr₂C₃, Cr₇C₃, γ-NiCr phase and relatively large amounts of Cr₂O₃. (3) The reason why such high hardness was obtained in VPS coating, has been considered due to their dense lamella structures.