Microstructure and properties of niobium carbide composite coatings prepared by plasma spraying

Niobium carbide composite coatings were prepared on titanium alloy surface by plasma spraying NbC–Al₂O₃, Nb–SiC and Nb–SiC–Al composite powders, respectively. The phase composition, microstructure and formation mechanism of the three composite coatings were analyzed and their microhardness, toughness and scratch resistance were compared. The phases of the NbC–Al₂O₃ system did not change during the plasma spraying process, and new phases (Nb₂C, NbC and Nb₃Si) were formed in the Nb–SiC and Nb–SiC–Al systems. TEM results of the Nb–SiC composite coating indicate that the new phases nanocrystalline Nb₂C, submicron NbC and nanocrystalline Nb₃Si were formed during the plasma spraying process. Compared with the NbC–Al₂O₃ composite coating, the microstructure of the Nb–SiC and the Nb–SiC–Al composite coatings were uniform, and the porosity were relatively low, and the hardness was higher. The Nb–SiC–Al composite coating was denser than the Nb–SiC composite coating, the lamellar structure was obvious and the number of pores in the coating was the least, which is attributed to the better molten state of the composite powder by the addition of the Al to the Nb–SiC system. The Nb–SiC–Al composite coating had better toughness and scratch resistance.