TEM characterization and reaction mechanism of composite coating fabricated by plasma spraying Nb-SiC composite powder

Niobium carbide composite coatings with Nb2C, NbC, Nb3Si as the main phases were prepared in situ on the surface of TC4 titanium alloy by plasma spraying Nb-SiC composite powder. The microstructure of the coating was characterized in detail by TEM, and the reaction mechanism of Nb-SiC was revealed. Sub-micron and nano -scale NbC grains dispersed in Nb3Si region, nano-Nb/Nb3Si cellular eutectic region, and equiaxed Nb2C nano -grains region were formed in the coating. The research results show that Nb and SiC reacted firstly to form cubic NbC and Nb3Si phases during the plasma spraying process. Then, NbC with a higher melting point took the lead in crystallization during the cooling process of the coating, forming sub-micron and nano-scale NbC granular fine grains. Nb3Si with a lower melting point crystallized around the sub-micron and nano-scale NbC granular fine grains in the subsequent cooling process. In the plasma spraying process, the molten droplets formed Nb/Nb3Si cellular eutectic structure under large temperature gradient and extremely fast cooling rate. The remaining Nb in the raw material powder formed a diffusion couple with NbC to generate fine and dispersed nano-equiaxed Nb2C with cubic structure. The present investigation provides a reference for the reaction synthesis of advanced nanocomposites using Nb-SiC system.