Nitride formation in synthesis of titanium aluminide matrix composite coatings by reactive RF plasma spraying

This paper presents an in situ process to form intermetallic matrix composite coatings by reactive radio frequency (RF) plasma spraying with premixed elemental powder. The typical splat morphology of impinged titanium droplets on a stainless steel substrate is a disk with an outer peripheral fringe. If the supplied titanium powder size becomes finer or the nitrogen partial pressure in the plasma gas increases, splats containing prominent asperities with a smaller flattening ratio appear along with the plain disk type. An increase in nitrogen content is detected in all the splats sprayed with finer titanium powder and/or higher nitrogen partial pressure. The splats containing prominent asperities, which correspond to TiN, are twice as high in nitrogen content than the plain disk type. Aluminum splats are also classified into two categories: a disk type with an irregular outer periphery and a seminodular type. Oxygen exists on the splat surfaces, on which there are nitrogen concentrated areas corresponding to AlN. Consequently, the nitride formation proceeds on titanium and aluminum droplets during the flight as well as on the substrate. If the substrate temperature is higher than 873 K just before spraying with premixed titanium and aluminum powder, the formation of TiAl and Ti₂AlN proceeds on the substrate because of negligible mutual collisions during the flight. Titanium aluminide matrix in situ composites sprayed with premixed titanium and aluminum powder contain more nitrides than those sprayed with TiAl compound powder, because of the higher nitrogen absorption in titanium and aluminum droplets that results in an exothermic reaction.