Preparation of Protective MoSi2 Coating on Niobium Substrate

A protective MoSi2 coating on Nb substrate was prepared by air plasma spraying and annealing followed by siliconizing. The influence of annealing for plasma-sprayed Mo layer on the subsequent preparation of MoSi2 coating was investigated. Oxidation behavior of the MoSi2 coating was investigated at 1200 A degrees C in air. Phase constituents and microstructure of the MoSi2 coating were characterized by x-ray diffraction and scanning electron microscopy. High-temperature annealing significantly decreases the porosity of the as-sprayed Mo coating (from 9.6 to 2.1%). After vacuum annealing, the splat boundaries and lamellar microstructure of the as-sprayed coating disappear and the recrystallized structure of the Mo coating appears. The MoSi2 coating falls off from the surface of the as-sprayed Mo coating during siliconizing process. The MoSi2 coating prepared on the annealed Mo layer surface has a dense microstructure, which shows a good interface bond between the coating and the Nb substrate. The coating has a multilayer structure, with MoSi2 outer layer, Mo middle layer, and Mo-Nb alloy inner layer. The MoSi2 coating exhibits an excellent oxidation resistance at 1200 A degrees C in air.