Effect of Al₂O₃-SiO₂ film at the interface on the microstructure formation and oxidation resistance of MoSi₂ coating on Nb-Si based alloy via slurry sintering method

This study aims to improve the coating density by introducing Al₂O₃-SiO₂ film as a diffusion barrier (DB) at the interface during the sintering process. The influence of the thickness of DB at the interface on the density, phase constituent and interface structure of the coating was studied. The results showed that the Al₂O₃-SiO₂ film can effectively reduce the inward diffusion of Si, facilitating the densification of MoSi₂ coating. Increasing the thickness of the DB correspondingly improves the diffusion blocking effect. Compared with the MoSi₂ coating without DB, the area porosity of the coating with a 5 μm thick DB decreased from 26.8 % to 15.8 %. The oxide DB is discontinuously distributed at the interface. However, although the density of the coatings was markedly increased, severe cracking occurred when the diffusion barrier was excessively thick. The microstructure of coating without DB and coatings with 3 and 5 μm thick DB is generally similar, with the inter-diffusion zone (IDZ) consisting of Mo₅Si₃, Nb₄Fe₃Si₅, and (Nb,X)₅Si₃ three layers. In contrast, the IDZ of coatings with 10 and 15 μm thick DB is comprised of a single (Nb,X)₅Si₃ layer. In comparison to the MoSi₂ coating without DB, the coating with a 5 μm thick DB exhibited a markedly enhanced oxidation resistance. The pores in the coating can be quickly filled by the as-formed SiO₂. The weight gain of coating with a 5 μm thick DB is about 9.4 mg/cm² after oxidation at 1250 °C for 100 h, and about 10.1 mg/cm² after oxidation at 1350 °C for 50 h.