Microstructure and oxidation behavior of plasma sprayed WSi₂-mullite-MoSi₂ coating on niobium alloy at 1500 °C

WSi₂ and mullite co-modified MoSi₂ (WMM) coatings were prepared by atmospheric plasma spraying (APS) on the siliconized Nb alloy, and the oxidation behavior was investigated at 1500 °C in air. Single MoSi₂ (M) and single mullite modified MoSi₂ (MM) coatings were also studied for comparison. The effective protection time (≥500 h) of WMM coating was at least 2.8 and 1.5 times longer than that of M (175 h) and MM (346 h) coatings, respectively. The mass loss of the WMM sample was only 4.41 mg/cm² after oxidation up to 500 h, which was 77% and 75% lower than that of M (18.88 mg/cm²) and MM (17.59 mg/cm²) samples, respectively. Such an attractive long-term oxidation protection performance of WMM coating was attributed to the incorporation of mullite and WSi₂. The presence of mullite and, in particular, of glassy SiO₂ contributed to seal cracks. On the other hand, WSi₂ gave rise to the growth of a new transition layer of (Nb, Mo, W)₅Si₃ in the coating, which restrained the outward diffusion of Nb atoms, thus kept the oxide layer intact and stable.