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

WSi 2 and mullite co-modified MoSi 2 (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 MoSi 2 (M) and single mullite modified MoSi 2 (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/cm 2 after oxidation up to 500 h, which was 77% and 75% lower than that of M (18.88 mg/cm 2 ) and MM (17.59 mg/cm 2 ) samples, respectively. Such an attractive long-term oxidation protection performance of WMM coating was attributed to the incorporation of mullite and WSi 2 . The presence of mullite and, in particular, of glassy SiO 2 contributed to seal cracks. On the other hand, WSi 2 gave rise to the growth of a new transition layer of (Nb, Mo, W) 5 Si 3 in the coating, which restrained the outward diffusion of Nb atoms, thus kept the oxide layer intact and stable.