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. •The APS-MoSi2 coating on Nb alloy was improved by incorporation of both mullite and WSi2.•The improved oxidation resistance at 1500 °C for 500 h was ascribed to the existence of an intact and stable oxide scale.•The outward diffusion of Nb element was restrained by introducing WSi2.•Mullite contributed to form a crack-free oxide scale.