Oxidation response determined by multiphase-dependent melting degree of plasma sprayed MoSi2 on Nb-based alloy

Abstract

Multiphase constituent is a typical characteristic of plasma sprayed MoSi2 coatings. In present work, the in-situ multiphase doping effect on oxidation behavior of plasma-sprayed MoSi2 compound coating on Nb-based alloy was comparatively investigated through two different fabrication methods: suspension plasma spray (SPS) and supersonic atmospheric plasma spray (SAPS). For the first time, the melting index, M.I., was proposed based on formation feature of each phase to illustrate the multiphase effect. Multi-phases including tetragonal MoSi2 (α), metastable MoSi2 (β), Mo5Si3, the amorphous and nanoparticle precipitates were found in both plasma sprayed MoSi2 coatings. The SAPS-MoSi2 coating exhibited a higher M.I. value than that of SPS-MoSi2 coating. Cyclic oxidation behavior at 1200 °C demonstrated that higher M.I. could bring better oxidation resistance of MoSi2 coating on Nb-based alloy. SAPS-MoSi2 coating possessed an extremely low thermally parabolic rate constant of 1.39 × 10−4 mg2cm−4h−1 under long-duration exposure in air at 1200 °C. The phase-dependent M.I. on thermally sprayed MoSi2 can balance negative factor of pesting oxidation and positive factor of forming protective glassy film in high-temperature cyclic oxidation experiment. High M.I.-value MoSi2 coating on Nb-based alloy could easily achieve a continuous, adhesive and protective surface scale as well as the absence of pesting oxidation.