A novel method to prepare composite MoSi2 coatings on Mo substrate: Microstructure, oxidation behavior and anti-oxidation mechanism at 1500 °C

Molybdenum (Mo) is a promising structural material for ultra-high temperature applications, but its catastrophic oxidative degradation remains a critical bottleneck. Conventional single-layer silicide coatings often fail prematurely due to either weak interfacial adhesion (spallation) or insufficient long-term structural stability (cracking). To overcome these limitations, a novel dual-layer composite coating was developed in this study. An inner Mo-Si diffusion layer (∼90 μm) was first fabricated by pack cementation process to ensure robust metallurgical bonding with the Mo substrate. Subsequently, an anti-oxidative outer layer (∼98 μm) containing MoSi 2 , boron and quartz was deposited using slurry sintering technique. Isothermal oxidation tests revealed that the composite coating exhibited excellent protective performance at 1500 °C in air, with only minimal mass loss of 0.178 mg/cm 2 after 9 h. Microstructural analysis demonstrated that bidirectional Si diffusion dynamically transformed the coating into a highly stable gradient structure of SiO 2 -Mo 5 Si 3 -MoSi 2 -Mo 5 Si 3 during high-temperature exposure. The superior oxidation resistance was attributed to a synergistic anti-oxidation mechanism: the inner layer physically anchored the coating to prevent spallation, while the outer layer continuously generated a B 2 O 3 -fluxed fluid borosilicate glass phase that effectively sealed microcracks and pores. This work provides an effective architectural design strategy for developing high-performance oxidation-resistant coatings on refractory metals in ultra-high temperature environments.