Microstructure evolution and growth mechanism of Si-MoSi2 composite coatings on TZM (Mo-0.5Ti-0.1Zr-0.02 C) alloy

The Si-MoSi2 composite coatings with a high silicon concentration were prepared on TZM alloy through a hot dip siliconizing method (HDS). The evolution behavior of microstructure and phase composition of Si-MoSi2 composite coatings with different diffusion times were studied. The Si-MoSi2 composite coatings have a very dense and uniform microstructure with a submicron roughness, and the lowest Sa, Sq, and Sz are only 0.258, 0.312, and 5.08 µm, respectively. A typical three-layer diffusion structure is observed at cross-section of coatings, the MoSi2 phase and carbide phases form in the outer layers, and the intermediate layer only consists of MoSi2 phase. Whereas the transition layer mainly consists of Mo5Si3 phases. The formations of globular SiO2 and carbide dispersion particles on coating surface are due to the reduction reaction between Si and (Ti,Zr)O2 dispersion phase, and the carbonation reactions. The growth process of silicide ceramic coating can be divided into four different stages: wetting stage, initial diffusion stage, stratification stage of diffusion layer, and final stage of diffusion layer formation.