Microstructure and properties of Mo-based double-layer MoSi2 thick coating by a new two-step method

A double-layer MoSi2 thick coating was prepared on Titanium-Zirconium-Molybdenum (TZM) alloy through a new two-step method, including molybdenum slurry vacuum sintering and halide-activated pack cementation. The coating was formed by a porous MoSi2 outer layer and a dense MoSi2 inner layer. The ball-milled fine Mo powder can effectively improve the coating density and the high temperature oxidation resistance. The oxidation protection time of the coatings prepared by coarse and fine Mo powders were 1 h and 6 h at 1700 degrees C, respectively. After oxidation, the coating exhibited multilayer structure consisted of SiO2 oxide layer, porous MoSi2 layer, dense MoSi2 layer and Mo5Si3 layer from the outer to the inner. With oxidation proceeding, the Mo5Si3 layer thickened, while the MoSi2 layer thinned and eventually transformed into Mo5Si3 completely, leading to coating failure because of the depletion of silicon.