To improve the oxidation resistance of Mo-based alloys, a novel (Mo,Hf)Si2-Al2O3 composite coating was fabricated on a Mo-based alloy by the method of slurry sintering. The isothermal oxidation behavior of the coating was evaluated at 1400 °C. The microstructure evolution and phase composition of the coating before and after oxidation exposure were characterized. The anti-oxidant mechanism for the good performance of the composite coating during high-temperature oxidation was discussed. The coating had a double-layer structure consisting of a MoSi2 inner layer and a (Mo,Hf)Si2-Al2O3 outer composite layer. The composite coating could offer more than 40 h of oxidation-resistant protection at 1400 °C for the Mo-based alloy, and the final weight gain rate was only 6.03 mg/cm2 after oxidation. A SiO2-based oxide scale embedded with Al2O3, HfO2, mullite, and HfSiO4 was formed on the surface of the composite coating during oxidation. The composite oxide scale exhibited high thermal stability, low oxygen permeability, and enhanced thermal mismatch between oxide and coating layers, thus improving the oxidation resistance of the coating.

To enhance the oxidation resistance of Mo-based TZM alloy (Mo-0.5Ti-0.1Zr-0.02C, wt%), a novel MoSi2-ZrB2 composite coating was applied on the TZM substrate by a two-step process comprising the in situ reaction of Mo, Zr, and B4C to form a ZrB2-MoB pre-layer followed by pack siliconizing. The as-packed coating was composed of a multi-layer structure, consisting of a MoB diffusion layer, an MoSi2-ZrB2 inner layer, and an outer layer of mixture of MoSi2 and Al2O3. The composite coating could provide excellent oxidation-resistant protection for the TZM alloy at 1600 °C. The oxidation kinetic curve of the composite coating followed the parabolic rule, and the weight gain of the coated sample after 20 h of oxidation at 1600 °C was only 5.24 mg/cm2. During oxidation, a dense and continuous SiO2-baed oxide scale embedded with ZrO2 and ZrSiO4 particles showing high thermal stability and low oxygen permeability could be formed on the surface of the coating by oxidation of MoSi2 and ZrB2, which could hinder the inward diffusion of oxygen at high temperatures. Concurrently, the MoB inner diffusion layer played an important role in hindering the diffusion of Si inward with regard to the TZM alloy and could retard the degradation of MoSi2, which could also improve the long life of the coating.

Cr3C2-modified NiCr–TiC composite coatings were prepared using the plasma spraying technique for different Cr3C2 contents on the microstructure and the properties of the Ni-based TiC cladding layer were investigated. The microstructures of the coatings were characterized using scanning electron microscopy, and the friction and wear performance of the coating was evaluated by the wear tests. The results revealed that the surfaces of the Cr3C2-modified NiCr–TiC composite coatings with varying Cr3C2 contents were dense and smooth. TiC was uniformly distributed throughout the entire coating, forming a gradient interface between the binder phase of the Ni-based alloy and the hard phase of TiC. At high temperatures, Cr3C2 decomposes, with some chromium diffusing and forming complex carbides around TiC, some chromium solubilizes with Fe, Ni, and other elements. An increase in chromium carbide content leads to an upward trend in hardness. The measured hardness of the coatings ranged from 600 to 850 HV3 and tended to increase with increasing Cr3C2 content. When the mass fraction of Cr3C2 reached 30%, the hardness increased to 850 HV3, and the cracks and defects were observed in the coating, resulting in a wear resistance decline.