Effects of TiB₂ contents on the microstructure and oxidation behavior of Mo-Si-B composite coatings

A novel Mo-Si-B-TiB₂ composite coating was prepared on Nb-Si based ultrahigh temperature alloy by slurry sintering. The effects of TiB₂ contents (1, 3, 5 and 10 wt%) on the microstructure and oxidation behavior of Mo-Si-B composite coatings were revealed. The results show that TiB₂ modified Mo-Si-B composite coatings display a bilayer structure, in which the outer layer is composed of MoSi₂, Mo₅Si₃ and TiB₂ phases, and the inner layer is composed of (Ti,Nb)₅Si₄, (Nb,X)₅Si₃ and (Nb,Ti)B₂ phases. The added TiB₂ particles are evenly distributed in the outer layer of the coatings. By introducing TiB₂ into the coating, the service life of Mo-Si-B composite coatings is extended from 50 h to 100 h at 1250 ℃, the oxidation rate of the coating is reduced from 1.62 to 0.02 mg² cm⁻⁴ h⁻¹. This indicates that the addition of appropriate TiB₂ effectively slows down the oxidation rate and significantly improves the oxidation resistance of Mo-Si-B composite coatings. The Mo-Si-B composite coating with 3 wt% TiB₂ had the lowest mass gain (3.9 mg/cm²) after oxidation at 1250 ℃ for 100 h, and it exhibited the best oxidation resistance. This is attributed to the formation of composite oxide scale with crack-free, good healing ability, suitable levels of differential thermal expansion and low oxygen permeability.