The microstructural evolution and oxidation resistance at 1500 °C of Mo-Si-Cr-B coating with a Cr₂Nb₄Si₅layer

To protect Nb-Si based ultrahigh temperature alloy from oxidation, a slurry-sintered Mo-Si-Cr-B protective coating was developed. The microstructural evolution of the coating during the high-temperature sintering and its high-temperature oxidation behavior at 1500 °C were investigated. With increase in sintering temperature, the main phase evolution sequence of the outer layer is as follows: (Mo, Cr, Si, B)→(MoSi₂, Mo₅Si₃, CrSi₂, Cr₅Si₃, Cr₃Si, CrB)→((Mo,Cr)Si₂, CrB)→((Mo,Cr)Si₂, (Mo,Cr)₅Si₃). The continuous inward diffusion of Si, Cr and B elements leads to the formation of an inner layer composed of Cr₂Nb₄Si₅, (Nb,Ti)B₂, (Ti,Nb)₅Si₄ and (Nb,X)₅Si₃. The coating exhibited excellent oxidation resistant performance during exposure at 1500 °C for 15 h. This is primarily because the outward diffusion of Cr promotes the formation of a dense and continuous CrNbO₄, which reduces the residual amount of liquid-phase Nb₂O₅ and enhances the stability of the oxide scale, thereby effectively suppressing the inward diffusion of oxygen. In addition, Cr₂Nb₄Si₅ layer provides a sufficient Cr source for the formation of CrNbO₄ during the long-term oxidation process, thus enhancing the integrity of the oxide scale.