Improvement in oxidation resistance of silicide coating on an Nb-Ti-Si based ultrahigh temperature alloy by second aluminizing treatment

Modified silicide coating was prepared on an Nb-Ti-Si based alloy by a two-step pack cementation process including first siliconizing and then aluminizing. The two-step coating is composed of an Al-rich outermost layer, a (Nb,X)Si₂ outer layer and a (Ti,Nb)₅Si₄ inner layer. The coating possesses an enhanced oxidation resistance at 1250°C due to the presence of the Al-rich outermost layer. The mass gain of the coating after oxidation at 1250°C for 100h is 3.7mg/cm², and a duplex scale consisting of an Al₂O₃ outer layer and a SiO₂-matrix inner layer forms.