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

• A second aluminizing treatment was performed to modify the siliconized coating.
• The structure change of the coatings during the two-step process was studied.
• The two-step coating possessed much better oxidation resistance at 1250 °C.
• Effect of second aluminizing on the oxidation behavior of the coatings was revealed.

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 100 h is 3.7 mg/cm2, and a duplex scale consisting of an Al2O3 outer layer and a SiO2-matrix inner layer forms.