Oxidation Behavior of an Al-Modified Silicide Coating on an Nb-Silicide-Based Ultrahigh-Temperature Alloy

The oxidation behavior of an Al-modified silicide coating prepared through a siliconization and then an aluminization pack cementation process on an Nb-silicide-based ultrahigh-temperature alloy was investigated in air at 1,250°C. The coating consisted of a 50-μm-thick (Nb,X)Si₂ (X represents Ti, Cr, and Hf elements) outer layer with (Nb,Ti)₃Si₅Al₂ distributed discontinuously in its superficial part and a 90-μm-thick (Nb,Ti)Al₃ inner layer containing Si-rich precipitates. The oxidation of this coating was controlled primarily by the preferential oxidation of Al with the formation of α-Al₂O₃ scale. The (Nb,Ti)Al₃ inner layer could act as an Al reservoir for forming scale and maintaining the existence of (Nb,Ti)₃Si₅Al₂ phase in the zone just beneath the scale. The parabolic oxidation rate constant of this coating was about 1.72 μm²/h, lower than that of the bare alloy by about 4 orders of magnitude.