Preparation and moderate temperature oxidation behavior of Ti- and Al-doped NbSi2-Si3N4 composite coatings on Nb alloy

In order to adjust the mismatch of coefficient of thermal expansion between NbSi2 coating and Nb alloy while suppressing the pest oxidation of NbSi2 coating at moderate temperatures, NbSi2-Si3N4 composite coatings with doped Ti and Al were prepared on Nb alloy by a spark plasma sintering process using an Nb-Si-Si3N4-Ti-Al powder mixture. The sintering reactions based on the thermodynamic feasibility for the constituent phase formation of the coatings were revealed, and the oxidation resistance of the coatings at 750 degrees C was investigated. The coatings had a multiple-phase structure and mainly consisted of (Nb,Ti)Si-2, Si3N4, (Ti,Nb)(5)Si-4, (Nb,Ti)(5)Si-3, and a small number of other nitride phases. After 50 h of oxidation, pest oxidation was observed on both the NbSi2 coating and NbSi2-Si3N4 coating, while no indication of pest oxidation was present on the coatings after doping with Ti and Al.