Structure and Oxidation Behavior of Zr-Y Modified Silicide Coatings Prepared on an Nb-Ti-Si-Cr Based Ultrahigh Temperature Alloy

Zr-Y modified silicide coatings have been prepared on an Nb-Ti-Si-Cr based ultrahigh temperature alloy by a pack cementation process. The effects of amount of Zr powder in the pack mixture and co-deposition temperature on the coating structure were assessed. The coatings prepared at 1,250 A degrees C with different amounts of Zr powders in the pack mixture have similar structures, mainly consisting of a thick (Nb,X)Si-2 (X represents Ti, Cr and Hf elements) outer layer, a thin (Ti,Nb)(5)Si-4 middle layer and a 1-2 A mu m thick (Nb,X)(5)Si-3 inner layer. The increased amount of Zr powders in the pack mixtures led to a significant decrease in the coating thickness. The coatings prepared at 1,150 and 1,200 A degrees C have similar structures to that prepared at 1,250 A degrees C. However, when the co-deposition temperature increases to 1,300 and 1,350 A degrees C, a (Ti,Nb)(5)Si-3 outermost layer formed in addition to the three layers mentioned above. The Zr-Y modified silicide coating can protect the base alloy from oxidation at least for 100 h at 1,250 A degrees C in air. The good oxidation resistance of the Zr-Y modified silicide coating is attributed to the formation of a dense scale consisting of SiO2 and TiO2.