Y and Al modified silicide coatings on an Nb-Ti-Si based ultrahigh temperature alloy prepared by pack cementation process

Y and Al modified silicide coatings were prepared on an Nb-Ti-Si based ultrahigh temperature alloy by co-depositing Si, Al and Y at 1150 degrees C for up to 10 h, respectively. The deposition of Al and Si occurred in a sequential manner during the pack cementation process. At the initial stage, the element deposited was primarily Al with very little Si and an Al(3)(Nb,X) (X represents Ti, Cr and Hf elements) layer formed preferentially. After a short period of holding time. Si started depositing and Si-Al co-deposition took place. However, this Si-Al co-deposition period was not long. When the holding time was longer than 1 h at 1150 degrees C, Si deposition dominated the coating growth process. The coating growth kinetics at 1150 degrees C followed a parabolic law. The coating prepared at 1150 degrees C for 10 h had a multi-layer structure, with a thick (Nb,X)Si(2) outer layer, a thin (Ti,Nb)(5)Si(4) middle layer and an Al, Cr-rich inner layer. The coating could protect the Nb-Ti-Si based alloy from oxidation at 1250 degrees C in air for at least 100 h. The excellent oxidation resistance of the coating was attributed to the formation of a dense scale mainly consisted of TiO(2). SiO(2) and Al(2)O(3). (C) 2011 Elsevier B.V. All rights reserved.