Structure and oxidation behavior of Si–Y co-deposition coatings on an Nb silicide based ultrahigh temperature alloy prepared by pack cementation technique

Abstract

In order to improve the oxidation resistance of silicide coatings on Nb silicide based alloys, Y-modified silicide coatings were prepared by co-depositing Si and Y at 1050, 1150 and 1250 °C for 5–20 h, respectively. It has been found that the coatings prepared by co-depositing Si and Y at 1050 and 1150 °C for 5–20 h as well as at 1250 °C for 5 h were composed of a thick (Nb,X)Si 2 (X represents Ti, Cr and Hf elements) outer layer and a thin (Nb,X) 5Si 3 inner layer, while the coatings prepared by co-depositing Si and Y at 1250 °C for 10–20 h possessed a thin outer layer composed of (Ti,Nb) 5Si 3 and Ti-based solid solution, a thick (Nb,X)Si 2 intermediate layer and a thin (Nb,X) 5Si 3 inner layer. EDS analyses revealed that the content of Y in the (Nb,X)Si 2 layers of all the coatings was about 0.34–0.58 at.% while that in the outer layers of the coatings prepared by co-depositing Si and Y at 1250 °C for 10–20 h was about 1.39–1.88 at.%. The specimens treated by co-depositing Si and Y at 1250 °C for 10 h were selected for oxidation test. The oxidation behavior of the coating specimens at 1250 °C indicated that the Si–Y co-deposition coating had better oxidation resistance than the simple siliconized coating because the oxidation rate constant of the Si–Y co-deposition coating was lower than that of the simple siliconized coating by about 31%. The scale developing on the Si–Y co-deposition coating consisted of a thicker outer layer composed of SiO 2 and TiO 2 and a thinner SiO 2 inner layer.