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

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)Si2 (X represents Ti, Cr and Hf elements) outer layer and a thin (Nb,X)5Si3 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)5Si3 and Ti-based solid solution, a thick (Nb,X)Si2 intermediate layer and a thin (Nb,X)5Si3 inner layer. EDS analyses revealed that the content of Y in the (Nb,X)Si2 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 SiO2 and TiO2 and a thinner SiO2 inner layer.