Microstructure and growth kinetics of Ce and Y jointly modified silicide coatings for Nb–Ti–Si based ultrahigh temperature alloyMicrostructure and growth kinetics of Ce and Y jointly modified silicide coatings for Nb–Ti–Si based ultrahigh temperature alloyretain

In order to protect NbTiSi based ultrahigh temperature alloy from oxidation, pack cementation processes were utilized to prepare Ce and Y jointly modified silicide coatings. The Ce and Y jointly modified silicide coating has a double-layer structure: a relatively thick (Nb, X)Si₂ (X represents Ti, Cr and Hf elements) outer layer and a thin (Ti, Nb)₅Si₄ transitional layer. The pack cementation experiments at 1150 °C for 8 h proved that the addition of certain amounts of CeO₂ and Y₂O₃ powders in the packs distinctly influenced the coating thickness, the contents of Si, Ce and Y in the (Nb, X)Si₂ outer layers, and the density of cavities in the coatings. In order to study the effects of Ce and Y joint modification in the silicide coatings, both only Ce and only Y modified silicide coatings were also prepared for comparison. The mechanisms of the beneficial effects of Ce and Y are discussed. A pack mixture containing 1.5CeO₂–0.75Y₂O₃ (wt%) powders was employed to investigate the growth kinetics of the Ce and Y jointly modified silicide coating at 1050, 1150 and 1250 °C. It has been found that the growth kinetics obeyed parabolic laws and the parabolic rate constants were 109.20 μm²/h at 1050 °C, 366.75 μm²/h at 1150 °C and 569.78 μm²/h at 1250 °C, and the activation energy for the growth of the Ce and Y jointly modified silicide coating was 197.53 kJ/mol.