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)Si 2 (X represents Ti, Cr and Hf elements) outer layer and a thin (Ti, Nb) 5 Si 4 transitional layer. The pack cementation experiments at 1150 1C for 8 h proved that the addition of certain amounts of CeO 2 and Y 2 O 3

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... wders in the packs distinctly influenced the coating thickness, the contents of Si, Ce and Y in the (Nb, X)Si 2 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 1.5CeO 2 -0.75Y 2 O 3 (wt%) powders was employed to investigate the growth kinetics of the Ce and Y jointly modified silicide coating at 1050, 1150 and 1250 1C. It has been found that the growth kinetics obeyed parabolic laws and the parabolic rate constants were 109.20 mm 2 /h at 1050 1C, 366.75 mm 2 /h at 1150 1C and 569.78 mm 2 /h at 1250 1C, and the activation energy for the growth of the Ce and Y jointly modified silicide coating was 197.53 kJ/mol.