Structure and High-Temperature Oxidation Performance of Si-Co Diffusion Coatings Prepared on a TiAl-Nb Alloy

Si-Co diffusion coatings were prepared on a Y-modified TiAl-Nb alloy using the pack cementation process. The structures of the coatings prepared at different temperatures (1050, 1080, and 1120 C-circle) and pack Co contents (5, 10, and 20 wt.%) were comparatively studied. The coatings possessed the typical structure of a (Ti,X)Si-2+Ti5Si4 (X represents Nb and Al elements) outer layer with a Co-rich superficial zone, a Ti5Si4+Ti5Si3 middle layer, and a TiAl2 inner layer. Increasing the co-deposition temperature in the range of 1050-1120 C-circle led to a larger coating thickness but a more-porous coating structure, while increasing the pack Co contents in the range of 5-20 wt.% caused a lower coating growth rate. The formation of the Si-Co diffusion coating followed an orderly process of depositing Si first and then Co. The Si-Co diffusion coating had much better anti-oxidation performance than both the TiAl-Nb substrate and pure silicide coating. After undergoing oxidation at 1000 C-circle for 100 h, the oxidation parabolic rate of the Si-Co diffusion coating was approximately 6.16 x 10(-3) mg(2)/cm(4)h(1), which was lower than those of the TiAl-Nb substrate by about two orders of magnitude and pure silicide coating by about one order of magnitude.