Effect of Y₂O₃ on microstructure of silicide coatings on nb-silicide-based ultrahigh temperature alloy

Si-Y₂O₃ co-deposition coatings on an Nb-silicide-based ultrahigh temperature alloy were prepared by pack cementation process at 1050°C for 10 h. The structure, phase constituents and compositional distribution of coatings were investigated by SEM, EDS and XRD and compared with those of the coating prepared by depositing Si under the same conditions. The results show that no evident new phases form on the surface layer of Si-Y₂O₃ co-deposition coating. Coatings prepared with different content of Y₂O₃ (in the range of 0.5%-5%, mass fraction) in the pack mixtures have the similar structure, which is composed of a (Nb, X)Si₂ (X represents Ti, Hf and Cr elements) layer, a (Nb, X)₅Si₃ transitional layer and an Al-rich diffusion zone. The Si-Y₂O₃ co-deposition coatings are more compact than the simple Si deposition coating. EDS analyses reveal that the distribution of Y in the coating are not uniform. The Y content appears the highest in the place near the interface between substrate and transitional layer and it gradually decreases from the substrate to the coating surface. Furthermore, the Y concentration in the coating firstly increases and then decreases with the increase of the content of Y₂O₃ in pack mixtures. The coating growth can be catalyzed obviously when the content of Y₂O₃ in the pack mixtures ranged from 1% to 2% (mass fraction).