Microstructure and high-temperature oxidation-resistant performance of several silicide coatings on Nb-Ti-Si based alloy prepared by pack cementation process

The microstructure and high-temperature oxidation-resistant performance of several silicide coatings on an Nb-Ti-Si based alloy were revealed in the present work. These silicide coatings were prepared respectively at 1250°C for 8 h by pack siliconizing process, Si-Y co-deposition process and Si-Al-Y co-deposition process (with different Al contents in the packs). The results showed that the purely siliconized coating was composed of a (Ti,Nb)5Si3 ouer layer, a (Nb,X)Si2 (X represents Ti, Cr and Hf elements) middle layer and a (Ti,Nb)5Si4 inner layer. A thicker and more compact double-layer structure including a (Nb,X)Si2 outer layer and a (Ti,Nb)5Si4 inner layer was observed in the Si-Y co-deposition coating. In addition, a higher Y content (about 0.34 at. %) in the outer layer of the Si-Y co-deposition coating was obtained, while the Y content was only about 0.06 at. % in the purely siliconized coating. The Si-Al-Y co-deposition coating possessed a (Nb,X)Si2 outer layer, a (Ti,Nb)5Si4 middle layer and an Al, Cr-rich inner layer. A suitable addition of Al powders (5 wt. %) in the packs was beneficial to thicken the (Nb,X)Si2 outer layer, while a sharp reduction in the coating thickness was found when excess Al powders (10 wt. %) was added in the packs. However, compared with the former coating, the later coating prepared with more Al powders in the packs resulted in a slight increase in the content of Al and Y in the (Nb,X)Si2 outer layer from about 0.21 and 0.54 at. % to 0.87 and 0.79 at. % respectively. The thickness and microstructure of the scales formed on above four coatings upon oxidation at 1250°C for either 5 h or 100 h were comparatively investigated. The oxidation resistance of these silicide-type coatings was notably enhanced by the addition of Y and Al. The Si-Al-Y co-deposition coating, which was prepared with 5 wt. % Al powders in the pack, possessed the best oxidation resistance due to its optimum dense and continuous scale and compact coating remained.