Formation mechanism and oxidation performance for a novel Mo(Si,Al)2 coating prepared using a modified pack cementation strategy

Abstract

Using a modified Si-Al deposition strategy in the process of halide-activated pack cementation, conventional and novel Mo(Si,Al)₂ coatings are fabricated on molybdenum substrate to assess their distinct formation mechanism and oxidation performance. The results show that the conventional coatings are composed of an inner layer containing pure C11b MoSi₂ and an outer layer containing C11b MoSi₂ with a little Al. By contrast, the new coating is composed of pure C22 Al₈Mo₃ as the inner layer and C40 Mo(Si,Al)₂ as the outer layer (unexpectedly low Al percentage of 13 at.%). Notably, the two kinds of coatings have different microstructures and formation characteristics. The former's synthesis pathway includes the sequential formation of C22 Al₈Mo₃, C40 Mo(Si,Al)₂, and C11b MoSi₂, accompanied by a reduction in the proportion of solidified Al in Mo(Si,Al)₂. The latter's synthesis path includes the transformation from C11b MoSi₂ to C40 Mo(Si,Al)₂, and the Al content is always maintained at the highest level with the coating growth. Meanwhile, the phase transition between Al₈Mo₃ and Mo(Si,Al)₂ is an irreversible process and neither coating forms the C54 structure of Mo(Si,Al)₂ due to the low temperature. Besides, the different oxidation mechanisms are elucidated on basic of the two deposition procedures and corresponding to oxidation performance. Hence, the newly developed Mo(Si,Al)₂ coating of first silicon and then aluminum in the process of pack-cementation can be effectively used to fabricate a protective α-Al₂O₃ barrier layer at a temperature of 1300 °C.