Improving oxidation resistance of Si coating by isolated-particle healing

Atmospheric plasma sprayed dense Si coatings have excellent potential for high-temperature oxidation resistance. However, numerous isolated spherical particles on the coating surface result in rapid oxidation and a large local TGO thickness. In the present study, isolated particle healing (IPH) treatment was proposed to eliminate these isolated particles. Then, the related structure evolution and oxidation kinetics of Si coatings were investigated. Finally, the failure mechanism of TGO after oxidation was identified. The results show that the IPH treatment makes isolated particles diffuse, connect with and then immerse themselves into the bulk coating surface, thereby contributing to both surface smoothing and coating densification. Consequently, the localized large thickness in the TGO scale is eliminated, and the overall oxidation rates of the IPH Si coating decrease by ∼24% compared with the as-sprayed coating. The β→α phase transformation of polymorphic cristobalite with a volume reduction of ∼4.5% is the main cause for TGO channel cracking and is not suppressed by IPH treatment, which points out future directions for better antioxidant Si coatings.