Failure mechanism of ytterbium silicate/silicon bi-layer environmental barrier coatings on SiCf/SiC composites upon long-time water vapor and oxygen corrosion test

Environmental barrier coatings (EBCs) are crucial for SiC f /SiC composites to resist water vapor-oxygen corrosion and maintain the stability of their structure and properties in a complex combustion environment at high temperatures. In this work, an EBC system, composing Yb 2 Si 2 O 7 topcoats and Si bond coats, has been prepared by an air plasma spraying (APS) route to completely wrap up SiC f /SiC composites. Then the corrosion and failure mechanisms of the EBC system in a water vapor-oxygen environment are systematically studied. The results indicate that the SiC f /SiC composites exhibit excellent stability under the full protection of the current EBCs. With the progression of corrosion, due to the difference of the thermal expansion coefficients (TECs) between the topcoat and bond coat materials, the vertical mud cracks generated by the thermal cycles become the intrusion passages for the corrosion species and then form transverse cracks and pores, resulting in the peeling off of the Yb 2 Si 2 O 7 topcoat. Moreover, in the later stage of thermal cycles, the residue of the Si bond coat is shown to play an important protective role against the rapid corrosion of the substrates. • The phase composition of topcoat changes dynamically during vapor-oxygen corrosion. • The increase of Yb 2 SiO 5 is the dictating factor causing the TEC mismatch. • The damage of Si bond coat leads to the peeling off of the coating system. • The failure mechanism of the current EBC system has been proposed.