A novel gradient SiC-ZrB₂-MoSi₂ coating for SiC coated C/C composites by supersonic plasma spraying

A gradient SiC-ZrB₂-MoSi₂ (SZM) coating was successfully prepared on SiC coated C/C composites using supersonic plasma spraying, which is dense and defect-free. The design of gradient structure could avoid cracking at the interface between single MoSi₂ coating and SiC coated C/C substrate. Result of X-ray diffraction shows that the residual tensile stress of SZM gradient coating (48.5 MPa) is much lower than that of single MoSi₂ coating (490.8 MPa). Finite element analysis results show that the stress concentration significantly decreased at the interface of the as-sprayed gradient coating and the inner SiC coating. After thermal cycles between 900 °C and room temperature for 20 times, penetrated cracks formed in the MoSi₂ coating, but no obvious defect was observed in the SZM gradient coating. The brittle cracking of MoSi₂ coating could be avoided by the design of SZM gradient structure, which attributes to the thermal stress release of the coating with gradient thermal expansion coefficient (CTE). The good thermal shock resistance of the gradient SZM coating in air was ascribed to the CTE gradient structure and the formation of oxygen insulation layer (Mo₅Si₃) beneath the outmost oxidation layer. Superior oxidation protective ability was also obtained by gradient SZM coating at 1450 °C, which is due to the formation of Zircon and SiO₂ phases to restrain the crack propagation and improve the oxidation resistance.