Bonding strength and thermal shock resistance of a novel bilayer (c-AlPO4-SiCw-mullite)/SiC coated carbon fiber reinforced CMCs

Mullite coating, SiC whiskers toughened mullite coating (SiC w -mullite), and cristobalite aluminum phosphate ( c -AlPO 4 ) particle modified SiC w -mullite coating ( c -AlPO 4 -SiC w -mullite) were prepared on SiC coated C/SiC composites using a novel sol-gel method combined with air spraying. Results show that molten SiO 2 formed by the oxidation of SiC whiskers and molten c -AlPO 4 improved the bonding strength between mullite outer coating and SiC–C/SiC composites due to their high-temperature bonding properties. The bonding strength between mullite, SiC w -mullite, c -AlPO 4 -SiC w -mullite outer coatings and SiC–C/SiC composites were 2.41, 4.31, and 7.38 MPa, respectively. After 48 thermal cycles between 1773 K and room temperature, the weight loss of mullite/SiC coating coated C/SiC composites was up to 11.61%, while the weight losses of SiC w -mullite/SiC and c -AlPO 4 -SiC w -mullite/SiC coatings coated C/SiC composites were reduced to 7.40% and 5.12%, respectively. The addition of appropriate SiC whiskers can considerably improve the thermal shock resistance of mullite coating owing to their excellent mechanical properties at high temperature. In addition, c -AlPO 4 particles can further improve the thermal shock resistance of SiC w -mullite coating due to their high-temperature bonding and sealing properties. No obvious micro-pores and cracks were observed on the surface of c -AlPO 4 -SiC w -mullite coating after 48 thermal cycles due to timely healing effect by formation of secondary mullite.