Microstructure and oxidation behavior of a novel bilayer (c-AlPO4-SiCw-mullite)/SiC coating for carbon fiber reinforced CMCs

A novel cristobalite aluminum phosphate particle (c-AIPO(4)) modified SiC whisker toughened mullite coating (c-AlPO4-SiCw-mullite) was prepared on SiC coated carbon fiber reinforced SiC composites (C/SiC) by a new sol-gel method combined with air spraying to improve the oxidation resistance of SiCw-mullitecoating. Results show that c-AlPO4-SiCw-mullite coatings with 10 and 20 wt.% of c-AlPO4 exhibited obviously improved oxidation resistance at 1773 K in ambient air for 100 h than SiC mullitecoating. Moreover, the oxidation resistance of cAlPO(4)-SiCw-mullite coatings were rapidly declined when the c-AIP04 in c-AlPO4-SiCw-mullite coating were set to 30 and 40 wt.%. The c-AlPO4-SiCw-mullite coating with 20 wt.% of c-AlPO4 showed most pronounced oxidation resistance, the weight loss rate after the oxidation in ambient air for 210 h was merely 3.00 x 10(-5) g.cm(-2) h(-1). The failure of c-AlPO4-SiCw-mullite coating with 20 wt.% of c-AlPO4 was due to the generation of penetrative micro-cracks and micro-holes in the coating, which cannot be self-healed by the silicate glass layer after long time oxidation at 1773 K.