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 -AlPO 4 ) modified SiC whisker toughened mullite coating ( c -AlPO 4 -SiC w -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 SiC w -mullite coating. Results show that c -AlPO 4 -SiC w -mullite coatings with 10 and 20 wt.% of c -AlPO 4 exhibited obviously improved oxidation resistance at 1773 K in ambient air for 100 h than SiC w -mullite coating. Moreover, the oxidation resistance of c -AlPO 4 -SiC w -mullite coatings were rapidly declined when the c -AlPO 4 in c -AlPO 4 -SiC w -mullite coating were set to 30 and 40 wt.%. The c -AlPO 4 -SiC w -mullite coating with 20 wt.% of c -AlPO 4 showed most pronounced oxidation resistance, the weight loss rate after the oxidation in ambient air for 210 h was merely 3.00 × 10 −5 g·cm −2  h −1 . The failure of c -AlPO 4 -SiC w -mullite coating with 20 wt.% of c -AlPO 4 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.