Effect of B₂O₃/SiO₂ mass ratio on oxidation resistant property of phosphate coating for C/C composites

To prevent carbon/carbon (C/C) composites from oxidation, a phosphate coating was prepared on the composites by a simple and low cost slurry technique. The effect of B₂O₃/SiO₂ mass ratio on the microstructure and oxidation resistant property of phosphate coating was studied. The coating characterised by scanning electron microscopy and X-ray diffraction is mainly composed of SiO₂, BPO₄ and Al(PO₃)₃. Oxidation tests show that, as the mass ratio of B₂O₃ and SiO₂ is 1:1, the coating possessed excellent oxidation resistant ability. After oxidation in air at 700°C for 30 h and thermal cycling between 900°C and room temperature for 50 times, the weight losses of the coated samples were only 4·3 and 0·35% respectively. The weight loss of the coated C/C composites was considered to arise from the formation of bubble holes and microcracks in the phosphate coating.