Influence of AlPO4 Crystalline Structure on Microstructure and Antioxidation Property of Multi-layer Coating

An SiC/AlPO4 multilayers coating was fabricated on the carbon/carbon (C/C) composites by using the pack cementation and the hydrothermal electrophoretic deposition method. Phase compositions and microstructures of the as-prepared multi-layers coating were characterized by XRD and SEM. The influences of AlPO4 crystalline structure on microstructure and antioxidation property of the as-prepared multi-layers coating were investigated respectively. Results show that AlPO4 crystalline structure has strongly influence on the microstructure and antioxidation property of multi-layers coating. The porous coating with many defects is obtained using berlinite-type (B-AlPO4) powder. Coating density and uniformity have been greatly improved by using tridymite-type AlPO4 (T-AlPO4) powder, but there are some small holes, and scale of cracks between the coating and the substrate can be observed. Very dense and homogeneous coating is obtained using cristobalite-type AlPO4 (C-AlPO4) powder. No visible micro-cracks and other defects at the interface between C-AlPO4 coating and substrate can be observed. The C-AlPO4 coating exhibits better antioxidation property. The as-prepared C-AlPO4 coating could effectively protect C/C composites from oxidation at 1500℃ in air for 21h with a weight loss of 0.41wt%.