Preparation of a SiC/Cristobalite-AlPO4 Multi-layer Protective Coating on Carbon/Carbon Composites and Resultant Oxidation Kinetics and Mechanism

In order to improve the oxidation resistance of carbon/carbon (C/C) composites, a SiC/C-AlPO4 multi-layer coating was fabricated on the C/C composites by a simple and low-cost method. The internal SIC bonding layer was prepared by a two-step pack cementation process and the external C-AlPO4 coating was deposited by hydrothermal electrophoretic deposition process. Phase compositions and microstructures of the as-prepared multi-layer coating were characterized by X-ray diffraction (XRD), scaning electron microspocy (SEM) and energy dispersive spectrometer (EDS). Anti-oxidation properties, oxidation behavior and the failure behavior of the coated composites were investigated. The results indicate that the multi-layer coating exhibits obviously two-layer structure. The inner layer is composed of beta-SiC, alpha-SiC phase with a scale of silicon phase. The outer layer is composed of cristobalite aluminum phosphate (C-AlPO4) crystallites. The SEM observation shows the good bonding between the inner and outer layers. The multi-layer coating displays an excellent oxidation resistance in air in the temperature range from 1573 to 1773 K, and the corresponding oxidation activation energy of the coated C/C composites is calculated to be 117.2 kJ/mol. The oxidation process is predominantly controlled by the diffusion of O-2 through the C-AlPO4 coating. The failure of the multi-layer coating results from the generation of the microholes that may be left by the escape of the oxidation gases.