Research On Production And Mechanism Of Sic/c-alpo₄ Coating On C/c Composites

Carbon/Carbon (C/C) composites are being widely used in aviation and space flight industries due to their excellent properties. However, C/C composite composites have a fatal weakness and can easily be oxidized at high-temperature oxidizing atmosphere. The oxidation of these composites limited their application. It is therefore important to increase its resistance towards air oxidation at high temperature. Oxidation resistant coating is considered to be one of the most reasonable choices for high temperature protection of C/C composites. It is also important to prepare dense and homogeneous coating with excellent bonding to matrix using a low cost and simple process, which is vital for improving the oxidation resistance of C/C composites under air and gas eroding atmosphere; and is all important for expanding their application areas at elevated temperature. In the present work, SiC/C-AlPO4 multi-layer coating was fabricated on the carbon/carbon (C/C) composites by a simple and low-cost method. The internal SiC layer was prepared by a two-step pack cementation process and the external C-AlPO4 coating was produced by hydrothermal electrophoretic deposition process. Phase compositions and microstructures of the coated C/C composites were characterised by XRD, SEM and EDS analyses. Anti-oxidation properties, oxidation mechanism and failure mechanism of the coating were investigated and discussed. The main contents and conclusions are as follows.Si/α-SiC/β-SiC bonding layer for C/C composites were prepared by a two-step pack cementation method using silicon, graphite powders as raw materials and Al2O3, B2O3 as infiltration additives. The results show that Si/β-SiC coatings are obtained after the first step pack cementation; and the Si/a-SiC/β-SiC multi-coatings can be achieved after the second step pack cementation. The as-prepared Si/α-SiC/β-SiC multi-coatings with the thickness of about 100μm possess the dense' surface and the cross-section microstructures. No pin-holes and microcracks are observed. The oxidation resistance capacity of the SiC coating prepared by the second step pack cementation is higher than the first step pack cementation. Isothermal oxidation tests in air at 1500℃for 16h show that the weight loss of the as-obtained Si/a-SiC/β-SiC coating coated C/C composites is only 1.42%. The failure of the SiC coating is due to the formation of the holes in the coating surface, which is generated by the escaping of SiO and CO.The charge mechanism and the stability of cristobalite aluminum phosphate (C-AlPO4) powder in suspension were studied. The results show that C-AIPO4 molecules have positive charges because of absorbing H+ broken by organic medium molecule. The stability of C-AlPO4 suspension is best in the isopropyl alcohol. The influences of the deposition temperature, deposition voltage and suspension powder content on the microstructure of the C-AlPO4 coatings prepared by hydrothermal electrophoretic deposition were explored. The optimization technological conditions for preparing the C-AIPO4 coating were the deposition voltage 220V, deposition time 25min and the deposition temperature 100℃. The AlPO4 crystal phase structure on microstructure of multi-layer coating was particularly investigated. Very dense and homogeneous coating prepared by C-AIPO4 powder is obtained. No visible micro-cracks and other defects at the interface between C-AlPO4 coating and substrate can be observed. The deposition process of the C-AlPO4 coatings on SiC coated C/C composite is controlled by the diffusion rate of the charged C-AlPO4 particles to the cathode, and the deposition activation energy is calculated to be 21.88kJ/mol.The as-prepared SiC/C-AlPO4 multi-layer coating exhibits an excellent oxidation resistance in air at the temperature from 1300℃to 1500℃, and the oxidation activation energy of the coated C/C is calculated to be 117.2 kJ/mol. The oxidation process is predominantly controlled by the diffusion of oxygen in the dense C-AlPO4 coating. The failure of the multi-layer is result from the generation of the microholes that may be left after the escaping of the oxidation gases.