Researching Of ZrB₂-SiC Ultra High Temperature Ceramic Coating For C/C Composites

C/C composite material is a new type of high temperature heat structural materials which has excellent performance such as low density, high specific strength, high specific modulus, high temperature resistance and erosion resistance, it represents the future of development direction of special vehicle and the ultra-high-speed space strategic weapons represented by hypervelocity missile. But the C/C composite material begins to be oxidized at 400 ℃ aerobic environment, which limits the usage of C/C composites. In order to improve the high temperature oxidation resistance of C/C composites, this paper puts forward using the pack cementation and slurry methods. These methods are simple and low cost for preparation SiC/ZrB₂-SiC/SiC ultra high temperature ceramic coating system on C/C composite surface. Moreover, both the high temperature oxidation resistance and microstructure of the coating samples are analysed.This thesis aim at modifying the C/C composites using the isothermal chemical vapor inflitration （CVI） and precursor infiltration pyrolysis （PIP） technology. After modification, the mechanical properties of C/C composites is excellent, bending strength, tensile strength, compressive strength are 320 Mpa,229 Mpa and 305 Mpa, respectively; Bending modulus and tensile modulus are 35.8 GPa,35.8 GPa, respectively; At the same time, the thermal physical properties of C/C composites are improved.With the modified C/C composite materials as the substrate, the SiC transition layer was prepared using the pack cementation method on the surface of the matrix. Different ratio of Si,60wt%Si,70wt%Si,80wt%Si, and different sintering temperature,1600 ℃,1700 ℃,1750 ℃,1800 ℃ and 1900 ℃, were adopted to fabricate the SiC transition layer. Effect of different composition ratio and sintering temperature on the microstructure of SiC coating was systematicly investigated. With the optimal sintering temperature and Si ratio of 1750 ℃ and 70wt%Si, the transition layer and outer coating exhibited less cracks and holes, better microstructure, and the oxidation weight loss ratio of the transition layer was 16% after static oxidation for 5 h at 1400 ℃.The oxygen barrier layer （ZrB₂-SiC） is prepared on the surface of the sample with SiC transition layer coated using slurry method. In this study, we formulated the ZrB₂-SiC oxygen barrier layer composition ratio （n （Zr）:n （Si）） of 3:1,4:1,5:1, and different sintering temperature of 1700 ℃,1800 ℃ and 1800 ℃. Effect of composition ratio and sintering temperature on the microstructure of the ZrB₂-SiC coating was investigated, which indicated that under the optimal sintering temperature of 1800 ℃ and components ratio of 4:1, the grain size of ZrB₂-SiC oxygen barrier layer is uniform, better integration with the substrate, and exhibited the minimum oxidation weight loss ratio of 1.4% after static oxidation for 5 h at 1400 ℃. Moreover, after thermal shock 10 times, the oxidation weight loss ratio of the sample was 6%. After oxidation, there are still a little holes in the surface of the coating can’t be healed, although its surface is dense glass phase. To further decrease the oxidation weight loss ratio via healing the holes and cracks, we prepared the SiC outer coating on the surface of SiC/ZrB₂-SiC coating.The SiC coating is prepared with the composition ratio of 70Si/1750 by pack cementation method. Investigation of the relationship between the oxidation weight loss ratio and the microstructure of SiC coating found that SiC outer coating can effectively heale the holes and cracks in the coating. Moreover, the dense glass phase on the surface of the coating could effectively stop the infiltration of oxygen into inner coating, and hence improve the oxidation resistance of the coating. The oxidation was only 0.8% after static oxidation for 5 h at 1400 ℃. After thermal shock for 10 times, the oxidation weight loss ratio of the sample was 11% due to the heat stress generated in thicker coating. Linear ablation rate of sample was 9.1 ×10-4 mm/s and 9.87×10-4 mm/s after 200 s×5 cyclic ablation and a 1000 s ablation, respectively. Linear ablative rate is always only in 10-4 orders of magnitude no matter use what kind of ablation, indicating the less-ablation characteristics of sample. Moreover, the sample demonstrated relative good macroscopic and microscopic morphology even after wind tunnel ablation.