Fabrication And Properties Investigation On Ultra High Temperature Ceramics Coatings Of C_f/SiC Composites

The coatings of carbon fiber reinforced silicon carbide (Cf/SiC) composites play crucial role in the anti-oxidation and anti-erosion properties of Cf/SiC composites. In this dissertation, the research & development of the coatings and ultra high temperature ceramics(UHTCs) coatings for Cf/SiC composites were reviewed at first, aiming to improve the anti-oxidation and anti-erosion properties of Cf/SiC composites, the coatings of Cf/SiC composites were fabricated and analyzed, and the techniques were optimized. In the paper, three coating systems were fabricated via brushing process and pack cementation method respectively in order to improve the high temperature resistance of Cf/SiC composites. The oxidation resistance, the microstructures and the compositions of the coating systems were investigated by means of some techniques such as flexural strength test, scanning electron microscope (SEM) and X-ray diffraction (XRD).The ZrB2-SiC coating was fabricated via brushing process using PCS-DVB as binder and ZrB2,SiC and B powders as fillers according to the design of the coating system. When solidified at low temperature and sintered at 1200℃,the ZrB2-SiC coating was fabricated at the surface of Cf/SiC composites, then the effect of the proportion of ceramic powders on the performance of coated Cf/SiC composites was studied by designing an orthogonal experiment. The results show that when the filler was composed of 60wt.%ZrB2, 4wt.% SiC, 6wt.%PCS, 4wt.%B and 26wt.%DVB, the coating showed the best performance, the adhesive strength was 2.01MPa. After soaked at 1200℃for 30min under static air, the coated Cf/SiC composites retained 99.46% of original mass and 97.3% of original flexural strength. While the uncoated Cf/SiC composites retained 89.63% of original mass and 38.7% of original flexural strength.The other two coating systems were fabricated via pack cementation method. The ZrC-Zr2Si coating , homogeneous and density, was formed through the reaction of Zr-Si with Cf/SiC composites, whit the thickness was 10μm, the coating was composed of a ZrC inner layer and a ZrC-Zr2Si outer layer. An orthogonal experiment was designed to investigate the influences of the process condition. It was found that the optimizing infiltration composition and process was: 60wt..%Zr-Si, 30wt..%PCS-DVB, 10wt..%Al2O3, holding 8 hours at 1400℃in Ar protecting atmosphere. The bond between coating and Cf/SiC composites was mainly metallurgical bond, so the adhesive strength was as high as 7.41MPa. After soaked at 1200℃for 30min under static air, the coated Cf/SiC composites retained 99.70% of original mass and 92.5% of original flexural strength. The coating had the remarkably improvement of oxidation-resistance of Cf/SiC composites. After reaction with B, when held for 3 hours at 1200℃in Ar protecting atmosphere, the ZrC-Zr2Si coating was translated to the ZrB2 coating which was homogeneous and density. After soaked at 1200℃for 30min under static air, the coated Cf/SiC composites retained 99.87% of original mass and 94.3% of original flexural strength. Compared to the ZrC-Zr2Si coating, the ZrB2 coating showed the better performance.