Study On The SiC Whisker-Toughened Silisides And SiC/Glass Oxidation Protective Coatings

Carbon/carbon (C/C) composites are provided with excellent properties at high temperature, and are considered as the most promising candidate materials for high temperature application such as in aviation, space flight fields. However, C/C composites are prone to oxidize at high temperature, which limits their applications as high-temperature structural materials. Anti-oxidation coatings are considered to be an effective method for preventing C/C composites from oxidation at high temperature. In order to exploit the oxidation protective coatings using at high temperature, SiC whisker-toughened silicides coating and SiC/glass double-layer coating were prepared by pack cementation and slurry. The phase composition, microstructure and element distributing of the as-prepared coatings were characterized by XRD, SEM and EDS analyses. The oxidation protective ability and thermal shock resistance of the as-obtained coatings were investigated. Moreover, the anti-oxidation mechanism and the failure reason of the coatings were also discussed. The main contents and conclusions are listed as follows:The SiC-CrSi₂ double-phase coating was produced by a two-step pack cementation technique. The microstructure and oxidation protective ability of the coating were investigated. The effect of Si/Cr with the different weight ratio on the microstructure and anti-oxidation property was discussed. The results show that the CrSi₂ phase can be filled in the holes and cracks of the SiC inner layer, resulting in the dense structure of the coating. With the increasing of Cr/Si in the initial pack power, the content of SiC phase in the coating decreases, CrSi₂ phase and the size of the cracks in the coating increase gradually, and the oxidation protective ability of the coating increase firstly and decrease later.SiC whisker (SiCw) toughened SiC-CrSi₂ oxidation protective coating was prepared by slurry and pack cementation. The effects of SiC whisker content on the microstructure, oxidation protective ability and thermal chock resistance were investigated. The oxidation behavior of the SiC whisker-toughened SiC-CrSi₂ coating at different temperatures were discussed and the failure mechanism of the coating was also analyzed. With the increasing of the SiCw content, the size of the cracks in the coating and the thickness of the coating decease, while the CrSi₂ content in the coating and the oxidation protective ability of the coating exhibit the trend of increase first and decrease afterwards. The coating containing with 15wt.% SiCw exhibits excellent oxidation protective ability and thermal shock resistance. The weight loss of the coated specimens was only 0.66% aider oxidation for 50h at 1500℃in air. The failure of the coated specimens mainly results from the oxidation between the oxygen diffusing though the cracks in the coating and C/C matrix during the thermal cycles between high temperature and room temperature.SiCw toughened Si-SiC coating was obtained by slurry and pack cementation technique. The effects of the SiCw content on the microstructure, oxidation resistance and thermal shock resistance were investigated and the preparing process was also optimized. The effect of the thermal shock on the mechanical property of the coated samples was discussed. The SiCw toughened Si-SiC coating is provided with excellent oxidation resistance, and can protect C/C composites from oxidation for more than 310 h at 1773℃and for 128h at 1873℃in air. After thermal cycle between 1873K air and 373K boiling water for 50 times, the weight loss of the SiC coated sample was 2.76% and the remaining strength percentage was 74.5%. The decreasing of the flexural strength during the thermal cycle was primarily due to the oxidation between the coated samples and oxygen or vapor.SiCw toughened MoSi₂-SiC-Si multi-phase coatings were produced on the surface of C/C composites by slurry and pack cementation. The effects of the SiCw content and the preparing temperature on the microstructure and oxidation protective ability of the as-obtained coating were investigated. The effect of the coating on the mechanical property of the samples was also discussed. The results show that when the SiCw content is 10wt.%, the preparing temperatures for the slurry method and pack cementation are 1800℃and 2000℃respectively, the coating is provide with excellent oxidation resistance, which could effectively protect C/C composites at 1500℃for 240h and the corresponding weight loss was only 0.33%. The flexural strength of the specimens increases after they are coated. The percentage of remaining flexural strength of the coated specimens were 81.97% and 89.63% respectively after thermal shock between 1500℃and room temperature for 10 times and isothermal oxidation at 1500℃for 60 min. The decreasing of the flexural strength is mainly attributed to the formation of the cracks and holes in the coating, through which the oxygen can diffuse into the C/C matrix and oxidize the C/C matrix.SiCw toughened SiCw-MoSi₂-SiC coating modified by B₂O₃ was produced by a two-step pack cementation technique. The effect of B₂O₃ content on the property of the coating was investigated. The result shows that as the B₂O₃ content is 5%, the coating exhibits better oxidation resistance. After oxidation at 1500℃for 242 hours, the weight gain of the coated specimens is 0.18%. The poor oxidation resistance at the changing temperature stage from 1600℃to 400℃is due to the formation of the open cracks at about 800℃.Using silicon-sol as the binder, glass power and B4C particles as main raw materials, the glass outer coating applied at 900℃was prepared on the surface of SiC inner coating. The effects of the B4C content and the structure of the SiC inner coating on the oxidation protective ability of the as-obtained coating were investigated. The failure reason of the coated specimens was also discussed. The results show that as the B4C content is 10 wt.% and the SiC inner layer is dense structure obtained by two-step pack cementation, the SiC/glass coating exhibits excellent oxidation protective ability, which can protect C/C composites at 900℃in air for 100h and the weight loss is only 0.14%. The slight weight loss is mainly attributed to the gradual volatilization of the glass coating during oxidation test.The glass oxidation protective coating for application at 1300℃was prepared on the surface of a porousβ-SiC bonding layer by slurry. The microstructure and oxidation resistance of the coated specimens was investigated. The oxidation mechanism of the coating was also discussed. The sealing layer is composed of the borosilicate glass phase with the dispersion of MoSi₂ particles. The weight loss of theβ-SiC/glass coated specimens is only 1.07% after oxidation for 150 hours at 1300℃and thermal cycles between 1300℃and room temperature for 20 times. The oxidation of the coated specimens mainly results from the cracks in the coating and the debonding of some glass coating.Using SiO₂, B₂O₃ and Al₂O₃ as the original materials, the glass coating for application at 1500℃was prepared on the surface of the SiC coated C/C composites by slurry. The microstructure and oxidation resistance of the coating was investigated. The oxidation mechanism of the coated specimens was also discussed. The coating can effectively protect C/C composites at 1500℃for 140 hours and the corresponding weight loss was only 0.98%. The weight loss of the coated specimens is mainly attributed to the crazing of the glass coating and the formation of holes on the coating surface.The oxidation behaviors of C/C composites with SiCw-Si-SiC/glass coating and SiCw-MoSi₂-SiC-Si/SiC/glass coating in combustion gas environment at 1512℃were studied respectively. The oxidation mechanism of the coated specimens was also nalyzed. The SiCw-Si-SiC/glass coating and SiCw-MoSi₂-SiC-Si/SiC/glass coating can protect C/C composites in wind tunnel environment at 1500℃for 16h and 53h respectively. The cracking of the coating under thermal impact and airflow impact environment is the main reason for the anti-oxidation failure of the coating.