| As an all-carbon composite,carbon/carbon(C/C)composite has excellent properties of both carbon matrix material and fiber reinforced material.carbon/carbon composites not only have the characteristics of low density and low coefficient of thermal expansion,but also have excellent mechanical properties such as high specific modulus and strength,high thermal conductivity,wear resistance and corrosion resistance.Especially,its mechanical strength does not decrease but increases with the increase of temperature,and is considered an ideal high-temperature material in aerospace and other industrial fields.However,carbon/carbon composites are highly susceptible to oxidation in high temperatures and aerobic environments.Oxidation begins at 643 K.When the temperature exceeds 773 K,the oxidation rate increases.When C/C composites are oxidized,their carbon matrix will be destroyed,resulting in a significant decrease in their mechanical properties and physical and chemical properties.This paper solves this problem by preparing a composite anti-oxidation coating.In this study,SiC coating was prepared on C/C composites by secondary embedding method,and SiB6/Al2O3 mixed coating,SiB6@Al2O3 coating with core-shell structure and SiB6@Al2O3 coating with glass protective layer were prepared on C/C samples coated with SiC coating(SiC-C/C)by hot dipping.The oxidation resistance of these coatings,and the effect of Al2O3 addition on the oxidation resistance of SiB6 coating and the effect of raw material concentration ratio on the oxidation resistance of SiB6@Al2O3 coating were studied.The composition,surface microstructure,section microstructure and element content analysis of coating samples were analyzed and tested by XRD,SEM,EDS and other characterization methods.The conclusions are as follows:(1)Based on the optimized process parameters of the previous research group,SiB6 coating with dense surface and good bonding with SiC layer and SiB6/Al2O3 coating with different ratios were successfully prepared by thermal impregnation.Although SiB6 coating can protect SiC-C/C samples from oxidation within a certain period of time,long-term high temperature oxidation will make it lose its protective effect.Adding Al2O3 can effectively improve the protective time of SiB6 coating on the carbon/carbon matrix.After adding 10wt%Al2O3 to the SiB6 coating,the oxidation weight loss rate of the sample after oxidation in 1573K air for 230h is only 0.57%,the oxidation mass loss rate per unit area of the composite coating is only 0.1850g/cm2,and the oxidation mass loss rate per unit area is 8.0435 ×10-4g/(cm2·h).Its excellent oxidation resistance comes from Al2O3,which can form mullite with SiB6 oxidation product SiO2,increase the melting point of the coating,reduce the volatilization of the coating,and play a certain role in toughening the coating.The failure of the coating is due to the reduction of SiO2 production under high temperature and long oxidation,resulting in the formation of pores and microcracks on the surface of the coating.These defects cannot be effectively cured.These defects will become oxygen channels,allowing oxygen to enter the carbon/carbon matrix,thereby damaging the C/C matrix.(2)SiB6@Al2O3 microcapsules with core-shell structure were prepared by sol gel method.The results showed that the coating effect was the best when the concentration ratio of SiB6 to Al2O3 was 1:2.SiB6@Al2O3 coating was prepared on SiC-C/C samples by hot dipping.SiB6@Al2O3-SiC composite coating can effectively protect C/C composites for a long time,in which the 1:2SiB6@Al2O3 coating sample after oxidized at 1573K in air for 540h,the oxidation weight loss rate was only 0.27%,and the weight loss per unit area was only 6.8027×10-4 g/cm2,after 250 thermal cycles,the weight loss of the sample is only-0.14%.The outer coating with core-shell structure can accurately heal the pores produced by the coating,and the Al2O3 shell can reduce the exposure time of SiB6 and its oxidation products under high temperature environment,thus reducing its gasification and volatilization,and showed excellent oxidation resistance.In the process of thermal shock,the outer coating not only forms pores generated by the healing of the borosilicate glass phase,but also produces mullite whiskers from the oxidation products of Al2O3 and nuclear materials.The presence of mullite whiskers plays a toughening role in the coating and increases the bonding ability between the coatings.(3)On the basis of SiB6@Al2O3 coating,the glass protective layer was prepared by hot dipping.The oxidation resistance of SiB6@Al2O3-SiC-C/C samples at 1573K was improved greatly by the presence of glass protective layer.Glass-1:2SiB6@Al2O3-SiC-C/C sample was oxidized at 1573K in air for 740h,the oxidation weight loss rate was only 0.36%,and the weight loss per unit area was 1.0182×10-3 g/cm2.After 250 thermal cycles,the weight loss of the sample was only-0.20%.The existence of glass protective layer significantly increases the weight gain time of the sample,so that SiB6@Al2O3-SiC coating loss in a certain period of time is less.Glass-1:2 SiB6@Al2O3 coating not only has excellent in situ self-healing and self-toughening ability,the Glass protective layer provides sufficient time for high temperature reaction to reduce the volatilization and gasification of its oxidation products,and the binding ability between coatings is further improved,showing excellent performance. |
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