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0xidation Behaviors Of 3d C/sic Composites In Oxidizing Environments

Posted on:2002-10-16Degree:DoctorType:Dissertation
Country:ChinaCandidate:X W YinFull Text:PDF
GTID:1101360032453967Subject:Materials science
Abstract/Summary:PDF Full Text Request
In this thesis, three main aspects of 3D C/SiC prepared by lower pressure chemical vapor infiltration (LPCVI) were researched: oxidation behaviors, characteristics, mechanisms and mode of C/SiC; effect of different oxidation-resistance approaches on oxidation behavior of C/SIC; effect of environments on oxidation behaviors. The main contents and results are as follows: I. The oxidation behaviors, mechanisms and modes of C/SiC-SiC were investigated. The oxidation behavior of C/SiC was controlled by defects. The influence of defects can be reduced by multi-deposition of SiC coating. Between 400 and 7000C, the oxidation rate of C/SiC was controlled by oxidation of C phase, the oxidation mode was uniform oxidation; between 700 and 10000C, the oxidation rate was controlled by diffusion of oxygen through coating microcracks and preparation defects, the oxidation mode was non-uniform oxidation; beyond 10000C, oxidation rate was controlled by diffusion of oxygen through coating defects, the oxidation mode was surface oxidation. 2. The oxidation behaviors of C/SiC-SiC were characterized by weight loss and residual flexural strength. The relation between residual flexural strength and weight loss of C/SiC-SiC was set up for the first time. The results showed the change of residual flexural strength was controlled by oxidation of carbon phase. The variation of flexural strength with weight loss followed the principle of exponential function. 3. Effect of different oxidation-resistance approaches on oxidation behaviors of C/SiC was investigated. The oxidation resistance of C/SiC was improved greatly by multi-deposition of SiC coating. However, the oxidation of C/SiC-SiC was controlled by diffusion of oxygen through coating microcracks and coating gap, which led to the weight losses of C/SiC-SiC in all range of temperature. The oxidation resistance of C/SiC-SiC can be improved below 1 1000C by employing glass coating, and the composites showed weight gain beyond 1 1000C by using alloy coating. By combining alloy coating with glass coating, the oxidation resistance of C/SiC-SiC can be improved in all range of temperature. 4. Effect of temperature gradient on the mechanical properties of composites was investigated in different oxidizing atmospheres. Oxygen may diffuse through the area below cracking temperature into the area beyond cracking temperature, which resulted in the decrease of strength of sample beyond cracking temperature. The diffusion flux of oxygen in combustion gas was higher than in static air, the area where strength decreased was larger in combustion gas than in air. 5. The variation of weight loss with oxygen partial pressures followed the principle of power function. The effect of water vapor (I .4x I O4Pa, 5x I O4Pa) on oxidationbehaviors of ClSiC-SiC was researched for the firSt time. Watr vapor can imProve theparabolic rate constant of SiC in oxygen, and the oxidahon of C fibers can berestrained between 700-l300"C at the sarne time, which resultCd in the deerease ofweight loss of ClSiC-SiC in oxygen.6. The variation of thermal-shock resistance of ClSiC-SiC composites in combustiongas under certain temPerature gradient was investigated for the first time. lt was fOundthat there existed a critic value for the influence of thermal-shock times on strength, andthe deCrease of strength resulted from thermal-shock damage of carbon fibers. Afterthermal-shocked for l00 times between 3oo and l300"C, the strength keeping rate ofcomposites still drived to 83%. The crihc temperature difference of C/SiC-SiC incombustion gas was about 700"C, and the crihc times of...
Keywords/Search Tags:C/SiC composites, Coating, LPCVI method, RMI method, Oxidaion, Thermal-shock, Combustion gas, Microstructure.
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