Investigation On Strength Properties Of Ultra-high-temperature Ceramics

The ultra-high-temperature ceramics (UHTCs) is a family of ceramic compositesthat have melting points higher than3000oC, can be used in the high temperature andoxidizing environment and have good chemical and physical stability. The uniqueproperties make these UHTCs strong potential candidates as ultra-high-temperaturestructures and thermal protection materials in aerospace applications. The studiesshowed that the addition of SiC could improve the oxidation resistance of UHTCs.However, many experiments and theories have showed that the cracking occurs aroundthe second-phase particles. Yet new flaw will occur too under high temperature as SiC iseasy to be oxidized. And the shapes of flaws are various and closely related to theshapes of added SiC particles. Those factors will affect the strength properties ofUHTCs. Due to the limitations of the experimental methods and the insignificance ofstrength theories under high temperature, a set of unified theory frames that accountingfor the effects of those factors at different temperatures on the strength properties needto be established. The main work and conclusions in this dissertation are as follows:①A thermo-damage strength model accounting for effects of residual stress wasestablished. Using the model, the effects of SiC particle size, the size of crackemanating from SiC particle and SiC particle volume fraction on strength correspondingto different temperatures were studied in detail. The study showed that in the range ofthe typical microstructure sizes of material and temperatures of the research, the effectsof SiC particle size and crack size on strength are serious dependent on their ratios andthe residual stress; and the particle size, crack size and their ratios will affect the roles inwhich the SiC particle volume fraction controls the strength.②The thermal shock resistance (TSR) models for UHTCs considering the effectsof SiC grain size, flaw size, temperature and residual stress were proposed. Therelations between the TSR and the microstructures were studied in detail. And theguidance on the improvement of TSR of UHTCs was provided.③The thermo-damage strength models corresponding to different crack shapeswere established. Using the models, the effects of crack shape on strength of UHTCswere studied in detail, and the conclusions were compared with the results obtainedusing the finite element software ABAQUS. The study will provide the valuableguidance on the shapes of added SiC particles.