| High temperature structural ceramics have been developed for the aerospace and other industrial field owing to their excellent mechanical and thermal properties. Among the high-temperature structural ceramics, ultra-high temperature ceramics have attracted most widely attention. The ultra-high-temperature ceramics are a family of transition-metal composites that have melting points higher than 3000 oC, and can be used in the high temperature(higher than 2000oC) and oxidizing environments and have good chemical and physical stability, such as ZrB2, TaC, HfN, HfB2, ZrC and so on.The ultra-high-temperature ceramics have been proved to be the best replacements of refractory metal and C/C(C/SiC), which are the most promising materials under ultra-high temperature field. The addition of reinforcing phase has been proved to be the most promising way of improving the mechanical properties and oxidation resistance of ultra-high-temperature ceramics. The good combination of melting point, hardness, thermal conductivity, strength and toughness at high temperatures and oxidation resistance have resulted in particulate-reinforced ultra-high-temperature ceramic matrix composites been developed for the leading edge and nose cap materials in hypersonic vehicles. For the ceramics used for high temperature applications, their strength performances at high temperatures and thermal shock resistance are faced with the severe challenges as they are usually exposed to the severe high temperature and oxidizing environments with large temperature fluctuations and are often subjected to rapid heating or cooling process. How to characterize and improve the strength performance at high temperatures and thermal shock resistance of ceramic materials has been a focus and hotspot in the researches of the high temperature structural ceramics.The researches of the progressive damage and failure mechanisms of high temperature structural ceramics under extreme high temperature environments, and the establishment of the characterization model having the profound physical background and including the effects of causative environments have important theoretical significance and engineering application background. The main work and conclusions in this dissertation obtained by using the combination methods of experiments and theoretical analysis are as follows:Based on the idea of modeling effects of heat energy and strain energy on fracture of materials and the classical fracture theories, a new temperature dependent fracture surface energy model of ceramics was developed. Based on the newly proposed temperature dependent fracture surface energy model and the Griffith fracture theories, a new temperature dependent fracture strength model for the particulate-reinforced ultra-high temperature ceramic matrix composites was proposed.The model has no fitting parameter and includes the combined effects of temperature, grain size, flaw size and temperature dependent residual stress. Specially, for some composites the flaw size of which changes obviously with the increase of temperature, the effect of temperature dependent flaw size was introduced into the model. Model predictions for some ultra-high temperature ceramic matrix composites were presented and compared with experimental data. The excellent agreement was obtained between predicted values and experimental data. Additionally, the model can also offer a more effective means for characterization of the temperature dependent critical flaw size of materials, which thus can help us to discover the control mechanisms of the material strength under different temperatures. Using the proposed model, the effects of different control mechanisms on the fracture strength of materials at high temperatures were studied in detail, and further the guidance and possible way for the improvement of the strength of materials at high temperatures and applying reliability were proposed.Based on the researches for the oxidative mechanisms of ultra-high temperature ceramic matrix composites, the theoretical model characterized the microstructure evolution during the formation process of SiC-depleted layer was obtained. Further, based on the studies of fracture mechanisms of ultra-high temperature ceramic matrix composites under normal and high temperatures, a thermo-damage strength model for the SiC-depleted layer including combined effects of oxidation temperature, time and phase transformation was proposed. Using the proposed model, the fracture strengths of the SiC-depleted layer at different oxidation stages were studied in detail. And the effects of various control mechanisms on the fracture strength of this oxidation layer were analyzed. The results revealed the damage evolution law and degradation law of fracture strength of the SiC-depleted layer, and analyzed the failure mechanism.Effects of cooling medium temperatures on the experimental characterization results of cooling thermal shock resistance of ceramics were studied. The results showed that the thermal shock behavior of ceramics is very sensitive to the cooling medium temperature, even which is near about room temperature. When characterizing the thermal shock resistance of ceramics using quench testing the effect of cooling medium temperature thus should be taken into account. A higher water-bath temperature may not correspond to a greater thermal shock resistance of ZrO2(3Y) ceramics. This indicates the one-sidedness of the current generally accepted conclusion that the higher water-bath temperature corresponds to the greater thermal shock resistance of ceramics.When the temperature difference in thermal shock is close to the critical temperature difference of rupture, a little change of which can cause a great change in the retained strength of materials. As the retained strength of materials is too sensitive to the temperature difference close to the critical temperature difference of rupture, we consider that it is unsuitable to use the critical temperature difference of rupture to characterize the cooling thermal shock resistance of ceramics.A new, simple and efficient experimental equipment for the ascending thermal shock testing of ceramics was proposed by refitting the traditional quenching furnace.During the ascending thermal shock testing, the experimental equipment designs a specimen fall into a high temperature environment with the temperature can be controlled accurately form a low temperature environment automatically and momentarily. The designed method solves the existing problems of the current used ascending thermal shock testing method that the central portion of the specimen is heated rapidly firstly, leading to the uneven temperature distribution in the surface of specimen, and the target temperature of thermal shock during heating is very difficult to be controlled. Using the proposed method, a series of ascending thermal shock testing of ceramics were carried out. The effects of target thermal shock temperature and specimen size on ascending thermal shock behavior were studied in detail. The results showed that our testing succeeds in developing the internal cracks in the shocked specimen, while there are no optical changes of surface morphology. This coincides with the failure mechanism of ascending thermal shock, and this is not reported before. For the ascending thermal shock resistance, we also proposed a new characterization method. Similarly to the cooling thermal shock resistance, this work showed for the first time that we can also define an evaluation index, the critical temperature difference of rupture of material, and the value of which is related to the microstructures and material properties. Further, a method for modelling the critical temperature difference of rupture of rectangular specimen is proposed based on the experimental results and theoretical analysis. |
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