| Hypersonic weapon with main technological character of hypersonic, high mobility and precise strike from long distance has been drawn much attention all over the world. A revolution on conception and mode of future war will bring profound effect on human's life. Comparing with conventional weapons, hypersonic weapons have a huge advantage of efficiency, which can decrease the response time of defense effectively, reinforce the abilities of break-through and anti defense, enlarge the scope of launch platform and further enhance the survival potential and operational efficiency of weapons. The hypersonic flying vehicles include ballistic missiles, interceptor missiles, hypersonic aerodynamic missiles, re-entry vehicles, cross-atmospherical vehicles and hypersonic airplane and so on. It is a great challenge for the thermal protection materials and structures used in weapon furnish demanding hypersonic (M>5) and long time. Especially, the essential performances of limiting temperature and durability, high temperature oxidation and light-weight strengthening and toughening under complex loadings are exacting.ZrB2-SiC have been regarded as promising candidate materials for use in thermal protection systems and propulsion systems in hypersonic aerospace vehicles, as a result of their unique combination of high melting temperature, high strength, thermal stability, and corrosion resistance. As a result of the application environment, the main types of failure were thermal shock destroy due to the temperature increase suddenly, oxidize destroy and property deterioration due to the high temperature during a long time. The failure mechanism and the evaluation method were not clear as a result of the initial step of the research.The key of improving the reliability and service life of UHTCs under oxidizing and complex load is to recognize the failure mechanism, to attribute the failure process and predict the relationship between properties and serviceability temperature and time. Therefore, theoretical, experimental and numerical methods have been adopted to investigate the failure mechanism during service process. The main influence factors of UHTCs properties during thermal shock, oxidation and under high temperature for a long time have been obtained. The results give a lot of help for improving the properties of UHTCs.The thermal shock properties have been studied first. The model of heat transfer considering surface heat transfer condition has been established. The difference scheme of thermal stress has been built. The surface heat transfer coefficient and characteristic heat transfer length were the main influence factors for thermal shock properties of UHTCs. To approve the conclusion above, the thermal shock behavior of UHTCs in different sizes has been investigated; results showed a strong size effect on thermal shock behavior of UHTCs which can be well described by the heat transfer model. The different room temperature failure mode and thermal shock failure mode of different materials containing different addition agent has been discussed as well as the influence of residual thermal stress on residual thermal shock intensity.Through the calculation above we can see that the thermal shock behavior of UHTCs can be strongly influenced by surface heat transfer coefficient. Therefore, a pre-oxidation method has been adopted in order to decrease the surface heat transfer coefficient. Through thermal shock test on pre-oxidized material we can see that the thermal shock behavior has been strongly improved by pre-oxidation, the amplitude is more than 40%. The temperature of pre-oxidation was a main influencing factor in improving the thermal shock behavior of the composites, while the influence of the length of the pre-oxidation time was slight. In addition, the defects on the surface can be closed by pre-oxidation and the radiation factor of oxide is high which can decrease the absorbing of heat quantity. Therefore, pre-oxidation method can improve the thermal shock behavior of UHTCs under true environment.The temperature of water bath is a main factor of thermal shock behavior tested by water quenching. Boiling water heat transfer rule has been introduced and the model of the effect of bubbles has been founded. The experimental phenomenon has been well explained by numerical simulation and the availability of the model has been confirmed by thermal shock test in liquid nitrogen.Due to the oxidize failure is one of the main failure mode, the oxidize failure process has been investigate. The oxidize model has been established in allusion to the SiC depletion layer. The relationship between porosity and oxidize time has been obtained by Arrhennius equation, mass and solid volume conservation. Results show that the porosity increases first and than decrease vs. oxidize time. The porosity of SiC depletion layer increases as a result of the increment of the content of SiC. The availability of the model has been confirmed by extracting the shade of gray of microphotograph of static oxidize test at 1800℃. Considering the phase change and pores, the elastic behavior of SiC depletion layer has been calculated by Meso-mechanical. The decay of the strength of SiC depletion layer has been calculated by pore evolution model.The model of grain boundary softening and flow has been established according to the property deterioration of UHTCs at high temperature during long time. The influencing factor of stress on grain boundary phase has been obtained. The results show that the inhomogeneity of grain size leads to the stress concentration on grain boundary, the stress concentration increase as the increment of the inhomogeneity of grain size. 20~30 doubled stress concentration also can be induced by the impurity on grain boundary. The ultimate reason of the cavity nucleation at high temperature during long time is the stress concentration induced by microstructure. The cavities propagate and confluence model has been established. The influence factor of cavity propagates to a grain size and the propagate time have been obtained. The rule of the cavity nucleation and evolvement has been given.In addition, The MD simulation method has been attempted to analyse the high temperature property and crack propagate mode of UHTCs. In this article, Tersoff three-body potential function has been adopted to simulate the high temperature property and failure process of SiC. Results show that the brittle failure was the main failure process when the temperature is low. However, when the temperature rise to 1200℃or more the failure model has been changed, damage zone has been appeared before crack tip. The failure mechanism has been changed from brittle failure to primary-secondary crack propagation. When the temperature rises higher, the primary-secondary crack propagation has been changed to the main failure mechanism. Preliminary study indicates that the MD simulation method has some value in simulating the high temperature property of materials. |
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