【作者】 王超；

【导师】 梁军；

【作者基本信息】 哈尔滨工业大学 ， 工程力学， 2009， 博士

【摘要】 以高超声速、高机动的远距离精确打击为主要技术特征的高超声速武器已成为世界军事热点,将对未来战争概念和模式带来一场革命,可以毫不夸张地说,高超声速飞行器的出现将给人类生活带来极为深远的影响。比起传统武器,高超声速武器具有极大的效能优势,可以有效减少防御响应时间,增强武器突防和反防御能力,扩大发射平台范围,提高武器生存能力、作战效能和效率。高超声速、长时间的服役特征对武器装备的防热材料和结构提出了严峻的挑战,尤其是对抗氧化材料的耐温极限和耐久性、高温氧化和复杂载荷条件下的轻质强韧化性能提出了苛刻的要求。ZrB2-SiC基超高温陶瓷材料具有高熔点、高硬度、高导热率、优异的高温强度和抗氧化性等特点,因此作为新一代热防护材料受到广泛关注,国外较先进的超高声速飞行器上已经对超高温陶瓷材料进行了一定的应用。由于超高温陶瓷材料的研究在国内尚处在起步阶段,对材料在使用过程中的失效机制并不是十分清楚,材料失效过程的表征与评价方法也比较有限。因此,认清材料在使用过程中的失效机制,表征材料在使用过程中的失效过程,预报材料性能与使用温度及使用时间的关系成为提高超高温陶瓷材料可靠性和使用寿命的关键。本文针对ZrB2-SiC基超高温陶瓷材料在使用过程中的失效机制进行了理论、实验以及数值模拟等方面的研究,获得了材料在热冲击、氧化以及高温长时使用过程中发生破坏的主要影响因素。为进一步提高材料的性能以及对材料进行设计提供了一定的理论依据。首先研究了超高温陶瓷的抗热冲击性能,建立了考虑表面换热条件下材料的热冲击传热模型以及热应力的差分格式,通过计算表明表面换热系数以及材料的特征尺寸是影响材料热冲击性能的主要因素。为了验证这一结论,实验研究了不同尺寸试样的热冲击性能,结果表明超高温陶瓷材料的热冲击性能存在明显的尺寸效应,这种尺寸效应能够通过上述模型很好地表征。在试验的过程中还讨论了不同添加剂对材料的室温破坏模式及热冲击破坏模式的影响,以及残余热应力对材料热冲击性能的影响。通过上述计算还可以发现材料的表面换热系数能够显著影响材料的抗热冲击性能,因此对超高温陶瓷材料进行了设计,通过预氧化过程中材料表面形成的氧化膜大幅降低材料表面的换热系数。对预氧化材料进行了热冲击实验表明,预氧化方法确实能够显著提高材料的抗热冲击性能,提高幅度在40%以上。预氧化过程中表面氧化物的成分与形貌是影响预氧化效果的主要因素,而预氧化时间对于效果的影响比较有限。预氧化方法除了能够提高材料的抗热冲击性能以外还能够弥合在加工过程中试样的表面缺陷;同时氧化物层的热辐射性能较高,在材料真实的使用过程中能够减少热量的吸收,因此预氧化方法对于提高材料在真实环境下的热冲击性能有较大的帮助。用水淬法测试材料热冲击性能的过程中水槽温度对结果的影响非常明显。通过引入沸腾换热的概念,建立了气泡对材料热冲击性能影响的模型,通过数值模拟很好的解释了实验中出现的现象,并通过材料在液氮中的热冲击实验验证了上述模型的有效性。由于氧化破坏是材料的主要破坏模式之一,本文研究了材料的氧化破坏过程,针对氧化过程中出现的SiC耗尽层建立了氧化动力学模型,以氧化过程中质量守恒以及固体相体积守恒为基础,并应用反应速率方程得到了SiC耗尽层孔隙率与氧化时间的关系。孔隙率随着氧化时间的增加先上升后下降,材料内的SiC含量越高SiC耗尽层的孔隙率越高。通过不同SiC含量的ZrB2-SiC材料在1800℃的静态氧化实验以及灰度提取与二值化处理技术验证了上述模型的有效性。考虑氧化过程中的相变以及出现的孔洞,利用细观力学计算了SiC耗尽层的弹性性能的衰减,并建立了二维孔洞演化模型,给出了由于孔洞的出现SiC耗尽层强度的衰减规律。针对材料在高温长时使用过程中的性能劣化建立了材料在高温下的晶界相软化流动模型,给出了晶界相受力的影响因素。通过计算表明材料内部颗粒尺寸的不均匀性会导致晶界上的应力集中,颗粒的尺寸越大分布越不均匀应力集中现象越明显;晶界上的杂质同样会导致20~30倍的应力集中,这些由于微结构引起的应力集中是材料在高温长时使用过程中缺陷形核的根本原因。在讨论了材料内部缺陷如何成核的基础上建立了晶界相孔洞扩展以及连通模型,得到了孔洞成核后扩展到完整晶界尺寸的影响因素,给出了材料在长时高温使用过程中内部缺陷的形成与演化规律。另外,对于超高温陶瓷材料在高温下的性能以及裂纹扩展模式尝试通过分子动力学方法进行分析,本文针对SiC材料利用Tersoff三体势函数模拟了材料在高温下的性能及破坏过程。结果表明在温度较低时,材料的破坏过程完全符合脆性断裂的特点。当温度高于1200℃后,材料的断裂模式发生了一定的转变,裂纹尖端开始出现损伤区,从完全的脆性解理断裂发展为脆性断裂与子母裂纹传播机制并存的裂纹扩展机制。当温度进一步升高,裂纹扩展过程中裂纹前缘损伤区的出现更加明显,子母传播机制成为裂纹扩展的主要机制。初步研究表明分子动力学方法在模拟材料高温性能方面具有一定的价值。更多还原

【Abstract】 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.更多还原