【作者】 齐菲；

【导师】 孟松鹤；

【作者基本信息】 哈尔滨工业大学 ， 工程力学， 2013， 博士

【摘要】 陶瓷材料具备耐高温、抗氧化、化学性质稳定等一系列优势，在国防、航空航天、新型工业等领域发挥着越来越重要的作用。但是，陶瓷因为其固有脆性，在快速加热或者由高温极速冷却的过程中容易产生微裂纹，微裂纹迅速增多、扩展及连通等损伤行为可能导致材料发生灾难性破坏。因此，提高脆性陶瓷材料抗热冲击性能一直是热结构材料应用中所面临的严峻挑战。如何表征陶瓷材料微观结构，并建立微结构与热冲击性能的联系，从而深入了解影响陶瓷抗热冲击性能的各种因素，是设计优化材料以至提高其抗热冲击性能的一个十分重要的课题。热冲击下陶瓷中裂纹成核与扩展等损伤行为严重影响了材料性能，是导致陶瓷破坏的最本质因素。本文通过水淬热冲击实验，研究了多晶单相Al₂O₃陶瓷在不同水淬温度下材料表面裂纹形貌以及强度衰减规律。基于ZrB₂-基超高温陶瓷良好的导电性，通过电阻热冲击实验，研究了多晶复相ZrB₂-SiC在不同热冲击次数及升温速率下剩余强度的变化规律，分析了氧化表面玻璃相以及氧化层内孔洞对剩余强度的耦合作用。针对传统欧式空间几何方法不能完全描述材料不规则、不均匀几何特征的客观事实，应用图像预处理方法合理地提取了陶瓷热冲击实验中裂纹、断口以及氧化表面的几何信息，并采用分形这种非线性几何方法对其进行刻画，定性地分析了微结构分形维数与陶瓷宏观力学性能的联系。对于Al₂O₃陶瓷水淬热冲击后的裂纹形貌，通过盒计维数法计算了扩展的表面宏观裂纹分形维数，在此基础上对比了不同晶粒尺寸陶瓷的裂纹分形维数变化趋势及剩余强度衰减规律。进一步通过相似维数法，建立了裂纹分叉分形维数模型，给出了分形维数随分叉角度的变化规律。此外，应用三维白光干涉技术测量陶瓷断口表面形貌，并计算出断口分形维数，揭示了不同材料断裂强度随断口分形维数的变化规律。计算了ZrB₂-SiC超高温陶瓷氧化表面玻璃相分形维数，从而度量了玻璃相在氧化表面的覆盖情况，结合氧化层内孔洞分形维数的变化趋势，初步分析了陶瓷热冲击后强度变化的原因。临界温差与热冲击后材料剩余强度是评价陶瓷抗热冲击性能的两个关键因素，有必要给出陶瓷微结构分形维数与如上抗冲击性能的定量关系。首先，通过分形裂纹长度与平直裂纹长度的投影关系，给出了裂纹做分形扩展与平直扩展的临界裂纹扩展力的关系。并针对陶瓷不同晶粒形状及断裂模式，计算了初始裂纹分形维数。在以上分析和计算的基础上，基于Hasslman经典抗热冲击临界温差模型，建立了多晶脆性陶瓷裂纹分形维数与热冲击临界温差之间的关系。指出陶瓷制备中通过改变材料微观结构，从而增大初始裂纹分形维数，可以提高材料的热冲击临界温差。其次，建立了陶瓷材料热冲击下统计剩余强度分形模型，给出了热冲击后陶瓷材料的强度衰减比，深入分析了裂纹数量以及裂纹分形维数对强度衰减比的影响。最后，考虑了陶瓷热冲击后裂纹分叉的影响，分析了分形维数及陶瓷断裂韧性随裂纹分叉角度的变化规律，进而拓宽了对陶瓷统计剩余强度分形模型的涵义。针对建立的热冲击临界温差分形模型与统计剩余强度分形模型，结合具体的陶瓷结构和性能，对分形模型有效性进行了验证。通过热冲击临界温差分形模型，对Al₂O₃、ZrB₂-SiC、ZrB₂-SiC-graphite陶瓷的临界温差进行了预测，预测结果与Hasslman模型计算结果及实验结果的对比，验证了模型有效性。基于Hasslman模型中裂纹长度与密度的关系，对不同水淬温度下试样中裂纹数量进行了估算。应用统计剩余强度分形模型，预测了晶粒尺寸为10μm Al₂O₃热冲击后剩余强度变化。结合热冲击后裂纹分叉角度分形维数对剩余强度的影响，说明了考虑裂纹分叉的必要性。更多还原

【Abstract】 Ceramic materials, which have high temperature resistance, oxidation resistance,chemical stability and a series of advantages, play an increasingly important role in na-tional defense, aerospace engineering and modern industry. However, during rapid heat-ing or cooling, ceramics as a kind of brittle materials, may create microcracks, whichmay lead to catastrophic failure to materials when the cracks rapidly increase, connectand expand to some extent. Thereby, increasing the thermal shock resistance of ceramicshas been becoming a challenge to thermal structural materials. How to characterize mi-crostructures of ceramics and establish the relationship between the microstructure andthermal shock properties, as a very important project, is helpful to understand the impactparameters to thermal shock resistance deeply, and efective to increase the thermal shockresistance of ceramics by designing materials optimallly.The nucleation and propagation of cracks make great influence on material proper-ties, which is the most essential reason for ceramics failure. In quenching experiments,we studied the crack morphology and strength weakening for polycrystalline single phaseAl₂O₃ceramic. With the well conductivity of ZrB₂-based UHTCs, the residual strengthof multiphase polycrystalline ZrB₂-SiC with diferent heating cycles were analyzed byelectric resistance method. What is more, coupling efect of glass phase and pores locatedin oxidization layer for residual strength was also studied.For traditional Euclidean geometry can not describe the irregular and uneven geom-etry of materials completely, fractal methods were used to characterize the cracks, frac-ture surfaces and oxidation surfaces after thermal shock experiments based on the imagepre-processing geometrical morphologies. Then the relationship between microstructurefractal dimension and macro-properties of ceramics were analyzed qualitatively. For crackmorphology of Al₂O₃ceramic after thermal shocks, the fractal dimension of cracks wascalculated by using box-counting method. Then, the crack fractal dimensions and resid-ual strength of ceramics with diferent grain sizes were compared. Furthermore, a crackbranching fractal dimension model was established by the similar dimension method, andthe variation of fractal dimension along with bifurcation angle was also given. In addition,three-dimensional white light interferometry technique was employed to measure fracturesurface and its fractal dimension, which reveals the relations between fracture strengthsand fractal dimensions. As a measure of the coverage on oxidation surface, the dimension of glass phase after UHTCs thermal shock, combined with the variation of dimension ofpores located in the oxide layer, the strength variation of the ceramic after thermal shockscan be explained.Critical temperature diference and residual strength after thermal shock are key fac-tors to evaluate the thermal shock resistance of ceramics. It is necessary to give the rela-tionship quantitatively between the fractal dimension of microstructures and the thermalshock performance mentioned above. Firstly, based on the projection relations of fractalcrack and straight crack, the relations of critical crack extension force between fractal ex-pansion and straight extension were established, and the initial crack fractal dimension forceramics with diferent grain shapes and fracture modes was calculated. Combined withthe above analyses, a fractal model of critical thermal shock temperature diference wasbuilt based on the Hasslman classical model. Enhance the critical temperature diferenceof ceramics can be realized by increasing the fractal dimension of initial crack. Secondly,fractal model of statistical residual strength was proposed and strength weakening ratiowas given after thermal shocks. Then, the efect of crack quantities and crack fractal di-mensions on strength weakening ratio were analyzed deeply. Finally, crack bifurcationafter thermal shock was considered, and the influence of fractal dimension and fracturetoughness of ceramics with various crack bifurcation angles was also stated, which mayextend the profound implication of the fractal model of statistical residual strength.Combined with microstructures and macroscopic properties of ceramics, the fractalmodel of critical temperature diference and residual strength were verified. The fractalmodel of critical temperature diference was used to forecast the critical temperature dif-ference of Al₂O₃, ZrB₂-SiC, ZrB₂-SiC-graphite ceramics, which were efective comparedwith the Hasslman results and experiment data. On the basis of the Hasslman model,crack quantities under diferent water quenching temperatures were estimated. Then frac-tal model was applied to predict the statistical residual strength of Al₂O₃with grain size10μm after thermal shocks. The crack bifurcation has influence on the statistical residualstrength of ceramics after thermal shocks, which should be obviously considered in thismodel.更多还原