Fractal Characteristics Of Microstructure And Damage Behavior In Polycrystalline Ceramics Under Thermal Shocks

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.