Crack-healing behavior of structural ceramics under the conditions of tensile stresses and high temperatures

本研究选取裂纹愈合能力优异的Si_3N_4/SiC、氧化铝/SiC和莫来石/SiC复合材料作为试验材料。引入100 μm的表面裂纹，在800 ~ 1200℃的高温下进行裂纹修复处理，并施加拉应力。基于应力下裂纹愈合试件的强度，研究了应力下的裂纹愈合行为。从得到的结果，得出以下结论：1。热处理试样具有与光滑试样相同的强度。在强度为断裂试样强度的70 ~ 80%的拉应力作用下，断裂试样能够自愈。在裂纹愈合温度下，应力下裂纹愈合的试样与热处理后的光滑试样具有相同的强度。而且，由于断裂起裂点主要在裂纹愈合部分的外侧，使得裂纹愈合部分具有足够高的强度。根据Newman-Raju的应力强度因子方程和各试件残余应力的应力强度因子，估计了预裂纹能够愈合的临界条件。在静应力和循环应力下，裂纹愈合的试样在裂纹愈合温度下具有足够高的静强度和循环疲劳强度。这些值对应于裂纹愈合试样的70-80%断裂强度。综上所述，本研究中使用的Si_3N_4/SiC、氧化铝/SiC和莫来石/SiC复合材料，在适当的温度、应力和气氛条件下，均能修复使用中产生的裂纹。

In this study, Si_3N_4/SiC, alumina/SiC and mullite/SiC composites having excellent crack-healing ability were selected to be test materials. Being introduced a surface-crack of 100 μm, they were then carried out the crack-healing treatment at elevated temperatures raging from 800 to 1200℃ to be applied tensile stress. Based on the strength of the specimens crack-healed under stress, crack-healing behaviors under stress were investigated. From the obtained results, the following conclusions were derived.1.The heat-treated specimens have same strength as smooth specimens. The cracked specimens could crack-healed under the tensile stress that is 70 to 80% strength of the cracked specimens. The specimens crack-healed under stress had the same strength as the heat-treated smooth specimen at the crack-healed temperature. Moreover, the crack-healed part had enough high strength, because fracture initiation was mostly outside the crack-healed part.2.The critical condition to be able to heal the pre-crack was estimated from the equation of the stress intensity factor expressed by Newman-Raju and the stress intensity factor of the residual stress for every specimen.3.The specimens crack-healed under static and cyclic stresses had enough high static and cyclic fatigue strengths at the crack-healing temperature. These values corresponded to 70-80% fracture strength of the crack-healed specimens.Based on the above conclusions, it is confirmed that Si_3N_4/SiC, alumina/SiC and mullite/SiC composites used in this study can heal cracks initiated in service if the temperature, stress and atmosphere conditions were adjusted.