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

本研究选取了具有良好裂纹愈合能力的氮化硅/碳化硅、氧化铝/碳化硅和莫来石/碳化硅复合材料作为试验材料。引入一个100μm的表面裂纹，然后在800~1200℃的高温下进行裂纹愈合处理，施加拉应力。基于应力作用下裂纹愈合试件的强度，研究了应力作用下的裂纹愈合行为。结果表明：1.热处理后的试件具有与光滑试件相同的强度。裂纹试件在拉应力为裂纹试件强度的70~80%的条件下即可实现裂纹愈合。应力下裂纹愈合的试件在裂纹愈合温度下具有与热处理光滑试件相同的强度。根据Newman-Raju表示的应力强度因子方程和每个试件的残余应力强度因子的方程，估算了裂纹愈合的临界条件。3.在静应力和循环应力下裂纹愈合的试件在裂纹愈合温度下具有足够高的静疲劳和循环疲劳强度。根据上述结论，本研究采用的氮化硅/碳化硅、氧化铝/碳化硅和莫来石/碳化硅复合材料，只要调节温度、应力和气氛条件，就可以修复在使用中产生的裂纹。

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.