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

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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.