Strength and Fatigue of Silicon Nitride/Silicon Nitride and Alumina/Alumina Joints

To investigate the fatigue behavior of bonded ceramics, joint specimens (Si3N4/Si3N4 and Al2O3/Al2O3 butt joints) were three-point-fatigue-tested under static and cyclic loading at temperatures ranging from 300 to 1038 K in air environment. Fractographic observation was also carried out using SEM after the fatigue test. Restriction of the strain in the joint layer due to the difference of Young's modulus between ceramics and metals causes triaxial tension in the tension side of the joint layer, accelerating the nucleation of microcavities. Therefore the process of strength degradation owing to fatigue is controlled by the rate of either the cavity growth or crack growth, depending on which one is dominant. At lower temperatures, fatigue process is mainly controlled by crack growth, on the other hand, cavity nucleation and its growth seem to be predominant factor in controlling the fatigue process at higher temperatures. Therefore, the triaxial tensile stress component is a more important factor in evaluating the fatigue life of joints at higher temperatures. However, a fairly good estimation of fatigue life is obtained using conventional bending stress in the present investigation. It is also found that fatigue life can be estimated using bending strength of the joints.