Strength and fatigue of silicon nitride/silicon nitride and alumina/alumina joints

To investigate the fatigue behavior of bonded ceramics, Si3N4/Si3N4 and Al2O3/Al2O3 joint specimens were three-point-fatigue-tested under static and cyclic loading at temperatures ranging from 300 to 1 038 K in air. Fractographic observation was also carried out using SEM after the fatigue test. Restriction of the strain in the joining layer due to the difference in Young's modulus between ceramics and metals causes triaxial tension in the tension side of the joining layer, accelerating the formation of small cavities. Therefore the process of strength degradation owing to fatigue is controlled by the rate of either the cavity growth or the crack growth, depending on which one is dominant. At low temperatures, the fatigue process is mainly controlled by crack growth; on the other hand, formation of small cavity and its growth seem to be predominant factors in controlling the fatigue process at high temperatures. Therefore, the triaxial tensile stress component is a very important factor in evaluating the fatigue life of joints at high temperatures. However, a fairly good estimation of the fatigue life is obtained using nominal bending stress in the present investigation. It is also found that the fatigue life can be estimated using the bending strength of joints.