Probabilistic strength estimation for ceramic-metal joints with various interlayer thicknesses

Residual stress generated by thermal expansion coefficient mismatch between ceramic and metal is an important problem affecting the strength of ceramic-metal joints. An interlayer, which is a ductile metal, is often inserted between ceramic and metal in order to relax this residual stress. In this study, the residual stress produced in the joint-cooling process is analyzed and 4-point bending tests are carried out. From the viewpoint of experimental observations and fracture mechanics, the effects of interlayer thickness on joint strength in ceramic/metal joints are discussed considering the superimposed stress distribution of the residual stress and the bending stress. Joint strength is then estimated based on fracture mechanics and fracture probability considering the superimposed stress, crack size and position of pre-existing defects in the ceramics. The optimum interlayer thickness for the present specimen is identified, and the estimation method is extended to the analysis of generic joint strength using normalized strength of ceramic by expressing joint strength in terms of normalized strength considering the scatter of joint strength and the effective volume.