THERMAL RESIDUAL-STRESSES IN CERAMIC-TO-METAL BRAZED JOINTS

When a ceramic-to-metal brazed joint is fabricated, residual stresses develop as the material is cooled from the brazing temperature to room temperature. These residual stresses reduce the strength of the brazed joint, and in some cases lead to catastrophic failure at or near an interface, during the brazing process itself. Finite element analysis modeling techniques are presented and used to determine the residual stresses inherent in the bonding process. Elastoplastic material behavior of the active filler metal, including its temperature-dependent stress-strain response, is accounted for. Various combinations of ceramics and metals are considered. Residual stresses are calculated and used to explain joint strength. The effects of the filler metal work-hardening rate and thickness on the levels of residual stresses are investigated. Analysis shows that the conclusions reached from a laboratory-brazed coupon test may not be applicable to the practical case in which ceramic tiles are brazed onto a metal substrate. There is qualitative agreement between analytical predictions and experimental findings regarding failure modes that occur during cooling from the brazing temperature to room temperature.