Interlayer selection and thermal stresses in brazed Si3N4− steel joints

The influence of Cu, Kovar, Mo, and W interlayers on the magnitude and distribution of thermal stresses and on the tensile strength of brazed Si3N4− steel joints is examined using a combination offinite element modelling (FEM) calculations and direct experiment. The FEM takes into account plastic flow and changes of mechanical properties of the interlayer, the steel substrate, and the filler metal as temperature decreases following the brazing operation. Joints made using low yield strength/high ductility interlayers, such as Cu, have lower thermal stresses and higher strengths than those made using low thermal expansivity/high yield strength interlayers, such as Mo or W. A composite interlayer comprising Cu and W will produce the lowest thermal stresses during brazing. Increasing the thickness of the interlayer decreases the thermal stresses produced during brazing, since the rigid restraint effect due to the high yield strength/high elastic modulus steel substrate is reduced. MST/1442