Diffusion bonding of silicon nitride to austenitic stainless steel with metallic interlayers

Abstract

This article deals with the diffusion bonding of hot-pressed silicon nitride (HPSN) to austenitic stainless steel using metallic interlayers. Experiments were carried out with Fe, Ni, invar, and kovar as the interlayer material. The best results were obtained with invar interlayers. Using these interlayers, joints having an average shear strength of 95 MPa were produced. The reaction layer formed at the ceramic-interlayer interface during the diffusion-bonding process consists of a porous zone anchored in the silicon nitride and a diffusion zone extending in the interlayer. No silicides were detected. The formation of the joint is governed by the decomposition of the silicon nitride. The free silicon generated by the decomposition reaction diffuses into the interlayer, whereas part of the free nitrogen disappears to the surface, the remainder being trapped in the form of pores. The mechanical strength of the joint depends strongly on the residual stress generated in the joint during cooling from diffusion-bonding temperature to room temperature due to the difference in thermal shrinkage between the ceramic and the metal. The residual stress level is directly related to the thickness of the reaction layer.