Diffusion Bonding of Si₃n₄ to Metals by Hip used for High Temperature Applications

Abstract

In the last few years there has been growing interest in the joining of metals to ceramics for use as structural materials. Diffusion bonding of Si₃N₄ to the new generation of ODS-superalloys, such as MA-6000, is a method considered to yield well joined metal/ceramic systems for high temperature applications. Joining of Si₃N₄ to MA-6000 bars was achieved by diffusion bonding during HI** at 100 to 200 MPa at 1100 to 1400 DC. Stresses caused by the largely different thermal expansion coefficients were reduced by multiphase interlayers. An INVAR-alloy layer with high formability at elevated temperatures and small thermal expansion coefficient at low temperatures were combined with various reactive layers of transition metals, such as Ti, Nb and V. The reactions at the interfaces and the fracture surfaces were investigated by SEM, EDX and WDX analysis.

During bonding of Si₃N₄ to metals brittle phases are frequently formed at the interfaces which may lead to a failure of the joint. In systems with a large thermal mismatch, stress relaxation by thin soft interlayers is very limited. However, the formation of brittle reaction layers can be controlled by an interdiffusion layer technique and by the bonding temperature and -time. At special HIP - conditions, the damage of the joint can additionally be reduced by microcrack induced stress relaxation. Based on the thermodynamics of multi component systems, the interfacial reactions and the compatibility of the interlayers were calculated and com. Pared with the experimental results. Calculations on the residual stresses in the metalfceramic multilayer joints are presented.