INTERFACE MICROCHEMISTRY OF SILICON-NITRIDE NICKEL-CHROMIUM ALLOY JOINTS

Silicon nitride ceramics were joined by diffusion bonding with a Ni-Cr alloy foil. The reactions between the alloy foil and the silicon nitride were examined using optical (OM) or scanning electron microscopy (SEM) and electron probe microanalysis and related to published thermodynamic, phase diagram, and diffusion data. Joints were produced using Cr-precoated or uncoated Si3N4 ceramics and Ni-20 pct Cr foils either at 1423 K by applying 100 MPa for 1 hour in an Ar atmosphere or at 1473 K in vacuum. All joints formed were bonded by bridging layers of alloyed CrN, whose thickness and Cr content were affected by the joining conditions. Bonding both in Ar and vacuum produced volume diffusion-controlled concentration gradients of Cr and Si in the Ni-Cr foil. Similarly, N2 diffusion from the ceramic into the metal foils produced grain boundary precipitation of Cr2N. For samples bonded in vacuum, a two-phase zone was additionally formed between the CrN bridging layers and the metal foils consisting of a Ni5Si2 matrix that was molten at the bonding temperature and undissolved islands of foil material. The interfacial microchemistries are discussed with reference to the mechanical properties and high-temperature stability of the joints.