Kinetic and microstructural aspects of the reaction layer at ceramic/metal braze joints

The formation and stability of the reaction layer when brazing non-oxide ceramic materials were studied. Si3N4-Si3N4, SiC-SiC and Si3N4-stainless steel braze joints were produced and investigated. Several filler metals, most Cu- and Ag/Cu-based, containing different amounts of titanium were used to evaluate the effect of titanium on the formation and growth of the reaction layer. Some braze joints were processed using filler metals containing precious metals for high-temperature and oxidation-resistant applications. It was established that the matrix composition of titanium-bearing filler metals affects the ceramic wetting characteristics and the reaction layer kinetics. In the Si3N4 braze joints, the reaction layer consisted of TiN and titanium silicides. An activation energy corresponding to the diffusion of nitrogen in TiN was calculated for the growth of the reaction layer. During fabrication of the braze joints with precious-metal-containing filler metals at 1250 degrees C, Si3N4 decomposed and a sound joint could not be processed. Premetallizing the Si3N4 with an AgCulnTi filler metal resulted in the formation of the reaction layer and permitted the fabrication of sound braze joints at 1250 degrees C. Attempts to produce SiC braze joints with CuTi filler metals were unsuccessful owing to the decomposition of the SiC; a TiC reaction layer had developed, but this did not prevent the diffusion of copper into the ceramic substrate, nor did it slow down the decomposition of the SiC.