KINETIC AND MICROSTRUCTURAL ASPECTS OF THE REACTION LAYER AT CERAMIC METAL BRAZE JOINTS/

The formation and stability of the reaction layer when brazing non-oxi de ceramic materials were studied. Si3N4-Si3N4, SiC-SiC and Si3N4-stai nless steel braze joints were produced and investigated. Several fille r metals, most Cu- and Ag/Cu-based, containing different amounts of ti tanium were used to evaluate the effect of titanium on the formation a nd growth of the reaction layer. Some braze joints were processed usin g filler metals containing precious metals for high-temperature and ox idation-resistant applications. It was established that the matrix com position of titanium-bearing filler metals affects the ceramic wetting characteristics and the reaction layer kinetics. In the Si3N4 braze j oints, the reaction layer consisted of TiN and titanium silicides. An activation energy corresponding to the diffusion of nitrogen in TiN wa s calculated for the growth of the reaction layer. During fabrication of the braze joints with precious-metal-containing filler metals at 12 50 degrees C, Si3N4 decomposed and a sound joint could not be processe d. Premetallizing the Si3N4 with an AgCulnTi filler metal resulted in the formation of the reaction layer and permitted the fabrication of s ound braze joints at 1250 degrees C. Attempts to produce SiC braze joi nts with CuTi filler metals were unsuccessful owing to the decompositi on of the SiC; a TiC reaction layer had developed, but this did not pr event the diffusion of copper into the ceramic substrate, nor did it s low down the decomposition of the SiC.