Rapidly solidified CoTi alloys as brazing foils for high-temperature joining of silicon nitride ceramic

Si3N4-Si3N4 braze joints were produced and investigated. Cobalt-based filler materials with different amounts of titanium were processed, but only with a CoTi10 alloy it was possible to produce sound joints. For 3 different brazing temperatures between 1550 K and 1770 K the formation and chemical composition of the reaction layer were investigated. It was established that the reaction layer consists of TiN mainly. Silicon diffusion into the braze is only little affected by the reaction layer and causes the formation of brittle silicides. Brazing of sound joints is furthermore complicated by the fact that decomposition of silicon nitride is drastically enhanced above 1570 K and vacuum conditions are necessary to prevent oxidation. The average flexural strength of brazed samples tested in the 4-point bending geometry is approximately 50 MPa.