IMPROVING THE STRENGTH OF BRAZED JOINTS TO ALUMINA BY ADDING CARBON-FIBERS

The addition of short, bare, carbon fibres to a silver-based active br azing alloy (63Ag-34Cu-2Ti-1Sn) resulted in up to 30% improvement in t he shear/tensile joint strength of brazed joints between stainless ste el and alumina. The optimum fibre volume fraction in the brazing mater ial was 12%. This improvement is attributed to the thinning and micros tructural simplification of the alumina/braze reaction product (titani um-rich) layer, the softening of the brazing alloy matrix, the strengt hening of the braze and the reduction of the coefficient of thermal ex pansion. The depth of titanium diffusion into the alumina was decrease d by the fibre addition. The first two effects are due to the absorpti on of titanium by the fibres. This absorption resulted in less titaniu m in the brazing alloy matrix, a braze/fibre particulate reaction prod uct (titanium-rich) on the fibres and the diffusion of titanium into t he fibres. In contrast, the use of an active brazing alloy with a lowe r titanium content but without carbon fibres gave much weaker joints.