Improving the strength of brazed joints to alumina by adding carbon fibres

Abstract

The addition of short, bare, carbon fibres to a silver-based active brazing alloy (63Ag-34Cu-2Ti-1Sn) resulted in up to 30% improvement in the shear/tensile joint strength of brazed joints between stainless steel and alumina. The optimum fibre volume fraction in the brazing material was 12%. This improvement is attributed to the thinning and microstructural simplification of the alumina/braze reaction product (titanium-rich) layer, the softening of the brazing alloy matrix, the strengthening of the braze and the reduction of the coefficient of thermal expansion. The depth of titanium diffusion into the alumina was decreased by the fibre addition. The first two effects are due to the absorption of titanium by the fibres. This absorption resulted in less titanium in the brazing alloy matrix, a braze/fibre particulate reaction product (titanium-rich) on the fibres and the diffusion of titanium into the fibres. In contrast, the use of an active brazing alloy with a lower titanium content but without carbon fibres gave much weaker joints.