Design of composite filler with hedgehog-like reinforcement clusters for effective stress relief in ceramic/metal joints

In ceramic/metal brazing, high residual stress usually causes fracture in the ceramic substrates. Therefore, composite fillers consisting of ductile matrix featuring low yield strength and elastic modulus, as well as reinforcements featuring low coefficient of thermal expansion (CTE), are employed. However, for AgCuTi-base composite fillers, the reinforcements are likely to weaken the deformability of brazing seams (BS), i.e. increase the elastic modulus and yield stress of BS. To address this issue, we developed a new AgCu-base composite filler with “hedgehog-like” reinforcement clusters. Herein, AgCu + NbB₂ composite filler was used to braze ZrB₂–SiC and TC4-TiB_w, wherein ductile Ag(s,s) and Cu(s,s) dominated BS, thereby ensuring high deformability; in addition, a NbB layer along with limited TiB whiskers was formed at the NbB₂/brazes interface with the orientation relationship of (-11-1)_NbB//(0-1-1)_TiB and [-314]_NbB//[12-2]_TiB, resulting in “hedgehog-like” reinforcement clusters. This structure exhibited the following advantages: (i) crystallographic misfit at the interface of reinforcement clusters/brazes and the controlled formation of TiB whiskers minimised the conventional fine grain strengthening and dispersion strengthening effects of BS; (ii) isolated reinforcement clusters decelerated the increase in the elastic property of BS; (iii) “hedgehog-like” structure absorbed fracture energy by deflecting cracks; and (iv) the formation of brittle compounds could be avoided. In other words, the designed reinforcement clusters reduced the deformability expense and compensated for the mechanical-resistance of BS. Therefore, the maximum joint strength was obtained with 2 wt% NbB₂, which increased the shear strength of the joint six times compared to that with 0 wt% NbB₂. For the joints failing inside ceramic substrates, (i) the key factor for limiting joint strength was the residual stress inside ceramics and (ii) the remission of the trade-off between thermal expansion mismatch and deformability expense contributed to relaxing residual stress, thereby confirming the reliability of our design.