Joining of SiO2–BN ceramic to Nb using a CNT-reinforced brazing alloy

Abstract

Brazing SiO₂–BN ceramics with Nb are often associated with the problems of high residual stress caused by the difference in the thermal expansion coefficients and poor mechanical properties under high temperature. To overcome these problems, here we report a new type of carbon nanotube (CNT)-reinforced TiNi brazing alloy via an “in situ growth” method by PECVD. The CNT/TiNi brazing alloy has very homogeneously dispersed CNTs within the TiNi brazing alloy, produced by in situ growth of CNTs on TiH₂ and Ni powders mixed evenly into the CNT/TiH₂ powders. It can be applied in the brazing of SiO₂–BN and Nb. Results show that the addition of CNTs into the TiNi brazing alloy is very beneficial for the dissolution and diffusion of Nb in the brazed joint, because it can promote more TiNi–(Nb,Ti) eutectics which emerge in most of the brazing seam. Furthermore, the average shear strength of the brazed joint at room temperature is raised from 49 to 85 MPa with 1.5 vol% CNTs added into the TiNi brazing alloy. In particular, at 800 °C the brazed joint still has a very high shear strength of 51 MPa, about 1.7 times stronger than that of the TiNi brazing alloy. These results suggest that the CNTs played a key role in reducing the residual stress and reinforcing the mechanical properties (at both room and high temperature) of the brazed joint. It provides a way for the development of CNT-reinforced brazing alloys.