Wetting and Joining of Mullite Ceramics by Active-Metal Braze Alloys

Mullite and ZrO₂-toughened mullite (ZTM) were joined with Ag─Cu eutectic braze alloys that contain Ti or Zr as active elements. Although neither the alloy compositions nor the processing conditions were optimized, four-point bend strengths of joined bars were as high as 108 MPa. Joining reactions were studied by conducting separate sessile drop experiments with molten Ti─Ag─Cu and Zr─Ag─Cu alloys on mullite for times and temperatures that were similar those used for joining. Detailed compositional and microstructural analyses of those metal–ceramic interfaces revealed a complex reaction zone in which the oxide of the active element was a principal reaction product. Compared with the Ti-containing alloys, those with Zr were more refractory, they were less reactive with the mullite, and the Zr did not segregate as completely to the interface during heating.