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 bends 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 to 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