Control of wetting and uniformity via ZrSi_x formation in ceramic-to-metal joints fabricated using Ag-Zr brazes

The deposition of a 2.0 µm SiO₂ film on the alumina surface in Kovar^TM/94% alumina joints enables the formation of a silicide reaction layer on the alumina during brazing with 97Ag2Zr1Cu. Additionally, the average and standard deviation of joint thickness decrease from 50 to 15 and 29 to 4 µm, respectively compared to joints without added SiO₂. Finally, the average failure stress of these braze joints was 45 MPa, while that of similar joints without added SiO₂ was 90 MPa. Sessile drop experiments of 98Ag2Zr on SiO₂ and 99.6% Al₂O₃ substrates show that the braze wets and spreads to 3x its original area on SiO₂ with a wetting angle near 0°, but remains the same area on 99.6% Al₂O₃ with a wetting angle of 106.6°. Focused-ion-beam scanning electron microscopy analysis of a cross-section of the 98Ag2Zr sessile drop on the SiO₂ substrate has shown that Zr reacts with SiO₂ to form Zr oxide and silicide layers. Scanning transmission electron microscopy diffraction and energy dispersive X-ray spectroscopy analysis indicate this silicide layer contains tetragonal Zr₅Si₄. Analysis shows the silicide layer enhances wetting and joint uniformity while unreacted SiO₂ embrittles the joint and degrades strength.