The effect of surface preoxidation of sintered silicon carbide on its wettability by silver–copper based brazing alloys in vacuo

The sessile drop method has been used to determine time dependence of contact angle at 850°C in vacuo for Ag–28wt.%Cu, Ag–35wt.%Cu–1.5wt.%Ti and Ag–22wt.%Cu–12wt.%In–2wt.%Ti on pressureless sintered SiC in the as-polished and high temperature preoxidized conditions. Contact angle was independent of time for the Ti-free alloy and was increased from 137 to 161 or 162° by preoxidising the SiC. Contact angle decreased with time when Ti was present typically from 80 to 110° initially to 5 to 20° after 90 min at 850°C, again values being higher for the preoxidised condition. These behaviours are attributed to the effects of preoxidation in replacing the thin amorphous silica film on the polished surface by a thicker crystalline film which retards the action of Ti in promoting the wettability desired for brazing purposes.