Brazing mechanism of SiC using the Ag-Cu-Ti alloys was investigated. The roles of titanium in active brazing of SiC have been studied extensively, while studies on the roles of silver and copper, which constitute the major parts of the active brazing alloys, have been disregarded. the effects of the relative content of silver and copper in the brazing alloy on the interfacial reactions and bond strength have been investigated in this study. The interfacial reactions were divided into the decomposition reaction of SiC by the brazing alloy melt and the interfacial reaction of titanium. Brazing by the Cu-5at%Ti alloy caused SiC to decompose, but the addition of silver in the brazing alloy suppressed the decomposition of SiC. The decomposition reaction of SiC was not brazing of SiC by the Ag-5at%Ti and $Ag_{0.6}Cu_{0.4}$-5at%Ti alloy. In the brazing of SiC by the Cu-5at%Ti alloy, three reaction layers were formed due to the decomposition of SiC and interfacial reaction of titanium and clearly identified. The decomposition caused a copper (or copper silicides) /graphite composite layer to be formed in contact with SiC. Some of graphite and silicon which were released from the decomposition reaction of SiC reacted with titanium in the Cu-5at%Ti alloy to produce TiC and $Ti_{5}Si_{3}$ respectively. The brazing alloy was changed from the copper phase to the copper silicides, $Cu_{7}Si$ and $Cu_{5}Si$, depending on the extent of the decomposition reaction. In the brazing of SiC by the Ag-5at%Ti alloy, TiC was formed as a thin layer in the contact with SiC, while $Ti_{5}Si_{3}$ was formed as a sparse band apart from the TiC layer. In the brazing by the $Ag_{0.6}Cu_{0.4}$-5at%Ti alloy, TiC and $Ti_{5}Si_{3}$ coexisted in one layer. TiC and $Ti_{5}Si_{3}$ were produced from the interfacial reactions of titanium independent of the brazing alloys. However, their morphologies and formation mechanisms differ greatly depending on the relative content of silver and copper. The four-point bend strength and fracture modes are also dependent on the relative content of silver and copper. A good bond strength of 174 MPa was obtained by brazing with the Ag-5at%Ti alloy at 985℃ for 600 sec and fracture initiated at the interface between SiC and the reaction product layer and propagated through SiC.
A study to substitute other element for titanium as active element in the copper alloy was conducted. Vanadium, niobium and chromium were considered as active element. Wetting, microstructure and bond strength were investigated. Cu-Cr alloy had the lowest wetting angle of 10℃ to 20℃ on SiC. Four-point bend tests showed that the brazement by the Cu-2at%Nb alloy had good bend strength of 154 MPa compared with 86 MPa in the case of the Cu-5at%Ti alloy. The bend strength had no correlation with wetting angle and microstructure at the bond interface. Niobium has a great potential to replace titanium a active element.
In PLS-SiC/PLS-SiC, HIP-SiC/HIP-SiC and HIP-SiC/mild steel brazing by the Ag-Ti alloy, the effects of brazing conditions on the bend strength were investigated. Changes of the content of titanium in the brazing alloy from 1. 9at% to 9at% and brazing temperature from 985℃ to 1100℃ did not affect on the four-point bend strength independently of the type of SiC and brazing couples. But the brazing gap had great effects on the four-point bend strength. The high bend strength of 250 MPa in PLS-SiC/PLS-SiC brazing and 212 MPa in HIP-SiC/mild steel brazing was obtained with the gap of 0.1mm by Ag-5at%Ti alloy at 985℃ for 600 sec.
The effects of alloying elements in the Ag-5at%Ti alloy on the four-point bend strength were investigated. Aluminum, iron, boron, silicon, cobalt and nickel were selected as alloying elements. The addition of 2at%Al and 2at%Si improved the bend strength greatly from the 174 MPa to 241 MPa and 249 MPa respectively. But the addition of 5at%Al, 2at%Co and 2at%Ni decreased the bend strength. Fracture in the most of the specimens was initiated at the interface between SiC and reaction layer and propagated through SiC.
