Joining of Cf/SiC composite with AuNi(Cu)-Cr brazing fillers and interfacial reactions

Carbon fiber reinforced SiC (Cf/SiC) ceramic matrix composites (CMC), which exhibit excellent properties at high temperatures, have been considered as a desirable high-temperature structural material in aeronautical turbines and advanced rocket propulsion thrust chambers. In many applications, the joining of Cf/SiC to itself or to metals is necessary to fabricate complex structural components and the joints must retain their properties at high temperatures. In the present paper, six AuNi(Cu)-Cr brazing fillers were designed for joining of Cf/SiC composite. The filler alloys were fabricated into foils with a thickness of 0.1 mm by a rapidly solidifying technique. The alloys’ wettability on the Cf/SiC composite was studied with the sessile drop method. The result showed that the wettability of AuNi(Cu)-Cr brazing alloys was improved with the increase of the Cr content. The microstructures of the Cf/SiC joints were examined with SEM, EDS and EPMA. It was found that element Cr participated in the interfacial reactions, and a thin Cr3C2 reaction layer was formed between the composite and the brazing fillers. The element Ni significantly diffused into the Cf/SiC matrix and reacted with SiC, and then Ni2Si compound was formed. The Cf/SiC joints brazed with Au-Ni20-Cr10 (wt, %) and Au70-Ni10-Cu10-Cr10 (wt, %) exhibit average three-point bend strengths of 154.5 and 171.6 MPa at room temperature, respectively. However, there is a remarkable difference for the two kinds of joints in bend strength at 600 °C, that is, the former is 75.2 MPa while the later decreased to 28.6 MPa. Element Cu in AuNi-Cr system alloys can increase room temperature strength of Cf/SiC joints, but it is harmful to the strength at elevated temperature. When the test temperature reached 700 °C, the bend strength of the two kinds of joints was further decreased to 17.7 and 9.8 MPa, respectively.