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

Carbon fiber reinforced SiC (C-f/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 C-f/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 C-f/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 C-f/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 C-f/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 C-f/SiC matrix and reacted with SiC, and then Ni2Si compound was formed. The C-f/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 degrees 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 C-f/SiC joints, but it is harmful to the strength at elevated temperature. When the test temperature reached 700 degrees C, the bend strength of the two kinds of joints was further decreased to 17.7 and 9.8 MPa, respectively.