JOINING OF ZrB2-MoSi2 COMPOSITE USING METALLIC INTERLAYER

Ultra-high temperatures ceramics (UHTCs) are the subject of intense worldwide research effort, and their stability in severe environments makes them candidates for aerospace, nuclear and solar energy applications. Widespread usage UHTCs requires the development of effective and reliable joining methods that facilitate the fabrication of large, complex-shaped, and potentially multimaterial components and devices. Brazing of ZrB2 that exhibit outstanding thermo-mechanical and thermochemical properties and good erosion and corrosion resistance, was the focus of the present study. MoSi2 is an effective sintering aid for ZrB2, resulting in dense bulk materials with excellent mechanical properties. ZrB2-10vol%MoSi2 composite was joined at 1500°C with both Ti and Zr interlayers. ZrB2 based composite with MoSi2 as a sintering aid was fabricated by pressure-less sintering method. Ti and Zr foils were employed as brazing interlayers with 50 micron initial thickness. Joining assemblies were hot-pressed at 1500 °C for various holding time under 20 Pa vacuum with a constant 8.5 MPa applied pressure, then the interfacial region of joint was systematically investigated by using EPMA and FESEM-EDS. The joints were mechanically tested with four-point bending both at room and high temperature, and after the strength test, fractographic investigations were performed. Figure 1 shows microstructural and microchemical analyses of the polished images of the reaction layer and related phases identified on the basis cross section of ZrB2-MoSi2 composite joints bonded at 1500 °C for 0 min holding. Joint-region characterization revealed well-bonded interfaces with microstructures strongly dependent on the interlayer. It was found that the Ti interlayer exhibited more intensive interfacial reaction with ZrB2 composite than the Zr interlayer, which is mainly attributed to the formation of Ti-Zr-Si ternary liquid phase at the bonding temperature. The formation of liquid phase at the interface region can promote diffusion and reaction. Additionally, the bending strength at room temperature reached 477.7±88 MPa for the Ti interlayer and 439±70 MPa for the Zr interlayer, respectively. Then at high temperature (1000 °C, under air), the bending strength exhibited 495.3± 45 MPa for the Ti interlayer and 190.7±84 MPa for the Zr interlayer, respectively. These results indicated that the strength of the joints were comparable at room temperature, however at elevated temperature, the joint with Zr interlayer revealed significantly reduced strength, which is due to the residual Zr at the interface and its oxidization during the bending test at 1000 °C under air.