Microstructure and interfacial stability of diffusion bonded HfB2-SiC and ZrB2-SiC based composites

Manufacturing of complex-shaped UHTCs occasionally requires joining of dissimilar materials. Herein diffusion bonded (1800 °C and 40 MPa) two dissimilar composites, ZrB₂-20vol%SiC (ZS) and HfB₂-20vol% SiC (HS), are compared with similar ones i.e., ZS-ZS. The ZS-ZS joints exhibit a seamless interface with uniform hardness (∼20 GPa) and Young’s Modulus (∼366 GPa). For ZS-HS, diffusion and reactivity between Hf and Zr created an interface with (Zr, Hf)B₂ solid solution, thus, decreasing interfacial hardness and Young’s modulus (by ∼17 %, ∼3 %) from HS sides, but were similar to that of ZS side. Under compressive stresses, both joints exhibited interfacial stability and comparable bending and shear strength, i.e., (∼259 MPa, ∼87 MPa for ZS-HS, and ∼296 MPa, ∼113 MPa for ZS-ZS). Though good thermal stability of joints is observed when oxidized at 1400 °C, 60 min, ZS-ZS joints with uniform mechanical properties and good structural and thermal stability are more promising for complex shape applications.