Effects of Re2O3 (Re = La,Nd, Y and Yb) addition in hot-pressed ZrB2–SiC ceramics

ZrB₂–20 vol% SiC composites with 3 vol% Re₂O₃ rare-earth oxide (Re = La, Nd, Y or Yb) were hot pressed to near full-density at 1900 °C. The La₂O₃ and Nd₂O₃ additions not only caused the formation of an amorphous grain boundary phase and enhanced densification, but also resulted in substantial ZrB₂ and SiC grain growth. In contrast, the Y₂O₃ and Yb₂O₃ additions resulted in the formation of crystalline (Y/Yb)₂Zr₂O₇ and enhanced densification without ZrB₂ and SiC grain growth. The hardness was improved by the rare-earth oxide addition, especially with Y₂O₃ and Yb₂O₃. The La₂O₃ and Nd₂O₃ only had a minor effect on the fracture toughness, whereas the Y₂O₃ and Yb₂O₃ additions increased the fracture toughness. The type of Re₂O₃ addition was found to influence the nature of the grain boundary and the concomitant fracture and toughening mechanisms.