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.