Reaction sintering and mechanical properties of B₄C with addition of ZrO₂

The effect of ZrO₂ addition on sintering behavior and mechanical properties of both hot-pressed and pressureless-sintered B₄C was investigated. The addition of ZrO₂ improved the densification behavior of B₄C remarkably via a reaction with the B₄C to form ZrB₂ at elevated temperatures. When B₄C was densified at 2000 °C by hot pressing, only a small amount (approximately 2.5 vol%) of ZrO₂ was necessary to achieve a full densification. Excellent mechanical properties (hardness, elastic modulus, flexural strength, and fracture toughness) were observed in those specimens. As the amount of ZrO₂ was increased further, the mechanical properties were reduced, except for the fracture toughness, apparently due to the formation of too much ZrB₂ in the specimen. Without the applied pressure, larger amounts of ZrO₂ should be added to obtain a body with high relative density. When the B₄C was sintered at 2175 °C with addition of 10 vol% ZrO₂, the specimen has a density higher than 95% of the theoretical, and hardness and flexural strength of 25 GPa and 400 MPa, respectively.