Mechanical properties and grain orientation evolution of zirconium diboride-zirconium carbide ceramics

The effect of ZrC on the mechanical response of ZrB2 ceramics has been evaluated from room temperature to 2000 degrees C. Zirconium diboride ceramics containing 10 vol% ZrC had higher strengths at all temperatures compared to previous reports for nominally pure ZrB2. The addition of ZrC also increased fracture toughness from similar to 3.5 MPa root m for nominally pure ZrB2 to 4.3 MPa root m due to residual thermal stresses. The toughness was comparable with ZrB2 up to 1600 degrees C, but increased to 4.6 MPa root m at 1800 degrees C and 2000 degrees C. The increased toughness above 1600 degrees C was attributed to plasticity in the ZrC at elevated temperatures. Electron back-scattered diffraction analysis showed strong orientation of the ZrC grains along the [001] direction in the tensile region of specimens tested at 2000 degrees C, a phenomenon that has not been observed previously for fast fracture (crosshead displacement rate = 4.0 mm min(-1)) in four point bending. It is believed that microstructural changes and plasticity at elevated temperature were the mechanisms behind the ultrafast reorientation of ZrC.