Processing, microstructure, and mechanical properties of zirconium diboride-boron carbide ceramics

The processing, microstructure, and mechanical properties of zirconium diboride-boron carbide (ZrB2-B4C) ceramics were characterized. Ceramics containing nominally 5, 10, 20, 30, and 40 vol% B4C were hot-pressed to full density at 1900 °C. The ZrB2 grain size decreased from 4 to 2 µm and B4C inclusion size increased from 3 to 5 µm for B4C additions of 5 and 40 vol% B4C, respectively. Elastic modulus decreased from 525 to 515 GPa and Vickers hardness increased from 15 to 21 GPa as the B4C content increased from 5 to 40 vol%, respectively, following trends predicted using linear rules of mixtures. Flexure strength and fracture toughness both increased with increasing B4C content. Fracture toughness increased from 4.1 MPa m½ at 5 vol% B4C to 5.3 MPa m½ at 40 vol% B4C additions. Flexure strength was 450 MPa with a 5 vol% B4C addition, increasing to 590 MPa for a 40 vol% addition. The critical flaw size was calculated to be ~30 µm for all compositions, and analysis of the fracture surfaces indicated that strength was controlled by edge flaws generated by machining induced sub-surface damage. Increasing amounts of B4C added to ZrB2 led to increasing hardness due to the higher hardness of B4C compared to ZrB2 and increased crack deflection. Additions of B4C also lead to increases in fracture toughness due to increased crack deflection and intergranular fracture.