Processing, Microstructure, and Mechanical Properties of Zirconium Diboride-Boron Carbide Ceramics

The processing, microstructure, and mechanical properties of zirconium diboride-boron carbide (ZrB ₂ -B ₄ C) ceramics were characterized. Ceramics containing nominally 5, 10, 20, 30, and 40 vol% B ₄ C were hot-pressed to full density at 1900 ⁰C. The ZrB ₂ grain size decreased from 4 to 2 μm and B ₄ C inclusion size increased from 3 to 5 μm for B ₄ C additions of 5 and 40 vol% B ₄ C, respectively. Elastic modulus decreased from 525 to 515 GPa and Vickers hardness increased from 15 to 21 GPa as the B ₄ C 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 B ₄ C content. Fracture toughness increased from 4.1 MPa m ^1/2 at 5 vol% B ₄ C to 5.3 MPa m ^1/2 at 40 vol% B ₄ C additions. Flexure strength was 450 MPa with a 5 vol% B ₄ C 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 B ₄ C added to ZrB ₂ led to increasing hardness due to the higher hardness of B ₄ C compared to ZrB ₂ and increased crack deflection. Additions of B ₄ C also lead to increases in fracture toughness due to increased crack deflection and intergranular fracture.