Microstructure and mechanical properties of reaction‐hot‐pressed zirconium diboride based ceramics

Abstract ZrB ₂ ceramics were prepared by in‐situ reaction hot pressing of ZrH ₂ and B. Additions of carbon and excess boron were used to react with and remove the residual oxygen present in the starting powders. Additions of tungsten were utilized to make a ZrB ₂ ‐4 mol%W ceramic, while a change in the B/C ratio was used to produce a ZrB ₂ ‐10 vol% ZrC ceramic. All three compositions reached near full density. The baseline ZrB ₂ and ZrB ₂ –ZrC composition contained a residual oxide phase and ZrC inclusions, while the W‐doped composition contained residual carbon and a phase that contained tungsten and boron. All three compositions exhibited similar values for flexure strength (~520 MPa), Vickers hardness (~15 GPa), and elastic modulus (~500 to 540 GPa). Fracture toughness was about 2.6 MPa m ^1/2 for the W‐doped ZrB ₂ compared to about 3.8 MPa m ^½ for the ZrB ₂ and ZrB ₂ –ZrC ceramics. This decrease in fracture toughness was accompanied by an observed absence of crack deflection in the W‐doped ZrB ₂ compared with the other compositions. The study demonstrated that reaction‐hot‐pressing can be used to fabricate ZrB ₂ based ceramics containing solid solution additives or second phases with comparable mechanical properties.