Microstructure and Mechanical Properties of Reaction-Hot-Pressed Zirconium Diboride based Ceramics

ZrB2 ceramics were prepared by in-situ reaction hot pressing of ZrH2 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 ZrB2-4 mol%W ceramic, while a change in the B/C ratio was used to produce a ZrB2-10 vol% ZrC ceramic. All three compositions reached near full density. The baseline ZrB2 and ZrB2-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 · m1/2 for the W-doped ZrB2 compared to about 3.8 MPa · m1/2 for the ZrB2 and ZrB2-ZrC ceramics. This decrease in fracture toughness was accompanied by an observed absence of crack deflection in the W-doped ZrB2 compared with the other compositions. The study demonstrated that reaction-hot-pressing can be used to fabricate ZrB2 based ceramics containing solid solution additives or second phases with comparable mechanical properties