Fracture peculiarities and high-temperature strength of bulk polycrystalline boron

Abstract

We report the mechanical behavior of a bulk boron ceramic prepared by spark plasma sintering of commercially-available β-boron powder. In order to fabricate polycrystalline boron ceramic, we used a protective tantalum foil reacted with carbon from the graphite die or graphite foil forming a thin layer of TaB2 and TaC covering the boron specimen. This is the first study to show the high-temperature flexural strength, toughness, and Young's moduli of boron up to 1400 °C. At 1600 °C and above, boron will react with testing environment forming an outer shell.The flexural strength and fracture toughness at room temperature reached an average of 340 MPa and 4.1 MPa m1/2, respectively. Despite showing clear signs of plastic deformation on the strain-stress curves, the yield strength of the monolithic boron ceramic exceed 1 GPa at 1200 °C. It was determined that fracture at elevated temperatures follows a quasi-transgranular mechanism, where the sub-grains of the boron fracture as plate-like structures. An interpretation for the observed fracture behavior was proposed.