Densification, microstructure and properties of ultra-high temperature HfB2 ceramics by the spark plasma sintering without any additives

Ultra-high temperature HfB2 ceramic with nearly full densification is achieved by using gradient sintering process of SPS without any additives. The effect of the sintering temperature on the densification behavior, relative density, microstructure, mechanical and thermionic properties is systematically investigated. The results show that the fast densification of HfB2 ceramic occurs at the heating stage, and the highest relative density of 96.75% is obtained at T =1950 degrees C, P = 60 MPa and t =10min. As the temperature is increased from 1800 to 1950 degrees C, the grain size of HfB2 increases from 6.12 +/- 1.33 to 10.99 +/- 2.25 mu m, and refined microstructure gives the excellently mechanical properties. The highest hardness of 26.34 +/- 2.1GPa, fracture toughness of 7.12 +/- 1.33 MPa m1/2 and bending strength of 501 +/- 10MPa belong to the HfB2 ceramic obtained at T =1950 degrees C. Moreover, both the Vickers hardness and fracture toughness obey the normal indentation size effect. HfB2 ceramic also exhibits the thermionic emission characterization with the highest current density of 6.12 A/cm2 and the lowest work function of 2.92 eV.