Beneficial effects of an ultra-fine alpha-SiC incorporation on the sinterability and mechanical properties of ZrB2

A fully dense ZrB2 ceramic containing 10 vol.?% ultra-fine alpha-SiC particulate was successfully hot pressed at 1900 °C for 20 min and 40-50 MPa of applied pressure. Faceted ZrB2 grains (average size about 3 microns) and SiC particles dispersed regularly characterized the base material. No extra secondary phases were found. The introduction of the ultra-fine alpha-SiC particulate was recognized as the key factor that enabled both the control of the diboride grain growth and the achievement of full density. The mechanical properties offered an interesting combination of data: 4.8±0.2 MPa m0.5 fracture toughness, 507±4 GPa Young's modulus, 0.12 Poisson's ratio, and 835±35 MPa flexural strength at room temperature. The flexural strength measured at 1500 °C (in air) provided values of 300±35 MPa. The incorporated ultra-fine alpha-SiC particulate was fundamental, sinterability apart, to enhancing the strength and oxidation resistance of ZrB2. The latter property was tested at 1450 °C for 20 h in flowing dry air. In such oxidizing conditions, the formation of a thin external borosilicate glassy coating supplied partial protection for the faces of the material exposed to the hot environment. The oxidation attack penetrated into the material's bulk and created a 200-micron-thick zirconia scale. The SiC particulate included in the oxide scale, lost by active oxidation, left carbon-based inclusions in the formerly occupied sites.