Advances in microstructure and mechanical properties of zirconium diboride based ceramics

The use of silicon nitride as a sintering aid (5 vol.%) greatly improves the powder sinterability of zirconium diboride, in comparison to additive free ZrB2. Nearly full dense monolithic material is obtained by hot pressing at 1700 °C. The microstructure consists of fine regular ZrB2 grains and of various secondary grain boundary phases (e.g. BN, t-ZrO2, BN-rich glassy phase), mainly located at triple points. The addition of 20 vol.% of silicon carbide as a reinforcing particulate phase to the ZrB2+5vol.%Si3N4 powder mixture slows down the densification rate of ZrB2, therefore a higher hot pressing temperature (i.e. 1870 °C) is necessary to achieve nearly full density. Further addition of oxide additives (1vol.%Al2O3+0.5vol.%Y2O3) to the ZrB2–20vol.%SiC–5vol.%Si3N4 system enables the production of near fully dense composites at lower hot pressing temperature (1760 °C). The presence of SiC particles in both the ZrB2-based composites effectively improves strength, hardness and toughness, compared to monolithic zirconium diboride. Some mechanical properties are very interesting: flexural strength up to 700 and 600 MPa are measured at room temperature and 1000 °C, respectively. The properties are discussed in terms of the microstructural features.