Advances in microstructure and mechanical properties of ZrB2 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 8C. 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 8C) 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 8C). 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 8C, respectively. The properties are discussed in terms of the microstructural features.