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 degreesC. 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 degreesC) is necessary to achieve nearly full density. Further addition of oxide additives (1vol.%Al2O3+0.5vol.%Y2O3) to the ZrB2-20vol.%SiC(5)vol.%Si3N4 system enables the production of near fully dense composites at lower hot pressing temperature (1760 degreesC). 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 degreesC, respectively. The properties are discussed in terms of the microstructural features. (C) 2002 Elsevier Science B.V. All rights reserved.