Pressureless sintering of Al2O3 containing up to 20 vol% zirconium diboride (ZrB2)

The production of particle composites by means of pressureless sintering provides a cost-effective alternative to production variants such as hot-pressing. However, minimal quantities of additives are sufficient to impede the densification of oxidic matrix components. This paper examines the sintering behaviour of alumina powder as a function of the volume fraction of ZrB2 (up to 20 vol%). A distinction can be made between two sintering ranges according to the temperature: at T⩽1700° a solid-state process applies. This process is decisively influenced by the boron oxide (B2O3) contained in the raw material ZrB2. The validity of the Lange model, which describes the influence of increasing volumes of inclusions on the densification behaviour of a crystalline matrix phase, is confirmed in this temperature range. At T>1700°C, an aluminium borate melt occurs, accelerating the sintering process substantially. As a result, the composites quickly attain matrix densities greater than 95% of the theoretical density. At higher firing temperatures the ZrB2 particles coalesce, resulting in the formation of an electrically conductive penetration structure at a content level of 20 vol%.