Bulk glasses and ultrahard nanoceramics based on alumina and rare-earth oxides

Abstract

Although often regarded as a network-former in conventional silicate glasses, Al₂O₃ alone cannot be obtained as a bulk glass. Until now, glasses comprising continuously linked [AlO_x] polyhedra have been prepared in only a few systems under very fast cooling conditions, which limits their dimensions to a few millimetres^1,2,3. Yet it is desirable to prepare bulk, or monolithic, alumina-rich glasses, with the prospect of superior mechanical, chemical and optical properties⁴. Here we report a novel process for preparing very-high-alumina glasses and nanoscale glass-ceramics. Fully dense bulk articles in net shape are obtained through viscous sintering of glass microbeads. Additional heat treatment of the consolidated glasses leads to fully crystallized transparent glass-converted nanoceramics with a hardness similar to that of alumina. This method avoids the impracticably high applied pressures (more than 1 GPa) that have been required in most cases to prepare nanocrystalline ceramics by sintering, owing to the concurrent nature of densification and grain growth under pressureless conditions^5,6. The reported techniques can be extended to form glasses and nanoceramics in other oxide systems that do not include a conventional glass-forming component.