The effect of composition on the optical properties and hardness of transparent Al-rich MgO·nAl2O3 spinel ceramics

The study investigates the transmittance and hardness of Al-rich spinel ceramics (MgOBLnAl2O3, 1 < n < 2.5) prepd. By reaction air sintering (up to closed porosity) of different ratios of fine and coarse-grained com. Al2O3 and MgO raw powders completed by subsequent hot isostatic pressing (HiP). Different compns give rise to a wide range of presintering temps. With starting compns 1 < n < 1.5, presintering results in a formation of single-phase spinel, in which the excess of Al is solved. With higher Al contents (n > 1.5), however, a biphasic ceramic of stoichiometric MgAl2O4 and residual alumina is formed first. This excess alumina is incorporated into the spinel lattice during the final HiP at a temp. of 1750°C. Single-phase, highly transparent spinel is obtained by increasing the Al-content up to n = 2.5, which gives about 85% in-line transmittance in the visible range of light and about 63% at aUV wavelength of 200 nm. Whereas the optical properties can be improved, the hardness (HV1) slightly decreases with increasing Al content. Depending on the raw powders, the hardness of samples prepd. by finer powders tend to higher values enabled by the development of a bimodal microstructure with a finer grain fraction (<2 mm) between coarser grains (<156 mm). In contrast, samples made of coarser powders need higher sintering temps and exhibit, then, a monomodal microstructure of very large grains (<622 mm) only.