Microstructural development of platelet alumina while hot-pressing to transparency

Abstract Transparent alumina specimens were produced using non-pre-aligned (NPA) and pre-aligned (PA) platelet alumina powder and hot-pressing at several different pressures. The effects of powder alignment and pressure on the crystallographic orientation, relative density, microstructure, and optical properties were studied. Additionally, samples were quenched at certain times during the hot-press run to investigate the microstructural evolution of the specimens. No significant trends were observed with increasing hot-pressing pressure. However, it is shown that while the pre-alignment procedure results in a higher degree of crystallographic orientation, the aligned platelets of the PA specimens result in a higher starting relative density, which causes faster densification to occur, yielding more grain-growth, porosity, and a lower final relative density. This ultimately results in minimal improvements in the in-line transmission compared to the NPA specimens, illustrating the importance of both high relative densities as well as crystallographic orientation for transparent alumina.