For solid-phase sintered 99.9% Al2O3 ceramics, the grain morphology is usually equiaxed. The microstructure of high-purity Al2O3 powder (containing 0.25% (wt) MgO) sintered at 1520℃ shows that the material is dense and uniform, the grain boundaries are clean, there is no glass phase, the grains are hexagonal or other polygonal equiaxed, and the average grain size is about 2μm. Hot pressing sintering or hot isostatic pressing sintering can obtain a microstructure with a smaller grain size (<1μm) but an equiaxed grain morphology.
Many engineering alumina ceramics contain 99%~99.7% alumina, so they contain very small amounts of glass phase. They are usually sintered at 1600~1700℃ in air, with a typical grain size of 2~25μm and high strength. Among these ceramics, fine-grained 99% alumina ceramics can be used for hip joint repair, while coarse-grained alumina ceramics can be used as electrical insulation components. However, for the sintering of this type of Al2O3 ceramics, if grain growth inhibitors (such as MgO) are not added or the sintering temperature is too high, abnormal grain growth will occur, that is, some large grains will recrystallize by consuming small grains, resulting in uneven microstructure and reducing the mechanical properties of the material.
Al2O3 ceramics with a purity of 99.9% have very high hardness (19.3GPa), flexural strength (550~600 MPa), and thermal conductivity (38.9W/(m·K)); 99%~99.7% Al2O3 ceramics also have good properties, but 99.7% Al2O3 ceramics without MgO have significantly reduced flexural strength due to recrystallization, only 160~300 MPa (William et al., 1994).
94.5%~99% alumina ceramics have more grain boundary glass phases, and the grain morphology is generally equiaxed, but a few may develop into short columns or plates. The microstructure of 96% alumina ceramics shows that the grains are not completely equiaxed.
The mechanical properties of alumina ceramics containing glass phases depend on the thermal conductivity and crystallization degree of the glass phase, especially the volume change caused by the crystallization of the glass phase at the grain boundary may produce microcracks and reduce the strength.
The glass phase at the grain boundary is available and can be used for metallization to achieve the connection between ceramics or ceramics and metals. Before connection, the surface of alumina ceramics is metallized, usually by mixing Mo, MnO, SiO2 with an organic binder into a paste, coating or screen printing on the surface of Al2O3 ceramics, and then sintering at 1300~1500℃ in hydrogen.