Plastic forming and microstructural development of α-alumina ceramics from highly compacted green bodies using extrusion

High density (>99% TD) and microstructurally controlled α-alumina ceramics were produced from seeded nano-size boehmite (γ-AlOOH) sols with a very fine crystallite size (2–3 nm). A totally wet processing technique comprising vacuum filtering and pressure filtration (PF) was applied in order to increase the solids-loading of the sol and hence form an extrudable paste suitable for plastic forming using extrusion. High packing densities (>68% TD in the green state) are achieved by PF starting from the slurry state resulting in the formation of a consolidated paste which is further consolidated by extrusion. This combined processing technique was successfully applied, in an attempt to reduce the γ-Al2O3 formation temperature, and hence lower the θ- to α-Al2O3 transition temperature. The microstructure of dense α-Al2O3 bodies derived from seeded boehmite sol contains very fine (250 nm) alumina grains after sintering at 1200 °C for 2 h. Although the DTA evidence points to a θ- to α-Al2O3 transition temperature of 1208 °C for a seeded (with 30 nm TiO2) sample, X-ray analysis indicates that a seeded, pressure filtrated and extruded sample is transformed to α-Al2O3 phase after sintering at 1100 °C for 2 h.