Effect of mechanical milling on the sintering behaviour of alumina

Temperatures around 1700ºC are normally required to sinter alumina powder into useful ceramic monoliths. Grain size reduction using high energy mechanical milling can be used to promote sintering at lower temperatures. Here, milling was conducted in alumina milling media at a low charge ratio to eliminate contamination which occurs when alumina is milled with milling media such as hardened steel. The milled powders were characterised by their apparent or agglomerate size and the true particle (or crystallite) size. X-ray diffraction peak broadening analysis showed considerable particle size reduction for long milling times. Milled powders were sintered and the progress of sintering monitored using in-situ high temperature XRD and contact dilatometry. Results show that some sintering occurs as low as 400ºC, and substantial sintering occurs at temperatures several hundred degrees below the normal sintering temperature. The low and high temperature regimes are discussed in terms of intra-agglomerate and inter-agglomerate consolidation.