Effects of ball-milling on fabrication of YAG ceramics by a phase transformation assisted spark plasma sintering

Abstract

YAG ceramics were fabricated by spark plasma sintering assisted by the YAH-YAG phase transformation. These YAH powders were prepared by field-assisted rapid synthesis of the amorphous precursors using the planetary ball-milling. Moreover, the effects of ball-milling speed on the thermal behavior and morphology of the precursors and calcined YAH powders, as well as the subsequent sintering process were systematically studied. The results of thermal analysis of the YAH powders suggest that the exothermic peak for phase transformation shifts to the lower temperature and becomes sharper by increasing the ball-milling speed. The YAH powders synthesized from 225 rpm-milled precursors exhibit a relatively shallow and weak exothermic peak, indicating a slow phase transformation. Therefore, the samples sintered at 950 °C have the homogeneous microstructures from the center to edge. The in-line transmittances are 32% and 60% at 680 nm and 1000 nm, respectively. In the case of 250 rpm, however, the sharp and strong exothermic peak indicates a rapid phase transformation, resulting in an extremely rapid shrinkage between 890 °C and 900 °C during sintering. Microstructure evidence suggests that differential densification occurs during rapid phase transformation. The exterior of the sample is dense, while large pores trapped in the interior lead the sample to be opaque.