Solid state amorphization and electric discharge consolidation of oxide ceramics

Full density powder processing, which consists of reaction ball milling and the instrumented electric discharge consolidation method, is proposed for the development of bulky oxide ceramic materials with nanoscale grain. We find that mechanical alloying of the powder mixture, ZrO2-20 mass%Al2O3 can produce solid state amorphization, when the conditions for planetary reaction ball milling yield the optima. When electric discharge consolidation is combined with high-rate heating of the mechanically alloyed amorphous ceramic powder, we obtain fully dense ZrD2-20 mol%Al2O3 at 1347 K when a pressure of 100 MPa is applied. The densification in the supercooled liquid of ZrO2-20 mol%Al2O3 is fairly well expressed by an Arrhenius type equation of viscous flow: eta=eta(0) exp (H/kT). The process viscosity (eta) is below 10(12) Pa . s at the glass temperature and the activation energy (H) decreases from 337 to 184 kJ . mol(-1) with increasing current applied to the graphite die. For cubic ZrO2-20 mol%Al2O3 with a monoclinic phase, synthesized via crystallization at 1366 K, the average grain size is estimated at 19.6 nm from X-ray line broadening. Vickers' pyramidal hardness is 1051 DPN when the initiation of cracks at the corners of indentation is avoided.