Densification of Magnesia Compacts during Hot-Pressing

Abstract

Hot-pressing experiments were carried out with magnesia in graphite dies at the temperatures ranging from 1200° to 1600°C for 30 minutes under the pressure of 200kg/cm2. Densification characters were measured by a dilatometric method.Rapid densification was observed at the early stage of hot-pressing. The densification behavior in this stage was analyzed with a plastic deformation model, in which plastic deformation taking place at the contact areas between the particles was thought to be the major cause of the densification because of the concentrated stress. The effective stress values at the contact areas obtained from this analysis seemed to be reasonable when they were compared with the yield stress values of single crystal and polycrystalline magnesia.The densification behavior in the later stage was analyzed with several rate equations. The activation energies were estimated according to the equations: 34kcal/mol after Murray's, 56kcal/mol after Rossi's rate equation, and 60kcal/mol for plastic flow and 73kcal/mol for diffusion mechanism after our rate equation. These activation energies were compared with those from diffusion, grain growth and creep experiments on magnesia. Predominant densification mechanism in the later stage was thought to be diffusional.