Microstructural evolution and mechanism of grain growth in magnesia ceramics prepared by high pressure and temperature with ultra-high heating rate

The fast densification method of combustion reaction plus quick pressing was adopted to prepare nanocrystalline ceramics. The densification process of magnesia compact with a particle size of 100 nm was investigated, under the applied pressure of up to 170 MPa, and the temperature range of 1740–2080 K with ultra-high heating rate (above 1700 K/min). High-purity magnesia ceramics with a relative density of 98.8% and an average grain size of 120 nm was obtained at 1740 K, and the grain growth during the densification process was effectively restrained. The characteristic morphology of evaporation-condensation was observed in the compact prepared at 2080 K, which revealed the actual process of mass transfer by gas diffusion. Moreover, the investigation on the microstructure evolution and mechanism of grain growth was carried out, on the basis of as-preserved nanocrystalline ceramics. The result indicated that the grain growth of the nanocrystalline MgO was controlled by the mechanism of evaporation-condensation rather than surface diffusion. Furthermore, the pressure had an influence of restraining the grain growth based on solid diffusion and strengthening the effect of gas diffusion with the increasing temperature. Under the particular conditions, there existed an appropriate temperature for the densification of nanocrystalline magnesia, while the excessive temperature would exaggerate grain growth and impede densification.