Pressure Sintering: Mechanisms and Microstructures for Alumina and Magnesia

Pressure sintering (or hot‐pressing) of pure alumina and magnesia in graphite dies and in ceramic dies over the range 1100° to 1700°C and 4000 to 10,000 psi has been shown to be very useful in fabricating these materials with controlled microstructures (e.g., very high relative density and fine grain size or controlled larger grain sizes) without the use of additives to control grain size. Apparent diffusion co‐efficients calculated for the densification process are orders of magnitude greater than for pressureless sintering, which might be explained by enhanced diffusion under stress. Alternatively, however, lower calculated coefficients which are more in agreement with pressureless‐sintering data result when pressure correction terms, modified by porosity, are applied to existing relations. It is concluded that the consistency in calculated diffusion coefficients as well as the reasonable comparison of activation energies suggests that, for the pressures used, densification beyond the initial stages is a diffusion‐controlled process.