EFFECT OF MAGNESIA SOLUTE ON SURFACE-DIFFUSION IN SAPPHIRE AND THE ROLE OF MAGNESIA IN THE SINTERING OF ALUMINA

Surface diffusion in sapphire and sintering of Al2O3 have been studied under identical firing conditions (1593°C, N2) as a function of MgO solute additions. The effect of MgO on surface diffusion has been investigated directly using the technique of multiple scratch smoothing on the (1123) surface of sapphire. For these conditions MgO additions enhance the decay of periodic profiles by a factor of 4; that is interpreted as reflecting an increase in the surface diffusivity by the same amount. Measurements of the grain sizedensity trajectory during final stages of sintering of Al2O3 reveal that MgO enhances the densification rate/coarsening rate ratio by a modest factor of 1.8. Since an increase in surface diffusivity alone would decrease the ratio, it is deduced that the primary role of MgO in the sintering of Al2O3 is other than the influence on surface diffusion. These observations in conjunction with independent measurements of the effect of MgO on grain‐boundary mobility confirm that the primary role of MgO is the suppression of grainboundary motion. Copyright © 1990, Wiley Blackwell. All rights reserved