Calcium Segregation to Grain Boundaries in Alumina

Abstract

The fracture surfaces of a series of CaO‐doped polycrystalline aluminas are examined by Auger electron spectroscopy (AES) and scanning electron microscopy (SEM). The AES results show a maximum in the detected calcium over the range of materials, whereas the SEM observations show a monotonic increase in the proportion of transgranular failure over the same range. AES measurements from as‐fractured and sputtered surfaces suggest that Ca is substituting for Al at the grain boundaries. By combining the AES and SEM measurements and compensating for the proportion of transgranular failure exposed on the fracture surfaces, the grain‐boundary concentrations of calcium are determined and are shown to be very different from the apparent concentrations suggested by AES measurements alone. The proportion of transgranular failure increases rapidly with grain‐boundary calcium concentration, although this concentration changes only slowly with changes in CaO‐dopant concentration and heat treatment. The driving force for segregation is concluded to be dominated by the misfit strain of the calcium ions substitutionally incorporated into the alumina lattice. Copyright © 1988, Wiley Blackwell. All rights reserved