Grain boundary chemistry of alumina by high-resolution imaging SIMS

The unique capabilities of the high-resolution scanning ion microprobe developed at the University of Chicago (UC-SIM) are described and its utility is demonstrated in a study of grain boundary chemistry of alumina ceramics. When polycrystalline alumina is doped singly with tither MgO or SiO2, strong segregation of the individual ions to grain boundaries is observed: (1) for Mg segregation C-gb/C-grain similar to 400; (2) for Si segregation C-gb/C-grain similar to 300 However, on codoping with both MgO and SiO2, grain boundary segregation is significantly diminished by a factor of five or more over single doping as both cations are redistributed into the bulk alumina lattice. A defect compensation mechanism is proposed to explain this mutual solid solubility of Mg and Si in alumina. One important consequence of this chemical redistribution is a change in abnormal grain growth morphology from facetted grains in SiO2 singly doped alumina, to non-facetted grains with curved boundaries in MgO and SiO2 codoped alumina. As the Mg/Si dopant ratio exceeds the equimolar concentration, abnormal grain growth development ceases. These findings provide a physical mechanism to explain the role of MgO as a sintering aid to control microstructure evolution in alumina. Another significant consequence of SiO2 redistribution on MgO doping is an observed improvement in the corrosion resistance of alumina to aqueous HF. Siliceous grain boundary films readily corrode and compromise the intrinsically good corrosion resistance of bulk alumina. MgO doping in amounts greater than the SiO2 concentration prevents the formation of these corrodable silica-based phases leading to the development of aluminas for use in aqueous HF-containing ambients. (C) 1999 Published by Elsevier Science Ltd on behalf of Acta Metallurgica Inc. All rights reserved.