Grain boundary chemistry of alumina by high-resolution imaging SIMS

The unique capabilities of the high-resolution scanning ion microprobe deve loped at the University of Chicago (UC-SIM) are described and its utility i s demonstrated in a study of grain boundary chemistry of alumina ceramics. When polycrystalline alumina is doped singly with tither MgO or SiO2, stron g segregation of the individual ions to grain boundaries is observed: (1) f or 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 b oundary 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 mutua l solid solubility of Mg and Si in alumina. One important consequence of th is chemical redistribution is a change in abnormal grain growth morphology from facetted grains in SiO2 singly doped alumina, to non-facetted grains w ith curved boundaries in MgO and SiO2 codoped alumina. As the Mg/Si dopant ratio exceeds the equimolar concentration, abnormal grain growth developmen t ceases. These findings provide a physical mechanism to explain the role o f MgO as a sintering aid to control microstructure evolution in alumina. An other significant consequence of SiO2 redistribution on MgO doping is an ob served improvement in the corrosion resistance of alumina to aqueous HF. Si liceous grain boundary films readily corrode and compromise the intrinsical ly good corrosion resistance of bulk alumina. MgO doping in amounts greater than the SiO2 concentration prevents the formation of these corrodable sil ica-based phases leading to the development of aluminas for use in aqueous HF-containing ambients. (C) 1999 Published by Elsevier Science Ltd on behal f of Acta Metallurgica Inc. All rights reserved.