Influence of an applied dc electric field on the plastic deformation kinetics of oxide ceramics

A modest dc electric field markedly reduced the tensile flow stress at high temperatures in three polycrystalline oxides, i.e. MgO, Al2O3 and yttria-stabilized tetragonal ZrO2 (Y-TZP). The reduction in flow stress Delta sigma(E) in Y-TZP consisted of three components: (i) Delta sigma(T) due to Joule heating, (ii) delta sigma*(E) a rapid, reversible component obtained in on-off and electric field step tests and (iii) Delta sigma(str)(E) the cumulative effect of the field on microstructure. Only Delta sigma(T) and delta sigma*(E) occurred in MgO and Al2O3. It is concluded that delta sigma*(E) results from a reduction in the electrochemical potential for the formation of vacancies corresponding to the diffusion of the rate-controlling ion in the space-charge at the grain boundary. The calculated magnitude of the space-charge zone width and its temperature and solute composition dependence are in accord with theory and experiment; Delta sigma s(E)(str) is attributed mainly to the retardation of grain growth by the field. The retardation could be due to one or more of the following effects of the field on the space-charge zone: (i) an increase in the segregated solute ions, (ii) a decrease in grain boundary energy and (iii) a decrease in solute ion mobility.