Ferroelectricity restoration via thermally driven recovery of surface damage layers in BiFeO₃-based lead-free ceramics

The effect of mechanically induced surface damage layers on the crystal structure and ferroelectric properties was systematically investigated in lead-free 0.99(0.67BiFeO₃−0.33BaTiO₃)−0.01BiMnO₃ ceramics. The surface damage layer formed by mechanical polishing was identified through diffraction peak broadening, increased lattice strain, and reduced remanent polarization. The damage layer was eliminated by thermal annealing at 800°C, which resulted in sharp diffraction peaks, reduced lattice strain, and complete recovery of polarization. Structure refinement confirmed that Bi³⁺ off-centering was suppressed by polishing, whereas Bi³⁺ displacement and nanoscale domain activity were restored by annealing. Phase-field simulations revealed that the damage layer functioned as a ferroelectrically inactive region and that the recovery of Bi³⁺ off-centering was essential for polarization restoration. It was demonstrated that the degradation and recovery of ferroelectric properties in BiFeO₃-based ceramics depended on the formation and relaxation of mechanically induced surface damage layers. These findings provide a clear processing–property relationship and establish a strategy to optimize the performance of lead-free piezoelectric ceramics for electronic and energy applications.