Stress dependence of reversible and irreversible domain switching in PZT during cyclic loading

Stresses suffered by lead zirconate titanate (PZT) components in actuators are the origin of the gradual degradation of the microstructure and piezoelectric capability that limits their lifetime. The stress–strain behavior of a PZT ceramic has been studied in compressive uniaxial cyclic loading using a constant loading rate, in order to determine the operating stresses that cause structural damage associated to the exhaustion of twinning. Domain switching strain curves have been calculated considering that stress-induced switching of 90° domains is the mechanism responsible of non-linear stress–strain behavior. Each load–unload cycle caused a permanent strain in the PZT. Successive cycles produced incremental increases in stress-induced permanent strain, up to a maximum value or ‘saturated cyclic permanent strain’, attributed to irreversible stress-induced domain switching. The dependence of the saturated permanent strain with the maximum cyclic load showed a characteristic non-linear behavior, with a steep slope at a stress level σI, that we called the ‘critical stress for irreversible domain switching’. Below σI, the majority of domain switching is reversible. We have called σR to the stress at which the increase in reversible domain switching is more pronounced. At stresses between σR and σI high reversible strains can be reached without resulting in permanent stress-induced depoling and thus without the exhaustion of available twinning during subsequent load cycles. At maximum cyclic stresses higher than σI, irreversible domain switching accounts for the majority of strain and increase rapidly toward values close to the maximum 90° domain switching strain available. The number of cycles to failure also had a strong dependence with the maximum cyclic stress. Cyclic stresses above the critical stress caused a rapid accumulation of permanent strains, so the saturated value is reached after a few cycles, resulting in early catastrophic failure, because of the exhaustion of reversible domain switching.