Effect of strain rate on the hardness of relaxor ferroelectric PMN-0.32PT single crystals

The relaxor ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) has been considered to be a potential choice for high performance actuators and sensors owing to their ultrahigh electromechanical properties. Recent experiments have shown that the hardness and indentation fracture toughness of PMN-PT varies with load and loading direction. However, it is not clear from these studies that how the loading rates would affect the indentation response of these materials. Therefore, nanoindentation experiments at different strain rates are performed on [001] and [011] oriented poled PMN-0.32PT single crystals. Results show that the hardness, H decreases with increase in strain rate. This behavior is in contrast to the trend reported for PMN-PT single crystal in the recent past. The indentation size effect (ISE) is noticed in both the orientation for all the strain rates. In order to analyze the observed ISE, various mechanistic models are used such as Meyer's law, Hays-Kendall (H-K) model, proportional specimen resistance (PSR) model, and modified PSR (MPSR) model. The analysis of nanoindentation data using MPSR model satisfactorily describes the ISE in terms of resistance offered by the specimen to indenter penetration, the interface friction, and surface residual stress.