Temperature dependent fracture toughness of KNN-based lead-free piezoelectric ceramics

The fracture toughness of unpoled and electrically poled lead-free KNN-based piezoelectric ceramics with the composition of 0.92KNN-0.02Bi(0.5)Li(0.5)TiO(3)-0.06BaZrO(3) was investigated. Results reveal that at room temperature, the intrinsic fracture toughness (K-10) of the unpoled samples, evaluated by the near-tip crack opening displacement (COD) technique, is the lowest with a value of 0.70 MPa.m(0.5); the long (through-thickness) crack fracture toughness (K-Ivnb), obtained by the single edge V-notch beam (SEVNB) technique, is the highest, with a value of 0.95 MPa.m(0.5); intermediate short surface crack fracture toughness (K-I(sc)) of 0.86 MPa.m(0.5) was determined by the surface crack in flexure (SCF) technique. These results were rationalized by the toughening behavior of the material combined with the crack geometry-dependent stress intensity evolution during crack propagation. With increasing temperature, K-Ivnb and K-I(sc) decrease, and become nearly identical at 350 degrees C, suggesting an absence of toughening. For electrically poled samples, their room temperature fracture toughness was characterized by both SCF and SEVNB techniques, with values of 0.88 MPa.m(0.5) and 0.99 MPa.m(0.5) respectively, slightly larger than the values measured for unpoled samples. Nonlinear electric field-strain and stress-strain analysis of the material was also employed during electric field loading, mechanical compression and four-point bending in order to quantify crack tip shielding by domain switching and the actual stress at the point of instable crack propagation. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.