R-Curve Effect and Process Zone in Coarse Grained BaTiO3

Abstract

The mechanical properties of coarse grained BaTiO3 doped with 0.5 mol% TiO2 and fine grained BaTiO3 doped with 3 mol% La2O3 were investigated. Double cantilever beam (DCB) specimens were used to measure fracture toughness and R-Curve effect. Coarse grained BaTiO3 had an average grain size of 20 μm compared to 0,4 μm for fine grained material. In the case of coarse grained BaTiO3, increasing crack resistance with crack length was observed for crack elongations up to 2 mm. The starting value of the R-curve was between 1.1 MPa√m and 1.4 MPa√m. The R-curve behavior can be contributed to mechanisms shielding the crack tip from applied loads. Ferroelastic domain switching in a process zone ahead of the crack tip was observed in situ. Domain switching events left a pattern on the polished surface of the DCB specimen. It was possible to visualize the domain switching ahead of the crack tip during crack propagation using Nomarski differential interference contrast. The method allowed measuring the size and shape of the process zone in unpoled and poled material. The maximum width of the process zone was 150 μm in both cases but the distributions of switchings showed remarkable differences.