R-curve effect, influence of electric field and process zone in BaTiO3 ceramics

Double cantilever beam (DCB) specimens were used to measure fracture toughness and R-curve effect of coarse grained BaTiO3 doped with 0.5 mol% TiO2 and fine grained BaTiO3 doped with 1.5 mol% La2O3 and 3.3 mol% TiO2. The coarse grained BaTiO3 had an average grain size of 20 μm compared to 0.4 μm for the fine grained material. Coarse grained BaTiO3 showed increasing crack resistance with rising crack length for crack elongations up to 2 mm. The R-curve behaviour can be attributed to mechanisms shielding the crack tip from applied loads, such as ferroelastic domain switching in a process zone ahead of the crack tip, which was observed in situ. Since domain switching events left a pattern on the polished surface of the DCB specimen, it was possible to visualise the domain switching during crack propagation using Nomarski differential interference contrast. The method allowed to measure the size and shape of the process zone in unpoled and poled material. The maximum width of the process zone was 120 μm in both cases but the spatial distribution of switching events showed remarkable differences. An electrical DC field applied to the DCB samples while the cracks were propagating promoted crack growth in coarse grained, unpoled samples. In specimens poled perpendicularly to the crack, a rising toughness was found, if negative electric fields were applied. Fine grained samples were not affected by electrical fields of up to 1 kV/mm.