Fatigue crack growth in ferroelectric ceramics below the coercive field

Abstract

Perovskite-type ferroelectric ceramics have found many applications in actuators, sensors and memories. One critical limitation on their performance is due to electric fatigue, which refers to the deterioration of material properties associated with electric cycling. The degradation of electrical properties in ferroelectrics, which appears in the hysteresis loop in the form of a decrease of remanant polarization and an increase of the coercive field, is a serious concern in application [1]. Due to a strong electro-mechanical coupling effect, an electrical field may also degrade the mechanical properties of ferroelectrics. The performance of ferroelectric ceramics in smart structures is often hampered by the cracks propagating in the devices [2]. It is essential to characterize the failure behavior of ferroelectric ceramics when subjected to a cyclic electric field. Cao and Evans [3] first attacked the issue of electrical field– induced fatigue crack growth. They reported that the growth of indented cracks was governed by the magnitude of an applied electrical field Ea relative to the coercive field Ec: When Ea≤ 0.9Ec, there was only a minor amount of growth (about 50 μm), and then the crack arrested; when Ea≥ 1.1Ec, the crack continued to grow and settled into a steadily growing state. They concluded that the fatigue effect occurred only at fields above the coercive field. For Ea<Ec, however, the intensified electric field near the flaw may exceed the coercive field. The electrical field concentration may cause local degradation. The analysis by Zhu and Yang [4] indicated that, for Ea<Ec, steady-state fatigue crack growth could be induced due to the effect of electrical field–induced domain switching at the crack tip. We report here a detailed experimental study of electrical field–induced crack growth for ferroelectrics under an electrical field below the coercive field. In the experiment, a long-focal-length optical microscope was used to observe the crack propagation process. Optical micrographs were taken to show crack growth associated with each electrical field reversal. The reported experimental phenomenon was interpreted in terms of the domain switching model. The material used for the experiment was PZT-5 provided by the Institute of Acoustics, Chinese Academy of Science. The material has a tetragonal crystal structure at room temperature and an average 3 μm grain size. Specimens were cut to dimensions of 4× 2× 15 mm. Gold electrodes were sputtered onto the opposing 2× 15 mm faces. The side faces were polished with 7, 5, 3.5 and 1μm grain-sized diamond abrasive pastes. The specimens were poled at 130 ◦C with a poling direction along the 4 mm dimension. The specimens were poled for 0.5 h under an electrical field of 2 kV/mm. To get the coercive field, Ec, the polarization hysteresis loop was measured as a function of the electrical field. The electric displacement was monitored using the Sawyer–Tower circuit. The measured ferroelectric hysteresis loop is shown in Fig. 1. The coercive field Ec is 1100 V/mm. The Vickers indentation technique was used to introduce the initial cracks in the electrical fatigue test. A Vickers indenter was placed in the center of the polished 4× 15 mm surface with a load of 29.4N. This created a square pyramid-shaped indentation with cracks emanating from the corners. The poled ferroelectric ceramics exhibited fracture toughness anisotropy [5]. The crack perpendicular to the poling direction was much longer than the crack parallel to it. Denoting c as the measured crack length (from one corner of the indentation to the tip of the crack), the fracture toughness of the ferroelectric ceramics can be expressed as [6]