Does charge trapping affect subcritical crack growth behavior of yttria-stabilized zirconia?

Abstract

Exposure to ionizing radiation has been known to affect the mechanical properties of solids; however, the exact mechanisms remain debated. In this study, we test the hypothesis that long lived metastable states formed by trapping of charges within defects influence subcritical cracking (SCC). Crack propagation rates were measured in 5 mol% Yttria-stabilized zirconia samples, with and without prior exposure to Co-60 gamma radiation (10 kGy absorbed dose). Crack growth was followed in situ by employing a double cantilever beam specimen inside an environmental scanning electron microscope (ESEM). In comparison with the unirradiated samples, an increased energy release rate of ∼10 J/m² was required to maintain SCC in the irradiated samples conforming to an increase in SCC fracture resistance. Raman and x-ray studies preclude any phase transformation and volume change due to irradiation; however, there was a significant change in optical absorption characteristics observed as the darkening of the irradiated sample. Thermally and optically stimulated luminescence measurements suggest that sample darkening is caused by metastable states that form due to charge trapping during radiation exposure. A closer examination of the SEM images demonstrates an increased number of microcracks ahead of the main crack in the irradiated specimens. We conclude that charge trapping in defects due to irradiation, and subsequent detrapping during crack propagation by mechanical stresses, initiate the formation of these microcracks. Consequently, energy is consumed during the interactions between the main crack and the developing microcracks, ultimately ensuing an overall increase in fracture resistance in the SCC regime.