13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Cohesive Zone Model for Intergranular Slow Crack Growth in Zirconia and Ceramic: Influence of the water concentration and Microstructure Bassem El Zoghbi1,*, Rafael Estevez1, Christian Olagnon2 1 Laboratoire SIMAP, UMR CNRS 5266, Grenoble-INP, UJF, Université de Grenoble, 38402, France 2 Laboratoire MATEIS, UMR CNRS 5510, INSA Lyon, Université de Lyon, 69621, France * Bassem.El-Zoghbi@simap.grenoble-inp.fr Abstract We describe intergranular slow crack growth (SCG) in Zirconia within a cohesive zone methodology. Stress corrosion being thermally activated, a rate and temperature dependent cohesive zone formulation is proposed and shown able to capture SCG. In the present study, the influence of microstructure in terms of anisotropic properties, the grain to grain disorientation, and the initial thermal stress state that originates from the ceramic processing are shown to determine the kinetics of SCG and the level of the load threshold below which no slow crack growth is observed. The influence of the water concentration (R.H.) on the magnitude of the minimum load threshold and on the kinetics of SCG is investigated. Ultimately, this work aims at providing reliable predictions in long lasting applications of ceramics. Keywords Zirconia, Ceramic, Slow crack growth, Cohesive Zone, Humidity 1. Introduction Zirconia has been one of the most important ceramics and one of the prominent mechanical properties used in various domains as thermal barriers, coatings, and in medical applications. Their intrinsic advantages are wear chemical resistance and inertness. When subjected to subcritical mechanical loads in presence of water, Zirconia is prone to a delayed damage mechanism called slow crack growth (SCG). This damage process is environmentally assisted by the diffusion and adsorption of water molecules in the crack tip or along the grain boundaries, which will reduced the energy required for failure. SCG is characterized by the variation of the crack velocity with load level. It is show experimentally [1, 2, 7] that beyond a load threshold K0, SCG takes place at a velocity that increases with load (regime I). Regime I depends strongly on the load level, temperature and water concentration. This process is influenced by the environment. Chevalier et al. [2] showed experimentally the influence of water concentration and temperature on SCG and on the threshold K0 in Zirconia polycrystals (see Fig. 1). Increasing the water concentration (cf. air 25°C vs water 25°C) induced a shift in the V-KI curve with an increase in the crack velocity and a decrease in the magnitude of K0. The same trend is observed with increasing temperature (cf. water 25°C vs water 75°C) with an additional decreasing of the threshold K0. The slope of regime I of the V-KI curve is not affected by the environment but the kinetics of SCG (velocity, K0) are strongly dependent on the magnitude of the mechanical load, water concentration and temperature. In the present study, we aim at predicting the load threshold K0, the regime I of the V-KI curve and at providing insight of the origin of their variation with the water concentration.
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