Young’s modulus of zirconia at high temperature

Excerpt

Zirconia products can be used in a wide range of engineering applications. The highly refractory nature and good resistance to aggressive environments, such as melted glass, are exploited in the high temperature chemical industry. Structural ceramics take advantage of the well-known possibilities of transformation-toughening. Oxygen probes have been developed because zirconia-based solid solutions exhibit ion conduction. However the polymorphism of zirconia, in particular the transition at about 1170 °C from the low temperature monoclinic phase to the tetragonal phase (stable up to 2370 °C) involves strong volume variations (3–5%) of the unit cell. There is a decrease at the monoclinic–tetragonal transition (M→T) and reciprocally, an increase when cooling down to the reverse transition (T→M below 1000 °C). This expansion involves significant internal stresses in pure polycrystalline zirconia sintered at temperatures higher than T _T→M, which cause drastic cracking on cooling. Therefore the manufacture of large parts of pure monoclinic zirconia is impossible by classical powder densification methods and additives have to be used to stabilize the material in either the tetragonal or cubic phase [1, 2]. However, it was found possible to obtain, using an electro-fused cast process, dense materials (EFZ for electro-fused zirconia) with more than 80 vol.% of pure polycrystalline monoclinic phase embedded into a glassy phase [3] which accommodates the internal stresses. These are caused, either by the thermal expansion mismatches between the phases, or by the volume changes accompanying the transformation of zirconia. The variation of the stress field within this EFZ material can be taken into account by the measurement of the elastic modulus trough a wide temperature range. So, in order to theoretically evaluate this parameter, elastic properties of pure zirconia and those of the glassy phase are required. This letter proposes an evaluation of the elastic properties of pure zirconia in the 20–1400 °C temperature range, starting from literature data for single crystals. This information is then used to interpret experimental variations of Young’s modulus versus temperature observed in EFZ. …