Influence of α-Al2O3 addition on sintering and grain growth behaviour of 8 mol% Y2O3-stabilised cubic zirconia (c-ZrO2)
Introduction
Zirconia ceramics consist of three polymorphs; monoclinic, tetragonal and cubic. These phases can be obtained depending on temperature and compositional ranges under equilibrium conditions [1], [2], [3]. Monoclinic zirconia is present below 1240 °C and is the stable room temperature phase of pure zirconia. Tetragonal zirconia is an intermediate phase, which lies between 1240 and 2370 °C. The retention of the tetragonal phase can be controlled as in the case of cubic zirconia by the addition of dopants. Y2O3 additions yield an extremely fine grained microstructure known as tetragonal zirconia polycrystal which has excellent mechanical properties. Cubic zirconia is the highest temperature phase which is present in the temperature range of 2370 and 2680 °C. However, upon the addition of a few percent of stabilisers; such as CaO, MgO or Y2O3, the cubic phase can be obtained at lower temperatures [1], [2]. The high-temperature cubic phase can also be retained at room temperatures as a non-equilibrium phase by rapid cooling such that diffusive transformation does not occur. The cubic form of stabilised zirconia ceramics are of technological importance due to their high oxygen ionic conductivity at around 1000 °C. Their use as solid state electrolytes has allowed the creation of novel application such as oxygen gas sensors, oxygen membrane separators and solid oxide fuel cells (SOFCs).
High-temperature deformation in fine-grained ceramics has been extensively studied in recent years. Large tensile elongations have been found in many ceramics and ceramic composites such as yttria-stabilised zirconia [4], [5], alumina [6], [7], hydroxyapatite [8], zirconia–alumina [9], [10], mullite–zirconia [11], silicon carbide [12], silicon nitride–silicon carbide [13], [14] and iron–iron carbide [15]. Of the above materials, tetragonal zirconia has been intensively investigated, beginning with the work of Wakai et al. [4], in which a tensile elongation to failure of 100% at 1723 K was obtained. In the ensuing years, tensile ductility in the same material has been improved and Kajihara et al. [16] have reported an elongation to failure of 1038% in 2.5Y-TZP containing 5 wt.% SiO2 at 1673 K and at 1.3×10−4 s−1. In contrast, such elongations have not been obtained in cubic zirconia despite attempts to attain the high-temperature ductility. As stated by Chen and Xue [17] microstructural superplasticity requires an ultra fine grain size that is stable against coarsening during sintering and high-temperature deformation. A low sintering temperature is a necessary, but not a sufficient condition for achieving the required microstructure. In many cases, it seems that the selection of an appropriate crystalline phase is also crucial for obtaining an ultra fine grain size; for instance, tetragonal zirconia is superplastic whereas cubic zirconia is not. Extensive ductilities in tetragonal zirconia are a consequence of grain size stability during sintering and high-temperature deformation. Compared to tetragonal zirconia, cubic zirconia suffers fast grain growth and shows almost no ductility.
The present study was, therefore, undertaken with the aim of investigating the effect of α-Al2O3 addition on sintering and grain growth behaviour of high purity 8 mol% yttria-stabilised cubic zirconia (c-ZrO2).
Section snippets
Experimental: materials and procedures
The materials used in the present work were 8 mol% yttria-stabilised cubic zirconia (c-ZrO2) powder and high purity (>99.999%) α-Al2O3 powder, supplied by Mandoval Ltd., Zirconia Sales (UK) Ltd. The average particle sizes were 0.3 μm for c-ZrO2 and 0.4 μm for α-Al2O3. The chemical composition of the powders provided from the manufactures is listed in Table 1.
A slip-casting method was used for the preparation of specimens for density, grain growth, phase content and lattice parameter measurements.
Experimental results and discussion
X-ray diffraction measurements were carried out on specimens containing Al2O3 dopant in amounts up to 5 wt.%. X-ray diffraction data showed that all specimens contained only cubic fluorite structure. Fig. 1 shows the variation of the average lattice parameter of the cubic lattice with dopant amount. As can be seen from this figure, the average lattice parameter varied linearly with the amount of Al2O3 dopant up to 0.3 wt.% and then levelled off. The lower values of the lattice parameter found for
Conclusions
Grain growth in doped c-ZrO2 occurs slowly and is more sluggish than that in undoped c-ZrO2. This is mainly due to the lower grain boundary mobility and energy which results from solute segregation in the grain boundary and its drag in doped c-ZrO2 but not in undoped c-ZrO2. The drag effect arises from any preferred segregation of an impurity either to or from grain boundary area because of size and charge differences. Al2O3 addition is expected to segregate to grain boundaries. This
Acknowledgements
One of the authors (S. Tekeli) thanks TÜBİTAK (the Science and Technology Research Council of Turkey) for providing financial support under BAYG-C program for this work.
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