Segregation of Yttrium Ions as $${\left\{ {2{{Y'}_{Zr}}:{V_{\ddot O}}} \right\}^x}$$ to the Surfaces of t-ZrO 2
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{
/ .. Segregation of Yttrium Ions as 2YZr : VO
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S.E. Redfern, C. R. Stanek, R.W. Grimes* and R.D. Rawlings Dept. Materials, Imperial College London, Prince Consort Road, London, SW7 2BP,UK
ABSTRACT Atomistic simulation has been used to predict the segregation of defect clusters containing two substitutional Y3+ ions and one charge compensating oxygen vacancy to the (100) and (101) surfaces of t-ZrO2. The most stable orientation of the defect cluster depends on its distance from the surface. Significantly, segregation energies vary greatly between surfaces. For example, the defect cluster is equally stable up to a depth of 9Å from the (100) surface but only to a depth of 4Å below the (101) surface. In both cases, segregation energies are negligible 12Å beneath the surface. INTRODUCTION At temperatures below ~1000°C the crystal structure of zirconia is monoclinic. Above this temperature it transforms to a structure with tetragonal symmetry. However, a suitable combination of small grain size and the addition of an appropriate amount of a stabilising oxide, e.g. 3mol% yttria (Y2O3), results in a metastable tetragonal structure (t-ZrO2) even at room temperature. This metastable material is commonly known as tetragonal zirconia polycrystalline ceramic (TZP) and has excellent mechanical properties. Unfortunately a complication arises at moderate temperatures (typically 60-200°C), when the presence of moisture causes the tetragonal phase to uncontrollably transform into the more stable monoclinic phase, thus degrading the mechanical performance1,2. Microstructural studies have shown that the reaction is nucleated at the surface and that the nuclei grow to form a monoclinic surface layer, which increases in thickness with time3-5. Unfortunately, the nucleation mechanism is not well understood. One suggestion for the origin of monoclinic nucleation is the preferential dissolution of yttria due to the presence of water6; this leaching causes a local reduction in yttria content, which may lead to the transformation. As part of this mechanism, it is proposed that yttrium ions segregate to surfaces of t-ZrO2 grains. Furthermore, this segregation and hence the leaching rate, is orientation dependent. The atomic simulation work reported here was designed to investigate this hypothesis under dry conditions by modelling the surface orientation dependence of the driving force for segregation. There have been a number of previous experimental studies concerning segregation effects in TZP. For example, yttrium ions have been explicitly shown to segregate to grain boundaries7 and to domain boundaries8. In addition, the segregation of Sr and Bi to surfaces of polycrystalline TZP has been studied using angular-resolved X-ray photoelectron spectroscopy9,10. Within the resolution limits of this technique, the results were consistent with there having been an increase in concentration of Bi in the top five atomic layers10 whereas for Sb results were consistent with segregation to the top fifteen lay
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