Grain growth resistant nanocrystalline zirconia by targeting zero grain boundary energies
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Mingming Gong Department of Chemical Engineering and Materials Science & NEAT ORU, University of California - Davis, Davis, California, USA; and State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, People’s Republic of China
Feng Liu State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, People’s Republic of China
Ricardo H.R. Castroa) Department of Chemical Engineering and Materials Science & NEAT ORU, University of California - Davis, Davis, California, USA (Received 1 May 2015; accepted 18 August 2015)
Nanocrystalline ceramics offer interesting and useful physical properties attributed to their inherent large volume fraction of grain boundaries. At the same time, these materials are highly unstable, being subjected to severe coarsening when exposed at moderate to high temperatures, limiting operating temperatures and disabling processing conditions. In this work, we designed highly stable nanocrystalline yttria stabilized zirconia (YSZ) by targeting a decrease of average grain boundary (GB) energy, affecting both driving force for growth and mobility of the boundaries. The design was based on fundamental equations governing thermodynamics of nanocrystals, and enabled the selection of lanthanum as an effective dopant which segregates to grain boundaries and lowers the average energy of YSZ boundaries to half. While this would be already responsible for significant coarsening reduction, we further experimentally demonstrate that the GB energy decreases continuously during grain growth caused by the enrichment of boundaries with dopant, enhancing further the stability of the boundaries. The designed composition showed impressive resistance to grain growth at 1100 °C as compared to the undoped YSZ and opens the perspective for similar design in other ceramics.
I. INTRODUCTION
Mechanical properties of polycrystalline materials are strongly dependent on the average grain size, as evidenced by the empirical Hall–Petch relationship.1 In principle, decreasing grain size is associated with an increased grain boundary (GB) density, consequently impeding dislocation movement and leading to strengthening of the material.2 Wollmershauser et al. recently showed this relation still applies for nanocrystalline materials, showing hardness increase from about 13.5 GPa for a microcrystalline magnesium aluminate to roughly 20.2 GPa for a sample with grain size 28 nm.3 The results contradict reports describing a low grain size limit below which transitions to diffusion controlled strain accommodation
Contributing Editor: Suk-Joong Kang a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.269 J. Mater. Res., Vol. 30, No. 20, Oct 28, 2015
would be expected, similarly to nanocrystalline metals4 and reported for MgO,5 and gives new breath for the development of new super-hard nanoceramics with ultrafine grains. On the other hand, thermally activated grain growth is commonly
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