Nanoindentation of nanocrystalline ZnO
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R.W. Siegeland Y.X. Liao Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439
W. D. Nix Department of Materials Science and Engineering, Stanford University, Stanford, California 94305 (Received 29 May 1991; accepted 26 November 1991)
A number of nanocrystalline ceramics have been fabricated by the gas phase condensation technique. The mechanical properties of one of the first ceramics produced by this method, nanophase TiO 2 , have been discussed in an earlier study.1 This paper reports a similar study undertaken to examine the properties of nanocrystalline ZnO. Nanoindenter techniques are used to determine hardness, Young's modulus, and strain rate sensitivity in ultra-fine grained ZnO. Significant properties variations are experienced within a given sample, indicating a large degree of microstructural inhomogeneity. Nevertheless, a distinct evolution in properties can be observed as a function of sintering temperature. Young's modulus and hardness values increase almost linearly with increasing sintering temperature, and, in addition, there also appears to be a linear correlation between the development of the two materials properties. In contrast, strain rate sensitivity is shown to have an inverse dependence on sintering temperature. This dependence appears to be linked to the strong influence of grain size on strain rate sensitivity, so that the lower sintering temperatures, which provide the finer grain sizes, tend to promote strain rate sensitivity. The results of this study are strikingly similar to those obtained earlier for nanophase TiO 2, and they indicate that the earlier results could probably be generalized to a much broader range of nanocrystalline ceramics.
I. INTRODUCTION
II. EXPERIMENTAL
Recently it has been shown that nanoindentation can serve as a useful tool for probing the mechanical properties of nanophase (nanocrystalline) materials.1 When only small quantities of nanocrystalline material are available, as is often the case, a micromechanical probe becomes highly desirable for testing purposes. Initial nanoindenter studies1 of the mechanical properties of nanophase TiO2 have yielded some rather intriguing results. Samples of gas phase condensationproduced TiO2 exhibited a high degree of microstructural inhomogeneity, a substantial level of low temperature sintering and diffusional activity, and a notable increase in strain rate sensitivity with decreasing grain size. It is clearly of interest to determine whether such trends are limited to nanophase TiO2 or whether they extend to nanocrystalline materials of different compositions. The current study is designed to determine whether a distinctly different nanocrystalline oxide—more specifically ZnO—follows the same trends in properties evolution as nanophase TiO 2 . These results should indicate whether the earlier results were system specific, or whether a more generalized interpretation can be applied.
Samples of ZnO were prepared by the gas phase condensation technique.2 Briefly, coarse-grained ZnO precu
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