Size Effect and Geometrical Effect of Polycrystals and Thin Film/Substrate System in Micro-indentation Test
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Size Effect and Geometrical Effect of Polycrystals and Thin Film/Substrate System in Micro-indentation Test Y. Wei, M. Zhao, X. Wang, S. Tang LNM, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100080, China ABSTRACT Micro-indentation test at scales on the order of sub-micron has shown that the measured hardness increases strongly with decreasing indent depth or indent size, which is frequently referred to as the size effect. Simultaneously, at micron or sub-micron scale, the material microstructure size also has an important influence on the measured hardness. This kind of effect, such as the crystal grain size effect, thin film thickness effect, etc., is called the geometrical effect. In the present research, in order to investigate the size effect and the geometrical effect, the micro-indentation experiments are carried out respectively for single crystal copper and aluminum, for polycrystal aluminum, as well as for a thin film/substrate system, Ti/Si3N4. The size effect and geometrical effect are displayed experimentally. Moreover, using strain gradient plasticity theory, the size effect and the geometrical effect are simulated. Through comparing experimental results with simulation results, the length-scale parameter appearing in the strain gradient theory for different cases is predicted. Furthermore, the size effect and the geometrical effect are interpreted using the geometrically necessary dislocation concept and the discrete dislocation theory. INTRODUCTION Indentation test is an important and effective experimental method, and has been used extensively to estimate the plastic properties of solids undergoing the plastic deformation. Through indentation test, the loading-unloading relation between hardness and indent depth is measured. Thereby, the material parameters, such as the yielding stress, strain hardening exponent, Young's modulus, etc. are estimated. Recently, with the advancement of experimental technique and measurement precision, it is possible to carry out the indentation tests at the scale levels of one micron or sub-micron for obtaining more detailed material information. Such small-scale indentation experiments are frequently referred to as micro-indentation tests (or nano-indentation tests). In the micro-indentation test, a new result, which is different from the conventional one, size-dependent hardness result has been revealed [1-5]. For metal materials, the measured hardness may double or even triple the conventional hardness as indent size (or depth) decreases to a fifth micron. Usually, the effect is referred to as the size effect. In addition, in our recent research, we found that if material was manufactured by some microstructures and if the microstructure size was comparable to the indent depth, the size-dependent hardness results mentioned above were additionally influenced by the geometrical size. We call this the geometrical effect. In the present research, micro-indentation experiments are carried out respectively for single crystal copper and aluminum, polycryst
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