Size effect measurement and characterization in nanoindentation test
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Nanoindentation test at scale of hundreds of nanometers has shown that measured hardness increases strongly with decreasing indent depth, which is frequently referred to as the size effect. Usually, the size effect is displayed in the hardness-depth curves. In this study, the size effect is characterized in both the load–displacement curves and the hardness–depth curves. The experimental measurements were performed for single-crystal copper specimen and for surface-nanocrystallized Al-alloy specimen. Moreover, the size effect was characterized using the dislocation density theory. To investigate effects of some environmental factors, such as the effect of surface roughness and the effect of indenter tip curvature, the specimen surface profile and the indentation imprint profile for single-crystal copper specimen were scanned and measured using the atomic force microscopy technique. Furthermore, the size effect was characterized and analyzed when the effect of the specimen surface roughness was considered.
I. INTRODUCTION
Indentation test is an important and effective experimental method and has extensively been used to estimate the plastic properties of solids undergoing plastic deformation. Through indentation test, the material parameters, such as yield strength, strain hardening exponent, and Young’s modulus, are estimated. Recently, with advancement in experimental technique and measuring precision, it is possible to carry out the indentation experiment at scale levels of tens to hundreds of nanometers. Such a small-scale indentation test is referred to as the nanoindentation test (or microindentation test). In the nanoindentation test, an important phenomenon, sizedependent indentation result, has been attained for metal materials;1–14 that is, the measured hardness may double or even triple the conventional hardness as indent size (or depth) decreases to one-fifth of a micrometer. The effect is often referred to as the size effect. In addition, at the micrometer scale when material microstructured size is comparable to the indent depth, the size-dependent hardness results mentioned above should be influenced additionally by the geometrical size.10 Regarding the sensitive zone size of the size effect, for the typical metals, such as Cu, Ag, and Al,2,7,9,11 within one or one-half micrometer of the indent depth, the hardness is sensitive a)
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J. Mater. Res., Vol. 19, No. 1, Jan 2004 Downloaded: 03 Apr 2015
to indent depth. However, for some uniquely high modulus metals, such as W and Ir,8,13 the size effect sensitive zone size is much larger, that is, within about tens of micrometers or even 100 micrometers the hardness is still sensitive to depth. The trends of both size effect and geometrical effect are at odds with the size-independence implied by the conventional elastic–plastic (CEP) theory. To predict the size effect phenomenon in nanoindentation test, several versions of strain gradient plasticity
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