Size Effect in the Initiation of Plasticity for Ceramics in Nanoscale Contact Loading
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0976-EE06-12
Size Effect in the Initiation of Plasticity for Ceramics in Nanoscale Contact Loading T.T. Zhu1, X.D. Hou1, C. J. Walker2, K. M.Y. P'ng1, D. J. Dunstan1 , and A. J. Bushby1 1 Department of Materials, Centre for Materials Research, Queen Mary University of London, Mile End Road, London, E1 4NS, United Kingdom
ABSTRACT In nanoindentation, the plasticity size effect has been observed for several years, where a higher hardness is measured as indenter size decreases. In this paper, we report the size effect on the initiation of plasticity in ceramics by using spherical indenters. Here, we show a clear method that is able to determine the details of the onset of plasticity in nanoindentation. This enables us to measure the yield pressure with a high degree of accuracy and over a large range of indenter radii (hundreds of nanometers to several tens of micrometers). Our data shows clearly that there is a significant yield strength enhancement, which is inversely proportional to the cube root of the indenter radius. Also after normalization by the bulk yield strength, the increase in yield strength with decreasing indenter radius is shown to follow a single relation for all the ceramics studied in agreement with recent results for metals [1], and consistent with critical thickness theory for the initiation of yielding over a finite volume.
INTRODUCTION Small-scale mechanical behaviour is at the cutting-edge of research in materials science and applied mechanics. Nanoindentation testing is one of the most popular means for testing the mechanical properties of materials at small scales, because the size of the specimen can be extremely small and the procedure is usually non-destructive. Compared to Berkovich or conical indenters, a spherical indenter induces a larger area of contact at a given depth, providing an entire stress-strain curve, which is useful in studying the smooth transition from an elastic region to a plastic one. Conventional plasticity mechanics has no length scale. Over recent decades, hardness measurements have been recognized to be size dependent, where hardness increase with decreasing indentation depth (or indenter size) [2, 3] However, few papers have reported the initial yield behavior. Quite recently, Spary et al. [1] demonstrated that the initial yield strength of metals increases linearly with inverse cube root of indenter radius, by using spherical nanoindentation together with finite elemental modeling. In this paper, we show more direct and accurate yield size effect in behavior of ceramics. Ceramics have very high hardness, so the elastic portion of the curve is significant and the departure from elastic to plastic behaviour is clear.
EXPERIMENTAL DETAILS Calibration of nanoindenters The successful application of spherical nanoindentation requires accurate calibration of indenter tip geometry. Diamond is one of the most common indenter materials due to its high
hardness and elastic modulus. However, the diamond indenter often deviates from an ideal spherical shape due to diffi
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