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The interaction between lateral size effect and grain size when scratching polycrystalline copper using a Berkovich indenter.

Kareer, A*; Hou, X D; Jennett, N M*; Hainsworth, S V* (2016) The interaction between lateral size effect and grain size when scratching polycrystalline copper using a Berkovich indenter. Philos. Mag., 96 (32-34). pp. 3414-3429.

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It has been reported previously that, for single and polycrystalline copper (fcc), the indentation size effect (ISE) and the grain size effect (GSE) can be combined in a single length-scale-dependent deformation mechanism linked to a characteristic length-scale calculable by a dislocation-slip-distance approach (Hou et al, Acta. Mater. 2012). Recently, we identified a ¿lateral size effect (LSE)¿ in scratch hardness measurements in single crystal copper, where the scratch hardness increases when the scratch size is reduced (Kareer et al., Philos. Mag. 2016). Grain size strengthening is widely used in industry to enhance component mechanical performance, including surface mechanical and tribological properties. Since the latter are shear-induced responses, it is very important to understand the interaction between the lateral and grain size effects in scratch (shear) contacts.

This paper investigates the effect of grain size on the scratch hardness of polycrystalline copper with average grain sizes between 1.2 µm and 44.4 µm, when using a Berkovich indenter. Exactly the same samples are used as in the indentation investigation by Hou et al. (Acta Mater. 2012). It is shown that, not only does the scratch hardness increase with decreasing grain size, but that the GSE and LSE combine (rather than superpose) in a very similar manner to indentation. Applying the same (as indentation) dislocation-slip-distance-based size effect model to scratch hardness yielded a good fit to the experimental data, strongly indicating that it is the slip-distance-like combined length-scale that determines scratch hardness. A comparison of the fit parameters obtained by indentation and scratch on the same samples is made and some distinct differences are identified. The most striking difference is that scratch hardness is over four times more sensitive to grain size than is indentation hardness. More research is needed to investigate this finding as, if true, it has significant implications for: the design of polycrystalline components subject to shear events such as scratching or other asperity contact and wear, and the use of indentation testing to inform those designs

Item Type: Article
Subjects: Advanced Materials
Advanced Materials > Mechanical Measurement
Identification number/DOI: 10.1080/14786435.2016.1240881
Last Modified: 02 Feb 2018 13:13
URI: http://eprintspublications.npl.co.uk/id/eprint/7344

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