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The existence of a lateral size effect and the relationship between indentation and scratch hardness.

Kareer, A*; Hou, X D; Jennett, N M*; Hainsworth, S V* (2016) The existence of a lateral size effect and the relationship between indentation and scratch hardness. Philos. Mag., 96 (32-34). pp. 3396-3413.

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Indentation size effects are well known in static indentation of materials that deform by dislocation based mechanisms. However, whilst instrumented indentation techniques have become rapidly established as a means of determining the near-surface mechanical properties of materials, scratch testing has been much less widely used. Hardness is used in wear models as a proxy for the yield stress, and the design of materials and hard coatings has often sought to exploit size-derived performance enhancements through length-scale engineering. Yet, it is not known directly whether (or not) length-scale effects also apply to scratch (and thus wear) performance at small scales, or what the functional form of this effect is. This work directly demonstrates that there is a lateral size effect and shows that there are questions to be answered if the use of hardness as an indicator of wear performance is to remain valid.
We report constant load scratch experiments using a Berkovich indenter on single-crystal, annealed copper, using a range of applied normal forces, and compare results from three scratch hardness calculation methods to indentation hardness (ISO 14577:2002) measured on the same sample at the same loads. Scratch tests were performed with the Berkovich indenter aligned either edge-forward or face-forward to the scratch direction. In all cases we demonstrate that there is a very significant (approximate factor of two) effect of scratch size (a lateral size effect) on scratch hardness. The results also show that the deformation mechanisms occurring in scratch tests are different to those occurring beneath a static indentation and that different mechanisms dominated for different stylus orientations (face-forward vs. edge-forward orientation). This is, to our knowledge, the first direct demonstration of a lateral size effect akin to the indentation size effect in metallic materials. The results have significant implications for using static indentation as a predictor of deformation during wear processes

Item Type: Article
Subjects: Advanced Materials
Advanced Materials > Mechanical Measurement
Identification number/DOI: 10.1080/14786435.2016.1146828
Last Modified: 02 Feb 2018 13:13
URI: http://eprintspublications.npl.co.uk/id/eprint/7345

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