Scratching the surface of Lateral Size Effects (LSE): A critical comparison between indentation and scratch hardness size effects

Conference Dates

October 1-6, 2017


All tribological interactions have (by definition) a shear component and it is the general experience (going back to the invention of the Mohs hardness scale) that a harder material is one that is more resistant to scratch deformation. Indentation hardness is, however, more reproducible, has a smaller footprint than a scratch test, and so has become the “go to” material parameter for predicting tribological performance. This implied relationship relies on the assumption that a material’s plastic properties depend only on the crystallographic direction of deformation and not on the test direction with respect to a free surface. Indentation size effects (ISE) and the realisation that material strength is genuinely length-scale dependent suggests anti-tribological wear applications. The question remains, however, as to whether further performance enhancement can be realised in practice and this requires study of scratch deformation and LSE itself.

This talk describes the efforts we have made to determine the relationship between indentation and scratch hardness and plasticity size effects. This is not a simple as it sounds and some practical reasons for this will be presented. One such issue is the presence of viscous drag in a scratch test; absent from indentation. Clear evidence of lateral (scratch) size effects, LSE, will be presented for single and poly crystal copper and directly compared to the ISE measurements made in the same samples [1,2]. Smaller scratches are harder. By scratching with a Berkovich geometry indenter it is possible to define two different scratch geometries, Edge Forward (EF) and Face Forward (FF), with the same lateral projected area. The lateral force generated for EF and FF scratches are very different. This can be correlated to the drag coefficients of the two geometries.

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