Abstract
Indentation size effects (ISEs) 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 (LSE) 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 on 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 (an LSE) 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 an LSE akin to the ISE in metallic materials. The results have significant implications for using static indentation as a predictor of deformation during wear processes.
Highlights
Hardness testing has evolved from early empirical attempts to rank which materials are ‘stronger’ and can damage other ‘weaker’ materials
1 3 of the contact area and the depth might be expected to increase by a factor of case, the contact area supporting the normal force is reduced to
It might be expected that the face forward (FF) scratch would be deeper than the edge forward (EF)
Summary
Hardness testing has evolved from early empirical attempts to rank which materials are ‘stronger’ and can damage other ‘weaker’ materials. Simplification of the scratch test to a single point force application has brought us the widely useful, and more reproducible indentation hardness test. This expresses hardness as a plastic yield pressure that is analogous to the uniaxial yield stress and proof/flow stress found in mechanical compression testing. Hardness is a simpler test than scratch, being static rather than dynamic, and acting in only one direction It is, not surprising that hardness testing is often used to specify a material and/or to indicate the likely friction and wear of a surface or coating, epitomised by the well-known Archard’s Law [2]. Bellemare and co-authors have related elasto-plastic parameters (Young’s modulus, yield stress, power law hardening exponent), to the steady state frictional sliding response using both computational and experimental methods [11]
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