Meshless microscale simulation of wear mechanisms in scratch testing M. Varga n , S. Leroch, S.J. Eder, M. Rodríguez Ripoll AC2T research GmbH, Viktor-Kaplan-Straße 2C, 2700 Wiener Neustadt, Austria article info Article history: Received 18 September 2016 Received in revised form 2 November 2016 Accepted 6 November 2016 Keywords: Scratch Abrasion Smooth particle hydrodynamics Tribology abstract Scratch testing is an inexpensive technique used for determining the mechanical/wear properties of materials and coatings. In contrast to instrumented indentation, where quantitative relations have been established, only few studies are devoted to quantitatively understanding the material response during scratch testing. Most modelling approaches rely on the finite element method, which is particularly suitable for calculating stresses during plastic deformation, but cannot straightforwardly reproduce gross deformations or material detachment. The present work relies on smooth particle hydrodynamics (SPH) for overcoming this issue. SPH approximates a continuous field using a set of kernel functions centred about so-called particles. These carry the physical properties of the system, e.g. mass, internal energy, or velocity, thus discretising the set of underlying partial equations. By using a discrete method, material detachment is intrinsically taken into account. A parameter study was undertaken on the basis of the elastic-viscoplastic material model of Johnson-Cook. The aim of this work is to investigate the role of material/wear properties and load on the scratch behaviour via a parameter study. The influences of strain hardening, yield strength and Young's modulus on the scratch behaviour are pointed out. The results of the scratch simulations are in good agreement with experimental data for single phase alloys. Relaxation phenomena known from scratch experiments are reproduced for the first time using a nu- merical method. & 2016 Elsevier B.V. All rights reserved. 1. Introduction The assessment of mechanical properties of materials requires specialised testing equipment. Uniaxial tensile tests are widely utilised for this purpose, but have the disadvantage of requiring test samples with a defined geometry, which implies the need of effortful sample preparation and restricts the applicability of the tests mostly to bulk materials. Furthermore, it is a destructive method and cannot be ap- plied to finished components. To overcome these issues, many attempts have been made over the past three decades to extract material prop- erties from instrumented indentation [1]. The principal advantage of indentation is that it is non-destructive and can be straightforwardly applied to coatings and thin films. The recording of force-displacement curves in instrumented indentation tests has allowed the establishment of routine procedures for determination of the Young's modulus, yield strength and material hardness without the need to measure the in- dentation imprint [2]. With the onset of the Finite Element Method (FEM), signi ficant effort has also been put into the determination of strain hardening parameters therefrom [3, 4]. An alternative test for determining the hardness properties of ma- terials relies on sliding an indenter of defined geometry over the surface of interest. This method, known as sclerometry or scratch test, was proposed as early as 1722 [5] and became popularised by Mohs in 1822 by his well-known scale of hardness based on 10 reference materials [5]. Since then, scratch testing has been widely used as a cost-effective method for determining the hardness of materials and components [6] and, more recently, to identify critical loads in thin coatings and poly- mers using load ramps [7, 8]. However, the intrinsic complexity of scratch tests have so far prevented the development of an appropriate analytical framework for extracting material properties and limited their use to rather qualitative observations. In analogy with instrumented indentation, the use of the Finite Element Method (FEM) allowed the performance of extensive compu- tational simulations with the hope of linking scratch data to plastic material properties. Most of the initial studies assumed ideal plastic materials for simplicity [9]. Bellemare et al. published the study in 2007 [10], varying material parameters in a wide range and taking strain hardening into account. They found noteworthy relationships of the dimensionless functions normalised scratch hardness, normalised pile- up height and apparent friction coefficient with the normalised yield strength. All in all, the number of works that take into account strain hardening is nowadays still scant [10–12], and all of the published works focus on scratch tests using an ideal conical indenter. However, Rockwell indenters usually have a spherical tip that in many cases is larger than the typical indentation depths used in practice, particularly for thin coatings [13]. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/wear Wear http://dx.doi.org/10.1016/j.wear.2016.11.023 0043-1648/& 2016 Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: markus.varga@ac2t.at (M. Varga). Wear 376-377 (2017) 1122–1129