Nanotribological simulations of multi-grit polishing and grinding

doi:10.1016/j.wear.2015.03.006

Wear

Volumes 340–341, 15 October 2015, Pages 25-30

https://doi.org/10.1016/j.wear.2015.03.006 Get rights and content

Highlights

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Molecular dynamics simulation of atomistic polishing and grinding.
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Multiple abrasive particles acting on Gaussian surface topography.
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Identification of individual wear particles and contact zones.
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Generation and interpretation of nanoscopic wear maps.
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Break-down of the wear volume into constituent contributions.

Abstract

A quantitative molecular dynamics (MD) study of nanoscopic wear under dry grinding and polishing conditions with multiple abrasive particles (grits) is presented. The initial topography of the monocrystalline iron surface has a pseudo-random Gaussian height distribution, and the 16 rigid abrasive grits have cuboid or spherical geometries. The grinding and the polishing process are differentiated via the kinematic constraints imposed on the abrasive grits. A post-processing scheme based on drift velocity analysis dynamically identifies atoms as either part of a wear particle, the substrate, or the sheared zone between the two. The knowledge of each atom׳s zone affiliation and a time-resolved, mesh-based evaluation of the substrate topography lead to a break-down of the asperity volume reduction into its constituents: pit fill-up volume, individual wear particles, shear zone, and sub-surface substrate compression. It was found that the initial geometric type of the abrasive grits as well as their kinematics strongly influences the quality of the final surface.

Graphical abstract

Introduction

As the degree of smoothness required in industrial applications enters the nanoscopic regime, the experimental analysis of workpieces becomes increasingly challenging. Molecular dynamics (MD) simulation can help understand the surface finishing processes at hand, which can lead to their improvement. An extensive overview of the simulation of the grinding process at several length scales can be found in [1]. Many ideas with respect to the atomistic simulation of two- and three-body wear were already laid out in the mid-1990s [2], [3], but even recent literature seems to be missing effective concepts for the implementation of surface roughness, multi-grit abrasion, and the subsequent possibility of evaluating atomistic wear in a quantitative fashion.

The authors have recently proposed a modelling and analysis approach based on MD [4] which can be applied to simulations of nano-scale polishing and grinding. While polishing and grinding are generally considered processes featuring mechanisms occurring mainly at the micrometer-scale, where the grain structure of the work piece material plays an important role, this work focuses on phenomena which take place at an even smaller length scale. This atomistic approach ensures a consistent representation of the substrate crystallography and the contact dynamics at the interface between the abrasive particles and the work piece, including the respective thermal and the nanomechanical states. Our entire model system can thus be seen as part of a single grain within a larger work piece, and the mechanisms involved in material removal at this scale do not include the breaking out of entire grains but rather exhibit atom-by-atom desorption and subsequent formation of debris. Our computational scheme laid out in [4] features the correct generation of atomistic surfaces with pseudo-random Gaussian roughness which satisfy periodic boundary conditions, dynamic wear particle and contact zone identification, a clustering algorithm to evaluate individual abrasive contributions to the wear volume and contact area, as well as grid-based topography analysis. While this approach is open to extension in many ways (e.g., multi-grain substrates, larger systems in terms of lateral substrate dimensions, as well as size, surface coverage, explicit bonding, and wear of the abrasive particles), our present work exploits the possibilities employing a basic model on a purely atomistic scale with a homogeneous material. We elucidate nanoscale grinding and polishing by defining, analysing, and discussing tribologically relevant assessment quantities such as surface topographies, contact zones, roughness parameters, and wear volumes for three distinct sets of process parameters in the case of high normal pressures.

Section snippets

Computational details

All MD calculations were performed using the open-source MD code LAMMPS [5]. For a comprehensive account of how the model was set up, how the periodic Gaussian surface topography was produced, and which computational constraints were imposed, the reader is referred to previous work by the authors [4]. Fig. 1(a) shows snapshots of the system geometries as well as a simplified schematic representation, and a top view of the initial surface topography can be seen in Fig. 1(b). The substrate is

Results and discussion

Fig. 6 shows the topographies after polishing with spherical grits, grinding with spherical grits, and grinding with cubic grits, once including the wear particles (a)–(c), and once after removing them and relaxing the surface (d)–(f). The bottom row is therefore a representation of the finished and cleaned surface. While the surface achieved by grinding with spherical grits is smooth (Fig. 6(e)), the cubic grits produce typical V-shaped grinding marks (Fig. 6(f)). The slight angle between

Summary and conclusion

In this paper, we presented and quantified the interaction phenomena on a local nanometric scale between individual abrasive grits and a rough substrate. By applying a recently proposed method, which allows the time-resolved quantitative evaluation of the wear volume and the real contact zone resulting from nano-grinding and nano-polishing using molecular dynamics simulations, we analysed the difference between three surface finishing processes. We found that, at equal sliding velocity and

Acknowledgements

This work was funded by the Austrian COMET-Program (Project K2 XTribology, Grant no. 824187) and carried out at the “Excellence Centre of Tribology”. The authors wish to thank Davide Bianchi for his topography evaluation routine.

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Nanotribological simulations of multi-grit polishing and grinding

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Computational details

Results and discussion

Summary and conclusion

Acknowledgements

CIRP Ann. Manuf. Technol.

Wear

Wear

Comput. Phys. Commun.

J. Comput. Phys.