A Molecular Dynamics Study of Polishing and Grinding. S.J. Eder, U. Cihak-Bayr, D. Bianchi, A. Vernes, G. Betz

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1 K2 centre Tribology A Molecular Dynamics Study of Polishing and Grinding S.J. Eder, U. Cihak-Bayr, D. Bianchi, A. Vernes, G. Betz AC²T research GmbH, Wiener Neustadt, Austria 1/many Austrian Ministry for Transport, Innovation and Technology Copyright AC2T research GmbH All rights reserved. Reproduction of any material, whether by photocopying, photographing or storing in any medium by electronic or other means is prohibited without prior written consent of AC2T research GmbH. Federal Ministry of Economy, Family and Youth

2 Modeling Grinding & Polishing with MD Currently, almost all attempts to atomistically model abrasive wear focus on the interaction between a single abrasive particle and the surface study the chipping from a perfectly flat surface using a tool (= abrasive) predominantly neglect polishing kinematics NEW at AC²T: statistical approach with 16 abrasive grits pseudo-random Gaussian initial topography 2/many grinding and polishing kinematics proposed / implemented

3 Model Setup abrasive grits (rigid) 3/many vx 28.5 nm y x Gaussian substrate roughness similar system with spherical grits 28.5 nm Grinding: simulations with cubic and spherical grits, dragged across surface at an angle of +7 with x-direction; relative positions and orientations of abrasive grits are locked Polishing: only with spherical grits pulled in x-direction, which may adjust their z-position individually (as in a slurry), rolling motion and free movement in y-direction allowed (according to topography) Comput. Phys. Commun. 185, 2456 (2014)

4 Computational Details Substrate (28.5 x 28.5 x 4 nm³) Finnis-Sinclair potential for Fe (Mendelev 2003) Langevin thermostat of two monolayers close to bottom (T = 300 K) pseudo-random Gaussian topography with Sq = 0.8 nm (RMS) 16 abrasive grits (spherical, cubic) 4/many placed on staggered 4x4 grid, randomly rotated grinding fix rigid single velocity set vx vx/8 NULL fix aveforce NULL NULL Fz (all grits) polishing fix rigid group velocity set vx NULL NULL fix aveforce NULL NULL Fz/16 (per grit) Substrate grit interactions: lj/cut (ε in ev) Grit grit interactions: lj/cut (ε in ev) boundary p p s 80 nm sliding distance, simulation runs at 2 sliding velocities: vx = 8 m/s, 16 m/s 3 normal loads: σz = Fz/A = 0.1 GPa, 0.5 GPa, 1.0 GPa

5 Initial Gaussian Topography 5/many Color according to z-value of surface atom: red = peaks, blue = valleys mov

6 6/many grinding (cubes) grinding (balls) polishing (balls) Topography after 5 16 m/s σz = 0.1 GPa σz = 0.5 GPa σz = 1.0 GPa

7 Definition of Zones via their Drift Velocities Dynamic identification of atoms as wear particles shear zone substrate 7/many Validation of identification scheme via radial distribution function g(r) υ (max) wear particle abrasive particle: υ = υ (max) υ contact zone ) ax (m.9 0 > υ (m υ 1. υ > 0 shear zone ax ) substrate: υ < 0.1 υ (max)

8 Development of Wear Particles By counting the number of atoms in every one of the defined zones, we obtain the time development of the Wear particles Plastically sheared zone Sub-surface compression Wear volume can be fitted to the macroscopic Barwell wear law which differentiates between running-in and steady-state wear atoms in wear particles plowing cutting 8/many atoms in shear zone surplus atoms in sub-surface zone

9 Cluster Identification Scheme Identification of individual wear particles break-down of wear volume into wear particle contributions 9/many polishing (balls) grinding (balls) grinding (cubes)

10 Wear height maps after 5 16 m/s 10/many 10/many polishing (balls) grinding (balls) grinding (cubes) Time-resolved evaluation of substrate topography (mapped to mesh) σz = 0.1 GPa σz = 0.5 GPa σz = 1.0 GPa

11 Comparison of Wear Volumes over Sliding Distance Integration of the wear height distributions over the lateral area yields volumetric wear components Positive contributions asperity volume reduction Negative contributions pit fill-up volume 11/many 11/many asperity volume reduction pit fill-up volume

12 Sq Roughness Parameter, Levelling Time-resolved evaluation of substrate topography allows keeping track of surface smoothing and planarization 12/many 12/many roughness parameter mean surface height

13 Wear Volume Break-down Mild vs. severe surface finishing conditions Asperity reduction volume (blue) can be broken down into contributions polishing, σz = 0.1 GPa 13/many 13/many grinding (cubes), σz = 1.0 GPa

14 Outlook: Bonded Abrasives Abrasive grits embedded by ~50% in LJ bond Bond characteristics governed by Total bond film thickness, bond-grit overlap Bond cohesion, strength of bond-grit interaction Stiff bond, grits tightly bound Soft bond, grits loosely bound 14/many 14/many plowing, cutting tumbling, bulldozing Bulk: blue, asperities: yellow, abrasives: green, bond: red

15 Outlook: Advanced Substrates Polycrystalline Periodically replicable polycrystal as workpiece Inclusion of roughness Various grain sizes/distributions annealing 15/many 15/many Current / future trends Explicit modelling of cementite (Fe3C) allows more realistic results for steel pearlite, martensite, austenite

16 Summary Atomistic simulation of grinding and polishing Rough Gaussian contact Multiple abrasive particles Quantitative evaluation of wear volume and real contact zone Clustering technique allows break-down of 16/many 16/many contact zone wear particles into single-asperity contributions Mesh-based topography evaluation time-dependent wear maps Break-down of asperity reduction volume into pit fill-up volume wear particles shear zone substrate compression Outlook Inclusion of bonded abrasives, more advanced substrates, liquid media, etc. Reproduction of experimental wear rates Improvement of grinding/polishing tools and parameters

17 Disclaimer All information in this publication and all further technical advice is based on our present knowledge. However, they imply no liability or other legal responsibility on our part, including with regard to existing third party intellectual property rights, especially patent rights including copyrights, trademarks and designs. In particular, we cannot give any warranty, whether express or implied, or guarantee product properties in the legal sense. We reserve the right to make any changes according to technological progress or further developments. The performance of products described herein should be verified by each user with experiments (designed for the respective application) which are to be carried out by qualified experts. Suggestions for uses or applications are only opinions. Reference to trade names used by other companies is neither a recommendation, nor does it imply that similar products cannot be used. 17/many 17/many