simulation group

2024

1. Elastic shakedown and roughness evolution in repeated elastic–plastic contact

   Lucas Frérot, and Lars Pastewka

   Tribol. Lett. 72, 23 (2024)

   Abstractdoi PDF

   Surface roughness emerges naturally during mechanical removal of material, fracture, chemical deposition, plastic deformation, indentation, and other processes. Here, we use continuum simulations to show how roughness which is neither Gaussian nor self-affine emerges from repeated elastic–plastic contact of rough and rigid surfaces on a flat elastic–plastic substrate. Roughness profiles change with each contact cycle, but appear to approach a steady-state long before the substrate stops deforming plastically and has hence “shaken-down” elastically. We propose a simple dynamic collapse for the emerging power-spectral density, which shows that the multi-scale nature of the roughness is encoded in the first few indentations. In contrast to macroscopic roughness parameters, roughness at small scales and the skewness of the height distribution of the resulting roughness do not show a steady-state. However, the skewness vanishes asymptotically with contact cycle.

2. Sound waves, diffusive transport, and wall slip in nanoconfined compressible fluids

   Hannes Holey, Peter Gumbsch, and Lars Pastewka

   Phys. Rev. Fluids 9, 014203 (2024)

   Abstractdoi PDF

   Although continuum theories have been proven quite robust to describe confined fluid flow at molecular length scales, molecular dynamics (MD) simulations reveal mechanistic insights into the interfacial dissipation processes. Most MD simulations of confined fluids have used setups in which the lateral box size is not much larger than the gap height, thus breaking thin-film assumptions usually employed in continuum simulations. Here we explicitly probe the long-wavelength hydrodynamic correlations in confined simple fluids with MD and compare to gap-averaged continuum theories as typically applied in, e.g., lubrication. Relaxation times obtained from equilibrium fluctuations interpolate between the theoretical limits from bulk hydrodynamics and continuum formulations with increasing wavelength. We show how to exploit this characteristic transition to measure viscosity and slip length in confined systems simultaneously from equilibrium MD simulations. Moreover, the gap-averaged theory describes a geometry-induced dispersion relation that leads to overdamped sound relaxation at large wavelengths, which is confirmed by our MD simulations. Our results add to the understanding of transport processes under strong confinement and might be of technological relevance for the design of nanofluidic devices.

3. The bumpy road to friction control

   Viacheslav Slesarenko, and Lars Pastewka

   Science 383, 150–151 (2024)

   Abstractdoi PDF

   The frictional properties of material interfaces can be rationally designed.

4. matscipy: materials science at the atomic scale with Python

   Petr Grigorev, Lucas Frérot, Fraser Birks, Adrien Gola, Jacek Golebiowski, Jan Grießer, Johannes L Hörmann, Andreas Klemenz, Gianpietro Moras, Wolfram G. Nöhring, Jonas A. Oldenstaedt, Punit Patel, Thomas Reichenbach, Thomas Rocke, Lakshmi Shenoy, Michael Walter, Simon Wengert, Lei Zhang, and James R. Kermode

   J. Open Source Softw. 9, 5668 (2024)

   Abstractdoi PDF

   Behaviour of materials is governed by physical phenomena that occur at an extreme range of length and time scales. Computational modelling requires multiscale approaches. Simulation techniques operating on the atomic scale serve as a foundation for such approaches, providing necessary parameters for upper-scale models. The physical models employed for atomic simulations can vary from electronic structure calculations to empirical force fields. However, construction, manipulation and analysis of atomic systems are independent of the given physical model but dependent on the specific application. matscipy implements such tools for applications in materials science, including fracture, plasticity, tribology and electrochemistry.

2023

1. Toward a continuum description of lubrication in highly pressurized nanometer-wide constrictions: The importance of accurate slip laws

   Andrea Codrignani, Stefan Peeters, Hannes Holey, Franziska Stief, Daniele Savio, Lars Pastewka, Gianpietro Moras, Kerstin Falk, and Michael Moseler

   Sci. Adv. 9, eadi2649 (2023)

   Abstractdoi PDF

   The Reynolds lubrication equation (RLE) is widely used to design sliding contacts in mechanical machinery. While providing an excellent description of hydrodynamic lubrication, friction in boundary lubrication regions is usually considered by empirical laws, because continuum theories are expected to fail for lubricant film heights h0 ≪ 10 nm, especially in highly loaded tribosystems with normal pressures pn ≫ 0.1 GPa. Here, the performance of RLEs is validated by molecular dynamics simulations of pressurized (with pn = 0.2 to 1 GPa) hexadecane in a gold converging-diverging channel with minimum gap heights h0 = 1.4 to 9.7 nm. For pn ≤0.4 GPa and h0 ≥5 nm, agreement with the RLE requires accurate constitutive laws for pressure-dependent density and viscosity. An additional nonlinear wall slip law relating wall slip velocities to local shear stresses extends the RLE’s validity to even the most severe loading condition pn = 1 GPa and h0 = 1.4 nm. Our results demonstrate an innovative route for continuum modeling of highly loaded tribological contacts under boundary lubrication.