FricLess

A seamless multi-scale model for contact, friction, and solid lubrication – FricLess

The main aim of this project is to build a multi-scale modeling technique to simulate the behavior of solids in contact. Attention is then placed on analyzing their deformation, frictional behavior and lubrication.

We started by extending[1] and merging two modelling techniques, Green’s Function Molecular Dynamics (GFMD) and Dislocation Dynamics (DD) to build a new one that we called Green’s Function Dislocation Dynamics (GFDD)[2]. The technique is suited for modeling contact between plastically deformable metal single crystals with dimensions at the micro-scale and a very accurate description of surface roughness.

Next, in order to have a basis for comparison of the new results for randomly rough plastic surfaces, simulations were firstly performed on elastic rough surfaces[3].

Simulations of contact between metal rough surfaces were then performed with the new GFDD model. Thanks to the model we found out that the local contact pressure during plastic deformation is much higher than reported in previous studies and that the plastic response is size-dependent. This is a critical point as it entails that classical plasticity theories largely overestimate the onset and amount of plastic deformation at the start of deformation[4][5].

We then performed simulations by loading metals tangentially[6] and found that they exhibit a pronounced roughening, which can affect the variation in contact area and deformation[7]. When contacts are adhesives, which is the case for polymers at any scale, and for metals at the nanoscale, friction and adhesion interact. To try and understand how, we built a macro-scale model, where the interfacial interactions are described by means of a coupled cohesive zone model[8]. Simulations using this model reproduce the typical stick-slip behavior, and show that the contact area decreases during the stick period, vanishes, and reattaches again[9]. Interestingly, the detachment of the contact is not symmetric, but the peeling occurs more on the trailing than on the leading edge[9]. It is critical that relative sliding of solids is driven not only by the adhesive and frictional behavior of the interface but also by the compliance of the bodies in contact[10]. In this regard there is a significant difference between the contact response of metals and that of viscoelastic materials[11][12][13]. To study viscoelastic solids the GFMD model was first extended[11] and then applied to simple contact problems, as the retraction of a cylinder from a viscoelastic semi-infinite body[12]. An open question regarding viscoelastic solids regards whether and how there is interplay between roughness and viscoelastic dissipation, as tuning this interaction can be applied to new devices in the fields of nano- and bio-engineering. Viscoelasticity is found to cause an increased effective work of adhesion due to stiffening of the contact, while roughness is responsible for elastic instabilities. At low retraction rates, the instabilities in the load-area curve typical of rough elastic contacts are suppressed by viscoelasticity: the contact stiffens to promote a stable decrease of the contact area with load[14]. Both roughness and viscoelasticity contribute to stiffening of the adhesive contact, and thus to a departure from short–ranged towards long–ranged adhesion. This is relevant because many theoretical and numerical predictions of adhesive contact behaviour rely heavily on the short-ranged adhesion assumption.

But back to metals: to capture dislocation nucleation as well as friction and wear as emergent phenomena, a dual scale model was built, consisting of an atomistic domain close to the contact, coupled with an elastic continuum DD domain away from the contact[15]. In metals, wear is found to be very sensitive to the interaction range of the adhesive potential. Wear can be reduced significantly by means of solid lubricants[16][17][18]. Few-layers graphene is found particularly effective as the sheets decrease interaction between rough surfaces due to their flexural rigidity, while they easily slide on each other.


This research has been funded by the European Research Council under the European Union’s Horizon 2020 research and innovation programme (ERC Consolidator Grant, grant agreement no. 681813).

References

  1. Syam P. Venugopalan and L. Nicola and Martin H. Müser (2017): Green's function molecular dynamics: Including finite heights, shear, and body fields. In: MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING, vol. 25, no. 3, 2017.
  2. Syam P. Venugopalan and Martin H. Müser and L. Nicola (2017): Green's function molecular dynamics meets discrete dislocation plasticity. In: MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING, vol. 25, no. 6, 2017.
  3. J. S. Dokkum and M. Khajeh Salehani and N. Irani and L. Nicola (2018): On the Proportionality Between Area and Load in Line Contacts. In: TRIBOLOGY LETTERS, vol. 66, no. 3, 2018.
  4. S. P. Venugopalan and L. Nicola (2019): Indentation of a plastically deforming metal crystal with a self-affine rigid surface: A dislocation dynamics study. In: ACTA MATERIALIA, vol. 165, pp. 709–721, 2019.
  5. S. P. Venugopalan and N. Irani and L. Nicola (2019): Plastic contact of self-affine surfaces: Persson's theory versus discrete dislocation plasticity. In: JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, vol. 132, 2019.
  6. N. Irani and L. Nicola (2019): Modelling surface roughening during plastic deformation of metal crystals under contact shear loading. In: MECHANICS OF MATERIALS, vol. 132, pp. 66–76, 2019.
  7. R Civiero and F Perez Rafols and L Nicola (2023): Modeling contact deformation of bare and coated rough metal bodies. In: MECHANICS OF MATERIALS, vol. 179, 2023.
  8. M. Khajeh Salehani and J. S. Dokkum and N. Irani and L. Nicola (2020): On the load-area relation in rough adhesive contacts. In: TRIBOLOGY INTERNATIONAL, vol. 144, 2020.
  9. M. Khajeh Salehani and N. Irani and L. Nicola (2019): Modeling adhesive contacts under mixed-mode loading. In: JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, vol. 130, pp. 320–329, 2019.
  10. F. Perez-Rafols and L. Nicola (2022): Incipient sliding of adhesive contacts. In: FRICTION, vol. 10, no. 6, pp. 963–976, 2022.
  11. J. S. Van Dokkum and L. Nicola (2019): Green's function molecular dynamics including viscoelasticity. In: MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING, vol. 27, no. 7, 2019.
  12. J. S. Van Dokkum and F. Perez-Rafols and L. Dorogin and L. Nicola (2021): On the retraction of an adhesive cylindrical indenter from a viscoelastic substrate. In: TRIBOLOGY INTERNATIONAL, vol. 164, 2021.
  13. A. Gandin and Y. Murugesan and V. Torresan and L. Ulliana and A. Citron and P. Contessotto and G. Battilana and T. Panciera and M. Ventre and A. P. Netti and L. Nicola and S. Piccolo and G. Brusatin (2021): Simple yet effective methods to probe hydrogel stiffness for mechanobiology. In: SCIENTIFIC REPORTS, vol. 11, no. 1, 2021.
  14. F Perez Rafols and JS Van Dokkum and L Nicola (2023): On the interplay between roughness and viscoelasticity in adhesive hysteresis. In: JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, vol. 170, 2023.
  15. Mohammad Aramfard and Francisco PEREZ RAFOLS and L. Nicola (2022): A 2D dual-scale method to address contact problems. In: TRIBOLOGY INTERNATIONAL, vol. 171, 2022.
  16. J. Bian and L. Nicola (2021): On the lubrication of rough copper surfaces with graphene. In: TRIBOLOGY INTERNATIONAL, vol. 156, 2021.
  17. J Bian and L Nicola (2022): Oscillation of a graphene flake on an undulated substrate with amplitude gradient. In: COMPUTATIONAL MATERIALS SCIENCE, vol. 211, 2022.
  18. Jianjun Bian and L. Nicola (2022): Lubrication of rough copper with few-layer graphene. In: TRIBOLOGY INTERNATIONAL, vol. 173, 2022.