We started by 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), see [1]. The technique is suited for modeling contact between plastically deformable metal crystals with dimensions at the micro-scale and a very accurate description of surface roughness.
Simulations performed with the model showed that the local contact pressure during plastic deformation is much higher than reported in previous studies and that the plastic response is size-dependent [2]. 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.
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 [3], with which we could show that wear can be reduced significantly by means of solid lubricants [4]. 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.
When contacts are adhesives, which is the case for polymers at any scale, but even for metals at the nanoscale, friction and adhesion interact. To try and understand how, we developed a macro-scale model, where the interfacial interactions are described by means of a coupled cohesive zone model [5]. Simulations using this model reproduce the typical stick-slip behavior, and moreover show that the contact area decreases during the stick period, detaches in a non-symmetric way, and reattaches again.
Clearly the relative sliding of solids does not only depend on surface properties but also on the compliance of the bodies in contact. In this regard there is a significant difference between the contact response of metals and that of viscoelastic materials. To study viscoelastic solids the GFMD model was first extended and then applied to increase our understanding on the interplay between roughness and viscoelasticity [6] as tuning this interaction can be applied to new devices in the fields of nano- and bio-engineering.
Both roughness and viscoelasticity are found to 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.
Results of this work have been disseminated through open access journal publications, conferences and workshops.
[1] S.P. Venugopalan, M.H. Mueser and L. Nicola `Green’s function molecular dynamics meets discrete dislocation plasticity’ Model Simul Mater Sc Eng 25 065018 (2017)
[2] S.P. Venugopalan and L. Nicola `Indentation of a plastically deforming metal crystal with a self-affine rigid surface: A dislocation dynamics' Acta Mater (2018)
[3] M. Aramfard, F. Perez-Rafols and L. Nicola, ‘A 2D dual-scale method to address contact problems’, Tribol Inter 171 Article number: 107509 (2022).
[4] J.J. Bian and L. Nicola, ‘Lubrication of rough copper with few-layer graphene’, Tribol Inter 173 Article number: 107621 (2022).
[5] M.K. Salehani, N. Irani; L. Nicola, ‘Modeling adhesive contacts under mixed-mode loading’, J Mech Phys Solids 130 Pages: 320-329 (2019)
[6] F. Perez-Rafols, J.S. Van Dokkum, L. Nicola, ‘On the interplay between roughness and viscoelasticity in adhesive solids’, J Mech Phys Solids 170 Article number: 105079 (2023).
