Periodic Reporting for period 6 - FricLess (A seamless multi-scale model for contact, friction, and solid lubrication)
Reporting period: 2023-06-01 to 2023-11-30
Friction and wear are liable for enormous losses in terms of energy and resources in modern society. Moreover, with the advent of miniaturisation, moving components have large surface-to-bulk ratio, and are therefore more susceptible to the effect of frictional forces. It is therefore important, both from a financial and technological viewpoint, to find solutions to reduce friction, and enhance it only when desirable for applications.
At present, our knowledge of the friction and lubrication of rough surfaces is not satisfactory and essentially phenomenological. This is because friction is only deceivingly a simple mechanisms, which instead requires understanding of physical phenomena simultaneously acting at different length scales. The change in contact size, which controls the friction stress, depends on material properties, interfacial properties and on nano-scale phenomena such as atomic displacements causing defect formation, slip or wear.
The objective of this work is to reach improved understanding of metal friction, wear and solid lubrication by means of a multi-scale model. The model developed is general and can be used to study different combination of materials, loading and environmental conditions.
One of the most relevant outcomes of the project lies in the understanding of how rough metal surfaces deform plastically under light contact loading: contrary to what one would expect the occurrence of plasticity does not fully squeeze the asperities in contact, even if the yield point of the metal is reached. At the nano and microscale the asperities are harder than their large counterparts. Note that this finding entails that using a contact model based on linear elasticity would largely overestimate contact pressures and areas, while using a model based on classical plasticity would largely underestimate them.
When considering viscoelastic bodies, instead, which are adhesive in nature, we found that roughness and viscoelasticity contribute to stiffening the 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.
At present, our knowledge of the friction and lubrication of rough surfaces is not satisfactory and essentially phenomenological. This is because friction is only deceivingly a simple mechanisms, which instead requires understanding of physical phenomena simultaneously acting at different length scales. The change in contact size, which controls the friction stress, depends on material properties, interfacial properties and on nano-scale phenomena such as atomic displacements causing defect formation, slip or wear.
The objective of this work is to reach improved understanding of metal friction, wear and solid lubrication by means of a multi-scale model. The model developed is general and can be used to study different combination of materials, loading and environmental conditions.
One of the most relevant outcomes of the project lies in the understanding of how rough metal surfaces deform plastically under light contact loading: contrary to what one would expect the occurrence of plasticity does not fully squeeze the asperities in contact, even if the yield point of the metal is reached. At the nano and microscale the asperities are harder than their large counterparts. Note that this finding entails that using a contact model based on linear elasticity would largely overestimate contact pressures and areas, while using a model based on classical plasticity would largely underestimate them.
When considering viscoelastic bodies, instead, which are adhesive in nature, we found that roughness and viscoelasticity contribute to stiffening the 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.
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).
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).
Compared to the state of the art the progress has been twofold: new modelling techniques have been developed and thanks to those techniques new insight has been obtained related to the contact response of solids.
Regarding insight, for metal contacts, results of the project have shown that contrary to common expectations, the occurrence of plasticity does not fully squeeze the asperities in contact, even if the yield point of the metal is reached. At the nano and microscale the asperities are harder than their large counterparts and they act as indenters on the metal substrate. Deformation occurs mostly a few micrometers underneath the surface, depending on the wavelength that describes the roughness. Note that these findings entail that using a contact model based on linear elasticity would largely overestimate contact pressures and areas, while using one based on classical plasticity would largely underestimate them. When describing the onset of plasticity in contact models it is critical to consider the size-dependence of plastic deformation.
When considering viscoelastic bodies, instead we found that roughness and viscoelasticity contribute to stiffening the 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.
Regarding insight, for metal contacts, results of the project have shown that contrary to common expectations, the occurrence of plasticity does not fully squeeze the asperities in contact, even if the yield point of the metal is reached. At the nano and microscale the asperities are harder than their large counterparts and they act as indenters on the metal substrate. Deformation occurs mostly a few micrometers underneath the surface, depending on the wavelength that describes the roughness. Note that these findings entail that using a contact model based on linear elasticity would largely overestimate contact pressures and areas, while using one based on classical plasticity would largely underestimate them. When describing the onset of plasticity in contact models it is critical to consider the size-dependence of plastic deformation.
When considering viscoelastic bodies, instead we found that roughness and viscoelasticity contribute to stiffening the 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.