List of Journal Publications
2022
9.
Aramfard, Mohammad, RAFOLS, Francisco PEREZ, Nicola, L.
A 2D dual-scale method to address contact problems Journal Article
In: TRIBOLOGY INTERNATIONAL, vol. 171, 2022.
Abstract | BibTeX | Tags: contact mechanics, friction, Indentation, Multiscale modeling | Links:
@article{Aramfard2022,
title = {A 2D dual-scale method to address contact problems},
author = {Mohammad Aramfard and Francisco PEREZ RAFOLS and L. Nicola},
doi = {10.1016/j.triboint.2022.107509},
year = {2022},
date = {2022-01-01},
journal = {TRIBOLOGY INTERNATIONAL},
volume = {171},
publisher = {ELSEVIER SCI LTD},
abstract = {A seamless 2D dual-scale computational scheme is developed to study contact problems. The model consists of an atomistic domain close to the contact, coupled with an elastic continuum domain away from the contact. The atomistic formulation provides a description of the contact interaction through interatomic potentials and permits to capture atomic wear and defect formation in the contact region. The fields in the continuum domain are calculated by an efficient FFT-based Green's function method. The novel scheme is validated against full atomistic simulations and applied to study the effect of adhesion on the scratching of a rough copper surface by a rigid smooth spherical tip.},
keywords = {contact mechanics, friction, Indentation, Multiscale modeling},
pubstate = {published},
tppubtype = {article}
}
A seamless 2D dual-scale computational scheme is developed to study contact problems. The model consists of an atomistic domain close to the contact, coupled with an elastic continuum domain away from the contact. The atomistic formulation provides a description of the contact interaction through interatomic potentials and permits to capture atomic wear and defect formation in the contact region. The fields in the continuum domain are calculated by an efficient FFT-based Green's function method. The novel scheme is validated against full atomistic simulations and applied to study the effect of adhesion on the scratching of a rough copper surface by a rigid smooth spherical tip.
8.
Bian, Jianjun, Nicola, L.
Lubrication of rough copper with few-layer graphene Journal Article
In: TRIBOLOGY INTERNATIONAL, vol. 173, 2022.
Abstract | BibTeX | Tags: friction, Graphene, Interlocking, Rough surface | Links:
@article{Bian2022a,
title = {Lubrication of rough copper with few-layer graphene},
author = {Jianjun Bian and L. Nicola},
doi = {10.1016/j.triboint.2022.107621},
year = {2022},
date = {2022-01-01},
journal = {TRIBOLOGY INTERNATIONAL},
volume = {173},
publisher = {ELSEVIER SCI LTD},
abstract = {It has been demonstrated through experiments and simulations that friction decreases significantly when graphene is used as a solid lubricant on various materials. However, the effect of increasing the number of graphene layers on lubrication is controversial. Some studies predict an increase of friction with the number of layers that can be imputed to increased contact area, others a decrease in friction attributed to increased flexural rigidity of the layers. Herein, atomistic simulations are performed to investigate the atomic mechanisms by which few-layers graphene lubricate rough copper surfaces when probed by a smooth tip. The results of the simulations show that increasing the number of graphene layers drastically reduces friction, while the deformation mechanism is found to change from atomic wear to recoverable flattening of surface steps, as the amount of interlocking between the surfaces is reduced.},
keywords = {friction, Graphene, Interlocking, Rough surface},
pubstate = {published},
tppubtype = {article}
}
It has been demonstrated through experiments and simulations that friction decreases significantly when graphene is used as a solid lubricant on various materials. However, the effect of increasing the number of graphene layers on lubrication is controversial. Some studies predict an increase of friction with the number of layers that can be imputed to increased contact area, others a decrease in friction attributed to increased flexural rigidity of the layers. Herein, atomistic simulations are performed to investigate the atomic mechanisms by which few-layers graphene lubricate rough copper surfaces when probed by a smooth tip. The results of the simulations show that increasing the number of graphene layers drastically reduces friction, while the deformation mechanism is found to change from atomic wear to recoverable flattening of surface steps, as the amount of interlocking between the surfaces is reduced.
7.
Müser, Martin H, Nicola, L.
Modeling the surface topography dependence of friction, adhesion, and contact compliance Journal Article
In: MRS BULLETIN, vol. 47, no. 12, pp. 1221–1228, 2022.
Abstract | BibTeX | Tags: Adhesion, Contact, friction | Links:
@article{Mueser2022,
title = {Modeling the surface topography dependence of friction, adhesion, and contact compliance},
author = {Martin H Müser and L. Nicola},
doi = {10.1557/s43577-022-00468-2},
year = {2022},
date = {2022-01-01},
journal = {MRS BULLETIN},
volume = {47},
number = {12},
pages = {1221–1228},
publisher = {SPRINGER HEIDELBERG},
abstract = {The small-scale topography of surfaces critically affects the contact area of solids and thus the forces acting between them. Although this has long been known, only recent advances made it possible to reliably model interfacial forces and related quantities for surfaces with multiscale roughness. This article sketches both recent and traditional approaches to their mechanics, while addressing the relevance of nonlinearity and nonlocality arising in soft- and hard-matter contacts.},
keywords = {Adhesion, Contact, friction},
pubstate = {published},
tppubtype = {article}
}
The small-scale topography of surfaces critically affects the contact area of solids and thus the forces acting between them. Although this has long been known, only recent advances made it possible to reliably model interfacial forces and related quantities for surfaces with multiscale roughness. This article sketches both recent and traditional approaches to their mechanics, while addressing the relevance of nonlinearity and nonlocality arising in soft- and hard-matter contacts.
2021
6.
Bian, J., Nicola, L.
On the lubrication of rough copper surfaces with graphene Journal Article
In: TRIBOLOGY INTERNATIONAL, vol. 156, 2021.
Abstract | BibTeX | Tags: friction, Graphene, Lubrication, Rough surface | Links:
@article{Bian2021,
title = {On the lubrication of rough copper surfaces with graphene},
author = {J. Bian and L. Nicola},
doi = {10.1016/j.triboint.2020.106837},
year = {2021},
date = {2021-01-01},
journal = {TRIBOLOGY INTERNATIONAL},
volume = {156},
publisher = {Elsevier Ltd},
abstract = {Graphene is well known as a solid lubricant for nanoscale devices and is generally used to decrease friction between flat surfaces. In this work, we investigate its performance as a lubricant for rough surfaces. To this end, the problem of a silicon tip sliding on a rough copper single crystal, bare or covered by a graphene layer, is addressed through molecular dynamics simulations. To simplify the analysis, the copper crystal is taken to be quasi-three dimensional, so that the roughness profile is constant along the short periodic dimension. Results show markedly different deformation mechanisms in copper, depending on whether the rough surface is bare, covered with a stretched graphene layer, or with a wrinkled graphene layer. The wrinkled layer appears to be the best solution to reduce friction.},
keywords = {friction, Graphene, Lubrication, Rough surface},
pubstate = {published},
tppubtype = {article}
}
Graphene is well known as a solid lubricant for nanoscale devices and is generally used to decrease friction between flat surfaces. In this work, we investigate its performance as a lubricant for rough surfaces. To this end, the problem of a silicon tip sliding on a rough copper single crystal, bare or covered by a graphene layer, is addressed through molecular dynamics simulations. To simplify the analysis, the copper crystal is taken to be quasi-three dimensional, so that the roughness profile is constant along the short periodic dimension. Results show markedly different deformation mechanisms in copper, depending on whether the rough surface is bare, covered with a stretched graphene layer, or with a wrinkled graphene layer. The wrinkled layer appears to be the best solution to reduce friction.
2018
5.
Vakis, A. I., Yastrebov, V. A., Scheibert, J., Nicola, L., Dini, D., Minfray, C., Almqvist, A., Paggi, M., Lee, S., Limbert, G., Molinari, J. F., Anciaux, G., Aghababaei, R., Restrepo, S. Echeverri, Papangelo, A., Cammarata, A., Nicolini, P., Putignano, C., Carbone, G., Stupkiewicz, S., Lengiewicz, J., Costagliola, G., Bosia, F., Guarino, R., Pugno, N. M., Müser, M. H., Ciavarella, M.
Modeling and simulation in tribology across scales: An overview Journal Article
In: TRIBOLOGY INTERNATIONAL, vol. 125, pp. 169–199, 2018.
Abstract | BibTeX | Tags: Adhesion, Contact, friction, Lubrication, Multiphysics modeling, Multiscale modeling, roughness, Tribochemistry, tribology, Wear | Links:
@article{Vakis2018,
title = {Modeling and simulation in tribology across scales: An overview},
author = {A. I. Vakis and V. A. Yastrebov and J. Scheibert and L. Nicola and D. Dini and C. Minfray and A. Almqvist and M. Paggi and S. Lee and G. Limbert and J. F. Molinari and G. Anciaux and R. Aghababaei and S. Echeverri Restrepo and A. Papangelo and A. Cammarata and P. Nicolini and C. Putignano and G. Carbone and S. Stupkiewicz and J. Lengiewicz and G. Costagliola and F. Bosia and R. Guarino and N. M. Pugno and M. H. Müser and M. Ciavarella},
doi = {10.1016/j.triboint.2018.02.005},
year = {2018},
date = {2018-01-01},
journal = {TRIBOLOGY INTERNATIONAL},
volume = {125},
pages = {169–199},
publisher = {Elsevier Ltd},
abstract = {This review summarizes recent advances in the area of tribology based on the outcome of a Lorentz Center workshop surveying various physical, chemical and mechanical phenomena across scales. Among the main themes discussed were those of rough surface representations, the breakdown of continuum theories at the nano- and microscales, as well as multiscale and multiphysics aspects for analytical and computational models relevant to applications spanning a variety of sectors, from automotive to biotribology and nanotechnology. Significant effort is still required to account for complementary nonlinear effects of plasticity, adhesion, friction, wear, lubrication and surface chemistry in tribological models. For each topic, we propose some research directions.},
keywords = {Adhesion, Contact, friction, Lubrication, Multiphysics modeling, Multiscale modeling, roughness, Tribochemistry, tribology, Wear},
pubstate = {published},
tppubtype = {article}
}
This review summarizes recent advances in the area of tribology based on the outcome of a Lorentz Center workshop surveying various physical, chemical and mechanical phenomena across scales. Among the main themes discussed were those of rough surface representations, the breakdown of continuum theories at the nano- and microscales, as well as multiscale and multiphysics aspects for analytical and computational models relevant to applications spanning a variety of sectors, from automotive to biotribology and nanotechnology. Significant effort is still required to account for complementary nonlinear effects of plasticity, adhesion, friction, wear, lubrication and surface chemistry in tribological models. For each topic, we propose some research directions.
2017
4.
Siang, Kelvin Ng Wei, Nicola, L.
Static friction of sinusoidal surfaces: a discrete dislocation plasticity analysis Journal Article
In: PHILOSOPHICAL MAGAZINE, vol. 97, no. 29, pp. 2597–2614, 2017.
Abstract | BibTeX | Tags: Contact, discrete dislocation plasticity, friction, Size effect | Links:
@article{NgWeiSiang2017,
title = {Static friction of sinusoidal surfaces: a discrete dislocation plasticity analysis},
author = {Kelvin Ng Wei Siang and L. Nicola},
doi = {10.1080/14786435.2017.1344785},
year = {2017},
date = {2017-01-01},
journal = {PHILOSOPHICAL MAGAZINE},
volume = {97},
number = {29},
pages = {2597–2614},
publisher = {Taylor and Francis Ltd.},
abstract = {Discrete dislocation plasticity simulations are carried out to investigate the static frictional response of sinusoidal asperities with (sub)-microscale wavelength. The surfaces are first flattened and then sheared by a perfectly adhesive platen. Both bodies are explicitly modelled, and the external loading is applied on the top surface of the platen. Plastic deformation by dislocation glide is the only dissipation mechanism active. The tangential force obtained at the contact when displacing the platen horizontally first increases with applied displacement, then reaches a constant value. This constant is here taken to be the friction force. In agreement with several experiments and continuum simulation studies, the friction coefficient is found to decrease with the applied normal load. However, at odds with continuum simulations, the friction force is also found to decrease with the normal load. The decrease is caused by an increased availability of dislocations to initiate and sustain plastic flow during shearing. Again in contrast to continuum studies, the friction coefficient is found to vary stochastically across the contact surface, and to reach locally values up to several times the average friction coefficient. Moreover, the friction force and the friction coefficient are found to be size-dependent.},
keywords = {Contact, discrete dislocation plasticity, friction, Size effect},
pubstate = {published},
tppubtype = {article}
}
Discrete dislocation plasticity simulations are carried out to investigate the static frictional response of sinusoidal asperities with (sub)-microscale wavelength. The surfaces are first flattened and then sheared by a perfectly adhesive platen. Both bodies are explicitly modelled, and the external loading is applied on the top surface of the platen. Plastic deformation by dislocation glide is the only dissipation mechanism active. The tangential force obtained at the contact when displacing the platen horizontally first increases with applied displacement, then reaches a constant value. This constant is here taken to be the friction force. In agreement with several experiments and continuum simulation studies, the friction coefficient is found to decrease with the applied normal load. However, at odds with continuum simulations, the friction force is also found to decrease with the normal load. The decrease is caused by an increased availability of dislocations to initiate and sustain plastic flow during shearing. Again in contrast to continuum studies, the friction coefficient is found to vary stochastically across the contact surface, and to reach locally values up to several times the average friction coefficient. Moreover, the friction force and the friction coefficient are found to be size-dependent.
2016
3.
Sun, Fengwei, Giessen, Erik Van Der, Nicola, L.
Dry frictional contact of metal asperities: A dislocation dynamics analysis Journal Article
In: ACTA MATERIALIA, vol. 109, pp. 162–169, 2016.
Abstract | BibTeX | Tags: dislocation dynamics, friction, Rough surface, Size effect | Links:
@article{Sun2016,
title = {Dry frictional contact of metal asperities: A dislocation dynamics analysis},
author = {Fengwei Sun and Erik Van Der Giessen and L. Nicola},
doi = {10.1016/j.actamat.2016.02.033},
year = {2016},
date = {2016-01-01},
journal = {ACTA MATERIALIA},
volume = {109},
pages = {162–169},
publisher = {Elsevier Ltd},
abstract = {Discrete dislocation plasticity simulations are performed to investigate the static frictional behavior of a metal asperity on a large single crystal, in contact with a rigid platen. The focus of this study is on understanding the relative importance of contact slip opposed to plasticity in a single asperity at the micrometer size scale, where plasticity is size dependent. Slip of a contact point is taken to occur when the shear traction exceeds the normal traction at that point times a microscopic friction coefficient. Plasticity initiates through the nucleation of dislocations from Frank-Read sources in the metal and is modeled as the collective motion of edge dislocations. Results show that plasticity can delay or even suppress full slip of the contact. This generally happens when the friction coefficient is large. However, if the flattening depth is sufficiently large to induce nucleation of a large dislocation density, slip is suppressed even when the friction coefficient is very small. This study also shows that when self-similar asperities of different size are flattened to the same depth and subsequently loaded tangentially, their frictional behavior appears size independent. However, when they are submitted to the same contact pressure, smaller asperities slip while larger asperities deform plastically.},
keywords = {dislocation dynamics, friction, Rough surface, Size effect},
pubstate = {published},
tppubtype = {article}
}
Discrete dislocation plasticity simulations are performed to investigate the static frictional behavior of a metal asperity on a large single crystal, in contact with a rigid platen. The focus of this study is on understanding the relative importance of contact slip opposed to plasticity in a single asperity at the micrometer size scale, where plasticity is size dependent. Slip of a contact point is taken to occur when the shear traction exceeds the normal traction at that point times a microscopic friction coefficient. Plasticity initiates through the nucleation of dislocations from Frank-Read sources in the metal and is modeled as the collective motion of edge dislocations. Results show that plasticity can delay or even suppress full slip of the contact. This generally happens when the friction coefficient is large. However, if the flattening depth is sufficiently large to induce nucleation of a large dislocation density, slip is suppressed even when the friction coefficient is very small. This study also shows that when self-similar asperities of different size are flattened to the same depth and subsequently loaded tangentially, their frictional behavior appears size independent. However, when they are submitted to the same contact pressure, smaller asperities slip while larger asperities deform plastically.
2015
2.
Dikken, Robbert Jan, Giessen, Erik Van Der, Nicola, L.
Plastic shear response of a single asperity: A discrete dislocation plasticity analysis Journal Article
In: PHILOSOPHICAL MAGAZINE, vol. 95, no. 34, pp. 3845–3858, 2015.
Abstract | BibTeX | Tags: Contact, discrete dislocation plasticity, friction, single asperity | Links:
@article{Dikken2015,
title = {Plastic shear response of a single asperity: A discrete dislocation plasticity analysis},
author = {Robbert Jan Dikken and Erik Van Der Giessen and L. Nicola},
doi = {10.1080/14786435.2015.1102982},
year = {2015},
date = {2015-01-01},
journal = {PHILOSOPHICAL MAGAZINE},
volume = {95},
number = {34},
pages = {3845–3858},
publisher = {Taylor and Francis Ltd.},
abstract = {We investigate the plastic shear response during static friction of an asperity protruding from a large FCC single crystal. The asperity is in perfectly adhesive contact with a rigid platen and is sheared by tangentially moving the platen. Using discrete dislocation plasticity simulations, we elucidate the plastic shear behaviour of single asperities of various size and shape, in search for the length scale that controls the plastic behaviour. Since plasticity can occur also in the crystal, identification of the length scale that controls a possible size-dependent plastic behaviour is far from being trivial. It is found that scaling down the dimensions of an asperity results in a higher contact shear strength. The contact area is dominant in controlling the plastic shear response, because it determines the size of the zone, in and below the asperity, where dislocation nucleation can occur. For a specific contact area, there is still a dependence on asperity volume and shape, but this is weaker than the dependence on contact area alone.},
keywords = {Contact, discrete dislocation plasticity, friction, single asperity},
pubstate = {published},
tppubtype = {article}
}
We investigate the plastic shear response during static friction of an asperity protruding from a large FCC single crystal. The asperity is in perfectly adhesive contact with a rigid platen and is sheared by tangentially moving the platen. Using discrete dislocation plasticity simulations, we elucidate the plastic shear behaviour of single asperities of various size and shape, in search for the length scale that controls the plastic behaviour. Since plasticity can occur also in the crystal, identification of the length scale that controls a possible size-dependent plastic behaviour is far from being trivial. It is found that scaling down the dimensions of an asperity results in a higher contact shear strength. The contact area is dominant in controlling the plastic shear response, because it determines the size of the zone, in and below the asperity, where dislocation nucleation can occur. For a specific contact area, there is still a dependence on asperity volume and shape, but this is weaker than the dependence on contact area alone.
1.
Song, H., Dikken, R. J., Nicola, L., Giessen, E. Van Der
Plastic ploughing of a sinusoidal asperity on a rough surface Journal Article
In: JOURNAL OF APPLIED MECHANICS, vol. 82, no. 7, 2015.
Abstract | BibTeX | Tags: contact mechanics, dislocation dynamics, friction | Links:
@article{Song2015,
title = {Plastic ploughing of a sinusoidal asperity on a rough surface},
author = {H. Song and R. J. Dikken and L. Nicola and E. Van Der Giessen},
doi = {10.1115/1.4030318},
year = {2015},
date = {2015-01-01},
journal = {JOURNAL OF APPLIED MECHANICS},
volume = {82},
number = {7},
publisher = {American Society of Mechanical Engineers (ASME)},
abstract = {Part of the friction between two rough surfaces is due to the interlocking between asperities on opposite surfaces. In order for the surfaces to slide relative to each other, these interlocking asperities have to deform plastically. Here, we study the unit process of plastic ploughing of a single micrometer-scale asperity by means of two-dimensional dislocation dynamics simulations. Plastic deformation is described through the generation, motion, and annihilation of edge dislocations inside the asperity as well as in the subsurface. We find that the force required to plough an asperity at different ploughing depths follows a Gaussian distribution. For self-similar asperities, the friction stress is found to increase with the inverse of size. Comparison of the friction stress is made with other two contact models to show that interlocking asperities that are larger than ∼2 μm are easier to shear off plastically than asperities with a flat contact.},
keywords = {contact mechanics, dislocation dynamics, friction},
pubstate = {published},
tppubtype = {article}
}
Part of the friction between two rough surfaces is due to the interlocking between asperities on opposite surfaces. In order for the surfaces to slide relative to each other, these interlocking asperities have to deform plastically. Here, we study the unit process of plastic ploughing of a single micrometer-scale asperity by means of two-dimensional dislocation dynamics simulations. Plastic deformation is described through the generation, motion, and annihilation of edge dislocations inside the asperity as well as in the subsurface. We find that the force required to plough an asperity at different ploughing depths follows a Gaussian distribution. For self-similar asperities, the friction stress is found to increase with the inverse of size. Comparison of the friction stress is made with other two contact models to show that interlocking asperities that are larger than ∼2 μm are easier to shear off plastically than asperities with a flat contact.
