List of Journal Publications
2022
5.
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.
2021
4.
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.
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.
Sun, Fengwei, Giessen, Erik Van Der, Nicola, L.
Interaction between neighboring asperities during flattening: A discrete dislocation plasticity analysis Journal Article
In: MECHANICS OF MATERIALS, vol. 90, pp. 157–165, 2015.
Abstract | BibTeX | Tags: contact mechanics, dislocation dynamics, Rough surface, Size effect | Links:
@article{Sun2015,
title = {Interaction between neighboring asperities during flattening: A discrete dislocation plasticity analysis},
author = {Fengwei Sun and Erik Van Der Giessen and L. Nicola},
doi = {10.1016/j.mechmat.2015.04.012},
year = {2015},
date = {2015-01-01},
journal = {MECHANICS OF MATERIALS},
volume = {90},
pages = {157–165},
publisher = {Elsevier},
abstract = {Discrete dislocation plasticity simulations are performed to investigate the role of interaction between neighboring asperities on the contact pressure induced by a rigid platen on a rough surface. The rough surface is modeled as an array of equispaced asperities with a sinusoidal profile. The spacing between asperities is varied and the contact pressure necessary to flatten the surface to a given strain is computed. Plasticity in the asperities and in the crystal below is described by the collective glide of dislocations of edge character. Results show that the mean contact pressure necessary to flatten closely spaced asperities is larger than that required to flatten widely separated asperities. A small dependence on asperity density is already observed for a purely elastic material, but it is enhanced for small asperities, in the presence of dislocation plasticity. Plastic strain gradients, dislocation limited plasticity and interaction between neighboring plastic zones all contribute to what we will call the asperity density effect. Since dislocation limited plasticity plays a dominant role, the asperity density effect will mainly be relevant for surfaces having small asperity roughness.},
keywords = {contact mechanics, dislocation dynamics, Rough surface, Size effect},
pubstate = {published},
tppubtype = {article}
}
Discrete dislocation plasticity simulations are performed to investigate the role of interaction between neighboring asperities on the contact pressure induced by a rigid platen on a rough surface. The rough surface is modeled as an array of equispaced asperities with a sinusoidal profile. The spacing between asperities is varied and the contact pressure necessary to flatten the surface to a given strain is computed. Plasticity in the asperities and in the crystal below is described by the collective glide of dislocations of edge character. Results show that the mean contact pressure necessary to flatten closely spaced asperities is larger than that required to flatten widely separated asperities. A small dependence on asperity density is already observed for a purely elastic material, but it is enhanced for small asperities, in the presence of dislocation plasticity. Plastic strain gradients, dislocation limited plasticity and interaction between neighboring plastic zones all contribute to what we will call the asperity density effect. Since dislocation limited plasticity plays a dominant role, the asperity density effect will mainly be relevant for surfaces having small asperity roughness.
2012
1.
Sun, Fengwei, der Giessen, Erik Van, Nicola, L.
Plastic flattening of a sinusoidal metal surface: A discrete dislocation plasticity study Journal Article
In: WEAR, vol. 296, no. 1-2, pp. 672–680, 2012.
Abstract | BibTeX | Tags: contact mechanics, dislocation dynamics, Rough surface, Size effect, Surface topography | Links:
@article{Sun2012,
title = {Plastic flattening of a sinusoidal metal surface: A discrete dislocation plasticity study},
author = {Fengwei Sun and Erik Van der Giessen and L. Nicola},
doi = {10.1016/j.wear.2012.08.007},
year = {2012},
date = {2012-01-01},
journal = {WEAR},
volume = {296},
number = {1-2},
pages = {672–680},
publisher = {Elsevier Ltd},
abstract = {The plastic flattening of a sinusoidal metal surface is studied by performing plane strain dislocation dynamics simulations. Plasticity arises from the collective motion of discrete dislocations of edge character. Their dynamics is incorporated through constitutive rules for nucleation, glide, pinning and annihilation. By analyzing surfaces with constant amplitude we found that the mean contact pressure is inversely proportional to the wavelength. For small wavelengths, due to interaction between plastic zones of neighboring contacts, the mean contact pressure can reach values that are about 1/10 of the theoretical strength of the material, thus significantly higher than what is predicted by simulations that do not account for size dependent plasticity. Surfaces with the same amplitude to period ratio have a size dependent response, such that if we interpret each period of the sinusoidal wave as the asperity of a rough surface, smaller asperities are harder to be flattened than large ones. The difference between the limiting situations of sticking and frictionless contacts is found to be negligible.},
keywords = {contact mechanics, dislocation dynamics, Rough surface, Size effect, Surface topography},
pubstate = {published},
tppubtype = {article}
}
The plastic flattening of a sinusoidal metal surface is studied by performing plane strain dislocation dynamics simulations. Plasticity arises from the collective motion of discrete dislocations of edge character. Their dynamics is incorporated through constitutive rules for nucleation, glide, pinning and annihilation. By analyzing surfaces with constant amplitude we found that the mean contact pressure is inversely proportional to the wavelength. For small wavelengths, due to interaction between plastic zones of neighboring contacts, the mean contact pressure can reach values that are about 1/10 of the theoretical strength of the material, thus significantly higher than what is predicted by simulations that do not account for size dependent plasticity. Surfaces with the same amplitude to period ratio have a size dependent response, such that if we interpret each period of the sinusoidal wave as the asperity of a rough surface, smaller asperities are harder to be flattened than large ones. The difference between the limiting situations of sticking and frictionless contacts is found to be negligible.
