{ "items": [ {"key":"nicola2005size", "id":"nicola2005size", "type": "Publication", "BibTeX":"nicola_bib.html#nicola2005size", "abstract": "Stress development and relaxation in polycrystalline thin films perfectly bonded to a stiff substrate is analyzed numerically. The calculations are carried out within a two-dimensional plane strain framework. The film-substrate system is subject to a prescribed temperature decrease, with the coefficient of thermal expansion of the metal film larger than that of the substrate. Plastic deformation arises solely from the glide of edge dislocations. The dislocations nucleate from pre-existing Frank\u2013Read sources, with the grain boundaries and film-substrate interface acting solely as impenetrable barriers to dislocation glide. At each stage of loading, a boundary value problem is solved to enforce the boundary conditions and the stress field and the dislocation structure are obtained. The results of the simulations show both film-thickness and grain size dependent strengthening of polycrystalline films. Limited plasticity occurs in films with a sufficiently small grain-size, mainly due to a reduced nucleation rate in the constrained grain geometry.", "pub-type": "article", "label": "Size effects in polycrystalline thin films analyzed by discrete dislocation plasticity", "journal": "Thin Solid Films", "number": "1", "author": [ "Nicola, L.", "Van der Giessen, E.", "Needleman, A." ], "pages": "329−338", "year": "2005", "doi": "http://dx.doi.org/10.1016/j.tsf.2004.12.012", "volume": "479" }, {"key":"nicola20012d", "id":"nicola20012d", "type": "Publication", "BibTeX":"nicola_bib.html#nicola20012d", "abstract": "The paper deals with a discrete dislocation dynamics study of plastic deformation in a thin film caused by thermal mismatch with its substrate. A unit cell analysis is carried out, with dislocations in the film being represented by line singularities in an isotropic linear elastic medium. Their mutual interactions as well as the interactions with the interface and the free surface are accounted for by means of a coupled dislocation dynamics-finite element technique. The formulation includes a set of constitutive rules to model generation, glide, annihilation and pinning of dislocations at point obstacles. The simulation tracks the evolution of the dislocation structure as thermal stress builds up as well as during relaxation under constant temperature, leading to dense dislocation distributions near the interface and a dislocation-free zone along the stress-free surface of the film.", "tags": [ "thin film", "dislocation dynamics", "thermal stress" ], "pub-type": "article", "label": "2D dislocation dynamics in thin metal layers", "journal": "Materials Science and Engineering: A", "author": [ "Nicola, L.", "Van der Giessen, E.", "Needleman, A." ], "pages": "274−277", "year": "2001", "doi": "http://dx.doi.org/10.1016/S0921-5093(00)01690-7", "volume": "309" }, {"key":"gao2015lattice", "id":"gao2015lattice", "type": "Publication", "BibTeX":"nicola_bib.html#gao2015lattice", "abstract": "Strain gradient effects are commonly modeled as the origin of the size dependence of material strength, such as the dependence of indentation hardness on contact depth and spherical indenter radius. However, studies on the microstructural comparisons of experiments and theories are limited. First, we have extended a strain gradient Mises-plasticity model to its crystal plasticity version and implemented a finite element method to simulate the load-displacement response and the lattice rotation field of Cu single crystals under spherical indentation. The strain gradient simulations demonstrate that the forming of distinct sectors of positive and negative angles in the lattice rotation field is governed primarily by the slip geometry and crystallographic orientations, depending only weakly on strain gradient effects, although hardness depends strongly on strain gradients. Second, the lattice rotation simulations are compared quantitatively with micron resolution, three-dimensional X-ray microscopy (3DXM) measurements of the lattice rotation fields under 100 mN force, 100 micron radius spherical indentations in <111>, <110>, and <001> oriented Cu single crystals. Third, noting the limitation of continuum strain gradient crystal plasticity models, two-dimensional discrete dislocation simulation results suggest that the hardness in the nanocontact regime is governed synergistically by a combination of strain gradients and source-limited plasticity. However, the lattice rotation field in the discrete dislocation simulations is found to be insensitive to these two factors but to depend critically on dislocation obstacle densities and strengths.", "tags": [ "key3", "key2", "" ], "pub-type": "article", "label": "Lattice rotation patterns and strain gradient effects in face-centered-cubic single crystals under spherical indentation", "journal": "Journal of Applied Mechanics", "number": "6", "author": [ "Gao, Y. F.", "Larson, B. C.", "Lee, J. H.", "Nicola, L.", "Tischler, J. Z.", "Pharr, G. M." ], "pages": "061007", "year": "2015", "doi": "http://dx.doi.org/10.1115/1.4030403", "volume": "82" }, {"key":"nicola2006plastic", "id":"nicola2006plastic", "type": "Publication", "BibTeX":"nicola_bib.html#nicola2006plastic", "abstract": "Experimental measurements and computational results for the evolution of plastic deformation in freestanding thin films are compared. In the experiments, the stress-strain response of two sets of Cu films is determined in the plane-strain bulge test. One set of samples consists of electroplated Cu films, while the other set is sputter-deposited. Unpassivated films, films passivated on one side and films passivated on both sides are considered. The calculations are carried out within a two-dimensional plane strain framework with the dislocations modeled as line singularities in an isotropic elastic solid. The film is modeled by a unit cell consisting of eight grains, each of which has three slip systems. The film is initially free of dislocations which then nucleate from a specified distribution of Frank-Read sources. The grain boundaries and any film-passivation layer interfaces are taken to be impenetrable to dislocations. Both the experiments and the computations show: (i) a flow strength for the passivated films that is greater than for the unpassivated films and (ii) hysteresis and a Bauschinger effect that increases with increasing pre-strain for passivated films, while for unpassivated films hysteresis and a Bauschinger effect are small or absent. Furthermore, the experimental measurements and computational results for the 0.2% offset yield strength stress, and the evolution of hysteresis and of the Bauschinger effect are in good quantitative agreement.", "pub-type": "article", "label": "Plastic deformation of freestanding thin films: Experiments and modeling", "journal": "Journal of the Mechanics and Physics of Solids", "number": "10", "author": [ "Nicola, L.", "Xiang, Y.", "Vlassak, J. J.", "Van der Giessen, E.", "Needleman, A." ], "pages": "2089−2110", "year": "2006", "doi": "http://dx.doi.org/10.1016/j.jmps.2006.04.005", "volume": "54" }, {"key":"shishvan2010bauschinger", "id":"shishvan2010bauschinger", "type": "Publication", "BibTeX":"nicola_bib.html#shishvan2010bauschinger", "abstract": "Two-dimensional (2D) discrete dislocation plasticity simulations are carried out to investigate the Bauschinger effect (BE) in freestanding thin films. The BE in plastic flow of polycrystalline materials is generally understood to be caused by inhomogeneous deformation during loading, leading to residual stress upon unloading. This inhomogeneity can be caused by dislocation pile-ups, variations in texture, grain orientations, and grain size. To study the BE, columnar-grained films as well as films with multiple grains across the thickness are considered. The film is modeled in a 2D framework by a unit cell consisting of an array of grains with different orientation. In order to capture the interaction among grains, we motivate and explore the use of an affine deformation assumption on the grain level to mimic the three-dimensional geometry in this framework. It is shown that the dispersion of grain size in a film together with the size-dependence of yield strength leads to significant BEs in bare films. Quantitative comparison of simulations with experimental data is provided.", "pub-type": "article", "label": "Bauschinger effect in unpassivated freestanding thin films", "journal": "Journal of Applied Physics", "number": "9", "author": [ "Shishvan, S. S.", "Nicola, L.", "Van der Giessen, E." ], "pages": "093529", "year": "2010", "doi": "http://dx.doi.org/10.1063/1.3407505", "volume": "107" }, {"key":"davoudi2012dislocation", "id":"davoudi2012dislocation", "type": "Publication", "BibTeX":"nicola_bib.html#davoudi2012dislocation", "abstract": "In this paper, dislocation climb is incorporated in a two-dimensional discrete dislocationdynamics model. Calculations are carried out for polycrystallinethin films, passivated on one or both surfaces. Climb allows dislocations to escape from dislocation pile-ups and reduces the strain-hardening rate, especially for fully passivated films. Within the framework of this model, climb modifies the dislocationstructures that develop during plastic deformation and results in the formation of pile-ups on slip planes that do not contain any dislocation sources.", "pub-type": "article", "label": "Dislocation climb in two-dimensional discrete dislocation dynamics", "journal": "Journal of Applied Physics", "number": "10", "author": [ "Davoudi, K. M.", "Nicola, L.", "Vlassak, J. J." ], "pages": "103522", "year": "2012", "doi": "http://dx.doi.org/10.1063/1.4718432", "volume": "111" }, {"key":"zhang2014lattice", "id":"zhang2014lattice", "type": "Publication", "BibTeX":"nicola_bib.html#zhang2014lattice", "abstract": "A number of recent experimental efforts such as electron back scattering technique and three-dimensional X-ray structural microscopy have revealed the intriguing formation of sectors of lattice rotation fields under indentation. In the case of wedge indentation, the in-plane rotation changes sign from one sector to another. Although the lattice rotation fields can be used to compute the geometrically necessary dislocation (GND) densities, it remains unclear how these sectors can be related to the hardness and therefore to the indentation size effects, i.e., the increase of indentation hardness with the decrease of indentation depth. Crystal plasticity simulations in this work reproduce the experimental findings at large indentation depth. On the contrary, discrete dislocation plasticity can only capture the sectors found experimentally when there is a high obstacle density and large obstacle strength. Obstacle density and strength, however, have little effect on the hardness. In other words, there is no one-to-one correspondence between the lattice rotation patterns and the indentation size effects. The presence of obstacles favors the dislocation arrangements that lead to the experimentally found rotation sectors. Using the similarity solutions of indentation fields and the solution of localized deformation fields near a stationary crack, a simple model is developed that explains the dislocation pattern evolution, its relationship to the lattice misorientations, and more importantly its dependence on obstacles.", "pub-type": "article", "label": "Lattice rotation caused by wedge indentation of a single crystal: Dislocation dynamics compared to crystal plasticity simulations", "journal": "Journal of the Mechanics and Physics of Solids", "author": [ "Zhang, Y.", "Gao, Y. F.", "Nicola, L." ], "pages": "267−279", "year": "2014", "doi": "http://dx.doi.org/10.1016/j.jmps.2014.04.006", "volume": "68" }, {"key":"nicola2004relaxation", "id":"nicola2004relaxation", "type": "Publication", "BibTeX":"nicola_bib.html#nicola2004relaxation", "abstract": "The development and relaxation of stress in metal interconnects strained by their surroundings (substrate and passivation layers) is predicted by a discrete dislocation analysis. The model is based on a two-dimensional plane strain formulation, with deformation fully constrained in the line direction. Plastic deformation occurs by glide of edge dislocations on three slip systems in the single-crystal line. The substrate and passivation layers are treated as elastic materials and therefore impenetrable for the dislocations. Results of the simulations show the dependence of the stress evolution and of the effectiveness of plastic relaxation on the geometry of the line. The dependence of stress development on line aspect ratio, line size, slip plane orientation, pitch length, and passivation layer thickness are explored.", "pub-type": "article", "label": "Relaxation of thermal stress by dislocation motion in passivated metal interconnects", "journal": "Journal of materials research", "number": "4", "author": [ "Nicola, L.", "Van der Giessen, E.", "Needleman, A." ], "pages": "1216−1226", "year": "2004", "doi": "http://dx.doi.org/10.1557/JMR.2004.0158", "volume": "19" }, {"key":"sun2012plastic", "id":"sun2012plastic", "type": "Publication", "BibTeX":"nicola_bib.html#sun2012plastic", "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.", "pub-type": "article", "label": "Plastic flattening of a sinusoidal metal surface: A discrete dislocation plasticity study", "journal": "Wear", "number": "1", "author": [ "Sun, F.", "Van der Giessen, E.", "Nicola, L." ], "pages": "672−680", "year": "2012", "doi": "http://dx.doi.org/10.1016/j.wear.2012.08.007", "volume": "296" }, {"key":"nicola2005two", "id":"nicola2005two", "type": "Publication", "BibTeX":"nicola_bib.html#nicola2005two", "abstract": "Two-dimensional discrete dislocation plasticity simulations of the evolution of thermal stress in single crystal thin films on a rigid substrate are used to study size effects. The relation between the residual stress and the dislocation structure in the films after cooling is analyzed using dislocation dynamics. A boundary layer characterized by a high stress gradient and a high dislocation density is found close to the impenetrable film-substrate interface. There is a material-dependent threshold film thickness above which the dislocation density together with the boundary layer thickness and stress state are independent of film thickness. In such films the stress outside the boundary layer is on average very low, so that the film-thickness-independent boundary layer is responsible for the size effect. A larger size effect is found for films thinner than the threshold thickness. The origin of this size effect stems from nucleation activity being hindered by the geometrical constraint of the small film thickness, so that by decreasing film thickness, the dislocation density decreases while the stress in the film increases. The size dependence is only described by a Hall-Petch type relation for films thicker than the threshold value.", "pub-type": "article", "label": "Two hardening mechanisms in single crystal thin films studied by discrete dislocation plasticity", "journal": "Philosophical Magazine", "number": "14", "author": [ "Nicola, L.", "Van der Giessen, E.", "Needleman, A." ], "pages": "1507−1518", "year": "2005", "doi": "http://dx.doi.org/10.1080/14786430500036611", "volume": "85" }, {"key":"nicola2003discrete", "id":"nicola2003discrete", "type": "Publication", "BibTeX":"nicola_bib.html#nicola2003discrete", "abstract": "A discrete dislocation plasticity analysis of plastic deformation in metalthin films caused by thermal stress is carried out. The calculations use a two-dimensional plane-strain formulation with only edge dislocations. Single crystalfilms with a specified set of slip systems are considered. The film-substrate system is subjected to a prescribed temperature history and a boundary value problem is formulated and solved for the evolution of the stress field and for the evolution of the dislocations structure in the film. A hard boundary layer forms at the interface between the film and the substrate, which does not scale with the film thickness and thus gives rise to a size effect. It is found that a reduction in the rate of dislocation nucleation can occur abruptly, which gives rise to a two-stage hardening behavior.", "tags": [ "thin film", "size effect", "dislocation dynamics", "" ], "pub-type": "article", "label": "Discrete dislocation analysis of size effects in thin films", "journal": "Journal of Applied Physics", "number": "10", "author": [ "Nicola, L.", "Van der Giessen, E.", "Needleman, A." ], "pages": "5920−5928", "year": "2003", "doi": "http://dx.doi.org/10.1063/1.1566471", "volume": "93" }, {"key":"zhang2014direct", "id":"zhang2014direct", "type": "Publication", "BibTeX":"nicola_bib.html#zhang2014direct", "abstract": "This paper addresses the feasibility of direct nanoimprinting and highlights the challenges involved in this technique. Our study focuses on experimental work supported by dislocation dynamics simulations. A gold single crystal is imprinted by a tungsten indenter patterned with parallel lines of various spacings. Dedicated dislocation dynamics simulations give insight in the plastic deformation occurring into the crystal during imprinting. We find that good pattern transfer is achieved when the lines are sufficiently spaced such that dislocation activity can be effective in assisting deformation of the region underneath each line. Yet, the edges of the obtained imprints are not smooth, partly due to dislocation glide.", "pub-type": "article", "label": "Direct nanoimprinting of single crystalline gold: Experiments and dislocation dynamics simulations", "journal": "Applied Surface Science", "author": [ "Zhang, J.", "Zhang, Y.", "Mara, N. A.", "Lou, J.", "Nicola, L." ], "pages": "301−307", "year": "2014", "doi": "http://dx.doi.org/10.1016/j.apsusc.2013.11.072", "volume": "290" }, {"key":"zhang2014competition", "id":"zhang2014competition", "type": "Publication", "BibTeX":"nicola_bib.html#zhang2014competition", "abstract": "Direct metal nanoimprinting of a gold thin layer is studied by means of quasi-static molecular dynamics simulations. The aim of this study is to understand if it is possible to obtain a reproducible nanopattern with features of a few nanometers, that closely resembles the shape of the template. The majority of our simulations show an unexpected competition between crack formation and dislocation plasticity upon retraction of the template, which leads in some cases to an imprint and in other cases to a flat surface. These results are at odds with previous simulations of metal nanoimprinting, which always predicted formation of an imprint. The reason for this discrepancy lies in the much lower (and thus more realistic) imprinting velocity used in this work. The most interesting finding of this paper is that the competition between crack and dislocations for certain loading conditions and geometry of the crystals is driven by thermal fluctuations of the atomic velocities. Local events, namely atomic fluctuations and dislocation nucleation, determine the global mechanical response of the system, i.e. whether an imprint is obtained or not. The relevance of thermal fluctuations is confirmed by the fact that any of the simulations presented here, if repeated at 10 K, leads to a brittle material behavior.", "pub-type": "article", "label": "Competition between dislocations and cracks in molecular dynamics simulations of metal nanoimprinting", "journal": "Computational Materials Science", "author": [ "Zhang, Y.", "Thijsse, B. J.", "Nicola, L." ], "pages": "95−105", "year": "2014", "doi": "http://dx.doi.org/10.1016/j.commatsci.2014.02.016", "volume": "94" }, {"key":"nicola2007surface", "id":"nicola2007surface", "type": "Publication", "BibTeX":"nicola_bib.html#nicola2007surface", "abstract": "The indentation of single crystals by a periodic array of flat rigid contacts is analyzed using discrete dislocation plasticity. Plane strain analyses are carried out with the dislocations all of edge character and modeled as line singularities in a linear elastic solid. The limiting cases of frictionless and perfectly sticking contacts are considered. The effects of contact size, dislocation source density, and dislocation obstacle density and strength on the evolution of the mean indentation pressure are explored, but the main focus is on contrasting the response of crystals having dislocation sources on the surface with that of crystals having dislocation sources in the bulk. When there are only bulk sources, the mean contact pressure for sufficiently large contacts is independent of the friction condition, whereas for sufficiently small contact sizes, there is a significant dependence on the friction condition. When there are only surface dislocation sources the mean contact pressure increases much more rapidly with indentation depth than when bulk sources are present and the mean contact pressure is very sensitive to the strength of the obstacles to dislocation glide. Also, on unloading a layer of tensile residual stress develops when surface dislocation sources dominate.", "pub-type": "article", "label": "Surface versus bulk nucleation of dislocations during contact", "journal": "Journal of the Mechanics and Physics of Solids", "number": "6", "author": [ "Nicola, L.", "Bower, A. F.", "Kim, K.-S.", "Needleman A.", "Van der Giessen, E." ], "pages": "1120−1144", "year": "2007", "doi": "http://dx.doi.org/10.1016/j.jmps.2006.12.005", "volume": "55" }, {"key":"kumar2009density", "id":"kumar2009density", "type": "Publication", "BibTeX":"nicola_bib.html#kumar2009density", "abstract": "Discrete dislocation dynamics simulations are carried out to systematically investigate the microstructural and geometrical size dependence of films under tension that have a varying number of grains through their thickness. By varying film thickness, grain size and aspect ratio, more insight is gained into the competition between grain boundary hardening and film thickness effects. This provides a seamless link between previous dislocation plasticity studies and qualitative agreement with experimental data. In the 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. Grain boundaries are treated as impenetrable to dislocation motion. The numerical results show that the grain size dependence of yield in thin films as well as in bulk polycrystals is controlled by the density of grain boundaries.", "pub-type": "article", "label": "Density of grain boundaries and plasticity size effects: A discrete dislocation dynamics study", "journal": "Materials Science and Engineering: A", "number": "1", "author": [ "Kumar, R.", "Nicola, L.", "Van der Giessen, E." ], "pages": "7−15", "year": "2009", "doi": "http://dx.doi.org/10.1016/j.msea.2009.08.072", "volume": "527" }, {"key":"nicola2005effect", "id":"nicola2005effect", "type": "Publication", "BibTeX":"nicola_bib.html#nicola2005effect", "abstract": "Gurtin recently proposed a strain-gradient theory for crystal plasticity in which the gradient effect originates from a defect energy that characterizes energy storage due to the presence of a net Burgers vector. Here we consider a number of different possibilities for this energy: specifically, working within a simple two-dimensional framework, we compare predictions of the theory with results of discrete-dislocation simulations of stress relaxation in thin films. Our objective is to investigate which specific defect energies are capable of capturing the size-dependent response of such systems for different crystal orientations.", "pub-type": "article", "label": "Effect of defect energy on strain-gradient predictions of confined single-crystal plasticity", "journal": "Journal of the Mechanics and Physics of Solids", "number": "6", "author": [ "Nicola, L.", "Van der Giessen, E.", "Gurtin, M. E." ], "pages": "1280−1294", "year": "2005", "doi": "http://dx.doi.org/10.1016/j.jmps.2005.02.001", "volume": "53" }, {"key":"song2015plastic", "id":"song2015plastic", "type": "Publication", "BibTeX":"nicola_bib.html#song2015plastic", "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 micron are easier to shear off plastically than asperities with a flat contact.", "tags": [ "key1", "key2", "key3" ], "pub-type": "article", "label": "Plastic ploughing of a sinusoidal asperity on a rough surface", "journal": "Journal of Applied Mechanics", "number": "7", "author": [ "Song, H.", "Dikken, R.-J.", "Nicola, L.", "Van der Giessen, E." ], "pages": "071006", "year": "2015", "doi": "http://dx.doi.org/10.1115/1.4030318", "volume": "82" }, {"key":"ngweisiang2016discrete", "id":"ngweisiang2016discrete", "type": "Publication", "BibTeX":"nicola_bib.html#ngweisiang2016discrete", "abstract": "A contact mechanical model is presented where both metal bodies can deform by discrete dislocation plasticity. The model intends to improve on previous dislocation dynamics models of contact, where only a plastically deformable body was considered, flattened by a rigid platen. The effect of the rigid platen was mimicked through boundary conditions acting on the deformable body. While the formulation is general, the simulations presented here are only performed for contact between a plastically deforming body with sinusoidal surface and a flat body that is either elastic or rigid. Results show that the contact conditions, i.e. frictionless and full stick, affect the morphology of the contact as well as the contact pressure distribution. This is because dislocations can glide through the frictionless contact and fragment it, but do not penetrate a sticking contact. Average quantities like mean apparent contact pressure and total plastic slip are, instead, independent of contact conditions and of the details of the contact area. A size dependence is observed in relation to the onset of plastic deformation, where surfaces with smaller wavelength and amplitude require a larger contact pressure to yield than self similar surfaces with larger wavelength. The size dependence is very pronounced when the flat body is rigid, but fades when the compliance of the flat body is large.", "pub-type": "article", "label": "Discrete dislocation plasticity analysis of contact between deformable bodies of simple geometry", "journal": "Modelling and Simulation in Materials Science and Engineering", "number": "4", "author": [ "Ng Wei Siang K.", "Nicola, L." ], "pages": "045008", "year": "2016", "doi": "http://dx.doi.org/10.1088/0965-0393/24/4/045008", "volume": "24" }, {"key":"sun2015interaction", "id":"sun2015interaction", "type": "Publication", "BibTeX":"nicola_bib.html#sun2015interaction", "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.", "pub-type": "article", "label": "Interaction between neighboring asperities during flattening: A discrete dislocation plasticity analysis", "journal": "Mechanics of Materials", "author": [ "Sun, F.", "Van der Giessen, E.", "Nicola, L." ], "pages": "157−165", "year": "2015", "doi": "http://dx.doi.org/10.1016/j.mechmat.2015.04.012", "volume": "90" }, {"key":"nicola2008multi", "id":"nicola2008multi", "type": "Publication", "BibTeX":"nicola_bib.html#nicola2008multi", "abstract": "Plane strain indentation of single crystals by a periodic array of flat rigid contacts is analyzed. The calculations are carried out, with the mechanical response of the crystal characterized by conventional continuum crystal plasticity or by discrete dislocation plasticity. The properties used in the conventional crystal plasticity description are chosen so that both theories give essentially the same response in uniform plane strain compression. The indentation predictions are then compared, focusing in particular on the effect of contact size and spacing. The limiting cases of frictionless contacts and of perfectly sticking contacts are analyzed. Conventional continuum plasticity predicts a size-independent response. Unless the contact spacing to size ratio is very small, the predicted deformation mode under the contacts is a wedging mechanism of the type described by slip line theory, which is only weakly sensitive to friction conditions. For the micron scale contacts analyzed, discrete dislocation plasticity predicts a response that depends on the contact size as well as on the contact spacing to size ratio. When contacts are spaced sufficiently far apart, discrete dislocation plasticity predicts that the deformation is localized beneath the contacts, whereas for more closely spaced contacts, deformation occurs by shear bands extending relatively far into the crystal. Unless the contacts are sufficiently close together so that the response is essentially one of plane strain compression, the mean contact pressure predicted by discrete dislocation plasticity is substantially greater than that predicted by conventional continuum crystal plasticity and is more sensitive to the friction conditions.", "pub-type": "article", "label": "Multi-asperity contact: A comparison between discrete dislocation and crystal plasticity predictions", "journal": "Philosophical Magazine", "number": "30-32", "author": [ "Nicola, L.", "Bower, A. F.", "Kim, K.-S.", "Needleman A.", "Van der Giessen, E." ], "pages": "3713−3729", "year": "2008", "doi": "http://dx.doi.org/10.1080/14786430802566372", "volume": "88" }, {"key":"zhang2010effect", "id":"zhang2010effect", "type": "Publication", "BibTeX":"nicola_bib.html#zhang2010effect", "abstract": "Dislocation dynamics simulations are performed to investigate the effect of template shape on the nanoimprinting of metal layers. To this end, metal thin films are imprinted by a rigid template made of an array of equispaced indenters of various shapes, i.e., rectangular, wedge, and circular. The geometry of the indenters is chosen such that the contact area is approximately the same at the final imprinting depth. Results show that, for all template shapes, the final patterns strongly depend on the dislocation activity, and that each imprint differs from the neighboring ones. Large material pile ups appear between the imprints, such that polishing of the metal layer is suggested for application of the patterns in electronics. Rectangular indenters require the lowest imprinting force and achieve the deepest retained imprints.", "tags": [ "nanoimprinting", "dislocation dynamics", "" ], "pub-type": "article", "label": "Effect of template shape on metal nanoimprinting: A dislocation dynamics study", "journal": "Journal of Zhejiang University SCIENCE A", "number": "10", "author": [ "Zhang, Y.", "Nicola, L." ], "pages": "722−726", "year": "2010", "doi": "http://dx.doi.org/10.1631/jzus.A1000175", "volume": "11" }, {"key":"davoudi2014bauschinger", "id":"davoudi2014bauschinger", "type": "Publication", "BibTeX":"nicola_bib.html#davoudi2014bauschinger", "abstract": "The effects of dislocation climb on plastic deformation during loading and unloading are studied using a two-dimensional discrete dislocation dynamics model. Simulations are performed for polycrystalline thin films passivated on both surfaces. Dislocation climb lowers the overall level of the stress inside thin films and reduces the work hardening rate. Climb decreases the density of dislocations in pile-ups and reduces back stresses. These factors result in a smaller Bauschinger effect on unloading compared to simulations without climb. As dislocations continue to climb at the onset of unloading and the dislocation density continues to increase, the initial unloading slope increases with decreasing unloading rate. Because climb disperses dislocations, fewer dislocations are annihilated during unloading, leading to a higher dislocation density at the end of the unloading step.", "pub-type": "article", "label": "Bauschinger effect in thin metal films: Discrete dislocation dynamics study", "journal": "Journal of Applied Physics", "number": "1", "author": [ "Davoudi, K. M.", "Nicola, L.", "Vlassak, J. J." ], "pages": "013507", "year": "2014", "doi": "http://dx.doi.org/10.1063/1.4861147", "volume": "115" }, {"key":"zhang2010discrete", "id":"zhang2010discrete", "type": "Publication", "BibTeX":"nicola_bib.html#zhang2010discrete", "abstract": "Simulations of rough surface flattening are performed on thin metal films whose roughness is created by nanoimprinting flat single crystals. The imprinting is carried out by means of a rigid template with equal flat contacts at varying spacing. The imprinted surfaces are subsequently flattened by a rigid platen, while the change of roughness and surface profile is computed. Attention is focused mainly on comparing the response of the film surfaces with those of identical films cleared of the dislocations and residual stresses left by the imprinting process. The aim of these studies is to understand to what extent the loading history affects deformation and roughness during flattening. The limiting cases of sticking and frictionless contact between rough surface and platen are analyzed. Results show that when the asperities are flattened such that the contact area is up to about one third of the surface area, the loading history strongly affects the flattening. Specifically, the presence of initial dislocations facilitates the squeezing of asperities independently of the friction conditions of the contact. For larger contact areas, the initial conditions affect only sticking contacts, while frictionless contacts lead to a homogeneous flattening of the asperities due to yield of the metal film. In all cases studied the final surface profile obtained after flattening has little to no resemblance to the original imprinted surface.", "pub-type": "article", "label": "Discrete dislocation simulations of the flattening of nanoimprinted surfaces", "journal": "Modelling and Simulation in Materials Science and Engineering", "number": "3", "author": [ "Zhang, Y.", "Van der Giessen, E.", "Nicola, L." ], "pages": "034006", "year": "2010", "doi": "http://dx.doi.org/10.1088/0965-0393/18/3/034006", "volume": "18" }, {"key":"berthold2000glass", "id":"berthold2000glass", "type": "Publication", "BibTeX":"nicola_bib.html#berthold2000glass", "abstract": "Glass-to-glass wafer bonding has recently attracted considerable interest. Especially for liquid manipulation applications and on-chip chemical analysis systems, all-glass sealed channels with integrated metal electrodes are very attractive. In this paper, we present a novel anodic bonding process in which the temperature does not exceed 400u00b0C. This is a crucial requirement if metal patterns are present on the wafers. A number of thin film materials available in most conventional IC processes deposited on the glass wafers have been tested as intermediate bonding layers. Successful bonding is obtained for various layer combinations and an explanation of the bonding mechanism is given.", "tags": [ "anodic bonding", "thin film", "" ], "pub-type": "article", "label": "Glass-to-glass anodic bonding with standard IC technology thin films as intermediate layers", "journal": "Sensors and Actuators A: Physical", "number": "1", "author": [ "Berthold, A.", "Nicola, L.", "Sarro, P. M.", "Vellekoop, M. J." ], "pages": "224−228", "year": "2000", "doi": "http://dx.doi.org/10.1016/S0924-4247(99)00376-3", "volume": "82" }, {"key":"sun2015effect", "id":"sun2015effect", "type": "Publication", "BibTeX":"nicola_bib.html#sun2015effect", "abstract": "Discrete dislocation (DD) plasticity simulations are carried out to investigate the effect of flattening and shearing of surface asperities. The asperities are chosen to have a rectangular shape to keep the contact area constant. Plasticity is simulated by nucleation, motion, and annihilation of edge dislocations. The results show that plastic flattening of large asperities facilitates subsequent plastic shearing, since it provides dislocations available to glide at lower shear stress than the nucleation strength. The effect of plastic flattening disappears for small asperities, which are harder to be sheared than the large ones, independently of preloading. An effect of asperity spacing is observed with closely spaced asperities being easier to plastically shear than isolated asperities. This effect fades when asperities are very protruding, and therefore plasticity is confined inside the asperities.", "tags": [ "key2", "" ], "pub-type": "article", "label": "Effect of plastic flattening on the shearing response of metal asperities: A dislocation dynamics analysis", "journal": "Journal of Applied Mechanics", "number": "7", "author": [ "Sun, F.", "Van der Giessen, E.", "Nicola, L." ], "pages": "071009", "year": "2015", "doi": "http://dx.doi.org/10.1115/1.4030321", "volume": "82" } ] }