arXiv subjects

S. Papanikolaou

Publications and source records attributed to S. Papanikolaou.

At least 19 recordsLinked to original sources

arXiv2026

High-temperature behavior of amorphous alumina coatings: Insights from in-situ nanoindentation and X-ray diffraction studies

Further development of nuclear power plant technology relies heavily on materials durability under operating conditions. Estimating the materials performance in the operando tests is crucial. In this paper, the mechanical behavior of thin amorphous nuclear-dedicated Al2O3 coatings deposited by pulsed laser deposition was investigated by nanoindentation over the temperature range of 25-650C. Experimental nanomechanical analysis was supported by MD simulations. The results indicate that the hardness of the amorphous coating experiences a gradual, constant decrease with temperature, while the Young modulus value remains constant in the whole temperature range. Observed phenomena confirm the increasing plasticity of the material and it is postulated to be related to the bond-switching mechanism that accelerates at high temperatures. The post-mortem transmission electron microscopy characterization confirmed that the loaded material was non-crystalline over the entire range of the indentation temperatures. The thermal stability of the structure was further studied in-situ up to 1050C by X-ray diffraction. The implemented methodology allowed us to follow the dynamic process of phase transitions occurring in the material above 650C. First, thermally activated crystallization was observed at 700C. Intermediate alumina phases were present up to 950C, while above this temperature, exclusively the thermodynamically stable alpha-Al2O3 was observed. The in-situ high-temperature characterization of the evolution of thin films boosts the understanding of the application limits of the coating systems at elevated temperatures. The added value is that the paper demonstrates the potential usefulness of combining high-temperature techniques to characterize the complete behavior of thin films at elevated temperatures.

cond-mat.mtrl-sci↗

arXiv2025

Dissociative Mechanism from NH3 and CH4 on Ni-Doped Graphene: Tuning Electronic and Optical Properties

In this study, we employ a multi-scale computational modeling approach, combining density functional theory (DFT) and self-consistent charge density functional tight binding (SCC-DFTB), to investigate hydrogen (H2) production and dissociation mechanisms from ammonia (NH3) and methane (CH4) on pristine and nickel-doped graphene. These two-dimensional materials hold significant potential for applications in advanced gas sensing and catalysis. Our analysis reveals that Ni-doped graphene, validated through work function calculations, is a promising material for gas separation and hydrogen production. The samples with adsorbed molecules are characterized by calculating chemical potential, chemical hardness, electronegativity, electrophilicity, vibrational frequencies, adsorbtion and Gibbs energies by DFT calculations. Methane molecules preferentially adsorb at the hexagonal ring centers of graphene, while ammonia inter-acts more strongly with carbon atoms, highlighting distinct molecular doping mechanisms for CH4 and NH3. Dynamic simulations show that CH4 splits into CH3+H, with Ni-doped graphene facilitating enhanced hydrogen transmission, while NH3 dissociates into NH2+H, which may lead to N2H4 formation. Our non-equilibrium Green's function (NEGF) simulations demonstrate increased H-atom transmission on Ni-doped graphene during gas interactions. These findings suggest that Ni-doped graphene is superior to pristine graphene for applications in gas separation, hydrogen production, and high-sensitivity sensors.

cond-mat.mtrl-sci↗

arXiv2025

Self-ion irradiation effects on nanoindentation-induced plasticity of crystalline iron: A joint experimental and computational study

In this paper, experimental work is supported by multi-scale numerical modeling to investigate nanomechanical response of pristine and ion irradiated with Fe2+ ions with energy 5 MeV high purity iron specimens by nanoindentation and Electron Backscatter Diffraction. The appearance of a sudden displacement burst that is observed during the loading process in the load-displacement curves is connected with increased shear stress in a small subsurface volume due to dislocation slip activation and mobilization of pre-existing dislocations by irradiation. The molecular dynamics (MD) and 3D-discrete dislocation dynamics (3D-DDD) simulations are applied to model geometrically necessary dislocations (GNDs) nucleation mechanisms at early stages of nanoindentation test; providing an insight to the mechanical response of the material and its plastic instability and are in a qualitative agreement with GNDs density mapping images. Finally, we noted that dislocations and defects nucleated are responsible the material hardness increase, as observed in recorded load-displacement curves and pop-ins analysis.

cond-mat.mtrl-sci↗

arXiv2025

CH$_4$ and CO$_2$ Adsorption Mechanisms on Monolayer Graphenylene and their Effects on Optical and Electronic Properties

In this study, we employ a computational chemistry-based modeling approach to investigate the adsorption mechanisms of CH$_4$ and CO$_2$ on monolayer GPNL, with a specific focus on their effects on optical adsorption and electrical transport properties at room temperature. To simulate the adsorption dynamics as closely as possible to experimental conditions, we utilize the self-consistent charge tight-binding density functional theory (SCC-DFTB). Through semi-classical molecular dynamics (MD) simulations, we observe the formation of H$_2$ molecules from the dissociation of CH$_4$ and the formation of CO+O species from carbon dioxide molecules. This provides insights into the adsorption and dispersion mechanisms of CH$_4$ and CO$_2$ on GPNL. Furthermore, we explore the impact of molecular adsorption on optical absorption properties. Our results demonstrate that CH$_4$ and CH$_2$ affects drastically the optical adsorption of GPNL, while CO$_2$ does not significantly affect the optical properties of the two-dimensional material. To analyze electron transport, we employ the open-boundary non-equilibrium Green's function method. By studying the conductivity of GPNL and graphene under voltage bias up to 300 mV, we gain valuable insights into the electrical transport properties of GPNL under optical absorption conditions. The findings from our computational modeling approach might contribute to a deeper understanding of the potential applications of GPNL in hydrogen production and advanced electronic devices.

physics.comp-ph↗

arXiv2025

Multiscale nanoindentation modeling of concentrated solid solutions: A continuum plasticity model

Recently developed single-phase concentrated solid-solution alloys (CSAs) contain multiple elemental species in high concentrations with different elements randomly arranged on a crystalline lattice. These chemically disordered materials present excellent physical properties, including high-temperature thermal stability and hardness, with promising applications to industries at extreme operating environments. The aim of this paper is to present a continuum plasticity model accounting for the first time for the behaviour of a equiatomic five-element CSA, that forms a face-centered cubic lattice. The inherent disorder associated with the lattice distortions caused by an almost equiatomic distribution of atoms, is captured by a single parameter α that quantifies the relative importance of an isotropic plastic contribution to the model. This results in multiple plasticity mechanisms that go beyond crystallographic symmetry-based ones, common in the case of conventional single element metals. We perform molecular dynamics simulations of equiatomic CSAs: NiFe, NiFeCr, NiFeCrCo, and Cantor alloys to validate the proposed continuum model which is implemented in the finite element method and applied to model nanoindentation tests for three different crystallographic orientations. We obtain the representative volume element model by tracking the combined model yield surface.

physics.comp-ph↗

arXiv2025

Nanoindentation of single crystalline Mo: Atomistic defect nucleation and thermomechanical stability

The mechanical responses of single crystalline Body-Centered Cubic (BCC) metals, such as molybdenum (Mo), outperform other metals at high temperatures, so much so that they are considered as excellent candidates for applications under extreme conditions, such as the divertor of fusion reactors. The excellent thermomechanical stability of molybdenum at high temperatures (400-1000$^{\rm o}$C) has also been detected through nanoindentation, pointing towards connections to emergent local dislocation mechanisms related to defect nucleation. In this work, we carry out a computational study of the effects of high temperature on the mechanical deformation properties of single crystalline Mo under nanoindentation. Molecular dynamics (MD) simulations of spherical nanoindentation are performed at two indenter tip diameters and crystalline sample orientations [100], [110], and [111], for the temperature range of 10-1000K. We investigate how the increase of temperature influences the nanoindentation process, modifying dislocation densities, mechanisms, atomic displacements and also, hardness, in agreement with reported experimental measurements. Our results suggest that the characteristic formation and high-temperature stability of [001] dislocation junctions in Mo during nanoindentation, in contrast to other BCC metals, may be the cause of the persistent thermomechanical stability of Mo.

cond-mat.mtrl-sci↗

arXiv2024

Collision Cascade-Driven Evolution of Vacancy Defects in Ni-Based Concentrated Solid-Solution Alloys

Concentrated solid--solution alloys (CSAs) in single--phase form have recently garnered considerable attention owing to their potential for exceptional irradiation resistance. This computational study delves into the intricate interplay of alloying elements on the generation, recombination, and evolution of irradiation-induced defects. Molecular dynamics simulations were conducted for collision cascades at room temperature, spanning a range of primary knock-on atom energies from 1 to 10 keV. The investigation encompasses a series of model crystals, progressing from pure Ni to binary CSAs such as NiFe$_{20}$, NiFe, NiCr$_{20}$, and culminating in the more intricate NiFeCr$_{20}$ CSA. We observe that materials rich in chromium actively facilitate dislocation emissions and induce the nucleation of stacking fault tetrahedra in the proximity of nanovoids, owing to Shockley partial interactions. This result is validated by molecular static simulations, which calculate the surface, vacancy, and defect formation energies. Among various shapes considered, the spherical void proves to be the most stable, followed by the truncated octahedron and octahedron shapes. On the other hand, the tetrahedron cubic shape is identified as the most unstable, and stacking fault tetrahedra exhibit the highest formation energy. Notably, among the materials studied, NiCr$_{20}$ and NiFeCr$_{20}$ CSAs stood out as the sole alloys capable of manifesting this mechanism, mainly observed at high impact energies.

cond-mat.mtrl-sci↗

arXiv2024

Atomistic-Level Analysis of Nanoindentation-Induced Plasticity in Arc--Melted NiFeCrCo Alloys: The role of stacking faults

Concentrated solid solution alloys (CSAs) have attracted attention for their promising properties; however, current manufacturing methods face challenges in complexity, high costs, and limited scalability, raising concerns about industrial viability. The prevalent technique, arc melting, yields high-purity samples with complex shapes. In this study, we explore nanoindentation tests at room temperature where arc-melted samples exhibit larger grain sizes, diminishing the effects of grain boundaries on the results. Motivated by these findings, our investigation focuses on the atomistic-level exploration of plasticity mechanisms, specifically dislocation nucleation and propagation during nanoindentation tests. The intricate chemistry of NiFeCrCo CSA influences pile-ups and slip traces, aiming to elucidate plastic deformation by considering both pristine and pre-existing stacking fault tetrahedra. Our analysis scrutinizes dynamic deformation processes, defect nucleation, and evolution, complemented by stress-strain and dislocation densities-strain curves illustrating the hardening mechanism of defective materials. Additionally, we examine surface morphology and plastic deformation through atomic shear strain and displacement mappings. This integrated approach provides insights into the complex interplay between material structure and mechanical behavior, paving the way for an enhanced understanding and potential advancements in CSA applications.

cond-mat.mtrl-sci↗

arXiv2023

Plastic deformation mechanisms during nanoindentation of W, Mo, V body-centered cubic single crystals and their corresponding W-Mo, W-V equiatomic random solid solutions

Deformation plasticity mechanisms in alloys and compounds may unveil the material capacity towards optimal mechanical properties. We conduct a series of molecular dynamics (MD) simulations to investigate plasticity mechanisms due to nanoindentation in pure tungsten, molybdenum and vanadium body-centered cubic single crystals, as well as the also body-centered cubic, equiatomic, random solid solutions (RSS) of tungsten--molybdenum and tungsten--vanadium alloys. Our analysis focuses on a thorough, side-by-side comparison of dynamic deformation processes, defect nucleation, and evolution, along with corresponding stress--strain curves. We also check the surface morphology of indented samples through atomic shear strain mapping. As expected, the presence of Mo and V atoms in W matrices introduces lattice strain and distortion, increasing material resistance to deformation and slowing down dislocation mobility of dislocation loops with a Burgers vector of 1/2 $\langle 111 \rangle$. Our side-by-side comparison displays a remarkable suppression of the plastic zone size in equiatomic W--V RSS, but not in equiatomic W--Mo RSS alloys, displaying a clear prediction for optimal hardening response equiatomic W--V RSS alloys. If the small-depth nanoindentation plastic response is indicative of overall mechanical performance, it is possible to conceive a novel MD-based pathway towards material design for mechanical applications in complex, multi-component alloys.

cond-mat.mtrl-sci↗

arXiv2022

Atomistic simulations of nanoindentation in single crystalline tungsten: The role of interatomic potentials

Computational modeling is usually applied to aid experimental exploration of advanced materials to better understand the fundamental plasticity mechanisms during mechanical testing. In this work, we perform Molecular dynamics (MD) simulations to emulate experimental room temperature spherical-nanoindentation of crystalline W matrices by different interatomic potentials: EAM, modified EAM, and a recently developed machine learned based tabulated Gaussian approximation potential (tabGAP) for describing the interaction of W-W. Results show similarities between load displacements and stress-strain curves, regardless of the numerical model. However, a discrepancy is observed at early stages of the elastic to plastic deformation transition showing different mechanisms for dislocation nucleation and evolution, that is attributed to the difference of Burgers vector magnitudes, stacking fault and dislocation glide energies. Besides, contact pressure is investigated by considering large indenters sizes that provides a detailed analysis of screw and edge dislocations during loading process. Furthermore, the glide barrier of this kind of dislocations are reported for all the interatomic potentials showing that tabGAP model presents the most accurate results with respect to density functional theory calculations and a good qualitative agreement with reported experimental data

physics.comp-ph↗

arXiv2022

Dynamic nanoindentation and short-range order in equiatomic NiCoCr medium entropy alloy lead to novel density wave ordering

Chemical short-range order (CSRO) is believed to be a key contributor to the exceptional properties of multicomponent alloys. However, direct validation and confirmation of CSRO has been highly elusive in most compounds. Recent studies for equiatomic NiCoCr alloys have shown that thermal treatments (i.e., annealing/aging) may facilitate and manipulate CSRO. In this work, by using molecular simulations, we show that nanomechanical probes, such as nanoindentation, may be utilized towards further manipulation of CSRO, providing explicit validation pathways. By using well established interatomic potentials, we perform hybrid Molecular-Dynamics/Monte-Carlo (MD/MC) at room temperature to demonstrate that particular dwell nanoindentation protocols can lead, through thermal MC equilibration, to the reorganization of CSRO under the indenter tip, to a density-wave stripe pattern (DWO). We characterize the novel DWO structures, that are directly correlated to incipient SRO but are highly anisotropic and dependent on local, nanoindentation-induced stress concentrations, and we show how they deeply originate from the peculiarities of the interatomic potentials. Furthermore, we show that the DWO patterns consistently scale up with the incipient plastic zone under the indenter tip, justifying the observation of the DWO patterning at any experimentally feasible nanoindentation depth.

cond-mat.mtrl-sci↗

arXiv2022

Mechanisms of strength and hardening in austenitic stainless 310S steel: Nanoindentation experiments and multiscale modeling

Austenitic stainless steels with low carbon have exceptional mechanical properties and are capable to reduce embrittlement, due to high chromium and nickel alloying, thus they are very attractive for efficient energy production in extreme environments. It is key to perform nanomechanical investigations of the role of chromium and the form of the particular alloy composition that give rise to the excellent mechanical properties of steel. We perform nanoindentation experiments and molecular dynamics (MD) simulations of FCC austenitic stainless steel 310S, using established interatomic potentials, and we use a comparison to the plastic behavior of NiFe solid solutions under similar conditions for the elucidation of key dislocation mechanisms. We combine EBSD images to connect crystalline orientations to nanoindentation results, and provide input data to MD simulations for modeling mechanisms of defects nucleation and interactions. The maps of impressions after nanoindentation indicate that the Ni-Fe-Cr composition in 310S steel leads to strain localization and hardening. A detailed analysis of the dislocation dynamics at different depths leads to the development of an experimentally consistent Kocks-Mecking-based continuum multiscale model. Furthermore, the analysis of geometrically necessary dislocations (GND) shows to be responsible for exceptional hardness at low depths, predicted by the Ma-Clarke's constitutive model.

physics.comp-ph↗

arXiv2021

Edge dislocations in multi-component solid solution alloys: Beyond traditional elastic depinning

High-entropy alloys (HEA) form solid solutions with large chemical disorder and excellent mechanical properties. We investigate the origin of HEA strengthening in face-centered cubic (FCC) single-phase HEAs through molecular dynamics simulations of dislocations, in particular, the equiatomic $\rm CrCoNi$, $\rm CrMnCoNi$, $\rm CrFeCoNi$, $\rm CrMnFeCoNi$, $\rm FeNi$, and also, $\rm Fe_{0.4}Mn_{0.27}Ni_{0.26}Co_{0.05}Cr_{0.02}$, $\rm Fe_{0.7}Ni_{0.11}Cr_{0.19}$. The dislocation correlation length $ξ$, roughness amplitude $R_{a}$, and stacking fault widths $W_{SF}$ are tracked as a function of stress. All alloys are characterized by a well defined depinning stress ($σ_c$) and we find a novel regime where exceptional strength is observed, and a fortuitous combination takes place, of small stacking fault widths and large dislocation roughness $R_{a}$. Thus the depinning of two partials seems analogous to unconventional domain wall depinning in disordered magnetic thin films. This novel regime is identified in specific compositions commonly associated with exceptional mechanical properties ($\rm CrCoNi$, $\rm CrMnCoNi$, $\rm CrFeCoNi$, and $\rm CrMnFeCoNi$). Yield stress from analytical solute-strengthening models underestimates largely the results in these cases. A possible strategy for increasing strength in multi-component single-phase alloys is the combined design of stacking fault width and element-based chemical disorder.

cond-mat.mtrl-sci↗

arXiv2018

Laning and Clustering Transitions in Driven Binary Active Matter Systems

It is well known that a binary system of non-active disks that experience driving in opposite directions exhibits jammed, phase separated, disordered, and laning states. In active matter systems, such as a crowd of pedestrians, driving in opposite directions is common and relevant, especially in conditions which are characterized by high pedestrian density and emergency. In such cases, the transition from laning to disordered states may be associated with the onset of a panic state. We simulate a laning system containing active disks that obey run-and-tumble dynamics, and we measure the drift mobility and structure as a function of run length, disk density, and drift force. The activity of each disk can be quantified based on the correlation timescale of the velocity vector. We find that in some cases, increasing the activity can increase the system mobility by breaking up jammed configurations; however, an activity level that is too high can reduce the mobility by increasing the probability of disk-disk collisions. In the laning state, the increase of activity induces a sharp transition to a disordered strongly fluctuating state with reduced mobility. We identify a novel drive-induced clustered laning state that remains stable even at densities below the activity-induced clustering transition of the undriven system.

cond-mat.soft↗

arXiv2017

Brittle to Quasi-Brittle Transition and Crack Initiation Precursors in Disordered Crystals

Crack initiation emerges due to a combination of elasticity, plasticity, and disorder, and it is heavily dependent on the material's microstructural details. In this paper, we investigate brittle metals with coarse-grained, microstructural disorder that could originate in a material's manufacturing process, such as alloying. As an investigational tool, we consider crack initiation from a surface, ellipsoidal notch: As the radius of curvature at the notch increases, there is a dynamic transition from notch-induced crack initiation to bulk-disorder crack nucleation. We perform extensive and realistic simulations using a phase-field approach coupled to crystal plasticity. Furthermore, the microstructural disorder and notch width are varied in order to study the transition. We identify this transition for various disorder strengths in terms of the damage evolution. Above the transition, we identify detectable precursors to crack initiation that we quantify in terms of the expected stress drops during mode I fracture loading. We discuss ways to observe and analyze this brittle to quasi-brittle transition in experiments.

cond-mat.mtrl-sci↗

arXiv2014

Statistical properties of Barkhausen noise in amorphous ferromagnetic films

We investigate the statistical properties of the Barkhausen noise in amorphous ferromagnetic films with thicknesses in the range between $100$ and $1000$ nm. From Barkhausen noise time series measured with the traditional inductive technique, we perform a wide statistical analysis and establish the scaling exponents $τ$, $α$, $1/σνz$, and $\vartheta$. We also focus on the average shape of the avalanches, which gives further indications on the domain wall dynamics. Based on experimental results, we group the amorphous films in a single universality class, characterized by scaling exponents $τ\sim 1.27$, $α\sim 1.5$, $1/σνz \sim \vartheta \sim 1.77$, values similar to that obtained for several bulk amorphous magnetic materials. Besides, we verify that the avalanche shape depends on the universality class. By considering the theoretical models for the dynamics of a ferromagnetic domain wall driven by an external magnetic field through a disordered medium found in literature, we interpret the results and identify an experimental evidence that these amorphous films, within this thickness range, present a typical three-dimensional magnetic behavior with predominant short-range elastic interactions governing the domain wall dynamics. Moreover, we provide experimental support for the validity of a general scaling form for the average avalanche shape for non-mean-field systems.

cond-mat.dis-nn↗

arXiv2012

Order and supersymmetry at high filling zero energy states on the triangular lattice

We perform exact diagonalization studies in d=2 dimensions for the Fendley and Schoutens model of hard-core and nearest-neighbor excluding fermions that displays an exact non-relativistic supersymmetry. Using clusters of all possible shapes up to 46 sites, we systematically study the behavior of the ground state phase diagram as a function of filling. We focus on the highly degenerate zero-energy states found at fillings between 1/7 and ~1/5. At the lower end of that interval, at filling 1/7, we explicitly show that the ground states are gapped crystals. Consistent with previous suggestions, we find that the extensive entropy of zero states peaks at a filling of ~0.178. At the higher end of the interval, we find zero energy ground states at fillings above 1/5, in contradiction to previous numerical studies and analytical suggestions; these display non-trivial amplitude degeneracies.

cond-mat.str-el↗

arXiv2008

Quantum dimer models and exotic orders

We discuss how quantum dimer models may be used to provide "proofs of principle" for the existence of exotic magnetic phases in quantum spin systems.

cond-mat.str-el↗