Nanoindentation into a bcc high-entropy HfNbTaTiZr alloy-an atomistic study of the effect of short-range order.

The plastic response of the Senkov HfNbTaTiZr high-entropy alloy is explored by means of simulated nanoindentation tests. Both a random alloy and an alloy with chemical short-range order are investigated and compared to the well understood case of an elementary Ta crystal. Strong differences in the dislocation plasticity between the alloys and the elementary Ta crystal are found. The high-entropy alloys show only little relaxation of the indentation dislocation network after indenter retraction and only negligible dislocation emission into the sample interior. Short-range order-besides making the alloy both stiffer and harder-further increases the size of the plastic zone and the dislocation density there. These features are explained by the slow dislocation migration in these alloys. Also, the short-range-ordered alloy features no twinning plasticity in contrast to the random alloy, while elemental Ta exhibits twinning under high stress but detwins considerably under stress relief. The results are in good qualitative agreement with our current knowledge of plasticity in high-entropy alloys.

An atomistic study of sticking, bouncing, and aggregate destruction in collisions of grains with small aggregates.

Molecular dynamics simulations are used to study central collisions between spherical grains and between grains and small grain aggregates (up to 5 grains). For a model material (Lennard-Jones), grain-grain collisions are sticking when the relative velocity v is smaller than the so-called bouncing velocity and bouncing for higher velocities. We find a similar behavior for grain-aggregate collisions. The value of the bouncing velocity depends only negligibly on the aggregate size. However, it is by 35% larger than the separation velocity needed to break a contact; this is explained by energy dissipation processes during the collision. The separation velocity follows the predictions of the macroscopic Johnson-Kendall-Roberts theory of contacts. At even higher collision velocities, the aggregate is destroyed, first by the loss of a monomer grain and then by total disruption. In contrast to theoretical considerations, we do not find a proportionality of the collision energy needed for destruction and the number of bonds to be broken. Our study thus sheds novel light on the foundations of granular mechanics, namely the energy needed to separate two grains, the difference between grain-grain and grain-aggregate collisions, and the energy needed for aggregate destruction.

Adsorption of Diclofenac and PFBS on a Hair Keratin Dimer.

Environmental pollution by man-made toxic and persistent organic compounds, found throughout the world in surface and groundwater, has various negative effects on aquatic life systems and even humans. Therefore, it is important to develop and improve water treatment technologies capable of removing such substances from wastewater and purifying drinking water. The two substances investigated are the widely used painkiller diclofenac and a member of the class of "forever chemicals", perfluorobutanesulfonate. Both are known to have serious negative effects on living organisms, especially under long-term exposure, and are detectable in human hair, suggesting adsorption to a part of the hair fiber complex. In this study, a human hair keratin dimer is investigated for its ability to absorb diclofenac and perfluorobutanesulfonate. Initial predictions for binding sites are obtained via molecular docking and subjected to molecular dynamics simulations for more than 1 µs. The binding affinities obtained by the linear interaction energy method are high enough to motivate further research on human hair keratins as a sustainable, low-cost, and easily allocatable filtration material.

The effect of collisions on the chemomechanics of ice-covered silica slabs: a molecular dynamics study.

Using molecular dynamics simulation and the REAX potential, we study the collision of two planar silica surfaces covered by water ice. Without the ice cover, the two surfaces stick at all velocities investigated (160-1800 m s-1), due to the formation of chemical bonds between the colliding surfaces. A narrow ice cover - here of thickness 2 nm - prevents the sticking above a characteristic velocity, the bouncing velocity νb. During the collision, reactions occur at the silica-water interface; in particular, water molecules are dissociated and silanols are formed at the surface of the silica slabs. Passivation of the silica surface by H atoms is of little consequence to the magnitude of vb but reduces the number of surface reactions occurring.

Strain-rate-dependent plasticity of Ta-Cu nanocomposites for therapeutic implants.

Recently, Ta/Cu nanocomposites have been widely used in therapeutic medical devices due to their excellent bioactivity and biocompatibility, antimicrobial property, and outstanding corrosion and wear resistance. Since mechanical yielding and any other deformation in the patient's body during treatment are unacceptable in medicine, the characterization of the mechanical behavior of these nanomaterials is of great importance. We focus on the microstructural evolution of Ta/Cu nanocomposite samples under uniaxial tensile loading conditions at different strain rates using a series of molecular dynamics simulations and compare to the reference case of pure Ta. The results show that the increase in dislocation density at lower strain rates leads to the significant weakening of the mechanical properties. The strain rate-dependent plastic deformation mechanism of the samples can be divided into three main categories: phase transitions at the extreme strain rates, dislocation slip/twinning at lower strain rates for coarse-grained samples, and grain-boundary based activities for the finer-grained samples. Finally, we demonstrate that the load transfer from the Ta matrix to the Cu nanoparticles via the interfacial region can significantly affect the plastic deformation of the matrix in all nanocomposite samples. These results will prove useful for the design of therapeutic implants based on Ta/Cu nanocomposites.

Adsorption of Diclofenac and Its UV Phototransformation Products in an Aqueous Solution on PVDF: A Molecular Modeling Study.

The presence of pharmaceuticals in drinking water has generated considerable scientific interest in potential improvements to polymeric membranes for water purification at the nanoscale. In this work, we investigate the adsorption of diclofenac and its ultraviolet (UV) phototransformation products on amorphous and crystalline poly(vinylidene difluoride) (PVDF) membrane surfaces at the nanoscale using molecular modeling. We report binding affinities by determining the free energy landscape via the extended adaptive biasing force method. The high binding affinities of the phototransformation products found are consistent with qualitative experimental results. For diclofenac, we found similar or better affinities than those for the phototransformation products, which seems to be in contrast to the experimental findings. This discrepancy can only be explained if the maximum adsorption density of diclofenac is much lower than that of the products. Overall, negligible differences between the adsorption affinities of the crystalline phases are observed, suggesting that no tuning of the PVDF surfaces is necessary to optimize filtration capabilities.

Spin-lattice-dynamics analysis of magnetic properties of iron under compression.

Compression of a magnetic material leads to a change in its magnetic properties. We examine this effect using spin-lattice dynamics for the special case of bcc-Fe, using both single- and poly-crystalline Fe and a bicontinuous nanofoam structure. We find that during the elastic phase of compression, the magnetization increases due to a higher population of the nearest-neighbor shell of atoms and the resulting higher exchange interaction of neighboring spins. In contrast, in the plastic phase of compression, the magnetization sinks, as defects are created, increasing the disorder and typically decreasing the average atom coordination number. The effects are more pronounced in single crystals than in polycrystals, since the presence of defects in the form of grain boundaries counteracts the increase in magnetization during the elastic phase of compression. Also, the effects are more pronounced at temperatures close to the Curie temperature than at room temperature. In nanofoams, the effect of compression is minor since compression proceeds more by void reduction and filament bending-with negligible effect on magnetization-than by strain within the ligaments. These findings will prove useful for tailoring magnetization under strain by introducing plasticity.

Simulated nanoindentation into single-phase fcc Fe[Formula: see text]Ni[Formula: see text] alloys predicts maximum hardness for equiatomic stoichiometry.

We investigate by molecular dynamics simulation the mechanical behavior of concentrated alloys under nanoindentation for the special example of single-phase fcc Fe[Formula: see text]Ni[Formula: see text] alloys. The indentation hardness is maximum for the equiatomic alloy, [Formula: see text]. This finding is in agreement with experimental results on the strength of these alloys under uniaxial strain. We explain this finding with the increase of the unstable stacking fault energy in the alloys towards [Formula: see text]. With increasing Fe content, loop emission from the plastic zone under the indenter becomes less pronounced and the plastic zone features a larger fraction of screw dislocation segments; simultaneously, the length of the dislocation network and the number of atoms in the stacking faults generated in the plastic zone increase. However, the volume of twinned regions in the plastic zone is highest for the elemental solids and decreases for the alloys. This feature is explained by the fact that twinning proceeds by the glide of dislocations on adjacent parallel lattice planes; this concerted motion is less efficient in the alloys. Finally, we find that surface imprints show increasing pile-up heights with increasing Fe content. The present results will be of interest for hardness engineering or generating hardness profiles in concentrated alloys.

A granular mechanics model study of the influence of non-spherical shape on aggregate collisions.

Collisions between granular aggregates influence the size distribution of dust clouds. Granular aggregates may possess non-spherical shapes as a result of, for instance, previous collision processes. Here, we study aggregate collisions using a granular mechanics simulation code. Collisions between spherical aggregates are compared to collisions of ellipsoidal aggregates of equal mass. As the most prominent result, we find that the growth velocity, i.e., the velocity above which the post-collision aggregates are smaller than before collision, is generally reduced for ellipsoidal aggregates. The reason hereto lies in the less compact structure of ellipsoids which allows for a larger degree of fragmentation in a 'rim peel-off' mechanism. On the other hand, relative fragment distributions are only little influenced by aggregate shape.

A nanodispersion-in-nanograins strategy for ultra-strong, ductile and stable metal nanocomposites.

Nanograined metals have the merit of high strength, but usually suffer from low work hardening capacity and poor thermal stability, causing premature failure and limiting their practical utilities. Here we report a "nanodispersion-in-nanograins" strategy to simultaneously strengthen and stabilize nanocrystalline metals such as copper and nickel. Our strategy relies on a uniform dispersion of extremely fine sized carbon nanoparticles (2.6 ± 1.2 nm) inside nanograins. The intragranular dispersion of nanoparticles not only elevates the strength of already-strong nanograins by 35%, but also activates multiple hardening mechanisms via dislocation-nanoparticle interactions, leading to improved work hardening and large tensile ductility. In addition, these finely dispersed nanoparticles result in substantially enhanced thermal stability and electrical conductivity in metal nanocomposites. Our results demonstrate the concurrent improvement of several mutually exclusive properties in metals including strength-ductility, strength-thermal stability, and strength-electrical conductivity, and thus represent a promising route to engineering high-performance nanostructured materials.

Collisions between CO, CO[Formula: see text], H[Formula: see text]O and Ar ice nanoparticles compared by molecular dynamics simulation.

Molecular dynamics simulations are used to study collisions between amorphous ice nanoparticles consisting of CO, CO[Formula: see text], Ar and H[Formula: see text]O. The collisions are always sticking for the nanoparticle size (radius of 20 nm) considered. At higher collision velocities, the merged clusters show strong plastic deformation and material mixing in the collision zone. Collision-induced heating influences the collision outcome. Partial melting of the merged cluster in the collision zone contributes to energy dissipation and deformation. Considerable differences exist-even at comparable collision conditions-between the ices studied here. The number of ejecta emitted during the collision follows the trend in triple-point temperatures and increases exponentially with the NP temperature.

Bouncing and spinning of amorphous Lennard-Jones nanoparticles under oblique collisions.

Collisions of Lennard-Jones nanoparticles (NPs) may be used to study the generic collision behavior of NPs. We study the collision dynamics of amorphous NPs for oblique collisions using molecular dynamics simulation as a function of collision velocity and impact parameter. In order to allow for NP bouncing, the attraction between atoms originating from differing NPs is reduced. For near-central collisions, a finite region of velocities - a 'bouncing window' - exists where the 2 NPs bounce from each other. At smaller velocities, energy dissipation and - at larger velocities - also NP deformation do not allow the NPs to surpass the attractive forces such that they stick to each other. Oblique collisions of non-rotating NPs convert angular momentum into NP spin. For low velocities, the NP spin is well described by assuming the NPs to come momentarily to a complete stop at the contact point ('grip'), such that orbital and spin angular momentum share the pre-collision angular momentum in a ratio of 5:2. The normal coefficient of restitution increases with impact parameter for small velocities, but changes sign for larger velocities where the 2 NPs do not repel but their motion direction persists. The tangential coefficient of restitution is fixed in the 'grip' regime to a value of 5/7, but increases towards 1 for high-velocity collisions at not too small impact parameters, where the 2 NPs slide along each other.

Molecular dynamics of rolling and twisting motion of amorphous nanoparticles.

Granular mechanics codes use macroscopic laws to describe the damping of rolling and twisting motion in granular ensembles. We employ molecular dynamics simulation of amorphous Lennard-Jones grains to explore the applicability of these laws for nm-sized particles. We find the adhesive force to be linear in the intergrain attraction, as in the macroscopic theory. However, the damping torque of rolling motion is strongly superlinear in the intergrain attraction. This is caused by the strong increase of the 'lever arm' responsible for the damping torque-characterizing the asymmetry of the adhesive neck during rolling motion-with the surface energy of the grains. Also the damping torque of twisting motion follows the macroscopic theory based on sliding friction, which predicts the torque to increase whit the cube of the contact radius; here the dynamic increase of the contact radius with angular velocity is taken into account.

Strength of Graphene-Coated Ni Bi-Crystals: A Molecular Dynamics Nano-Indentation Study.

Nanoindentation simulations are performed for a Ni(111) bi-crystal, in which the grain boundary is coated by a graphene layer. We study both a weak and a strong interface, realized by a 30 ∘ and a 60 ∘ twist boundary, respectively, and compare our results for the composite also with those of an elemental Ni bi-crystal. We find hardening of the elemental Ni when a strong, i.e., low-energy, grain boundary is introduced, and softening for a weak grain boundary. For the strong grain boundary, the interface barrier strength felt by dislocations upon passing the interface is responsible for the hardening; for the weak grain boundary, confinement of the dislocations results in the weakening. For the Ni-graphene composite, we find in all cases a weakening influence that is caused by the graphene blocking the passage of dislocations and absorbing them. In addition, interface failure occurs when the indenter reaches the graphene, again weakening the composite structure.

Bouncing of Hydroxylated Silica Nanoparticles: an Atomistic Study Based on REAX Potentials.

Clean silica surfaces have a high surface energy. In consequence, colliding silica nanoparticles will stick rather than bounce over a wide range of collision velocities. Often, however, silica surfaces are passivated by adsorbates, in particular water, which considerably reduce the surface energy. We study the effect of surface hydroxylation on silica nanoparticle collisions by atomistic simulation, using the REAX potential that allows for bond breaking and formation. We find that the bouncing velocity is reduced by more than an order of magnitude compared to clean nanoparticle collisions.

Boron nitride nanotubes as containers for targeted drug delivery of doxorubicin.

Using molecular dynamics simulations, the adsorption and diffusion of doxorubicin drug molecules in boron nitride nanotubes are investigated. The interaction between doxorubicin and the nanotube is governed by van der Waals attraction. We find strong adsorption of doxorubicin to the wall for narrow nanotubes (radius of 9 Å). For larger radii (12 and 15 Å), the adsorption energy decreases, while the diffusion coefficient of doxorubicin increases. It does, however, not reach the values of pure water, as adsorption events still hinder the doxorubicin mobility. It is concluded that nanotubes wider than around 4 nm diameter can serve as efficient drug containers for targeted drug delivery of doxorubicin in cancer chemotherapy.

Molecular dynamics simulations of the mechanical behavior of alumina coated aluminum nanowires under tension and compression.

For materials with high oxygen affinity, oxide layers will significantly change the material properties. This is of particular importance for aluminum nanowires which have many applications because of their ultrahigh strengths. Recent studies show that thin amorphous oxide shell layers on aluminum surfaces significantly change the responses of the material. However, the relations between the thickness of the oxidized layer, the strain rate and the mechanical response of nanowires to compression and tension have not been investigated intensively. In this study, we use a ReaxFF potential to analyze the influences of oxide shell layers on the material responses of the nanowires under uniaxial tension and compression at different strain rates. The Al-O interface leads to an increased defect nucleation rate at the oxide interface preventing localized deformation. During tension, we observe a reorganization of the structure of the oxide layer leading to bond healing and preventing fracture. While ductility is increasing with coating thickness during tension, the thickness of the coating is less decisive during compression.

The Influence of Lubrication and the Solid-Fluid Interaction on Thermodynamic Properties in a Nanoscopic Scratching Process.

Liquid lubricants play an important role in contact processes; for example, they reduce friction and cool the contact zone. To gain better understanding of the influence of lubrication on the nanoscale, both dry and lubricated scratching processes in a model system are compared in the present work using molecular dynamics simulations. The entire range between total dewetting and total wetting is investigated by tuning the solid-fluid interaction energy. The investigated scratching process consists of three sequential movements: A cylindrical indenter penetrates an initially flat substrate, then scratches in the lateral direction, and is finally retracted out of the contact with the substrate. The indenter is fully submersed in the fluid in the lubricated cases. The substrate, the indenter, and the fluid are described by suitably parametrized Lennard-Jones model potentials. The presence of the lubricant is found to have a significant influence on the friction and on the energy balance of the process. The thermodynamic properties of the lubricant are evaluated in detail. A correlation of the simulation results for the profiles of the temperature, density, and pressure of the fluid in the vicinity of the chip is developed. The work done by the indenter is found to mainly dissipate and thereby heat up the substrate and eventually the fluid. Only a minor part of the work causes plastic deformation of the substrate.