A molecular dynamics study of nanoindentation on a methyl methacrylate ultrathin film on a Au(111) substrate: interface and thickness effects

A novel degradation mechanism of the elastic modulus of wet polymer substrates under nanoindentation

We demonstrated that the formation and solidification of a continuous confined water film played a very important role in changing the elastic modulus of the wet polymer substrate in a nanoindentation process by a coarse-grained molecular dynamics simulation of this process. It was found that as the water content increased, the elastic modulus of the wet polymer substrate showed a non-monotonic change. Relative to the dry polymer substrate, the elastic modulus of the wet polymer first decreased. This is because the appearance of a confined water film caused the force between the polymer substrate and the indenter to change from repulsion to attraction. Subsequently, as the confined water film gradually solidified and then weakened, the elastic modulus of the wet polymer slowly increased and then rapidly increased due to a large number of interstitial water molecules gradually penetrating the polymer substrate. Therefore, it is unreasonable to explain the wet polymer degradation during nanoindentation only from the plasticization and anti-plasticization effects based on the hydrogen bond breaking and formation during stretching. The above-mentioned results will help to more comprehensively understand the degradation mechanism of the polymers' encounter with water, thus promoting further practical applications for polymers.

Atomistic simulations on nanoimprinting of copper by aligned carbon nanotube arrays under a high-frequency mechanical vibration

Nanoimprinting behaviors of copper substrates and double-walled carbon nanotubes with interwall sp ³ bonds are investigated using molecular dynamics simulations. A high-frequency mechanical vibration with various amplitudes is applied on the carbon nanotube (CNT) mold and copper substrate in different directions. Results show that exciting mechanical resonances both on the CNT and substrate drastically decrease the maximum imprint force and interfacial friction up to 50% under certain amplitudes. Meanwhile, it is demonstrated that defects occur in the {111} plane in the copper substrate during nanoimprinting. For different CNT array densities, a higher grafting density needs more imprint force to transfer patterns. The maximum imprint force for a large range of CNT array densities can be reduced by vibrational perturbations, while reduction rates depend on the CNT grafting density. This work sheds deep insights into the nanoimprint process at the atomic level, suggesting that vibration perturbation is an effective approach for improving the nanoimprinting accuracy and preventing the fracture of nanopatterns.

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.

Nanomechanics of RDX Single Crystals by Force-Displacement Measurements and Molecular Dynamics Simulations

Nanoenergetic material modifications for enhanced performance and stability require an understanding of the mechanical properties and molecular structure-property relationships of materials. We investigate the mechanical and tribological properties of single-crystal hexahydro-1,3,5-trinitro-s-triazine (RDX) by force-displacement microscopy and molecular dynamics (MD). Our MD simulations reveal the RDX reduced modulus (Er) depends on the particular crystallographic surface. The predicted Er values for the respective (210) and (001) surfaces are 26.8 and 21.0 GPa. Further, our simulations reveal a symmetric and fairly localized deformation occurring on the (001) surface compared to an asymmetric deformation on the (210) surface. The predicted hardness (H) values are nearly equal for both surfaces. The predicted Er and H values are ∼33% and 17% greater than the respective experimental values of 0.798 ± 0.030 GPa and 22.9 ± 0.7 GPa for the (210) surface and even larger than those reported previously. Our experimental H and Er values are ∼19% and 9% greater than those reported previously for the (210) surface. The difference between the experimental values reported here and elsewhere stems in part from an inaccurate determination of the contact area. We employ the parameter √H/Er, which is independent of area, as a means to compare present and past results, and find excellent agreement, within a few percent, between our predicted and experimental results and between our results and those obtained from previous nanoindentation experiments. Also, we performed nanoscratch simulations of the (210) and (001) surfaces and nanoscratch tests on the (210) surface and present values of the dynamic coefficient of deformation friction.

Analysis of local properties during a scratch test on a polymeric surface using molecular dynamics simulations

This work demonstrates a possible route to connect a particle (chain) based understanding with continuum mechanical questions about contact mechanics. The bond orientation, chain conformation and stress field of a polymer film were analyzed during scratch tests (tangential contact) using a molecular dynamics (MD) simulation approach. Scratch tests with a conical tip at constant scratching velocity were simulated on linear amorphous polymer surfaces at various temperatures and roughnesses of the tip and for various interactions between the tip and the particles of the polymer chains. The second Legendre polynomial (computed for small domains around the tip) gave the bond orientation inside the polymer film during sliding of the tip. The gyration tensor (layer-resolved in the direction of the polymer film thickness) provided information about the conformation of the polymer chains. These results allowed us to argue in favor of Briscoe's hypothesis (thin film sheared vs. "bulk" compressive behavior) concerning the friction properties of the polymer surfaces. Finally, the first stress measurements of the virial stress tensor (in sub-boxes placed in the MD cell) revealed a complex combination between compressive hydrostatic pressure and shear stress, which may be interpreted as a complex sheared domain at the interface.

Determination of Interphase Thickness and Mechanical Properties of Effective Nanofillers in Polymer Nanocomposites by Molecular Dynamic Simulation

The properties of interphase in polymer composites are often different from those of bulk polymer matrix, which may include chemical, physical, microstructural, and mechanical properties. The nature of interphase is critical to the overall properties and performance of polymer materials, in particular in nanofiller reinforced composites. Experimental efforts have been made to determine the effective interphase thickness and its properties, for example, by nanoindentation and nanoscratch techniques. Yet, it is very difficult to quantify the interphase and its properties because of its nanoscale nature and the unclear boundary. In this regard, computer simulation, e.g., molecular dynamics, provides an effective tool to characterize such interphase and the properties. In this work, molecular dynamics simulations are applied to quantify the interphase thickness in clay-based polymer nanocomposites. Then, the mechanical properties of the so-called effective nanofiller (i.e., the physical size of nanofiller plus the thickness of interphase) will be determined by a series of simulations.