Pair vs many-body potentials: Influence on elastic and plastic behavior in nanoindentation of fcc metals

Molecular-dynamics simulation can give atomistic information on the processes occurring in nanoindentation experiments. In particular, the nucleation of dislocation loops, their growth, interaction and motion can be studied. We investigate how realistic the interatomic potentials underlying the simulations have to be in order to describe these complex processes. Specifically we investigate nanoindentation into a Cu single crystal. We compare simulations based on a realistic many-body interaction potential of the embedded-atom-method type with two simple pair potentials, a Lennard-Jones and a Morse potential. We find that qualitatively many aspects of nanoindentation are fairly well reproduced by the simple pair potentials: elastic regime, critical stress and indentation depth for yielding, dependence on the crystal orientation, and even the level of the hardness. The quantitative deficits of the pair potential predictions can be traced back: (i) to the fact that the pair potentials are unable in principle to model the elastic anisotropy of cubic crystals and (ii) as the major drawback of pair potentials we identify the gross underestimation of the stable stacking fault energy. As a consequence these potentials predict the formation of too large dislocation loops, the too rapid expansion of partials, too little cross slip and in consequence a severe overestimation of work hardening.     

Application and comparison of algorithms for the evaluation of interferograms

Organic molecules are easily absorbed by polymers. This interaction induces changes in the volume and the refractive index of the resulting mixture phase. The change in the bulk refractive index is directly related to the volume fraction of the organic substance in the resulting mixture. It is essential for an understanding of the underlying phenomena to study the change in volume and the change in refractive index separately. A novel method is presented to separate both effects by white light interferometric measurements on thin polymer films. This method is based on a spectral fitting of amplitude and phase shift effects in the interferograms.Dedicated to Professor K. Doerffel on the occasion of his 70th birthday     

Effect of material stiffness on hardness: A computational study based on model potentials

We investigate the dependence of the hardness of materials on their elastic stiffness. This is possible by constructing a series of model potentials of Morse type; starting with modelling natural Cu, the model potentials exhibit an increased elastic modulus, while keeping all other potential parameters (lattice constant, bond energy) unchanged. Using molecular-dynamics simulation, we perform nanoindentation experiments on these model crystals. We find that the crystal hardness scales with the elastic stiffness. Also the load drop, which is experienced when plasticity sets in, increases in proportion to the elastic stiffness, while the yield point, i.e. the indentation at which plasticity sets in, is independent of the elastic stiffness.     

In-situ characterization of thin polymer films for applications in chemical sensing of volatile organic compounds by spectroscopic ellipsometry

Polymer coatings are applied in many kinds of chemical sensors. The interaction with organic vapours changes the physical properties of the coating material. For optical sensors, changes in the coating volume and the complex refractive index are most important. Spectroscopic ellipsometry has been applied for the first time to the in-situ characterization of thin poly(dimethylsiloxane) films in contact with tetrachloroethene, toluene and cyclohexane vapours. The differences in bulk refractive index between organic solvent and polymer are large for toluene and tetrachloroethene and both effects were studied separately. Cyclohexane has a bulk refractive index very close to the investigated poly(dimethylsiloxane) films. Therefore the calculation of the volume fraction of the analyte in the mixture phase with the polymer is subject to large errors for vapour concentrations below 5000 ppm. Received: 8 December 1995 / Revised: 5 March 1996 / Accepted: 6 March 1996     

Why nanoprojectiles work differently than macroimpactors: the role of plastic flow

Atomistic simulation data on crater formation due to the hypervelocity impact of nanoprojectiles of up to 55 nm diameter and with targets containing up to 1.1×10(10) atoms are compared to available experimental data on μm-, mm-, and cm-sized projectiles. We show that previous scaling laws do not hold in the nanoregime and outline the reasons: within our simulations we observe that the cratering mechanism changes, going from the smallest to the largest simulated scales, from an evaporative regime to a regime where melt and plastic flow dominate, as is expected in larger microscale experiments. The importance of the strain-rate dependence of strength and of dislocation production and motion are discussed.     

Periodic orbit analysis of the Lorenz attractor

We present an application of the periodic orbit formalism to the Lorenz attractor at the standard parameter values. The symbolic encoding of trajectories, the effects of symmetries and scaling properties of trajectories are discussed. Good results for the Hausdorff dimension and the Lyapunov exponent are obtained. The classical spectral density is computed and positions and widths of resonances are compared with those found in correlation functions.     

On the Schrödinger equation with a time dependent mass parameter: Nuclear fragmentation theory

We study the dynamics of a system with a coordinate-dependent inertia tensorB _ik (x ₁,...,x _n ), in which one of the coordinates is treated classically,x _j →x _j (t), so that the associated mass parameter becomes time-dependent. As a result, the Hamiltonian must contain a pure imaginary term in order that probability be conserved. The nuclear fragmentation theory is an example of such a system. The inertia parameter associated with the mass fragmentation degree of freedom depends on the relative motion coordinate and, hence, on time. The time dependence of the imaginary term is derived from a fully quantum mechanical treatment, by going to the classical limit in the relative motion variable. A connection is made with the closely related situation which arises if one transforms from an inertial system to one which depends non-linearly on the time.