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We show that discrete detonation chemistry can be studied using molecular dynamics simulations. A model 2D semi-infinite energetic molecular solid described by reactive many-body potentials is shown to support a chemically sustained shock wave with properties that are consistent with experimental results and the classic continuum theory of planar detonations. These promising results demonstrate for the first time that simulations using reactive many-body potentials provide a powerful probe of the interplay between the continuum properties of shock waves and the atomic-scale chemistry they induce in condensed-phase detonations.PACS numbers: 82.40.Py, 47.40.Nm, 62.50.+p Solid explosives rest quietly in their metastable states but when struck can undergo rapid exothermic chemical reactions, often with catastrophic results. Once begun, a detonation propagates as a shock wave inducing the exothermic chemical reactions that sustain it [1]. This shock front separates the unreacted material by only a few lattice spacings from the shocked material which typically experiences pressures of hundreds of kilobars, while flowing at velocities of several kilometers per second [1,2]. Propagating as a shock wave, a detonation consumes the explosive at velocities several times the speed of sound in the quiescent material resulting in the release of chemical energy at rates that can exceed 10 n W for a 10 cm 2 detonation front [2].Processes at condensed-phase shock fronts can occur on such short time (subpicosecond) and length (subnanometer) scales that they are ideal for classical molecular dynamics (MD) simulations [3-5] which follow individual atomic trajectories. Although starting from an atomicscale description, MD simulations have also proven able to treat enough atoms for long enough times to describe continuum properties of planar shock waves in nonenergetic materials [3] -including such complex hydrodynamic behavior as shock-wave splitting caused by a polymorphic phase transition [4]. Therefore, MD simulations hold great promise both for studying discrete shock-induced chemistry in energetic materials [5] and for directly relating this atomic-scale chemistry to the continuum properties of planar detonations successfully described by the hydrodynamic theory of compressive reactive flows [1,2]. A better understanding of the relationship between atomic-scale chemistry caused by shock waves and the continuum properties of condensed-phase detonations could aid in the design of safer, more reliable explosives.MD simulations of chemically sustained shock waves, however, require new many-body potentials capable of simultaneously following the dynamics of thousands of atoms in a rapidly changing environment, while including the possibility of exothermic chemical reactions proceeding along chemically reasonable reaction paths from cold solid-state reactants to hot gas-phase molecular products.In addition, for a molecular solid (which is typical of many energetic materials), these potentials must incorporate both the strong intramo...

We have used molecular dynamics simulations to examine friction when two diamond (1 11) surfaces are placed in sliding contact. We find that flexible hydrocarbon species, chemically bound to the diamond surface, can lead to a significant reduction of the calculated friction at high loads. In addition to clarifying the effects of such species on atomic-scale friction at diamond interfaces, these simulations might also yield insight into more complicated systems, e.g., Langmuir-Blodgett films, and aid in the design of low-friction coatings.The friction and wear of surfaces are two of the more costly problems facing industry today. Liquid lubricants and boundary layer additives are widely used to reduce wear and increase equipment lifetime. There are, however, many applications, such as those dealing with the vacuum of outer space and those at high or low temperatures, where liquid lubricants cannot be used. As a result, some solid-state materials, such as molybdenumdisulfide and diamond, that can be deposited as thin films have become important tribological materials.Diamond and diamond films deposited by chemical vapor deposition (CVD) show relatively low friction and wear.14 While inroads have been made into the qualitative understanding of friction and wear on the macroscopic scale,M little is known about them on the atomic scale. Recent advances in scientific instrumentation have allowed, for the first time, the study of atomic-scale friction and wear leading to the emergence of a new field called nanotribology.7 For example, the surface force apparatus has been used to study the rheology of molecularly thin liquid 1ayers,b11 a quartz crystal microbalance has been used to measure the sliding friction of molecularly thin adsorbed films,1213 and the atomic force microscope (AFM) has been used to measure the frictional force between a sharp tip (possibly a single asperity) and a flat surface during ~liding.16~~ These innovative experiments have also stimulated theoretical work. For instance, atomic-scale friction has been investigated com-* To whom corrtepondencc should bc addressed.0022-3654 f 93 f 2O91-6513$Q4.QO f 0 putationally using analytical models,2O-23 first principles calculations,u-26 and molecular d~n a m i c s .~~-'~ In this letter, we use molecular dynamics simulations to investigate the atomic-scale friction between two diamond (1 11) surfaces in sliding contact. Our previous work focused on the behavior of the friction coefficient, 1.

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