The carbon nano-onion can be considered as a new kind of interesting lubricating nanoparticle. Used as lubricant additives, carbon nano-onions lead to a strong reduction of both friction and wear, even at low temperature. To better elucidate the mechanisms by which these processes occur, coupled experimental and computational investigations are carried out. In addition, it is found that lubricious iron oxide nanoparticles are generated in the core of the steel contact through mechanisms that are not yet known. The molecular dynamics simulations of carbon onions placed between sliding diamond-like carbon surfaces at high contact pressure indicate that the lubrication mechanism of the onions is based on a coupled process of rolling and sliding inside the contact area. We conclude that most of carbon onions seem to remain intact under friction processes and do not generate graphitic planes, which is in contrast to the previously determined behavior of MoS 2 fullerenes that are mainly exfoliated inside the contact area and liberate lubricating lamellar sheets of h-MoS 2 .

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Molecular dynamics simulations and experimental measurements were used to investigate the thermal and mechanical properties of cross-linked phenolic resins as a function of the degree of cross-linking, the chain motif (ortho−ortho versus ortho−para), and the chain length. The chain motif influenced the type (interchain or intrachain) as well as the amount of hydrogen bonding. Ortho− ortho chains favored internal hydrogen bonding whereas ortho−para favored hydrogen bonding between chains. Un-cross-linked ortho− para systems formed percolating 3D networks of hydrogen bonds, behaving effectively as "hydrogen gels". This resulted in differing thermal and mechanical properties for these systems. As crosslinking increased, the chain motif, chain length, and hydrogen bonding networks became less important. Elastic moduli, thermal conductivity, and glass transition temperatures were characterized as a function of cross-linking and temperature. Both our own experimental data and literature values were used to validate our simulation results. ■ INTRODUCTIONThermosetting resins, such as phenolic, polycyanurates, epoxies, and polyimides, have numerous applications as adhesives, coatings, and constituents for composite materials. The irreversible three-dimensional cross-linked networks formed during cure distinguish them from their thermoplastic analogues. Cross-linked structures result in stiffer mechanical properties even at elevated temperatures. Phenolic resins in particular are an important component of ablative thermal protection materials due to their high strength, low thermal conductivity, and high char yield. Ablative composites are stateof-the-art heat shield materials that protect space vehicles from extreme atmospheric entry conditions; examples of this class of materials include PICA 1−3 and Carbon Phenolic. 4,5 Experimental characterization of phenolic resins has spanned many decades due to their versatile applications in industry, 6,7 academics, 8−11 and government. 12−16 Numerous experimental results on isolated phenolic resins as well as in composites have been published to understand their characteristics and properties as a function of cure, 17−22 processing, 23−27 and composite design. 10,28−33 In addition, experimental properties such as the coefficients of thermal expansion, 12,20,34 thermal conductivity, 18,19,32 and elastic moduli 15,35,36 have been reported. Phenolic resins also have disadvantages, however, which include void formation and shrinkage that occur during processing as well as its brittle nature when highly cured. Recent experimental work has shown that thermomechanical properties can be improved substantially by introducing additives and varying processing conditions. 2,3 Understanding the relationship between chemical structures, properties, and processing will lead to improved, high-performance resins for this important class of materials.Advanced simulation studies are expected to play an important role in complementing and guiding experimental design for these systems. Recently, Li ...

Inorganic fullerene-like (IF) nanoparticles made of metal dichalcogenides have previously been recognized to be good friction modifiers and anti-wear additives under boundary lubrication conditions. The tribological performance of these particles appears to be a result of their size, structure and morphology, along with the test conditions. However, the very small scale of the IF nanoparticles makes distinguishing the properties which affect the lubrication mechanism exceedingly difficult. In this work, a high resolution transmission electron microscope equipped with a nanoindentation holder is used to manipulate individual hollow IF-WS(2) nanoparticles and to investigate their responses to compression. Additional atomistic molecular dynamics (MD) simulations of similarly structured, individual hollow IF-MoS(2) nanoparticles are performed for compression studies between molybdenum surfaces on their major and minor axis diameters. MD simulations of these structures allows for characterization of the influence of structural orientation on the mechanical behavior and nano-sheet exfoliation of hollow-core IF nanoparticles. The experimental and theoretical results for these similar nanoparticles are qualitatively compared.

The mechanical behavior of different types of single-walled and double-walled MoS2 nanotubes when subjected to external compressive, tensile, and torsional loading is investigated using classical molecular dynamics simulations. The forces on the atoms are determined using a reactive empirical bond-order potential parameterized for Mo-S systems. The simulations report on the elastic properties of the different MoS2 nanotube systems as well as the interrelationships between the buckling behavior and the structural parameters of the nanotubes, such as length, diameter, chirality, and number of walls. The simulations predict that the most important factor influencing mechanical response is the number of walls present and, to a lesser extent, the diameters of the nanotubes, with the other structural parameters predicted to have little effect on the results over the range investigated. These findings are consistent with reported density functional theory calculations and experimental data for WS2 and MoS2 nanotubes.