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Mechanisms governing the tribological behavior of polymer-on-polymer sliding were investigated by molecular dynamics simulations. Three main mechanisms governing frictional behavior were identified. Interfacial “brushing” of molecular chain ends over one another was observed as the key contribution to frictional forces. With an increase of the sliding speed, fluctuations in frictional forces reduced in both magnitude and periodicity, leading to dynamic frictional behavior. While “brushing” remained prevalent, two additional irreversible mechanisms, “combing” and “chain scission”, of molecular chains were observed when the interfaces were significantly diffused. [ABSTRACT FROM AUTHOR]

Molecular simulations of the sliding processes of polymer-on-polymer systems were performed to investigate the surface and subsurface deformations and how these affect tribological characteristics of nanometer-scale polymer films. It is shown that a very severe deformation is localized to a band of material about 2.5 nm thick at the interface of the polymer surfaces. Outside of this band, the polymer films experience a uniform shear strain that reaches a finite steady-state value of close to 100%. Only after the polymer films have achieved this steady-state shear strain do the contacting surfaces of the films show significant relative slippage over each other. Because severe deformation is limited to a localized band much thinner than the polymeric films, the thickness of the deformation band is envisaged to be independent of the film thickness and hence frictional forces are expected to be independent of the thickness of the polymer films. A strong dependency of friction on interfacial adhesion, surface roughness, and the shear modulus of the sliding system was observed. Although the simulations showed that frictional forces increase linearly with contact pressure, adhesive forces contribute significantly to the overall friction and must therefore be accounted for in nanometer-scale friction. It is also shown that the coefficient of friction is lower for lower-density polymers as well as for polymers with higher molecular weights. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations of ZnO nanowires under tensile loading were performed and compared with simulations of TiO2wires to present size-dependent mechanical properties and super ductility of metal oxide wires. It is shown that while large surface-to-volume ratio is responsible for their size effects, ZnO and TiO2wires displayed opposite trends. Although the stiffness of both wires converged monotonically to their bulk stiffness values as diameter increases, bulk stiffness represented the upper bound for ZnO nanowires as opposed to the lower bound for TiO2wires. ZnO nanowires relaxed to either completely amorphous or completely crystalline states depending on wire thickness, whereas a thin amorphous shell is always present in TiO2nanowires. It was also found that when crystalline ZnO nanowires are stretched, necking initiated at localized amorphous regions to eventually form single-atom chains which can sustain strains above 100%. Such large elongations are not observed in TiO2nanowires. Using the analogy of a clothesline, an explanation is offered for the necessary conditions leading to super ductility. [ABSTRACT FROM AUTHOR]

The mechanisms governing the tensile behavior of TiO2nanowires were studied by molecular dynamics simulations. Nanowires below a threshold diameter of about 10 Å transformed into a completely disordered structure after thermodynamic equilibration, whereas thicker nanowires retained their crystalline core. Initial elastic tensile deformation was effected by the reconfiguration of surface atoms while larger elongations resulted in continuous cycles of Ti−O bond straightening, bond breakage, inner atomic distortion, and necking until rupture. Nanowires have much better mechanical properties than bulk TiO2. Nanowires below the threshold diameter exhibit extraordinarily high stiffness and toughness and are more sensitive to strain rate. [ABSTRACT FROM AUTHOR]