tailieunhanh - Modern Tribology Handbook 2010 Part 8

Tham khảo tài liệu 'modern tribology handbook 2010 part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | commensurate surfaces in Figure . One may expect that the friction would decrease if incommensurate walls were used. However the rolling motion of methane might be prevented in such geometries. Perry and Harrison 1996 1997 compared their results to earlier simulations Harrison et al. 1993 where hydrogen terminations on one of the two surfaces were replaced by chemisorbed methyl -CH3 ethyl -C2H5 or tt-propyl -C3H7 groups Section . The chemisorbed molecules were seen to give a considerably smaller reduction of the friction with respect to the physisorbed molecules. Perry and Harrison note that the chemisorbed molecules have fewer degrees of freedom and so are less able to avoid collisions that dissipate energy. Corrugated Surfaces The presence of roughness can be expected to alter the behavior of lubricants particularly when the mean film thickness is comparable to the surface roughness. Gao et al. 1995 1996 and Landman et al. 1996 used molecular dynamics to investigate this thin film limit. Hexadecane tt-C16H34 was confined between two gold substrates exposing only 111 surfaces Figure . The two outer layers of the substrates were completely rigid and were displaced laterally with a constant relative velocity of 10 to 20m s at constant separation. The asperities on both walls were modeled by flat-topped pyramidal ridges with initial heights of 4 to 6 atomic layers. The united atom model Section was used for the interactions within the film and the embedded atom method was used for Au-Au interactions. All other interactions were modeled with suitable 6-12 Lennard-Jones potentials. The alkane molecules and the gold atoms in the asperities were treated dynamically using the Verlet algorithm. The temperature was kept constant at T 350 K by rescaling the velocities every 50th time step. Simulations were done in three different regimes which can be categorized according to the separation Ahaa that the outer surfaces of the asperities would

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