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Research Papers: Tribochemistry & Tribofilms

Quantification of the Surface Morphology of Lubricant Films With Polar End Groups Using Molecular Dynamics Simulation: Periodic Changes in Morphology Depending on Film Thickness

[+] Author and Article Information
Susumu Ogata1

Department of Electronic-Mechanical Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

Hedong Zhang

Department of Complex System Science, Nagoya University, Furo-cho Chikusa-ku, Nagoya 464-8601, Japanzhang@nuem.nagoya-u.ac.jp

Kenji Fukuzawa

Department of Micro-Nano Systems Engineering, Nagoya University, Furo-cho Chikusa-ku, Nagoya 464-8603, Japanfukuzawa@nuem.nagoya-u.ac.jp

Yasunaga Mitsuya2

Department of Micro-Nano Systems Engineering, Nagoya University, Furo-cho Chikusa-ku, Nagoya 464-8603, Japanmitsuya@nuem.nagoya-u.ac.jp

1

Present address: Fujitsu Laboratories Ltd., 10-1 Morinosato-Wakamiya, Atsugi 243-0197, Japan. E-mail: ogata.susumu@jp.fujitsu.com

2

Corresponding author.

J. Tribol 130(2), 022301 (Mar 13, 2008) (9 pages) doi:10.1115/1.2842297 History: Received January 09, 2007; Revised January 06, 2008; Published March 13, 2008

Using a coarse-grained molecular dynamics simulation based on the bead-spring polymer model, we reproduced the film distribution of molecularly thin lubricant films with polar end groups coated on the disk surface and quantified the film-surface morphology using a molecular-probe scanning method. We found that the film-surface morphology changed periodically with increasing film thickness. The monolayer of a polar lubricant that entirely covers the solid surface provides a flat lubricant surface by exposing its nonpolar backbone outside of the monolayer. By increasing film thickness, the end beads aggregate to make clusters, and bulges form on the lubricant surface, accompanying an increase in surface roughness. The bulges continue to grow even though the averaged film thickness reaches or exceeds the bilayer thickness. With further increases in film thickness, the clusters start to be uniformly distributed in the lateral direction to clearly form a third layer. As for the formation of fourth and fifth layers, the process is basically the same as that for the second and third layers. Through our calculations of the intermolecular potential field and the intermolecular force field, these values are found to change periodically and are synchronized with the formation of molecule aggregations, which explains the mechanism of forming the layered structure that is inherent to a polar lubricant.

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Copyright © 2008 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Bead-spring polymer model, interactions between beads and those between the beads and the surface

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Figure 2

Calculation volume

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Figure 3

Calculation procedure: (a) initial bead allocation and (b) equilibrated film

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Figure 4

Snapshot of surface morphology calculated from the molecular-probe scanning method

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Figure 5

Snapshots of molecular distribution for the Zdol2000 model after 160,000 steps

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Figure 6

Relationship between polymer number and average film thickness

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Figure 7

Lubricant surface morphology depending on average film thickness t for thinner film cases (<1nm)

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Figure 8

Lubricant surface morphology depending on average film thickness t for thicker film cases (>7nm)

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Figure 9

Relationship between the rms roughness of lubricant film surface and the average film thickness

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Figure 10

End-bead distributions inside the lubricant film for thinner film cases (<7nm)

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Figure 11

End-bead distributions inside the lubricant film for thicker film cases (>7nm)

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Figure 12

Molecular interaction potential field and number density of end beads inside the lubricant film

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Figure 13

Average intermolecular force field and number density of end beads inside the lubricant film

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