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Pressure Dependence of Shear Strengths of Thin Films on Metal Surfaces Measured in Ultrahigh Vacuum

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Abstract

The friction coefficient is measured for systems consisting of a thin potassium chloride film deposited onto a variety of clean, flat metal substrates, namely Pb, Sn, Au, Ag, Cu, Pd, Fe, Ta, and two types of steel, which are rubbed by a tungsten carbide pin in an ultrahigh vacuum. The friction coefficients are plotted versus 1/H S, the inverse of the substrate hardness, where two regimes are found. In the first regime, where deformation at the asperity tips is suggested to be plastic, the observed variation in friction coefficient with substrate hardness is rationalized by assuming that the shear strength S for sliding on a KCl film varies with contact pressure P as S = S 0 + aP, yielding values for a of 0.14 ± 0.02 and S 0 of ~60–70 MPa. In the second regime, it is proposed that the softer, film-covered Pb and Sn substrates are closer to being in conformal contact with the rough tribopin. These values of S 0 and a, along with the measured surface asperity height distribution of the tribopin and the value of the friction coefficient for a KCl monolayer on the metal, are used to rationalize the observed increase in friction coefficient with increasing film thickness.

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Acknowledgments

We gratefully acknowledge the support for this work by the Chemistry Division of the National Science Foundation under Grant No. CHE-9213988. We thank Prof. Ken Johnson for extremely useful discussions and Prof. H. Lopez for his help with the Hardness measurements.

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Authors and Affiliations

  1. Department of Chemistry and Biochemistry, and Laboratory for Surface Studies, University of Wisconsin-Milwaukee, Milwaukee, WI, 53211, USA

    Feng Gao, Octavio Furlong & W. T. Tysoe

  2. Benz Oil, Inc, Milwaukee, WI, USA

    Peter V. Kotvis

Authors
  1. Feng Gao
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  2. Octavio Furlong
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  3. Peter V. Kotvis
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  4. W. T. Tysoe
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Correspondence to W. T. Tysoe.

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Gao, F., Furlong, O., Kotvis, P.V. et al. Pressure Dependence of Shear Strengths of Thin Films on Metal Surfaces Measured in Ultrahigh Vacuum. Tribol Lett 31, 99–106 (2008). https://doi.org/10.1007/s11249-008-9342-1

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  • DOI: https://doi.org/10.1007/s11249-008-9342-1

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