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Meccanica

, Volume 50, Issue 7, pp 1915–1922 | Cite as

Frictional characteristics of molecular length ultra-thin boundary adsorbed films

  • I. S. Y. Ku
  • W. W. F. Chong
  • T. Reddyhoff
  • H. Rahnejat
Brief Notes and Discussions

Abstract

The paper presents measurements of friction of any ultra-thin film entrained into the contact of a pair of very smooth specimen subjected to entrainment in a converging micro-wedge of a special-purpose micro-tribometer. An ultra-thin film is expected to form at the boundary solids through adsorption of boundary active molecules. Fluids with linear and branched molecules are used in the investigation. It is found that the frictional characteristics of these films can be adequately described through use of Eyring thermal activation energy and a potential energy barrier to sustain conjunctional sliding motion. The combined experimental measurement and the simple activation energy approach shows that the thin molecular adsorbed films act like hydro Langmuir–Blodgett layers, the formation and frictional characteristics of which are affected by the competing mechanisms of adsorption, forced molecular re-ordering and discrete-fashion drainage through the contact by the solvation effect. This process is a complex function of the contact sliding velocity as well as a defined Eyring activation density (packing density of the molecules within the conjunction). It is shown that the contribution of solvation to friction is in the form of energy expended to eject layers of lubricant out of the contact, which unlike the case of micro-scale hydrodynamic films, is not a function of the sliding velocity.

Keywords

Ultra-thin boundary films Thermal activation energy Solvation Friction 

List of symbols

kB

Boltzmann constant (0.008314 kJ/mol/K)

v

Sliding velocity (m/s)

v0

Characteristic velocity (m/s)

E

Barrier height (kJ/mol)

P

Contact pressure (Pa)

Q

Process activation energy (kJ/mol)

T

Contact temperature (K)

ϕ

Shear stress activation volume (m3/mol)

τ

Shear stress (Pa)

τ0

Pressure dependent Eyring shear stress (Pa)

τ1

Sliding velocity dependent Eyring shear stress (Pa)

θ

Rate of change for friction at a given sliding velocity for constant applied load (−)

ξ

Rate of change for friction at an applied load for constant sliding velocity (−)

Ω

Pressure activation volume (m3/mol)

Notes

Acknowledgments

The authors acknowledge sponsorship provided by the EPSRC through Grant Nos. EP/D04099X and EP/L001624/1 (covering experimental work) along with the EPSRC ENCYCLOPAEDIC program Grant (covering theoretical work).

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Copyright information

© Springer Science+Business Media Dordrecht 2015

Authors and Affiliations

  • I. S. Y. Ku
    • 1
  • W. W. F. Chong
    • 1
    • 2
  • T. Reddyhoff
    • 3
  • H. Rahnejat
    • 4
  1. 1.National Centre for Advanced Tribology (nCATS), Faculty of Engineering and the EnvironmentUniversity of SouthamptonSouthamptonUK
  2. 2.Faculty of Engineering and the EnvironmentUniversity of Southampton Malaysia CampusNusajayaMalaysia
  3. 3.Tribology Group, Department of Mechanical EngineeringImperial College LondonLondonUK
  4. 4.Wolfson School of Mechanical and Manufacturing EngineeringLoughborough UniversityLoughboroughUK

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