Tribology Letters

, Volume 53, Issue 2, pp 477–486

Friction Reduction in Elastohydrodynamic Contacts by Thin-Layer Thermal Insulation

  • M. Björling
  • W. Habchi
  • S. Bair
  • R. Larsson
  • P. Marklund
Original Paper

Abstract

Reducing friction is of utmost importance to improve efficiency and lifetime of many products used in our daily lives. Thin hard coatings like diamond-like carbon (DLC) have been shown to reduce friction in full-film-lubricated contacts. In this work, it is shown that contrarily to common belief, the friction reduction stems mainly from a thermal phenomenon and not only a chemical/surface interaction one. It is shown that a few micrometer-thin DLC coating can significantly influence the thermal behavior in a lubricated mechanical system. The presented simulations, validated by experiments, show that applying a thin DLC coating to metal surfaces creates an insulating effect that due to the increased liquid lubricant film temperature at the center of the contact, locally reduces lubricant viscosity and thus friction. The results of the investigation show that the addition of thin insulating layers could lead to substantial performance increases in many applications. On a component level, the contact friction coefficient in some common machine components like gears, rolling element bearings, and cam followers can potentially be reduced by more than 40 %. This will most likely open up the way to new families of coatings with a focus on thermal properties that may be both cheaper and more suitable in certain applications than DLC coatings.

Keywords

Diamond-like carbon (DLC) EHL Insulation Friction Coating Thermal effects Ball-on-disk 

List of symbols

βK

Temperature coefficient of K0 (K1)

χ

Dimensionless heat capacity scaling parameter

η

Generalized (shear dependent) viscosity (Pa s)

γ

Shear rate (s−1)

κ

Dimensionless conductivity scaling parameter

Λ

Limiting stress pressure coefficient

λR

Relaxation time at TR and ambient pressure (s)

μ

Limiting low-shear viscosity (Pa s)

μR

Low shear viscosity at TR and ambient pressure (Pa s)

μ

Viscosity extrapolated to infinite temperature (Pa s)

ρ

Lubricants density (kg)

τ

Shear stress (Pa)

τL

Limiting shear stress (Pa)

φ

Dimensionless viscosity scaling parameter

φ

Viscosity scaling parameter for unbounded viscosity

A

Coefficient in the dimensionless conductivity scaling parameter

av

Thermal expansivity defined for volume linear with temperature (K−1)

BF

Fragility parameter in the new viscosity equation

C0

Parameter in the heat capacity function (J/m3 K)

Ck

Parameter in the conductivity function (W/m K)

cp

Specific heat capacity (J/kg K)

Cv

Lubricants volumetric heat capacity

F

Load (N)

G

Effective shear modulus (Pa)

g

Thermodynamic interaction parameter

k

Thermal conductivity (W/m K)

K0

Pressure rate of change of isothermal bulk modulus at p = 0

K00

K0 at zero absolute temperature (Pa)

K0

Isothermal bulk modulus at p = 0 (Pa)

L

Contact load (N)

m

Parameter in the heat capacity function (J/m3 K)

n

Power law exponent

p

Pressure (Pa)

q

Coefficient in the dimensionless conductivity scaling parameter

R

Ball radius (m)

s

Exponent in the conductivity scaling model

SRR

Slide to roll ratio

T

Temperature (K)

TR

Reference temperature (K)

Ue

Mean entrainment speed (m/s)

ui

Surface velocity (m/s)

V

Volume (m3)

V0

Volume at p = 0 (m3)

VR

Volume at reference state, TR, p = 0 (m3)

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Copyright information

© Springer Science+Business Media New York 2013

Authors and Affiliations

  • M. Björling
    • 1
  • W. Habchi
    • 2
  • S. Bair
    • 3
  • R. Larsson
    • 1
  • P. Marklund
    • 1
  1. 1.Division of Machine Elements, Department of Engineering Science and MathematicsLuleå University of TechnologyLuleåSweden
  2. 2.Department of Industrial and Mechanical EngineeringLebanese American UniversityByblosLebanon
  3. 3.Georgia Institute of Technology, Centre for High Pressure RheologyG.W. Woodruff School of Mechanical EngineeringAtlantaUSA

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