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Journal of Materials Science

, Volume 41, Issue 19, pp 6441–6452 | Cite as

Analysis of the apparent friction of polymeric surfaces

  • S. Lafaye
  • C. Gauthier
  • R. Schirrer
Article

Abstract

The apparent friction coefficient is the ratio between the tangential force and the normal load applied to moving body in contact with the surface of a material. This coefficient includes a so-called “true local friction” at the interface and a “geometrical friction” which is the ploughing effect. The material underneath a moving tip may display various types of behaviour: elastic, elastic–plastic where elastic and plastic strain are present in the contact area, or fully plastic. As is usual in polymers, the material behaviour is time and temperature dependent and may exhibit strain hardening. A surface flow line model of a scratching tip which links the apparent friction to the local friction and contact geometry was recently proposed. An inverse analysis is used in the present work to estimate the local friction from the measured apparent friction and a knowledge of the contact area and tip shape. The polymer true friction coefficient displays temperature and sliding speed dependency, which may be attributed to the surface thermodynamics. It is shown that the local friction depends on the level of strain in the polymer at the contact interface.

Keywords

PMMA Contact Pressure Real Contact Area Glass Temperature Plastic Contact 

Symbols

μapp

Apparent friction coefficient

μ

True friction

fad

Adhesive friction coefficient

μplough

Ploughing friction coefficient

fvisco

Viscoelastic friction coefficient

fplast

Plastic friction coefficient

Ft

Tangential force

Fad

Adhesive force

Fn

Normal load

τapp

Apparent interfacial shear stress

τ(or τtrue)

Shear stress at the moving contact area

τplough

Ploughing shear stress

pm

Contact pressure

σy

Yield stress

p

Local pressure at the contact

pmy

Normalised contact pressure

Sn

Real normal contact area

St

Tangential contact area

ds

Contact surface element

K

A constant

H

Hardness

tanδ

Loss factor

θ

Half apex angle of the conical tip

ω

Rear contact angle

A,B,C,D

Elementary action integrals of the local pressure and shear

dɛ/dt (or \(\mathop\varepsilon\limits^\bullet\))

Mean effective strain rate

V

Sliding speed

l

Scratch contact width

a

Contact radius

Rtip

Radius of the tip

T

Temperature

\(\vec{x}\vec{y}\vec{z}\)

Axes moving with the tip

\(\vec{z}\)

Axis of the indentation direction

\(\vec{\chi}\)

Axis of the scratching direction

\(\vec{N}\)

Elementary normal load vector

\(\vec{T}\)

Elementary tangential load vector

\(\vec{n}\)

Normal unit vector

\(\vec{t}\)

Unit vector tangential to the flow lines

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Copyright information

© Springer Science+Business Media, LLC 2006

Authors and Affiliations

  1. 1.Institut Charles Sadron, CNRS-UPR 22StrasbourgFrance

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