Page 170 - Handbook of Surface Improvement and Modification

11

The Coefficient of Friction

11.1 METHODS AND MECHANISMS OF IMPROVEMENT OF
THE COEFFICIENT OF FRICTION

The following topics are discussed in this chapter
• friction − principles
• properties of polymeric materials
• polymeric products
• particles as lubricating additives
• polytetrafluorethylene
• other additives
• effect of degrading environments
1
Friction is the force resisting movement of two sliding bodies given by the follow-
2
ing equations
F A = τ [11.1]

F Aτ
μ = ----- = ------- [11.2]
W W
where:
F friction force
A sum of areas of contact asperities (real area of contact)
τ shear strength of contacts
μ coefficient of friction
W applied normal load.

The contact area is related to the surface roughness of a sliding pair. The surface of
materials in contact may change due to the formation of tribo-film and formation of wear
debris as well as the time of contact (dwell time) affecting friction properties of materials
by changes in a real contact area. Also, dynamic and static frictions are considered as dif-
ferentiating factors, therefore, the speed of sliding is an essential variable. The energy of
friction is converted into thermal energy which affects physical properties of materials in
contact including their friction properties.
The above list of the essential parameters is far from being complete. For example,
surface physics explains friction in terms of the formation of adhesive junctions by inter-
acting asperities and their breaking away by shearing, whereas continuum mechanics
interprets friction by the interlocking and fracture of asperities. The reaction of materials
during sliding depends on their mechanical properties and forces involved. In real materi-
als, nonelastic and nonlinear conditions are involved and they are making an evaluation of