Tribology | 2. Friction Mechanisms and Theories

Introduction to Friction

Friction is the tangential resistance to motion. The
occurrence of friction is a part of everyday life. It is needed
so that we have control on our walking. On the other hand, in
most of running machines friction is undesirable (energy loss,
leading to wear of vital parts, deteriorating performance due
to heat generation) and all sorts of attempts (i.e. using low
friction materials, lubricating surfaces with oil or greases,
changing design so that sliding can be reduced) have been made
to reduce it.

Often coefficient of friction(μ) is considered a constant value
for a pair of material. In addition, the value of μ is
accounted much lesser than 1.0.

When assessing a system’s tribology need, we must consider

  •  Materials, Coating, Lubricant 
  •  Contact Area, Geometry, Stress
  •  Surface Roughnesses
  •  Sliding Speed
  •  Sliding Mode (unidirectional, reciprocating,
    multi directional)
  •  Duty Cycle (continuous contact, intermittent
  •  Environment
  •  Temperature, Humidity,  
  •  Atmosphere (air, exhaust gases, vacuum)

Friction is NOT a Material Property .Friction is a
“System” Property.

Frictional resistance has two types :

When two objects touches each other- there will produce
friction forces between them.

1) Microscopic forces of  molecular  Adhesion.
 (includes electrostatic, Van der Waals, metallic

2) Microscopic forces of  mechanical
( Deformation)  [includes elastic and
plastic deformation]

To understand the effect of material pair, role of lubrication,
and environmental factors let us start with dry friction. The
dry friction is also known as solid body friction and it means
that there is no coherent liquid or gas lubricant film between
the two solid body surfaces. Four
given by Leonardo da Vinci, Amonton,
Coulomb and Tomlison
for dry lubrication are explained in
following paragraph.

Leonardo da vinci(Earliest experimenter, 1452-1519)

  • As per Leonardo, “Friction made by same weight will be of
    equal resistance at the beginning of movement, although contact
    may be of different breadths or length”.
  • “Friction produces the double the amount of effort if
    weight be doubled”. In other words, F α W.

G.Amontons, 1699 :

  •  The friction force is independent of the nominal area
    (F ≠ A) of contact between two solid surfaces. The friction
    force is directly proportional F α N to the normal component of
    the load. He considered three cases(Fig. 1) and showed that
    friction force will vary as per the angle of application of
    load. As per Amontons μ = 0.3 for most of materials.

Amonton`s work.
Fig.1 Amonton`s

C.A.Coulomb 1781 (1736-1806) :

• Clearly distinguished between static & kinetic frictions.
Friction due to interlocking of rough surfaces.

• Contact at discrete points μstatic ≥ μkinetic.

As per coulomb friction force is independent of sliding speed.
But this law applies only approximately to dry surfaces for a
reasonable low range of sliding speeds, which depends on heat
dissipation capabilities of tribo-pairs.

TOMLINSON’s Theory of Molecular attraction, 1929

As per Tomlison due to molecular attraction between
metal, cold weld junctions are formed. Generally load on
bearing surface is carried on just a few points. These are
subjected to heavy unit pressure, and so probably weld
together. Adhesion force developed at real area of contact.

There are two main friction sources:
Adhesion and Deformation. Force needed to
plough asperities of harder surface through softer. In
lubricated tribo-pair case, friction due to adhesion will be
negligible, while for smoother surfaces under light load
conditions deformation component of friction will be

Adhesion and deformation wear

 Fig. 2 demonstrates the adhesion (cold weld)
between two surfaces. Some force, Fa, is required to tear the
cold junction. Fig. 3 demonstrates the deformation
It shows a conical asperity approaching to a
softer surface. To move upper surface relative to lower surface
some force is required.

Two friction sources : Deformation and Adhesion.

• Resulting friction force (F) is sum of two contributing
(Fa & Fd) terms.

• Lubricated tribo-pair case — negligible adhesion.

• Smoother surfaces under light load conditions – Negligible

Adhesion and Ploughing in

This theory is based on the fact that all surfaces are made of
atoms. All atoms attract order one another by attractive
force. For examples, if we press steel piece over indium
piece (as shown in Fig. 4) they will bind across the region of
contact. This process is sometimes called “cold welding,” since
the surfaces stick together strongly without the application of
heat. It requires some force to separate the two surfaces. If
we now apply a sideways force to one of surfaces the junctions
formed at the regions of real contact will have to be sheared
if sliding is to take place. The force to do this is the
frictional force.

Cold welding in steel and indium
Fig.4 Cold welding in steel and

Theory of ADHESIVE Friction :

Bowden and Tabor developed theory of adhesive friction.
As per this theory on application of W, initial contact at some
of higher asperity tips occurs. Due to high stress those
asperities suffer plastic deformation, which permits strong
adhesive bonds among asperities. Such cold formed junctions are
responsible for the adhesive friction. The real area of
contact, A can be estimated by applied load W and hardness of
the soft material, H. If s is shear stress of softer material,
then force Fa required to break these bonds can be estimated by
Equation Fa = As. The coefficient of friction due to adhesive
friction is given by ratio of friction force to applied load W.
Fig. 5 shows the formulation and breakage of cold junctions.

• Two surfaces are pressed together under load W.

• Material deforms until area of contact (A) is sufficient to
support load W, A = W/H.

• To move the surface sideway, it must overcome shear strength
of junctions with force Fa. 

• μ = Fa ⁄ W = s ⁄ H.

In other words shear strength(s) and hardness(H) of soft
material decides the value of μ. This means whatever properties
of the other harder pairing material, μ would not change.

Summary of adhesion and ploughing.
Fig.5 Summary of adhesion and


This theory is based on the fact that contact between
tribo-pairs only occurs at discrete points, where the
asperities on one surface touch the other. The slope of
asperities governs the friction force. Sharp edges cause more
friction compared to rounded edges. Expression for coefficient
of friction can be derived based on the ploughing effect.
Ploughing occurs when two bodies in contact have different
hardness. The asperities on the harder surface may penetrate
into the softer surface and produce grooves on it, if there is
relative motion.

Refer figure 5

Summary : Three frictional theories

In first expression it is shown that friction
depends on the lowest shear strength of the contact tribo-pair.
Reducing shear strength and increasing the hardness reduces the
coefficient of friction.

• Second expression shows the dependence of
coefficient of friction on the angle of conical

• Third expression indicates lesser sensitivity of
coefficient of friction compared to that of conical