SMEX1033 INDUSTRIAL TRIBOLOGY

UNIT-I

1.1 Introduction

The word tribology was first reported in a landmark report by Jost (1966). The word is

derived from the Greek word tribos meaning rubbing, so the literal translation would be "the

science of rubbing." Its popular English language equivalent is friction and wear or lubrication

science, alternatively used. The latter term is hardly all-inclusive. Dictionaries define tribology as

the science and technology of interacting surfaces in relative motion and of related subjects and

practices. Tribology is the art of applying operational analysis to problems of great economic

significance, namely, reliability, maintenance, and wear of technical equipment, ranging from

spacecraft to household appliances. Surface interactions in a tribological interface are highly

complex, and their understanding requires knowledge of various disciplines, including physics,

chemistry, applied mathematics, solid mechanics, fluid mechanics, thermodynamics, heat transfer,

materials science, rheology, lubrication, machine design, performance, and reliability.So study of

friction ,wear and lubrication(tri-three discipline ) it’s called Tribology as shown in fig.1.1

Fig.1.1 tri-three discipline

1.1.1 Industrial Tribology

Tribology locates the applications in all industrial sectors to investigate the question of

lubrication education, research and the needs of industry as followed.

Tribology

Lubrication

Friction

Wear

Aerospace industries

Automotive industries

Construction industries

Textile industries

Mining industries

Petrochemical machinery & tools

paper mill , transport

Power plant

Steel plant

Cement plant

Military vehicles

Transport

Metallurgy Equipments

Agriculture and food industries

1.1.3 Friction

Friction is a force that opposes the relative motion between two surfaces of objects in contact.

The force of friction always acts in a direction of opposite to the applied force as shown in fig

1.2. The Friction exists between two surfaces due to irregularities on the surfaces of the objects

in contact or interlocking of micro-level irregularities of the two surfaces or ploughing of harder

surfaces into smoother surfaces.

1.2 Friction

1.1.4 Types of friction

The Friction is classified in to four types according to the contact between two surfaces.

(i) Object is not moved

(ii) Object is moved

(iii) Objects is rolling and objects moving in liquid or air as shown in fig 1.3

Fig.1.3 Types of friction

1.2 Dry friction

When two solid surfaces are in micro contact or macro contact without aid of air or liquid it’s

described the dry friction and examples of dry friction is static and sliding friction. The fluid is

resist to the object while object is moving to the fluid surface it’s called as fluid friction, in this

chapter gives details information of dry friction and theory of dry friction

i) Static friction: The Pushing force and opposing forces are equal then object is not moved its

called as static friction or else solid object is motionless with self weight. The examples are

Pushing the wall

Pushing the lathe machine

Books rest on table

Man rest on chair or standing on ground etc…

Friction

Static Sliding Rolling Fluid

ii) Sliding friction: When the two surfaces encompass sliding relative to each other then friction

two surfaces is known as sliding friction or solid friction .the examples are

Pushing or pulling the book on table

Ice skating

Computer mouse moving on table

Oldham’s coupling rectangular bar sliding over the one surface to another surface.

Reciprocating mechanism

Tail stock moving over the Lathe bed etc….

iii) Rolling friction: when one surface is rolling over to another surface then generated friction

between the two surfaces is known as rolling friction. The examples are

Tyre moving on road surface

Train wheel on track

Elevator wheel on rail

Iron balls rotating in bearing (rolling iron ball between inner recess and outer recess)

iv) Fluid friction: when solid object slides through the water or air it’s known as fluid friction

Water flow in pipe (water slides against pipe)

Swimming (man sliding on water)

Flight flying (flight sliding on air)

Fig. 1.4 Static, Sliding and Rolling Friction

The static friction, sliding friction and rolling friction coefficient are high, medium and low

respectively as shown in fig. Here assume the pushing force and block is same for all the friction,

Solid is not moved (static friction)

Solid is moved at X1 distance (Sliding friction)

Solid is moved X2 (Rolling Friction)

1.2.1 Variables in dry friction

In automobile braking application after applying braking force the brake shoe contact with the

brake drum in dry conditions then the contacting surfaces are free from oxide and moisture. The

variables in dry friction are

Friction co efficient of soft material

External load

Surface roughness

Linear velocity or angular velocity

Types of contact and energy dissipation and dry friction as show in fig 1.5

Fig.1.5 Dry friction

1.2.2 Laws of dry friction

(i) The friction force is directly proportional to the normal load between two solid surfaces

in contact.

(ii) The friction force is independent of the apparent area of between two solid surface in

contact

(iii) The friction force is independent of the sliding speed

The first two laws presented by French engineer Guillaume Amotonts in 1699 and

third is called after coulomb proposed it in 1785 the laws of friction as shown in fig.1.6

Fig. 1.6 Laws of Friction

1.3 Boundary friction

When the two solid surfaces are in contacts with presence of thin lubricant layer is known as

boundary friction, the lubricant layer in separated by two solid or lubricant molecules contact

between them here lubricants acts as a boundary so it’s known as boundary friction as shown in

fig.1.7. In example engine oil is the boundary between two mating gears

W α F

2W α 2 F

W α F

Area of contact is independent

Same

Fig.1.7 Boundary friction

1.3.1 Examples of boundary friction

piston and piston rings sliding in engine block

Friction between gear teeth with liquids lubricants

Ball bearing with grease

Cooling oil used in machining operation

Power screws with lubricants

Wire drawings etc…

1.3.2 Variables in boundary friction

Friction force

Temperature

Surface roughness (Ra) value

Vibration

Lubricants properties

1.4 Oiliness or Greasy

“Oiliness of a lubricant is a measure of its capacity to glue on the surfaces of engine parts, under

conditions of heavy weight. When a lubricating oil of poor oiliness is subjected to high pressure, it

has a tendency to be squeezed out of the lubricated machine parts, so its lubrication action stops.

Asperities contact

with lubricants Lubricants is

the boundary

for two layer

On the other hand, lubricants, which have good oiliness stay in-between the lubricated surfaces,

when they are subjected to high pressure”.

“Oiliness is very important property of lubricants, particularly for extreme-pressure lubrication.

Mineral oils are very poor oiliness and vegetable oils are good oiliness. So, in order to improve

oiliness of mineral oils, additives are added like vegetable oils and higher fatty acids (such as oleic

and stearic acids)

1.5 Surface topography

The Deterministic surface textures may be studied by simple analytical methods. However no

solid surfaces are perfectly smooth at atomic scale and all solid engineering surfaces are having

rough to some extent the study of rough solid surface is called surface topography.

The textures are random, either isotropic or anisotropic, and either Gaussian or non-

Gaussian.

The exact type of surface texture depends on the nature of the processing technique.

The so-called cumulative processes, such as peening, lapping and electro-polishing.

Where the final shape of each region is the cumulative outcome of a large number of random

discrete local events and independent of the distribution governing each individual event,

produce surfaces that are governed by the Gaussian form. The Classification of solid surfaces is

shown in fig 1.8. The direct result of the central limit theorem of statistical theory is the Extreme-

value processes, such as grinding and milling and single-point processes such as turning and

shaping, usually, produce anisotropic and non-Gaussian surfaces.

Fig.1.8 Classification of solid surface

1.5.1 Beilby layer

If viewing the metal in microscopic level its consists of several layers in microns as shown in fig 1.9

Adsorbed gases and mixed with water vapour

Oxide

Solid surface

Homogeneous Heterogeneous

Anisotropic Isotropic

Gaussian Non Gaussian

Beilby layer is an amorphous layer (without regular crystalline structure) formed on the

surface of metals via mechanical working, wearing, or mechanical polishing.

Worked layer

Bulk material

Fig.1.9 Typical surface layer

1.5.2 Surface parameter

Roughness – the finest of the asperities

Waviness - concern the more widely spaced ones

Flaw – surface defect

Lay – The direction of the asperities

1.5.2.1 Arithmetic roughness average

This method is also known as roughness average and by two earlier term; arithmetic average (AA)

and center-line average (CLA)

The roughness average is the arithmetic average of the absolute values of the deviation from the

profile height measured from the centerline along a specified sampling length. The average

roughness as shown in fig

Fig 1.11 average roughness

1.5.2.2 Root-Means-Square roughness (Ra or RMS)

Closely related to the roughness average (Ra)

Square the distances, average them, and determine the square root of the result

The resulting value is the index for surface texture comparison

Usually 11% higher than the Ra value

1.5.2.3 Maximum Peak-Valley Roughness (Rmax or Rt)

Determine the distance between the lines that contact the extreme outer and inner point of the

profile it is the Second most popular method in industry

Ten-Point Height (Rz)

Averages the distance between the five peaks and five deepest valleys within the

sampling length

Average Peak-to-Valley Roughness (R or H or Hpl)

Average the individual peak-to-valley heights

Use the height between adjacent peaks and valleys, not measure from a center line

to peak valleys

Average Spacing of Roughness Peaks (Aror AR)

Average the distance between the peaks without regard to their height

1.6 Contact between surfaces

When two solid surfaces are loaded together there will always be some distortion of each of them.

Deformations may be purely elastic or may involve some additional plastic, and so permanent,

changes in shape. Such deflections and modifications in the surface profiles of the components can

be viewed at two different scales. Consider, for example, the contact between a heavily loaded roller

and the inner and outer races in a rolling element bearing. The degree of flattening of the rollers can

be expressed as a proportion of their radii. i.e.. at a relatively is macroscopic scale. On the other

hand since on the micro scale no real surface, such as those of either the roller or the race, can be

truly smooth, it follows that when these two solid bodies are pushed into contact they will touch

initially at a discrete number of points or asperities. Some deformation of the material occurs on a

very small scale at, or very close to these areas of true contact. It is within these regions that the

stresses are generated whose total effect is just to balance the applied load. Classical contact

mechanics assumes the deforming materials to be isotropic and homogeneous; in principle, its

results can be applied both to global contacts and to those between interacting asperities.

Micro scale: Is the real area of contact and Summation of all contact area of each asperity

Macro scale: is nominal contact area and void and non contact region its includes real contact

area the two different contact areas are in show in fig 1.12

The geometrical contacts are classified in to three type design

(i) Surface to surface contact

(ii) Line to line contact

(iii) Point to point contact

Fig. 1.12 Real contact area and nominal contact area

Ar= A1 + A2

Ff = 𝜎. Ar

Ar = Real area of contact

1.6.1 Hertzian contact

Contact between elastic curved bodies it’s called Hertzian contact it’s as shown in fig .1.12

• It is the Initial Contact bodies (spheres) touch at origin

• Crated the curvatures R1, R2

• Defined elastic parameters (E1,ν1) (E2,ν2)

• Curvature changed into two separation (parabolas)

Fig 1.12 Hertzian contact

1.7 Energy dissipation during the friction

When the solid body relative movement each one another the results is work being carried out or

work done, the energy get slow by slow the during the friction it’s called energy dissipation. In the

practical engineering situation all the frictions were discussed in this chapter so far on an individual

basis which means interact with each other in a complicated way. The shown Fig.1.13 is an attempt

to visualize all the possible steps of friction-induced energy dissipations.

In general, frictional work is dissipated at two different locations within the contact zone. The first

location is the interfacial region characterized by high rates of energy dissipation and usually

associated with an adhesion model of friction. The other one involves the bulk of the body and the

larger volume of the material subjected to deformations. Because of that, the rates of energy

dissipation are much lower. Energy dissipation during ploughing and asperity deformations takes

place in this second location.

It should be pointed out, however, that the distinction of two locations being completely

independent of one another is artificial and serves the purpose of simplification of a very complex

problem. The various processes depicted in Fig1.13 .can be briefly characterized as follows:

(i) plastic deformations and micro-cutting

(ii) viscoelastic deformations leading to fatigue cracking and tearing, and subsequently to

subsurface excessive heating and damage

(iii) true sliding at the interface leading to excessive heating and thus creating the conditions

favorable for chemical degradation (Polymers)

(iv) Interfacial shear creating transferred films, true sliding at the interface due to the

propagation of Schallamach waves (elastomers).

Fig.1.13 Energy dissipation during the friction

1.7.1 Friction theories

i) Mechanical inter locking ( Amontons ) Theory

The metallic friction can be attributed to the mechanical interlocking of surface roughness

elements. This mechanism gives an explanation for the existence of a static coefficient of friction,

and it also explains dynamic friction as the force required to lift the asperities of the upper surface

over those of the lower one.

ii ) Molecular Attraction (Tomlinson and Hardy ) Theory

The friction forces to energy dissipation when the atoms of one material are "plucked" out

of the attractive range of their counter-parts on the mating surface. Later work attributed adhesion

friction to a molecular-kinetic bond rupture process in which energy is dissipated by the stretch,

break, and relaxation cycle of surface and subsurface molecules.

iii) Electrostatic friction theory (stick-slip phenomena)

The Forces According to this theory presented as recently as 1961, stick-slip phenomena between

rubbing metal surfaces can be explained by the initiation of a net flow of electrons, which produces

clusters of charges of opposite polarity at the interface. These charges are assumed to hold the

surfaces together by electrostatic attraction.

iv). Welding, Shearing, and Ploughing (bowdon theory -1950)

This is the most recent theory widely accepted for metal friction. The high pressures

developed at individual contact spots cause local welding, and the junctions thus formed are

sheared subsequently by relative sliding of the surfaces Ploughing by the asperities of the harder

surface through the matrix of the softer material contributes the deformation component of

friction.

1.7.2 Mathematical model of molecular attraction theories

Two main factors are likely for generation of attraction between two asperities as shown in fig1.14

Adhesion in real contact asperities

Deformation of asperities

Fig1.14 Asperities contact ,ploughing ,welding and shearing

F = Fa + Fd

F = friction force

Fa = friction force due to adhesion

Fd = friction force due to deformation

Similarly friction co-efficient for adhesion and deformation

µ = µa + µd

Fa = A x S

A = true area of contact

S = shear strength of junction

F dformation = F Ploughing

Normal load (w) = A X p

µ = 𝐹

𝑤

𝐹

𝑤 =

A S

𝐴 𝑃

A S

𝐴 𝑃=

𝑆

𝑃

µ = 𝐹

𝑤 =

A S

𝐴 𝑃 =𝑆

𝑃

Here “p” is the yield strength of material

1.8 Fretting corrosion and prevention (Tribo corrosion)

Fretting is a phenomenon of friction which occurs between two mating surfaces subjected

to cyclic relative motion of extremely small amplitude of vibrations. Fretting appears as pits or

grooves surrounded by corrosion products. The deterioration of material by the conjoint action of

fretting and corrosion is called 'Fretting Corrosion.' Fretting is usually accompanied by corrosion in

a corrosive environment. It occurs in bolted parts, engine components and other machineries.

Fretting of the blade roots of tube blades.

Loosening of the wheels from axles. For example, the coming out of railroad car

wheel from the axle due to vibrations.

Fretting of electrical contacts.

Fretting damage of riveted joints.

Surgical implantations.

Factor affecting Fretting corrosion

Contact load

Amplitude

Number of cycle

Temperature and

Relative humanity

Prevention fig1.15 Fretting corrosion

Increase the magnitude of load at the mating surfaces to minimize the occurrence of slip.

Keep the amplitude below the level at which fretting occurs, if known for a particular

system.

There is lower threshold limit below which fretting does not occur.

Decrease the load at the bearing surfaces; however, it may not always work.

Use materials which develop a highly resistant oxide film, at high temperature to minimize

the adverse effect of temperature on fretting.

Use gaskets to absorb vibration.

Increase the hardness of the two contacting metals,

Use low viscosity lubricating oils.

Use thick resin bonded coatings.

1.9 Friction characteristics of metal and non metal

Sl.no Friction characteristics

Metal Non-metal

1 Real area of contact Metal to metal contact

metal to non metal contact

2 Friction coefficient High 0.3 to 0.6 low 0.2 to 0.3

3 lubrication Lubricants used metal to metal contact

No lubricants( self lubricants)

4 Temperature With stand high temperature Low temperature

5 Rubbing noise level Without lubricants noise level high

Very minimum

6 Debris form Powder or layer Powder or layer

7 Load With load high load Low load

8 Surface roughness (Ra)

Need secondary operation Not required secondary operation

9. Fretting Due to amplitude of vibration

No vibration

10. Forming process Applicable Is not possible

11. Example Mild steel on brass Mild steel on copper

Mild steel on PTPF

1.9.1 Friction coefficient of metal and non metal

Surfaces μs μk

Steel on steel 0.74 0.57 Aluminum on steel 0.61 0.47 Copper on steel 0.53 0.36 Rubber on concrete 1.0 0.8 Wood on wood 0.25 - 0.5 * 0.2 Glass on glass 0.94 0.4 Waxed wood on wet snow 0.14 0.1 Waxed wood on dry snow - 0.04 Metal on metal (lubricated) 0.15 0.06 Ice on ice 0.1 0.03 Teflon on Teflon 0.04 0.04 Synovial joints in humans 0.01 0.003

1.10 MEASUREMENT OF FRICTION

a) Gravitational method

b) Rotary method

(i) Tilting Plane method

To measure the coefficient of friction for several combinations of materials, making use of

an inclined plane.

To study equilibrium and non-equilibrium of a body on an inclined plane under the action of

forces.

The forces acting on a body on an inclined plane are shown in figure 1.16. W is the weight of

the body (W = mg), N is the normal force due to the plane, f is the frictional force, and P is the

applied force present when a string is attached to the body to pull it up the plane. Generally this

situation is analyzed by resolving the forces into components parallel and perpendicular to the

plane, the free body diagram as shown in Fig.1.17

The components of the weight are:

W = Wx sin θ

W = Wy cos θ

Fig. 1.17 Tilting Plane Apparatus

Fig.1.16 Tilting Plane free body diagram

The angle of uniform slip is that angle of the plane which causes the free block to slide down the

plane with constant velocity

µ = tan θ

Working method

Lay the wooden block on the plane with its large face down. Apply a force, P, to it by means of a

string running over a pulley to a weight hanger. Add weights to the hanger until the block begins to

move. In this way determine the approximate value of starting friction. Determine the value more

accurately by starting just below this value and adding weight in very small increments.

(ii) Adaptation of the lifting plane

The incline plane surface is can be lifted at one end with certain mechanical arrangement, so

that the incline angle “θ” with horizontal direction can be varied. This set up enable to change the

gravitational pull on the block in the direction of incline surface and work with incline of different

angles for the study of motion on the incline. The forces, in the direction normal to the incline plane,

WEIGHT

are balanced as there is no motion in that direction. On the other hand, the component of weight

parallel to contact surface, mg sinθ, tends to initiate motion of the block along the incline plane.

For small value of "θ", the component of force parallel to contact surface is small and is

counteracted by friction, acting in the opposite direction Angle of repose. Adaptation of the lifting

plane is used find only the static friction co efficient as shown in fig.

μk = tan θk

(iii) Weight and pulley method

Two blocks of mass m and M are connected via pulley with a configuration as shown. This method

used to find coefficient of static friction.

(iv) Spring balance method

Spring Balance Pull a spring balance connected to the block and slowly increase the force until the

block begins to slide. Make sure the spring balance is parallel to the surface. The reading on the

spring balance scale when the load begins to slide is a measure for the static friction, while the

reading when the block continues to slide is a measure of dynamic friction. The coefficient of

friction is simply µ = F spring /F normal = F spring /(m block · g ), g=9.81 m/s²

Rotary type friction measurement (Motorized Tribometers)

Motorized Tribometers as shown in the figure the friction coefficient is

measured in fresh contacts, not after running in. The coefficient of friction may change significantly

during first half hour of sliding. The time necessary to obtain a stable value of the coefficient of

friction can be observed in a motorized tribometer by monitoring the friction over time. This

method is common for measuring the specific wear rate and the contact temperature during

operation.

Fig. (a) Pin-on-disc (b) Pin-on-cylinder and (c) pin-on- cross cylinder