RECENT ADVANCES IN TRIBOLOGY Page 1 of 52 1. INTRODUCTION 1.1 BACKGROUND Tribology in a traditional form has been in existence since the beginning of recorded history. There are many well documented examples of how early civilizations developed bearings and low friction surfaces. The scientific study of tribology also has a long history, and many of the basic laws of friction, such as the proportionality between normal force and limiting friction force, are thought to have been developed by Leonard0 da Vinci in the late 15th century. However, the understanding of friction and wear languished in the doldrums for several centuries with only fanciful concepts to explain the underlying mechanisms. For example it was proposed by Amonton in 1699 that surfaces were covered by small spheres and that the friction coefficient was a result of the angle of contact between spheres of contacting surfaces. A reasonable value of friction coefficient close to 0.3 was therefore found by assuming that motion was always to the top of the spheres. The relatively low priority of tribology at that time meant that nobody really bothered to question what would happen when motion between the spheres was in a downwards direction. Unlike thermodynamics, where fallacious concepts like 'phlogiston' were rapidly disproved by energetic researchers such Department of Mechanical Engineering Bheemanna Khandre Institute Of Technology, Bhalki
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RECENT ADVANCES IN TRIBOLOGY Page 1 of 35
1. INTRODUCTION
1.1BACKGROUND
Tribology in a traditional form has been in existence since the beginning of recorded history.
There are many well documented examples of how early civilizations developed bearings and
low friction surfaces. The scientific study of tribology also has a long history, and many of the
basic laws of friction, such as the proportionality between normal force and limiting friction
force, are thought to have been developed by Leonard0 da Vinci in the late 15th century.
However, the understanding of friction and wear languished in the doldrums for several centuries
with only fanciful concepts to explain the underlying mechanisms. For example it was proposed
by Amonton in 1699 that surfaces were covered by small spheres and that the friction coefficient
was a result of the angle of contact between spheres of contacting surfaces. A reasonable value
of friction coefficient close to 0.3 was therefore found by assuming that motion was always to
the top of the spheres. The relatively low priority of tribology at that time meant that nobody
really bothered to question what would happen when motion between the spheres was in a
downwards direction. Unlike thermodynamics, where fallacious concepts like 'phlogiston' were
rapidly disproved by energetic researchers such as Lavoisier in the late 18th century, relatively
little understanding of tribology was gained until 1886 with the publication of Osborne Reynolds'
classical paper on hydrodynamic lubrication. Reynolds proved that hydrodynamic pressure of
liquid entrained between sliding surfaces was sufficient to prevent contact between surfaces even
at very low sliding speeds. His research had immediate practical application and lead to the
removal of an oil hole from the load line of railway axle bearings. The oil, instead of being
drained away by the hole, was now able to generate a hydrodynamic film and much lower
friction resulted. The work of Reynolds initiated countless other research efforts aimed at
improving the interaction between two contacting surfaces, and which continue to this day. As a
result journal bearings are now designed to high levels of sophistication. Wear and the
fundamentals of friction are far more complex problems, the experimental investigation of which
is dependent on advanced instrumentation such as scanning electron microscopy. Therefore, it
has only recently been possible to study these processes on a microscopic scale where a true
understanding of their nature can be found. Tribology is therefore a very new field of science,
most of the knowledge being gained after the Second World War. In comparison many basic
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RECENT ADVANCES IN TRIBOLOGY Page 2 of 35
engineering subjects, e.g. thermodynamics, mechanics and plasticity, are relatively old and well
established. Tribology is still in an imperfect state and subject to some controversy which has
impeded the diffusion of information to technologists in general. The need for information is
nevertheless critical; even simple facts such as the type of lubricant that can be used in a
particular application, or preventing the contamination of oil by water must be fully understood
by an engineer.
As our technological civilization expands, material and energy conservation is
becoming increasingly important. Wear is a major cause of material wastage, so any reduction of
wear can effect considerable savings. Friction is a principal cause of energy dissipation and
considerable savings are possible by improved friction control. Lubrication is the most effective
means of controlling wear and reducing friction. Thus tribology, which is the science and
technology of wear of friction, lubrication and wear, is of considerable importance in material
and energy conservation. The history of this relatively new science which is concerned with
problems that have always presented man with a challenge has been recorded, and the
fundamentals reviewed.
1.2 MEANING OF TRIBOLOGY
Tribology, which focuses on friction, wear and lubrication of interacting surfaces in relative
motion, is a new field of science defined in 1967 by a committee of the Organization for
Economic Cooperation and Development. Tribology’ is derived from the Greek word ‘tribos’
meaning rubbing or sliding. After an initial period of scepticism as is inevitable for any newly
introduced word or concept, the word ’tribology’ has gained gradual acceptance. As the word
tribology is relatively new, its meaning is still unclear to the wider community and humorous
comparisons with tribes or tribolites tend to persist as soon as the word ‘tribology’ is mentioned.
Wear is the major cause of material wastage and loss of mechanical performance and any
reduction in wear can result in considerable savings. Friction is a principal cause of wear and
energy dissipation. Considerable savings can be made by improved friction control. It is
estimated that one third of the world’s energy resources in present use is needed to overcome
friction in one form or another. Lubrication is an effective means of controlling wear and
reducing friction. Tribology is a field of science which applies an operational analysis to
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problems of great economic significance such as reliability, maintenance and wear of technical
equipment ranging from household appliances to spacecraft.
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2.PRACTICAL OBJECTIVES OF TRIBOLOGY
Film formation between any pair of sliding objects is a natural phenomenon which can occur
without human intervention. Film formation might be the fundamental mechanism preventing the
extremely high shear rates at the interface between two rigid sliding objects. Non-mechanical
sliding systems provide many examples of this film formation. For example, studies of the
movement between adjacent geological plates on the surface of the earth reveal that a thin layer
of fragmented rock and water forms between opposing rock masses. Chemical reactions between
rock and water initiated by prevailing high temperatures (about 6OOOC) and pressures (about
100 [MPa]) are believed to improve the lubricating function of the material in this layer 131.
Laboratory tests of model faults reveal that sliding initiates the formation of a self-sliding layer
of fragmented rock at the interface with solid rock. A pair of self-sealing layers attached to both
rock masses prevent the leakage of water necessary for the lubricating action of the inner layer of
fragmented rock and water [31. Although the thickness of the intervening layer of fragmented
rock is believed to be between 1 - 100 [m] , this thickness is insignificant when compared to the
extent of geological plates and these layers can be classified as ‘films’. Sliding on a geological
scale is therefore controlled by the properties of these ‘lubricating films’, and this suggests a
fundamental similarity between all forms of sliding whether on the massive geological scale or
on the microscopic scale of sliding between erythrocytes and capillaries. The question is, why do
such films form and persist? A possible reason is that a thin film is mechanically stable, i.e. it is
very difficult to completely expel such a film by squeezing between two objects. It is not
difficult to squeeze out some of the film but its complete removal is virtually impossible.
Although sliding is destructive to these films, i.e. wear occurs, it also facilitates their
replenishment by entrainment of a 'lubricant' or else by the formation of fresh film material from
wear particles. Film formation between solid objects is intrinsic to sliding and other forms of
relative motion, and the study and application of these films for human benefits is the raison d
'etre of tribology.
In simple terms it appears that the practical objective of tribology is to minimize
the two main disadvantages of solid to solid contact: friction and wear, but this is not always the
case. In some situations, as illustrated in Figure 1.1, minimizing friction and maximizing wear or
minimizing wear and maximizing friction or maximizing both friction and wear is desirable. For
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example, reduction of wear but
not friction is desirable in
brakes and lubricated clutches,
reduction of friction but not
wear is desirable in pencils,
increase in both friction and
wear is desirable in erasers.
3. FRICTION
The friction force is the resistance encountered when one body moves relative to another body
with which it is in contact. The static friction force is how hard you have to push something to
make it, whilst the dynamic friction force is how hard you push to keep it moving. The ratio of
the frictional force F to the normal force W is called the co-efficient of friction and given the
Greek symbol m (pronounced mew).
Friction is the dissipation of energy between sliding bodies. Four basic empirical
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laws of friction have been known for centuries since the work of da Vinci and
Amonton:
the tangential friction force is proportional to the normal force in
sliding;
there is a proportionality between the maximum tangential force
before sliding and the normal force when a static body is subjected to
increasing tangential load;
friction force is independent of the contact area;
friction force is independent of the sliding speed.
In the early studies of contacts between the real surfaces it was assumed that since the contact
stresses between asperities are very high the asperities must deform plastically . This assumption
was consistent with Amonton's law of friction, which states that the friction force is proportional
to the applied load, providing that this force is also proportional to the real contact area.
However, it was later shown that the contacting asperities after an initial plastic deformation
attain a certain shape after which the deformation is elastic . It has been demonstrated on a model
surface made up of large irregularities approximated by spheres with superimposed smaller set of
spheres which were supporting an even smaller set , that the relationship between load and
contact area is almost linear despite the contact being elastic. It was found that a nonlinear
increase in area with load at an individual contact is compensated by the increasing number of
contacts. A similar tendency was also found for real surfaces with random topography. It
therefore became clear that Amonton’s law of friction is also consistent with elastic deformations
taking place at the asperities providing that the surface exhibits a complex hierarchical structure
so that several scales of microcontact can occur.
The proportionality between friction force and normal load has lead to the definition of ‘kinetic’
and ‘static’ coefficients of friction. In many reference books, coefficients of friction are quoted
as ’properties’ of certain combinations of materials. This approach, however, is very simplistic
since the coefficients of friction are dependent on parameters such as temperature and sliding
speed and in some instances there is no exact proportionality between friction force and normal
load. The underlying reasons for the laws of friction listed above have only recently been
deduced. It has been found that much of the characteristics of friction are a result of the
properties of rough surfaces in contact.
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4.WEAR
Wear may be defined as the undesired displacement or removal of surface material, although
under some circumstances, the initial stages of wear or mild wear which tends to smooth
surfaces, may be beneficial for the running-in of mechanisms. The economic implications of
wear cause concern in industry, as a reasonable life is required of mechanical equipment to cover
capital and maintenance costs. It certainly causes a great deal of expenditure on maintenance t h
a t must take place; such maintenance is costly in itself , but also costly in lost productivity
whilst it is being carried out. Progress i n wear control and prevention can be made only after a
better understanding of the mechanisms by which it occurs and of the controlling factors has
been acquired.
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TYPES OF WEAR
ABRASIVE WEAR
EROSIVE WEAR
CAVITATION WEAR
1. ABRASIVE WEAR
It was originally thought that abrasive wear by grits or hard asperities closely resembled cutting
by a series of machine tools or a file. However, microscopic examination has revealed that the
cutting process is only approximated by the sharpest of grits and many other more indirect
mechanisms are involved. The particles or grits may remove material by microcutting,
microfracture, pull-out of individual grains or accelerated fatigue by repeated deformations as
illustrated.
Fig :- Mechanisms of abrasive wear: microcutting, fracture, fatigue and grain pull-out.
2. EROSIVE WEAR
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Erosive wear involves several wear mechanisms which are largely controlled by the particle
material, the angle of impingement, the impact velocity, and the particle size. If the particle is
hard and solid then it is possible that a process similar to abrasive wear will occur. Where liquid
particles are the erodent, abrasion does not take place and the wear mechanisms involved are the
result of repetitive stresses on impact.
The term 'erosive wear' refers to an unspecified number of wear mechanisms
which occur when relatively small particles impact against mechanical components. This
definition is empirical by nature and relates more to practical considerations than to any
fundamental understanding of wear. The known mechanisms of erosive wear are illustrated.
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Fig :- Possible mechanisms of erosion; a) abrasion at low impact angles, b) surface fatigue during low speed,
high impingement angle impact, c) brittle fracture or multiple plastic deformation during medium speed, large
impingement angle impact, d) surface melting at high impact speeds, e) macroscopic erosion with secondary
effects, f) crystal lattice degradation from impact by atoms.
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3. CAVITATION WEAR
The characteristic feature of cavitation is the cyclic formation and collapse of bubbles on a solid
surface in contact with a fluid. Bubble formation is caused by the release of dissolved gas from
the liquid where it sustains a near-zero or negative pressure. Negative pressures are likely to
occur when flow of liquid enters a diverging geometry, i.e. emerging from a small diameter pipe
to a large diameter pipe. The down-stream face of a sharp sided object moving in liquids, e.g.
ship propeller, is particularly prone to cavitation. The ideal method of preventing cavitation is to
avoid negative pressures close to surfaces, but in practice this is usually impossible. When a
bubble collapses on a surface the liquid adjacent to the bubble is at first accelerated and then
sharply decelerated as it collides with the surface. The collision between liquid and solid
generates large stresses which can damage the solid. Transient pressures as high’as 1.5 [GPa] are
possible. The process of bubble collapse together with experimental evidence of a hole formed in
a metal surface by bubble collapse are shown in Figure.
Fig :- Mechanism of cavitation wear; mechanism of bubble collapse
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5. LUBRICATION
Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close
proximity.
Types of lubrication :-
HYDROSTATIC LUBRICATION
ELASTOHYDRODYNAMIC LUBRICATION
EXTREME PRESSURE LUBRICATION
SOLID LUBRICATION
HYDROSTATIC LUBRICATION
In hydrostatic lubrication the bearing surfaces are fully separated by a lubricating film of liquid
or gas forced between the surfaces by an external pressure. The pressure is generated by an
external pump instead of by viscous drag as is the case with hydrodynamic lubrication. As long
as a continuous supply of pressurized lubricant is maintained, a complete film is present even at
zero sliding speed. Hydrostatic films usually have a considerable thickness reaching 100 [pm]
and therefore prevent contact between the asperities of even the roughest surfaces. This ensures a
complete absence of sticking friction. Furthermore, the friction generated by viscous shear of the
lubricant decreases to zero at zero sliding speed. Hydrostatic bearings can support very large
masses and allow them to be moved from their stationary positions with the use of minimal
force. These extraordinary features of zero static friction and high load capacity were applied, for
example, in the 5.08 [ml diameter Mount Palomar telescope and in many radar installations.
With other types of bearing, starting friction is inevitable and can cause distortion and damage to
large structures. This problem is critical to the design of large telescopes which rely on extreme
accuracy of telescope positioning.
ELASTOHYDRODYNAMIC LUBRICATION
Elastohydrodynamic lubrication can be defined as a form of hydrodynamic lubrication where the
elastic deformation of the contacting bodies and the changes of viscosity with pressure play
fundamental roles. The influence of elasticity is not limited to second-order changes in load
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capacity or friction as described for pivoted pad and journal bearings. Instead, the deformation of
the bodies has to be included in the basic model of elastohydrodynamic lubrication. The same
refers to the changes in viscosity due to pressure. Elastohydrodynamic lubrication can be defined
as a form of hydrodynamic lubrication where the elastic deformation of the contacting bodies
and the changes of viscosity with pressure play fundamental roles. The influence of elasticity is
not limited to second-order changes in load capacity or friction as described for pivoted pad and
journal bearings. Instead, the deformation of the bodies has to be included in the basic model of
elastohydrodynamic lubrication. The same refers to the changes in viscosity due to pressure.
EXTREME PRESSURE LUBRICATION
In many practical applications there are cases where the operating conditions are such that
neither hydrodynamic nor EHL lubrication is effective. The question then is, how are the
interacting machine components lubricated and what is the lubrication mechanism involved? The
traditional name for this type of lubrication is 'boundary lubrication' or 'boundary and
extreme-pressure lubrication'. Several specialized modes of lubrication such as, adsorption,
surface localized viscosity enhancement, amorphous layers and sacrificial films are commonly
involved in this lubrication regime to ensure the smooth-functioning and reliability of machinery.
Boundary and E.P. lubrication is a complex phenomenon. The lubrication mechanisms involved
can be classified in terms of relative load capacity and limiting frictional temperature.
These lubrication mechanisms are usually controlled by additives present in the oil. Since
the cost of a lubricant additive is usually negligible compared to the value of the mechanical
equipment, the commercial benefits involved in this type of lubrication can be quite large.
SOLID LUBRICATION
Solid lubricants have many attractive features compared to oil lubricants, and one of the obvious
advantages is their superior cleanliness. Solid lubricants can also provide lubrication at extremes
of temperature, under vacuum conditions, or in the presence of strong radioactivity. Oil usually
cannot be used under these conditions. Solid lubrication is not new, the use of graphite as a
forging lubricant is a traditional practice. The scope of solid lubrication has, however, been
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greatly extended by new technologies for depositing the solid film onto the wearing surface. The
lubricant deposition method is critical to the efficiency of the lubricating medium, since even the
most powerful lubricant will be easily scraped off a wearing surface if the mode of deposition is
incorrect.
Specialized solid substances can also be used to confer
extremely high wear resistance on machine parts. The economics of manufacture are already
being transformed by the greater lifetimes of cutting tools, forming moulds, dies, etc. The wear
resistant substances may be extremely expensive in bulk but when applied as a thin film, they
provide an economical and effective means of minimizing wear problems.
Fig :- Mechanism of lubrication by lamellar solids
RECENT ADVANCES IN TRIBOLOGY
The below mentioned advances are the most recent developments achieved in the field of
tribology which will be discussed in detail later in this report
1. SOYBEAN OIL AS FUTURE LUBRICANT FOR IC ENGINES.
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2. CHEMICAL VAPOR DEPOSITION (CVD)
3. CHEMICAL VAPOR DEPOSITION (CVD)
4. TRIBOLOGY CONCERNS IN MEMS DEVICES
5. DIAMOND COATING
6. ULTRANANOCRYSTALLINE DIAMOND (UNCD)
7. SELF ASSEMBLED MONOLAYERS (SAMS)
8. CANTILEVER BEAM ARRAY TECHNIQUE
1. SOYBEAN OIL AS FUTURE LUBRICANT FOR IC ENGINES
Using plant-derived oils like soybean oil as a form of lubrication is nothing new to companies
that operate and maintain machinery. The idea of using soy as a replacement for petroleum has
been around for decades and is becoming increasingly important due to volatile petroleum prices
and heightened concern with dependency on foreign sources of petroleum. Soy also adds natural
lubrication to machinery and enhances engine performance. Soybean oil is a vegetable oil
extracted from soybean seeds. It is easily available at low prices.
PROPERTIES OF SOYBEAN OIL
It has a high viscosity index up to 223.
Has comparatively high flash point 610°F.
Has good fire point about 650°F.
It has high pour point, it can be reduced by winterizing the soybean oil.
MERITS OF SOYBEAN OIL
Soybean oil is biodegradable, in general it is less toxic.
It is a renewable oil, so it reduces dependency of foreign petroleum products.
Ease of processing.
2. CHEMICAL VAPOR DEPOSITION
Chemical vapor deposition (CVD) is a chemical process used to produce high-purity, high-
performance solid materials. The process is often used in the semiconductor industry to
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