LUBRICANTS

Introduction
Machines have movable parts and the surfaces of the moving or sliding or rolling parts
rub against each other. This causes friction which in the long run causes a lot of wear
and tear of surfaces.

A lubricant can be introduced between two moving surfaces in order to reduce the
friction (or frictional resistance) between them. The main purpose of a lubricant is to
keep the moving/sliding surfaces apart, so that friction and consequent destruction of
material is minimized. This process is called lubrication.

Function of Lubricants:
1. Reduction of wear and tear of the surfaces
2. Lubrication reduces expansion of metal due to frictional heat and destruction of
material
3. It acts as a coolant of metal due to heat transfer media
4. It avoids unsmooth relative motion
5. It reduces maintenance cost
6. It also reduces power loss in internal combustion engines

THEORIES OF FRICTION

Welding theory: All metal surfaces, regardless how much finely finished they are,
appear as a series of peaks (or asperities) and valleys. So when two solid surfaces are
pressed one over the other, only the peaks of the two surfaces come in real contact.
Under the action of a load, the local pressure at the peaks becomes sufficiently great to
cause deformation of the peaks to create weld junctions between them.

Mechanical Interlocking: When one surface moves over another, the peaks and valleys
present on the surface undergo interlocking; restrict the movement of one surface over
the other. This accounts for static friction.

Molecular Attraction: Atoms of one material are plucked out of the attractive range of
their counterparts on the mating surface, lead to the friction.

Electrostatic Attraction: When stick-slip phenomenon takes place between rubbing

metal surfaces, a net flow of electrons takes place producing clusters of charges of
opposite polarity at the interface. These charges are responsible for holding the surfaces
together by electrostatic attraction.

MECHANISMS OF LUBRICATION

(a) Thick-Film lubrication (Fluid-Film or hydrodynamic lubrication)

In hydrodynamic lubrication, the moving surfaces are separated by a thick film of fluid
(at least 1000 A° thick). The lubricant film covers/fills the irregularities of moving
surfaces and forms a thick layer between them, so that there is no direct surface to
surface contact and welding of welding of junctions rarely occurs.

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Note:
- The lubricant chosen should have minimum viscosity to reduce the internal
resistance between the particles of the lubricant under working conditions
- The lubricant should remain in place and separate the surfaces.
o Example of lubricant; Hydrocarbon oils (mineral oils which are lower
molecular weight hydrocarbons with about 12 to 50 carbon atoms)
o In order to maintain the viscosity of the oil in all seasons of year, ordinary
hydrocarbon lubricants are blended with selected long chain polymers.

(b) Thin Film lubrication (Boundary lubrication)

This type of lubrication is preferred where a continuous film of lubricant cannot persist.
In such cases, the clearance space between the moving/sliding surfaces is lubricated by
such a material which can get adsorbed on both the metallic surfaces by either
physical or chemical forces. This adsorbed film helps to keep the metal surfaces away
from each other at least up to the height of the peaks present on the surface.

- Vegetable and animal oils and their soaps can be used in this type of lubrication
because they can get either physically adsorbed or chemically react in to the
metal surface to form a thin film of metallic soap which can act as lubricant.
- Although these oils have good oiliness, they break down at high temperatures.

***Mineral oils are thermally stable and the addition of vegetable/animal oils
to mineral oils can improve their oiliness. Graphite and molybdenum
disulphide are also suitable for thin film lubrication.

(c) Extreme Pressure lubrication

A high local temperature is attained when moving (sliding) surfaces are at high pressure
and speed. In such a case, liquid lubricants fail to stick and/or may decompose and even
vaporize. At such conditions, extreme pressures additives are added to minerals
oils.These additives form more durable films (capable of withstanding very high loads
and high temperatures) on metal surfaces. Important additives are organic compounds
having active radicals or groups such as chlorine (as in chlorinated esters), sulphur (as in
sulphurized oils) or phosphorus (as in tricresyl phosphate). These compounds react with

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metallic surfaces, at existing high temperatures, to form metallic chlorides, sulphides or
phosphides.

CLASSIFICATION OF LUBRICANTS

Lubricants are classified on the basis of their physical state, as follows;

a. Gaseous Lubricants
b. Liquid lubricants or Lubricating Oils,
c. Semi-solid lubricants or Greases and
d. Solid lubricants.

(a) Gaseous Lubricants

These will give very low resistance against the flow. In that case the coefficient of
friction will be very low but, because of the molecular structure they are not able to
sustain much load that means they can be used only for very low load applications

(b) Liquid lubricants or Lubricating oils - further classified into three categories;
i. Animal and Vegetables oils,
ii. Mineral or Petroleum oils and
iii. Blended oils.

Animal and Vegetables oils

- Animal oils are extracted from the crude fat and vegetables oils such as cotton
seed oil and caster oils.
- These oils possess good oiliness and hence they can stick on metal surfaces
effectively even under elevated temperatures and heavy loads.
- However;
o They are costly
o Undergo easy oxidation to give gummy products
o Hydrolyze easily on contact with moist air or water.
There are rarely used for lubrication on their own, but they are still used as blending
agents in petroleum based lubricants to get improved oiliness .

Mineral or Petroleum oils

These are basically low molecular weight hydrocarbons with about 12 to 50 carbon
atoms. They are cheap, available in abundance and stable under service conditions,
hence they are widely used.
However: The oiliness of mineral oils is low ... This can be improved by addition of
higher molecular weight compounds e.g. oleic acid and stearic acid

Blended oils
No single oil possesses all the properties required for a good lubricant and hence
addition of proper additives is essential to make them perform well.
Examples: The addition of higher molecular weight compounds like oleic acid, stearic
acid, palmetic acid, etc or vegetables oil like coconut oil, castor oil, etc increases the
oiliness of mineral oil.

Characteristic of good lubricating oils

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 (1) High boiling point
(2) Low freezing point
(3) Adequate viscosity for proper functioning in service
(4) High resistance to oxidation and heat
(5) Non-corrosive properties and
(6) Stability to decomposition at the operating temperatures

(c ) Semi-solid Lubricants or Grease

- They are obtained by combining lubricating oil with thickening agents. The
lubricating oil can be either petroleum oil or a synthetic hydrocarbon of low to
high viscosity.
- The thickeners consist primarily of special soaps of Li, Na, Ca, Ba, Al, etc.
- Non-soap thickeners include carbon black, silica gel, polyureas and other
synthetic polymers, clays, etc.

Grease can support much heavier load at lower speed. Internal resistance of grease
is much higher than that of lubricating oils; therefore it is better to use oil instead of
grease. Compared to lubricating oils, grease cannot effectively dissipate heat from
the bearings, so work at relatively lower temp.

(d) Solid lubricants

They are preferred where;
1. The operating conditions are such that a lubricating film cannot be secured by
the use of lubricating oils or grease
2. Contamination (by the entry of dust particles) of lubricating oils or grease is
unacceptable
3. The operating temperature or load is too high, even for grease to remain in
position and
4. Combustible lubricants must be avoided.

They are used either in the dry powder form or with binders to make them stick firmly
to the metal surfaces while in use. They are available as dispersions in nonvolatile
carriers like soaps, fats, waxes, etc and as soft metal films.

- The most common solid lubricants are graphite, molybdenum disulphide,

tungsten disulphide and zinc oxide. They can withstand temperature up to 650°
C and can be applied in continuously operating situations.
- They are also used as additives to mineral oils and greases in order to increase
the load carrying capacity of the lubricant.
- Other solid lubricants in use are soapstone (talc) and mica.

Graphite:
It is the most widely used of all the solid lubricants and can be used either in the
powdered form or in suspension. It is soapy to touch; non-inflammable and stable up to
a temperature of 375° C. Graphite has a flat plate like structure and the layers of
graphite sheets are arranged one above the other and held together by weak van der
Waal’s forces. These parallel layers which can easily slide one over other make graphite
an effective lubricant. Also the layer of graphite has a tendency to absorb oil and to be
wetted of it.

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Molybdenum Disulphide

It has a sandwich-like structure with a layer of molybdenum atoms in between two

layers of sulphur atoms. Poor interlaminar attraction helps these layers to slide over one
another easily. It is stable up to a temperature of 400° C.

PROPERTIES OF LUBRICANTS
1. Viscosity
2. Flash Point and Fire Point
3. Cloud Point and Pour Point
4. Aniline Point and
5. Corrosion Stability

1. Viscosity
The resistance to its own flow of liquid is known as viscosity (unit poise). It is the most
important single property of any lubricating oil, because it is the main determinant of
the operating characteristics of the lubricant.
***
- If the viscosity of the oil is too low, a liquid oil film cannot be maintained
between two moving/sliding surfaces.
- If the viscosity of the oil is too high, excessive friction will result.****

Effect of temperature on viscosity:

- Viscosity of liquids decreases with increasing temperature (consequently, the
lubricating oil becomes thinner as the operating temperature increases).
- Hence, viscosity of good lubricating oil should not change much with change in
temperature, so that it can be used continuously, under varying conditions of
temperature.

The rate at which the viscosity of lubricating oil changes with temperature is measured
by an arbitrary scale, known as Viscosity Index (V. I) . If the viscosity of lubricating oil falls
rapidly as the temperature is raised, it has a low viscosity index. On the other hand,
if the viscosity of lubricating oil is only slightly affected on raising the temperature, its
viscosity index is high.

2. Flash Point and Fire Point:

- Flash point is the lowest temperature at which the lubricant oil gives off enough
vapours that ignite for a moment, when a tiny flame is brought near it;
- Fire point is the lowest temperature at which the vapours of the lubricant oil burn
continuously for at least five seconds, when a tiny flame is brought near it.
o In most cases, the fire points are 5° C to 40° C higher than the flash points.
- A good lubricant should have flash point at least above the temperature at which
it is to be used. This safeguards against risk if fire, during the use of lubricant.

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 3. Cloud Point and Pour Point
- Cloud point is the temperature at which a lubricant becomes cloudy or hazy in
appearance when it is cooled slowly
- The temperature, at which the lubricant oil ceases to flow or pour, is called its
pour point.
o Cloud and pour points indicate the suitability of lubricant oil in cold
conditions.
- Lubricant oil used in a machine working at low temperatures should possess low
pour point; otherwise solidification of lubricant oil will cause jamming of
machine. It has been found that presence of waxes in the lubricant oil raise pour
point.

4. Aniline Point
- Aniline point of the lubricant oil is defined as the minimum equilibrium solution
temperature for equal volumes of aniline and lubricant oil samples.
- It gives an indication of the possible deterioration of the lubricant oil in contact
with rubber sealing; packing, etc.
- Aromatic hydrocarbons have a tendency to dissolve natural rubber and certain
types of synthetic rubbers. Consequently, low aromatic content in the lubricant
oil is desirable. A higher aniline point means a higher percentage of paraffinic
hydrocarbons and hence, a lower percentage of aromatic hydrocarbons.
Aniline point is determined by mixing mechanically equal volumes of the lubricant oil
samples and aniline in a test tube. The mixture is heated, till homogenous solution is
obtained. Then, the tube is allowed to cool at a controlled rate. The temperature at
which the two phases (the lubricant oil and aniline) separate out is recorded at the
aniline point.

5. Corrosion Stability

Corrosion stability of the lubricant oil is estimated by carrying out a corrosion test.
- A polished copper strip is placed in the lubricant oil for a specified time at a
particular temperature.
- After the stipulated time, the strip is taken out and examined for corrosion
effects.
- If the copper strip has tarnished, it shows that the lubricant oil contains any
chemically active substances which cause the corrosion of the copper strip.

A good lubricant oil should not effect the copper strip. To retard corrosion effects of
the lubricant oil, certain inhibitors are added to them. Commonly used inhibitors are
organic compounds containing P, As, Cr, Bi or Pb.

Characteristics of a good lubricant are as follows

1. It should have a high viscosity index.
2. It should have flash and fire points higher than the operating temperature of the
machine.
3. It should have high oiliness.
4. The cloud and pour points of a good lubricant should always be lower than the
operating temperature of the machine.
5. The volatility of the lubricating oil should be low.
6. It should deposit least amount of carbon during use.
7. It should have higher aniline point.
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 8. It should possess a higher resistance towards oxidation and corrosion.
9. It should have good detergent quality

TRIBOLOGY - The science and study of the mechanisms of friction, lubrication and wear
of interacting surfaces that are in relative motion.

Deals with friction, wear and lubrication

• Two aspects – Science: Basic mechanisms
– Technology: Design, manufacture, maintenance

Four Elements of Tribology

- Surface interactions, including lubrication and lubricants
- Generation and transmission of forces at the interface
- Response of materials to the force generated at the interface
- Design of tribological systems
-
Some of the Basic Questions
- What is friction
- How is the friction force generated?
- How do materials wear?
- What is Friction
- How do you lower friction?
- How should we reduce the wear rate of materials?

Friction force - The force exerted by a surface as an object moves across it or makes an
effort to move across it. There are at least two types of friction force - sliding and static
friction.
Friction results from the two surfaces being pressed together closely, causing intermolecular
attractive forces between molecules of different surfaces. As such, friction depends upon the
nature of the two surfaces and upon the degree to which they are pressed together.

Friction is affected by the following:

1. Presence of wear particles and externally introduced particles at the sliding

interface
2. Relative hardness of the materials in contact
3. Externally applied load and/or displacement
4. Environmental conditions such as temperature and lubricants
5. Surface topography
6. Microstructure or morphology of materials
7. Apparent contact area
8. Kinematics of the surfaces in contact (i.e., the direction and the magnitude of
the relative motion between the surfaces

Composition of Lubricants

A lubricant is a balanced mixture of a number of components. The formulation of a

lubricant is made up of base oils and additives that combine to determine behavior
when in use (in terms of performance and duration). The final quality of lubricating oil
usually depends on the quality of the base oils used. Base oils can be:
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 - Mineral oils (low molecular weight hydrocarbons): obtained from the distilling
process in the refining of crude oil or
- Synthetic oils: which are derived from particular physical/chemical laboratory
treatments

Compared to mineral base oils, synthetic base oils offer the following advantages:

1. A lower level of volatility to a comparable level of viscosity (which leads to lower

consumption during use)
2. A higher viscosity index (a wider temperature gap)
3. Greater chemical stability at high temperatures (longer useful life)

Mineral oils
- Naphthenic
- Paraffinic
- Hydrocracked

Additives

The formulation of a lubricant is generally defined by the performance requirements

(e.g. volatility, viscosity, longer life), environmental considerations (non-toxicity,
biodegradability) or by marketing demands (synthetic oil = high technology oil). The
following can be added to a base oil to improve its properties;

1. Polymers
Recall: Polymers are made of long chain molecules of varying sizes and distributions.
Polymers tend to be relatively viscous above their melt temperature and “Sticky” above
their melt temperature thus are added in lubricating oils and greases as viscosity
modifiers, viscosity index improvers, adhesion agents and rheology modifiers. They can
also be used as thickeners to improve consistency

External = Insoluble
- Typically provide lubrication between the polymer and the metal surface of the
processing equipment
- Classic types: Polyethylene waxes, Oxidized Polyethylene waxes, Paraffins, Metal
Soaps, Esters (high esterification), Amides, Fatty Acids

Internal = Semi-Soluble (Plasticizer)

- Typically reduce bulk viscosity through partial compatibility with the polymer,
thus opening the polymer chain with the lubricant’s soluble component while
providing intermolecular lubrication with the less soluble portion of the
molecule.
- Classic types: Fatty alcohols, Esters (low esterification), EVA Wax, others

REMEMBER!
Most lubricants provide a combination of internal and external effects. It is the
balancing of these effects in the formulation that will determine the ultimate and
overall effectiveness of the lubricant!
AND
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Lubricants will act differently in different polymer compounds due to chemical
solubility. The solubilities change relative to polymer chemistry and other
additive (Pigment!) chemistries!

GENERAL CHEMISTRIES OF LUBRICANTS

Acid Amides
- Primary Amides: Erucamide, Oleamide, Stearamide
- Secondary Amides: EBS, EBO

Acid Esters
- PEMS, PEDS, PETS, PEAS, GMS, GMO, Montan Wax,
- Stearyl Stearate, Distearyl Pthalate
Fatty Acids
- Saturated: Lauric (C12), Myristic (C14), Palmitic (C16),
Stearic (C18)
- Unsaturated: Oleic (C18), Erucic
Hydrocarbon Waxes
- Polyethylene, Polypropylene, OPE, Paraffin
Metallic Soaps
- Calcium, Zinc, Magnesium, Lead, Aluminum, Sodium, Tin, Barium, Cobalt, etc.
Stearate

2. ADHESIVES
Adhesive-type additives are highly branched, relatively low molecular weight, low melt
temperature (lower than the polymer) materials that disperse within both polymer and
filler. The adhesives will form mechanical bonds between the moving surfaces.

- Function to increase interfacial forces created by surface attachment (I.e.,

mechanical bonding)
- Increase the energy required to break adhesive bonds causing increased shear

The adhesion leads to increased shear forces, required to break the mechanical bonds,
increased temperatures, therefore dispersion is improved.

3. SURFACTANTS
Surfactants (“Surface Active Agents”) are made up of a tail group which is typically
soluble in non-polar region (internal) and a head group is typically soluble in polar
region or adsorbs to surfaces of polymer, filler or metal. The adsorption is typically via
hydrogen bonding and forms a monolayer with tail group providing lubricating effects.
Surfactants:
- Create a surface active film via polar and non-polar ends
- Polar end adsorbs/bonds to a surface
- Wetting of the filler allows for improved low energy dispersion
- Lubricant-like end effect