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Dr.V.S.Gayathri & Dr.K.Yamuna Page 1 11/18/2008

LUBRICANTSTopics

Definition Classification Mechanism of lubrication Properties Solid lubricantsOn viewing under an electron microscope, the surface of even a mirror polished metallic surface exhibits

irregularity. The peaks are called asperities and the valleys are called pits

Scanning Electron Microscopic Picture of Mirror Polished Platinum Surface

Asperities and Valleys on two sliding surfaces


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Types of Friction: There are two types of friction (Static and Dynamic)

Sliding Friction: Two flat Surfaces in motion Rolling Friction: At least one moving surface is spherical

Frictional Heat and Welded Joints

Ar= Real area of contact, junction, high stress, plastic deformation, adhesive bonding

B = Area filled with oxides, lubricant or air

Micro weld is the area where invisible welded joints are formed

N = Normal force

F = Frictional force

Definition

Substances introduced between moving surfaces in order to reduce friction and thusprevent welded joints and seizure, are called Lubricants

The process of prevention of metal to metal contact by means of an intervening layer offluid or fluid like material is termed as Lubrication

Functions of Lubricants

It acts as a thermal barrier and reduces friction and wear and prevents welded joints Avoids seizure of moving surfaces Acts as coolant Acts as a seal and prevents entry of dust, moisture, & dirt between moving parts

Ar B


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Some lubricants acts as corrosion inhibitors thus reduce operational costClassification: Three Different types of classifications are done

Based on occurrence / Source Based on lubricant Physical State

Based on Lubrication process

Liquid lubricant examples: 1. Animal oils - Lard oil, Tallow oil, Whale oil 2. Vegetable oil - Castor oil, Palm oil, Coconut oil 3. Mineral based - Petroleum fractions 4. Emulsions

O/ W Cutting liquids

W / O Cooling liquids

5. Synthetic oils blended mineral oils

Source

NaturalRubber, graphite

SyntheticSilicones, greases

Physical

SolidMoS2 ,Talc , Mica Silicone, Graphite and

Semisolid

Greases VaselineLiquid (oil)

1. 432

O/W W/O


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Thick Film / Fluid film / Hydrodynamic Lubrication

In this type a continuous fluid film exists between the sliding surfaces

Employed when the sliding velocity is moderate and load is low Used for machineries operated continuously In fluid film lubrication, the fluid immediately adjacent to each surface travels at the

same speed and direction of each surface.

Film thickness is 1000 A Factors which affect the thickness of hydrodynamic fluid film include lubricantviscosity, rotation speed or RPM, oil supply pressure, and component loading.

An increase in speed or viscosity increases oil film thickness. An increase in load decreases oil film thickness. Coefficient of friction is 0.001 to 0.03 cm/sec Blended Mineral oils are used to introduce a thick film between sliding surfaces Finds application in watches, clocks and scientific instrument (stop watches) Hydrodynamic lubrication often occurs in components such as cylinders, gears, and plain

bearings

Mechanism

Thick film

Greases, Vaseline

Thin film

Blended mineral oils

Extreme Pressure

Oil with chlorinated ester


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Boundary Lubrication / Thin film

Boundary lubrication often occurs during the startup and shutdown of equipment, or when

loading becomes excessive.

Low speed , high load & non continuous operation Thickness is 20 30 A

Coefficient of friction is 0.05 0.15 cm /sec Film formation by either Physisorption or chemisorption Oiliness should be high Fatty acids and their soaps are used as additives Used in automobiles and heavy machinery Certain types of gear sets that need to withstand sliding pressures and shock loading,

such as hypoid gears use boundary lubricants

a and b Thick film lubrication

c and d - Thin film lubrication


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Extreme Pressure Lubricants

Extreme pressure lubricants interact with metal surfaces in a chemical process, at molecular

level, to create a protective compound which increases the thermal stability of metal surfaces.

This is not a film or coating over the metal. It is a permanent modification of the metalsurface

Molecules in the extreme pressure lubricants are polarized and actually bond with the metal

surface

It is applicable in High load and high speed condition

Metal surface should be active (Fe, Cu etc.,) Lubricants containing Chlorinated ester/ Sulphurised oils / Tricresyl phosphate are used Surface compounds is formed due to chemisorption (ex.sulphurised oil containing

lubricants reacts with Iron and forms Iron sulphide whose thermal stability is better thanmere iron)

The thickness of the surface compound is a few molecule layer Co. efficient of friction 0.1 0.4 cm /sec It finds application in Air crafts and space crafts Unique character of this type of lubricants is its replenish ability / easy of

replacement

Extreme pressure

lubricant


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Important Properties

1. Viscosity and Viscosity Index

Viscosity is defined as Resistance to flow Viscosity Index is the measure of resistance to change in viscosity Viscosity Index (V.I) is value representing the degree for which the oil viscosity changes

with temperature.

If change in viscosity ( ie decrease in viscosity with increase in temperature) is small , theoil is said to have a high viscosity index.

Low viscosity does not mean Low VI (Ex. Viscosity of water is less but VI of water ishigh)

Viscosity Index is measured using Red wood (Common Wealth Countries) Viscosity Index is measured using Say bolt viscometer (USA) Viscosity Index is measured using Engler viscometer (European countries) Naphthalenic base Gulf oils have Low viscosity index (VI =0 , L series oils) Paraffinic base Pennsylvanian oils have high viscosity index (VI =100, H series oils)

The viscosity of test oil is measured at 100

o

F and 210

o

F

An oil possessing the same viscosity as that of test oil at 210 oFis chosen from both L and H series

Their corresponding viscosities at 100oF is noted from the series ( L and H) If the viscosity of the test oil at 100oF is considered as U then its VI can be

calculated using the formula

V.I. = (L U / L H) x 100

Polymers and copolymers of olefins, methacrylates, dienes or alkylated styrenes areused as viscosity index improvers

When the oil gets heated up due to frictional heat, the viscosity of the oil decreases . The solubility of a polymeric molecule increases with increase in temperature which

results in the increase in viscosity


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Hence at elevated temperatures, the decrease in the viscosity of the oil will be balancedby the solubility of the polymer in oil. Thus polymers act as good VI improvers

Good motor oil has a high V.I.(For diagram, description and working of Red wood viscometer refer Text book - 190OR class notes)

2. Tackiness / oiliness / lubricity

Oiliness is defined as the ability of the oil to stick firmly to a solid surface Oiliness should be high for a good lubricant Can be improved using additives long chain fatty acids and soaps are generally used as

tackiness improvers

Very essential in Boundary lubrication as tackiness improves replenish ability of thesurface film

3. Cloud and Pour Points

Cloud point is defined as the minimum temperature at which oil becomes hazy Pour point is defined as the minimum temperature at which oil stops pouring( or ceases

to flow)

When oil is cooled , wax starts crystallizing , and the oil becomes hazy Tiny wax crystals agglomerate and form bigger crystal lattice, at this stage the oil

solidifies completely and does not pour out on tilting.

Both cloud and Pour points should be very less than the operating temperature Cloud and pour points can be reduced by adding depressants Polymeric additives such as poly amino alcohols are added to decrease cloud and pour

points

Other pour point depressants include alkylated naphthalenes and phoenolic polymers,poly methacrylates, maleata/fumarate and copolymer esters

The additives lower the pour point either by forming a thin coating over wax and preventit from agglomerisation or co precipitate along with wax , thus reducing the freezingpoint


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(For diagram, description and working of cloud and pour point apparatus refer Text

book - 194 OR class notes)

4.Flash and fire Points

Flash point is defined as the lowest temperature at which the oil gives off just enoughvapours which gives out a flash when an open (tiny ) flame is brought near the vapours

Fire point is the lowest temperature at which the oil gives off enough vapours whichburns continuously at least for 5 seconds

Flash and Fire points should be high for a good lubricant Flash and fire point additives are used to increase the performance of a lubricant

CTFE is added to increase flash and fire points as it increases the vaporization point oflubricants

Usually there is a difference of 5 40o C between both for a good lubricant

(For diagram, description and working of Pensky Marten apparatus refer Text book -

192 OR class notes)

Blended oils

Practically no single lubricant will be able to satisfy all the properties of a good lubricant.

Hence it is essential to add various additives to achieve the required performance.

The process of adding different substances to impart a special character to the base oil is called

blending and the resulting oil is called blended oil.

The following table gives the various additives and their function in synthetic oils / blended oils

Name of the additive Substance added to

lubricating oils

Function

Oiliness

Fatty acids like stearic acid,

palmitic acid, oleic acidVegetable oils

Increases lubricity

Prevent rupture of film


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Extreme Pressure

additives

Organic chlorine

compounds / organicsulphur compounds /

phosphorous compounds

Form surface compounds

which has better thermalstability & wear resistance

Viscosity Index improvers n-hexanol, polyalkylbenzene, polyisobutylene

Reduce the rate of changeof viscosity with

Temperature

Pour point depressants Phenols, chlorinated

hydrocarbons

Prevent the agglomerisation

of wax which separates outfrom the lubricating oils

Thickeners Polystyrene, polyesters Increase the viscosity oflubricating oils

Antioxidants Phenolic compds, aromatic

amines

Prevent oxidation of oil.

Prevent gum formation

Deposit inhibitors /

detergents/ deflocculents

Salts of carboxylic acid /salts of phenols/

sulphonates

Reduce deposits in engineswhich block the passage of

oil

Corrosion inhibitors Organic compounds ofPhosphorous / antimony

Get adsorbed on metalsurface and protect the

surface from attack by

moisture

Solid lubricants

Solid lubricants are deposited in the valleys to create a smoother surface.

They are designed to protect against metal contact by coming between two peaks at

the moment of contact.

This results in the deformation of the peak, rather than formation of welded joints.


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Even between highly loaded stationary surfaces the lamellar structure is able to

prevent contact.

Large lubricant particles perform better on relative rough surfaces and at low speed,while finer particle perform better on smooth surface and higher speeds

(1stand 2ndare sliding surfaces and 3rdis solid lubricant filling the valley)

Layered Compounds like graphite , boron nitride ,molybdenum di sulphide , talc ,teflon, mica, calcium fluoride , cerium fluoride, tungsten disulfide etc are as

solid lubricants used

Graphite (organic substance) and molybdenum disulfide (MoS2 is inorganicsubstance) are the predominant materials used as solid lubricant due to their

lamellar structure

The lamellas orient parallel to the surface in the direction of motion and shearsover each other easily, resulting in a low friction.

Graphite

Graphite is a form of carbon which exists as a stack of 'sheets' of carbonatoms, each sheet having a hexagonal arrangement of atoms


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Three valencies are satisfied by covalent bonds and the fourth is weak VanderWaals Force

Both crystalline and amorphous form of graphite find application as lubricant,however amorphous graphite does not possess high shear strength hence usedonly when load is low.

Both natural and synthetic graphite can be used as lubricants Graphite is soapy to touch and non inflammable The C-C bond length is 1.42A and the distance between two hexagonal planes is

3.4A

It is stable up to 375oC (in presence of air) Water vapor is a necessary component for graphite lubrication. The adsorption of water reduces the bonding energy between the hexagonal

planes of the graphite Because water vapor is a requirement for lubrication, graphite is not effective

in vacuum Graphite is usually mixed with oil / water/ greases Graphite mixed with oil is called oil dag and graphite mixed with water is called

aqua dag

It is used to lubricate air compressors , railway track joints , food stuffindustries , IC engines etc.,

Molybdenum di Sulphide

Just like graphite, MoS2 also has a hexagonal crystal structure with theintrinsic property of easy shear

MoS2lubrication performance often exceeds that of graphite and is effectivein vacuum also

Mo layer is sandwiched between two sulphur layers Like graphite it has weak van der Waals forces between the basal planes.

The bonds between the sulfur layers are weaker than the bonds between themolybdenum layers.

The Metallic bond length , Mo-Mo is 3.08A , the covalent bond length S-S is3.15 A and the distance between two lamellas is 3.13A

It is stable up to 425oC in presence of air and up to 800oC in absence of air


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It is chemically inert It may be used in the form of powder/ mixed with oil or greases Water is not essential for its lubricity hence can be used in vacuum too in fact

its lubricity decreases in presence of water

It is used in IC engines

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