MAE 493N 593T Lec13

8/8/2019 MAE 493N 593T Lec13

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“Tribology in Mechanical Engineering”

MAE 493N/593T

Dr. Konstantinos

A.

Sierros

West Virginia University

Mechanical & Aerospace Engineering

ESB Annex

263

[email protected]

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Boundary lubrication and friction

• Introduction

• Mechanism of boundary lubrication

‐ Importance of molecular films

‐ Effect of temperature on surface films

• Elasto‐hydrodynamic lubrication

‐Lubrication of soft surfaces

• Breakdown of lubrication: Scuffing

‐The Blok

temperature hypothesis

• Test methods in boundary lubrication

• Solid lubricants

http://www.stle.org/resources/lubelearn/lubrication/default.aspx



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Introduction

•

Satisfactory operation of bearings requires that solid bearing surfaces are

completely separated by the intervening fluid film

•

The bearing surfaces are not in contact and resistance to tangential motion is due

to viscous loses in the lubricant

•

If lubricant is Newtonian (constant viscosity) the coefficient of friction will

increase with the value of the tangential sliding velocity

**A

Newtonian fluid

is a

fluid

whose

stress

versus

strain

rate

curve is linear and passes

through the origin. The constant of proportionality is known as the viscosity

W

LUn

22

7=μ

μ

is friction coefficient

U is

relative

sliding

speed

of

surfaces

W/L is normal load supported per unit length

n is Newtonian viscocity

U

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U

Introduction

W

LUn

22

7=

μ

• However, when

the

load

is

very

high

or

the

speed

too

slow

it

is hard

to

build

up

a thick film that entirely separates the two surfaces

•

Therefore some mechanical interaction will be observed between opposing

asperities

• The highest

regions

of

the

surfaces

may

be

protected

by

lubricant

films

that

are

only a few molecules thick – This is known as boundary lubrication

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• Hydrodynamic lubrication – two surfaces are separated by a fluid film

•

Elastohydrodynamic

lubrication – two surfaces are separated by a very thin fluid

film

• Mixed lubrication – two surfaces are partly separated, partly in contact

•

Boundary lubrication – two surfaces mostly are in contact with each other even

though a fluid is present.

Lubrication regimes

Hydrodynamic BoundaryMixed




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Boundary lubrication

•

For

boundary

lubrication,

the

bulk

properties

of

the

fluid

(density,

viscosity)

are

of little importance

•

However, the chemical compositions of both the fluid and the underlying

substrate become important

•

Coefficient

of

friction

can

increase

substantially

with

transition

to

boundary

conditions (as much as two orders of magnitude compared to fully

hydrodynamic

regime!!)

• For journal bearings the changes are described by the Stribeck diagram

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Stribeck diagram

ω is rotational speed

p is

nominal

bearing

pressure

n is Newtonian viscosity

• Above A bearing operation is hydrodynamic

• Below B considerations of surface physics and chemistry predominate

• For

small

nω/p,

considerable

amounts

of

solid‐

to‐

solid

contact

exist

and

coefficient

of

friction

depends on the particular set of materials

•

Materials operating near points C and D show surface damage and

effective lubrication may

breakdown (scuffing)

• Intermediate region from A to B is known as mixed lubrication

•

Minimum

point

in

Stribeck

curve

associated

with

very

low

viscous

shear

losses

in

the

lubricant

film and low friction

journal

bearing

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Mechanism of boundary lubrication

Importance of molecular films

•

Boundary lubrication – We need to distinguish between surface greasiness and

inherent oiliness/lubricity of an exceedingly thin film of the lubricant

Oiliness (from SAE): differences in

friction

greater

than

can

be

accounted

for

on

the

basis of viscosity when comparing different lubricants under identical test conditions

•

We need to appreciate the very small scale at which these films operate. The

molecular structure

of

the

lubricant

becomes

important

•

Commercial lubricating oils contain small additions of chemical

compounds designed

to enhance boundary lubrication by forming protective surface films

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• Stearic

acid is a ‘classic’

boundary lubricant

•

When a drop of stearic

acid is floated on a bath of clear water, it spreads until it is

everywhere one molecule thick

•

In

addition,

all

the

molecules

will

be

oriented

with

their

longest

dimension

perpendicular to the surface

Surface

Number of layers

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• Best boundary lubricants – Long chain molecules with an active end group

‐Organic alcohols, amines, fatty acids

• Polar end

groups

are

attached

to

the

surface

•

Monolayers of such films may breakdown to allow mechanical contact of the two

surfaces

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Oiliness additives

•

In general, the longer the chain length of the attached molecule the greater the

degree of separation of the sliding surfaces and the lower the friction coefficient

•

Effectiveness of boundary lubricant depends on tenaciousness of

the bond between

the

active

end

of

the

protecting

molecule

and

the

metal

surface

to

which

it

adheres• The bond can be formed:

‐Physically from electrostatic or dipole forces and is completely

reversible

‐

Chemically by reaction between the metal surface and the lubricant and is

irreversible (ie can

not

be

removed

without

leaving

some

‘mark’ on

the

surface)

Surface

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Oiliness additives

•

Some results on slow speed friction of a steel slider against metal surfaces lubricated

by paraffin oil and paraffin oil+1% lauric

acid C12

H23

COOH (highly polar compound)

•

Upper four metals are not reacting with lauric

acid whereas lower four are more

reactive•

The results indicate that lubrication is mostly affected by the

product of reaction

between the metal surface and the acid which form a metallic soap

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Effect of temperature on surface films

•

During sliding the surface temperatures of contacting solid surfaces can be

significantly higher than those in the bulk

•

When using a boundary lubricant, the temperature increase will make the molecule

more mobile

and

will

tend

to

gradually

reduce

the

extent

of

surface

coverage

•

Increased temperature minimizes lubricant effectiveness and leads to an increased

coefficient of friction

•

The degree of surface coverage (density of ‘pile on the carpet’) is influenced by the

concentration of

the

active

molecules

(solute)

in

the

base

oil

(solvent)

•

The lower the concentration the greater the tendency for the surface to be

unprotected

Surface

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•

There is always a dynamic equilibrium between the mechanisms of

attachment and

detachment from the substrate

•

The reversible nature of the physical effects

lead to gradual increase in friction

coefficient with

temperature

for

paraffin

oil

(curve

A)

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•

If we have a chemical reaction (not reversible) then the increase in temperature will

tend to increase the rate of reaction (curve B) – Eg. fatty acids

•

This can be also thought of as a mild form of surface corrosion

since it involves a

reaction of

the

surface

of

the

component

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•

For surface temperatures higher than 150

oC, physical adsorption becomes negligible

and most metallic soaps undergo thermal decomposition

•

At such temperature levels frictional forces increase significantly and lead to

increased surface

damage.

We

then

say

that

surfaces

start

to

scuff

Study of a novel anti‐wear additive used in ashless anti‐wear hydraulic fluid

Longhua

et al, Ind. Lub. Technol. 61,

2009

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•

To provide surface protection against scuffing, extreme pressure (e.p.) additives are

used

• They help lubricating at relatively high temperatures (300‐400 oC)

• Compounds of

chlorine,

sulphur,

phosphorus

• At low temperatures these additives remain inert

• Curve C illustrates the e.p. behaviour

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• Curve D shows a combination of an oil based on both fatty acid and an e.p. additive

•

We have to be careful when transition from boundary to e.p. activity so our surface

will not get exposed

•

Transition

period

between

losing

the

physically

adsorbed

layer

and

the

formation

of

the protecting chemical layer –

Temperature distress gap

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Transition period between losing the physically adsorbed layer and the formation of

the protecting chemical layer –

Temperature distress gap

Illustrated by the ‘hump’

in the friction trace of the chlorinated paraffin



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•

A range of additives has been developed for surface protection at high working

temepratures

–

antiwear

additives

• ZDDP: zinc dialkyl

dithiophosphate

(1% in motor oils)

•

Phosphorous

‐containing

antiwear

additives

that

form

relatively

thick

(0.05

‐0.5

μm)

polymeric viscous layers

Study of a novel anti‐wear additive used in ashless anti‐wear hydraulic fluid

Longhua

et al, Ind. Lub. Technol. 61,

2009

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Summary

• Introduction

• Mechanism of boundary lubrication

-


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Surface

MAE 493N 593T Lec13

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