MAE 493N 593T Lec11

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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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Wear

• Introduction‐Definition and measurement of wear

‐Classification of wear

• Mechanisms of wear

‐Seizure

‐ Melt wear

‐ Oxidation‐dominated wear

‐ Mechanical wear processes

‐Fatigue

wear

in

rolling

contacts

‐ Fretting and corrosion wear

‐ Erosive wear

• Third bodies and wear

‐Wear

by

abrasive

contaminants

‐ Interfacial ‘third’

bodies

‐ Debris analysis

• Further reading

http://www.engineering.leeds.ac.uk/ietsi/Research_Projects/Lubricant‐surface.shtml

DLC coating wear track in oil lubricated conditions

http://www.engineering.leeds.ac.uk/ietsi/Research_Projects/Lubricant-surface.shtml




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Abrasive wear

•

During abrasive wear harder asperities/harder particles trapped

at the interface

damage the

other

surface

•

If both surfaces are soft, the hard particles can be contaminants from the outside

environment

• The hard particles can be formed in‐situ by oxidation

• Chemical or

mechanical

processes

can

produce

hard

particles

•

Abrasion takes place because the counterface is both rough and intrinsically

harder than the wearing component

•

If

particles

are

entrapped

within

the

contact

boundaries

the

situation

is

known

as

three‐body abrasion

Wear mechanisms

Three‐body abrasionTwo‐body abrasion

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Wear mechanismsAbrasive wear

•

The surface topography during abrasive wear consists of long parallel grooves

running in

the

sliding

direction

http://maxxtorque.com/dieselcommunity/winter‐2009/oil‐bypass‐filtration‐system‐for‐lmm‐dmax

• Volume and

size

of

grooves

varies

from

light

scratching

up

to

severe

scratching

• Abrasive wear accounts for up to 50% of wear problems

•

The rate of damage to a surface in three‐body abrasion is relatively insensitive to

the hardness of the particles if they are at least 20% harder than the surface itself

http://maxxtorque.com/dieselcommunity/winter-2009/oil-bypass-filtration-system-for-lmm-dmax
















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Wear mechanismsAbrasive wear

•

The most common contamination in industry comes from quartz or silica

minerals (60%

of

Earth’s

crust)

•

These particles have hardness around 8 GPa

and they can damage hardened

steel components (7‐8 GPa

hardness)

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Abrasive wear

•

During abrasive wear (using an equation of the same form as Archard’s) the

volumetric loss of a material is proportional to the distance slid and the intensity

of loading

If: Abrasive wear resistance = (wear volume)‐1

Then, Abrasive wear resistance is proportional to Hardness

Wear mechanisms

This has been confirmed experimentally in the early 1950s

(Kruschov

1957)

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Wear mechanisms

Kruschov

1957

Abrasive wear resistance is proportional to Hardness

Two‐body abrasion

(a)Pure metals

(b)Steels with varying

compositions and heat

treatments

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Wear mechanismsKruschov

1957

Abrasive wear resistance is proportional to Hardness

•

In such experiments specimens of each material in the form of cylindrical pins

are rubbed against abrasive paper/cloth carrying SiC or quartz particles

•

Relative wear resistance is equal to wear volume of

the sample divided by that of some standard material

tested under the same conditions

•

Relative wear resistance of a range of pure metals is

proportional to hardness

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Wear mechanisms• For alloys the situation is more complicated

• Steels

of

different

compositions

and

heat

treatments

(i.e.

different

hardnesses)

exhibit linear relationships but of different slopes from that of pure metals

• This is attributed to details in the microstructures of such alloys

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Wear mechanisms

•

Models of abrasive wear always assume that the deformations of the harder

surface are

negligible

compared

to

those

of

the

softer

surface

•

They are also based on a single hard asperity moving across a softer previously

unreformed surface

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Wear mechanisms

•

A conical asperity of semi‐angle 90o

–

θ

carrying a normal

load

W indents

a soft

surface to depth h

•

θ

represents the ‘average’

roughness of

the abrading surface

•

If

the

asperity

starts

sliding,

the

wear

rate

will be given by;

ϑ cot2×= hw

The depth h can be related to hardness H and the normal load W can be expressed in

terms of h and H as follows;

H hW ××=2)cot(

2

ϑ π

And finally;

H

W

w ×=π

ϑ tan2

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Wear mechanisms

H

W

w ×=π

ϑ tan2

This is like Archard’s equation

K (Wear coefficient)

The above suggests that rougher surfaces (larger θ) will exhibit larger K and thus

more wear damage

• However, the simple equation overestimates the wear observed in

reality

• If we set θ=1o , the predicted value of K is around 0.01

In reality, measured values of K in two‐body abrasion of metals are observed to lie

in the

range

of

5x10‐

3

–

50x10‐

3

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Wear mechanisms

•

The discrepancy between predicted and measured values of K is due to our

assumption that

all

material

from

the

groove

is

lost

from

the

surface

•

In an actual fact, experimental observation showed that some

material is

actually detached (due to microcutting) while the remainder is piled up at the

wear track edges (due to ploughing)

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Wear mechanisms

•If we attempt to model the asperity as a symmetrical pyramid then we can have

only some

material

lost

from

the

surface

•

The extent of subsurface deformation can be judged form the extent of the

wedge/prow of material in front of the leading edge

h

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Wear mechanisms

•The material in the ridges does not contribute greatly to wear because it is not

detached

from

the

surface•

The relative volumes appearing in the ridges and the produced chips depend

on the geometry of the hard asperity

•

In the symmetrical pyramid case the geometry is described by angles ψ

and φ

as

shown

in

figure

5.8b

Angle between leading edge and direction of

motion is called attack angle

ψ

Angle at the base of pyramidal asperity (2φ) is

called dihedral angle

If the

attack

angle

exceeds

some

critical

value

ψc

, there is a significant contribution to wear

by micromachining

If the angle is less than the critical value, the

process is

dominated

by

ploughing

or

rubbing

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W h i

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Ploughing: The surface topography is much modified but a small proportion

of the displaced material is actually detached from the surface

Micromachining:

A

much

higher

proportion

of

the

plastically

deforming

material is lost as wear debris

Wear mechanisms

Wear mechanisms

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Wear mechanismsTransition from ploughing to micromachining

•The dihedral angle can also influence the wear mode

• If 2φ

is small, the asperity is like a knife cutting through the surface

• As φ

increases

the

asperity

gets

‘bluffer’

•

When 2φ=180o the asperity will be moving with one of its flat faces forward

rather than with one of its edges

‐

The

effect

of

both

angles

is

illustrated

on

the

wear‐

mode

diagram

above

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Wear mechanismsSli li fi ld

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ea

ec a s sSlip‐line field

• Fig 5.10a shows a slip‐line field for this deformation mode

•

If friction between the hard asperity and the deforming surface

is high, the

plastic wave

may

shear

off

from

the

surface

along

the

path

AB

(Fig

5.10b)

•

When one prow has been removed as a wear fragment the indenter digs once

more and the process restarts

•

Larger values of attack angle ψ

favour cutting. h is depth of presentation of the

asperity into

the

softer

surface

(Fig

5.10c)

Wear mechanismsUpper bound approach

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Upper bound approach

• The 3D aspect of the problem is maintained in this case

• Forces and geometry are related

• Indenter is assumed to be rigid and the abraded material to be perfectly plastic

• The material flows at a constant shear stress h

•

This approach is also challenging but it has been shown that reasonable

agreement

between

theory

and

experiment

can

be

achieved‐Mainly for the transition from ploughing to micromachining

‐ And the magnitude of the tangential force

Wear mechanisms

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In reality…

• A wear surface is formed as a result of many wear events

• These wear events follow one another during the lifetime of the

component

• The interaction of individual wear events is not well understood to date

•

There

is

no

fully

satisfactory

way

of

predicting

abrasive

wear

rates

(component

lives) in terms of their initial surface geometry/topography and

material properties

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Summary

• Wear mechanisms

‐

Abrasive wear

MAE 493N 593T Lec11

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