railway tribology chp4 wear lect. & all figs ver. e 110613(3)

7/28/2019 Railway Tribology Chp4 Wear Lect. & All Figs Ver. E 110613(3)

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CHAPTER 4

INTRODUCTION TOWEAR & WEAR MECHANISMS

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4.1 Introduction

Wear is the surface damage orthe removal of thematerial from the surface of a solid body as a result

of mechanical action of the counter body. /sliding,rolling or impact motion/.

Wear may combine effects of various physical and

chemical processes proceeding during the frictionbetween two counteracting materials:

micro-cutting

micro-ploughing

plastic deformation

cracking

fracture

welding and melting 3


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Wear is not a material property, it is a system

response. Operating conditions affect interface wear.

It is some times assumed that high-friction interfaces

exhibits high wear rate. This is necessarily not true.

interfaces with solid lubricants and polymers exhibit relatively

low friction and relatively high wear

whereas ceramics exhibit moderate friction but extremely low

wear.

Wear can be either good or bad.

productive wear / writing with pencil, machining, polishing, andshaving / required controlled wear.

Wear is undesirable in almost all machine applications such as

/bearings, seals, gears and cams/. 4


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There are five different types of wear (wear

mechanisms): adhesive, abrasive, fatigue, corrosive

and erosive wear.

In many cases, the combinations of the adhesive,

corrosive and abrasive forms of wear occur:

two-thirds of all wear encountered in industrial situations occurs

because of adhesive-and abrasive wear mechanisms.

Wear by all mechanisms except by fatigue

mechanism, occurs by gradual removal of material.

Objective: to understand the wear mechanisms and

control methods.5


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4.2 Wear mechanisms

Wear occurs by mechanical action and is generally

accelerated by frictional heating (or thermal means).

Wear includes five principal, quite distinct

phenomena that have only one thing in common: the

removal of solid material from rubbing surfaces.

Types of wear are:

1. Adhesive wear

2. Abrasive wear

3. Fatigue wear

4. Corrosive wear

5. Erosive wear6


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4.2.1 Adhesive wear

Adhesive wear occurs when two normally flat

bodies are in sliding contact .

The load applied is so high that adhesion (or

bonding) and deformation occurs at the asperity

contacts at the interface, and these contacts are

sheared by sliding.

The motion of the rubbing counter bodies result in

rupture of the micro-joints. Thus some of the material

is transferred by its counter body.

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Strong adhesion between the asperities of wearingsurfaces has two effects:

a) a large component of frictional force is generated

and the asperities may be removed from the surfaceto form wear particles.

b) transfer layers.

Numerous tests on a wide variety of metalcombinations have shown that when there is strongadhesion, transfer of the weaker metal to the strongeroccurs fig-chp4\chp4-fig1.pptx

fig-chp4\chp4-fig2.pptx shows the typicalappearance of scuffed gear teeth. Other examples oftransfer layers fig-chp4\chp4-fig3.pptx and fig-

chp4\chp4-fig4.pptx 8
http://fig-chp4/chp4-fig1.pptxhttp://fig-chp4/chp4-fig2.pptxhttp://fig-chp4/chp4-fig3.pptxhttp://fig-chp4/chp4-fig4.pptxhttp://fig-chp4/chp4-fig4.pptxhttp://fig-chp4/chp4-fig4.pptxhttp://fig-chp4/chp4-fig4.pptxhttp://fig-chp4/chp4-fig3.pptxhttp://fig-chp4/chp4-fig2.pptxhttp://fig-chp4/chp4-fig1.pptx


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The mechanism of shearing and cracking to form a

transfer particle in the adhesive contact between

asperities is illustrated schematically in fig-

chp4\chp4-fig5.pptx.

In the contacts between asperities which do not

produce wear particles there may still be extensive

plastic deformation as illustrated in fig-chp4\chp4-fig6.pptx.

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http://fig-chp4/chp4-fig5.pptxhttp://fig-chp4/chp4-fig5.pptxhttp://fig-chp4/chp4-fig6.pptxhttp://fig-chp4/chp4-fig6.pptxhttp://fig-chp4/chp4-fig6.pptxhttp://fig-chp4/chp4-fig6.pptxhttp://fig-chp4/chp4-fig5.pptxhttp://fig-chp4/chp4-fig5.pptx


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a) Two- body abrasive wear:

If there are only two rubbing parts involved in the friction

process.

In this case the wear of the softer material is caused by the

asperities on the harder surface.

Example: the action of sand paper on a surface (Hard asperities

or rigidly held grits passes over the surface like a cutting tool).b) Three body abrasive wear:

If the wear is caused by a hard particle (grit) trapped between

the rubbing surfacesThe particle may be either free or partially embedded into one of

the mating materials (the grits are free to roll as well as slide

over the surface, since they are not held rigidly).

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Means of Control of Abrasive Wear

The basis of abrasive wear resistance of materials ishardness and it is generally recognized that hard

materials allow slower abrasive wear rates than softermaterials.

Since abrasive wear is the most rapid form of wear

and causes the largest costs to industry (smallamounts of abrasive can severely affect its overallperformance, e.g. in hydraulic systems), severalmethods have been developed to minimize the losses

incurred. The basic method of abrasive wear control is :

to raise the hardness of the worn surface.

apply hard surface coatings. 13


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4.2.3 Fatigue wear

Fatigue wear of a material is caused by a repeated(cycling) application of loads that produce stresses in

and under the contacting surfaces.

Fatigue occurs if the applied load is higher than thefatigue strength of the material.

Mechanisms of Fatigue Weara) Strains caused by shearing in sliding are presentsome depth below the surface reaching the extreme

values at the surface.The strain levels in the deformed surface layer are

illustrated schematically in fig-chp4\chp4-fig10.pptxandfig-chp4\chp4-fig11.pptx.

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http://fig-chp4/chp4-fig10.pptxhttp://fig-chp4/chp4-fig11.pptxhttp://fig-chp4/chp4-fig11.pptxhttp://fig-chp4/chp4-fig10.pptx


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b) Surface and subsurface modes of crack mechanisms.The Mechanism of surface crack initiated fatiguewear is illustrated schematically in fig-chp4\chp4-

fig12.pptx. Fatigue wear formation:

Fatigue cracks start at the material surface and spread to thesubsurface regions.

The cracks may connect to each other resulting in separation ofthe material pieces.

Examples :

o surface and subsurface initiated spalls are shown in fig-chp4\chp4-fig14.pptx .

o surface failure due to high surface temperature and heavyload is shown fig-chp4\chp4-fig15.pptx.

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http://fig-chp4/chp4-fig12.pptxhttp://fig-chp4/chp4-fig12.pptxhttp://fig-chp4/chp4-fig14.pptxhttp://fig-chp4/chp4-fig14.pptxhttp://fig-chp4/chp4-fig15.pptxhttp://fig-chp4/chp4-fig15.pptxhttp://fig-chp4/chp4-fig14.pptxhttp://fig-chp4/chp4-fig14.pptxhttp://fig-chp4/chp4-fig12.pptxhttp://fig-chp4/chp4-fig12.pptx


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Means of Control of Fatigue Wear

The tendency for surfaces to fail in fatigue can be

reduced by :

Application of high strength materials (hardness increases

resistance to surface fatigue)

Decreasing load and decreasing sliding

Better lubrication

Better surface condit ion

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Assignment:

1. What is the difference between pitting and spalling.

2. What is the difference between scuffing andscoring.

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4 2 4 C i


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4.2.4 Corrosive wear

The fundamental cause of these forms of wear is achemical reaction between the worn material and a

corroding medium. If a material (metal) is corrode to produce a film onits surface while it is simultaneously subjected to a

sliding contact then one of the three followingprocesses may occur:

A durable lubricating film which inhibits both corrosion and wearmay be formed

a weak film which has a short life-time under sliding contactmay be produced and a high rate of wear may occur.

the protective films may be worn (e.g. by pitting) and may resultin rapid corrosion of the worn area on the surface.

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Models of corrosive wear are illustrated schematically

in fig-chp4\chp4-fig16.pptx

The first process is dominated by the formation of durable

lubricating films. If such films prevail then the worn contactsare well lubricated and corrosive wear does not occur.

Unfortunately, very few corrosion product films are durable so

that this category of film formation is rarely seen in practice.

The second process is related to the formation of short life-time corrosion product films consist of brittle oxides or other

ionic compounds. For example, the oxides of iron are

extremely brittle at all but very high temperatures.

The third process relates to wear in highly corrosive media.

while the fourth process is effectively limited to extremely

corrosive media where the corrosion products are very weak

and are probably soluble in the liquid media.19
http://fig-chp4/chp4-fig16.pptxhttp://fig-chp4/chp4-fig16.pptx


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Examples of corrosive wear

Corrosive wear can be found in situations when

overly reactive E.P additives are used in oil or when

methanol, used as a fuel in engines, is contaminated

with water and the engine experiences as rapid wear.

Corrosive wear, is that of cast iron in the presence of

sulphuric acid.

Abrasion can accelerate corrosion by the repeated

removal of passivating films and a very rapid form

of material loss may result (particularly significant

in the mineral processing industries) . The generally

accepted model of corrosive-abrasive wear is shown

in fig-chp4\chp4-fig17.pptx. 20

M f C t lli C i W


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Means of Controlling Corrosive Wear

The addition of corrosion inhibitors to thelubricating oil or process fluid can be an effective

means of controlling corrosive wear. The corrosioninhibitor may, however, displace adsorbed layers oflubricants and promote adhesive wear.

The severity of corrosion and wear determines theselection of an optimum corrosion inhibitor: When corrosion is severe but wear is mild, then a corrosion inhibitor which

forms a passivating film is the most suitable.

When loads or wear are severe but corrosion is relatively mild, then aninhibitor which functions by adsoption to produce a lubricating layer is themost suitable. In this case, even a weak corrosion inhibitor may beeffective.

When both corrosion and wear are severe, an effective corrosion inhibitor

which adsorbs strongly to the worn surface is essential. 21

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4.2.5 Erosive wear

Erosive wear is caused by the impact of particles of

solid or liquid against the surface of an object.

Typical examples of erosive wear are:

damage to gas turbine blades when an aircraft flies through dust

clouds, and

the wear of pump impellers in mineral slurry processing systems.

Mechanisms of Erosive Wear

The known mechanisms of erosive wear are

illustrated in fig-chp4\chp4-fig18.pptx

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i i l l h i


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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.

Impingement angle can range from 0-90o as shown in fig-chp4\chp4-fig19.pptx. The effect of impingement angle is

illustrated on fig-chp4\chp4-fig20.pptx.

The speed of the erosive particle has a very strong effect on the

wear process. If the speed is very low then stresses at impact

are insufficient for plastic deformation to occur and wear

proceeds by surface fatigue.

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T i l l f i i h i fi
http://fig-chp4/chp4-fig19.pptxhttp://fig-chp4/chp4-fig19.pptxhttp://fig-chp4/chp4-fig20.pptxhttp://fig-chp4/chp4-fig20.pptxhttp://fig-chp4/chp4-fig19.pptxhttp://fig-chp4/chp4-fig19.pptxhttp://fig-chp4/chp4-fig21.pptxhttp://fig-chp4/chp4-fig21.pptx


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Typical example of erosive wear is shown in fig-

chp4\chp4-fig21.pptx. And Erosive wear results in:

Dimensional changes

Leakage

Lower efficiency

Generated particles contribute more wear

Means of Controlling Erosive Wear

Material characteristics exert a strong effect on

erosive wear. High wear resistance can be achieved:

increase hardness of the material

proper alloy content

Two contrasting erosive wear protection mechanisms

are illustrated in fig-chp4\chp4-fig22.pptx 24

4 3 W A l i P
http://fig-chp4/chp4-fig21.pptxhttp://fig-chp4/chp4-fig21.pptxhttp://fig-chp4/chp4-fig22.pptxhttp://fig-chp4/chp4-fig22.pptxhttp://fig-chp4/chp4-fig21.pptxhttp://fig-chp4/chp4-fig21.pptx


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4.3 Wear Analysis Process

Wear is a system characteristic or phenomenon; it is not

a material property. It is necessary to examine and

characterize a number of different parameters, not simplythe worn part.

A tribosystem consists of various parameters that

influence the wear process. The basic elements of a tribosystem are:

Contacting materials,

Geometrical parameters

Relative motion

Loading

Type of lubrication and Environment

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W l i i l d


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Wear analysis process includes :

Examination

Characterization

Modeling and Evaluation

Testing

The study of wear is common in many disciplines:

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W l i i d b i i


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Wear analysis process is used by engineers in

industry to solve wear problems on the existing

equipments or new designs.

a) Existing equipments:

Improve wear life

Develop prototype to investigate wear life

b) New design

Design new equipments

Evaluation of the effect of design change

Evaluation of the effect of new or extended application

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W l i


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Wear analysis process:

requires blend of theoretical and experimental

techniques

Focuses on all aspects of the problem not just

materials: e.g. contact geometry, lubrication, motion ,

etc.

Activities in wear analysis process are shown as

follows (Fig.)

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Fig. Wear analysis process 29

1 W i ti


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1. Wear examination:

data gathering phase

Involves consideration of the whole tribosystem notjust the worn component (reason for the wear

problem may be related to a different part of the

machine)

a) Component information

Geometry

Dimensions Material

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b) Contact condition


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b) Contact condition

Orientation

Location

Loading

Motion

c) Lubrication information

Type of lubrication Lubricant

Condition

d) Environmental information Temperature

Humidity

Contamination 31

) W i f ti ( 2 ll t t t t lt)


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e) Wear information (e.g. 2 roller contact test result)

Amount of wear

Usage Appearnance

Location

The examination of the tribosystem should includealso the inspection and measurement of the wear

scars.

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2 Wear characteri ation


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2. Wear characterization

Provides the basis for selecting appropriate models

for wear behavior for model selection

Aids in the application of the models to the wear

situation

Basically it involves synthesising data gathered into

a useful description of the wear situation.

Should contain the following elements: description

of motion, contact geometry, nature of the loading,

description of the materials, type of lubricant.

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The wear situation is described in terms of the


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The wear situation is described in terms of the

contact velocity, contact area contact pressure and

entry angle.

The purpose of the examination and characterization

is to be able to define the tribosystem at the point of

contact or wear site.

For some engineering situations, a very crude

description might be sufficient, such as describing the

tribosystem as being a lightly loaded, lubricated

contact at low sliding speed in an ambient roomenvironment.

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The shape morphology and locaiton of the wear


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The shape, morphology, and locaiton of the wear

scars provide important information generally needed

to characterize the tribosystem and the wear process.

Quantifying the amount of wear, particularly in

terms of depth, generally is useful as well.

The magnitude of the wear can support the

characterization of the wear behavior and aid in the

identification of a solution when used in conjunction

with various models and analytical relationships.

Methods to examine wear scar such as visual, low-power optical, and scanning electron microscopy

(SEM). In many situations, magnification between 30

and a few hundred are most useful. 35

Other methods to characterize materials measure


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Other methods to characterize materials, measure

dimensions and surface roughness's can also be

applied,

In general, the amount of wear or root cause that

results in the failure should be identified. (A criterion

for acceptable wear also should be identified). Both

pieces of information generally are important indeveloping an economical and practical solution.

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3 Modeling and Evaluation


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3. Modeling and Evaluation

This stage is the core of the wear analysis

Involves selecting an appropriate analyticalrelationship to describe wear and select design

parameters

There are four steps in this stage

Determine relationships

Develop mathematical model relating wear life to design

parameters

Verify the model to check accuracy

Optimize parameters and establish design changes

required

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4 Testing


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4. Testing

It is not an intrinsic part of the process, but may be

significant

if material data is not available

to evaluate materials

as part of the verification process

to define wear coefficients for analytical models

Testing must conform to available standards and a

key elements to ensure that the wear is the same in

the test as it is in the actual applications.

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39


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40Fig. 4-1 Process of metal transfer due to adhesion
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Fig. 4-2 Adhesion between gear teeth resulting in scuffing


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42

Fig. 4-3 Example of metallic film transfer ; a) brass film

transfer on alumina, b)Al-Si alloy transfer film onto a piston

ring.


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43Fig. 4-4 Al-Si alloy surface worn by adhesive.


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44

Fig. 4-5 Schematic diagram of the formation of an

adhesive transfer particle


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45

Fig. 4-6 Alternative model of deformation in adhesive

asperity contact.


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Fig. 4-7 Mechanisms of abrasive wear


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47Fig. 4-8 Two-body abrasive wear

Soft material

Hard material


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48

Soft material

Hard material

Fig. 4-9 Three-body abrasive wear


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49Fig. 4-10 Strain levels in a deformed surface


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50

Fig. 4-11 Accumulation of material on the surface due to the

passage of blunt wedge and resulting plastic deformation.


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Fig. 4-12A Schematic illustration of the process of surface

crack initiation and propagation
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Fig. 4-12B Fatigue wear
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Fig. 4-13 Schematic illustration of the surface and

subsurface modes of contact fatigue.


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54Fig. 4-14 Surface and subsurface initiated spalls.

(a) Surface initiated spall (b) Subsurface initiated spall


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55

Case-Carburized Steel

Fn = 19.2 kN (H 2.12 GPa )

N2 = 0.5107

1.0 mm

Large-pit

, Ts = 470 K

Fig. 4-15 Appearance of surface due to high surface temperature and

heavy load.


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Fig. 4-16 Models of interaction between a corrosive agent

and a worn surface


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Fig. 4-17 Cyclic removal of corrosion product films by

abrasion


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Fig. 4-18 Possible mechanisms of erosion; a) abrasion at low impact

angle, b) surface fatigue during low speed, high

impingement angle impact, c) multiple plastic deformation

or brittle fracture during medium speed, large impingement

angle impact (Contd)

Contd


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59

Fig. 4-18 (Contd) d) surface melting at high impact speeds,

e) macroscopic erosion with secondary effects

Contd


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60

Fig. 4-19 Impingement angle of a particle causing erosion of surface.


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Fig. 4-20 Schematic representation of the effect of impingement

angle on wear rates of ductile and brittle materials.


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Fig. 4-21 Typical erosive wear on bearing surface


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Fig. 4-22 Comparison of the high and low elastic modulus

railway tribology chp4 wear lect. & all figs ver. e 110613(3)

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