Railway Tribology Chp4 Wear Lect. & All Figs Ver. E 110613(3)
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Transcript of 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
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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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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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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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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 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
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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
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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
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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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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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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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Fig. 4-5 Schematic diagram of the formation of an
adhesive transfer particle
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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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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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Fig. 4-18 (Contd) d) surface melting at high impact speeds,
e) macroscopic erosion with secondary effects
Contd
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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