ED7012 SE_notes

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KIT- KALAIGNARKARUNANIDHI INSTITUTE OF TECHNOLOGY
COIMBATORE-641402
M.E. ENGINEERING DESIGN
ED7012 - SURFACE ENGINEERING
Academic Year : 2015-16
Faculty : R.SARAVANAN, M.E
ASSISTANT PROFESSOR

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UNIT I FRICTION
Surface engineering is a multidisciplinary activity intended to tailor the
properties of the surfaces of engineering components so that their function
and serviceability can be improved. The ASM Handbook defines surface
engineering as ―treatment of the surface and near-surface regions of a
material to allow the surface to perform functions that are distinct from
those functions demanded from the bulk of the material‖
The desired properties or characteristics of surface-engineered
components include:
• Improved corrosion resistance through barrier or sacrificial protection
• Improved oxidation and/or sulfidation resistance
• Improved wear resistance
• Reduced frictional energy losses
• Improved mechanical properties, for example, enhanced fatigue or
toughness
• Improved electronic or electrical properties
• Improved thermal insulation
• Improved aesthetic appearance.
Friction
 Any force that resists motion
 It involves objects that are in contact with each other.
 This is the force that keeps an object from sliding down and incline
plane.
 Some scientists believe that friction is caused by uneven surfaces of
the touching objects – when rubbed together resistance is offered.
 Experiments have shown that tiny particles are actually torn from
one surface and imbedded in the other.
 If two surfaces were carefully polished, there is a limit to the
amount by which friction may be reduced. If made too smooth, the
friction between them actually increases.
Principles of Friction
 Friction acts parallel to the surface that is in contact. The direction
that friction acts is OPPOSITE the direction of the motion (or
intended motion).

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SURFACE TOPOGRAPHY
Controlling the friction is becoming increasingly significant due to
constant demands for improved reliability and effectiveness of mechanical
parts and especially the reduction in frictional loses. In recent years
surface texturing was introduced as a way of reducing friction. With
employment of different patterns in the form of micro dimples or groves
on the surface, reduction of friction can be obtained [1] to [13]. There are
some texturing parameters like the shape of the dimples, their depth and
width, their area density and orientation, which exert an influence on
friction and wear. However, a lot of research work has been done in the
field of surface texturing, while modification of surface topography by
texturing is still mainly based on trial and error approach. A possible way
of designing surface texturing parameters, which would result in contact
surfaces with lower friction, is by treating surface texturing as a controlled
roughness. By knowing what kind of surface topography in terms of
roughness parameters results in lower friction, we would be able to select
proper surface texturing parameters. However, for this knowledge about
the correlation between surface roughness and friction is essential.
Surface roughness and topography, which are used to characterize contact
surfaces are described with surface roughness parameters. Unfortunately,
standard surface roughness parameters normally used by designers do not
describe contact surfaces sufficiently, with completely different surfaces
showing similar or even the same values of standard roughness
parameters and the other way round – similar surfaces having much
different standard roughness parameters. In addition, different standards
use different parameters.
A very good overall description of height variations, but does not give any
information on the wavelength and it is not sensitive to small
changes in profile. Root mean square deviation of the assessed profile .
However, there are also other roughness parameters defined by ISO 4287
standard, which give better surface description and sensitive on
occasional deep valleys or hig peaks. Zero skewness reflects in
symmetrical height distribution, while positive and negative skewness
describe surfaces with high peaks or filled valleys, and with deep
scratches or a lack of peaks, respectively. On the other hand, kurtosis
describes the probability density sharpness of the profile. For surfaces is
less than 3, and more than 3 for surfaces with high peaks and low
valleys.The load bearing ratio, as well as the maximum contact
pressure increased.
When two surfaces rub together, the peak region usually gets

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worn out; the core region bears the load and has influence on the life of
the product and valley region which acts as a lubricant reservoir.
Therefore, the aim of the present research was to investigate surface
topography of contact surfaces in terms of different surface roughness
parameters and to correlate surface topography changes to friction.
Furthermore, to investigate the possibility of using roughness parameters
as design parameters for surface topography modification, real roughness
profiles were virtually altered to achieve virtually textured surfaces and
roughness parameters calculated using NIST Surface Metrology
Algorithm Testing System (SMATS) softgauge. Virtually altered
roughness profiles were investigated in terms of influence of texture size
and shape on skewness and kurtosis parameters.
Fundamentals of Sliding Friction
Friction modeling involves a systematic process, which is very much like
any process used to model a physical phenomenon.
When friction modelers use simplifying assumptions they risk neglecting
influential variables. Some theorists introduce quantities with elegant-
sounding names but which cannot be measured directly. These ―adjustable
models‖ confirm the notion that at least some of the basic physics of
friction still remain elusive. While simplified models can adequately
describe specific cases, their general applicability is limited. Therefore, the
following three important elements are needed to develop accurate and
predictive models for static or kinetic friction:
1. Process identification—knowledge of the dominant interfacial processes
of friction and their relative stability over time
2. Understanding of scale—knowledge of the size scale at which these
processes.
3. Definition of functional relationships—identification of the rules that
translate the external stimulus to the response of the tribal system,
consistent with interfacial processes and scale of interaction.
At each level of interaction, different physical processes come into play,
and a hierarchy of effects emerges (see Figure 4.1).3 If the lubrication
regime is adequate to promote full separation of the surfaces (zone I), be
effectively modeled using lubrication theories. If the conditions of
lubrication are insufficient to avoid solid contact (zone II), then the
characteristics of the solid bodies, surface structure and third bodies play a
role. Should there be no effective lubrication in the system and if the shear
forces generated by sliding cannot be fully accommodated by shearing the
materials immediately adjacent to the interface, then these forces are
transmitted to the fixtures holding the solid bodies together, and

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accounting must be made for the characteristics of the surrounding
structure (e.g., its stiffness, natural frequency). The latter effects are
depicted as zone III.
STATIC FRICTION AND STICK-SLIP
Historically, static friction has been linked to adhesion and the breaking of
bonds between atoms on the opposing surfaces. The Random House
Collegiate Dictionary (1973) defines adhesion as: ―Physics, the molecular
force of attraction in the area of contact between two unlike bodies that
acts to hold them together.‖ If all possible causes for friction are to be
fairly considered, it is reasonable to inquire whether there are other means
to cause bodies to stay together without the requirement for molecular
bonding. For example, a keeper bar holds to a magnet without adhesively
bonding to it. Velcro works by mechanical interlocking.
Surfaces may adhere, but adherence is not the same as adhesion because
there is no requirement for molecular bonding. If a certain material is cast
between two sur-faces and, after penetrating and filling irregular voids in
the two surfaces, solidifies so as to form a network of interlocking
―fingers,‖ there may be a strong mechanical joint produced but no
adhesion.
Invoking adhesion (i.e., electrostatically balanced attraction/chemical
bonding) in friction theory meets the need for an explanation of how one
body can transfer shear forces to another. It is convenient to assume that
molecular attraction is strong enough to allow the transfer of force
between bodies, and in fact that assumption has led to many of the most
widely promulgated friction theories. From another perspec-tive, is it not
equally valid to consider that if one pushes two rough bodies together so
that their asperities penetrate and then attempts to move those bodies
tangentially, that the atoms may approach each other closely enough to
repel strongly, thus caus-ing a ―back force‖ against the bulk materials and
away from the interface? (See the schematic illustration in Figure 4.9.) The
repulsive force parallel to the sliding direction
Adhesive forces impede relative movement
Repulsive forces impede relative movement

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must be overcome to move the bodies tangentially, whether
accommodation occurs by asperities climbing over one another or by
deforming one another. In the lat-ter conception, it is repulsive forces and
not adhesive bonding that produce sliding resistance.
Solid adhesion experiments have been conducted for many years. Ferrante
et al.23 have provided a comprehensive review of the subject. In addition,
an excel-lent discussion of adhesion and its relationship to friction can be
found in the book by Buckley.24
Atomic force microscopes came into prominence during the closing
decades of the twentieth century. They permit investigators to study and
map adhesion and lateral forces between surfaces on the molecular scale.
Operators of such instru-ments are familiar with the tendency of the fine
probe tip to jump to the sur-face as the approach distance becomes very
small. The force required to shift the two bodies tangentially must
overcome whichever bonds are holding the sur-faces together. In the case
of dissimilar metals with a strong bonding preference, the shear strength of
the interfacial bonds can exceed the shear strength of the weaker of the
two metals, and the static friction force (FS) will depend on the shear
strength of the weaker material (τm) and the area of contact (A). In terms
of the static friction coefficient µs ,
FS = µsP* = τmA (4.5)
or
_s
_m A
P
*A (4.6)
where P* is the normal force that is the sum of the applied load and the
adhe-sive force normal to the interface. Under specially controlled
conditions, such as friction experiments with clean surfaces in a high
vacuum, the static friction coefficients can be much greater than 1.0, and
the experiment becomes more of a measurement of the shear strength of
the solid materials than of interfacial friction.
Historically, scientific understanding and approaches to modeling friction
have been strongly influenced by our concepts of solid surfaces and by the
instru-ments available to study them. Atomic force microscopes and
scanning tunneling microscopes have permitted views of surface atoms
with amazing resolution and detail. Among the first to study nanocontact

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frictional phenomena were McClellan et al.,25,26 who used the instrument
diagrammed in Figure 4.10. A tungsten wire with a very fine tip was
brought down to the surface of a highly oriented, cleaved basal plane of
pyrolytic graphite as the specimen was oscillated at about 10 Hz using a
piezoelectric driver. The cantilevered wire was calibrated so that its spring
constant was known (2500 N/m) and the normal force could be determined
by measuring the deflection of the tip with a reflected laser beam. As
normal force was decreased, the contributions of individual atoms to the
tangential force became increasingly appar-ent (see Figure 4.11). At the
same time, it appeared that the motion of the tip became less uniform,
exhibiting what some might call nano scale stick-slip.
Friction might simply be defined as the resistance to relative motion
between two contacting bodies parallel to a surface that separates them.
Motion at the atomic scale is invariably unsteady. One struggles to define
the physical boundaries of the ―surface that separates them.‖ In
nanocontact, accounting for the tangential com-ponents of thermal
vibrations of the atoms thus affects our ability to clearly define ―relative
motion‖ between surfaces. Furthermore, under some conditions it may be
possible to translate the surface laterally while the adhesive force between
the probe tip and the opposite surface exceeds a small externally applied
tensile force. When the sense of the applied normal force is reversed,
should the friction coefficient of necessity take on a negative value?
Progress in instrumentation and experimental sophistication forces us to
consider whether the term ―friction‖ has a traditional meaning at the
atomic scale. Perhaps a new term, such as ―frictomic forces,‖ should be
introduced to avoid such concerns.
A parallel development, molecular dynamics (MD) modeling on high-
speed computers, has further broadened the possibilities of modeling
friction on the finest scale.30 Landman et al.31 reviewed progress in this
field, drawing together develop-ments in experimental techniques as well
as computer simulations. By conducting MD simulations of an Ni tip and a
flat Au surface, Landman et al. illustrated how the tip can attract atoms
from the surface simply by close approach and without actual indentation.
A connective neck or ―bridge‖ of surface atoms was observed to form as
the indenter was withdrawn. The neck can exert a force to counteract the
withdrawal force on the tip, and the MD simulations indicate the tendency
to transfer material between opposing asperities under pristine surface
conditions. Landman et al. subsequently conducted numerous other MD
simulations, including complete indentation and indentation in the
presence of organic species between the indenter and the substrate. Belak
and Stowers32, using a material volume containing 43,440 atoms in 160

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layers, simulated many of the deformational features associated with
metals, such as edge dislocations, plastic zones, and point defect
generation. Cal-culated shear stresses for a triangular indenter passing
along the surface exhibited erratic behavior, not unlike that observed
during metallic sliding under clean condi-tions. Pollock and Singer33 have
compiled a fascinating series of papers on atomic-scale approaches to
friction.
Frictional Properties of Non-Metallic Materials for Use
in Sliding Bearings
Sliding bearings with a restoring force element are a very useful type of
isolation hardware. During earthquakes, they transmit shear force from the
ground to the structure above up to the point at which sliding initiates.
After this point, the force transmitted depends on the dynamic coefficient
of friction of the sliding interface, not on the magnitude of the earthquake
itself. This is a very attractive property, as it allows structures to be
designed independently of the seismicity of the area. By lowering the
friction of the sliding interface, forces transmitted to the structure are
lowered as well. friction in Axon bearings was tested. Small scale Axon
bearings, previously used in the shaking table tests performed by Wolff
(2001), consist of a flat sliding interface and a urethane ring to provide
damping and restoring force. Past studies of the sliding interface have
focused on the use of high friction materials in hopes of providing
increased energy dissipation. Instead, this study concentrates on low
friction materials, which provide a greater reduction of transmitted forces.
ADHESION THEORY OF FRICTION
Friction and wear characteristics can best be explained using the adhesion
theory of friction. This theory of friction is based on strong adhesive forces
between contacting asperties. the asperities come into metallic contact,
with resulting high stresses at the true contact area. The true area of
contact is so small that, following elastic deformation, the stress quickly
reaches the yield stress of one of the two materials. It is obtained at the
contact area (some of the surface contaminants are forced out). The face
relative to the other requires shear at these welded junctions. This adhesion
theory of friction was advanced As the load is applied Hence, plastic flow
occurs and a "cleaning" action Because local areas are now somewhat
clean and the stress is Moving one surface.
The adhesion theory of friction states that the friction force is the
first a shear term, the second The shear-term is that force required to equal
t o the sum of two terms: ploughing or roughness term.

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shear at the welded junctions (note that shear may take place in the
junction itself or adjacent to it in either of the two contacting materials).
―he Ploughing term is that force which results from displacement of the
softer of the two metals by an asperity of the hard m e t a l . In many
instances, the Ploughing or roughness term is negligible in comparison
with the show term.
Friction:
F = S + P = Shear + Ploughing = As + A‘p
where
A r e a l area of contact
A‘ ploughing area
p flow pressure
S shear strength of junction
UNIT II WEAR
Wear is a process of removal of material from one or both of two solid
surfaces in solid state contact, occurring when two solid surfaces are in
sliding or rolling motion together according to Bhushan and Gupta (1991).
The rate of removal is generally slow, but steady and continuous. Figure
1.4 shows the five main categories of wear and the specific wear
mechanisms that occur in each category. Each specific mode of wear looks
different to the next, and may be distinguished relatively easily.
Wear is the gradual removal of material obtained at contacting
surfaces in relative motion. While friction results in important energy
losses, wear is associated with increased maintenance costs and costly
machine downtime. Wear phenomena are intimately linked to frictional
processes. Recall that friction forces are generally the result of two main
physical processes: shearing and ploughing. If solid surfaces in relative
motion are not separated in some way, wear can be expected. Lubricants
are used to separate contacting surfaces in relative motion and thus to
reduce wear. Lubricants may completely separate the surfaces, as in fluid
film lubrication or allow solid-solid contact only at a restricted number of
locations (mitigated solid contact) as in boundary lubrication. The
tribology of lubricants is discussed in another chapter. Here the focus is on
wear resulting from direct soli-solid contact.
Wear phenomena are heavily influenced by the fact that most
engineering surfaces are rough (and hence surfaces come in contact at
single asperities and the real area of contact is usually much smaller that
the nominal contact area). Furthermore, wear behavior is also influenced
by the presence of adsorbed species and/or surface layers. Many different

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wear mechanisms have been identified. A first classification of
mechanisms is based on their relative importance in engineering practice.
According to this, the following types of wear are often
encountered:
• Adhesive Wear (plus Fretting Wear). Encountered in 23-45 % of cases.
• Abrasive Wear (plus Erosive Wear). Encountered in 36-58 % of cases
• Surface Fatigue Wear. Encountered in 14-15 % of cases.
• Corrosive Wear. Encountered in 4-5 % of cases.
Wear phenomena can also be classified based on the underlying physics.
According to this, mechanistic-based classification, the following types of
wear have been identified:
• Adhesion and Transfer. Wear takes place by gradual removal of adhered
fragments of material picked up by the contacting surfaces during
frictional interaction.
• Corrosion Film Wear. Wear is associated with the removal of fragments
of protective corrosion/passivating layers from the surface of the worn
material.
• Cutting. Wear is the result of intermittent or continuous chip formation in
the soft material due to cutting action by a harder tool.
• Plastic Deformation. Wear being associated with the removal of sheared
layers resulting from excessive plastic deformation.
• Surface Jetting. Wear resulting from interfacial instabilities associated
with localized softening at the contact interface.
• Surface Fracture. Wear resulting from breakage of atomic bonds in
embrittled surface layers.
• Surface Fatigue. Wear resulting from subsurface cracking and fracture
induced by cyclic loading.
• Surface Reactions. Wear associated with the removal of reaction
products from the surface which in turn were produced by frictional flash
heating.
• Melting. Wear associated with the melting transition.
• Electrochemical. Gradual wear associated with anodic reactions on the
worn surface.
(1) Abrasive Wear
Abrasive wear occurs when material is removed from one surface by
another harder material, leaving hard particles of debris between the two
surfaces. It can also be called scratching, gouging or scoring depending on
the severity of wear. Abrasive wear occurs

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under two conditions:
1. Two body abrasion; In this condition, one surface is harder than the
other rubbing surface as shown in figure 1.5(a). Examples in mechanical
operations are grinding, cutting, and machining.
2. Three body abrasion; In this case a third body, generally a small particle
of grit or abrasive, lodges between the two softer rubbing surfaces, abrades
one or both of these surfaces, as shown in figure 1.5(b).
Cutting Tool Softer Surface
In the microscale, the abrasive wear process is where asperities of the
harder surface press into the softer surface, with plastic flow of the softer
surface occurring around the harder asperities This often leads to what is
known as microploughing, microcutting, and microcracking, when a
tangential motion is imposed.
Abrasive wear may be reduced by the introduction of hydrodynamic or
elastohydrodynamic lubricants at various film thickness to separate the
surfaces and to .wash out. any contaminant particles. Research has shown
that using the correct coating material and various thermally sprayed
techniques including the HVOF process, greatly benefits resistance to
abrasive wear.
(2) Erosive Wear
The impingement of solid particles, or small drops of liquid or gas often
cause what is known as erosion of materials and components. Solid
particle impact erosion has been receiving increasing attention especially
in the aerospace industry. Cavitation erosion occurs when a solid and a fluid
are in relative motion, due to the fluid becoming unstable and bubbling up
and imploding against the surface of the solid Cavitation damage generally
occurs in such fluid-handling machines as marine propellors, hydrofoils,
dam slipways, gates, and all other hydraulic turbines.
(3) Adhesive Wear
Adhesive wear is often called galling or scuffing, where interfacial
adhesive junctions lock together as two surfaces slide across each other
under pressure, according to Bhushan and Gupta (1991). As normal
pressure is applied, local pressure at the asperities become extremely high.
Often the yield stress is exceeded, and the asperities deform plastically
until the real area of contact has increased sufficiently to support the
applied load. In the absence of lubricants, asperities cold-weld
together or else junctions shear and form new junctions. This wear
mechanism not only destroys the sliding surfaces, but the generation of
wear particles which cause cavitation and can lead to the failure of the

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component. An adequate supply of lubricant resolves the adhesive wear
problem occurring between two sliding surfaces.
(4) Surface Fatigue
When mechanical machinery move in periodical motion, stresses to the
metal surfaces occur, often leading to the fatigue of a material. All
repeating stresses in a rolling or sliding contact can give rise to fatigue
failure. These effects are mainly based on the action of stresses in or below
the surfaces, without the need of direct physical contact of the surfaces
under consideration. When two surfaces slide across each other, the
maximum shear stress lies some distance below the surface, causing
microcracks, which lead to failure of the component. These cracks initiate
from the point where the shear stress is maximum, and propagate to the
surface. Materials are rarely perfect, hence the exact position of ultimate
failure is influenced by inclusions, porosity, microcracks and other factors.
Fatigue failure requires a given number of stress cycles and often
predominates after a component has been in service for a long period of
time.
(5) Corrosive Wear
In corrosive wear, the dynamic interaction between the environment and
mating material surfaces play a significant role, whereas the wear due to
abrasion, adhesion and fatigue can be explained in terms of stress
interactions and deformation properties of the mating surfaces. In
corrosive wear firstly the connecting surfaces react with the environment
and reaction products are formed on the surface asperities. Attrition of the
reaction products then occurs as a result of crack formation, and/or
abrasion, in the contact interactions of the materials. This process results in
increased reactivity of the asperities due to increased temperature and
changes in the asperity mechanical properties. Thermally sprayed coatings
applied to various material surfaces, such as those depositing using the
HVOF process, have proved an effective tool in the prevention of
corrosion.
The Economic Effects of Corrosion and
Wear
The progressive deterioration, due to corrosion and wear, of metallic
surfaces in use in major industrial plants ultimately leads to loss of plant
efficiency and at worst a shutdown. Corrosion and wear damage to
materials, both directly and indirectly, costs the United States hundreds of

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billions of dollars annually. For example, corrosion of metals costs the
U.S. economy almost $300 billion per year at current prices. This amounts
to about 4.2% of the gross national product.
However, about 40% of the total cost could be avoided by proper
corrosion prevention methods. Table 2 provides a breakdown of the cost of
metallic corrosion in the United States. Similar studies on wear failures
have shown that the wear of materials costs the U.S. economy about $20
billion per year (in 1978 dollars) compared to about $80 billion annually
for corrosion during the same period. Table 3 illustrates the extent of wear
failures by various operations within specific industrial segments.
Highway vehicles alone use annually 14,600 _ 1012 Btu/ton of energy
represented in lost weight of steel and 18.6% of this energy could be saved
through effective wear-control measures.
Methods to Control Corrosion
Owing to its many favorable characteristics, steel is well suited and widely
used for a broad range of engineering applications and is referenced here
to demonstrate the various corrosion- control steps that can be considered.
Steel has a variety of excellent mechanical properties, such as strength,
toughness, ductility, and dent resistance.
Steel also offers good manufacturability, including formability,
weldability, and paintability. Other positive factors include its availability,
ferromagnetic properties, recyclability, and cost. Because steel is
susceptible to corrosion in the presence of moisture, and to oxidation at
elevated temperatures, successful use of these favorable characteristics
generally requires some form of protection.
Methods of corrosion protection employed to protect steel include:
• Altering the metal by alloying, that is, using a more highly alloyed and
expensive stainless steel rather than a plain carbon or low-alloy steel
• Changing the environment by desiccation or the use of inhibitors
• Controlling the electrochemical potential by the application of cathodic
or anodic currents, that is, cathodic and anodic protection
• Applying organic, metallic, or inorganic (glasses and ceramics) coatings.
Application of corrosion-resistant coatings is one of the most widely used
means of protecting steel. As shown in Table 1, there are a wide variety of
coatings to choose from, and proper selection is based on the component
size and accessibility, the corrosive environment, the anticipated
temperatures, component distortion, the coating thickness attainable and

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costs. Painting is probably the most widely used engineering coating used
to protect steel from corrosion. There are a wide variety of coating
formulations that have been developed for outdoor exposure, marine
atmospheres, water immersion, chemical fumes, extreme sunlight, high
humidity, and moderately high temperatures (less than about 200 °C, or
400 °F). The most widely used corrosion-resistant metallic coatings are hot
dipped zinc, zinc-aluminum, and aluminum coatings. These coatings
exhibit excellent resistance to atmospheric corrosion and are widely used
in the construction, automobile, utility, and appliance industries.
Other important coating processes for steels include electroplating,
electroless plating, thermal spraying, pack cementation aluminizing (for
high-temperature oxidation resistance), and cladding (including weld
cladding and roll-bonded claddings).
The six traditional techniques applied to materials to deal with wear
produced in the preceding tribosystems include:
• Separate conforming surfaces with a lubricating film
• Make the wearing surface hard through the use of hardfacing, diffusion
heat treatments, hard chromium plating, or more recently developed vapor
deposition techniques or high-energy processes (e.g., ion implantation).
• Make the wearing surface resistant to fracture. Many wear processes
involve fracture of material from a surface; thus toughness and fracture
resistance play a significant role in wear-resistant surfaces. The use of very
hard materials such as ceramics, cemented carbides, and hard chromium
can lead to fracture problems that nullify the benefits of the hard surface.
• Make the eroding surface resistant to corrosion. Examples include the
use of cobalt-base hardfacing alloys to resist liquid erosion, cavitation, and
slurry erosion; aluminum bronze hardfacing alloys to prevent cavitation
damage on marine propellers or to repair props that have suffered
cavitation damage; nickel-base hardfacing alloys to resist chemical attack;
and epoxy-filled rebuilding cements used to resist slurry erosion in pumps.
• Choose material couples that are resistant to interaction in sliding (metal-
to-metal wear resistance). Hard facing alloys such as cobalt base and
nickel-chromium-boron alloys have been used for many years for
applications involving metal-to-metal wear. Other surface engineering
options include through-hardened tool steels, diffusion (case)-hardened
surfaces, selective surface-hardened alloy steels, and some platings.
• Make the wearing surface fatigue resistant. Rolling-element bearings,
gears, cams, and similar power-transmission devices often wear by a
mechanism of surface fatigue. Repeated point or line contact stresses can
lead to subsurface cracks that eventually grow to produce surface pits and

15
eventual failure of the device. Prevention is possible through the use of
through-hardened steels, heavy casehardened steels, and flame-, induction-
, electron beam-, or laser hardened steels.
UNIT III- CORROSION
INTRODUCTION:
Corrosion is an undesirable process. Due to corrosion there is limitation of
progress in many areas. The cost of replacement of materials and
equipments lost through corrosion is unlimited.
Metals and alloys are used as fabrication or construction materials in
engineering. If the metals or alloy structures are not properly maintained,
they deteriorate slowly by the action of atmospheric gases, moisture and
other chemicals. This phenomenon of destruction of metals and alloys is
known as corrosion.
Corrosion of metals is defined as the spontaneous destruction of metals in
the course of their chemical, electrochemical or biochemical interactions
with the environment. Thus, it is exactly the reverse of extraction of metals
from ores.
CONSEQUENCES (EFFECTS) OF CORROSION:
The economic and social consequences of corrosion include
i) Due to formation of corrosion product over the machinery, the
efficiency of the machine gets failure leads to plant shut down.
ii) The products contamination or loss of products due to corrosion.
iii) The corroded equipment must be replaced
iv) Preventive maintenance like metallic coating or organic coating is
required.
v) Corrosion releases the toxic products.
vi) Health (eg., from pollution due to a corrosion product or due to the esc
aping chemical from a corroded equipment).
CAUSES OF CORROSION:
In nature, metals occur in two different forms.
1) Native State
(2) Combined State
Native State:
The metals exist as such in the earth crust then the metals are present in a
native state.

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Native state means free or uncombined state. These metals are non-
reactive in nature. They are noble metals which have very good corrosion
resistance. Example: Au, Pt, Ag, etc.,
Combined State
: Except noble metals, all other metals are highly reactive in nature which
undergoes reaction with their environment to form stable compounds
called ores and minerals. This is the combined state of metals. Example:
Fe2O3, ZnO, PbS, CaCO3, etc.,
Metallic Corrosion:
The metals are extracted from their metallic compounds (ores). During the
extraction, ores are reduced to their metallic states by applying energy in
the form of various processes. In the pure metallic state, the metals are
unstable as they are considered in excited state (higher energy state).
Therefore as soon as the metals are extracted from their ores, the reverse
process begins and form metallic compounds, which are
thermodynamically stable (lower energy state). Hence, when metals are
used in various forms, they are exposed to environment, the exposed
metal surface begin to decay (conversion to more stable compound). This
is the basic reason for metallic corrosion.
conductivity are lost.
CLASSIFICATION OR THEORIES OF CORROSION
Based on the environment, corrosion is classified into
(i) Dry or Chemical Corrosion
(ii) Wet or Electrochemical Corrosion
DRY or CHEMICAL CORROSION:
This type of corrosion is due to the direct chemical attack of metal surfaces
by the atmospheric gases such as oxygen, halogen, hydrogen sulphide,
sulphur dioxide, nitrogen or anhydrous inorganic liquid, etc. The chemical
corrosion is defined as the direct chemical attack of metals by the
atmospheric gases present in the environment.
TYPES OF DRY or CHEMICAL CORROSION:
1.Corrosion by Oxygen or Oxidation corrosion
2.Corrosion by Hydrogen
3.Liquid Metal Corrosion
Humans have been trying to understand and control corrosion for as long
as they have been using metal objects. Corrosion occurs by oxidation-
reduction reactions. In some areas, salt is spread on the roads to melt ice
and snow. Although accidents are prevented, the salt accelerates the

17
formation of rust on car bodies, bridges, and other steel structures. Near
the ocean, the salt suspended in the air assists in corroding metal objects.
In each of these cases, the oxidation of the metal to its oxide is central to
the corrosion process.
In this activity, you will organize yourselves into in groups of four and
design an investigation into corrosion or corrosion prevention. Following
one week of data collection and results, you will present your findings to
the class. Then, as a class, you will compile all information regarding
corrosion and corrosion prevention.
Part One: Corrosion Investigation
Work in groups of four to design an investigation into corrosion.
Some ideas:
 Determine which types of substances will undergo corrosion.
 Investigate which factors cause a particular substance to corrode.
 Investigate which factors can reduce or stop corrosion.
Prepare a report according to the following components and rubric:
 State your objective of the investigation. (What is the question you
want to answer?)
 Include a hypothesis statement.
 Determine and list the equipment that you will require.
 Include a method to describe how you are going to carry out the
investigation.
 Carry out the investigation for one week, recording observations and
results (pictures would be helpful to have during your presentation).
 Discuss your experimental design. What about the investigation
could be added or improved?
Part Two: Presentation
 Prepare a short oral presentation (2-4 minutes) about some aspects
of your investigation into corrosion.
 Each member of the group must present some information.
Your presentation must include:
 What factor about corrosion did you investigate and why you chose
this factor.
 What did you think would happen in your experiment?
 What observations did you record? How did you record them?
 What conclusion did you draw from your observations?

18
 Were there any problems associated with your experimental design.
How do you think that you could overcome these problems?
 State some further factors about corrosion that you would like to
investigate.
 Comment on the importance of being able to reduce corrosion of
substances.
UNIT IV SURFACE TREATMENTS
Metals and plastics are treated to change their surface properties for:
decoration and reflectivity, improved hardness and wear resistance,
corrosion prevention and as a base to improve adhesion of other treatments
such as painting or photosensitive coatings for printing. Plastics, which are
cheaply available and easily moulded or formed, retain their own
properties such as insulation and flexibility while the surface can be given
the properties of metals. Printed circuit boards (PCBs) are a special case
where intricate electronic circuits are manufactured using metals on the
surface of plastics.
STM does not in itself form a distinct vertical sector as it provides a
service to a wide range of other industries. PCBs might be considered
products but are widely used in manufacturing, for example, computers,
mobile phones, white goods, vehicles, etc.
The market structure is approximately: automotive 22 %, construction 9%,
food and drink containers 8 %, electrical industry 7 %, electronics 7 %,
steel semis (components for other assemblies) 7 %, industrial equipment 5
%, aerospace 5 %, others 30 %. The range of components treated varies
from screws, nuts and bolts, jewellery and spectacle frames,
components for automotive and other industries to steel rolls up to 32
tonnes and over 2 metres wide for pressing automotive bodies, food and
drink containers, etc. The transport of workpieces or substrates varies
according to their size, shape and finish specification required:
jigs (or racks) for single or small numbers of workpieces and high quality,
barrels (drums) for many workpieces with lower quality and continuous
substrates (ranging from wires to large steel coils) are processed on a
continuous basis. PCBs have particularly complex production sequences.
All activities are carried out using jig equipment, therefore the activities
are described and discussed for jig plants, with supporting sections
describing specific issues for barrel, coil and PBC processing.

19
While no overall figures exist for production, in 2000 the large scale steel
coil throughput was about 10.5 million tonnes and about 640000 tonnes of
architectural components were anodised.
Another measure of the industry size and importance is that each car
contains over 4000 surface treated components, including body panels,
while an Airbus aircraft contains over two million. About 18000
installations (IPPC and non-IPPC) exist in EU-15, although the loss of
engineering manufacturing, largely to Asia, has reduced the industry by
over 30 % in recent years. More than 55 % are specialist sub-contractors
(‗jobbing shops‘) while the remainder provide surface treatment within
another installation, usually an SME. A few large installations are owned
by major companies although the vast majority are SMEs, typically
employing between 10 and 80 people. Process lines are normally modular
and assembled from a series of tanks. However, large installations are
typically specialist and capital intensive.
UNIT V ENGINEERING MATERIALS
Since the earliest days of the evolution of mankind , the main
distinguishing features between human begins and other mammals has
been the ability to use and develop materials to satisfy our human
requirements. Nowadays we use many types of materials, fashioned in
many different ways, to satisfy our requirements for housing, heating,
furniture, clothes, transportation, entertainment, medical care, defense
and all the other trappings of a modern, civilised society.
Most materials doesn't exist in its pure shape , it is always exist
as a ores . During the present century the scope of metallurgical
science has expanded enormously , so that the subject can now be
studied under the following headings :
a) Physical metallurgy
b) Extraction metallurgy
c) Process metallurgy
In the recent years studying the metallurgy science gave to
humanity an ever growing range of useful alloys. Whilst many of
these alloys are put to purposes of destruction, we must not forget
that others have contributed to the material progress of mankind
and to his domestic comfort.
This understanding of the materials resources and nature enable
the engineers to select the most appropriate materials and to use
them with greatest efficiency in minimum quantities whilst causing
minimum pollution in their extraction, refinement and manufacture.

20
Selection of materials :
Let’s now start by looking at the basic requirements for selecting
materials that are suitable for a particular application. For example
figure 1 shows a connecter joining electric cables. The plastic
casing has been partly cut away to show the metal connector.
Plastic is used for the outer casing because it is a good electrical
insulator and prevents electric shock if a person touches it. It also
prevents the conductors touching each other and causing a short
circuit. As well as being a good insulator the plastic is cheap, tough,
and easily moulded to shape. It has been selected for the casing
because of these properties – that is, the properties of toughness,
good electrical insulation, and ease of moulding to shape. It is also
a relatively low cost material that is readily available.
The metal joining piece and its clamping screws are made from
brass. This metal has been chosen because of its special
properties. These properties are good electrical conductivity, ease
of extruding to shape, ease of machining (cutting to length, drilling
and tapping the screw threads ), adequate strength and corrosion
resistance. The precious metal silver is an even better conductor,
but it would be far too expensive for this application and it would
also be too weak and soft.
Figure 1. The electrical connector.
Another example as in figure 2 shows the connecting rod of a
motor car engine. This is made from a special steel alloy. This alloy
has been chosen because it combines the properties of strength and
toughness with the ability to be readily forged to shape and finished
by machining.

21
Figure 2. The connecting rod of motor car engine.
Thus the reasons for selecting the materials in the above
examples can be summarized as :
Commercial factorssuch as:
Cost, availability, ease of manufacture.
 Engineering properties of materials such as: 
Electrical conductivity, strength, toughness, ease of forming by 
extrusion, forging and casting, machinability and corrosion
resistance.
1. Metals
1.1 Ferrous metals
 These are metals and alloys containing a high proportion
of the element iron. 
 They are the strongest materials available and are used
for applications where high strength is required at
relatively low cost and where weight is not of primary
importance. 
 As an example of ferrous metals such as : bridge
building, the structure of large buildings, railway lines,
locomotives and rolling stock and the bodies and highly
stressed engine parts of road vehicles. 
2. Non – ferrous metals
 These materials refer to the remaining metals
known to mankind. 
 The pure metals are rarely used as structural
materials as they lack mechanical strength. 
 They are used where their special properties such
as corrosion resistance, electrical conductivity and thermal
conductivity are required. Copper and aluminum are used
as electrical conductors and, together with sheet zinc and
sheet lead, are use as roofing materials. 
 They are mainly used with other metals to improve
their strength. 



22
Alloy Steels
Introduction :
Steels are, essentially, alloys of iron and carbon, containing up to
1.5 % of carbon. Steel is made by oxidizing away the impurities that
are present in the iron produced in the blast furnace.
The earliest attempt to produce an alloy steel was in 1822 and it
has progressing in producing the alloy steel because of using alloy
steel in those industries upon modern civilization largely depends.
Pure metal objects are used where good electrical conductivity,
good thermal conductivity, good corrosion resistance or a combination
of these properties are required. Therefore alloys are mainly used for
structural materials since they can be formulated to give superior
mechanical properties.
It is called as alloy steel because there are other elements added to
the iron beside the carbon with specific amount for each element. These
elements improve the properties of the alloying steel and make it used
with applications more than the carbon steel. So the most used elements
with the alloy steel and with their amount as a percentage of :
- 2 % Manganese (Mn)
- 0.5 % Chrome (Cr) or Nickel (Ni)
- 0.3 % Tungsten (W) or Cobalt
- 0.1 % Molybdenum (Mo) or Vanadium
- different amount of Aluminum (Al), Copper (Cu) and Silicon (Si).

23
Alloys steels are generally classified into two major types
depending on the structural classification :
- Low alloy steels :
It is one that possesses similar microstructure to, and requires
similar heat treatment to, plain carbon steels. These generally contain
up to 3 – 4 % of one or more alloying elements for purpose of
increasing strength, toughness and hardenability. The applications of
low alloy steels are similar to those of plain carbon steels of similar
carbon contents. Low alloy steels containing nickel are particularly
suitable for applications requiring resistance to fatigue.
- High alloy steels :
Those steels that possess structures, and require heat treatments,
that differ considerably form those of plain carbon steels. A few
examples of high alloy steels are given below:
1. High-speed tool steels
Tungsten and chromium form very hard and stable carbides.
Both elements also raise the critical temperatures and, also, cause an
increase in softening temperatures. High carbon steels rich in these
elements provide hard wearing metal-cutting tools, which retain their
high hardness at temperature up to 600˚C. a widely used high-speed
tool steel composition is containing 18% of tungsten, 4% of chromium,
1% of vanadium and 0.8% of carbon .
This high-alloy content martensite dose not soften appreciably unit it
is heated at temperatures is excess of 600˚C making them usable as
cutting tools at high cutting speeds.
2. Stainless steels
When chromium is present in amounts in excess of 12% , the steel
becomes highly resistance to corrosion, owing to protective film of
chromium oxide that forms on the metal surface. Chromium also
raises the α to γ transformation temperature of iron, and tends to
stabilize ferrite in the structure.

24
There are several types of stainless steels, and these are summarized below:
a) Ferritic stainless steels contain between 12- 25% of chromium
and less than 0.1% of carbon.
b) Martensitic stainless steels contain between 12-18 % of
chromium, together with carbon contents ranging from 0.1 to 1.5 %
c) Austenitic steels contain both chromium and nickel. When
nickel is present, the tendency of nickel to lower the critical
temperatures over-rides the opposite effect of chromium, and
the structure may become wholly austenitic.
3. Maraging steels
These are very high-strength materials that can be hardened to give
tensile strengths up to 1900 MN/m
2
. They contain 18% of nickel, 7%
of cobalt and small amounts of other elements such as titanium, and
the carbon content is low, generally less than 0.05% .
A major advantage of marging steels is that after the solution treatment
they are soft enough to be worked and machined with comparative ease.
Alloy elements cab be classified depending on the using of the
alloy (its application) or according to the basic influence of the
element on the alloy steel properties as follow:
1. alloying elements tend to make carbides such as Cr, W, Ti, V and
Mo it is used in the applications that needs to higher hardness.
2. alloying elements due to analyzing carbides such as Ni, Al, Co and
Si.
3. alloying elements stabilizing uestinet γ such as Ni, Co, Cu and Mn.
4. alloy elements stabilizing ferret α such as Cr, W, V, AL and Si.

25
The principal effects which these alloying elements have on the
microstructure and properties of a steel can be classified as follows:
1. The Effect on the Allotropic Transformation Temperatures.
Some elements, notably nickel, manganese, cobalt and copper, raise the
A4 temperature and lower the A3 temperature, as shown in figure 1.
In this way these elements, when added to a carbon steel, tend to
stabilise austenite (γ) and increase the range of temperature over
which austenite can exist as a stable phase.
°/o ALLOYING ELEMENT
Figure 1.—Relative Effects of the Addition of an Alloying Element on the Allotropic
Transformation Temperatures at A3 and A4 which tending to stabilise y.
Other elements, the most important of which include chromium,
tungsten, vanadium, molybdenum, aluminum and silicon, have the
reverse effect, in that they tend to stabilize ferrite (α) by raising the A3
temperature and lowering the A4 , as indicated in figure 2.
°/o ALLOYING ELEMENT
Figure 2. Relative Effects of the Addition of an Alloying Element on the Allotropic
Transformation Temperatures at A3 and A4 tending to stabilize α.

26
2. The Effect on the Stability of the Carbides.
Some of the alloying elements form very stable carbides when added
to a plain carbon steel. This generally has a hardening effect on the
steel, particularly when the carbides formed are harder than iron carbide
itself. Such elements include chromium, tungsten, vanadium,
molybdenum, titanium and manganese [ these elements are called the
carbide stabilizer]. When more than one of these elements are present,
a structure containing complex carbides is often formed.
3. The effect on grain growth.
The rate of crystal growth is accelerated, particularly at high
temperatures, by the presence of some elements, notably chromium.
Fortunately, grain growth is retarded by other elements, notably
nickel and vanadium, whose presence-thus produce a steel which is
less sensitive to the temperature conditions of heat-treatment.

27
4.The Displacement of the Eutectoid Point.
The addition of an alloying element to carbon steel displaces the
eutectoid point towards the left of the equilibrium diagram. That is, a
steel can be completely pearlitic even though it contains less than
0.83% carbon. For example, the addition of 2.5% manganese to a
steel containing 0.65% carbon produces a completely pearlitic
structure in the normalized condition as shown in figure 3.
Figure 3. The Effects of Manganese and Titanium on the Displacement of the Eutectoid
Point in Steel.
Similarly, although a high-speed steel may contain only 0.7%
carbon, its microstructure exhibits masses of free carbide due to the
displacement of the eutectoid point far to the left by the effects of the
alloying elements which are present. Whilst Figure 4. shows the
extent to which some elements raise or lower the eutectoid
temperature. This latter will move in sympathy with the A3 point.

28
Figure 4. The effect of alloying elements on the eutectoid temperature.
5. The Retardation of Transformation Rates.
By adding alloying elements, we reduce the critical cooling rate which is
necessary for the transformation of austenite to martensite to take place. This
feature of the alloying of steels has obvious advantages and all alloying
elements, with the exception of cobalt, will reduce transformation rates.
In order to obtain a completely martensitic structure in the case of a plain
0.83% carbon steel, we must cool it from above 723˚ C to room temperature in
approximately one second. This treatment involves a very drastic quench,
generally leading to distortion or cracking of the component. By adding small
amounts of suitable alloying elements, such as nickel and chromium, we
reduce this critical cooling rate to such an extent that a less drastic oil-quench
is rapid enough to produce a totally martensitic structure. Further increases in
the amounts of alloying elements will so reduce the rate of transformation that
such a steel can be hardened by cooling in air.

29
Engineering Materials Msc. Shaymaa Mahmood
"Air-hardening" steels have the particular advantage that
comparatively little distortion is produced during hardening. This
feature of alloying is one of the greatest significance.
6. The Improvement in Corrosion-resistance.
The corrosion-resistance of steels is substantially improved by the
addition of elements such as aluminum, silicon and chromium. These
elements form thin but dense and adherent oxide films which protect
the surface of the steel from further attack.
7. Effects on the Mechanical Properties.
One of the main reasons for alloying is to effect improvements in
the mechanical proper ties of a steel. These improvements are
generally the result of physical changes already referred to.
For example:
hardness is increased by stabilising the carbides; strength is
increased when alloying elements dissolve in the ferrite; and
toughness is improved due to refinement of grain.
Some alloying elements may improve the magnetic properties
such as cobalt and tungsten while other elements can made the alloy
steel without any particles such as vanadium.
Some general effects of adding an alloying elements is shown in table 1:

30
Nickel Steel
Nickel is extensively used in alloy steels for engineering purposes,
generally in quantities up to about 5.0%. When so used, its purpose is
to increase tensile strength and toughness. It is also used in stainless
steel. The main sources of nickel are the Sudbury mines in Northern
Ontario, Canada; Cuba and the one-time cannibal island of New
Caledonia in the Pacific.
The addition of nickel to a plain carbon steel tends to stabilize the
austenite phase over an increasing temperature range, by raising the
A4 point and lowering the A3 point. Thus, the addition of 25% nickel
to pure iron renders it austenitic, and so non-magnetic, even after
slow cooling to room temperature. The structure obtained after slow
cooling to room temperature can be estimated for a nickel steel of
known composition by referring to the type of diagram devised by
Guillet (Figure 5). These diagrams show the approximate relationship
between microstructure, carbon content and the amount of alloying
element added.
Figure 5. The Effects of Nickel as an Alloying Element of the Guillet diagram.

31
At the same time nickel makes the carbides unstable and tends to
cause them to decompose to graphite. For this reason it is inadvisable to
add nickel by itself to a high-carbon steel, and most nickel steels are low-
carbon steels. If a higher carbon content is desired, then the manganese
content is usually increased, since manganese acts as a stabiliser of
carbides.
In addition to improving tensile strength and toughness, nickel has
a grain-refining effect which makes the low-nickel, low-carbon steels
very suitable for case-hardening, since grain growth will be limited
during the prolonged period of heating in the region of 900˚ C.
The 3% and 5% nickel steels are the most widely employed.
Those with the lower carbon contents are used mainly for case-
hardening, whilst those with up to 0.4% carbon are used for structural
purposes, shafting, gears, etc.
Chromium steels
The main producers of chromium are the South Africa, the
Philippines, Jugoslavia, New Caledonia, Rhodesia, Cuba and
Turkey, though not all of these countries are important exporters.
Sierra Leone, a relatively small producer, is at present expanding its
output.
The main function of chromium when added in relatively small
amounts to a carbon steel is to cause a considerable increase in
hardness. At the same time strength is raised with some loss in
Ductility, though this is not noticeable when less than 1.0% chromium is
added. The increase in hardness is due mainly to the fact that
chromium is a carbide stabiliser, and forms the hard carbides Cr7C3 or
Cr23C6.
Chromium lowers the A4 temperature and raises the A3
temperature, forming the closed γ-loop already mentioned. In this
way it stabilises the α-phase at the expense of the γ-phase. The
latter is eliminated entirely, as shown in Figure 6, if more than 11%
chromium is added to pure iron, though with carbon steels a greater

32
amount of chromium would be necessary to have this effect.
Figure 6. The Effects of Chromium as an Alloying Element.
The main disadvantage in the use of chromium as an alloying
element is its tendency to promote grain growth, with the attendant
brittleness that this involves. Care must therefore be taken to avoid
overheating or .holding for too long at the normal heat-treatment
temperature.
Steels containing small amounts of chromium and up to 0.45%
carbon are used for axle shafts, connecting-rods and gears; whilst
those containing more than 1.0% carbon are extremely hard and are
useful for the manufacture of ball-bearings, drawing dies and parts
for grinding machines.
Chromium is also added in larger amounts up to 21%
and has a pronounced effect in improving corrosion - resistance due
to the protective layer of oxide formed. This oxide layer is extremely
thin, and these steels take a very high polish. They contain little or
no carbon and are therefore completely ferritic and non-
hardening (except by cold-work). They are used widely in the
chemical- engineering industry; for domestic purposes, such as
stainless-steel sinks; and in food containers, refrigerator parts, beer
barrels, cutlery and table-ware. The best-known alloy in this group is
"stainless iron", containing 13% chromium and usually less than
0.05% carbon.

33
If the carbon content exceeds 0.1% the alloy is a true stainless
steel and is amenable to hardening by heat treatment. The most
common alloy in this group contains 13% chromium and
approximately 0.3% carbon. Due to displacement of the eutectoid
point to the left, this steel is of approximately eutectoid composition.
It is widely used in stainless-steel knives.
Nickel - Chromium Steels
Certain disadvantages attend the addition of either nickel or
chromium, singly, to a carbon steel. Whilst nickel tends to prevent grain
growth during heat-treatment, chromium accelerates it, producing
the attendant brittleness under shock. On the other hand, whilst
chromium tends to form stable carbides, making it possible to produce
high-chromium, high-carbon steels, nickel has the reverse effect in
assisting graphitisation. The deleterious effects of each element can be
overcome, therefore, if we add them in conjunction with each other.
Then, the tendency of chromium to cause grain growth is nullified by
the grain-refining effect of the nickel, whilst the tendency of nickel to
favor graphitisation of the carbides is counteracted by the strong
carbide-forming tendency of the chromium.
At the same time other physical effects of each element are
additive, so that they combine in:
- increasing strength,
- corrosion-resistance
- the retardation of transformation rates during heat-treatment.
- heat resistance
- increase and stablies of carbides that leads to increase in the hardness.
- making the drastic water-quenches avoidable.
These advantages make the Ni – Cr steels used at very important

34
fields such as : stations of electricity generation, the tools of surgery ,
valves, the tools of cooking and it is etc.

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