Kwame Nkrumah University of

Science & Technology, Kumasi, Ghana

MSE 454
Surface Treatment of Materials

David Sasu Konadu (PhD)

Department of Materials Engineering
Faculty of Mechanical and Chemical Engineering
College of Engineering
Online Lectures:
Teaching Assistants:
 Course Objectives
1. Understand the principles of tribology and basis of contact
phenomena and friction

2. Understand how surfaces can be improved with surface

engineering/treatments techniques

3. Select appropriate surface treatment methodologies for predicting,

measuring, and preventing wear and corrosion of materials

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 Learning Outcomes
1. Explain the concept of tribology

2. Identify the different forms of wear

3. Select suitable surface treatment method to control wear and

corrosion.

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 Recommended Books

1. Surface Engineering, ASM International, J.R. Davis

2. Tribology: Friction and wear of engineering materials, I
Hutchings and P. Shipway
3. Corrosion Engineering, Fontana
4. Handbook of Corrosion Engineering, Roberge

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 Forms of Assessment

Quizzes 10
Mid-Sem Exam 20
Final Exam 70
Total 100

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 Course Outline
Week Topic
Introduction to surface treatment:
Purpose of surface treatment processes; Surfaces that need surface treatment;
1-2
Effect of surface treatment on the surfaces of materials; Solid surfaces and
surface texture; Various surface treatment processes
Friction and wear:
3-4 Friction and wear; Modes of friction; Types of wear; Wear mechanisms
Lubrication processes

Processes for surface modification:

Electron beam processes; Plasma processes; Thermochemical diffusion
5-6 processes; Thermal surface hardening treatments; Mechanical surface
treatments; Industrial applications

7 Mid Semester Exams

Coating and coating technologies:
8-12 Coating techniques/technologies; Pre-treatments and surface preparation for
coating; Characteristics of coatings; Application and limitations of coatings
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 Kwame Nkrumah University of
Science & Technology, Kumasi, Ghana

UNIT 1

Introduction to Surface Treatment

 Unit Content
1. Purpose of surface treatment processes

2. Surfaces that need surface treatment

3. Effect of surface treatment on the surfaces of materials

4. Solid surfaces and surface texture

5. Various surface treatment processes

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 Learning Outcomes
1. Determine why surface treatment processes need to be carried out

2. Understand the effect of treating a surface

3. Understand the requirements that go into selecting a treatment

process
4. Identify the microlayers present on a surface and how they affect
surface treatment processes
5. Understand the various types of surface treatment processes

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 Introduction
• The movement of one solid surface over another is fundamentally
important to the functioning of many kinds of mechanisms
• Tribology is ‘the branch of science and technology concerned with
interacting surfaces in relative motion and with associated matters’
• The study of friction, wear, lubrication and the design of bearings
constitute tribology
• Mechanical systems in which surfaces do not slide or roll against
each other are rare
• Friction plays a central role in the performance of many
mechanical systems
• Low friction is desirable and even essential

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 Introduction
• But low friction is not necessarily beneficial in all cases
• Whenever surfaces move over each other, wear will occur
• Damage to one or both surfaces, generally involves progressive
loss of material
• Wear causes increased clearances between the moving
components, unwanted freedom of movement and loss of precision
• It often leads to vibration, to increased mechanical loading and yet
more rapid wear, and sometimes to fatigue failure
• Small amounts of material wear can be enough to cause complete
failure of large and complex machines
• High wear rates are sometimes desirable

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 Introduction
• A key method of reducing friction and also wear is to lubricate the
system
• Surface engineering is a discipline of science, encompassing:
• 1. manufacturing processes of surface layers, thus, in accordance
with the accepted terminology - superficial layers and coatings,
produced for both technological and end use purposes
• 2. performance effects obtained by them
• Surface engineering encompasses the total field of research and
technical activity aimed at the design, manufacture, investigation
and utilization of surface layers with properties better than the
core, such as anti-corrosion, anti-fatigue, anti-wear and decorative

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 Introduction
• In effect, the surface properties of materials are improved,
enhanced and controlled by surface engineering

Fig 1: Schematic
representation of
the area of activity
of surface
engineering.

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 Introduction
• Surface engineering draws inspiration from
• 1. Fundamental sciences: physics, chemistry, partially mathematics
and constitutes their application to material surface
• 2. Applied (technical) sciences
• a. sciences dealing with materials science and material
engineering, with special emphasis on heat treatment
• b. construction and use of machines, with special emphasis on
material strength, primarily fatigue, tribology and corrosion
protection
• c. electrical engineering, electronics, optics, thermokinetics, the
science of magnetism, etc

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 Purpose of Surface Treatment
• Surface treatment is a multidisciplinary activity intended to tailor
the properties of surfaces of engineering components so that their
function and serviceability can be improved.

• Two substantial domains of surface technologies:

1. Tribology
2. Corrosion

• Tribological damage causes a loss of about 1% of the GNP.

• The economic effect of corrosion damage is about 3.5–4.5% of

the GNP.

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 Purpose of Surface Treatment

Abrasive wear Adhesive wear

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 Purpose of Surface Treatment
• Surfaces are responsible for all mechanical, thermal, chemical,
and electrochemical interactions with the environment.

• Desired characteristics of surface-engineered parts include:

– Improve corrosion and oxidation resistance
– Improve wear resistance
– Reduction in frictional energy losses
– Improve mechanical properties
– Improve electronic and electrical properties etc
– Improving aesthetic appearance

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 Purpose of Surface Treatment
• Ensure best properties of the surface by
✓ resistance to oxidation and other forms of corrosion, including
high temperature corrosion and corrosion in environments of
different aggressiveness
✓ resistance to sliding, abrasive and erosion wear
✓ raising static and dynamic (fatigue) strength
✓ giving the surface special physical properties, e.g., improving
electrical conductivity
✓ facilitating the carrying out of subsequent technological
operations

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 Purpose of Surface Treatment
• Various materials and their compositions are surface treated by
various means of immersion, spray, sputtering with
• – metals and alloys,
• – non-metals (e.g., C, N, B),
• – intermetallic compounds,
• – silicates (metals, ceramic and glass),
• – paint products (paints and varnishes),
• – plastics,
• – oils, greases, wax, paraffin, gum, India rubber, tar, bitumens.

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 Surfaces requiring modification
• Surface treatments are sometimes called post-processing
methods
• They affects either
– thin layer on the surface of the part itself or
– add a thin layer on top of the surface of the part
• Surface treatment can be applied on
– Old and worn-out components
– New components during manufacturing stage

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 Nature of interfaces in surface treated
components
• Properties of surface treated component depends on
1. Properties of surface treated layer
2. Properties of the substrate
3. Properties of the interface
• Interface can be grouped into 2 – sharp and diffuse

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 Nature of interfaces in surface treated
components
• When a surface engineered component is subjected to a
tribological ‘challenge’
• its response is associated with material’s behaviour and the
material below the surface
• In severe conditions, the engineered surface layer may be
removed by delamination
• Detachment at the interface rather than by progressive wear
• For systems with sharp interfaces, it is not straightforward either
to describe or to measure the strength of the interface

Progressive wear Delamination

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 Nature of interfaces in surface treated
components
• Interface debonding will depend on
– the strength of the substrate-coating bond
– intensity of the tribological conditions and
– thickness of the coating layer

Progressive wear Delamination

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 Solid Surfaces

Schematic representation of a metal surface, adapted from

Bhushan and Gupta (1991)

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 Surface morphology
• The topography of a surface can be represented by a
dataset that contains the co-ordinates of points which lie on
the surface
• Stylus profilometry and atomic force microscopy are both
techniques in which the co-ordinates of points on a surface
are measured by interactions with a probe
• The surface topography is described by a dataset
describing those co-ordinates, either along a line or across
an area
• With AFM, surface co-ordinates can be measured to a
much greater accuracy
• Once the topographic data has been collected, it is often
represented graphically or one or more quantitative
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 Surface morphology
• Three dimensional plots of (a) a grit blasted steel surface;
(b) a ground steel surface

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 Surface morphology
The surface shape or topology depends upon the process used for
forming, be it moulding, casting, or cutting and abrading.

Surface asperities of a nominal smooth surface

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 Surface Texture
• The surface of any component made of crystalline materials is
characterized by
– The nature of surface irregularities which is quantified by surface
roughness
– The sub-surface region which is composed of 5 distinct zones

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 Surface properties and their
modifications
Chemical properties
• Composition (near surface layer composition can be modified – e.g
nitriding, boronizing, carborizing)

• Chemical affinity (gases interact with metals – e.g steel in the

presence of oxygen)

• Oxidation/Corrosion (metals react in given environment)

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 Surface properties and their
modifications
Mechanical properties
• Hardness (indentation, abrasion, adhesive wear, hard coatings)
• Strength
• Ductility
• Fracture toughness
• Bond strength (coatings)

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 Surfaces Treatment Methods

Two major approaches:

1. Bringing change in one or more of the zones in the subsurface

2. Developing another layer of suitable material at the surface to

achieve the desirable properties

Changes at the surface/subsurface layers

1. Changing the structure of surface layer using thermal and
mechanical methods without compositional change

2. Changing the composition of the surface and subsurface layers

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Various Surfaces Treatment Methods

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Various Surfaces Treatment Methods

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 UNIT 2

Wear and Friction

 Unit Content

1. Wear and Friction

2. Types of wear
3. Wear mechanisms
4. Lubrication processes

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 Unit Objectives

1. Understand interaction between wear, friction, and lubrication

2. Differentiate between the various types of wear and their

mechanisms

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 Learning Outcomes
1.Identify the different types of wear and how they occur

2.Note the effect of friction and wear on the performance of materials

3.Have fair knowledge of lubrication processes and its effect on the

performance of materials

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 What is Tribology?
• Tribology is a science and engineering of friction, wear and
lubrication of elements in relative motion

• Examples: hinges on doors, joints (hips, knees, ...),

workpiece/tool, car windshield wiper, tyres on the road, brakes

• Tribology in industries and companies???

• Economic effects in businesses???

Tribology more than just lubrication and friction!!!

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 Wear, Friction, Lubrication

• Wear is the main cause of material wastage. It occurs

when there is material loss.

• Friction is the main cause of energy dissipation. It is

the force that resist motion.

• Lubrication reduce friction and consequently wear.

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 Wear, Friction, Lubrication

• Friction may be defined as the resistance encountered

by one body in moving over another
• Two important classes of relative motion: sliding and
rolling
A force, F, is
needed to
overcome friction
and cause motion
by (a) rolling or (b)
sliding

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 Wear, Friction, Lubrication
• A tangential force F is needed to move the upper body
over the stationary counterface
• The coefficient of friction μ is the ratio between this
frictional force and the normal load W
• This is given by μ = F/W
• μ can vary over a wide range, from about 0.001 in a
lightly loaded rolling bearing to greater than 10 for two
identical clean metal surface
• Sliding in air in the absence of a lubricant ranges from
about 0.1 to 1

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 Wear
• A process of removal of material from one or both of two solid
surfaces in solid state contact

• Occurs when two solid surfaces are in sliding or rolling motion

together.

• Most tribological contacts are lubricated to reduce CoF and

associated wear and damage.

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 Wear
• Wear needs to be evaluated, measured, and classified.

• Measuring includes loss of material: mass or volume

• There are five main categories of wear. Each has a specific wear
mechanisms that occur.

• In practice, more than one mechanism are involved simultaneously

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Classification of Wear

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 Classification of Wear
Abrasive wear:
It is defined as the wear due to hard particles or hard protuberances forced against
and moving along the solid surface

Adhesive wear:
Wear due to localized bonding between contacting solid surfaces leading to
material transfer between the two surfaces or the loss from either surface

Erosion:
Removal of material from a surface due to mechanical interaction between that
surface and a fluid, a multicomponent fluid, or impinging liquid or solid particles

Fatigue wear:
Fraction of material from a solid surface caused by the cyclic stresses produced by
repeated rolling or sliding on a surface.

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Abrasion - Low stress abrasion
Examples:
Particles sliding on chutes, plowing sandy soils, sliding systems in
dirty environments, ash handling equipment, mineral handling
equipment.

Applicable surface treatment:

Hard plating, case hardening, selective hardening, CVD coating

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Abrasion - High stress abrasion
Examples:
Milling of minerals, rollers running over dirty tracks, earth
moving equipment, heavily loaded metal to metal sliding systems
in dirty environment

Applicable surface treatments:

Heavy carburized case, cemented carbide wear tiles, heavy flame
hardening, cast white iron wear plates

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Abrasion - Gouging
Examples:
Hammer mills, gyratory crusher parts, ball mill parts, agricultural
equipment in rocky soils.

Applicable surface treatment:

Hardfacing

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 Abrasion - Polishing
Example:
Mixing device for grains and fine solids, high concentration of soot
in engine oil

Applicable surface treatment:

Hard plating, thin film hard compound, case hardening, selective
hardening

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Control of Abrasive Wear

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Erosion - Solid particle impingement
Examples:
Fans in dirty environment, abrasive blasting, aircraft operating in sand
or dirty, exhaust systems carrying particles

Applicable surface treatment:

Carbide and ceramic wear tiles

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 Erosion - Liquid impingement
Examples:
Rain impingement on aircraft,
liquid spray deflectors, steam
turbine vanes

Applicable surface treatment:

Ceramic and carbide wear tiles,
elastomer and plastic cladded
surface, corrosion resistant
plating
Process of materials’ damage by liquid
impingement erosion

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 Erosion - Cavitation
Examples:
Ship propellers, pipelines, pumps,
mixing device, ultrasonic agitators

Applicable surface treatment:

Ion implantation, ceramic tiles,
corrosion resistant plating

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 Erosion - Slurry
Examples:
Slurry pipelines, slurry pumps, mineral floatation system, cement
handling equipment

Applicable surface treatment:

Hard plating, ceramic and carbide wear tiles, chromized steel, plastic
lined pipe

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Effect of impact velocity on erosive wear

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Effect of angle of impingement on erosion wear

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 Adhesive - Fretting
Examples:
Bearing on shafts with a loose
fit, clamping faces of injection-
moulding cavities, metal parts
vibrating in tract or rail transit

Applicable surface treatment:

Hardfacing of co-based alloys

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 Adhesive - Seizure

Examples:
Hinge pins, overheated autoengine
causing seizure by thermal expansion
of pistons in cylinders, valves,
unlubricated sliding system

Applicable surface treatment:

Lubricating thin film coating, case
hardening, selective hardening.
Insufficient clearance

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 Adhesive - Galling
Examples:
Fitted sliding members, plug
valves, gate valves

Applicable surface treatment:

Chromium plating, hard coating,
case hardening, ceramic and
carbide coating

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 Adhesive - Oxidation
Examples:
Dry sliding systems on gauge and fixtures, hinge assemblies,
conveyors, sliding parts on machine tools

Applicable surface treatment:

Case hardening surface, thermal barrier coating

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Mechanism of Adhesion Wear

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 Fatigue - Pitting wear
Examples:
Cam paths, gear teeth, rail and metal tires

Applicable surface treatment:

Carburizing, selective hardening

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 Fatigue - Spalling
Examples:
Coated cam and gears, plated mechanical stops, thin plating on
reciprocating systems

Applicable surface treatment:

Hardfacing

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 Fatigue - Impact wear
Examples:
Hammer heads, riveting tools, pneumatic drills

Applicable surface treatment:

Hardfacing

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 Fatigue - Brinelling
Examples:
Static overload on mating surface: wheels on rails, on rolling element
bearing, on mold faces

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 Wear Rate
The loss of material per unit of sliding distance is measured.

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 Visual Classification of Wear of Metals
Mild wear Severe wear
High (100 – 1000 times
Rates of wear Low greater than those observed
in mild wear)
Surface
Smooth Rough
morphology
Heavily deformed metallic
Oxides (from both material (from both
Zone 3 material
surfaces) surfaces) with incorporated
oxide particles
Fine oxide
Debris Coarse metallic flakes
particles
Electrical contact
High Low www.knust.edu.gh
resistance
 The Archard Wear Equation

Q = the volume worn per unit sliding distance

W = the normal load
H = the surface hardness of the wearing material
K = wear coefficient (dimensionless)
 Wear Dependence
1. Normal load

2. Relative sliding speed

3. Initial temperature

4. Thermal, mechanical, chemical properties of the

material in contact
 Wear measurement
• Laboratory investigations of wear is to examine the
mechanisms by which wear occurs,
• or to simulate practical applications and provide useful
design data on wear rates and coefficients of friction
• Selection, control and measurement of all the variables
that may influence wear are very important
• Careful choice of conditions and their close control and
monitoring are important as useful tests
• In a sliding wear test, the main aspects that must be
considered are:
 Wear measurement
➢ the materials of which each of the two bodies in the couple
are made

➢ the test geometry including both the shape and dimensions

of the samples

➢ the applied load and contact pressure

➢ the sliding speed; and

➢ the test environment

 Wear Measurement
• The word tribometer was
an instrument intended
to measure friction and
more recently the term
tribotester and its
associated verb have
been coined.
• The most common test
rigs employ a pin or block
pressed against a disc
• Either on the flat face or
on the rim
Wear Measurement
 Friction
• Friction is defined as the resistance against movement of a body

• Degree of friction is expressed as coefficient of friction (μ).

Laws of friction
1. Friction force is proportional to applied load (F = μ.W)
2. Friction force is independent of contact area
3. Friction is independent of sliding velocity
 Friction
Laws of friction
Friction force is independent of contact area
Friction testing
 Factors Affecting Friction
1. Presence of wear particles and externally introduced particles at the
sliding interface

2. Relative hardness of the materials in contact

3. Externally applied load and/or displacement

4. Environmental conditions such as temperature and lubricants

5. Surface topography and contact area

6. Microstructure or morphology of materials

7. Kinematics of the surfaces in contact (i.e., the direction and the

magnitude of the relative motion between the surfaces)
 Lubricants
• CoF of sliding friction for dry metals, ceramics and polymers are rarely
below 0.5.

• Such high values of CoF in engineering applications lead to intolerably

high friction forces and frictional energy losses

• Lubricants are used to reduce the frictional force between surfaces

• A lubricant functions by introducing between the sliding surfaces a layer

of material with a lower shear strength than either of the surfaces
themselves, or the interface
 Lubricants
• The lubricant will always reduce the rate of sliding wear, and this
is a substantial benefit of lubrication
• In hydrodynamic lubrication the surfaces are separated by a fluid
film, which is usually thick in comparison with the asperity heights
on the bearing surfaces
• Elastohydrodynamic describes the case where the local pressures
in the lubricant are so high and the lubricant film so thin that
elastic deformation of the surfaces can no longer be neglected
• In boundary lubrication, the surfaces are separated by adsorbed
molecular films, usually laid down from an oil or grease containing
a suitable boundary lubricant
 Lubricants
• Solid lubricants function by providing a solid interfacial film
which either exhibits low shear strength or results in an interface
with low shear strength
 Function of Lubricants
• Reduce friction
• Separate surfaces, reduce wear
• Provide cooling
• Remove wear particles and debris
• Reduce vibrations
• Prevent corrosion
 Types of Lubricants
• Lubricants consist of
• Base oil (additive‐free liquid base)
• Additives (enhance, reduce or create certain properties (5‐20%)

• Base oil determines the main physical properties of a lubricant

• Chemically inert (contains some impurities)

• Main base oils:

• Biological (of plant or animal origin)
• Mineral oils (contain hydrocarbons)
• Synthetic oils
 UNIT 3

Processes for Surface Modifications

No Compositional Change
 Unit Content

1. Thermal surface hardening treatments

2. Mechanical surface treatments
3. Thermochemical diffusion processes
 Unit Objectives

1. Understanding of the different modification methods affecting

the surface or subsurface of a material

2. Understand the different modification techniques involving

diffusion and non-diffusion processes
 Learning Outcomes
1. Identify surface modification techniques
2. Identify the characteristics of selected technique
3. Determine the limitations and advantages of each technique
4. Determine the appropriate technique required for a specific
application

20
 Introduction
• Steels can be heat treated to high hardness and strength levels

• High hardness and strength levels are important – structural

components subjected to high operating stresses need high
strength of a hardened structure
 Quenching Media
• The most commonly used quenching media are:
1. Brine – the fastest cooling rate
2. Water – moderate cooling rate
3. Oil – slowest cooling rate
4. Liquid nitrogen – very fast cooling rate
Phase Transformations
Microstructure and Properties
 Kwame Nkrumah University of
Science & Technology, Kumasi, Ghana

MSE 454
Surface Treatment of Materials

Anthony Andrews (PhD)

Associate Professor
Department of Materials Engineering
Faculty of Mechanical and Chemical Engineering
College of Engineering
Website: www.anthonydrews.wordpress.com
Online Lectures: classroom.knust.edu.gh

Teaching Assistant: Ms Freda Quaye / Ms Mabel Agyare

 Kwame Nkrumah University of
Science & Technology, Kumasi, Ghana

UNIT 3

Processes for Surface Modifications

No Compositional Change
 Unit Content

1. Thermal surface hardening treatments

2. Mechanical surface treatments
3. Thermochemical diffusion processes

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 Unit Objectives

1. Understanding of the different modification methods affecting

the surface or subsurface of a material

2. Understand the different modification techniques involving

diffusion and non-diffusion processes

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 Learning Outcomes
1. Identify surface modification techniques

2. Identify the characteristics of selected technique

3. Determine the limitations and advantages of each technique

4. Determine the appropriate technique required for a specific

application

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 Introduction

• Steels can be heat treated to high hardness and strength levels

• High hardness and strength levels are important – structural

components subjected to high operating stresses need high
strength of a hardened structure

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 Quenching Media

• The most commonly used quenching media are:

1. Brine – the fastest cooling rate
2. Water – moderate cooling rate
3. Oil – slowest cooling rate
4. Liquid nitrogen – very fast cooling rate

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Phase Transformations

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Microstructure and Properties

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Factors Affecting the Hardening
Process

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Wear Resistant Metal Systems

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 Selection of material for wear
applications

Factors to consider include

1.Service conditions (temperature, pressure, mechanical
properties requirements)
2.Wear mechanisms
3.Combination of properties
4.Selecting suitable material

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 Important Heat Treatments

HEAT TREATMENT

BULK SURFACE

ANNEALING NORMALIZING HARDENING

THERMAL THERMO-
& CHEMICAL
TEMPERING
Full Annealing Carburizing
MARTEMPERING Flame
Recrystallization Annealing Induction Nitriding

Stress Relief Annealing AUSTEMPERING LASER Carbo-nitriding

Spheroidization Annealing Electron Beam

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Surface Metallurgy

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Methods to Surface Harden a Component

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 Transformation Hardening Methods
The surface material is rapidly and
selectively transformed by heating to
austenite
Then quenched to form martensite and
subsequently tempered
The method is applied only to ferrous
alloys
Applications: gear teeth, camshafts and
crankshafts, cutter blades and various
bearing surfaces
In surface hardening, the heat is
applied quickly to the surface so that
diffusion of the heat away from the
surface and into the bulk is restricted
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Transformation Hardening Methods
Care needs to be exercised to ensure that melting does not occur

The heat is removed either by an external colder medium or by

conduction into the colder bulk material, to achieve a cooling
rate that results in martensite formation

The depth of surface transformation hardening thus depends on

the rate of heat input into the surface layer, the time for which
that occurs, and the subsequent rate of heat loss, both from the
surface and by conduction into the cooler underlying material

Common methods use oxy-acetylene or oxy-propane flames

(flame hardening), high frequency electrical induction heating
(induction hardening), and lasers or electron beams
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Transformation Hardening Methods
An external quenchant is used to cool the workpiece commonly,
water jets or a water bath

Depths of hardening in the range 0.25–6 mm can be achieved by

both methods

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 Heating Methods
Flame Hardening
• High intensity oxy-acetylene flame is applied to selective region
• Temperature is high enough to be in the γ region
• The heated region is quenched (water jets) to achieve desired
hardness
Large gear

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 Heating Methods
Induction Hardening
• Steel part is placed inside electrical coil which has A.C. through
it.
• This energizes the steel part and heats it up.
• Rate and depth of heating can be controlled better than flame
hardening

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 Comparison of Flame and Induction
Characteristics Flame Induction
Equipment Oxyfuel torch, quench Power supply, quench
system system
Application Ferrous alloys, carbon Same
material steels, cast iron

Speed of heating Few seconds to few 1-10s

minutes
Depth of 1.2-6.2 mm 0.4-1.5 mm
hardening
Processing One part at a time Same
Part size No limit Must fit in coil
Control of Attention required Very precise
process
 Heating Methods
Laser Beam Hardening
• Laser used as a heat source (eg. CO2, Nd-YAG, diode)
– Selected based on wavelength and beam shape
• Selected area is exposed to laser energy heating it up.
• Parts are then quenched and tempered.

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 Heating Methods

Electron beam hardening

• An electron beam focused to a diameter of about 3 mm provides
the energy

• The beam is moved over the surface by electromagnetic deflection

• The electron beam process must be performed in a moderate

vacuum and depths of hardening of up to about 2 mm can be
achieved

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 Heating Methods
• Hardness of untampered martensite as a function of carbon
content for plain carbon steels

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 Surface Melting
• Uses liquid – solid phase transformation by locally melting materials
and allowing it to re-solidify

• High input power density is needed for localised melting

o Use of laser or electron beam heating, GTAW

• Rapid solidification causes

o homogenization and refinement in microstructure
o supersaturation and formation of non-equilibrium phases
o glass formation in suitable materials

• Method applicable to both ferrous and nonferrous alloys

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Controlled Surface Layer
Deformation

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 Shot Peening
Shot peening is a cold working
process used to produce a
compressive residual stress layer and
modify the mechanical properties of
metals

The main benefit is the delay or

prevention of cracks in highly tensile
stressed alloy components

The compressive stress helps to

prevent crack initiation as cracks
cannot propagate in the compressive
environment generated by peening
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Shot Peening

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 Shot Peening
Parameters influencing the results of shot peening
treatment

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 Burnishing
Burnishing is the plastic deformation of a surface due to
sliding contact with another object

It is a squeezing operation under cold working and makes the

surface shinier

Burnishing processes are used in manufacturing to improve

the size, shape, surface finish, or surface hardness of a
workpiece

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Burnishing

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 Friction Stir Processing
Friction stir welding (FSW) is a solid-state joining process that uses a
non-consumable tool to join two facing workpieces without melting
the workpiece material.

High temperature is developed at the joint by the relative motion of the

contact surfaces
A forging pressure is applied on the softened surfaces and the relative
motion is stopped
Material is extruded from the joint to form an upset, and a solid phase
bond is formed
A flywheel is used to store energy welds which is dissipated during
joint production
Energy supply may be divided into several operating modes
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Friction Stir Processing

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Other Mechanical Surface Treatments
• Water-jet peening: uses a jet of water at high
pressures, e.g. 400 MPa

➢ Laser peening: surface is hit by tiny impulses from a

laser
➢ Expensive process. Used to improve fatigue strength of jet
blades and turbine impellers

• Explosive hardening: layer of explosive coated on the

surface is blasted.
– Used to harden train rails

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 Kwame Nkrumah University of
Science & Technology, Kumasi, Ghana

MSE 454
Surface Treatment of Materials

David Konadu (PhD)

Department of Materials Engineering
Faculty of Mechanical and Chemical Engineering
College of Engineering
Online Lectures: classroom.knust.edu.gh

Teaching Assistant: Bright

 Kwame Nkrumah University of
Science & Technology, Kumasi, Ghana

UNIT 4

Processes for Surface Modifications

With Compositional Change

 Case Hardening - Introduction
• Transformational hardening and surface melting is limited by the
composition of the starting material. Properties restricted

• The composition of the surface is locally altered to produce

properties that are completely different from those of the substrate

• 1st case: Diffusion of small atoms into the surface leads to the
formation of an interstitial solid solution - Carburizing and
carbonitriding

• 2nd case: Chemical reaction occurs between the diffusing atoms

and constituents of the substrate, a distinct layer is formed -
nitriding, nitrocarburizing, boronizing and chromizing
• Depth of diffusion exhibit time-temperature relationship
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 Case Hardening - Objectives

There are two main objectives:

1. Increasing the surface hardness
2. Induce residual compressive stresses

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 The Process
• Depend on diffusion of species into the component at high
temperature
• Carburizing involves the diffusion of atomic carbon into steel
from the surface to produce an enhanced carbon
concentration
• Plain carbon or low alloy steels of low initial carbon content,
typically 0.15 wt.%–0.2 wt.% C
• Temperatures of 900°C or higher at the austenitic region is
used as diffusion of carbon in austenite is rapid
• The carbon concentration in the surface layer may be
enhanced to 0.7 wt.%–0.9 wt.% by carburizing
• A maximum hardness of up to about 900 HV is obtained

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 The Process
• It is either quenched immediately or machined to final
dimensions in a soft condition before the final heat treatment

• The martensitic transformation also causes distortion of the

component due to the associated change in lattice volume

• Applications: rotating shafts and bearing components, cam

followers, gears and camshafts

• The martensitic transformation produces a compressive

residual stress in the surface that substantially increases the
fatigue life

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 Case Depth
• Depth of hardening and the hardness achieved depend on the
time, temperature and carbon activity

• Both vacuum and plasma carburizing are energy-efficient

processes, and the high process temperature result in much
deeper case hardening than lower temperature methods of the
same duration

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Case Depth Measurement

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 Carburising - The Process
• Expose of low carbon steel to carbon
rich atmosphere at an elevated
temperature (austenitic range). C at > 900oC then quenched

• Initial carbon content 0.15 wt% to 0.2

wt% 𝜶 → 𝜸 + 𝑪 → 𝜶′

• Carbon concentration in surface layer

𝜶 → 𝜸 → 𝜶 𝒐𝒓 𝜶′
is 0.7 wt% to 0.9 wt%

• Maximum hardness = 900 HV

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Variation of Hardness / Carbon / Residual
stress from the surface to the core

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 Methods of adding carbon
• Pack (solid) carburizing
– Used to obtain thick cases
– Components heated (850 to 950oC) in sealed boxes containing charcoal
and barium carbonate
– Less preferred

• Gas carburizing
– Components heated to about 900oC in carbon monoxide, hydrogen and
nitrogen atmosphere (methanol and nitrogen)

• Vacuum carburizing
– Operate at higher temperatures (1050oC) in the absence of oxygen
– Components heated in moderate vacuum and methane or propane
introduced to the furnace at low pressure
– Reaction of the gas at surface of hot steel provides source of carbon
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 Methods of adding carbon
• In plasma carburizing, a glow discharge in methane at low
pressure is used to deposit carbon on the surface of the hot
substrate which is held at a negative potential

• Carbonitriding involves the simultaneous diffusion of both

carbon and nitrogen into austenite in a low carbon steel

• The process is carried out at 800–900°C to produce case

depths from 0.05 to 0.75 mm

• Oil quenching rather than water quenching can be used

• Carbonitrided steels are more resistant to sliding wear than

those carburized to the same hardness
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 Methods of adding carbon
• Variation of hardness with depth in a plain 0.18 wt.% carbon
steel carburized by different methods

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 Kwame Nkrumah University of
Science & Technology, Kumasi, Ghana

MSE 454
Surface Treatment of Materials

David Konadu (PhD)

Department of Materials Engineering
Faculty of Mechanical and Chemical Engineering
College of Engineering
Website: www.anthonydrews.wordpress.com
Online Lectures: classroom.knust.edu.gh

Teaching Assistant: Ms Freda Quaye / Ms Mabel Agyare

 Question

Nickel is plated from a Watts bath at a current density of 3 A dm-2.

The current efficiency is 96%.

The molar mass of nickel is 58.71 g mol-1.
The density of nickel is 8.90 g cm-3.
The Faraday constant is 96 485 C mol-1.
Exposed area is 100 cm2

What will be the averaged plating thickness in 1 hour?

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 Kwame Nkrumah University of
Science & Technology, Kumasi, Ghana

UNIT 4

Processes for Surface Modifications

With Compositional Change

 Thermochemical Reaction -
Introduction
• Formation of surface layers by thermochemical reactions with the
component material

• Reaction occurs between a constituent of the substrate and an

externally-supplied chemical species

• Substrate is hardened through precipitation of reaction product or

forming a hard layer of the reaction product at the surface

• Distinct layer forms sharp interface

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 Nitriding – The Process
• Base: special steels with strong nitride
forming elements (Al, Cr, V, W, Mo). N at 500 - 570oC
• Heat and hold in atmosphere of atomic
nitrogen.
✓ 500 – 600oC for a given time
• Nitrogen diffuse into steels to form a
thin hard surface.
• Slow cool – therefore no distortion from
cooling or phase transformation.
• Cases are harder than carburized cases
(1100 HV)
• Excellent wear and fatigue resistance

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Effect of time on nitriding depth

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 Methods of nitriding
• The process is applied to steels that contain the nitride-
forming solute elements aluminium, chromium, molybdenum,
titanium, tungsten or vanadium
• Atomic nitrogen is formed at the surface of the steel and
diffuses inwards
• React with the solute atoms to form very fine nitride
precipitates typically 5–15 nm in size
• Two main methods are used for nitriding steels
• In gas nitriding, the parts are heated to 530°C in a stream of
ammonia gas
• Process times are long, measured in days rather than hours:
4 days may be needed to develop a hard layer 500 μm thick

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 Methods of nitriding
• In plasma nitriding or ion nitriding, the steel component is
placed in a chamber containing nitrogen and hydrogen at a
pressure of 10–1000 Pa
• A plasma discharge is established at a potential of 500–1000
V with the workpiece as cathode
• The electrical power dissipation heats the steel surface,
which is bombarded with nitrogen ions
• The process is energy-efficient and about three times as fast
as gas nitriding at the same temperature

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Plasma Nitriding

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 Process Characteristics

• Reliable and uniform build up of nitrided cases.

• Retention of core hardness.
• No dimensional changes of work-pieces.
• Wide range of treatment temperatures.
• Completely non-toxic and environmentally clean.
• Low energy consumption and single step operation.

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 Gas Nitriding
• Single-stage or double-stage process
– Single-stage process at 495-525oC
– Double-stage process at 550-565oC

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Plasma vs. Gas

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 Boriding (Boronizing)
• Diffusion of boron into the metal surface to form hard layer of
metal boride
– Parts to be coated is packed with B-containing compounds
– Activators added to enhance production of B-rich gas at the
part surface
– Hold at 800-1050°C for several hours

• Can be applied to ferrous, nonferrous and cermet materials

• In steel, two phases formed: Outer FeB layer and inner Fe2B layer

• Hardness > 1500 HV

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 Others
• Chromizing
✓ Chromium diffuses into surface to form corrosion resistant
layer.
✓ Care must be taken with carbon steels as surface will
decarburize.
✓ Rate-controlling step is the diffusion of carbon towards the
surface of the steel.

• Aluminizing
✓ Used to increase the high temperature corrosion resistance of
steels and superalloys.

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 Hardness attainable in steels

Process (and compound formed) Hardness (HV)

Thermal hardening (0.5 wt%C steel) 700
Carburizing and carbonitriding 850 – 900
Nitriding
C—Mo steel 650
Cr-Mo-V steel 900
Cr-Mo-Al steel 1100
Nitrocarburizing (forms Fe2(C,N)) 500-650
Boronizing (forms FeB, Fe2B) 1500
Chromizing (forms Fe2Cr3) 1500

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Comparison of the process temperature
and depth of hardened material

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 Kwame Nkrumah University of
Science & Technology, Kumasi, Ghana

MSE 454
Surface Treatment of Materials

Anthony Andrews (PhD)

Associate Professor
Department of Materials Engineering
Faculty of Mechanical and Chemical Engineering
College of Engineering
Website: www.anthonydrews.wordpress.com
Online Lectures: classroom.knust.edu.gh

Teaching Assistant: Ms Freda Quaye / Ms Mabel Agyare

 Kwame Nkrumah University of
Science & Technology, Kumasi, Ghana

UNIT 4

Processes for Surface Modifications

With Compositional Change

 Ion Implantation
• Involves the bombardment of a solid material with medium- to
high-energy ionized atoms and accelerated by electric fields to
high velocities

• Offers the ability to alloy any elemental species into the near
surface region of any substrate.

• Ions commonly used include N+, N2+, C+, B+, Ti+, Al+

• Applications: nitrogen implanted tool steels

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Advantages of Ion Implantation

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Disadvantages of Ion Implantation

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 Physical vapor deposition
• Physical vapour deposition (PVD techniques) of
metals or ions in a vacuum consists of
▪ bringing the deposited metal (with a high melting
point) to the vapour state, with the utilization of
resistance, arc, electron and laser beam heating,
▪ introduction of gas,
▪ ionization of metal and gas vapours,
▪ deposition on the surface of a cold or insignificantly
heated substrate, of a single metal, or compounds
(e.g. nitrides, carbides, borides, silicides, oxides) of
that metal with the gas or with the substrate metal

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 Ion Plating
• Atoms or molecules of the coating material are
evaporated from a hot source into a glow discharge
• The vapour source may be heated resistively or by an
electron beam
• Some atoms of the vapour become positively ionized
and are accelerated towards the substrate held at a
negative potential of 2–5 kV
• The high energies of the atoms together with the
scattering provided by collisions with the argon ions
provides a uniformly distributed coating with good
adhesion

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 Ion Plating
• Cleaning of the substrate before coating is readily
carried out by sputtering in the glow discharge

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 Sputtering
• Sputtering is a term used to describe the mechanism in which
atoms and ions are ejected from the surface of a material when
that surface is struck by sufficient energetic particles.

• Metallic films: Al-alloys, Ti, Ta, Ni, Co, Au, etc

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 Sputtering
• The energy to transport material from the source to the substrate
is supplied by energetic heavy gas ions

• Positive ions are formed which strike the solid source material

• This ion bombardment causes atoms to be sputtered from the

target which then strike the substrate a short distance away
• A direct current source is good for
coatings of conducting materials but
alternating source removes it

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Reasons for Sputtering

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The Process

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 Chemical Vapour Deposition
• CVD involves thermally-induced chemical reactions at the surface
of a substrate, with reagents supplied in gaseous form
• CVD is a chemical process used to produce high purity, high
performance solid materials

• The substrate is exposed to one or more volatile precursors which

react and decompose on the substrate surface to produce the
desired deposit

• Volatile by-products are also produced, which are removed by gas

flow through the reaction chamber

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 Steps in CVD
• Transport of reactants by forced convection to the
deposition region
• Transport of reactants by diffusion from the main gas
stream to the substrate surface
• Adsorption of reactants in the substrate surface
• Chemical decomposition and other surface reactions take
place
• Desorption of by-products from the surface
• Transport of by-products by diffusion
• Transport of by-products by forced convection away from
the deposition region

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 Steps in CVD

Transport of by-
by diffusion products by
from the main diffusion
gas
Transport
of
reactants
by forced
convection

Adsorption of reactants in Desorption of by-

the substrate surface products from the
surface
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Types of CVD

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 Advantages of CVD

Advantages:
• High coating hardness; for example, TiN coatings
have a hardness of 2500 HV.
• Good adhesion (provided the coating is not too thick)
• Good throwing power (i.e., uniformity of coating)

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 Disadvantages of CVD

Disadvantages:
• High-temperature process (can affect the structure
and mechanical properties of the substrate itself -
distortion)
• Shard (spike or chip) edge coating is difficult due to
thermal expansion mismatch stresses.
• Limited range of materials can be coated.
• Environmental concerns about process gases

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 Kwame Nkrumah University of
Science & Technology, Kumasi, Ghana

MSE 454
Surface Treatment of Materials

Anthony Andrews (PhD)

Associate Professor
Department of Materials Engineering
Faculty of Mechanical and Chemical Engineering
College of Engineering
Website: www.anthonydrews.wordpress.com
Online Lectures: classroom.knust.edu.gh

Teaching Assistant: Ms Freda Quaye / Ms Mabel Agyare

 Kwame Nkrumah University of
Science & Technology, Kumasi, Ghana

UNIT 5

Processes for Surface Modifications

Development of Surface Layer

 Methods of applying suitable layers

• Chemical reaction

• Electrochemical

• Welding

• Thermal spraying

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 Hardfacing Terminology
Rebuilding: restoration of a part to its initial dimensions
when its geometry has been changed by wear

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 Hardfacing
Factors in choosing a suitable filler metal for rebuilding

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 Buffer Layer

Why use a buffer layer?

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 Hardfacing
“Hardfacing” is the deposition of a surface layer by welding, which is
harder than the base material

The surface of the substrate melts, mixes with and to some extent
dilutes the coating material

This is to give wear resistance

Characterized by soundness, toughness and environmental stresses

like corrosion and high temperatures

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Preventive and Remedial Hardfacing

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Selection of a suitable hardfacing
process

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 Benefits of hardfacing

• Reduced maintenance
• Reduced operation costs
• Lower repair costs
• Extended equipment lifetime

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 Process Types

• Gas tungsten arc welding process

• Shielded metal arc welding
• Flux cored arc welding
• Submerged arc welding

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Advantages of Hardfacing

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Disadvantages of Hardfacing

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 Thermal Spray Coatings

• Process in which a material in the form of powder, wire or rod

are fed to a torch with which they are heated to or slightly
above Tm
• Substrate remains relatively cool
• Coating material is formed and built up by solidification of
droplets as they strike the surface
• Coatings are deposited via a series of individual particles which
form splats in the microstructure
• These splats form an oxide layer which can be a source of
coating weakness in tribologically aggressive environments
• Cracking is often observed to proceed along splat boundaries

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Sources of heat in thermal spraying
• Combustion of a liquid or gaseous fuel
• Flame spraying
• High-velocity oxy-fuel (HVOF)

• Electric discharge
• Electric arc spraying
• Plasma spraying

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 Components
1. An energetic gas flow

2. Feedstock

3. Auxiliary gas feed

4. Controlled atmosphere or a soft vacuum

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Characteristics of Substrate

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Advantages of Thermal
Spraying

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Disadvantages of Thermal
Spraying

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 Applying Suitable Layer via
Chemical/Electrochemical Reactions

• Electroless plating

• Hot dipping

• CVD / PVD

• Electroplating

• Anodizing

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 Electroless deposition
• Electroless deposition: this process uses only one electrode and
no external source of electric current.

• Electroless deposition: the solution needs to contain a reducing

agent so that the reaction can proceed:

• Metal ion + Reduction solution

Catalytic
surface
Metal solid + oxidation solution

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 Types of Metal Deposition

1. Electroless deposition

2. Electroplating

3. Immersion deposition

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Typical thickness vs. time profiles

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 Immersion deposition
• A displacement reaction occurs on the surface of the anode.
• The work piece (anode) dissolves to metal ions. Metal ions in
solution deposits at the cathode, in the absence of an external
power source.
• This is a spontaneous reaction, driven by the electrode
potential of the reaction.

anode cathode

Fe2+
Fe Cu2+ Cu

Cu

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Limitation of immersion deposition

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 Electroless Plating Bath
1. Provides an electrolyte

2. Provides a source of the metal to be plated

3. Contains a reducing agent

4. Wets the cathode work-piece

5. Helps to stabilise temperature

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 Composition of the Bath (Ni-P)
• Ions of the metal to be plated, e.g.
– Ni2+ (nickel ions) added as the chloride

• Conductive electrolyte
– NiCl2, H2PO2-, CH3COO-

• Complexant
– Acetate, succinate

• Reducing agent
– Hypophosphite ion = H2PO2-

• Additives
– Wetters, stabilisers, brightners, stress modifiers…
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 Common Metals for Electroless
Deposition
• Copper

• Nickel-Phosphorus (3-15%wt P)

• Ni-P + PTFE particles

• Ni-P + SiC particles

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 Hydrogen Embrittlement
• To describe the presence of hydrogen in metal deposit.

• In electroless deposition or electroplating, H atom or H2 molecules

could be entrapped or absorbed into the metal deposits.

• Induces a high physical stress in the coating.

.
• Coatings may delaminate from the substrate or crack.

• Reduce the mechanical properties of coating.

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 Electroless Plating - Summary
• Electroless deposition provides important, speciality
• (e.g., Ni-P based) coatings on steel or aluminium or
• Cu printed circuit board tracks

• High degree of control over deposit thickness

• By controlling bath chemistry, temperature and time.

• The process requires no external current

• But is more expensive than electroplating

• The substrate must be made autocatalytic

• For deposition to start and continue

• The ‘throwing power’ is very good

• uniform coatings, even on screw threads
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 Metals for Hot Dipping
• Galvanizing - zinc coated onto steel or iron
– Most important hot dipping process

• Aluminizing - coating of aluminum onto a substrate

– Excellent corrosion protection, in some cases five times more
effective than galvanizing

• Tinning - coating of tin onto steel for food containers, dairy

equipment, and soldering applications

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 Hot Dipping Process

• Loading

• Degreasing

• Pickling

• Pre-fluxing

• Hot dip galvanizing

• Quenching

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Loading

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Degreasing

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Acid Pickling

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Pre-Fluxing

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Hot Dip Galvanizing

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Quenching

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 Electro Deposition
• Electro deposition is the process of coating a thin layer of one
metal on top of a different metal to modify its surface properties.

• Done to achieve the desired electrical and corrosion resistance,

reduce wear and friction, improve heat tolerance and for
decoration

• ED is a surface coating method that forms an adherent layer of

one metal on another.

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Electrochemical Setup

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 Parameters that may influence the
quality of electrodeposits

1.Current density (low to high current)

2.The nature of anions/cations in the solution
3.Bath composition, temperature, fluid flow
4.Type of current waveform
5.The presence of impurities in the bath
6.Physical and chemical nature of the substrate surface

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www.knust.edu.gh
 Typical steps in the electroplating
of metals
1. Cleaning with organic solvent or aqueous alkaline; to remove
dirt or grease.

2. Acid cleaning to remove oxides.

3. Rinse with water to neutralise the surface.

4. Electroplate metals under controlled condition.

5. Rinse with water and dry.

6. Additional step: heat treatment in air or vacuum environment

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 Functions of the Bath
• Provides an electrolyte
– to conduct electricity, ionically
• Provides a source of the metal to be plated
– as dissolved metal salts leading to metal ions
• Allows the anode reaction to take place
– usually metal dissolution or oxygen evolution
• Wets the cathode work-piece
– allowing good adhesion to take place
• Helps to stabilise temperature
– acts as a heating/cooling bath
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 Composition of the Bath (Watts Nickel)
• Ions of the metal to be plated, e.g.
– Ni2+ (nickel ions) added mostly as the sulphate

• Conductive electrolyte
– NiSO4, boric acid, NiCl2

• Nickel anode dissolution promoter

– NiCl2 provides chloride ions

• pH buffer stops cathode getting too alkaline

– Boric acid (H3BO3)

• Additives
– Wetters, levellers, brighteners, stress modifiers..
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Typical Bath Composition (Watts Nickel)

Component Concentration/g L-1

Nickel sulphate 330

Nickel chloride 45
Boric acid 40
Additives various
Temperature 60 oC
pH 4
Current density 2-10 A dm-2

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 Faraday’s Law of Electrolysis
Amount deposited at any electrode is proportional to the quantity
of electricity passed through the electrolyte

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 Faraday’s Law of Electrolysis:
Average thickness

M .I .t
w= w = weight (mass) of metal
M = molar mass of metal
z .F I = current
t = time
z = number of electrons
M .I .t
x= F = Faraday constant
 .A.z .F x = thickness of plating
A = area of the work-piece

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 Anodizing
Anodizing is the successful development and control of a natural
oxidation process that occurs when aluminum/titanium is exposed
to the atmosphere

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Reaction in Anodizing Process

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Purpose of Anodizing

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 Characteristics of Anodizing
• Hard, comparable to sapphire
• Transparent, similar to glass
• Insulative and static resistant
• Wide variety of colors and finishes
• Integral with aluminum surfaces, non-flaking

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 Benefits of Anodizing
Anodizing can improve the properties of aluminum:
• Corrosion resistance
• Wear Resistance
• Surface Hardness
• Electrical Resistance
• Fire Protection

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 Types of Anodizing
Bright Anodizing
Bright anodizing is a special type of anodizing (in combination with
polishing) when glossy or shiny surfaces are required

Hard Anodizing
Hard anodizing is a term used to describe the production of anodic
coatings with film hardness or abrasion resistance as their primary
characteristic

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