Embed Size (px)
Citation preview
Surface Engineering
By
Rasikh Tariq (ME113006)
Khawar Shahzad (ME113009)
Mohammad Adam (ME-113125)
A project report submitted to the
Department of Mechanical Engineering
in partial fulfillment of the requirements for the course of
MANUFACTURING PROCESSES-I
Page 1 of 30
Copyright 2013 MAJU Students
All rights reserved. Reproduction in whole or in part in any form requires the
prior written permission of Rasikh Tariq, Khawar Shahzad, and Mohammad
Adam.
Page 2 of 30
Declaration It is declared that this is an original piece of my own work, except where
otherwise acknowledged in text and references. This work has not been
submitted in any form for another internship program or diploma at any
institution for tertiary education and shall not be submitted by me in future for
obtaining any degree from this or any other Institution.
Rasikh Tariq
ME-113006
Khawar Shahzad
ME-113009
Mohammad Adam
ME-113125
November, 2013
Page 3 of 30
Table of Contents
Chapter 1: Surface Engineering – An Introduction
Definition .......................................................................................................... 7
Why Surface Engineering? ............................................................................... 7
Surface Integrity ............................................................................................... 8
Surface Finish Measurement ........................................................................... 11
Mechanical Cleaning and Finishing Blast Cleaning ........................................ 11
Blast Finishing ............................................................................................. 11
Shot Peening ................................................................................................ 11
Tumbling or Barrel Finishing ....................................................................... 12
Vibratory Finishing...................................................................................... 12
Media .......................................................................................................... 12
Compounds ................................................................................................. 13
Summary of Mass-Finishing Methods .......................................................... 13
Chemical Cleaning ......................................................................................... 13
General Considerations in Cleaning ........................................................... 13
Chemical Cleaning Processes ..................................................................... 14
Alkaline cleaning ..................................................................................... 14
Solvent cleaning ....................................................................................... 14
Acid cleaning ........................................................................................... 15
Chapter 2: Coating Processes
Painting, Wet or Liquid ................................................................................... 17
Paint Application Methods .............................................................................. 18
Dipping ....................................................................................................... 18
Spray Painting ............................................................................................. 18
Hand spraying ............................................................................................. 18
Drying ............................................................................................................ 19
Powder Coating .............................................................................................. 19
Hot-Dip Coating .............................................................................................. 21
Chemical Conversion Coatings ...................................................................... 21
Page 4 of 30
Blackening or Coloring Metals ....................................................................... 22
Electroplating ................................................................................................. 22
Chapter 3: Surface Enhancement Processes
Vaporized Deposition Processes .................................................................... 25
Physical Vapor Deposition .......................................................................... 25
Chemical Vapor Deposition ........................................................................ 26
Surface Hardening .......................................................................................... 27
Carburizing ................................................................................................. 28
Nitriding ...................................................................................................... 28
Carbonitriding ............................................................................................ 29
Chromizing .................................................................................................. 29
Boronizing ................................................................................................... 29
Clad Materials ................................................................................................ 29
References ...................................................................................................... 30
Page 5 of 30
Abstract The pupils who were studying Manufacturing Processes-I course were
supposed to do a project within the domain of manufacturing processes. We
had elected “Surface Engineering” as our project.
Surface engineering plays a dynamic part in all the manufacturing parts
irrespective if it is as small as a nut, bolt or as large as a space-shuttle. All the
mechanical modules, after their manufacturing, requires surface finishing.
As a result, of this project we got acquainted with the fundamentals of
surface engineering. We got acquainted with some of the industrialized
machines used in surface engineering. We had started our project with the
introduction of surface engineering leading towards surface texture,
mechanical cleaning, coating processes, heat treatment then to clad materials.
Page 6 of 30
SURFACE ENGINERRING –
AN INTRODUCTION Chapter 1
Mohammad Adam ME-113125
Page 7 of 30
Surface engineering is the sub-branch of material sciences and manufacturing
engineering which deals with the surface of solid material. It has applications
to chemistry, mechanical engineering, and electrical engineering (particularly
in relation to semiconductor manufacturing).
Definition
Surface engineering is a multidisciplinary activity intended to tailor the
properties of the surfaces of manufactured components so that their function
and serviceability can be improved.
Why Surface Engineering?
A manufacturing module usually fails when its surface cannot effectively resist
the external forces or environment to which it is subjected. The selection of a
surface material with the suitable thermal, optical, magnetic and electrical
properties and sufficient resistance to wear, corrosion and degradation, is vital
to its functionality. Sometimes technological progress and manufacturing
competence may be embarrassed only by surface requirements. For example,
the fuel efficiency and power output of gas turbines or diesel engines are
limited by the ability of key components to withstand high temperatures.
However, it is often not practical, inefficient or uneconomical to manufacture
components from a bulk material merely for its surface properties - far better
to use a cheaper, more easily formed underlying material and coat it with a
suitable high performance film. The resulting product conserves limited
material resources, performs better than the original and may well be cheaper
to produce.
Improving the functionality of an existing product is only one aim of
surface engineering. New coatings and treatment processes may also create
opportunities for new
products which could not
otherwise exist. For example,
satellites could not function,
nor could modern power
plants operate safely, without
the application of advanced
surface engineering
techniques.
The economic benefits
of surface engineering are
vast. According to a report by
Page 8 of 30
RCSE staff, in 2005 the value of the UK coating market is approximately £21.3
billion, and those coatings critically affect products with a value greater than
£143 billion (Source: "2005 Revisited; The UK Surface Engineering Industry to
2010", A Matthews, R Artley and P Holiday).
Many manufacturing processes influence surface properties, which in
turn may significantly affect the way the component function in service. The
demands for greater strength and longer life in components often depend on
changes in the surface properties rather than the bulk properties. These
changes may be mechanical, thermal, chemical, and/or physical and therefore
are difficult to describe in general terms. For example, two different surface
finishes on Inconel 718 can have a marked effect on the fatigue life, changing
the fatigue limit from 69 ksi after gentle grinding to as low as 22 ksi using
electrical discharge machining.
In brief, surface engineering is relevant to all types of products. It can
increase performance, reduce costs and control surface properties
independently of the substrate, offering enormous potential for:
improved functionality
the solution to previously insurmountable engineering problems
the possibility to create entirely new products
conservation of scarce material resources
reduction of power consumption and effluent output
Surface Integrity
The term surface integrity was coined by Field and Kahles in 1964 in reference
to the nature of the surface condition that is produced by the manufacturing
process. If we view the process as having five main components (workspace,
tool, machine tool, environment and process variables), we observer the
following properties are altered by the following:
High temperature involved in the machining process
Plastic deformation of the work material(residual stress)
Surface geometry(roughness, cracks, distortion)
Chemical reactions particularly between the tool and the work piece
Surface integrity has two aspects:
Topography
Surface layer characteristics
Page 9 of 30
Topography is made up of surface roughness, waviness, errors of form and
flaws. Machining processes produce surface flaws, waviness, and roughness
that can influence the performance of the component.
Surface layer characteristics includes the peaks and valleys that are
considered from waviness. Changes in the surface layer, as a result of
processing, include plastic deformation, residual stresses, cracks and other
metallurgical changes like hardness, over aging, phase changes,
recrystallization, inter-granular attack.
The material removal processes generate a wide variety of surfaces
textures, known as surface finish. The cutting process generate a wide variety
of surface textures on the material of which three are the important terms;
Page 10 of 30
surface roughness, waviness and lay. Roughness refers to the finely spaced
surface irregularities (it results from machining operations in case of machined
surfaces).Waviness is surface irregularity of greater spacing than in roughness
(it results from warping, vibration or the work being deflected during
machining).Lay is the term used to designate the direction of the predominant
surface pattern produced by the machining process.
The variety of surface instrument is available for measuring surface
roughness and surface profiles. The majority of these devices use a diamond
stylus that is moved at a constant rate across the surface, perpendicular to the
lay surface, perpendicular to the lay pattern. The rise and fall of the stylus is
detected electronically by LVDT (Linear Variable Differential Transformer) is
amplified and recorded on a strip-chart or is processed electronically to
produce average or root mean square readings for meter.
In most cases, the arithmetical average (AA) is used. In terms of measurements,
the AA or RA would be as follows:
𝑅𝐴 =∑ 𝑦𝑖𝑛𝑖−1
𝑛
yi is the vertical distance from the center line and n is the total number of
vertical measurements taken within a specified cut off distance. Cuttoff distance
refers to the sampling length used for the calculation of the roughness height.
When it is not specified, a value of 0.030in. (0.8mm) is assumed.
Page 11 of 30
Surface Finish Measurement
All of the processes used to manufacture components are important if their
effects are present in the finished part. It is convenient to divide processes that
are used to manufacture parts into three categories: traditional, nontraditional
and finishing treatments. In traditional processes the tool contacts the work
piece. Examples are grinding, milling and turning. In nontraditional processes
have intrinsic characteristics even if well controlled, will change the surface; in
these processes the work piece does not touch the tool. Electrochemical
machining (ECM), Electrical Discharge machining (EDM), Laser machining are
its examples. Finishing treatments can be used to negate or remove the impact
of both the traditional and nontraditional processes as well as provide good
surface finish. For example residual stresses can be removed by the shot
peening. Chemical milling can remove the recast layer left by EDM.
The objectives of the surface-modification processes can be quite
varied. Some are designed to clean surfaces and remove the kinds of defects
that occur during processing or handling (such as scratches, pores, fins).
Others further improve or modify the products, bring smoothness, texture and
color.
Mechanical Cleaning and Finishing Blast Cleaning
Mechanical cleaning involves the physical removal of soils, scales, or films from
the work surface of the work part by means of abrasives or similar mechanical
action. The processes used for mechanical cleaning often serve other functions
in addition to cleaning, such as debarring and improving surface finish.
Blast Finishing
Blast finishing uses the high-velocity impact of particulate media to clean and
finish a surface. The most well-known of these methods is sand blasting, which
uses grits of sand (SiO2) as the blasting media. Various other media are also
used in blast finishing, including hard abrasives such as aluminum oxide
(Al2O3) and silicon carbide (SiC), and soft media such as nylon beads and
crushed nut shells. The media is propelled at the target surface by pressurized
air or centrifugal force. In some applications, the process is performed wet, in
which fine particles in a water slurry are directed under hydraulic pressure at
the surface.
Shot Peening
In shot peening, a high-velocity stream of small cast steel pellets (called shot)
is directed at a metallic surface with the effect of cold working and inducing
compressive stresses into the surface layers. Shot peening is used primarily to
Page 12 of 30
improve fatigue strength of metal parts. Its purpose is therefore different from
blast finishing, although surface cleaning is accomplished as a by-product of
the operation.
Tumbling or Barrel Finishing
Tumbling (also called barrel finishing and tumbling barrel finishing) involves
the use of a horizontally oriented barrel of hexagonal or octagonal cross-
section in which parts are mixed by rotating the barrel at speeds of 10 to 50
rev/min. Finishing is performed by a ‘‘landslide’’ action of the media and parts
as the barrel revolves. As pictured in Figure 28.1, the contents rise in the barrel
due to rotation, followed by a tumbling down of the top layer due to gravity.
This cycle of rising and tumbling occurs continuously and, over time, subjects
all of the parts to the same desired finishing action.
Diagram of tumbling (barrel finishing) operation showing ‘‘landslide’’
action of parts and abrasive media to finish the parts.
Vibratory Finishing
Vibratory finishing is a type of mass finishing manufacturing process used
to deburr, radius, descale, burnish, clean, and brighten a large number of
relatively small work pieces. In contrast to the barrel processing, vibratory
finishing is performed in open containers as shown in the figure. Tubs or bowls
are loaded with work pieces and media are vibrated at frequencies between
900 and 3600 cycles per minute. The
process is less noisy and easily
controlled and automated.
Media
Media serves the purpose of
separating parts from each other and
interacting with each individual part
Page 13 of 30
to do the required finishing. Media is made from a variety of materials such as
ceramic, plastic, carbon or stainless steel, wood, leather, corn cob, nut shells,
river rock. The success of any mass finishing process is dependent on the
media selection and the ratio of media to the parts, their ratios are represented
in the following table. Natural abrasives include slag, sand, corundum; granite
etc Synthetic media contain 50 to 70% of abrasives such as Alumina (Al2O3),
flint, Silicon Carbide (SiC).
Compounds
A variety of functions are performed by the compounds that are added in
addition to the media and work pieces. These compounds can be liquid or
dry, abrasive or no abrasive and acid, neutral or alkaline. They are often
designed to assist in debarring, burnishing and abrasive cutting as well as to
provide cleaning, descaling.
Summary of Mass-Finishing Methods
The barrel and vibratory finishing processes are quite simple and economical
and can process large number of parts. Soft, nonferrous parts can be finished
in a little as 10 minutes, while the harder steels may require 2 hours or more.
Sometimes the operations are sequenced, using progressively finer abrasives.
The following figure shows the variety of parts before and after the mass
finishing operation using the triangular abrasive shown with each component.
Chemical Cleaning
Chemical cleaning operations are effective mean of removing oil, dirt, scale
other foreign material that may adhere to the surface of the product.
Manufacturers must ask themselves if a part really has to be cleaned, what soils
have to be removed, hoe clean the surface have to be. Selection of cleaning
method will depend on the cost of equipment, power, cleaning material etc.
General Considerations in Cleaning
There is no single cleaning method that can be used for all cleaning tasks. Just
as various soaps and detergents are required for different household jobs
(laundry, dishwashing, pot scrubbing, bathtub cleaning, and so forth), various
cleaning methods are also needed to solve different cleaning problems in
industry. Important factors in selecting a cleaning method are (1) the
contaminant to be removed, (2) degree of cleanliness required, (3) substrate
material to be cleaned, (4) purpose of the cleaning, (5) environmental and
safety factors, (6) size and geometry of the part, and (7) production and cost
requirements.
Page 14 of 30
A simple test is a wiping method, in which the surface is wiped with a clean
white cloth, and the amount of soil absorbed by the cloth is observed. It is a
non-quantitative but easy test to use.
Chemical Cleaning Processes
Chemical cleaning uses various types of chemicals to effect contaminant
removal from the surface. The major chemical cleaning methods are (1)
alkaline cleaning, (2) emulsion cleaning, (3) solvent cleaning, (4) acid cleaning,
and (5) ultrasonic cleaning. In some cases, chemical action is augmented by
other energy forms; for example, ultrasonic cleaning uses high-frequency
mechanical vibrations combined with chemical cleaning. In the following
paragraphs, we review these chemical methods.
Alkaline cleaning
Alkaline cleaning is the most widely used industrial cleaning method. As its
name indicates, it employs an alkali to remove oils, grease, wax, and various
types of particles (metal chips, silica, carbon, and light scale) from a metallic
surface. Alkaline cleaning solutions consist of low-cost, water-soluble salts such
as sodium and potassium hydroxide (NaOH, KOH), sodium carbonate
(Na2CO3), borax (Na2B4O7), phosphates and silicates of sodium and
potassium, combined with dispersants and surfactants in water. The cleaning
method is commonly by immersion or spraying, usually at temperatures of
50_C to 95_C (120_F–200_F). Following application of the alkaline solution, a
water rinse is used to remove the alkali residue. Metal surfaces cleaned by
alkaline solutions are typically electroplated or conversion coated.
Following are its types:
Electrolytic cleaning
Emulsion cleaning
Solvent cleaning
Organic soils such as oil and grease are removed from a metallic surface by
means of chemicals that dissolve the soils. Common application techniques
include hand-wiping, immersion, spraying, and vapor degreasing. Vapor
degreasing uses hot vapors of solvents to dissolve and remove oil and grease
on part surfaces. The common solvents include trichlorethylene (C2HCl3),
methylene chloride (CH2Cl2), and perchlorethylene (C2Cl4), all of which have
relatively low boiling points.In vapor degreasing process consists of heating
the liquid solvent to its boiling point in a container to produce hot vapors. Parts
to be cleaned are then introduced into the vapor, which condenses on the
relatively cold part surfaces, dissolving the contaminants and dripping to the
bottom of the container. Condensing coils near the top of the container prevent
any vapors from escaping the container into the surrounding atmosphere.
Page 15 of 30
Acid cleaning
It removes oils and light oxides from metal surfaces by soaking, spraying, or
manual brushing or wiping. The process is carried out at ambient or elevated
temperatures. Common cleaning fluids are acid solutions combined with
water-miscible solvents, wetting and emulsifying agents. Cleaning acids
include hydrochloric (HCl), nitric (HNO3), phosphoric (H3PO4), and sulfuric
(H2SO4), the selection depending on the base metal and purpose of the
cleaning. For example, phosphoric acid produces a light phosphate film on the
metallic surface, which can be a useful preparation for painting. A closely
related cleaning process is acid pickling, which involves a more severe
treatment to remove thicker oxides, rusts, and scales; it generally results in
some etching of the metallic surface, which serves to improve organic paint
adhesion.
Page 16 of 30
COATING PROCESSES Chapter 2
Rasikh Tariq ME-113006
Page 17 of 30
Each of the surface finishing methods is material removal process, designed to
clean, smooth, and otherwise reduce the size of the part. Many other techniques
have been developed to add material to the surface of a part. If the material is
deposited as a liquid or organic gas (or from a liquid or a gas medium), the
process is called coating. If the added material is a solid during deposition, the
process is known as cladding.
Painting, Wet or Liquid
Most of today’s commercial paints are synthetic organic compounds that
contain pigments and dry by polymerization or by a combination of
polymerization and adsorption of oxygen. Heat can be used to accelerate the
drying, but many of the synthetic paints and enamels will dry in less than an
hour without the use of additional heat. The older oil-based materials have a
long drying time and require excessive environmental protection measures.
For these reasons they are seldom used in manufacturing applications.
Paints are used for variety of reasons, usually to provide protection and
decoration but also to fill or conceal surface irregularities, change the surface
friction, or modify the light or heat absorption or radiation characteristics.
Following table provides a list of the more commonly used organic finishes,
along with their significant characteristics.
COMMONLY USED ORGANIC FINISHES AND THEIR QUALITIES
Material
Durability
(Scale of 1-
10)
Relative
Cost
(Scale of 1-
10)
Characteristics
Nitrocellulose
lacquers 1 2
Fast drying; low
durability
Epoxy esters 1 2 Good chemical
resistance Akyd-amine 2 1 Versatile; low adhesion
Acrylic lacquers 4 1.7 Good color retention;
low adhesion
Acrylic enamels 4 1.3
Good color retention;
though; high baking
temperature
Vinyl solutions 4-7 2 Flexible; good chemical
resistance; low solids
Silicones 4-7 10 Good gloss retention;
low flexibility
Flouropolymers 10 10 Excellent durability;
difficult to apply
Page 18 of 30
Paint Application Methods
In most cases, at least two coats are required. The first (or prime) coat serves
to:
Ensure adhesion
Provide a leveling effect by filling in minor porosity and other surface
blemishes, and
Improve corrosion resistance and thus prevent from being dislodged in
service.
In manufacturing, almost all painting is done by one of four methods:
Dipping is a simple and economical means of paint application when all
surfaces of the part are to be coated. The products can be manually immersed
into a paint bath or passed through the bath while on or attached to a conveyor.
Dipping is attractive for applying prime coats and for painting small parts
where spray painting would result a significant waste due to overspray.
Conversely, the process is unattractive where only some of the surfaces require
painting or where a very thin, uniform coating would be adequate, as on
automobile bodies other difficulties are associated with the tendency of paint
to run, production both a wavy surface and a final drop of paint attached to the
lowest drip point. Good-quality dipping requires that the paint be stirred at all
time and be of uniform viscosity.
Spray Painting is probably the most widely used paint application process
because of its versatility and the economy in the use of paint. In the
conventional technique, the paint is atomized and transported by the flow of
compressed air. In a variation known as airless spraying, mechanical pressure
forces the paint through an orifice at pressures between 500 and 4500 psi. This
provides sufficient velocity to produce atomization and also propel the particles
to the work piece. Because no air pressure is used for atomization, there is less
spray loss (Paint efficiency may be as
high as 99%) and less generation of
gaseous fumes.
Hand spraying is probably the most
adaptable means of an application but
can be quite costly in terms of labor and
production time. When air or
mechanical means provide the
atomization, workers must exercise
considerable skill to obtain the proper Basic Electroplating Process
Page 19 of 30
coverage without allowing the paint to “run” or “drape.” Only a very thin fill
can be deposited at one time, usually less than 0.001 in. As a result, several
coats may be required intervening time for drying.
Both manual and automatic spray painting can benefit from the use of
electrostatic deposition. A DC electrostatic potential is applied between the
atomizer and are therefore repelled. The oppositely charged work piece then
attracts the particles, with the actual path of the particle being a combination of
the kinetic trajectory and the electrostatic attraction. The higher DC voltage,
the greater the electrostatic attraction. Over-spraying can be reduced by as
much as 60 to 80%, as can the generation of airborne particles and other
emissions. Unfortunately, part edges and holes receive a heavier coating than
flat surfaces due to concentration of electrostatics lines of force on any sharp
edge. Depressed areas will receive a reduced amount of paint, and a manual
touch-up may be required using conventional spray techniques. Despite these
limitations, electrostatic spraying is an extremely attractive means of painting
complex-shaped products where the geometry would tend to create large
amount of overspray. The process is particularly attractive for applying the
prime coat to complex structures, such automobile bodies, where good
corrosion resistance is requirement.
Drying
Most paints and enamels used in manufacturing require from 2 to 24 hours to
dry at normal room temperature. This time can be reduced to between 10
minutes and 1 hour of the temperature can be raised to between 275o and 450o
F. As a result, elevated temperature drying is often preferred.
Elevated-temperature drying is rarely a problem with metal parts, but
other materials can damage by exposure to the moderate temperatures. For
example, when wood is heated, the gases, moisture, and residual liquid are
expanded and driven to the surface beneath the hardening paint.
Powder Coating
It is a variation of electrostatic spraying, but here the particles are solid rather
than liquid. Several coats, such as primer and finish, can be applied and then
followed by a single baking, in contrast to the baking after each coat that is
required in the conventional spray processes. In addition, the overspray
powder can often be collected and reused.
Page 20 of 30
Modern powder
technology can produce a high-
quality finish with superior
surface properties and usually
at a lower cost than liquid
painting. Powder painting is
more efficient in the use of
materials and lower energy
requirements. The process is
not good for large objects
(massive tanks) or heat-
sensitive objects. It is not easy to
produce film thickness less than
0.03mm.
Following table shows some thermosetting powder and their useful properties:
THERMOSETTING POWDER COATING (DRY PAINTING) HAVE A WIDE VARIETY OF PROPERTIES AND APPLICATIONS
Properties Epoxy
Epoxy/Po
lyester
Hybrid
TGIC
Polyester
Polyester
Urethane
Acrylic
Urethane
Application Thickness 0.5-20 mils a 0.5-10 mils
0.5-10
mils 0.5-10 mils 0.5-10 mils
Cure Cycle (Metal temperatures)b
450oF – 3
min
250oF –
30min
450oF – 3
min
325oF – 25
min
400oF – 7
min
310oF – 20
min
400oF – 7
min
325oF – 17
min
400oF – 7
min
360oF – 25
min
Outdoor weatherability
Poor Poor Very
Good Very Good Excellent
Pencil Hardness HB-5H HB-2H HB-2H HB-3H H-3H Direct Impact resistance, in lbc 80-160 80-160 80-160 80-160 20-60
Chemical Resistance Excellent
Very Good
Least
expensive
Good Good
Very
Good Most
expensive Cost (Relative) 2 1 3 4 5
Applications
Furniture,
cars,
ovens,
appliances
Water
heaters,
radiators,
office
furniture
Architect
ural
aluminum
, outdoor
furniture
Car
wheels/ri
ms,
playgroun
d
equipment
Washing
machine,
refrigerato
rs, ovens
Schematic Machine Diagram of Powder Coating
Page 21 of 30
a Thickness up to 150 mils can be applied via multiple coats in a fluidized bed. b Time and temperature can be reduced, by utilizing accelerated curing mechanisms while maintaining the same general properties c Tested at a coating thickness of 2.0 mils
Hot-Dip Coating
Large quantities of metal products are given corrosion-resistant coatings by
direct immersion into a bath of molten metal. The most common coating
materials are zinc, tin, aluminum, and tene (an alloy of lead and tin).
Hot-dip galvanizing is the most widely used method of imparting
corrosion resistance to steel. After the products, or sheets, have been cleaned
to remove oil, grease, scale, and rust, they are fluxed by dipping into a solution
of zinc ammonium chloride and dried. Next, the article is completely immersed
in a bath of molten zinc. The zinc and iron react metallurgical to produce a
coating that consists of a series of zinc-iron compounds and a surface layer of
nearly pure zinc.
The primary limitations to hot-dip galvanizing are the size of the product
and the “damage” that might occur when a metal is exposed to the
temperatures of the molten material.
Tin coatings can also be applied by immersing in a bath of molten tin
with a covering of flux material. Because of the high cost of tin and the relatively
thick coatings applied by hot dipping, most tin coatings are now applied by
electroplating.
Chemical Conversion Coatings
In chemical conversion coating, the surface of the metal is chemically treated
to produce a nonmetallic, nonconductive surface that can impart a range of
desirable properties. The most popular types of conversion coatings are
chromate and phosphate. Aluminum, magnesium, zinc and copper (as well as
cadmium and silver) can all be treated by a chromate conversion process that
usually involves immersion in a chemical bath. The surface of the metal is
convened into a layer of complex chromium compounds that can impart colors
ranging from bright color through blue, yellow, brown, olive drab, and black.
Most of the films are soft and gelatinous when they are formed but harden upon
drying. They can be used to:
Impart exceptionally good corrosion resistance.
Act as an intermediate bonding layer for paint, lacquer, or other organic
finishes, or
Page 22 of 30
Provide specific colors by adding dyes to the coating when it is in its soft
condition.
Blackening or Coloring Metals
Many steel parts are treated to produce a black, iron oxide coating – a lustrous
surface that is resistant to rusting when handled. Since this type of oxide forms
at elevated temperatures, the parts are usually heated in some form of special
environment, such as spent carburizing compound or special blackening salts.
Chemical solutions can also be used to blacken, blue and even “brown”
steels. Brown, black and blue colors can also be imparted to tin, zinc, cadmium,
and aluminum through chemical bath immersion or wipes. The surfaces of
copper and brass can be made to be black, blue, green, or brown, with a full
range of shades in between.
Electroplating
Large quantities of metal and plastic parts are electroplated to produce metal
coating that imparts corrosion or wear resistance, improves appearance, or
increases the overall dimensions. Virtually all commercial metals can be
plated, including aluminum, copper, brass, steel, and zinc-based die castings.
Plastics can be electroplated, provided that they are first coated with an
electrically conductive material.
The most common platings
are zinc, chromium, nickel,
copper, tin, gold, platinum, and
sliver. The electro-galvanized zinc
platings are thinner than the hot-
dip coatings and can be produced
without subjecting the base metal
to the elevated temperatures of
molten zinc. Nickel plating
provides good corrosion
resistance but is rather expensive
and does not retain its lustrous
appearance. Consequently, when
lustrous appearance is desired, a chromium plate is specified. An initial layer
of copper provides a leveling effect and makes it possible to reduce the
thickness of the nickel layer that typically follows to less than 0.0006 in. The final
layer of chromium then provides the attractive appearance. Gold, silver, and
platinum platings are used in both the jewelry and electronic industries, when
Electroplating Process
Page 23 of 30
the thin layers impart the desired properties while conserving the precious
metals.
Hard chromium plate, with Rockwell hardnesses between 66 and 70, can
be used to build up worn parts to larger dimensions and to coat tools and other
products that need reduced surface friction and good resistance to both wear
and corrosion. Hard chrome coatings are always applied directly to the base
material and are usually much thicker layers than the decorative treatments,
typically ranging from 0.003 to 0.010 in. thick. Even thicker layers are used in
applications such as diesel cylinder liners. Such hard chrome plate does not
have a leveling effect, defects or roughness in the base surface will be
amplified. If smooth surfaces are desired, subsequent grinding and polishing
may be necessary.
The figure in the margin shows the base process of electroplating
coating. A DC voltage is applied to the material that is to be coated and the
metal that will be used as a coating. Coating metal is provided with anode
voltage whereas material to be coated resides on cathode voltage. The
container also contains a solution can conducts electricity such as brine
solution. Electrons passes through the brine solution and making layers on the
surface of cathode (material to be coated).
The surface to be plated must also be prepared properly if satisfactory
results are to be obtained. Pinholes, scratches, and other surface defects must
be removed if a smooth, lustrous finish is desired. Combinations of degreasing,
cleaning, and pickling are used to ensure a chemically clean surface, one to
which the plating material can adhere.
Page 24 of 30
SURFACE ENHANCEMENT
PROCESSES Chapter 3
Khawar Shahzad ME-113009
Page 25 of 30
Vaporized Deposition Processes
The vapor deposition processes form a thin coating on a substrate by either
condensation or chemical reaction of a gas onto the surface of the substrate.
The processes can be classified into two main categories: physical vapor
deposition and chemical vapor deposition.
Physical Vapor Deposition
Physical vapor deposition (PVD) is a group of thin film processes in which a
material is converted into its vapor phase in a vacuum chamber and condensed
onto a substrate surface as a very thin layer.
In the Physical Vapor Deposition process, a negatively charged
electrode is slowly disintegrated by molecular bombardment. The PVD
medium is typically argon because this gas generates sufficient momentum to
free atoms from the target. In a vacuum environment, these free target atoms
deposit themselves on the surface of the material and form the desired coating
or plating.
Maintaining a specified gas mass flow
rate to the vacuum chamber is critical
during the PVD process. Typically,
vacuum pumping stations require a
throttle valve or orifice-limiting device
to control the pump's output when the
PVD gas is introduced. This method is
extremely pressure sensitive and can
result in inefficient gas delivery and
poor product quality.
Vaporized Deposition Processes
Physical Vapor Deposition
Vaccum Vapor Deposition
Sputtering
Ion Plating
Chemical Vapor Deposition
Page 26 of 30
Applications of PVD include thin decorative coatings on plastic and metal
parts such as trophies, toys, pens and pencils, watchcases, and interior trim in
automobiles.
Chemical Vapor Deposition
Chemical vapor deposition (CVD) involves the interaction between a mixture
of gases and the surface of a heated substrate, causing chemical decomposition
of some of the gas constituents and formation of a solid film on the substrate.
The reactions take place in an enclosed reaction chamber. The reaction
product (either a metal or a compound) nucleates and grows on the substrate
surface to form the coating. Most CVD reactions require heat. However,
depending on the chemicals involved, the reactions can be driven by other
possible energy sources, such as ultraviolet light or plasma. CVD includes a
wide range of pressures and temperatures; and it can be applied to a great
variety of coating and substrate materials. Modern interest in CVD is focused
on its coating applications such as coated cemented carbide tools, solar cells,
depositing refractory metals on jet engine turbine blades, and other
applications where resistance to wear, corrosion, erosion, and thermal shock
are important. In addition, CVD is an important technology in integrated circuit
fabrication.
Advantages typically cited for CVD include
Capability to deposit refractory materials at temperatures below their
melting or sintering temperatures;
Control of grain size is possible;
The process is carried out at atmospheric pressure it does not require
vacuum equipment;
Good bonding of coating to substrate surface.
Disadvantages include
Corrosive and/or toxic nature of chemicals generally necessitates a
closed chamber as well as special pumping and disposal equipment;
Certain reaction ingredients are relatively expensive;
Material utilization is low.
Summary of Physical Vapor Deposition (PVD) Processes
PVD Process Features and
Comparisons Coating Materials
Vacuum Evaporation
Equipment is relatively
low-cost and simple:
deposition of compounds
is difficult: coating
adhesion not as good as
other PVD processes
Ag, Al, Au, Cr, Cu, Mo,
W
Page 27 of 30
Sputtering
Better throwing power
and coating adhesion than
vacuum evaporation, can
coat compounds, slower
deposition rates and more
difficult process control
than vacuum evaporation
Al2O3, Au, Cr, Mo, SiO2,
Si3N4, TiC, TiN
Ion Plating
Best coverage and
coating adhesion of PVD
processes, most complex
process control higher
deposition rates than
sputtering.
Ag, Au, Cr, Mo, Si3N4,
TiC, TiN
Surface Hardening
Surface hardening refers to any of several thermochemical treatments applied
to steels in which the composition of the part surface is altered by addition of
carbon, nitrogen, or other elements. The most common treatments are
carburizing, nitriding, and carbo-nitriding. These processes are commonly
applied to low carbon steel parts to achieve a hard, wear-resistant outer shell
while retaining a tough inner core. The term case hardening is often used for
these treatments.
Surface Hardening Treatments
Carburizing
Pack Carborizing
Gas Carborizing
Liquid CarborizingCarbonitriding
Chromizing
Bronizing
Nitriding
Gas Nitriding
Liquid Nitriding
Page 28 of 30
Carburizing
Carburizing is the most common surface-hardening treatment. It involves
heating a part of low carbon steel in the presence of a carbon-rich environment
so that C is diffused into the surface. In effect the surface is converted to high
carbon steel, capable of higher hardness than the low-C core. The carbon-rich
environment can be created in several ways. One method involves the use of
carbonaceous materials such as charcoal or coke packed in a closed container
with the parts.
This process, called pack carburizing, produces a relatively thick layer
on the part surface, ranging from around 0.6 to 4 mm (0.025 to 0.150 in).
Another method, called gas carburizing, uses hydrocarbon fuels such
as propane (C3H8) inside a sealed furnace to diffuse carbon into the parts. The
case thickness in this treatment is thin, 0.13 to 0.75 mm (0.005 to 0.030 in).
Another process is liquid carburizing, which employs a molten salt bath
containing sodium cyanide (NaCN), barium chloride (BaCl2), and other
compounds to diffuse carbon into the steel. This process produces surface layer
thicknesses generally between those of the other two treatments. Typical
carburizing temperatures are 875 to 925 C (1600 to 1700 F), well into the
austenite range.
Carburizing followed by quenching produces a case hardness of
around HRC=60. However, because the internal regions of the part consist of
low carbon steel, and its hardenability is low, it is unaffected by the quench and
remains relatively tough and ductile to withstand impact and fatigue stresses.
Nitriding
Nitriding is a treatment in which nitrogen is diffused into the surfaces of special
alloy steels to produce a thin hard casing without quenching. To be most
effective, the steel must contain certain alloying ingredients such as aluminum
(0.85% to 1.5%) or chromium (5%or more). These elements form nitride
compounds that precipitate as very fine particles in the casing to harden the
steel.
Nitriding methods include: gas nitriding, in which the steel parts are
heated in an atmosphere of ammonia (NH3) or other nitrogen rich gas mixture;
and liquid nitriding, in which the parts are dipped in molten cyanide salt baths.
Both processes are carried out at around 500o C (950o F). Case thicknesses
range as low as 0.025 mm (0.001 in) and up to around 0.5 mm (0.020 in), with
hardnesses up to HRC 70.
Page 29 of 30
Carbonitriding
As its name suggests, carbonitriding is a treatment in which both
carbon and nitrogen are absorbed into the steel surface, usually by heating in
a furnace containing carbon and ammonia. Case thicknesses are usually 0.07 to
0.5 mm (0.003 to 0.020 in), with hardnesses comparable with those of the other
two treatments.
Two additional surface-hardening treatments diffuse chromium and
boron, respectively, into the steel to produce casings that are typically only
0.025 to 0.05 mm (0.001 to 0.002 in) thick.
Chromizing
Chromizing requires higher temperatures and longer treatment times than the
preceding surface-hardening treatments, but the resulting casing is not only
hard and wear resistant, it is also heat and corrosion resistant.
The process is usually applied to low carbon steels. Techniques for
diffusing chromium into the surface include: packing the steel parts in
chromium-rich powders or granules, dipping in a molten salt bath containing
Cr and Cr salts, and chemical vapor deposition.
Boronizing
Boronizing is performed on tool steels, nickel- and cobalt-based alloys, and
cast irons, in addition to plain carbon steels, using powders, salts, or gas
atmospheres containing boron.
The process results in a thin casing with high abrasion resistance and
low coefficient of friction. Casing hardnesses reach 70 HRC. When boronizing
is used on low carbon and low alloy steels, corrosion resistance is also
improved.
Clad Materials
Clad materials re actually a form of composite in which the components are
joined as solids, using techniques such as roll bonding, explosive welding, and
extrusion. The most common form is a laminate, where the surface layer
provides properties such as corrosion resistance, wear resistance, electrical
conductivity, thermal conductivity, or improved appearance, while the
substrate layer provides strength or reduces overall cost. Alclad aluminum is a
typical example. Here surface layers of weaker but more corrosion-resistant
single-phase aluminum alloys are applied to a base of high-strength but less
Page 30 of 30
corrosion-resistant age-hardenable material. Aluminum-clad steel meets the
same objective but with a heavier substrate, and stainless steel can be sused to
clad steels, reducing the need for nickel- and chromium-alloy additions
throughout.
Wires and rods can also be made as claddings. Here the surface layer
often imparts conductivity, while the core provides strength or rigidity.
Copper-clad steel rods that can be driven into the ground to provide electrical
grounding for lightning rod systems are one example.
References “Wood Modification - Chemical, Thermal and Other Processes” by
C. Hill (Wiley, 2006) BBS. “Process Engineering for Manufacturing” by Eray Johnson.
“Coatings Technology Handbook” by Arthur A. Tracton.
BASF Handbook on Basics of Coating Technology (American Coatings
Literature)
Surface Engineering of Light Alloys - Al., Mg. and Ti. Alloys - H. Dong
(Woodhead, 2010) BBS “Fundamentals of Modern Manufacturing – Materials, Processes,
and Systems” by M.P. Groover.
DeGarmo's Manufacturing Engineering and Technology, 10th Edition.
Coating Tribology (Properties, Mechanisms, Techniques and
applications in Surface Engineering)
http://www.packaging-int.com/video/Slot-Curtain-Coating.html
International Society of Coating Science and Technology
(http://www.iscst.org/)
http://www.plasmacoatings.com/coating-types.html
http://www.tstcoatings.com/types-of-coatings.html
http://www.tstcoatings.com/
http://www.gordonengland.co.uk/coldspray.htm
