Surface Engineering Technologies

Question 1

Name the different types of Surface Coatings

Answer 1

Electrolytic

Fusion

Non-fusion

Vapour phase

Question 2

Name the different surface modifications

Answer 2

Mechanically induced

With structural transformation

With changed chemical composition

Question 3

For the following applications, what surface modification treatment
would you recommend and why?

1) steel pipe for transport of rock in a mining operation
2) drill for producing holes in mass produced printed circuit boards
3) cylinder bore in a petrol engine
4) skis and sledge runners
5) lathe tool for cutting stainless steel at high speed

Answer 3

1) abrasive wear resistance, dense thick layer (e.g. HVOF cermets
(ceramic and metal mixes) like tungsten-carbide cobalt)

2) low friction, wear resistance (e.g. PVD TiN, TiAlN, TiCN)
3) thermal and wear resistance (e.g. PEO, hard anodizing)
4) low friction in sub-zero temp. (polymer coating e.g. PTFE)
5) reduce wear and chip pickup (e.g. PVD TiAlN, TiCN, CrN)

Question 4

Name the Electrolytic Surface Coating methods

Answer 4

Anodising

Plating

PEO

Question 5

Name the Fusion Surface Coating methods

Answer 5

Thermal Spraying

Weld Overlays

ESO

Question 6

Name the Non Fusion Surface Coating methods

Answer 6

Cold Spray

Question 7

Name the Vapour Phase Surface coating methods

Answer 7

PVD

CVD

Question 8

Describe the Anodising process with a diagrams (Process, Pros, Cons, Applications)

Answer 8

electrolytic passivation process

Increases the thickness of the natural oxide layer on the
metallic surface

Part to be treated forms the anode electrode of an
electrical circuit

Increases corrosion resistance and wear resistance

Also used to prevent galling of threaded components

Most commonly applied to protect aluminium alloys, although
processes also exist for titanium, zinc, magnesium, niobium,
and tantalum

Not a useful treatment for iron or carbon steel because these
metals ex-foliate when oxidized

Part is immersed in an electrolyte consisting of an acid/water solution.

A current is applied causing the water to break down, depositing oxygen on the anode.

Oxygen combines with aluminium to form an oxide thus building up an outer oxide film on the surface.

Question 9

Describe the process of Plating

Answer 9

Surface treatment process in which a metal is deposited on a
conductive surface- Can be done via Electroplating or Electro-less Plating

Used to decorate objects, for corrosion inhibition, to improve
solder ability, to harden, to improve wear, to reduce
friction, to improve paint adhesion, to alter conductivity.

Jewellery typically uses plating to give a silver or gold finish

Typically Cr or Ni on Steel are used but also gold plating, silver plating, rhodium plating, zinc plating,
tin plating, alloy plating

Typically 10’s μm to several mm thick

Question 10

Describe Electroplating including a diagram

Answer 10

Deposition of a metal coating by putting a
negative charge on component and putting it
into a solution which contains a metal salt.

Positively charged metal ions are attracted to
the negatively charged object and are “reduced” to metallic form.

Question 11

Describe Electroless plating including a diagram

Answer 11

Chemical reduction process which depends
upon the catalytic reduction process of metal
ions in an aqueous solution and the
subsequent deposition of metal without the
use of electrical energy.

The driving force for the reduction of metal
ions and their deposition is supplied by a chemical reducing agent in solution.

Question 12

What is PEO

Answer 12

Electro-chemical surface treatment process for generating
oxide coatings on metals

Similar to anodizing, but employs higher potentials, so that
discharges occur and the resulting plasma modifies (and
enhances) the structure of the oxide layer

Process can be used to grow thick oxide coatings on metals such as aluminium, magnesium and titanium

Due to high hardness and a continuous barrier, these coatings
can offer protection against wear, corrosion or heat as well as
electrical insulation

The coating is a chemical conversion of the substrate metal into
its oxide, and grows both inwards and outwards from the
original metal surface (excellent adhesion)

Question 13

Describe the Method of PEO including a diagram

Answer 13

Conventional anodizing oxide layer is grown on the surface of the
metal by the application of electrical potential, while the part is
immersed in an acidic electrolyte.

In PEO process high potential
is applied (200V) resulting in in localized plasma reactors, with
conditions of high temperature and pressure which modify the
growing oxide.

Processes include melting, melt-flow, resolidifcation,
sintering and densification of the growing oxide.

Question 14

Explain how the coating properties vary with PEO

Answer 14

PEO coatings are generally
recognized for high
hardness, wear resistance and corrosion resistance.

However, the coating
properties are highly
dependent on the substrate used, as well as on the
composition of the
electrolyte and the electrical regime used.

Even on aluminium, the coating properties can vary strongly according to the exact alloy composition.

Question 15

Outline the GENERAL method for Thermal Spraying with a diagram

Answer 15

Processes which heats a consumable and then sprays the
heated consumable onto a substrate

Coating build-up by the stacking of deformed particles
Coating thickness is 0.1 to 5mm

Substrate remains relatively cool

Coatings have a mechanical bond with the substrate

Question 16

What are the Benefits of thermal Spraying

Answer 16

Choice of coating materials: metals, alloys, ceramics, cermets
and carbides

Thick coatings can be applied at high deposition rates

Coatings are mechanically bonded to the substrate - can
often spray coating materials which are metallurgically
incompatible with the substrate

Components can be sprayed with little or no pre- or post-heat
treatment, and component distortion is minimal

Parts can be rebuilt quickly and at low cost, and usually at a
fraction of the price of a replacement

Coatings may be applied both manually and automatically

Variety methods: flame, arc, plasma, HVOF

Question 17

What are the different types of Thermal Spraying

Answer 17

Powder Flame Spraying

Wire Flame Spraying

Arc Spraying

Plasma Spraying

High Velocity Oxyfuel Spraying

Question 18

Outline Flame Spraying including a diagram

Answer 18

Uses the heat from the combustion of a fuel
gas (usually acetylene or propane) with oxygen to melt the
coating material,

This is fed into the spraying gun as a
powder, wire or rod.

The consumable types give rise to the two
process variants:
• powder flame spraying
• wire flame spraying

Question 19

Outline Arc Spraying including a diagram

Answer 19

It is the highest productivity thermal spraying process.

A DC electric arc is struck between two continuous consumable wire
electrodes that form the spray material.

Compressed gas atomises the molten
spray material into fine
droplets and propels them towards the substrate.

Question 20

Outline Plasma Spraying including a diagram

Answer 20

Process uses a DC electric arc to generate a stream of high temperature ionised plasma gas, which acts as the spraying heat source.

The coating material, in powder form, is carried in an inert gas stream into the plasma jet
where it is heated and propelled towards the substrate.

Question 21

Outline High Velocity Oxyfuel Spraying (HVOF) including a diagram

Answer 21

It employs higher
flow rates and pressures compared with conventional flame
spraying.

These factors combined with internal combustion within the HVOF gun allows a supersonic flame to be produced.

Question 22

Outline the process of Weld Overlays

Answer 22

Generally used to apply “sacrificial” material where there is high abrasive wear

Coating applied by standard welding methods - Oxy-acteylene,
Arc, MIG, TIG, etc.

Deposits are typically several mm thick - can be a lot thicker

Generally applied to Steels

Typically applied:
o austenitic (Mn) steels
o martensitic steels
o cast irons containing carbide formers
o WC / Co

The PTA (Plasma Transferred Arc) process welds a metallic coating material in powder form to a substrate to produce a hard, wear-resistant coating that is metallurgically bonded to the substrate.

The powder is injected into the stream of
plasma gas, depositing it onto the work-piece.

Question 23

Outline Electro Spark Deposition including a diagram

Answer 23

Ionized material (electrode) is transferred to the substrate
surface, producing an alloy with the substrate. The deposited layer
has a metallurgical bond to the substrate.

Typical electrodes: carbides (W, Ti, Cr etc) stainless steel,
Inconel, Aluminium

Energy transferred to a consumable electrode for a very short
duration 1/1000s; tip temp. 8000 – 25000°C

Typically up to 0.2mm layer thickness

Coating features are controlled by
the process parameters: spark energy,
tension, spark duration, inductivity,
frequency, temperature,
number of passes, pressing force ,
speed etc

Question 24

Outline cold spraying including a diagram

Answer 24

High Power power and gas are both fed in

High velocity particles have high kinetic energy

The particles collide with the prepared substrate and deform on impact thus building up a coating

Question 25

Outline the general idea of Physical Vapour Deposition

Answer 25

In PVD processes material is vaporized from a solid or liquid source in the form of atoms or molecules.

It is transported in the form of a vapour through a vacuum or low pressure gaseous
environment to the substrate where it condenses.

The substrates can range in size from very small to very large
and range in shape

PVD processes can be used to deposit films of elements and alloys
as well as compounds using reactive deposition processes

Deposition Rates : 1-10 nm/s

Film thickness few nanometres-10 micrometres

Process Temperatures 200-300 Celsius - MUCH LOWER THAN CVD

Question 26

Outline the THREE fundamental PVD steps

Answer 26

1. Vapour phase generation from coating material stock
2. The transfer of the vapour phase from source to substrate
3. Deposition and film growth on the substrate

These steps can be independent or superimposed on each other depending on the desired coating characteristics.

The final result of the coating/substrate composite is a function of each materials individual properties, the interaction of the materials and any process constraints that may exist

Question 27

What are the advantages of PVD

Answer 27

Low deposition temperature means can coat prior heat treated steels and also means minimal component distortion.

Shorter time cycle (4-5 hours) than CVD (15-20 hours)

Most coatings have high temperature and good impact strength,
excellent abrasion resistance

More environmentally friendly than
traditional coating processes
such as electroplating

More than one technique can be used
to deposit a given film

Question 28

What are the Limitations of PVD

Answer 28

Moderate throwing power (less than CVD) – need component
rotation

Line-of-sight transfer (coating annular shapes practically
impossible)

Relatively small loading capacity

Some PVD technologies typically operate at very high vacuums,
requiring special attention by operating personnel

Requires a cooling water system to dissipate large heat loads

Selection of the best PVD technology may require some
experience and/or experimentation

High capital costs

Question 29

What are the applications of PVD

Answer 29

Tool coatings:
Cutting and forming tools, moulds and dies

Tribological coatings: Machinery and automotive engine
components such as fuel injection, piston rings, gears, bearings

Decorative coatings: Bath/kitchen/door hardware, watches,
spectacle frames, mobile phones

Question 30

Outline Chemical vapour Depostition including a diagram

Answer 30

Deposition of a solid on a heated surface from a chemical reaction in a vapour phase

A heat-activated process, CVD
relies on the reaction of gaseous chemical compounds with
suitably heated and prepared substrates

Combines several scientific and
engineering disciplines including thermodynamics, plasma physics, kinetics, fluid dynamics
and chemistry

Reactants first diffuse through boundary layer

Absorption of reactants on the substrate

Then a chemical reaction takes place

Desorption of adsorbed species then occurs

The by-products finally diffuse out

Question 31

What are the advantages of Chemical Vapour Deposition

Answer 31

Relatively uncomplicated and flexible technology which can
accommodate many variations

Can be used for a wide range of metals and ceramics; forms
alloys

Materials in excess 99.9% of theoretical density are commonly
produced

It is possible to coat almost any shape of almost any size

Unlike other thin film techniques can also be used to produce
fibres, monoliths, foams and powders

Deposition rate is high and thick coatings possible (in some cases
centimetres thick)

Conforms homogeneously to contours of substrate surface

Has high throwing power, controllable thickness and morphology

Question 32

What are the limitations of Chemical Vapour Deposition

Answer 32

• Major disadvantage: most versatile at temperatures of 600 C and above so many substrates are not thermally stable at these temperatures

Requirement of having chemical precursors with high vapour pressure which are often hazardous and at times extremely toxic

Also by-products of the CVD reactions are also toxic and corrosive
and must be neutralised, which may be a costly operation

Question 33

What are the Applications of Chemical Vapour Deposition?

Answer 33

1) Semiconductor industry (estimated ¾ of all CVD production);
e.g. diffusion barrier layers for advanced semiconductor integrated
circuits

2) Metallurgical-coating industry
Technique used to deposit large
number of wear-resistant coatings such as nitrides, borides, carbides, oxides.

Question 34

Outline the Mechanical methods for Surface Modification

Answer 34

Surface work hardening

Increasing internal stresses
within the structure due to
increased the dislocation density

use controlled impingement:
- e.g. shot - “peening”

Low cost, automated process

Room temperature

Line of sight process

Little effect on wear resistance

Good for fatigue resistance.

Examples: valve spring wire, leaf
springs, gears

Question 35

Outline Heat Induced Phase Transformation- Different Types (DIAGRAMS)

Answer 35

Basic concept is to heat steel surface to austenitic range, then
quench it to form surface martensite

Heating Methods:
• Flame Treatment
• Induction Heating
• Electron Beam /

Laser Beam Hardening

is especially suitable for selective hardening of complex shaped parts, bores or edges, and parts where low
distortion is critical.

A High - power electron beam is scanned over surface by electromagnetic
deflection

Question 36

What are the three Thermochemical surface treatments?

Answer 36

Carburizing
– Heat steel to austenitic range (850-950 ºC) in a carbon rich
environment, then quench and temper

Nitriding
– Nitrogen diffusion into steels occurs around 500-560 ºC to form a
thin hard surface
– Good for Cr, V, W, and Mo steels. Will embrittle surface of
Aluminum.

Metallising
– Chromizing – chromium diffuses into surface to form corrosion
resistant layer (take care with carbon steels as surface will
decarburize)
– Boronising – diffuse boron into surface to form iron boride layer
Anodising
Thermochemical Treatments

Question 37

there are a few more

Answer 37

…….