Surface Engineering Technologies Flashcards
Name the different types of Surface Coatings
Electrolytic
Fusion
Non-fusion
Vapour phase
Name the different surface modifications
Mechanically induced
With structural transformation
With changed chemical composition
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
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)
Name the Electrolytic Surface Coating methods
Anodising
Plating
PEO
Name the Fusion Surface Coating methods
Thermal Spraying
Weld Overlays
ESO
Name the Non Fusion Surface Coating methods
Cold Spray
Name the Vapour Phase Surface coating methods
PVD
CVD
Describe the Anodising process with a diagrams (Process, Pros, Cons, Applications)
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.
Describe the process of Plating
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
Describe Electroplating including a diagram
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.
Describe Electroless plating including a diagram
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.
What is PEO
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)
Describe the Method of PEO including a diagram
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.
Explain how the coating properties vary with PEO
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.
Outline the GENERAL method for Thermal Spraying with a diagram
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
What are the Benefits of thermal Spraying
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
What are the different types of Thermal Spraying
Powder Flame Spraying
Wire Flame Spraying
Arc Spraying
Plasma Spraying
High Velocity Oxyfuel Spraying
Outline Flame Spraying including a diagram
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
Outline Arc Spraying including a diagram
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.
Outline Plasma Spraying including a diagram
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.
Outline High Velocity Oxyfuel Spraying (HVOF) including a diagram
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.
Outline the process of Weld Overlays
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.
Outline Electro Spark Deposition including a diagram
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
Outline cold spraying including a diagram
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
Outline the general idea of Physical Vapour Deposition
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
Outline the THREE fundamental PVD steps
- Vapour phase generation from coating material stock
- The transfer of the vapour phase from source to substrate
- 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
What are the advantages of PVD
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
What are the Limitations of PVD
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
What are the applications of PVD
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
Outline Chemical vapour Depostition including a diagram
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
What are the advantages of Chemical Vapour Deposition
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
What are the limitations of Chemical Vapour Deposition
• 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
What are the Applications of Chemical Vapour Deposition?
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.
Outline the Mechanical methods for Surface Modification
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
Outline Heat Induced Phase Transformation- Different Types (DIAGRAMS)
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
What are the three Thermochemical surface treatments?
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
there are a few more
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