Surface Engineering Technologies Flashcards
(37 cards)
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
