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Terms in blue are links to further information.
A
Abradable coatings: Coatings which are designed to
rub down against the action of a mating surface to form a
tight gas or air seal.
Abrasive blasting: A pressurised stream of hard metal
or oxide grit material used to clean and/or roughen surfaces
prior to coating.
Abrasive Wear: Wear caused by hard, sharp particles.
Acoustic emission: Acoustic emissions are sound or
ultrasound pulses generated during crack initiation or propagation
in materials and coatings as a result of being subjected to
stress. Acoustic emissions can be detected by transducers.
Adhesive Wear: Wear caused by sliding surfaces, where
surface asperities interact and adhere.
Agglomerated powder: A mechanically mixed combination
of fine particles of different materials held together with
an organic binder and formed into power particles.
Aluminising (gas): High temperature (approx 900oC)
pack or gaseous diffusion of aluminium into the surface of
a component to enhance high temperature corrosion and oxidation
resistance.
Alkyd resin: A type of polyester resin used in paints
and other surface coatings. The original alkyd resins were
made by co-polimerising phathalic anhydride with glycerol,
to give a brittle cross. Linked polymer.
Aluminising (hot dip): An aluminium coating process
based on submersion in liquid metal, usually with a strip
steel product being continuously fed through the bath. Provide
galvanic corrosion protection.
Aluminium Ion Plating: The deposition of aluminium
by a vacuum evaporative process. Provides galvanic corrosion
resistance. Normally given a passivation treatment.
Anodising: The production
of an oxide layer on aluminium alloys. The process is electrolytic,
a typical electrolyte being sulphuric acid. Treatment at room
temperature produces thin, decorative layers with some corrosion
protection. Treatment at 0oC produces hard, thicker
layers (up to 100µ) with wear resistance. They can be post sealed
to give improved corrosion resistance.
Arc Wire Spraying: A thermal spray process in which
two electrically conducting wires are brought together to
form an electric arc. The consequent molten metal is then
projected by a air stream towards the workpiece to form a
coating.
Autoclaving: The production of a stable, protective
oxide on steel parts by treatment in a pressurised, high temperature
steam containing atmosphere.
B
Blasting: A pressurised stream of some materials (Glass,
plastic, metal, , etc) applied on a surface to clean and/or
roughen. It can be, depending on the media, abrasive an non-
abrasive.
Bond: This represents the state of adhesion between
the coating and the substrate. It's strength will depend on
the details of the spraying process and the materials used.
Bonding mechanisms may be mechanical, physical or metallurgical
or a combination of these.
Bond coat: A coating applied as an intermediary between
the main or top coating and the substrate in order to improve
the bond strength.
Bond strength: The strength of the adhesion between
the coating and the substrate. A number of test methods are
in use to measure the bond strength of coatings.
Boronising: The diffusion of boron into the surface
of a component (usually steel) by a high temperature (approx
900oC) gas or pack process. Produces hard phases
within the surface (Typically 100µ deep).
C
Cadmium plating: The electrolytic
deposition of cadmium to provide galvanic corrosion protection.
Restricted by environmental considerations.
Cadmium ion plating: The deposition of cadmium by
a vacuum process to provide galvanic corrosion protection.
Carbide diffusion: A salt bath treatment at about
900oC for high carbon tool steels. Produces a very
hard layer of vanadium carbide, typically 10µ thick.
Carbonitriding: Similar to Carburising (see below).
Diffusion of carbon and nitrogen at about 900oC (by pack,
gas, salt bath or plasma process) into low carbon steel, followed
by quenching and tempering to produce martensitic case (typically
1mm thick).
Carburising (also called Case Hardening): Diffusion
of carbon at about 900oC (by pack, gas, salt bath or plasma
process) into low carbon steel, followed by quenching and
tempering to produce martensitic case (typically 1mm thick).
Case-hardening: See Carburising
Cavitation Erosion: A form of erosion causing material
to be removed by the action of vapour bubbles in a very turbulent
liquid.
Cermet powders: A composite powder of metal and ceramic
constituents produced by methods such as agglomeration, sintering
and spray drying. Examples include WC-Co, TiC-Ni.
Chemical Vapour Deposition (CVD): The deposition of
a coating by means of a chemical reaction in gases in a chamber
producing components which deposit on and adhere to the substrate.
Chromating: Chromate conversion
is a process which completely degreases and removes all traces
of the oxide film, replacing it by immersion (a rinse) with chromate
coating which can then be painted. It is used as a post-treatment
for cadmium, zinc and aluminium coatings
Chromising: High temperature (approx 900oC)
pack or gaseous diffusion of chromium into the surface of
a component to enhance high temperature corrosion and oxidation
resistance.
Cladding: The application of a thick (generally above
1mm) coating which melts or diffuses into the substrate. Processes
include weld cladding and plasma transferred arc (PTA).
Coating: The application of a thin (generally less
than 1mm) layer of material onto the surface of a substrate.
Composite: Mixture of two or more materials. Nearly
all have a reinforcing material(wood, glass, etc), called
filler, and a natural or artificial resin, called matrix to
achieve specific characteristics and required properties.
Corrosion: Chemical or electrochemical reaction between
a metal and the local environment whether wet or dry which
results in deterioration in the properties of the metal.
Crushed powder: Powder formed from a solid which is
then crushed to the appropriate size for spraying.
Copper plating: The electrolytic
deposition of copper to provide either a corrosion barrier (often
as an undercoat for hard chrome plate) or for reclamation of worn
parts.
CVD: See Chemical Vapour Deposition
D
Detonation Gun: A thermal spraying process in which
the coating material is heated and accelerated to the workpiece
by shock waves from a series of detonations or explosions
from gas mixtures. Also known as D-Gun (Praxair).
Diamond-like Carbon: A thin
carbon-based coating applied by either PVD or PACVD. It has high
hardness and low friction.
E
Electroless Nickel: The autocatalytic
deposition of nickel/phosphorous and nickel/boron have many useful
corrosion and tribo/corrosion applications. Unlike the electrolytic
processes, they produce a deposit with completely uniform coverage.
In the case of Ni P, deposits around 25 to 50 microns thick with
a hardness of about 500Hv is obtained, but thermal ageing at temperatures
around 400°C can develop hardness values in excess of 1000Hv.
Elasticity: The property of certain materials that
enables them to return to their original dimensions after
an applied stress.
Electroplating: The application of a layer of metal
onto a substrate in a conducting solution of metal slats.
Erosion: Removal of material from a surface caused
by the flow of particles within a liquid or gas.
Exothermic reaction or material: Certain materials
undergo chemical reactions when heated in a arc or plasma
and produce extra heating. This can be useful in improving
adhesion of the coating to the substrate. There is also a
potential explosive or fire hazard when handling powders which
are exothermic.
F
Fatigue: A cumulative effect causing a metal to fail
after repeated applications of stress none of which exceeds
the ultimate tensile strength. The fatigue strength (or fatigue
limits) is the stress that will cause failure after specified
number cycles.
Flame hardening: The localised surface heating of
a medium carbon steel by an impinging gas flame so that the
temperature is raised above 900oC. The part is
quenched (or self-quenches by virtue of the remaining cool
bulk of the component) and tempered to produce a hard martensitic
structure at the surface.
Flame spraying: A thermal
spraying process in which the particles are heated and accelerated
in a flame produced from the combustion of oxygen and fuel.
Filler: A solid inert material added to a synthetic
resin or rubber, either to change its physical properties
or simply to dilute it for economy.
Fretting: Surface damage caused by very small relative
movement between two surfaces usually under heavy load.
Fused and crushed powder: Powder formed from a fused
solid mass which is then crushed to the appropriate size for
spraying.
G
Galling: Damage to the surfaces of materials sliding
in contact with each other, usually caused by the localised
welding together of high spots. Common for materials like
stainless steel, aluminium alloys and titanium.
Gas carburising: See Carburising
Gas flow rate: The flow rate of gas (eg litres per
minute) through the spraying torch.
Gas nitriding: see Nitriding
Gas nitrocarburising: See Nitrocarburising
Galvanising: A hot dip process for deposition of zinc
for galvanic corrosion protection of steel.
Gold plating: The electrolytic deposition of gold
for decorative or electrical applications.
Grit blasting: A pressurised stream of hard metal
or oxide grit material used to clean and/or roughen surfaces
prior to coating.
H
Hard Chrome plating: The
electrolytic deposition of chromium to form a very hard (1000Hv),
tough coating with good wear resistance. The structure is micro-cracked.
Hardfacing: The application of a cladding or coating
of material designed to resist wear.
Hardness test: A test designed to assess the resistance
to penetration from a load. The surface is indented under
a defined load and the depth of penetration is observed.
High Velocity Oxy-fuel Spraying (HVOF): A Thermal
spray process. The spray powder particles are injected into
a jet formed by the combustion of oxy-fuel, heated and accelerated
to the workpiece.
HIPPING: The high temperature/high pressure consolidation
of a powder metallurgy component or thermally sprayed coating.
Density is greatly increased and metallurgical changes provide
enhanced corrosion and wear properties.
HVOF: See High Velocity Oxy-fuel spraying
I
Induction hardening: The localised surface heating
of a medium carbon steel by an induction coil so that the
temperature is raised above 900oC. The part is
quenched (or self-quenches by virtue of the remaining cool
bulk of the component) and tempered to produce a hard martensitic
structure at the surface.
Ion-Implantation: A process in which a beam of positive
ions is projected towards and into the surface. It is carried
out in partial vacuum and the ions diffuse into the surface
layer of the substrate. Typically this is carried out with
nitrogen giving a nitrided effect.
Ion nitriding: Also called plasma nitriding. A vacuum
glow discharge technique of nitriding. See Nitriding.
Ion plating: A process in which positive ions produced
in a glow discharge are attracted to the substrate which is
connected as the cathode. The ions are typically made by evaporation.
J
K
L
Laser alloying: The application of a powder to a surface
followed by fusing and alloying into the surface via the heat
from an impinging laser.
Laser glazing: The melting and quenching of a surface
to form a fine grained structure or 'glaze'.
Laser hardening: The localised surface heating of
a medium carbon steel by an incident laser so that the temperature
is raised above 900oC. The part is quenched (or
self-quenches by virtue of the remaining cool bulk of the
component) and tempered to produce a hard martensitic structure
at the surface.
LPPS: See 'Vacuum or Low Pressure Plasma Spraying.'
M
Magnetron sputtering: See Sputtering. In this PVD
process, the sputtering action is enhanced by intense magnetic
fields.
Micrograph: A micrograph is produced when a section
of the coating is taken, polished to show the particulate
layers and then photographed through a microscope.
Mechanical bonding: Usually represented by mechanical
interlocking of the deposited particles with the rough heights
on the substrate surface produced during grit blasting.
Metallurgical bonding: Produced by chemical bonding
between areas of the coating and substrate in intimate contact
or even by diffusion interaction between the coating and substrate.
Metallurgical bonding can be enhanced by post spraying diffusion
heat treatments.
Micro-hardness: The hardness of a coating as measured
on a microscopic scale.
Microtrack: A device for measuring powder particle
size distributions.
N
Nickel plating: The electrolytic
deposition of nickel to forma corrosion barrier or to reclaim
a worn part. Can also include hard ceramic
particles to from a wear resistant composite coating.
Nitriding: The diffusion of nitrogen into alloy steel
to form hard nitrides in the surface layer (typically 250µ).
Performed at between 500 and 750oC from a gas,
salt bath or plasma glow discharge.
Nitrocarburising: The diffusion of nitrogen and carbon
into alloy steel or mild steel to form hard nitrides in the
surface layer (typically 250µ). Performed at between 500 and
750oC from a gas, salt bath or plasma glow discharge.
O
Oxidation: Chemical reaction between the surface elements
and oxygen causing oxides of the elements to be formed.
Oxidising: The production of a stable oxide layer
on a steel component by heating in a controlled atmosphere.
Provides corrosion protection and reduced friction.
P
Pack carburising: See Carburising
Painting: The application of organic based layers
(acrylics, etc) for corrosion protection and decorative purposes.
Particle chemistry: The elements contained within
the particles of a spray powder.
Passivating: The post treatment (usually by chromating)
of nickel, cadmium or zinc coatings to reduce their corrosion
rates.
Peening: A stream of sharp material particles which
break superficial fibres, reducing internal stress fields.
Physical Vapour Deposition: A term covering all the
vapour deposition processes including Ion plating, It does
not include CVD as this is chemical not physical.
Phosphating: A conversion treatment to produce a thin
phosphate-based layer on a steel surface, providing improved
corrosion protection.
Photo-thermal NDE: An NDE technique for spayed coatings.
A repeated pulse of heat, from a laser source, flows through
the coating and substrate. The thermal signature is detected
and related to the input signal thereby indicating coating
thickness.
Plasma Carburising: See Carburising
Plasma jet: A jet of highly ionised gas usually produced
from a plasma torch. An electric arc is struck between a cathode
and anode and is then blown through a nozzle to form the jet.
Plasma Nitriding: Also called Ion Nitriding. See Nitriding
Plasma Spraying: A thermal
spraying process in which the heat source is a plasma jet.
Plasma Transferred Arc: See Transferred Arc
Polyester: A condensation polymer formed by the interaction
of polyhydric alcohols and polybasic acids. They are used
in the manufacture of glass-fibres products. See Alkyd resin.
Polymer: A substance having large molecules consisting
of repeated units. There are a number of natural polymers,
such as polysaccharides synthetic polymer are extensively
used in plastics.
Porosity: The presence of pores or voids in a coating
whether connected or not. Porosity is not the same as pull-out.
Powder coating (PTA): A polymeric coating deposited
via electrostatic attraction
Powder gas flow rate: The flow rate of the gas propelling
the powder into the plasma jet in plasma spraying.
Powder injection angle: The angle from which the powder
is injected into the plasma jet in plasma spraying.
Pull-out: Pull-out occurs when particles are removed
from the coating cross-section by the action of polishing.
It is sometimes confused with porosity.
PVD: See Physical Vapour Deposition
Q
Quality Control: All aspects of the control of the
spraying process including the surface preparation, spraying,
control of thickness deposited and the oxide and porosity
levels, surface finish and NDE checks as specified.
R
Resin: A synthetic or naturally occurring polymer
Rhodium plating: The electro-deposition of rhodium
for oxidation resistance combined with surface hardness.
S
Salt Bath Carburising: See Carburising
Salt Bath Nitriding: See Nitriding
Salt Bath Nitrocarburising: See Nitrocarburising
Shot peening: The bombardment of a component surface
with steel or ceramic shot. Produces a residual compressive
stress in the surface and improves fatigue and stress corrosion
performance.
Shroud: A gaseous and/or mechanical or physical barrier
placed around the spraying process designed to reduce the
ingress of air into the system and so reduce oxidation of
the of the particles being sprayed.
Silver plating: The electro-deposition
of silver for electrical, decorative or anti-fretting properties.
Size analysis: Analysis of the size of the particles
being deposited by spraying processes.
Size distribution: The distribution of sizes within
a size analysis. The distribution may be normal or skewed
in some way due to the powder manufacturing process.
Spalling: The lifting or detachment of a coating from
the substrate.
Spray chamber: A chamber in which the spraying process
is carried out. It may merely be an acoustic chamber for plasma
spraying or a vacuum chamber for vacuum plasma spraying.
Spray dried powder: Powder formed by the spray drying
process.
Spray-fused coatings: A process in which the coating
material is deposited by flame spraying and then fused into
the substrate by the addition of further heat. This can be
applied by flame induction heating or by laser.
Sputtering: This is a glow discharge process whereby
bombardment of a cathode releases atoms from the surface which
then deposit onto a nearby target surface to form a coating.
Steam tempering: The production of a stable oxide
on steel parts by treatment in steam at about 300oC.
Improves corrosion performance and reduces friction.
Strain: A measure of the extent to which a body is
deformed when it is subjected to a stress.
Stress: The force per unit area on body that tends
to cause it to deform. It is a measure of the internal forces
in a body between particles of the material of which it consists
as they resist separation, compression, or sliding.
Substrate: The parent or base material to which the
coating is applied.
Surface preparation: Cleaning and roughening the surface
to be sprayed, usually by grit or bead blasting. This is to
increase the adhesion of the coating to the substrate.
Surfacing: The application of a coating or cladding
to a surface to impart a change in its surface behaviour.
T
Tensile strength: A measure of the resistance that
a material offers to tensile stress. It is defined as the
stress, expressed as the force per unit cross sectional area,
required to break it.
Tensile stress: Axial forces per unit area applied
to a body that tend to extend it.
Thermal barrier coating: A coating produced to present
an insulating barrier to a heat source and to protect the
substrate.
Thermochemically formed coatings: A painted, dipped
or sprayed chromium oxide based coating consolidated by repeated
deposition and curing cycles (about 500oC).
Thermal spraying: A process
in which coating material is heated and accelerated from a spray
torch towards the workpiece. The deposited material forms a coating
on the surface.
Thermography: An NDE technique in which the coating
is flash heated and then viewed with an infra red camera.
"Hot spots" indicate areas of poor bonding or greater
coating thickness.
Transferred arc: In a plasma torch the plasma jet
is emitted from the torch and the current flows from the internal
cathode to the internal anode represented by the nozzle of
the torch. When the jet is carried to another anode with it
being electrically favourable to do so the current will then
transfer to the second anode, usually the workpiece and the
arc is said to be transferred.
U
Ultrasonic: An NDE technique which relies on an ultrasonic
beam passing through a coating and substrate and providing
a signal from the back wall which is then detected. The height
of this backwall echo depends on the discontinuity in impedance
from the sprayed coating to the substrate. Bonding flaws can
be easily seen by the weakening of the back wall echo.
V
Vacuum or Low Pressure Plasma Spraying: Plasma spraying
carried out in a chamber which has been evacuated to a low
partial pressure of oxygen. It is then usually partially backfilled
with argon to avoid the possibility of forming a glow discharge.
W
Wire spraying: A thermal
spray process whereby the supply for the coating material is fed
into the gun in the form of a continuous wire.
Wear: Loss of material from a surface by means of
relative motion between it and another body. Third bodies
i.e. grit
X
Y
Z
Zinc Plating: The electro-deposition
of zinc or zinc alloys (eg Zn/Ni, Zn/Sn) to provide galvanic corrosion
protection.
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