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Glossary of Surface Engineering Terms

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Click on the letter above to take you to the section required. 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 100) 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 100 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 10 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 400C 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 250). 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 250). 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.