Gordon England Surface Engineering Forum

A to Z of Terms related to the Thermal Spray Process and Surface Engineering



Abradable coatings
Coatings which are designed to be abraded by a mating surface to form a tight gas or air seal, while retaining good erosion resistance.
Abrasive blasting
A process for cleaning and roughening a surface by means of an abrasive directed at high velocity against the work piece.
Abrasive Wear
Wear due to hard particles or hard protuberances forced against and moving along a solid surface.
Acetylene C2H2 (ethyne)
Unsaturated aliphatic hydrocarbon gas. Used as a fuel gas in combustion thermal spray processes, welding and cutting. Acetylene has the highest flame temperature and requires the smallest amount amount of oxygen to form a neutral flame.
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 due to localized bonding between contacting solid surfaces leading to material transfer between the two surfaces or the loss from either surface.
Several particles adhering together.
Agglomerated powder
A powder made up of agglomerates
Air classification
The separation of powder into particle size fractions by means of an air stream of controlled velocity.
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-polymerising 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.
Aluminising (Thermal Spray Method)
Thermal sprayed coatings of aluminium usually on substrates of steel or nickel chromium alloys which are subsequently heat treated to aluminise the surface.
Aluminium Ion Plating
The deposition of aluminium by a vacuum evaporative process. Provides galvanic corrosion resistance. Normally given a passivation treatment.
non-crystalline, or devoid of regular structure.
A negatively charged ion
Positively charged electrode (nozzle in plasma gun) - the electrode of an electrolytic cell at which oxidation is the principle reaction. (Electrons flow away from the anode in the external circuit. Its usually the anode where corrosion occurs and metal ions enter solution.)
Anodic coating
A coating that becomes the anode in an electrochemical cell with the substrate (cathode). The only metals in common use for thermal spraying which are anodic to iron and steel are zinc and aluminium.
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.
Apparent density
The weight of a unit volume of powder or coating
Apparent hardness
The value obtained by testing a coating or sintered material with standard indentation hardness equipment. (See macrohardness) Since the reading reflects a composite of pores and solid material, (which may be particles relatively poorly bonded together) it is usually lower than that of an equivalent solid wrought or cast material. Not to be confused with particle hardness (See microhardness).
Atmosperic Plasma Spray Proces.
Arc Wire Spraying
A thermal spray process where two electrically conducting wires are brought together to form an electric arc. Molten material formed in the arc is projected by a compressed gas stream towards the work piece to form a coating.
Argon (Ar)
Monatomic noble gas, atomic number 18, one of the most inert elements. Commonly used as a primary plasma gas in the plasma spraying process and providing inert environments for many processes.
The dispersion of a molten material into particles by a rapidly moving gas or liquid stream or by mechanical dispersion.
Atomised powder
A powder produced by the dispersion of a molten material into particles by a rapidly moving gas or liquid stream or by mechanical dispersion.
The production of a stable, protective oxide on steel parts by treatment in a pressurised, high temperature steam containing atmosphere.


A cementing medium used in producing composite or agglomerate powders.
A pressurised stream of particulates (ceramic, plastic, metal, etc.) applied on a surface to clean, peen or abrade.
Blended Powder
A powder consisting of two or more different powder materials thoroughly mixed.
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, chemical 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 and/or to provide a corrosion or oxidation barrier.
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.
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 100µm deep).


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.
Similar to Carburising. 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).
Carrier gas
Usually nitrogen or argon gas that carries powder into the thermal spray process.
See Carburising
Negatively charge electrode. Electrode of an electrolytic cell at which reduction is the principle reaction. (Electrons flow towards the cathode in the external circuit.) Cathodic coating Coatings which become the cathode in an electrochemical cell with the substrate (anode). This type of coating protects the substrate from corrosion only by being a complete barrier. If the coating allows the environment to reach the substrate, accelerated corrosion of the substrate will occur.
Cathodic protection
A technique to reduce the corrosion rate of a metal by making it the cathode of an electrochemical cell. Thermal spray zinc and aluminium coatings provide this protection to steel substrates, the coating being the anode and the steel being the cathode.
A positively charged ion.
The formation and rapid collapse within a liquid of cavities or bubbles that contain vapour or gas or both.
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 blended or composite powder of metal and ceramic constituents.
Chemical conversion coating
A protective or decorative non-metallic coating produced in situ by chemical reaction of a metal with a chosen environment. (It is often used to prepare the surface prior to the application of an organic coating.)
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.
Chromate conversion is a process which completely degreases and removes all traces of the oxide film, replacing it by immersion with a chromate coating which can then be painted. It is used as a post-treatment for cadmium, zinc and aluminium coatings
High temperature (approx. 900oC) pack or gaseous diffusion of chromium into the surface of a component to enhance high temperature corrosion and oxidation resistance.
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).
The application of a layer of material onto the surface of a substrate.
Cold welding
Cohesion between two surfaces of a metal, generally under the influence of externally applied pressure at room temperature.
Cold Spray
or cold gas-dynamic spraying process uses the energy stored in high pressure compressed gas to propel fine powder particles at very high velocities at a substrate to form a coating.
Mixture of two or more materials. Nearly all have a reinforcing material (metal, wood, glass, etc.), called filler, and a natural or artificial resin, called matrix to achieve specific characteristics and required properties.
Composite Coating
Mixture of two or more materials. Many thermal spray coatings could be considered as composites.
Composite Powder
A powder in which each particle consists of two or more distinct materials joined together.( Not the same as a powder blend.)
Chemical or electrochemical reaction between a material and it's environment which results in deterioration in the properties of the material.
Corrosion fatigue
The process in which a metal fractures prematurely underconditions of simultaneous corrosion and repeated cyclic loading at lower stress levels or fewer cycles than would be required in the absence of the corrosive environment.
Corrosion potential
The potential of a corroding surface in an electrolyte relative to a reference electrode measured under open circuit conditions.
Corrosive wear
Wear in which chemical or electrochemical reaction with the environment is significant.
Crevice corrosion
Localized corrosion of a metal surface at, or immediately adjacent to, an area that is shielded from the full exposure to the environment because of close proximity between the metal and the surface of another material.
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.
See Chemical Vapour Deposition


The removal of grease and oil from a surface. Degreasing by immersion in liquid organic solvents or by solvent vapours condensing on the parts to be cleaned.
Deposit corrosion
Localized corrosion under or around a deposit or collection of material on a metal surface. (See also crevice corrosion.)
Detonation Gun
A thermal spray process in which the coating material is heated and accelerated to the work piece by a series of detonations or explosions from oxy-fuel gas mixtures.
Diamond-like Carbon
A thin carbon-based coating applied by either PVD or PACVD. It has high hardness and low friction.
Diffusion Coating
An alloy coating produced by applying heat to one or more coatings deposited on a basis metal.
The ability of a material to deform plastically without fracturing.


Electrochemical cell
An electrochemical system consisting of an anode and a cathode in metallic contact and immersed in an electrolyte. (The anode and cathode may be different metals or dissimilar areas on the same metal surface.
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.
A conducting medium in which the flow of current is accompanied by movement of matter. A substance that is capable of forming a conducting liquid medium when dissolved or melted.
Production of chemical changes of the electrolyte by the passage of current through an electrochemical cell.
Electromotive Force Series (EMF Series)
A list of elements arranged according to their standard electrode potentials, with "noble" metals such as gold being positive and "active" metals such as zinc being negative.
The property of certain materials that enables them to return to their original dimensions after an applied stress.
The electrodeposition of an adherent metallic coating upon an electrode for the purpose of securing a surface with properties or dimensions different from those of the substrate material.
The severe loss of ductility or toughness or both, of a material, usually a metal or alloy.
Removal of material from a surface due to mechanical interaction between that surface and a fluid, a multicomponent fluid, or impinging liquid or solid particles
Associated action involving corrosion and erosion in the presence of a corrosive substance.
A roughened surface produced by chemical, electrochemical or mechanical means. To dissolve unevenly a part of the surface of a material to highlight microstructure in metallography.
Corrosion that proceeds laterally from the sites of initiation along planes parallel to the surface, generally at grain boundaries or coating interfaces, forming corrosion products that force metal or coating away from the body of the material, giving rise to a layered appearance.
Exothermic reaction or material
Certain materials undergo chemical reactions when thermally sprayed and produce extra heating. This can be useful in improving adhesion of the coating to the substrate.


A cumulative effect causing a material 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.
Fatigue wear
Wear of a solid surface caused by fracture arising from material fatigue.
The portion of a powder composed of particles which are smaller than the specified size
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 (combustion flame, plasma flame). Old term for thermal spray process.
A solid inert material added to a synthetic resin or rubber, either to change its physical properties or simply to dilute it for economy.
Small amplitude oscillatory motion, usually tangential, between two solid surfaces in contact.
Fretting corrosion
A form of fretting wear in which corrosion plays a significant role.
Fretting wear
Wear arising as a result of fretting (see fretting).
The reaction force resulting from surface interaction and adhesion during sliding. The friction Coefficient is defined as the friction force divided by the load.
Fused coatings
A process in which the coating material is deposited by thermal spraying and then fused by post heat treatment. This can be done by flame, induction heating, furnace or by laser.
Fused and crushed powder
Powder formed from a fused solid mass which is then crushed to the appropriate size for spraying.


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.
Galvanic corrosion
Accelerated corrosion of a metal because of an electrical contact with a more noble metal or nonmetallic conductor in a corrosive electrolyte.
Galvanic Series
A list of metals and alloys arranged according to their relative corrosion potentials in a given environment.
Gas carburising
See Carburising
Gas flow rate
The flow rate of gas (e.g. litres per minute) through the spraying torch.
Gas nitriding
see Nitriding
Gas nitrocarburising
See Nitrocarburising
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.
Granular powder
Particles having approximately equidimensional nonspherical shapes.
The removal of material by the use of fixed abrasives like grinding wheels or emery paper.
Grit blasting
A pressurised stream of hard metal or oxide grit material used to clean and roughen surfaces prior to coating.


Hard Chromium Plating
The electrolytic deposition of chromium to form a very hard (1000Hv), tough coating with good wear resistance. The structure is micro-cracked.
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 or area of penetration is measured.
Helium (He)
Monatomic noble gas, Most inert element, atomic number 2. Used as a plasma gas in the plasma spraying process.

High Velocity Air-fuel Spraying (HVAF)
A Thermal spray process. The spray powder particles are injected into a high velocity jet formed by the combustion of fuel with air, heated and accelerated to the work piece.
High Velocity Oxy-fuel Spraying (HVOF)
A Thermal spray process. The spray powder particles are injected into a high velocity jet formed by the combustion of oxygen and fuel, heated and accelerated to the work piece.
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.
Hot dip coating
A metallic coating obtained by dipping the substrate metal into a molten metal.
See High Velocity Air fuel spraying
See High Velocity Oxygen fuel spraying
Hydrogen (H2)
Diatomic gas, atomic number 1, The lightest element, very reactive and powerful reducing agent. Used as a secondary plasma gas in the plasma spraying process and as a fuel gas in combustion thermal spray processes.
Hydrogen embrittlement
Hydrogen induced cracking or severe loss of ductility caused by the presence of hydrogen in the metal. Hydrogen absorption may occur during electroplating, pickling etc. ( The use of hydrogen as a secondary gas in plasma spraying does not appear to effect substrates and the majority of coatings, one exception being titanium coatings.)
Tending to absorb water.
Tending to repel water or lacking affinity for water


A process of filling the pores of a coating with resin, wax or oil. (See sealer, vacuum impregnation)
Induction heating
The heating of a electrically conductive material by an induction coil producing alternating magnetic fields which induce alternating electric currents to flow in the material and cause heating by resistance. Used in many heating process (induction fusing, induction plasma, induction hardening etc..)
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.
A process resulting in a continuing succession of impacts between (liquid or solid) particles and a solid surface.
Impingement corrosion
a form of erosion-corrosion generally associated with the impingement of a high velocity, flowing liquid containing air bubbles against a solid surface.
Interconnected porosity
A network of pores in and extending to the surface of a coating.
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.
Irregular powder
Particles lacking symmetry.
Intergranular corrosion
Preferential corrosion at or adjacent to the grain boundaries of a metal or alloy.
Internal oxidation
The formation of isolated particles of corrosion products beneath the surface of the metal or coating. (This occurs as a result of preferential oxidation of certain alloy constituents by inward diffusion of oxygen, nitrogen, sulphur, etc.



Liquid fuel used in some HVOF thermal spray processes.


Rubbing two surfaces together, with or without abrasives, for the purpose obtaining extreme dimensional accuracy or superior surface finish.
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.
Liquid impingement erosion
Progressive loss of material from a solid surface due to continue exposure to impacts by liquid drops or jets.
Localized corrosion
Corrosion at discrete sites, for example, pitting, crevice corrosion, and stress corrosion cracking.
See 'Vacuum or Low Pressure Plasma Spraying.'
Any substance interposed between two surfaces for the purpose of reducing the friction or wear between them.


The hardness of a coating as measured on a macroscopic scale, which shows the coatings bulk properties.
Magnetron sputtering
See Sputtering. In this PVD process, the sputtering action is enhanced by intense magnetic fields.
The continuous phase of a material or coating in which separate particles of another constituent are embedded. (Like tungsten carbide particles in a cobalt matrix)
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 diffusion or chemical bonding between the coating and substrate.
A micrograph is produced when a section of the coating is taken, polished to show the particulate layers and then photographed through a microscope.
The hardness of a coating as measured on a microscopic scale. Can show the hardness of individual phases within the coating and avoid the effects of porosity.
One millionth of an inch, 0.000001".
Micrometer (µm)
One millionth of a metre, 0.001mm.
A device for measuring powder particle size distributions.
One thousandth of an inch, 0.001" (Common in USA)
Minus sieve
The portion of a powder sample which passes through a standard sieve of specified number e.g. -140 mesh +325 mesh. ( See plus sieve)


Nickel plating
The electrolytic deposition of nickel to form a corrosion barrier or to reclaim a worn part. Can also include hard ceramic particles to from a wear resistant composite coating.
The diffusion of nitrogen into alloy steel to form hard nitrides in the surface layer (typically 250µm). Performed at between 500 and 750oC from a gas, salt bath or plasma glow discharge.
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.
Nitrogen (N2)
Diatomic gas. Used as a primary and secondary gas in plasma spraying. Inert to most materials, with some exceptions like titanium.
Noble metal
A metal that does not readily tend to furnish ions, and therefore does not dissolve readily, nor easily enter into such reactions as oxidation, etc. The opposite of base metal.
Nodular powder
Irregular particles having Knotted, rounded, or similar shapes.


Loss of electrons by a constituent of a chemical reaction. (Also refers to the corrosion of a metal that is exposed to an oxidising gas at elevated temperatures.)
An environment or material which promotes oxidation
Oxygen (O2 )
Gas used to support combustion of fuel gases in combustion thermal spray processes. Achieves much higher flame temperatures than using air.


Pack carburising
See Carburising
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.
Particle size
The controlling lineal dimension of an individual particle as determined by analysis with sieves or other suitable means.
Particle size distribution
The percentage by weight, or by number, of each fraction into which a powder sample has been classified with respect to sieve number or microns.
The process in metal corrosion by which metals become passive. (See passive)
A type of inhibitor which appreciably changes the potential of a metal to a more noble (positive) value.
The state of a metal surface characterized by low corrosion rates in a potential region that is strongly oxidising for the metal.
Blasting process using spherical shaped beads or shot for cleaning and/or modifying surface properties.
A property measured as a rate of passage of a liquid or gas through a coating
Plus sieve
The portion of a powder sample retained on a sieve of specified number. (See minus sieve.)
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.
A conversion treatment to produce a thin phosphate-based layer on a steel surface, providing improved corrosion protection and good surface for painting.
Photo-thermal NDT
An NDT 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.
A form of wear characterized by the presence of surface cavities the formation of which is attributed to processes such as fatigue, local adhesion, or cavitation.
corrosion of a metal surface, cofined to a point or small area, that takes the form of cavities
Plasma Carburising
See Carburising
Plasma jet or plasma flame
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 flame or jet.
Plasma Nitriding
Also called Ion Nitriding. See Nitriding
Plasma Spraying
A thermal spraying process in which the heat source is a plasma flame.
Plasma Transferred Arc (PTA)

Similar to the plasma spray process in that powder is sprayed through a plasma, but instead of being heated via a neutral plasma (carrying no electric current) the arc is transfered to the substrate (made to be the anode). This is a hot process and produces coatings similar to fused or weld hardfacing coatings.
The formation of grooves by plastic deformation of the softer of two surfaces in relative motion.
A condensation polymer formed by the interaction of polyhydric alcohols and polybasic acids. They are used in some coatings and the manufacture of glass-fibres products. See Alkyd resin.
Organic 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.
The presence of pores or voids in a coating, usually expressed as a percentage by volume.
Powder coating
A polymeric coating deposited via electrostatic attraction
Powder injection angle
The angle from which the powder is injected into the plasma jet in plasma spraying.
Pre-alloyed powder
A powder composed of two or more elements which are alloyed in the powder manufacturing process and in which the particles are of the same nominal composition throughout.
The smoothing of a material surface by means of the action of abrasive particles attached usually to a fabric cloth. The final mechanical step in metallographic preparation.
Propane (C3 H8)
Aliphatic hydrocarbon gas used as a fuel gas in thermal spray processes.
Propylene (C3 H6)
Hydrocarbon gas used as a fuel gas in thermal spray processes. Higher flame temperature than hydrogen and propane.
Pull-out occurs when particles are plucked from the coating during machining or grinding. Also occurs during metallographic preparation. It is sometimes confused with porosity.
See Physical Vapour Deposition


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.


Reducing Agent
A substance that cases reduction, thereby itself becoming oxidised.
A reaction in which electrons are added to the reactant. More specifically, the addition of hydrogen or the abstraction of oxygen.
A synthetic or naturally occurring polymer
RFI Shielding
Thermal spray coatings of electrically conductive metals such as zinc, aluminium and copper are used on non-conducting composite casing materials to shield sensitive electronic devices from radio frequency electromagnetic interference.
Rhodium plating
The electro-deposition of rhodium for oxidation resistance combined with surface hardness.


Sacrificial coating
A coating that provides corrosion protection wherein the coating material corrodes in preference to the substrate,thereby protecting the latter from corrosion.
Salt Bath Carburising
See Carburising
Salt Bath Nitriding
See Nitriding
Salt Bath Nitrocarburising
See Nitrocarburising
A severe form of wear characterized by the formation of extensive grooves and scratches in the direction of sliding.
The mechanical removal or displacement, or both, of material from a surface by the action of abrasive particles or protuberances sliding across the surfaces.
Sealant, Sealer
A preparation of resin or wax type materials for sealing the porosity in coatings.
A process which, by absorption of a sealer into thermal spray coatings, seals porosity and increases resistance to corrosion of the underlying substrate material.
Self-bonding coatings
A name given to thermal spray coatings that are capable of bonding to clean smooth surfaces. Bond and "one-step" coatings are normally in this group. These are paticularly important where grit blasting or surface roughening processes must be omitted.
A decrease in dimensions of a coating during processing.
Shrinkage stress
The residual stress in a coating caused by shrinkage during processing.
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.
Sieve classification
That portion of a powder sample which passes through a standard sieve of specified number and is retained by some finer sieve of specified number.
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 materials 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.
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.
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.
A measure of the extent to which a body is deformed when it is subjected to a 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.
Stress corrosion cracking
A cracking process that requires the simultaneous action of a corrodent and sustained tensile stress.
The parent or base material to which the coating is applied.
Surface preparation
Cleaning and roughening the surface to be sprayed, usually by grit blasting. This is to increase the adhesion of the coating to the substrate.
The reaction of a metal or alloy with a sulphur containing species to produce a sulphur compound that forms on or beneath the surface of the metal or alloy.
Surface topography
The geometrical detail of a surface, relating particularly to microscopic variations in height.
The application of a coating or cladding to a surface to impart a change in its surface behaviour.


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 forming an insulating barrier to a heat source 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 work piece. The deposited material forms a coating on the surface.
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 work piece and the arc is said to be transferred.
The science and technology concerned with interacting surfaces in relative motion.


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.


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.


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.
Loss of material from a surface by means of some mechanical action.
-in tribology, the bonding between surfaces in direct contact, at any temperature.
Welding Process
The joining of materials by the application of heat or friction. Usually involves the localized fusion of both contacting surfaces.



Yttria-stabilized zirconia (YSZ)
ceramic in which the crystal structure of zirconium dioxide is made stable at room temperature by an addition of yttrium oxide.


Zinc (Zn)
Thermal spray coatings of zinc or zinc alloys (eg Zn/Al, Zn/Sn) provide galvanic corrosion protection.
Zinc Plating
The electro-deposition of zinc or zinc alloys (eg Zn/Ni, Zn/Sn) to provide galvanic corrosion protection.

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Thermal Spray Processes:

Combustion Wire Thermal Spray Process

Combustion Powder Thermal Spray Process

Arc Wire Thermal Spray Process

Plasma Thermal Spray Process

HVOF Thermal Spray Process

HVAF Thermal Spray Process

Detonation Thermal Spray Process

Plasma Flame Theory

Cold Spray Coating Process

Wear and Use of Thermal Spray Coatings

Corrosion and Use of Thermal Spray Coatings

Glossary of Thermal Spray and Surface Engineering Terms

Image Directory for Thermal Spray Coatings

Plasma Gas Flow Information

Plasma Gas Flow Correction Calculator

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Links to other interesting sites related to thermal spray and surface engineering

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Periodic Table of the Elements

SI Units

Calculators for Conversion between Units of Measurement

Hardness Testing

Surface Engineering Message Board Archive

Surface Engineering Message Board Archive Index

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