A coating is a covering that is applied to the surface of an object, usually referred to as the

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (locomotion), the surface over which an organism lo ...

. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as

liquid A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, a ...

s, gases or solids e.g. Powder coatings.

Paint Paint is any pigmented liquid, liquefiable, or solid mastic composition that, after application to a substrate in a thin layer, converts to a solid film. It is most commonly used to protect, color, or provide texture. Paint can be made in many ...

s and

lacquer Lacquer is a type of hard and usually shiny coating or finish applied to materials such as wood or metal. It is most often made from resin extracted from trees and waxes and has been in use since antiquity. Asian lacquerware, which may be ca ...

s are coatings that mostly have dual uses of protecting the substrate and being decorative, although some artists paints are only for decoration, and the paint on large industrial pipes is for preventing corrosion and identification e.g. blue for process water, red for fire-fighting control etc. Functional coatings may be applied to change the surface properties of the substrate, such as adhesion, wettability,

corrosion resistance Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...

, or wear resistance. In other cases, e.g. semiconductor device fabrication (where the substrate is a wafer), the coating adds a completely new property, such as a magnetic response or electrical conductivity, and forms an essential part of the finished product. A major consideration for most coating processes is that the coating is to be applied at a controlled thickness, and a number of different processes are in use to achieve this control, ranging from a simple brush for painting a wall, to some very expensive machinery applying coatings in the electronics industry. A further consideration for 'non-all-over' coatings is that control is needed as to where the coating is to be applied. A number of these non-all-over coating processes are printing processes. Many industrial coating processes involve the application of a thin film of functional material to a substrate, such as paper, fabric, film, foil, or sheet stock. If the substrate starts and ends the process wound up in a roll, the process may be termed "roll-to-roll" or "web-based" coating. A roll of substrate, when wound through the coating machine, is typically called a web.

Functions of coatings

Coatings are not just designed to be aesthetically pleasing and for decorative purposes, but also have other functions. Sometimes a coating can be both decorative and have a specific function. An example would be the coating of a pipe carrying water for a fire suppression system that is coated with a red (for identification) anticorrosion paint to reduce degradation. In fact, most surface coatings or

paint Paint is any pigmented liquid, liquefiable, or solid mastic composition that, after application to a substrate in a thin layer, converts to a solid film. It is most commonly used to protect, color, or provide texture. Paint can be made in many ...

s are to some extent protecting the substrate e.g. general maintenance coatings/paints for metals and concrete. The decorative aspect of coatings is not just to impart a specific color, but also to create a particular reflective property such as high gloss, satin or flat/matt appearance. Some coatings though, are specifically designed to be very chemically resistant. A major use of coatings is to protect metal, and these are generally known as anticorrosion coatings. This use includes preserving machinery, equipment and structures Automobiles have improved in design over the years. Most are still made of metal for crashworthiness. The external coating and the underbody are coated. Coatings are also used to seal the surface of concrete. This would include Seamless polymer/resin flooring, bund wall/containment lining, Waterproofing and damp proofing of concrete walls, and concrete bridge decks.

Roof coating A roof coating is a monolithic, fully adhered, fluid applied roofing membrane. Many roof coatings are elastomeric, that is, they have elastic properties that allow them to stretch and return to their original shape without damage. Typical roof coa ...

s have been developed and improved over the years. They are designed primarily for waterproofing and also sun reflection to help keep a building cool. They tend to be

elastomeric An elastomer is a polymer with viscoelasticity (i.e. both viscosity and elasticity) and with weak intermolecular forces, generally low Young's modulus and high failure strain compared with other materials. The term, a portmanteau of ''elastic p ...

to allow for movement of the roof without cracking the coating membrane. The coating, sealing and waterproofing of wood has been going on since biblical times, with God commanding

Noah Noah ''Nukh''; am, ኖህ, ''Noḥ''; ar, نُوح '; grc, Νῶε ''Nôe'' () is the tenth and last of the pre-Flood patriarchs in the traditions of Abrahamic religions. His story appears in the Hebrew Bible (Book of Genesis, chapters 5– ...

to build an

ark Ark or ARK may refer to: Biblical narratives and religion Hebrew word ''teva'' * Noah's Ark, a massive vessel said to have been built to save the world's animals from a flood * Ark of bulrushes, the boat of the infant Moses Hebrew ''aron'' * ...

and then coat it. Wood was and is a key material of construction since ancient times so its preservation by coating has received much attention. Efforts to improve the performance of wood coatings continues. Coatings are used to alter tribological properties and wear characteristics. Other functions of coatings include: *

UV coating A UV coating (or more generally a radiation cured coating) is a surface treatment which either is cured by ultraviolet radiation, or which protects the underlying material from such radiation's harmful effects. UV coatings on pipe and tube UV c ...

s *

Anti-reflective coating An antireflective, antiglare or anti-reflection (AR) coating is a type of optical coating applied to the surface of lenses, other optical elements, and photovoltaic cells to reduce reflection. In typical imaging systems, this improves the effic ...

s for example on spectacles. *

Non-stick A non-stick surface is engineered to reduce the ability of other materials to stick to it. Non-stick cookware is a common application, where the non-stick coating allows food to brown without sticking to the pan. Non-stick is often used to refer ...

PTFE Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chemour ...

coated cooking pots/pans. *

Optical coating An optical coating is one or more thin layers of material deposited on an optical component such as a lens, prism or mirror, which alters the way in which the optic reflects and transmits light. These coatings have become a key technology in th ...

s are available that alter optical properties of a material or object. * Anti-Friction, Wear and Scuffing Resistance Coatings for

Rolling-element bearings In mechanical engineering, a rolling-element bearing, also known as a rolling bearing, is a bearing which carries a load by placing rolling elements (such as balls or rollers) between two concentric, grooved rings called races. The relative m ...

* Coatings that alter or have magnetic, electrical or electronic properties. * Antimicrobial coatings. * Anti-fouling coatings * Flame retardant coatings.

Coating analysis and characterization

Numerous destructive and non-destructive evaluation (NDE) methods exist for characterizing coatings. The most common destructive method is

microscopy Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of micr ...

of a mounted

cross-section Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture & engineering 3D *Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) **Ab ...

of the coating and its substrate. The most common non-destructive techniques include ultrasonic thickness measurement, X-ray fluorescence (XRF),

X-Ray diffraction X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

(XRD) and micro hardness indentation. X-ray photoelectron spectroscopy (XPS) is also a classical characterization method to investigate the chemical composition of the nanometer thick surface layer of a material. Scanning electron microscopy coupled with energy dispersive X-ray spectrometry (

SEM-EDX Energy-dispersive X-ray spectroscopy (EDS, EDX, EDXS or XEDS), sometimes called energy dispersive X-ray analysis (EDXA or EDAX) or energy dispersive X-ray microanalysis (EDXMA), is an analytical technique used for the elemental analysis or charac ...

, or SEM-EDS) allows to visualize the surface texture and to probe its elementary chemical composition. Other characterization methods include transmission electron microscopy (TEM),

atomic force microscopy Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very-high-resolution type of scanning probe microscopy (SPM), with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the op ...

(AFM), scanning tunneling microscope (STM), and Rutherford backscattering spectrometry (RBS). Various methods of Chromatography are also used, as well as thermogravimetric analysis.

Coating formulation

The formulation of a coating depends primarily on the function required of the coating and also on aesthetics required such as color and gloss. The four primary ingredients are the resin (or binder), solvent which maybe water (or solventless), pigment(s) and additives.

Coating processes

Coating processes may be classified as follows:

Vapor deposition

Chemical vapor deposition

* Metalorganic vapour phase epitaxy *

Electrostatic spray assisted vapour deposition Electrostatic spray-assisted vapour deposition (ESAVD) is a technique (developed by a company called IMPT) to deposit both thin and thick layers of a coating onto various Substrate (materials science), substrates. In simple terms chemical precursor ...

(ESAVD) *

Sherardizing Sherardising is a process of galvanization of ferrous metal surfaces, also called vapour galvanising and dry galvanizing. The process is named after British metallurgist Sherard Osborn Cowper-Coles (son of naval inventor Cowper Phipps Coles) who ...

* Some forms of

Epitaxy Epitaxy refers to a type of crystal growth or material deposition in which new crystalline layers are formed with one or more well-defined orientations with respect to the crystalline seed layer. The deposited crystalline film is called an epit ...

** Molecular beam epitaxy

Physical vapor deposition

* Cathodic arc deposition * Electron beam physical vapor deposition (EBPVD) *

Ion plating Ion plating (IP) is a physical vapor deposition (PVD) process that is sometimes called ''ion assisted deposition'' (IAD) or ''ion vapor deposition'' (IVD) and is a modified version of ''vacuum deposition''. Ion plating uses concurrent or periodi ...

* Ion beam assisted deposition (IBAD) *

Magnetron sputtering Sputter deposition is a physical vapor deposition (PVD) method of thin film deposition by the phenomenon of sputtering. This involves ejecting material from a "target" that is a source onto a "substrate" such as a silicon wafer. Resputtering is re ...

* Pulsed laser deposition *

Sputter deposition Sputter deposition is a physical vapor deposition (PVD) method of thin film deposition by the phenomenon of sputtering. This involves ejecting material from a "target" that is a source onto a "substrate" such as a silicon wafer. Resputtering is re ...

Vacuum deposition Vacuum deposition is a group of processes used to deposit layers of material atom-by-atom or molecule-by-molecule on a solid surface. These processes operate at pressures well below atmospheric pressure (i.e., vacuum). The deposited layers can r ...

* Vacuum evaporation,

evaporation (deposition) Evaporation is a common method of thin-film deposition. The source material is evaporated in a vacuum. The vacuum allows vapor particles to travel directly to the target object (substrate), where they condense back to a solid state. Evaporation ...

* Pulsed electron deposition (PED)

Chemical and electrochemical techniques

* Conversion coating ** Autophoretic, the registered trade name of a proprietary series of autodepositing coatings specifically for ferrous metal substrates **

Anodising Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. The process is called ''anodizing'' because the part to be treated forms the anode electrode of an electr ...

Chromate conversion coating Chromate conversion coating or alodine coating is a type of conversion coating used to passivate steel, aluminium, zinc, cadmium, copper, silver, titanium, magnesium, and tin alloys. The coating serves as a corrosion inhibitor, as a primer t ...

** Plasma electrolytic oxidation **

Phosphate (coating) Phosphate conversion coating is a chemical treatment applied to steel parts that creates a thin adhering layer of iron, zinc, or manganese phosphates, to achieve corrosion resistance, lubrication, or as a foundation for subsequent coatings or pai ...

* Ion beam mixing * Pickled and oiled, a type of plate

steel Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...

coating * Plating ** Electroless plating **

Electroplating Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the reduction of cations of that metal by means of a direct electric current. The part to be ...

Spraying

* Spray painting * High velocity oxygen fuel (HVOF) * Plasma spraying * Thermal spraying * Kinetic metallization (KM) *

Plasma transferred wire arc thermal spraying Plasma transferred wire arc (PTWA) thermal spraying is a thermal spraying process that deposits a coating on the internal surface of a cylindrical surface, or external surface of any geometry. It is predominantly known for its use in coating the Bor ...

* The common forms of Powder coating

Roll-to-roll coating processes

Common roll-to-roll coating processes include: *

Air knife An air knife is a tool used to blow off liquid or debris from products as they travel on conveyors. Air knives are normally used in manufacturing or as the first step in a recursive recycling process to separate lighter or smaller particles from ...

coating * Anilox coater *

Flexo Flexography (often abbreviated to flexo) is a form of printing process which utilizes a flexible relief plate. It is essentially a modern version of letterpress, evolved with high speed rotary functionality, which can be used for printing on a ...

coater * Gap Coating ** Knife-over-roll coating * Gravure coating *

Hot melt coating Hot melt coating is the application of a layer to a substrate by pre-melting the desired material and then allowing or forcing the material to cool, solidifying the layer. The process is widely used in industry, including pressure-sensitive adhes ...

- when the necessary coating viscosity is achieved by temperature rather than solution of the polymers etc. This method commonly implies slot-die coating above room temperature, but it also is possible to have hot-melt roller coating; hot-melt metering-rod coating, etc. * Immersion dip coating * Kiss coating * Metering rod (Meyer bar) coating * Roller coating ** Forward roller coating **

Reverse roll coating Reverse roll coating is a roll-to-roll coating method for wet coatings. It is distinguished from other roll coating methods by having two reverse-running nips. The metering roll and the applicator roll contra-rotate, with an accurate gap between ...

* Silk Screen coater ** Rotary screen * Slot Die coating - Slot die coating was originally developed in the 1950s. Slot die coating has a low operational cost and is easily scaled processing technique for depositing thin and uniform films rapidly, while minimizing material waste. Slot die coating technology is used to deposit a variety of liquid chemistries onto substrates of various materials such as glass, metal, and polymers by precisely metering the process

fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shear ...

and dispensing it at a controlled rate while the coating die is precisely moved relative to the substrate. The complex inner geometry of conventional slot dies require

machining Machining is a process in which a material (often metal) is cut to a desired final shape and size by a controlled material-removal process. The processes that have this common theme are collectively called subtractive manufacturing, which utilizes ...

or can be accomplished with

3-D printing 3-D, 3D, or 3d may refer to: Science, technology, and mathematics Relating to three-dimensionality * Three-dimensional space ** 3D computer graphics, computer graphics that use a three-dimensional representation of geometric data ** 3D film, a ...

. *

Extrusion coating Extrusion coating is the coating of a molten web of synthetic resin onto a substrate material. It is a versatile coating technique used for the economic application of various plastics, notably polyethylene, onto paperboard, corrugated fiberboa ...

- generally high pressure, often high temperature, and with the web travelling much faster than the speed of the extruded polymer **

Curtain coating Curtain coating is a process that creates an uninterrupted curtain of fluid that falls onto a substrate. The substrate is transported on a conveyor belt or calender rolls at a regulated speed through the curtain to ensure an even coat of the die. ...

- low viscosity, with the slot vertically above the web and a gap between slotdie and web. ** Slide coating- bead coating with an angled slide between the slotdie and the bead. Commonly used for multilayer coating in the photographic industry. ** Slot die bead coating- typically with the web backed by a roller and a very small gap between slotdie and web. ** Tensioned-web slotdie coating- with no backing for the web. * Inkjet printing * Lithography *

Flexography Flexography (often abbreviated to flexo) is a form of printing process which utilizes a flexible relief plate. It is essentially a modern version of letterpress, evolved with high speed rotary functionality, which can be used for printing on a ...

Physical coating processes

* Langmuir-Blodgett * Spin coating * Dip coating