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, an ...

s, gases or

solid Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structura ...

s 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 lacquers 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 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 engi ...

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 Adhesion is the tendency of dissimilar particles or surfaces to cling to one another ( cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another). The forces that cause adhesion and cohesion can b ...

, 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 engi ...

, or wear resistance. In other cases, e.g.

semiconductor device fabrication Semiconductor device fabrication is the process used to manufacture semiconductor devices, typically integrated circuit (IC) chips such as modern computer processors, microcontrollers, and memory chips such as NAND flash and DRAM that are pres ...

(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 Printing is a process for mass reproducing text and images using a master form or template. The earliest non-paper products involving printing include cylinder seals and objects such as the Cyrus Cylinder and the Cylinders of Nabonidus. The ...

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 Waterproofing is the process of making an object or structure waterproof or water-resistant so that it remains relatively unaffected by water or resisting the ingress of water under specified conditions. Such items may be used in wet environme ...

and damp proofing of concrete walls, and concrete bridge decks. Roof coatings 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 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 In monotheistic thought, God is usually viewed as the supreme being, creator, and principal object of faith. Swinburne, R.G. "God" in Honderich, Ted. (ed)''The Oxford Companion to Philosophy'', Oxford University Press, 1995. God is typically ...

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 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 coatings * Anti-reflective coatings for example on spectacles. * Non-stick

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 Chem ...

coated cooking pots/pans. * Optical coatings are available that alter optical properties of a material or object. * Anti-Friction, Wear and Scuffing Resistance Coatings for Rolling-element bearings * Coatings that alter or have magnetic, electrical or electronic properties. * Antimicrobial coatings. *

Anti-fouling Biofouling or biological fouling is the accumulation of microorganisms, plants, algae, or small animals where it is not wanted on surfaces such as ship and submarine hulls, devices such as water inlets, pipework, grates, ponds, and rivers that ...

coatings *

Flame retardant The term flame retardants subsumes a diverse group of chemicals that are added to manufactured materials, such as plastics and textiles, and surface finishes and coatings. Flame retardants are activated by the presence of an ignition source a ...

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 mi ...

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

transmission electron microscopy Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a gr ...

(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 opt ...

(AFM), scanning tunneling microscope (STM), and Rutherford backscattering spectrometry (RBS). Various methods of

Chromatography In chemical analysis, chromatography is a laboratory technique for the separation of a mixture into its components. The mixture is dissolved in a fluid solvent (gas or liquid) called the ''mobile phase'', which carries it through a system ( ...

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 In polymer chemistry and materials science, resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focuses on n ...

(or binder),

solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...

which maybe

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

(or solventless),

pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic comp ...

(s) and additives.

Coating processes

Coating processes may be classified as follows:

Vapor deposition

Chemical vapor deposition

Metalorganic vapour phase epitaxy Metalorganic vapour-phase epitaxy (MOVPE), also known as organometallic vapour-phase epitaxy (OMVPE) or metalorganic chemical vapour deposition (MOCVD), is a chemical vapour deposition method used to produce single- or polycrystalline thin films ...

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 substrates. In simple terms chemical precursors are sprayed across an electr ...

(ESAVD) * Sherardizing * Some forms of Epitaxy ** 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 period ...

* 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 Pulsed laser deposition (PLD) is a physical vapor deposition (PVD) technique where a high-power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited. This material is vaporized from the t ...

* Sputter deposition *

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 ...

* Vacuum evaporation, evaporation (deposition) * 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 ** 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 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 redox, reduction of cations of that metal by means of a direct current, direct electric cur ...

Spraying

Spray painting Spray painting is a painting technique in which a device sprays coating material (paint, ink, varnish, etc.) through the air onto a surface. The most common types employ compressed gas—usually air—to atomize and direct the paint particles. ...

High velocity oxygen fuel Thermal spraying techniques are coating processes in which melted (or heated) materials are sprayed onto a surface. The "feedstock" (coating precursor) is heated by electrical (plasma or arc) or chemical means (combustion flame). Thermal spray ...

(HVOF) * Plasma spraying *

Thermal spray Thermal spraying techniques are coating processes in which melted (or heated) materials are sprayed onto a surface. The "feedstock" (coating precursor) is heated by electrical (plasma or arc) or chemical means (combustion flame). Thermal spray ...

ing * Kinetic metallization (KM) * Plasma transferred wire arc thermal spraying * The common forms of Powder coating

Roll-to-roll coating processes

Common roll-to-roll coating processes include: * Air knife coating * Anilox coater * Flexo coater * Gap Coating ** Knife-over-roll coating *

Gravure Rotogravure (or gravure for short) is a type of intaglio printing process, which involves engraving the image onto an image carrier. In gravure printing, the image is engraved onto a cylinder because, like offset printing and flexography, i ...

coating * Hot melt coating- 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 * 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 Glass is a non-Crystallinity, crystalline, often transparency and translucency, transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most ...

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typi ...

, and

polymer A polymer (; Greek ''poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic and ...

s 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 shea ...

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 or can be accomplished with 3-D printing. * Extrusion coating - generally high pressure, often high temperature, and with the web travelling much faster than the speed of the extruded polymer ** Curtain coating- 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 Lithography () is a planographic method of printing originally based on the immiscibility of oil and water. The printing is from a stone ( lithographic limestone) or a metal plate with a smooth surface. It was invented in 1796 by the German ...

* Flexography

Physical coating processes

* Langmuir-Blodgett * Spin coating * Dip coating