A coating is a covering that is applied to the surface of an object, usually referred to as the substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquids, 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 structur ...

s e.g. Powder coatings. Paints 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 ...

, 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 engin ...

, 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 ...

(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 Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, different fabric types, etc. At first, the word "textiles" only referred to woven fabrics. However, weaving is not ...

, 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 paints 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 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 ''elast ...

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 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 ...

s * Anti-reflective coatings 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 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 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 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) **Abs ...

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 ...

(TEM), atomic force microscopy (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 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 nat ...

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

Molecular beam epitaxy Molecular-beam epitaxy (MBE) is an epitaxy method for thin-film deposition of single crystals. MBE is widely used in the manufacture of semiconductor devices, including transistors, and it is considered one of the fundamental tools for the dev ...

Physical vapor deposition

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

Ion beam assisted deposition Ion beam assisted deposition or IBAD or IAD (not to be confused with ion beam induced deposition, IBID) is a materials engineering technique which combines ion implantation with simultaneous sputtering or another physical vapor deposition techn ...

(IBAD) * Magnetron sputtering * Pulsed laser deposition * 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) 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 electro ...

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 p ...

** Plasma electrolytic oxidation ** Phosphate (coating) * 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-resistan ...

coating * Plating ** Electroless plating ** Electroplating

Spraying

* Spray painting * High velocity oxygen fuel (HVOF) * Plasma spraying * Thermal spraying * 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 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 Screen printing is a printing technique where a mesh is used to transfer ink (or dye) onto a Substrate (printing), substrate, except in areas made impermeable to the ink by a blocking stencil. A blade or squeegee is moved across the screen t ...

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- crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenchin ...

metal A metal (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, e ...

, 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 a ...

s by precisely metering the process fluid 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 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 - 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 * Flexography

Physical coating processes

* Langmuir-Blodgett * Spin coating * Dip coating