Coating. A coating is a covering that is applied to the surface of an object, or substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquids, gases or solids e.g. powder coatings.

Paints and lacquers are coatings that mostly have dual uses, which are protecting the substrate and being decorative, although some artists paints are only for decoration, and the paint on large industrial pipes is for identification (e.g. blue for process water, red for fire-fighting control) in addition to preventing corrosion.  Along with corrosion resistance, functional coatings may also be applied to change the surface properties of the substrate, such as adhesion, wettability, or wear resistance. In other cases the coating adds a completely new property, such as a magnetic response or electrical conductivity (as in semiconductor device fabrication, where the substrate is a wafer), and forms an essential part of the finished product.

A major consideration for most coating processes is controlling coating thickness.  Methods of achieving this range from a simple brush to expensive precision machinery in the electronics industry. Limiting coating area is crucial in some applications, such as printing.

"Roll-to-roll" or "web-based" coating is the process of applying a thin film of functional material to a substrate on a roll, such as paper, fabric, film, foil, or sheet stock. This continuous process is highly efficient for producing large volumes of coated materials, which are essential in various industries including printing, packaging, and electronics. The technology allows for consistent high-quality application of the coating material over large surface areas, enhancing productivity and uniformity.

Applications 
Coatings can be both decorative and have other functions. A pipe carrying water for a fire suppression system can be coated with a red (for identification) anticorrosion paint. Most coatings to some extent protect the substrate, such as maintenance coatings for metals and concrete. A decorative coating can offer a particular reflective property, such as high gloss, satin, matte, or flat appearance.

A major coating application is to protect metal from corrosion. Automotive coatings are used to enhance the appearance and durability of vehicles. These include primers, basecoats, and clearcoats, primarily applied with spray guns and electrostatically.
The body and underbody of automobiles receive some form of underbody coating. Such anticorrosion coatings may use graphene in combination with water-based epoxies.

Coatings are used to seal the surface of concrete, such as seamless polymer/resin flooring, bund wall/containment lining, waterproofing and damp proofing concrete walls, and bridge decks.

Most roof coatings are designed primarily for waterproofing, though sun reflection (to reduce heating and cooling) may also be a consideration. They tend to be elastomeric to allow for movement of the roof without cracking within the coating membrane.

Wood has been a key material in construction since ancient times, so its preservation by coating has received much attention. Efforts to improve the performance of wood coatings continue.

Coatings are used to alter tribological properties and wear characteristics.  These include anti-friction, wear and scuffing resistance coatings for rolling-element bearings

Other
Other functions of coatings include:
 Anti-fouling coatings
 Anti-microbial coatings. 
 Anti-reflective coatings for example on spectacles.
 Coatings that alter or have magnetic, electrical or electronic properties.
 Flame retardant coatings. Flame-retardant materials and coatings are being developed that are phosphorus and bio-based. These include coatings with intumescent functionality.
 Non-stick PTFE coated cooking pots/pans.
 Optical coatings are available that alter optical properties of a material or object.
 UV coatings

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

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 may be water (or solventless), pigment(s) and additives. Research is ongoing to remove heavy metals from coating formulations completely.

For example, on the basis of experimental and epidemiological evidence, it has been classified by the IARC (International Agency for Research on Cancer) as a human carcinogen by inhalation (class I) (ISPESL, 2008).

Processes
Coating processes may be classified as follows:

Vapor deposition

Chemical vapor deposition 

 Metalorganic vapour phase epitaxy
 Electrostatic spray assisted vapour deposition (ESAVD)
 Sherardizing
 Some forms of Epitaxy
 Molecular beam epitaxy

Physical vapor deposition 

 Cathodic arc deposition
 Electron beam physical vapor deposition (EBPVD)
 Ion plating
 Ion beam assisted deposition (IBAD)
 Magnetron sputtering
 Pulsed laser deposition
 Sputter deposition
 Vacuum deposition
 Vacuum evaporation, evaporation (deposition)
 Pulsed electron deposition (PED)

Chemical and electrochemical techniques
 Conversion coating
 Autophoretic, the registered trade name of a proprietary series of auto-depositing coatings specifically for ferrous metal substrates
 Anodising
 Chromate conversion coating
 Plasma electrolytic oxidation
 Phosphate (coating)
 Ion beam mixing
 Pickled and oiled, a type of plate steel coating
 Plating
 Electroless plating
 nickel plating coating using a different material to preserve mechanical properties
 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- 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 an 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 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 slot-die and web.
 Slide coating- bead coating with an angled slide between the slot-die 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 slot-die and web.
 Tensioned-web slot-die coating- with no backing for the web.
 Inkjet printing
 Lithography
 Flexography

Physical 
 Langmuir-Blodgett
 Spin coating
 Dip coating

See also

 Adhesion Tester
 Deposition
 Electrostatic coating
 Film coating drugs
 Food coating
 Formulations
 Langmuir-Blodgett film
 Nanoparticle deposition
 Optically active additive, for inspection purposes after a coating operation
 Paint
 Paper coating
 Plastic film
 Polymer science
 Printed electronics
 Seal (mechanical)
 Thermal barrier coating
 Thermal cleaning
 Thin-film deposition
 Thermosetting polymer
 Vitreous enamel

References

Further reading
 
 
 Titanium and titanium alloys, edited by C. Leyens and M. Peters, Wiley-VCH, , table 6.2: overview of several coating systems and fabriction processes for titanium alloys and titanium aluminides (amended)
 Coating Materials for Electronic Applications: Polymers, Processes, Reliability, Testing by James J. Licari; William Andrew Publishing, Elsevier, 
 High-Performance Organic Coatings, ed. AS Khanna, Elsevier BV, 2015, 

 
Corrosion
Materials science
Printing