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 solids 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 and identification e.g. blue for process water, red for fire-fighting control. Functional coatings may be applied to change the surface properties of the substrate, such as adhesion, wettability, corrosion resistance, 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.
Applications
Coating applications are diverse and serve many purposes. 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, or a flat or matte appearance.
A major coating application is to protect metal from corrosion. This use includes preserving machinery, equipment, and structures. Most automobiles are made of metal. The body and underbody are typically coated with underbody coating. 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.
Roof coatings are designed primarily for waterproofing and sun reflection to reduce heating. They tend to be elastomeric to allow for movement of the roof without cracking the coating membrane.
The coating, sealing, and waterproofing of wood have been going on since biblical times, with God commanding Noah to build an ark and then coat it. 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.
Automotive coatings are used to enhance the appearance and durability of vehicles. These coatings include primers, basecoats, and clearcoats, and they are applied using various techniques, including electrostatic and spray gun applications.
Coatings are used to alter tribological properties and wear characteristics. Other functions of coatings include:
- Anti-fouling coatings
- Anti-Friction, Wear and Scuffing Resistance Coatings for Rolling-element bearings
- 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) 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 maybe water (or solventless), pigment(s) and additives. Research is ongoing to remove heavy metals from coating formulations completely.
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
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
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
