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. The coating itself may be an all-over coating, completely covering the substrate, or it may only cover parts of the substrate. An example of all of these types of coating is a product label on many drinks bottles- one side has an all-over functional coating (the adhesive) and the other side has one or more decorative coatings in an appropriate pattern (the printing) to form the words and images.
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 presumably only for the function of preventing corrosion.
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.
Functions of coatings
- Adhesive – adhesive tape, pressure-sensitive labels, iron-on fabric
- Changing adhesion properties
- Optical coatings
- Catalytic e.g. some self-cleaning glass
- Light-sensitive as previously used to make photographic film
- Protective coatings
- Most surface coatings or paints are to some extent protecting the substrate e.g.
- Sealing and waterproofing wood
- Sealing the surface of concrete
- Waterproofing and damp proofing of concrete walls
- Roof coating
- Concrete bridge deck membranes
- Sealing and waterproofing of masonry
- Preserving machinery, equipment and structures
- Maintenance coatings/paints for metals, alloys and concrete
- Chemical resistant coatings
- Wear resistance
- Anti-corrosion- ensure metal components have the longest possible lifespan.
- Underbody sealant for cars
- Many plating products
- Preserving equipment and structural steel from degradation
- Under thermal insulation and under protective fireproofing for structural steel
- Adding glass flakes and zinc flakes to improve resistant of water and chemicals permeating
- Passive fire protection
- Waterproof fabric and waterproof paper
- Antimicrobial surface
- Foul release and anti-fouling
- Most surface coatings or paints are to some extent protecting the substrate e.g.
- Magnetic properties such as for magnetic media like cassette tapes, floppy disks, and some mass transit tickets
- Electrical or electronic properties
- Scent properties such as scratch and sniff stickers and labels
- Decorative- often to impart a specific colour, but also to create a particular reflective property such as gloss or matt.
Numerous methods exist for evaluating coatings, including both destructive and non-destructive methods. The most common destructive method is microscopy of a mounted cross-section of the coating and substrate. The most common non-destructive techniques include ultrasonic thickness measurement, XRF coatings thickness measurement, and ultra-micro hardness testing.
Coating processes may be classified as follows:
Chemical vapor deposition
Physical vapor deposition
Chemical and electrochemical techniques
- Conversion coating
- Ion beam mixing
- Pickled and oiled, a type of plate steel 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 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 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
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- Film Coating drugs
- Adhesion Tester
- Langmuir-Blodgett film
- Nanoparticle deposition
- Optically active additive, for inspection purposes after a coating operation
- Plastic film
- Printed electronics
- Seal (mechanical)
- Thermal barrier coating
- Thermal cleaning
- Thin-film deposition
- Paper coating
- Thermosetting polymer
- Vitreous enamel
- S. Grainger and J. Blunt, Engineering Coatings: Design and Application, Woodhead Publishing Ltd, UK, 2nd ed., 1998, ISBN 978-1-85573-369-5
- Mutyala, Kalyan C.; Singh, Harpal; Evans, R. D.; Doll, G. L. (23 June 2016). "Effect of Diamond-Like Carbon Coatings on Ball Bearing Performance in Normal, Oil-Starved, and Debris-Damaged Conditions". Tribology Transactions. 59 (6): 1039–1047. doi:10.1080/10402004.2015.1131349.
- Mutyala, Kalyan C.; Ghanbari, E.; Doll, G.L. (August 2017). "Effect of deposition method on tribological performance and corrosion resistance characteristics of Cr x N coatings deposited by physical vapor deposition". Thin Solid Films. 636: 232–239. doi:10.1016/j.tsf.2017.06.013. ISSN 0040-6090.
- Glass flakes
- Fristad, W. E. (2000). "Epoxy Coatings for Automotive Corrosion Protection". SAE Technical Paper Series. 1. doi:10.4271/2000-01-0617.
- US2681294A, "Method of coating strip material", issued 1951-08-23
- Beeker, L.Y. (March 2018). "Open-source parametric 3-D printed slot die system for thin film semiconductor processing" (PDF). Additive Manufacturing. 20: 90–100. doi:10.1016/j.addma.2017.12.004. ISSN 2214-8604.
- "Slot Die Coating - nTact". nTact. Retrieved 2018-11-24.
- "Open Source 3D printing cuts cost from $4,000 to only $0.25 says new study - 3D Printing Industry". 3dprintingindustry.com. Retrieved 2018-11-24.
- "What and why: Langmuir-Blodgett films" (PDF).
- Titanium and titanium alloys, edited by C. Leyens and M. Peters, Wiley-VCH, ISBN 3-527-30534-3, 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, ISBN 0-8155-1492-1
- High-Performance Organic Coatings, ed. AS Khanna, Elsevier BV, 2015, ISBN 978-1-84569-265-0