Coating

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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.^[1]

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 etc.

Functional coatings may be applied to change the surface properties of the substrate, such as adhesion, wettability, corrosion resistance, or wear resistance.^[2] 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.

Coatings may be applied as liquids, gases or solids.

Functions of coatings[]

• Adhesive – adhesive tape, pressure-sensitive labels, iron-on fabric
• Changing adhesion properties
  • Non-stick PTFE coated- cooking pans
  • Release coatings for example silicone-coated release liners for many self-adhesive products
  • primers encourage subsequent coatings to adhere well (also sometimes have anti-corrosive properties)
• Optical coatings
  • Reflective coatings for mirrors
  • Anti-reflective coatings example on spectacles
  • UV- absorbent coatings for protection of eyes or increasing the life of the substrate
  • Tinted as used in some coloured lighting, tinted glazing, or sunglasses
• 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
      • Film-forming sealers and floor paint
      • Seamless polymer/resin flooring
      • Bund wall/containment lining
    • Waterproofing and damp proofing of concrete walls
    • Roof coating
    • Concrete bridge deck membranes
    • Sealing and waterproofing of masonry
    • Preserving machinery, equipment and structures^[3]
      • Maintenance coatings/paints for metals, alloys and concrete
      • Chemical resistant coatings
      • UV coating
    • Wear resistance
      • Anti-Friction, Wear and Scuffing Resistance Coatings for Rolling-element bearings^[4]
      • Hard anti-scratch coating on plastics and other materials e.g. of titanium nitride to reduce scratching and abrasion loss
      • Barrier coatings on concrete, metals and alloys subject to erosion/abrasive attack
  • Anti-corrosion,^[5][6]- ensure metal components have the longest possible lifespan by preventing substrates from external environmental parameters
    • 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^{[7][circular reference]}
  • Passive fire protection
  • Insulation
  • Waterproof fabric and waterproof paper
  • Anti-graffiti
  • Antimicrobial surface
  • Foul release, anti-fouling and Biomimetic antifouling coating
• Magnetic properties such as for magnetic media like cassette tapes, floppy disks, and some mass transit tickets
• Electrical or electronic properties
  • Conformal Antenna, e.g., metal coatings on plastic airframes
  • Conductive coatings e.g. to manufacture some types of resistors
  • Insulating coatings e.g. on magnet wires used in transformers
• 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.

Coating Analysis[]

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 (X-ray fluorescence) coatings thickness measurement, and ultra-micro hardness testing.

Coating formulation[]

The formulation of the coating depends primarily on the function 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.^[8]

Coating 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)
• (PED)

Chemical and electrochemical techniques[]

• Conversion coating
  • Autophoretic, the registered trade name of a proprietary series of autodepositing coatings specifically for ferrous metal substrates^[9]
  • Anodising
  • Chromate conversion coating
  • Plasma electrolytic oxidation
  • Phosphate (coating)
• Ion beam mixing
• Pickled and oiled, a type of plate steel 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- 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.^[10] 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.^[11] 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.^[12] The complex inner geometry of conventional slot dies require machining or can be accomplished with 3-D printing.^[13]
• 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^[14]
• Spin coating
• Dip coating

See also[]

Look up coating in Wiktionary, the free dictionary.

References[]

• 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