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