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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. 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.
Coatings may be applied as liquids, gases or solids.
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:
Common roll-to-roll coating processes include:
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Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.
Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are called alloys. Metallurgy encompasses both the science and the technology of metals; that is, the way in which science is applied to the production of metals, and the engineering of metal components used in products for both consumers and manufacturers. Metallurgy is distinct from the craft of metalworking. Metalworking relies on metallurgy in a similar manner to how medicine relies on medical science for technical advancement. A specialist practitioner of metallurgy is known as a metallurgist.
Paint is any pigmented liquid, liquefiable, or solid mastic composition that, after application to a substrate in a thin layer, converts to a solid film. It is most commonly used to protect, color, or provide texture to objects. Paint can be made or purchased in many colors—and in many different types, such as watercolor or synthetic. Paint is typically stored, sold, and applied as a liquid, but most types dry into a solid. Most paints are either oil-based or water-based and each have distinct characteristics. For one, it is illegal in most municipalities to discard oil based paint down household drains or sewers. Solvents for clean up are also different for water based paint than they are for oil based paint. Water-based paints and oil-based paints will cure differently based on the outside ambient temperature of the object being painted Usually the object being painted must be over 10 °C (50 °F), although some manufacturers of external paints/primers claim they can be applied when temperatures are as low as 2 °C (35 °F).
Electroplating is a general name for processes that create a metal coating on a solid substrate through the reduction of cations of that metal by means of a direct electric current. The part to be coated acts as the cathode of an electrolytic cell; the electrolyte is a solution of a salt of the metal to be coated; and the anode is usually either a block of that metal, or of some inert conductive material. The current is provided by an external power supply.
Corrosion is a natural process that converts a refined metal into a more chemically stable form such as oxide, hydroxide, or sulfide. It is the gradual destruction of materials by chemical and/or electrochemical reaction with their environment. Corrosion engineering is the field dedicated to controlling and preventing corrosion.
Chrome plating, often referred to simply as chrome, is a technique of electroplating a thin layer of chromium onto a metal object. The chromed layer can be decorative, provide corrosion resistance, ease cleaning procedures, or increase surface hardness. Sometimes, a less expensive imitator of chrome may be used for aesthetic purposes.
Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts.
A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films is a fundamental step in many applications. A familiar example is the household mirror, which typically has a thin metal coating on the back of a sheet of glass to form a reflective interface. The process of silvering was once commonly used to produce mirrors, while more recently the metal layer is deposited using techniques such as sputtering. Advances in thin film deposition techniques during the 20th century have enabled a wide range of technological breakthroughs in areas such as magnetic recording media, electronic semiconductor devices, Integrated passive devices, LEDs, optical coatings, hard coatings on cutting tools, and for both energy generation and storage. It is also being applied to pharmaceuticals, via thin-film drug delivery. A stack of thin films is called a multilayer.
Plating is a surface covering in which a metal is deposited on a conductive surface. Plating has been done for hundreds of years; it is also critical for modern technology. Plating is used to decorate objects, for corrosion inhibition, to improve solderability, to harden, to improve wearability, to reduce friction, to improve paint adhesion, to alter conductivity, to improve IR reflectivity, for radiation shielding, and for other purposes. Jewelry typically uses plating to give a silver or gold finish.
Metallizing is the general name for the technique of coating metal on the surface of objects. Metallic coatings may be decorative, protective or functional.
Titanium nitride is an extremely hard ceramic material, often used as a coating on titanium alloys, steel, carbide, and aluminium components to improve the substrate's surface properties.
A primer or undercoat is a preparatory coating put on materials before painting. Priming ensures better adhesion of paint to the surface, increases paint durability, and provides additional protection for the material being painted.
A superalloy, or high-performance alloy, is an alloy with the ability to operate at a high fraction of its melting point. Several key characteristics of a superalloy are excellent mechanical strength, resistance to thermal creep deformation, good surface stability, and resistance to corrosion or oxidation.
Electroless nickel-phosphorus plating is a chemical process that deposits an even layer of nickel-phosphorus alloy on the surface of a solid substrate, like metal or plastic. The process involves dipping the substrate in a water solution containing nickel salt and a phosphorus-containing reducing agent, usually a hypophosphite salt. It is the most common version of electroless nickel plating and is often referred just by that name. A similar process uses a borohydride reducing agent, yielding a nickel-boron coating instead.
Thermal spraying techniques are coating processes in which melted materials are sprayed onto a surface. The "feedstock" is heated by electrical or chemical means.
Gas dynamic cold spraying or cold spraying (CS) is a coating deposition method. Solid powders are accelerated in a supersonic gas jet to velocities up to ca. 1200 m/s. During impact with the substrate, particles undergo plastic deformation and adhere to the surface. To achieve a uniform thickness the spraying nozzle is scanned along the substrate. Metals, polymers, ceramics, composite materials and nanocrystalline powders can be deposited using cold spraying. The kinetic energy of the particles, supplied by the expansion of the gas, is converted to plastic deformation energy during bonding. Unlike thermal spraying techniques, e.g., plasma spraying, arc spraying, flame spraying, or high velocity oxygen fuel (HVOF), the powders are not melted during the spraying process.
Substrate is a term used in materials science to describe the base material on which processing is conducted to produce new film or layers of material such as deposited coatings.
Adhesive bonding describes a wafer bonding technique with applying an intermediate layer to connect substrates of different types of materials. Those connections produced can be soluble or insoluble. The commercially available adhesive can be organic or inorganic and is deposited on one or both substrate surfaces. Adhesives, especially the well-established SU-8, and benzocyclobutene (BCB), are specialized for MEMS or electronic component production.
Cold Spray Additive Manufacturing (CSAM) is a particular application of the cold spraying able to fabricate freestanding parts or to build features on existing components. During the process, fine powder particles are accelerated in a high-velocity compressed gas stream, and upon the impact on a substrate or backing plate, deform and bond together creating a layer. Moving the nozzle over a substrate repeatedly, a deposit is building up layer-by-layer, to form a part or component. If an industrial robot or computer controlled manipulator controls the spray gun movements, complex shapes can be created. To achieve 3D shape, there are two different approaches. First to fix the substrate and move the cold spray gun/nozzle using a robotic arm, the second one is to move the substrate with a robotic arm, and keep the spray-gun nozzle fixed. There is also a possibility to combine these two approaches either using two robotic arms or other manipulators. The process always requires a substrate and uses only powder as raw material.