Coating services apply a thin layer of material onto parts or products provided by a client. These services are used by companies that do not have coating technology and equipment in house.

Once a part or finished product is ready for coating, the manufacturer will contact the coating shop with material and process specifications required to correctly coat the part. Parts are delivered to the coating shop where the coating procedure is done with minimal turnaround time. Based on the type of product, coating processes can involve individual parts or components, or can be web-based. Web-based coating involves the application of a coating to a continuous sheet of material.

Coating Methods

Determining the coating method needed for the application is the first and most important step in selecting the right coating service. The method defines how coating materials are applied to part surfaces; most methods are specific to certain coatings and substrate materials.

Anodizing

Anodizing is an electrocoating process that involves the use of negatively charged particles to increase the thickness of a metal's natural oxide layer. In the process, the target metal is submerged in a charged electrolytic solution and acts as the negatively charged anode. This creates a buildup of metal-oxide particles on the metal's surface. This oxide layer creates a more corrosion and wear resistant surface, and allows paint primers and glues to adhere easier than to bare metal. Anodizing is most commonly done to aluminum, but titanium, zinc, magnesium, niobium, and tantalum metals can also be anodized.

Conversion Coating

Conversion coating includes any process involving chemical or electrochemical treatment of a metal surface. The coatings provide corrosion resistance and prepare a surface for better adhesion of primers and paints. These are not intended to be decorative coatings. Chemical finishes include black oxide, chromate conversion, and phosphate coatings.

Cladding and Hardfacing

Cladding is a process where another type of material is metallurgically bonded to a base metal, usually through a rolling or high pressure process. Usually the base metal is less costly, but less corrosion or wear resistant. Stainless steel clad to carbon steel would be an example of a clad or bimetal material. Hardfacing is a process which welds materials with different properties to the substrate. Hardfacing processes are very useful for improving wear and corrosion resistance to selected areas of machinery, such as cutting edges of earth-moving machinery. Common hardfacing techniques include arc, torch, and other processes.

Plating

Plating is a process used to deposit a coating onto a metal or conductive surface using a metal salt solution of the metal coating to be deposited. Electroplating requires the application of an electric current, which turns the substrate into a cathode which attracts positive metal ions to its surface. Electroless plating uses a chemical deposition process instead of an electrical current to deposit the metallic layer, and provides a more even plate layer, even on complex geometries. Nickel is the most plating material deposited in electroless plating processes, but gold, silver, and copper can also be applied.

Sealing and Impregnation

Sealing and impregnation coating processes use vacuum, pressure, vacuum-pressure, and/or wicking action techniques to drive resins or coating materials into parts to seal open porosity.

Vacuum-pressure impregnation (VPI) is a sealing and impregnation process that uses a combination of pressure and vacuum to drive the resin into the open porosity or surface cavity defects. VPI is used to seal porous castings, powder metal parts and transformer or motor windings. The sealing process improves integrity and the resistance of the parts to corrosive environmental elements.

Spray Coating

In spray coating, the coating, sealant or resin is applied using a sprayer or atomizing applicator. Common applicators include pressurized nozzles or spray guns and rotary atomizers. An electrostatic charge can also be applied to reduce overspray.

Thermal spraying is a continuous coating by melting the consumable material into droplets and impinging these droplets on the substrate. The thickness of the coatings may range from 25 µm to 2.5 mm (.001 - .1 in). The thermal spray coatings compete with plating and paint coating for atmospheric corrosion resistance in water tanks, TV towers, bridges, and other large steel structures.

Thermal Spray Coating Process

The following video illustrates thermal spraying utilizing a robot.

Thermal spraying. Video Credit: ARC Specialties

Powder coating is a finishing technique in which dry paint particles are electrostatically charged and applied to a grounded part. Electrostatic attraction holds powder particles on the part surface until heat is added to flow the powder together and cure it. Parts must be electrically conductive. Although powder coating is most widely used on metal parts, recent technological developments have broadened the type of substrates to include glass, special plastics and wood.

Thin Film Coating

Thin film coatings play a prominent role in the manufacture of many electrical devices. They are used to apply dopants and sealants to chips and other microelectronic parts. Physical vapor deposition (PVD) and chemical vapor deposition (CVD) are two most common types of thin film coating methods.

Physical vapor deposition involves the condensation of a vaporized film onto a workpiece surface using physical processes rather than chemical reaction. These processes include cathodic arc, electron beam, evaporative, pulsed laser, and sputter.

Chemical vapor deposition involves the reaction or decomposition of a precursor substance on a wafer or other substrate. This results in a deposited coating on the surface of the substrate. CVD processes are used to create semiconductors integrated circuits and solar cells, to coat certain metals (including as molybdenum, tantalum, titanium, nickel, and tungsten), and to produce synthetic diamonds.

Other Coating Methods

Dip coating or immersion coating processes involve dipping or immersing parts into a bath or tank filled with the liquid coating solution.

Dip coating. Video Credit: Youtube - ibhooshan

Galvanization is the practice of immersing clean, oxide-free iron or steel into molten zinc in order to form a zinc coating that is metallurgically bonded to the iron or steel surface. The zinc coating protects the surface against corrosion by providing protection to the iron or steel in two ways. It shields the base metal from the atmosphere and because it is more electronegative than iron or steel, the zinc reacts with corroding agents first, providing a longer service lifetime for the part.

A diagram of the galvanization process.

Painting is physically applying a coating directly to the substrate via a brush or spray. Paint contains a vehicle which carries the solid and resin (film-forming) components of the coating, and evaporates to allow the coating to adhere to the applied surface. Paints can be used for decoration and/or protection.

Types of Coatings

In addition to the coating method, there are a large range of coating types that coating services can apply to surfaces and materials.

Abrasives / superabrasives - Resin and metal bonds (braze, electroplate or sinter) are used to coat a surface with an abrasive layer. Abrasive surfaces are coarse and rough, and are used either for increased grip or traction, or as tools to grind or wear down other surfaces.

Adhesive / PSA - Surfaces are coated with a layer of adhesive or pressure sensitive adhesives which promote adherence to other surfaces (e.g. tapes).

Ceramic coatings - Ceramic coatings exhibit excellent resistance to wear, heat, and aqueous corrosion. In addition, the coatings are excellent electrical insulators.

Dry lubricant coating - Dry lubricants are used to coat surfaces such as machine parts, fittings, cams, springs, gears, cables, springs, leather, plastic and elastomeric gaskets. The most common forms of dry lubricant are molybdenum disulfide (MoS2) and polytetrafluoroethylene (PTFE), possibly mixed together, or with adhesion promoters. Desirable properties as the result of coating with dry lubricants include reduced friction, high load performance, reduced power input, wear reduction, prevention of metal to metal contact, anti-stick properties, prevention of galling and scoring, cleanliness, and protection against moisture and corrosion.

Black oxide coatings - Black oxide coatings or finishes are a type of chemical or conversion coating. Black oxide coatings are formed through controlled oxidation of the base metal. The thin, dense oxide film retards corrosion and provides a suitable pretreatment or base layer for subsequent organic coatings (paint, varnish, oil or wax).

Phosphate coating - Phosphate coating is a treatment used on steel, iron, castings, and steel-based substrates to protect metal from corrosion and improve coating adhesion. In the phosphating process, the surface is treated with a solution of phosphoric acid and other chemicals, which react with the metal to form a mildly protective layer of insoluble crystalline. There are many types of phosphate coatings, including zinc and manganese.

Conformal coating - Conformal coatings encapsulate circuit boards and their electronic components in order to prevent the ingress of moisture, fungus, dust and other environmental contaminants. There are several basic types of conformal coatings. Electrically conductive products provide low resistivity and are often used to prevent electrostatic discharge (ESD), electromagnetic interference (EMI), and radio frequency interference (RFI). They are also used in thick film metallization and to provide electrical interconnections at both the device and circuit board level.

Liner - applied / spray-on - Spray-on liners, applied lining systems, or high build coatings are formed in place through a spray, troweling, gunning or thermal deposition process. Some protective liners and lining systems are available as fluoroplastic films or barrier membranes applied to a surface with an adhesive. Other applied linings are available as coated cloths or tarps. Liners and high build coatings are thicker compared to typical paint and coating applications (i.e. liners are measured in inches while coatings are measured in millimeters). For more information on liners, visit the Protective Liners and Lining Systems Specification Guide on Engineering360.

Optical coatings - Optical coatings are deposited onto optical surfaces such as mirrors or lenses to change how these surfaces interact with (transmit and reflect) light. They are designed for specific incident angles of light and for specific polarization of light depending on the properties required. They are created by depositing dielectric and metallic materials such Ta₂O₅ and/or Al₂O₃ in very thin (wavelength-sized) layers.

Titanium nitride coating - Titanium nitride (TiN) is an extremely hard, thin film coating that is applied mostly to precision metal parts. It is the most common hard coating in use today. TiN has an excellent combination of performance properties, attractive appearance, and safety (meets FDA requirements for surgical tools and implants as well as food contact applications).

Rubber coating - Rubber resins are typically applied via dip molding. They are used as protective, decorative, and/or insulating coatings.

Plastic coating - Plastic resins, like rubber resins, can be used in a variety of applications as protective, decorative and/or insulating coatings.

Functions

Coating services may provide services based on specific functions the coating provides.

Abrasion resistant - The coating provides resistance to damage by abrasion.

Adhesive - The coating provides adhesive properties.

Antireflective - The coating has an anti-reflective property applied to the substrate to reduce glare. Anti-reflective coatings enrich the color black by decreasing the amount of light reflected off the substrate by outside sources.

Anti-slip / abrasive - Anti-slip coatings produce a textured, abrasive, sticky or roughened surface layer to prevent personnel from slipping and falling on floors, steps, and walkways.

Chemical resistant - The coating provides resistance to damage by acids, alkalis, general chemicals and oils.

Conductive - The coating forms an electrically conductive layer on the substrate.

Corrosion / rust preventive - A type of coating that prevents rust by preventing moisture from reaching the metal or underlying substrate or by providing a sacrificial layer. Resin based coatings are corrosion and chemically resistant and provide a barrier to protect the substrate. Zinc or aluminum coatings provide a sacrificial layer that galvanically protects the ferrous surface, even if the layer is breached in places. Zinc phosphate, barium metaborate, and strontium chromate (all pigments) are common ingredients in corrosion-inhibitive coatings. These pigments absorb any moisture that enters the coating film. Lubricant, oil, and grease coating provide a water repellent barrier, which inhibits corrosion. Rust preventative coatings are designed to minimize rust or iron alloy corrosion when applied directly to ferrous metals such as carbon or alloy steels.

Dielectric - Optical coatings made from dielectric (nonconducting) materials. They are high-reflection coatings made from a stack of alternate layers of high- and low-refractive-index material, with each layer in the stack having an optical thickness of a quarter wave at the design wavelength.

Friction reducing / low friction - Specialty coatings applied to reduce friction between two materials.

Heat Resistant / high temperature - Coatings resistant to damage by heat or the coating is formulated for use in high temperature environments.

Mold release / nonstick - Release and nonstick coatings are applied to a substrate to prevent materials from sticking. Mold releases are applied to the surface of a mold or die cavity and allow the molded component to be easily ejected or removed.

Oil & grease resistant - Coatings resistant to degradation when in contact with oils, lubricants, grease and other petroleum fluids. Oil repellent or oleophobic coatings are not wet by oils.

Porosity / surface sealing - Coatings function as surface sealants or porosity sealers. The sealing material can be assisted using vacuum, pressure, and vacuum-pressure impregnation (VPI) techniques or the natural wicking action of the component.

Protective - Coatings protect substrates from the outside environment, physically and/or chemically.

Reflective - Reflective coatings consist of mirror like deposits for reflectance of light.

VOC compliant - Coatings are compliant with VOC standards. Volatile organic chemicals (VOCs) are any carbon compounds that evaporate under standard test conditions. Essentially, all paint solvents except water are VOCs. Federal and state governments limit the amount of volatile organics found in paint because of environmental and health effects.

Waterproof / water repellant - Exterior clear finishes that are specially formulated to cause water to bead up on the surface and minimize penetration of water into the substrate.

Wear / erosion resistant - The coating is resistant to wear or erosion. Wear is usually defined as wear produced from a sliding action between two or more components. Erosion is damage or material removal as a result of particle or slurry impact against a surface.

Service Location

In addition to the types of services that coating service providers offer, the location of the company is often in important to consider. In most applications, products need to be delivered from the client to the coating shop. Thus, depending on the quantity and size of the products, location can have a big effect on the total cost of the service and the total project time. Additional factors come into play when requesting services from companies located in different countries.

Image Credits:

Anoplate Corporation | Superior Metal Technologies, LLC. | Hi-tech Plating & The Tinning Company | Meldrum Mechanical Services, Inc. | Galvanizers Association