A coating is a chemical compound (either organic or inorganic in nature) that can either be applied to a surface or used to convert the chemical structure of a surface. This application is generally for the purpose of protection of the substrate material from various forms of environmental attack but can also be for other purposes such as stress indication and temperature indication.
Coatings are generally used to protect a material substrate from some form of environmental attack. Alternatively, a coating is applied for its cosmetic properties and must therefore be self-protecting from the elements. The forms of attack a material may be exposed to will typically be comprised of one or more elements of the following list:
- Electrical Energy
- Mutual Abrasion
Materials such as wood, paper, fiberboard and fabric are typically substrates in need of moisture protection. Coatings for these applications range from something as simple as airplane dope used on wing fabrics all the way to two-part catalyzed polymerics such as clear penetrating epoxy sealers, and polyester-saturated aliphatic polyurethanes for clear coating of fine hardwoods for marine or outdoor applications.
For metallic corrosion protection, the most popular modern innovation has been the widespread use of powdercoatings—powdered paints applied as a fine powder, held in place by static charge, and then heat cured to an almost ceramic-like finish.
Another common approach for metal protection is through the use of what are called “conversion coatings” that change the surface chemistry of a given metal alloy to create a protective layer. Examples of this method are evidenced by the “Alodine” process, the gun bluing process, the black oxide process and many others.
Sunlight (ultraviolet) Protection:
Ultraviolet radiation from the sun (or from artificial sources) can be the single most damaging environmental attack one has to overcome for many materials. Organic coatings are particularly susceptible to UV damage and therefore enlist the use of UV absorbers in their formulation.
Electrical Insulation Or Conductivity:
There are occasions when a surface must not only protect from one or more of the elements described above but also be a dielectric (electrically insulated). Interestingly, the flip side of this is the need in some applications for a surface to protect from the above-described elements yet remain electrically conductive.
For these applications an interesting coating material exists called Parylene which is applied to corrosion-susceptible metals like magnesium alloys where there is a need to be able to attach grounding lugs or terminals that can carry current without scratching or scraping away a coating from the pad location.
There are frequently applications in which one material is in close contact with another material and their respective surfaces must be protected from mutual abrasion.
One such coating is called “Plastisol” which is liquid PVC (polyvinyl chloride) and is generally applied through a dip process.
Coatings can also be made to be sensitive to stress and strain experienced by the substrate. Stresscoat is one example of this. It is applied just like a lacquer-based paint. When it cures it will show a crack pattern in the areas of high stress.
Coatings can also be applied to surfaces for the purpose of indicating high localized temperatures. As with Stresscoat, these coatings are applied like paint. Once cured, they will change colors in the areas of elevated temperatures.