Coating Technology

Summary

A coating is a thin layer or film of a substance spread over a surface for protection or decoration. Coatings can significantly improve the performance of the technology. Applications of coatings are far-ranging, from corrosion protection of metals in vehicles to thermal protection in jet engine turbine blades. Functional coatings are also used to generate electricity in fuel and photovoltaic cells and can reduce thermal emissions from buildings through windows. There are numerous methods of applying these coatings, each satisfying a different demand.

Definition and Basic Principles

Coating technologies have been applied to the surface of materials for centuries. Corrosion of steel can cause structural degradation in buildings and vehicles. When exposed to the atmosphere, metals can corrode, forming ceramic materials such as oxides, which are much more brittle and can lead to mechanical failure. Coatings can significantly alter the performance of the substrates to which they are applied. Coating the surfaces of metals can prevent or delay the onset of detrimental corrosion, but modern coating technologies can do much more than that. Some examples of coatings on polymers and glasses include antireflective and ultraviolet protection on sunglasses and thermochromic windows, which reduce heat influx into buildings in summer and prevent heat emissions in winter. Electrochemically active thin coatings can act as photovoltaic cells, supercapacitors, fuel cells, and electrodes of batteries. Thin films of the right chemical composition and microstructure typically increase the device’s efficiency. Patterned corrosion-protection coatings applied to semiconductors in photolithography prevent chemical etching and determine the size, shape, and the semiconductor’s functionality. Magnetic coatings are used as data storage devices. Ceramic-glaze coatings create colors on pottery pieces. Biomimetic coatings can prevent surgical implants from being attacked by the human immune system. Coating technologies can be physical, involving some type of melting or volatilization of a source material followed by deposition and solidification on a surface, or the solution or suspension of a coating material in a solvent, followed by the evaporation of the solvent after the deposition. Alternatively, the coating method can involve a chemical reaction that binds the coating to the substrate or chemically alters the substrate surface.

Background and History

Early humans developed pottery methods that still exist. By heating specific ceramic particles deposited from slurry on the surface of pre-fired clay, the pottery develops a dense glaze. This technology is one of the earliest surface-coating technologies. Paint, initially developed for artwork, has also been used during the last centuries as a protective coating.

Infrastructural corrosion of metallic structures in the United States alone amounts to billions of dollars in damages each year. Coatings have been applied to infrastructure and machinery for hundreds of years to prolong the useful lifetime of the equipment. Early coatings included the application of paint, which often contained metal-oxide materials. Paints usually involve particles suspended in a colloidal solution. Once the paint is applied to a surface, the solvent evaporates over time, leaving behind a coating. These coatings often did not stick well to their substrates and were often permeable to the environment, leading to their flaking off. In the early 1820s, the English chemist Sir Humphry Davy discovered that combining steels with easily corroding materials such as zinc results in significantly reduced corrosion rates—at the cost of corroding the sacrificial zinc.

Modern coating technologies have been developed as a result of the need to have devices with higher efficiencies and longer lifetimes. Although vehicles produced until the 1990s still exhibit an affinity to form rust, modern vehicles are produced with improved corrosion-protection coatings and rarely rust.

How It Works

Coatings add a thin film between the surface of a material and the environment with which the material comes in contact. They can be applied as a design feature or with specific material properties in mind.

To apply a coating, the correct deposition method must be selected. Vehicle corrosion is addressed by applying several functional coatings, usually in a rapid, high-quality process called dip coating, in which entire car components are immersed in a cataphoretic bath, which ensures complete, dense, and continuous coatings of all surfaces inside the structure. Further coatings may include color pigmentation, dyes, and a clear surface finish. Coatings can also be applied to much smaller substrates than vehicle parts. Small pigments of around a few micrometers such as mica and silica can change color if various oxide layers are applied by slurry casting since the refraction of light is changed because of these surface oxide layers. For example, the chemical company Merck manufactures such coatings under the name Ronasphere for cosmetics and vehicles.

Corrosion-protection coatings are supposed to be dense and not swell from water intake, but other coatings must intentionally be porous. For example, gas turbine engines in airplanes can be made more efficient by increasing the temperature inside the combustion chamber. However, the turbine blades are rotating at a high velocity and high temperature, and consequently, the metal surface of the blades may oxidize, and some deformation may occur because of creep. To prevent these effects, a 0.4-millimeter thin, porous layer of yttrium-stabilized zirconia (a type of beach sand) is applied to the surface of the turbine blades. This is usually performed in an open environment using a hot deposition gun capable of melting even ceramic zirconia, which has a melting point above 2,750 degrees Celsius. Heat is delivered by plasma spraying and high-velocity oxy-fuel, in which the ceramic material passes at high velocities through a flame to impact in liquid form and solidify on the surface of the turbine blades. The resultant coating is very porous, which is quite advantageous. Different materials expand at varying rates when heated, and dense ceramic coatings may develop such high stresses on the surface of metals, that they spall when exposed to varying temperatures. In porous coatings, on the other hand, these stresses are dispersed due to the porosity, and the coatings remain on the surface of the turbine blades. All these thermal-spraying methods are line-of-sight methods and cannot consequently be used if intricate three-dimensional structures that cannot be evenly positioned into the incoming material stream are to be coated evenly.

Similar hot spraying methods can also be used to create multiple graded functional layers in electrochemical devices such as batteries, fuel cells, solar cells, or capacitors. For example, the National Research Council Canada developed a reactive spray deposition method, in which metals are dissolved in an organic solvent, vaporized at elevated temperatures, and sprayed through a short flame on various substrate surfaces as a deposition straight from the gas phase. This method allows very good control of the microstructure of the functional layers.

For high-quality thin film coatings, physical vapor deposition methods have been developed that volatilize materials under extremely high vacuum conditions from high-purity emission sources referred to as targets. The growth rates of coating layers resulting from such a deposition are slow, and it may take several hours to form a layer with a thickness of even one micrometer. However, the layers are clean, pure, dense, and thin. Even something simple like a potato chip bag can have a thin metallic coating inside to improve the quality of the sealed products.

Chemical vapor deposition is another method to deposit well-adhered thin layers. Here, a chemical reaction occurs on the usually heated surfaces of the substrates, and the desired coating forms. Some deposition methods involving chemical reactions may include harsh chemicals such as hydrofluoric acid.

Biomedical coatings can have several functions. First and foremost they must prevent the human body from rejecting an implant. Furthermore, implants have coatings that either reduce the growth of scar tissue in soft tissues or aid in tissue growth inside bones, facilitating a solid attachment between bones and the implant and consequently a longer lifetime of the implant. In heart surgery, for example, very thin (less than 0.01 millimeter), drug-laced hydroxyapatite coatings on stents can be used to create devices that will not become covered in human tissue, thus reducing the risk of difficult follow-up operations and replacements.

Applications and Products

Corrosion Protection. One of the largest application areas of coatings is corrosion protection. As a result of corrosion, fatal accidents can occur. For example, vessels that are under pressure or carry corrosive and abrasive liquids may rupture and corroding bridges may collapse. In 2010, a small oil pipeline in Wyoming ruptured because of corrosion, resulting in a spill of 85,000 gallons of crude oil. To prevent the corrosion of infrastructure, protective coatings are applied to the metal surfaces. In steel production, alloying elements such as nickel or chromium can be added to the liquid metals. Once exposed to oxygen, they form stable, protective oxide-layer coatings, which are very thin and invisible to the naked eye. This kind of metal is consequently called stainless steel. Although stainless steel is common, the alloying elements are more expensive than the iron-containing raw materials, and they are less often used in structural applications or rebars within reinforced concrete. The most commonly produced stainless steel is called type 304. It contains 18 percent chromium and 8 percent nickel.

Once a type of steel has been produced, the metal surface can be protected by coatings that are applied externally. A method that was developed by French engineer Stanislas Sorel in 1837 is called galvanization. It involves dipping a metal part into a hot bath of zinc solution, causing a visible zinc coating to form on the surface. Although steels are protected by this method, they degrade fast in corrosive environments—salted roads in winter and the ocean. An improved method to protect steels from corrosion results in cataphoretically coating the substrates. The entire substrate is immersed in a conductive aqueous solution, and a dense, protective coating precipitates on the substrate surface due to an applied electric field. The German engineering firm Dürr uses this coating method in working with many global industries.

Surfaces can also be coated without involving electricity. Electroless nickel plating, for example, involves pretreating the surface of any material, including non-conductive materials, with a catalyst like sodium hypophosphite. This treated surface is immersed in a heated nickel-phosphorous or nickel-boron solution. The metal ions from the solution are reduced to metal in contact with the catalyst and form a dense alloy layer on the treated surface.

Biomedical Coatings. Blood vessel stems are often used in implants. Early implants were plagued by inflammation around the implant and the resulting growth of tissue around the implant as a defense mechanism to isolate the implant from the body. Companies such as MIV Therapeutics use the application of thin ceramic hydroxyapatite coatings to facilitate a better uptake of the implant in the body. These coatings are porous and can also be used to release anti-inflammatory drugs into the wound, locally and long term, without the need to medicate the patient with high doses of potentially dangerous drugs. In places where cellular growth is desired, such as in bone scaffolding and artificial joints, the outsides of the material in contact with the bone are coated with porous materials, such as stainless steel or titanium foams or beads that match the three-dimensional structure of the bone. It has been shown that these surfaces are overgrown and integrated into the bone much more easily than smooth metallic surfaces and constitute a significant improvement in implant lifetime. Because this is a large global market, there are a significant amount of international companies involved. Johnson & Johnson operates a subsidiary in North America for these implants called DePuy Orthopaedics. Other North American manufacturers include Stryker Orthopedics, Wright Medical Technologies, and Zimmer Biomet.

Nonstick Coatings. Other coatings applied in artificial blood vessels and large engineering pipes carrying liquids are nonstick surfaces such as polytetrafluoroethylene, marketed under the trademark Teflon. Blood clotting inside the artificial blood vessel is prevented by using a functional polymer coating. In engineering pipelines, liquid flow is slowed because of the friction of the liquid on the vessel walls. Nonstick coatings can significantly decrease the friction in the pipelines, reducing the power required to transport liquids, while simultaneously providing a chemical barrier coating between the liquid and the inside of the pipe. In addition to polymer nonstick coatings such as Teflon, the insides of pipes can be coated with thin layers of ceramic glass. Manufacturers like the Swedish company Trelleborg provide coating and sheathing solutions for transoceanic pipelines that carry various liquids, including oil products. Of course, one of the better-known applications for nonstick coatings is cookware. Nonstick coatings reduce the likelihood of heated materials sticking to the inside of a metallic pan since it does not chemically react with other materials. For this coating to stick to the inside of the pan, the metal is prepared with groves or porosity generated by sandblasting and coated with a porous primer.

Optical Coatings. Most ceramic glasses permit infrared radiation but will block ultraviolet radiation from the sun. Consequently, in summer, the insides of buildings are heated, and energy-intensive air-conditioning is required to cool the building. Some ceramics, such as vanadium oxide when mixed with small amounts of tungsten oxide, can block infrared radiation, effectively preventing solar heat radiation from reaching the insides of buildings. Applied to the glass, they automatically insulate the building from heat, silently and efficiently. However, this material is even smarter. Infrared radiation is blocked only above 29 degrees Celsius. In winter, the infrared radiation can pass into the building, heating it. Polymers, on the other hand, seldom block ultraviolet radiation. Most eyeglasses and sunglasses are manufactured using polymers coated with materials that block ultraviolet radiation to protect the eyes. Similarly, polymers and polymer fabrics can be protected from degradation due to ultraviolet radiation by applying thin coatings of zinc and titanium oxides. Other transparent coating materials, such as indium tin oxide, can be applied to glass surfaces to make them conductive, as with solar cells, where the transparent layer facing the sun acts as one of the electrodes. Schott and Carl Zeiss are Europe-based companies that manufacture high-quality specialty glasses with any number of functional coatings for the global market.

Magnetic Coatings. Videotapes, cassettes, floppy discs, and zip drives are technologically obsolete, but they have one thing in common—using magnetic materials to store data. In the case of magnetic tapes, a polymer film was coated with magnetic material, for example, iron oxide. The chemical company BASF used to be one of the largest providers of magnetic tapes. Modern computer hard disks typically use cobalt-based alloys to store the data, allowing faster and safer magnetization.

Careers and Course Work

Although it may seem trivial to pick up a paint roller to add color to a wall, the technological developments necessary to produce even a simple, stable paint are massive and have been ongoing for a long time. Detailed knowledge of the material properties, the chemistry of the solvents, and the fundamental physics of the optical, mechanical, thermal, and magnetic properties of coatings are essential to develop novel, innovative products for the global market. Career advancement in global coatings technology typically requires an undergraduate degree, ideally with many industrial internships for practical experience.

Social Context and Future Prospects

Vehicles manufactured as late as the 1980s were prone to corrosion, reducing the optical appearance of a vehicle and impairing mechanical stability and safety. Modern coating technologies have advanced significantly. All surfaces of a car body can now be coated, no matter what shape, and they are less permeable to air and absorb significantly less water. Coatings adhere better to surfaces, are dense, and have higher impact and scratch resistance. As a result, vehicles manufactured in the early twenty-first century corrode far less frequently. This development also applies to aeronautic and marine applications. The coated devices, such as ships, must be re-coated less often, reducing downtime and preventing fatal material failures. The same applies to biomedical devices. If a hip implant with a modern coating must be renewed inside a patient every fifteen years instead of every five years, the result is a significant improvement in the patient’s quality of life.

Coating technologies are far from perfect, evidenced by the huge global research effort in developing all types of coatings. Modern automotive coatings can typically involve more than 200 chemicals, and the exact physical and chemical interaction between all these chemicals can be evaluated only in small steps. Likewise, coating technology methods are constantly evolving. Consequently, there is a huge potential in coatings research and development with applications in almost every technology.

Further Reading

Gevaert, Paul. "UV Coatings on Plastics." Paint and Coatings Industry Magazine, 5 Feb. 2020, www.pcimag.com/articles/107018-uv-coatings-on-plastics. Accessed 20 May 2024.

Lakhtakia, Akhlesh, and Russell Messier. Sculptured Thin Films: Nanoengineered Morphology and Optics. SPIE, 2005.

Lüth, Hans. Solid Surfaces, Interfaces, and Thin Films. 6th ed. Springer-Verlag, 2018.

Smeets, Stefan, Egbert Boerrigter, and Stephan Peeters. “UV Coatings for Plastics.” European Coatings Journal, vol. 24, no. 6, 2004, pp. 42-48. doi.org/10.1016/j.ijadhadh.2004.01.006.

Sun, Jiaoxia, et al. "The Surface Degradation and Release of Microplastics from Plastic Films Studied by UV Radiation and Mechanical Abrasion." Science of the Total Environment, vol. 838, no. 3, 2022. doi.org/10.1016/j.scitotenv.2022.156369.

Mattox, D. M. The Foundations of Vacuum Coating Technology. 2nd ed., William Andrew Applied Science Publishers, 2018.

Stine, Keith J. Materials Processing for Production of Nanostructured Thin Films. Multidisciplinary Digital Publishing Institute, 2021.

Streitbergerm Hans-Joachim, and Artur Goldschmidt. BASF Handbook: Basics of Coating Technology. 3rd. rev. ed. Vincentz Network, 2018.

Weldon, Dwight G. Failure Analysis of Paints and Coatings. Rev. ed. John Wiley & Sons, 2009.

Yamaguchi, Hisato, et al. "Work Function Lowering of LAB6 by Monolayer Hexagonal Boron Nitride Coating for Improved Photo- And Thermionic-Cathodes." Tohoku University, 5 Dec. 2023, www.tohoku.ac.jp/en/press/new‗surface‗coating‗technology‗increases‗materials‗electron‗emission.html. Accessed 20 May 2024.