A Comprehensive Guide to Coatings Selection for Engineering Application

16 March 2025

Coatings are complex materials that offer engineers an expanded design space when developing products and solutions. Coatings are multifunctional materials, thin films adhered to an underlying structure, known as a substrate, which creates an interacting material system to achieve a baseline performance requirement. From mundane to complex applications, coatings are enabling technologies capable of protecting substrate materials, with some providing novel functionalities that substrates alone cannot support.  So is the case with exciting stealth or infrared suppression technologies, where the goal of coatings is to hide structures from enemy detection with special optical properties not inherent to typical materials used in construction.  While most material and process selection considerations prioritize factors like cost, schedule, material properties (mechanical, electrical, etc.), and complexity, selecting the right coating requires careful consideration of unique factors. In this article, we discuss three key drivers engineers should consider when choosing the right coating for their design needs to make informed decisions and achieve robust solutions.

The significance of a clear design intent cannot be overstated.  Although it may be clear and well understood in simpler applications, practical engineering design experience has shown that sometimes these intentions, if not well understood (or under control from change) could result in a coating unable to meet expectations.  The designer or design team should enter the coating material trade with a clear intent to mitigate the risk of a faulty selection, and the final coating selection should meet this intent.  

Application and Manufacturing Methods

Following the definition of design intent, manufacturing and application methodologies should be considered next. Coatings are only useful if applied to the target surfaces that require them. If coatings are not applied to the necessary surfaces, they cannot provide the desired performance and the initial coating selection would be incorrect. While minimizing the impact on the underlying substrate is one obvious driver, processing considerations should also be included.

Coatings can be applied to surfaces using methods that are either line-of-sight, non-line-of-sight, or either, depending on the nature of the chosen coating. A good example of a line-of-sight process is the application of a thermal barrier coating to protect hot-section components in turbine engines from excessive heat.  Here, coating feedstock material is introduced into a very high-temperature plasma jet, melting raw coating materials and linearly propelling them directly onto a workpiece to attach and cool.   Processes like these do not lend themselves to applying such coatings on surfaces without a direct line of sight.  This contrasts with a non-line-of-sight process, such as applying a chemical conversion coating to aluminum alloys, where parts can be dipped into an immersion bath, and all external surfaces may be covered. While chemical conversion coatings can also be applied with line-of-sight processes, such as spraying with a spray gun, not all coatings can be applied by either means.  

By assessing processing methods during coating trade studies, known restrictions can be considered to narrow down the coatings worthy of further investigation. Coatings that cannot be applied to the required surfaces due to processing limitations should not be considered in final designs.

Durability and Adhesion throughout Service Life

Coatings are only useful if they remain adhered to the surfaces where they are applied, within the environments they are exposed to, throughout their required service life. Coatings must stick to and remain attached to their intended surfaces for as long as needed, while simultaneously providing the intended performance enhancements. These properties of durability and adhesion are central to coating selection, because if coatings do not remain where they should, performance loss will result in consequences whose severity depends on the type of enhancement lost. While small chips of a corrosion-resistant coating falling off a large, oversized bridge carrying heavy traffic may be minor, the same size chip of a thermal barrier coating falling off a hot-section turbine component of a power-generating engine could be detrimental.

Ideally, coatings would be durable throughout their entire service life.  But more often than not, a coating's ability to remain intact and attached degrades over time. Such degradation modes are typically coupled with the exposure environment and include mechanical wear, chemical makeup, temperature exposures and/or cycles, among other factors. Note that both degradation severity and degradation time are both significant factors and must be accounted for. For example, while a wear coating might remain intact during a rare, higher-than-normal loading condition over a short period, long-term or cyclic exposure at the same loading could result in premature wear. Additionally, while thermally grown oxides beneath some thermal barrier coatings can help control adhesion, excessive exposure to elevated temperatures can cause excessive oxide growth and result in coating spallation. In either case, where the coating is consumed or detached, underlying substrates become exposed, original designs are compromised, and product functionality is at risk.

Coatings that cannot survive the service life while maintaining their functions should not be considered in final designs. However, if coating degradation is accounted for upfront during initial coating selection, studies to assess the likelihood of coating survival during the expected service life, and/or design accommodations to compensate for degradation like coating maintenance can be useful tools for the design team when faced with limited coating options.

Final Thoughts

Coatings can certainly expand innovation opportunities for engineers.  Understanding their capabilities and limitations, however, is crucial for success when considering them in design.  Without clearly defining the intent, understanding application methods, or assessing coating durability and adhesion over the expected lifespan, coating failure could result and lead to unintended ramifications. Careful study regarding these key points will better enable engineers to make informed decisions for robust engineering solutions. It is important to recognize that the final coating choice need not be the best performer in all the aforementioned areas. Like other engineering design challenges where cost and schedule are highly influential, meeting minimum expectations for performance instead is often sufficient.

Thx, m