The science of aerospace coatings takes us beyond aerospace paints and aircraft interior coatings. Coatings play a huge role in the aircraft turbine industry.
Coatings for use at high temperatures on a superalloy substrate can be defined as a surface layer of material, either ceramic or metallic or combinations thereof, that is capable of precluding or inhibiting direct interaction between the substrate and a potentially damaging environment. This damage can either be metal recession due to oxidation or corrosion, or a reduction in substrate mechanical properties due to the diffusion of harmful species into the alloy at high temperature. Aerospace coatings used on superalloys do not function as inert barriers. Rather, they provide protection by interacting with oxygen in the environment to form dense, tightly adherent oxide scales that inhibit the diffusion of damaging species such as oxygen, nitrogen, and sulfur into the substrate. Coatings must therefore be rich in those elements (such as Al, Cr, or Si) that readily participate in the formation of these protective scales. Essentially, they are reservoirs of those elements; the supply is continually being used to reform new scale to replace that which spalls as a result of thermal cycling or mechanical damage. Thus, by the nature of its protective mechanism, the usable life of a coating is governed by its ability to form the desired protective scale and to retain or replace that scale as needed.
By far the largest use of coatings on superalloys is on components in the hotgas section of turbine engines, hot section components, that is, combustors, blades, and vanes. The need for such coatings surfaced in the aircraft engine business in the 1950s when it became apparent that substrate compositional requirements for improved high-temperature strength and optimum high-temperature environmental protection were not compatible. Increasing operating temperatures caused excessive oxidation of the high-strength nickel- and cobalt-base superalloys being used for turbine blades and vanes. This led to the development of the simple aluminide diffusion coatings that solved the oxidation problem. Several of these aluminide coatings are still in use today.
Hot corrosion first became a serious problem in those larger industrial and power generation gas turbines that burned low-quality fuels contaminated with sulfur, sodium, and other impurities, or that were located in areas where deleterious species Powdercoating could be ingested through the air intakes, for example, marine or desert environments. The aluminide coatings developed for aircraft engines to solve the oxidation problem were not effective in inhibiting severe hot-corrosion attack. This sparked development of other classes of coatings aimed specifically at combatting hot corrosion. More recently, another distinct mechanism of corrosion, known as low-temperature hot corrosion, was identified. Its successful inhibition has required coating compositions different from those developed for resistance to classical hot corrosion. Thermal barrier coatings (TBCs), which utilize a ceramic layer to reduce the temperature seen by the superalloy component, have been developed to permit substrate materials to be used at engine operating temperatures that might otherwise exceed their capability. Thus, different classes of coating compositions, and processes for applying them, have evolved to meet the differing needs of various applications.
The factors affecting coating selection are numerous. Obviously, environmental protection is the primary reason for using a coating, and this is governed by the design and application of the part. Possible effects of the coating or coating process on the mechanical or thermal properties of the superalloy to be coated must be considered, including the effects of interdiffusion between the coating and the substrate during high-temperature service exposure. Part geometry may govern what coating process is to be used since some application techniques are line-of-sight. Finally, the cost of aerospace coatings is always an important consideration and is often the controlling factor in selection.