Principles of Superalloy Protective Coating

A superalloy is a metal specially designed for applications requiring a high level of resistance to elevated temperatures, tensile strength, and resistance to corrosion. These alloys are usually categorized under three possible types: a cobalt base, a nickel base, and a nickel-iron base. Superalloys are often used in chemical conversion plants, industrial turbines, and aerospace turbines. Although they have high resistance properties, some superalloys may need additional protective coatings to maintain performance levels, particularly if they are employed under temperatures near their incipient melting points or under extreme load-bearing conditions.

Most superalloy coatings are used to shield components from potentially harmful environmental effects and stresses, with an emphasis on increasing heat tolerance and structural integrity for materials functioning at temperatures up to 1,850 degrees Fahrenheit (F) and higher. A protective coating is a layer of material that blocks or inhibits interaction between a substrate and damaging environmental conditions. This damage can take the form of metal wastage from oxidation and corrosion, or a loss of mechanical properties from high-temperature diffusion of contaminants into the substrate. Most protective coatings are designed to shield the superalloy substrate from these effects.

Coating Compatibility

Most coatings that are applied to superalloys are not designed to be in equilibrium with the substrate, meaning they are not inert coatings and feature chemical and physical properties different from those of the material they protect. Through the reactions between aluminum and chromium with environmental oxygen, superalloy coatings create a dense, closely bonded oxide scale that impedes the diffusion of contaminants, such as nitrogen or sulfur, into the substrate. The coatings need to contain enough reactive aluminum, chromium, and silicon to continuously form the protective scale, but they must also be moderately compatible with the substrate.

For compatibility, it is important to use coating materials and application methods that reduce the potential for unwanted reactions between the coating and the substrate, as well as diffusion of coating elements into the base surface. These problems can cause material flaws, such as cracking, spalling, or void formation, that undermine the mechanical properties of the superalloy. Differences in thermal expansion rates can also affect compatibility, so coatings that feature some degree of ductility are usually effective.

Oxidation Effects

Along with hot corrosion, oxidation is one of the two basic types of deteriorative effects that superalloy coatings are designed to prevent. Oxygen reactions are some of the most common environmental threats for alloy materials, and although superalloys continue functioning with low risk at temperatures below 1,600 F, at higher temperatures oxidation can seriously degrade material quality and performance, particularly for nickel and cobalt base superalloys. At lower temperature levels, chromium content is the primary defense against oxygen reactions, while at higher temperatures (above 1,800 F), aluminum becomes more important because it is the base in a compound that forms a protective oxide scale.

When a superalloy’s aluminum content is insufficient for preventing oxidation, a protective coating can be used to maintain service life by preventing selective oxidation at the grain boundaries and surface carbides, and by inhibiting internal oxidation. Chromium can sometimes decrease high-temperature mechanical strength, so many superalloys include a lower chromium content to maintain tensile strength, but this lowers their hot corrosion resistance, necessitating a hot corrosion protective coating to preserve performance levels.

Hot Corrosion Effects

Hot corrosion is one of the most common accelerated oxidation processes, and is usually caused by environmental contaminants, such as salt, or the sulfur found in fuel. Although the chromium content in most cobalt base and nickel-iron base superalloys is usually sufficient for impeding hot corrosion, some nickel base varieties are prone to this deteriorative effect, particularly those with improved rupture strength at higher temperatures. Protective coatings, especially overlay coatings, and sometimes environmental inhibitors may be needed to shield high-strength nickel base superalloys.

Coating material can diffuse inward into the base metal, which may cause a decrease in the metal’s incipient melting temperature. Once a superalloy has been heated past its incipient melting point, it causes a drop in grain-boundary strength and ductility and these properties cannot be restored via heat treatment. The degree of oxidation and hot corrosion usually determines the upper temperature limit for a superalloy, while wrought alloys used in lower temperature applications, such as rotating seals or turbine disks, can generally function without protective coatings.

For extensive resources on superalloys, visit the Cambridge University Superalloy Resource.