Oxidation of high-temperature coatings

Nickel-based superalloys used in stressed situations at high temperatures tend to be optimized for mechanical properties and have insufficient aluminium to form a continuous surface layer in service to provide long-term protection against oxidation. This deficiency can be met through the use of coatings that have higher aluminium contents and which form a protective alumina layer during service. These can take the form of metallic overlays or diffusion coatings, both of which can also form a bondcoat layer within thermal barrier coatings (TBCs). Factors which lead to loss of aluminium through oxidation and interdiffusion are discussed in this article. A consequence of excessive aluminium depletion is the formation of non-protective oxides which can be particularly deleterious in TBCs.