The high-pressure turbine of a jet engine provides one of the most severe environment faced by man-made materials. To temperatures approaching the substrated melting point, one must add the considerable stresses caused by rotation at more than 10,000 rpm.

With today's jet engine operating temperatures, thermal barrier coating failure results in melting of the blade. But even without reaching such catastrophic failure, blades suffer from accelerated oxidation and, depending on the environment, hot corrosion. Coatings can considerably enhance the oxidation/hot corrosion resistance of these components, as illustrated below:

The result of 2500 h low altitude sea flight service on an uncoated and NiAl coated blade turbine blade, from Eskner, 2004.

Oxidation is the reaction between the coating (or in its absence, base alloy) with the oxidants present in the hot gases.
Hot corrosion occurs from surface reactions with salts deposited from the vapour phase.

As discussed in the next section, different service conditions result in different degradation mechanisms being predominant.
In addition of oxidation and hot corrosion, coatings will evolve through diffusion with the substrate alloy, as they are not in thermodynamic equilibrium with the latter.
This is of concern, not only because it may modify the carefully designed mechanical properties of the substrate, but also because loss of Al to the substrate reduce the oxidation life of the coating.

Bibliography

1. Eskner M., PhD thesis, Royal Institute of Technology, Stockholm, 2004