The following table compares the severity of the different surface-related problems for gas turbine applications:

Recent generations of superalloys for single crystal turbine blades contain relatively high percentages of refractory elements such as Ta, W or Re which enhance the high-temperature mechanical properties (SCT 1997 CHEN).

This, however, is done at the expense of Cr and Al. Given the severe environmental conditions in which the blades operate, the removal of the elements (beneficial for oxidation resistance) implies even greater degradation problems.

To palliate for this lack of appropriate oxidation.corrosion resistance, an external coating is applied to the blades. Its purpose is to allow for the growth of a resistant oxide layer. Of all possible oxides, α-Al₂O₃ offers excellent protection and very low growth rates (in a minority of cases, Cr oxides are preferred). The composition of the coating must therefore be chosen carefully so as to ensure growth of α-Al₂O₃.

Optimum coating composition in relation to oxidation and hot-corrosion resistance, after M. Schütze, Corrosion and Environmental Degradation Vol. II, Wiley-VCH, 2000.

The two most widely used types of coatings are aluminides (NiAl or Ni₂Al₃) and MCrAlY (`emcrawlee', where M is Fe and/or Cr) coatings. The formers are obtained by surface enrichment by diffusion, the later by plasma spray or EBPVD. An additional ceramic coating is often applied to high-temperature components (TBC, thermal barrier coating). In this context, the oxidation resistant coatings also provide a `transition' layer on which the TBC adheres better than on the substrate. For this reason, the oxidation resistant layer is also referred to as bond coat.

When the Al content of the coating is too low, other oxides than α-Al₂O₃ may form, the nature of which depends on the exact composition of the coating. For reasons which will be explained later on, this can result in failure and is therefore to be avoided.

Blade integrity is now critically dependent on these coatings and possibly additional ceramic thermal barrier coatings. Unfortunately, `prime-reliability', the concept of a coating whose life does no longer condition that of the blade, has not been achieved yet. With the increasing cost of the blades themselves, the practise of coating renewal has developed. In this operation, a gas turbine is taken apart, each blade is dismantled, its surface cleaned and a new coating applied.

With, on one hand, the prospect of coating failure leading to catastrophic blade loss, and on the other hand an extremely expensive maintenance process, it is no surprise that much research is dedicated to improving the evaluation of remnant life.