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Figure 11-9. .lade coating re.uirements and coating evolution
Hot corrosion is distinctly different from the pure oxidation of an aircraftenvironment;hence， coatings for heavy-duty gas turbines have different capabilities compared to coatings for aircraft engines. In addition to hotcorrosion， high-temperature oxidation and thermal fatigue resistance have become important criteria inthe higher firing gas turbines， as shown inFigure 11-9. In today's advanced machines， oxidation is of concern not onlyfor external blade surfaces， but also for internal passages such as coolingholes， due to the high temperature of the coolingair， which in turn is due to the high pressure ratio in the compressor.
The main requirements of a coating are to protect blades against oxida-tion， corrosion， and cracking problems. Coatings are there to prevent the base metal from attack. Other benefits of coatings include thermal fatiguefrom cyclic operation， surface smoothness and erosion in compressor coatings， and heat flux loading when one is considering thermal barriers. A sec-ondary consideration， but perhaps rather more relevant to thermal bar-riers， is their ability to tolerate damage from light impacts without spalling to an unacceptable extent because of the resulting rise in the local metaltemperatures. Coatings also extendlife， provide protection by enduring theoperational conditions， and protect the blades by being sacrificial by allow-ing the coating to be restripped and recoated on the same base metal.
The past and future trends in the development of coatings are shown in Figure 11-10. Present-day coatings last 10-20 times longer than coatings used 10 years ago. Coated blades last up to two times longer than uncoated blades in the field. Figure 11-11 is a comparison between the various types of
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coatings on the comparative resistance in the areas of Oxidation，Corrosion， and Cracking. To improve the oxidation protection an increase of aluminum content in the outer region of the coating matrix is needed. The higher aluminum content forms a more protective aluminum oxide layer that greatly improves the high-temperature oxidation resistance.
Life of coatings depends onComposition，Thickness， and the Standard of Evenness to which it has been deposited. Most of the new coatings are applied by vacuum Plasma Spray technique to ensure that the coating has been applied in a uniform and controlled manner. Coatings help extend the life ofbladings by protecting them against Oxidation， Corrosion， Cracking， ThermalFatigue， Temperature excursions， and foreign object damage (FOD) damage.Oxidization is a prime consideration in ""clean fuel'' regime， while corrosion is due to higher metal temperatures and emphasis in not so clean a fuel.
For a given combination of loadings， coating life is governed by:
1. Composition of the coating that includes environmental and mechan-ical properties such as thermal fatigue.
