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Increasing efficiency with the use of 2500 oF (1371 oC) tolerant uncooled ceramic blades provides an improvement in fuel consumption of more than 20% from an 1800 oF (982 oC) turbine inlet temperature. This rate represents almost a 50% improvement in specific air consumption. This improvementimplies that for the same sizeengine， power almost doubles， or conversely (and possibly more important to automakers)， engine flow size could be cut in half and retain the same horsepower output.
Ceramics are quite tolerant of such contaminants as sodium and vana-dium， which are present in low-cost fuels and highly corrosive to currently used nickel alloys. Ceramics are also up to 40% lighter than comparable high-temperature alloys-another plus in application. But the biggest plus is material cost. Ceramics cost around 5% the cost of super alloys.
Despite all the advantages ofceramics， they are brittle， and unless this problemisovercome， the use of ceramics in gas turbines will not be practical.
Coatings
Blade coatings were originally developed by the aircraft engine industry for aircraft gas turbines. Metal temperatures in heavy-duty gas turbines arelower than those in aircraft engines. However， heavy-duty gas turbines are generally subjected to excessive contamination or accelerated attack known as hot corrosion.
Blade coatings are required to protect the blade from corrosion， oxidation， and mechanical property degradation. As super alloys have become morecomplex， it has been increasingly difficult to obtain both the higher strength levels that are required and a satisfactory level of corrosion and oxidationresistance without the use of coatings.Thus， the trend toward higher firing temperatures increases the need for coatings. The function of all coatings is to provide a surface reservoir of elements that will form very protective andadherent oxidelayers， thus protecting the underlying base material from oxidation and corrosion attack and degradation.
Experience has shown that the lives of both uncoated and coated bladesdepend to a large degree on the amount of fuel and air contamination， as well as the operating temperature of the blade.The effect of sodium， acommon contaminant， on bucket life at 1600 oF (871 oC) is seen in Figure11-8.认hen sodium sulfate (Na 2SO4) ispresent， hot corrosion is greatly accelerated. Sodium sulfate is a product of combustion. The presence of only a few parts per million (ppm) of sodium and sulfate is sufficient to cause extensive hot corrosion damage. Sulfur is present as a natural contaminantin the fuel. Sodium can be introduced as a natural contaminant in thefuel， or in the atmosphere of sites located near salt water or contaminated areas.
The PT-Al coating is a precious metal applied by uniformly electroplatinga thin layer (0.00025 inch) of platinum onto the bucket at the airfoilsurface， followed by pack-diffusion steps to deposit a layer of aluminum and chro-mium. The resulting coating has an outer skin of an extremely corrosionresistant， platinum-aluminum intermetallic composition. As seen in Figure 11-8
60 50 40 IN738 + PtAI Coating30 20
IN738 Uncoated 10
U700 Uncoated 0
0.5 1.0 1.5 2.0 Equivalent Sodium (Fuel, Air, Water Mix), ppm
Percentiles
for Nat. Gas: 50% 90%
Commonly True Distillates: 50% 90%
Used Fuels Treated Ash Forming: 50% 90%
Figure 11-.. Effect of sodium corrosion on blade life
test was conducted for comparative corrosion on coated and uncoated IN-738 blades. The blades were run side-by-side in the same machine under severe corrosive conditions. The two blades were removed for interim eva-luation after11，300 service hours (289 starts). The unit burnt sour natural gas containing about 3.5% ppm sulfur and was located in a region where the soil surrounding the site contains up to 3% sodium.
The uncoated blade showed an 0.005 inch corrosion attack over 50%of the airfoil concaveface， with about 0.010 inch penetration at the base of the airfoil. Examination of the coated blade revealed no visual evidence ofattack， except for one small roughened spot on the leading edge about 1 inchup from the platform， and a second spot in the middle of the convex side about 1 inch down from the tip.
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