2. Coating Thickness that provides a greater protective reservoir ifthicker. However， thicker coatings may have lower thermal fatigue resistance.
3.
Standard of deposition such as thickness uniformity， or defined thick-ness variation and coating defects.
100
90
80
70
60
PtAl
50 Plasma40 Plasma+TBC
30
20
10
0
There are three basic types ofcoatings， thermal barrier coatings， diffusioncoatings， and plasma sprayed coatings. The advancements in coating have also been essential in ensuring that the blade base metal is protected at these high temperatures. Coatings ensure that the life of the blades are extendedand in many cases are used as sacrificial layer， which can be stripped and recoated. The general type of coatings is very little different from the coat-ings used 10-15 years ago. These include various types of diffusion coatings such as Aluminide Coatings originally developed nearly 40 years ago. The thickness required is between 25-75 μmthick， these coatings consisted of NijCo二 about 30% Al. The new aluminide coatings with Platinum (Pt)increase the oxidation resistance， and also the corrosion resistance. Platinumin the coating increases the activity of aluminum in the coating， enabling a very protective and adherent Al2O3 scale to form on the surface.
Coatings developed some 30-35 years ago， commonly known as MCrAlY， have a wide range of composition tailored to the type of performance required and are Ni/Co based as shown in these three common types of coatings:
1.Ni，
18%Cr， 12%Al， 0.3% Y
2.Co，
29%Cr， 3%AI， 0.3% Y
3.Co，
25%Ni， 20%Cr， 8%Al， 0.3% Y
These coatings are usually 75-500 μm thick and sometimes have other minorelement additions used to improve environmental resistance such asPt，Hf，Ta， and Zr. Carefullychosen， these coatings can give very good performance.
The thermal barrier coatings have an insulation layer of 100-300 μmthick， and are based on ZrO2 -Y2O3 and can reduce metal temperatures by 90-270 oF (50-150 oC). This type of coating is used in combustioncans，transition pieces， nozzle guide vanes， and also blade platforms.
Oxidation Corrosion Cracking
The interesting point to note is that some of the major manufacturers are switching away from corrosion protection biased coatings to coatings thatare not only oxidation resistant， but also oxidation resistant at higher metal temperatures. Thermal barrier coatings are being used on the first few stages in all the advanced technology units. The use of internal coatings is gettingpopular due to the high temperature of the compressor discharge， which results in oxidation of the internal surfaces. Most of these coatings are aluminide type coatings. The choice is restricted due to access problems toslurrybased， or gas phase/chemical vapor deposition. Care must be taken in production otherwise internal passages may be blocked. The use of pyrom-eter technology on some of the advanced turbines has located blades with internal passages blocked causing that blade to operate at metal tempera-tures of 50-100 oF (28-56 oC) higher than the neighboring blades.
Shroud Coatings
New high temperature gas turbines operate at considerably higher tem-peratures than previous heavy-duty gas turbines. Therefore， to provide adurable stationary shroud component， coatings are being used to coat thesurface of this high-temperature， inner shroud component. The coating of shrouds was developed and has been used extensively in aircraft engines. This provides an extremely oxidation-resistant surface and a rub-tolerant coating in the event that the blade tips rub against the stationary shroud. The coating also reduces the leakage between the blades and the shroud thus reducing tip losses.
Future Coatings
The investigation of even more corrosion-resistant coating materials has been an area of intensive research and development for the past few years. The goals of this research are to further improve the oxidation-resistance and thermal fatigue resistance of high-temperature bucket coatings. In addi-tion to these environmentally resistant coating developmentefforts， work is also underway to develop advanced thermal barrier coatings (TBCs) for application to stationary and rotating gas path components. By carefulprocess control， the structure of these TBCs may be made more resistant to thermal fatigue and their lives greatly extended.
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