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Coatings for High-Temperature Structural Materials Trends and Opportunities (1996) / Chapter Skim
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5 ENGINEERING CONSIDERATIONS
Pages 34-38

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From page 34...
... The beneficial properties of the new alloy were restored after a slight modification to the chemistry of the base material eliminated the formation of the brittle phase (Walston et al., 1993~. This case illustrates the sensitivity of carefully optimized superalloy compositions to slight changes in chemistry.
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From page 35...
... However, if there is a large CTE mismatch, even a ductile coating may crack or spell due to the large stresses and strains incurred during thermal cycling that result from thermal expansion mismatch. Another consideration is environmentally enhanced thermomechanical fatigue, which is a special case of low-cycle fatigue that is not adequately predicted with isothermal low-cycle fatigue data and models.
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From page 36...
... Thus, while the change in efficiency varies with engine operating conditions, there was a significant advantage to a smoother surface finish. While smooth surfaces improve aerodynamic performance, controversy exists regarding the type of surface finish that should be required for engine components, particularly airfoils.
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From page 37...
... Poor mixing is evidenced by the passage of unburnt hydrocarbons through the turbine.3 This often happens during startup, when the fuel-to-air ratio is high; the smell of unburnt hydrocarbons is usually detectable at airports. Incomplete combustion of the carbon can also result in the emission of CO from the combustor, which may not oxidize to CO2 3Some aviation turbines deposit dense pyrolitic carbon at the combustor, and eventually this detaches and passes through Me turbine where it may cause erosion or foreign object damage.
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From page 38...
... COATINGS FOR HIGH-TEMPERATURE STRUCI URAL MATERIALS combustor is moderated by cool air flowing over the inner surface, which is known as "film cooling." Film cooling can quench gaseous reactions in the combustion process near the wall and lead to enhanced CO and unburnt hydrocarbons emissions. TBCss can be used to shield the combustor liner from the combustion heat, reducing the amount of cooling air needed and lowering the amount of CO and unburnt hydrocarbons formed.
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Key Terms

  • surface finish
  • unburnt hydrocarbons
  • gas turbines
  • coating processes
  • heat transfer

  • coating process
  • surface roughness
  • turbine blades
  • combustion process
  • base material

  • strain tolerance
  • physical vapor
  • chemical vapor
  • operating conditions
  • grit blasting

  • combustion zone
  • film cooling
  • turbine section
  • environmental resistance
  • thermal expansion

  • surface treatment
  • sequential design
  • accommodate strain
  • thermal cycling
  • structural material

  • design process
  • aerodynamic efficiency
  • transfer rate
  • combustion products
  • gas turbine


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