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Coatings for High-Temperature Structural Materials Trends and Opportunities (1996) / Chapter Skim
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4 FAILURE MODES
Pages 26-33

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From page 26...
... The chapter concludes with a series of recommendations on future directions in research on environmental degradation modes. DEGRADATION MECHANISMS OF STRUCTURAL MATERIAIS Degradation mechanisms for structural materials are a function of the engine operating conditions, engine mechanical design, and the component base materials.
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From page 27...
... The structural materials degradation modes that are moderated by the traditional metallic protective coatings include oxidation and hot corrosion. In addition, TBCs insulate the underlying structure against the full effect of gas-path heat.
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From page 28...
... Figure 4-2 shows the result of high-temperature oxidation of a coating and its penetration to the base metal. In this example, an adherent, relatively uniform external oxide scale remains, but internal oxidation of the coating and base metal has occurred.
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From page 29...
... These failure modes are also a key concern in the retention of TBCs. These ceramic coatings are inherently more vulnerable to impact damage and erosion than metallic coatings.
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From page 30...
... A CASE STUDY: DEGRADATION OF THERMAL BARRIER COATINGS Experimental and theoretical thermal characterization of TBCs has shown that they can provide significant thermal benefits to components in operation, but only if coating integrity is maintained. TBCs are applied on top of a MCrAlY coating; the TBC provides thermal insulation, and the MCrAlY protects the substrate against oxidation and hot corrosion.
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From page 31...
... Spallation TBC spallation is a mechanical event that results in the removal of the ceramic layer by delamination from the bondcoat. Numerous processes may trigger this mechanical event, but the two main culprits are bondcoat oxidation and the strain generated by a thermal expansion mismatch between the ceramic top coat and the metallic components of the system (Miller and Lowell, 1982; Strangman, 1985; Hillery et al., 1988; Demasi-Marcin et al., 1989~.
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From page 32...
... The difference apparently arises because POD coatings are less susceptible to thermal cycling damage (Mannin~-Meier et al., 1991~. Continued growth of the oxide scale is believed to increase the compressive stresses applied to the ceramic layer during thermal cycling (Rigney et al., 1995~.
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From page 33...
... BITE ~ Research ~ roqui~d to improve Ad apply models that can help optimize coating development and to provide more accurate csUmmes of the remaining coadng IN for in-se~icc cDgines. More speciAcaIly, advances To requiIcd in (1)
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Key Terms

  • failure modes
  • ceramic layer
  • oxide scale
  • thermal expansion
  • protective scale

  • structural materials
  • typically occurs
  • operating temperatures
  • thermomechanical fatigue
  • thermal barrier

  • molten salts
  • protective oxide
  • potassium sulfates
  • compressive stresses
  • thermal cycling

  • typically sodium
  • expansion mismatch
  • degradation modes
  • environmentally induced
  • base metal

  • metallic coatings
  • degradation mechanisms
  • impact damage
  • tbc failure
  • engine operating

  • crack growth
  • coating failure
  • amplitude cyclic
  • delamination cracks
  • fatigue cracking


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