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High-temperature materials and creep

Which statement about the structure of a thermal barrier coating (TBC) applied to gas turbine blades is correct?

  1. ABeneath a low-conductivity zirconia (YSZ) top coat lies a bond coat such as MCrAlY, and a thin Al₂O₃ thermally grown oxide (TGO) forming at their interface protects the substrate from oxidation
  2. BThe top coat is dense, highly conductive alumina that quickly carries heat out of the blade to cool the metal, while the bond coat absorbs the thermal expansion mismatch with the substrate
  3. CThe YSZ top coat lets no oxygen through at all, so no bond coat is needed, and YSZ sprayed directly onto the substrate blocks both oxidation and heat
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Answer: A. Beneath a low-conductivity zirconia (YSZ) top coat lies a bond coat such as MCrAlY, and a thin Al₂O₃ thermally grown oxide (TGO) forming at their interface protects the substrate from oxidation

A TBC is a coating that lowers the metal temperature with an insulating surface layer while the inside of the blade is cooled by cooling air. The YSZ (yttria partially stabilized zirconia) top coat has a low thermal conductivity of about 1 to 2 W/(m·K) and, for a ceramic, a large thermal expansion coefficient. It is made as a porous sprayed coating or a columnar vapor-deposited coating to relieve the strain from the thermal expansion mismatch with the substrate. YSZ conducts oxide ions well and cannot stop oxygen, so Al in the bond coat oxidizes to form a thin Al₂O₃ TGO, which protects the substrate from oxidation. When the TGO thickens or undergoes thermal cycling, the top coat spalls from near it, which is the main factor limiting the life. A material with high thermal conductivity cannot serve as insulation.

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More High-temperature materials and creep questions

  1. A heat flux of 1.0 MW/m² passes uniformly through the thickness of the top coat of a thermal barrier coating (thickness 250 µm, thermal conductivity 1.2 W/(m·K)). Treating it as steady one-dimensional heat conduction, which is closest to the temperature difference between the two faces of the top coat?
  2. Which statement about the three stages of a creep curve (strain versus time) measured at constant load and temperature is correct?
  3. The steady-state creep rate of a heat-resistant alloy follows the power law ε̇ = Aσⁿexp(−Q/RT) with a stress exponent of n = 5. If the stress is raised by 20% at the same temperature, by roughly what factor does the steady-state creep rate increase?
  4. An alloy has a steady-state creep rate that follows ε̇ = Aσⁿexp(−Q/RT) with an activation energy of Q = 300 kJ/mol. If the temperature is raised from 1000 K to 1050 K at the same stress, by roughly what factor does the steady-state creep rate increase? Take R = 8.314 J/(mol·K).
  5. A fine-grained polycrystalline metal was creep tested at a low stress at about 0.5 times its melting point (in absolute temperature). The steady-state creep rate was nearly proportional to stress (stress exponent n ≈ 1) and nearly inversely proportional to the cube of the grain size. Which creep mechanism is most likely to be operating?