2. Contents:
• Thermal barrier coatings (TBCs) for superalloys.
• Ceramic–matrix composites (CMCs) as replacements for superalloys
• Thermal/ environmental barrier coatings (T/EBCs) for CMCs
3. Aerospace propulsion system
• Aerospace propulsion systems are broadly divided
into air-breathing engines and rocket engines3 . As
the name suggests, air-breathing engines use
atmospheric air, and are further classified into
rotating gas-turbine engines (subsonic, supersonic)
and hypersonic engines (>Mach 5) without rotating
parts
4. Thermal barrier coatings
• he introduction of ceramic TBCs has boosted the maximum temperature in the
hottest part of the gas-turbine engine (gas inlet) to unprecedented levels
(>1,500 °C), resulting in extraordinary efficiency and performance gains, and a
cleaner exhaust.
• TBCs are thin oxide-ceramic coatings (100 μm to 1 mm in thickness) applied to
metallic (typically Ni-based superalloys) components in the hot section of the
engine.
5. Thermal
Barrier coating
Composition
• TBCs, typically made of ZrO2 partially stabilized by
~7 wt% Y2O3 (7YSZ), have worked remarkably
well. They have sufficient porosity and
microstructural defects to reduce their thermal
conductivity and make them compliant (strain-
tolerant) in accommodating thermal strain. Most
importantly, 7YSZ falls in a narrow composition
range where the ferroelastic toughening
mechanism is active, making 7YSZ TBCs
mechanically robust
6. Limitations of 7YSZ:
• First, 7YSZ TBCs begin to lose their phase stability above ~1,300 °C and their
strain tolerance due to sintering
• Second, although 7YSZ TBCs have low thermal conductivity (~1 W m–1 K–1),
there is a need for TBCs with even lower thermal conductivities
• 8 . Third, at a TBC surface temperature above ~1,200 °C, silicates ingested by
the engine from the atmosphere (runway debris, dust, sand, volcanic ash) melt
and deposit on the TBC surface. The molten silicate, collectively referred to as
CMAS (calcia–magnesia– alumino–silicate), penetrates deep into 7YSZ TBCs
causing them to fail prematurely
7. Alternative of 7YSZ
• TBC compositions (for example, Gd2Zr2O7, ref. 11; 2ZrO2·Y2O3, ref. 12) are
being pursued that have higher temperature capabilities and lower thermal
conductivities, but are also resistant to CMAS attack8,10. However, those
compositions lack the ferroelastic toughening that is unique to 7YSZ.
8. Ceramics Metal Composites (CMC’s)
• CMCs are inherently lightweight, about
a third of the weight of superalloys, and
hence have high specific strength.
• CMCs are also more resistant to high-
temperature oxidation and creep
compared with superalloys.
• Unlike bulk ceramics, CMCs are
damage-tolerant and notch-insensitive
9. Ceramics Metal Composites (CMC’s)
• Typical CMCs comprise SiC-based matrix reinforced by SiC fibres, with a
moderately weak fibre/ matrix interface (for example, BN, C) that enables
extensive crack bridging by the fibres and frictional pull-out, imparting damage
tolerance
• Carbon fibres are also used to reinforce SiC matrices to result in higher-
strength C/SiC CMCs
• Oxide CMCs on the other hand are oxidation-resistant, but have lower
strength and creep resistance
10. Thermal/environmental barrier
coatings
• The popular SiC-based CMCs are subject to active oxidation and recession in
the water vapour containing high-velocity hot-gas stream, which have been
attributed to the formation and volatilization of Si(OH)4 primarily
• Thus, SiC-based CMCs need to be protected by ceramic EBCs
11. Composition of EBC’s
• T/EBCs are invariably multilayered, with the first layer being the bond coat.
Silicon is found to be a good bond-coat material, but it melts at 1,414 °C. Thus,
Si-based bond-coat materials with higher melting points are being considered,
prime among them are RE–Si alloys with Hf and/or Zr additions (RE represents
a rare earth element)
• Typically, the second layer is the dense, low-Coefficent of thermal expansion
EBC, where RE silicates with various additions (for example, HfO2 or Al2O3)
are being considered
12. Composition of EBC’s
• The fourth layer (the overcoat) performs the function of a TBC, which has (i)
low thermal conductivity and scatters photons, (ii) high strain-tolerance, and
(iii) resistance to CMAS attack
