The role of thermal barrier coatings (TBC) is, as their name suggests, to provide thermal insulation of the blade. A coating of about 1-200 μm can reduce the temperature by up to 200 C.

A TBC can be used either:

• to reduce the need for blade cooling (by about 36%), while maintaining identical creep life of the substrate.
• increase considerably the creep life of the blade while maintaining level of blade cooling (and therefore allowing the blade to operate at lower temperature for an identical turbine entry temperature).

The benefits of TBC in terms of engine efficiency are comparable to those brought be the replacement of directionally solidified alloys for single crystal ones (Strangman, 1985).

Because they allow surface temperatures higher than the melting point of the substrate, ensuring the integrity of TBCs is critical. Understanding the mechanisms of TBC failure is an active area of research. The failure is linked with large residual compression in the underlying thermally grown oxide (TGO), but details of the mechanism only begin to be understood (Evans et al., 2001).

Bibliography

• Strangman T. E., Thin Solid Films, 127:1985, 93--105, 'Thermal barrier coatings for turbine airfoils'.
• Evans A. G. et al., Prog. Mater. Sci., 46:2001, 505-553, 'Mechanisms controlling the durability of thermal barrier coatings'.