Present day TBCs generally consist of a yttria-stabilized zirconia (YSZ) coating deposited onto an oxidation-resistant bond-coat alloy that is first applied to a nickel-based superalloy component. In diesel engine applications where the temperatures are usually lower, the YSZ coating is generally applied directly onto the alloy. Two main types of coating are in use. For relatively small components such as blades and vanes in aerospace turbines, the coatings can be applied by electron-beam physical vapor deposition (EB-PVD). For larger components such as the combustion chambers and the blades and vanes of power generation, stationary turbines, the coatings are usually applied by plasma-spraying (PS). In many respects, the choice of materials and their production represent a mature materials technology. While improvements in their capabilities continue, there is a growing realization that new TBC systems will be required for the next generation turbines presently being designed. To set the stage for coming developments, we first review the selection of materials used in present YSZ coatings, some of the new insights that have been gained in understanding how YSZ coatings alumina, and a number of alloys including nickel-based superalloys are crossplotted against thermal conductivity. In the absence of any strain compliance, for instance due to a decreased elastic modulus, the large elastic mismatch would generate very large stresses and lead to spontaneous failure on cooling. Second, the coating material must exhibit thermodynamic compatibility with the oxide, usually aluminum oxide, formed on the bond-coat alloy at high temperatures. Third, with the continual quest to run engines at higher temperatures and the increasing dif- ficulty of increasing the metal temperature, it is increasingly likely that designers will seek “prime reliant” coatings, namely ones that can be used with assurance that they will not fail. Prime reliant thermal coatings are ones that are necessary to prevent the temperature of the metal from exceeding its maximum temperature, much in the same way that the tiles on the space shuttle prevent the underlying aluminum airframe from being exposed to temperatures in excess of their melting temperature on re-entry.
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