(Courtesy of Nitin Padture, The Ohio State University. Reprinted with permission from Padture, N. P., et al., Science (2002) 296, 280. © American Association for the Advancement of Science.)Because of their comparatively low cost, gas turbines are providing an increasing fraction of the base-load electric power generating capacity around the world. Likewise, gas turbines provide the power to propel aircraft and ships, or pump fluids, etc.
Obviously, with the increasing cost of fossil-derived fuels, all these systems could profit from an improvement in thermal efficiency. For gas turbines, this correlates directly to a higher gas temperature at the turbine inlet. Thus, since the mid-1970s, gas-turbine manufacturers have been developing thermal barrier coatings (TBCs) to permit an increase in temperature of the hot combustion product gas without a corresponding improvement in the temperature capability of the Ni- and Co-base superalloys that comprise the blades, vanes, and combustor components.
A TBC is a thin (100–500 μm), thermally insulating ceramic coating that is applied over the conventional oxidation-resistant aluminide diffusion coating on the superalloy (see figure). The TBC must provide significant thermal insulation at the external surface of the part while remaining adherent, i.e. providing compliance to thermal stresses introduced by low- and high-frequency thermal cycles.
As the purpose of the TBC is to permit an increase in the hot gas temperature without a corresponding increase in the metal temperature, the coating is only applied to parts provided with internal air cooling channels. In this way, the temperature gradient across the insulating ceramic overcoat achieves a drop of 100–300°C – an important fraction of the temperature of the hot gas.
In the best case scenario, the added TBC does not significantly affect the performance of the underlying traditional superalloy/coating substrate.
The ceramic currently used for TBCs is a yttria-stabilized zirconia (YSZ) with 6–8 wt.% yttria. This exists as a thermally stable tetragonal (t') phase. YSZ has one of the lowest thermal conductivities of all ceramics and a thermal expansion coefficient that approaches that for superalloy substrates.
To provide compliance to stresses induced by thermal cycling, the overcoat is sometimes deposited (via an electron-beam physical vapor deposition process) as a fine columnar structure perpendicular to the substrate. Otherwise, a less expensive plasma spray process yields a porous, defective, but compliant structure.
The real problem is to achieve and maintain adherence to the substrate, a problem that is exacerbated because the aluminide diffusion coating in contact with the TBC inherently exhibits a series of protruding ridges that must been removed by physical or chemical polishing prior to TBC deposition.
Of course, even after a TBC has been carefully and successfully deposited onto the substrate, the particular service environment can conspire to cause problems or even coating failure. For example, residual fuels rich in V can deposit a fused alkali vanadate on the coating that may leach out the yttria component and ‘destabilize’ the structure. Sometimes, the ingested combustion air contains an alkaline earth silicate deposit that ‘freezes up’ the stress-compliant columnar structure of the ceramic. With naval gas turbines, the deposition of sea salt can introduce a hot corrosion problem for the substrate.
At best, the growth of an alumina protective scale beneath the TBC already has its own problems with adherence as the scale thickens upon exposure. Depending upon the service environment, any TBC would have a restricted life, and scheduled refurbishment would be required.
TBC coatings are currently in use for military and commercial aircraft, but the application is associated with the risk that failure (spallation) of the TBC would expose the critical metallic engine parts to excessively high temperatures causing reduced service lives and performance. YSZ TBCs are well suited, and find application, in electric utility gas turbines where long campaigns with minimal temperature cycling are the norm. Certainly, as new improvements are made to TBCs, the coatings will find applications in more systems where energy efficiency can be improved.