Creating an Effective Barrier
In the last several decades, thermally deposited ceramic coatings on metallic turbine blades have enabled turbine engines to operate at higher temperatures, and, according to the laws of thermodynamics, higher efficiencies.1,2 Ceramic thermal barrier coatings have also provided improved performance in turbine engines for propulsion and power generation. Applying a coating of a refractory insulating ceramic to metal turbine blades and vanes allows the engine to run at higher temperatures while minimizing deleterious effects on the metal blades.
Ongoing advances in high-tech materials are providing even more opportunities in these areas. By combining these new materials with a good understanding of coating engineering principles and application technologies, coating manufacturers will be able to offer additional performance improvements in the future.
Engineering for QualityTo improve coating performance, several important engineering principles must be considered regarding the quality of the ceramic coating. First, the coating material should be selected so that it is refractory enough to withstand the high temperatures at the surface and have a low bulk thermal conductivity to minimize heat transfer to the metallic blade underneath. In addition, the thermal expansion of the selected material should closely match that of the metallic substrate (> 9 x 10-6 C-1) to minimize potential stresses. Yttria stabilized zirconia (YSZ) (8 weight % yttria) is the industry standard “first generation” coating material in use today.
Second, the coating must have a grain and pore structure that will minimize thermal conduction to the metal-ceramic interface. A low-density coating is commonly formed using state-of-the art deposition processes and is excellent for providing an insulating barrier. The coating should have enough porosity so that it reduces the thermal conductivity while simultaneously adhering to the metal turbine bond-coat layer. A significant amount of microstructural engineering in thermal barrier coatings is ongoing, and the patent literature is full of tailored double- and triple-layered microstructures for special applications.
Finally, the coating should stick to the turbine blade during operation. Failure of the adhesion (spalling) would suddenly expose the metallic blade to high temperatures, causing severe corrosion, localized creep or melting. Generally, a metallic bond coat that shows good adhesion to both the metallic turbine and the ceramic coating is applied.
Applying with CareIt is also important that the ceramic coating be homogenously applied to the surface of the turbine blade. This is achieved by either electron beam physical vapor deposition (EB-PVD) or the arc-plasma sprayable (APS) powder method.
EB-PVD is the process currently recommended for high-quality coatings. In this technique, a cylindrical ingot of the coating material is vaporized with an electron beam, and the vapor uniformly condenses on the surface of the turbine blade. One of the most important advantages of the EB-PVD process is the strain-tolerant coating that is produced. This columnar strain-elastic structure (see Figures 1 and 2) is said to reduce the elastic modulus in the plane of the coating to values approaching zero, thereby enhancing the lifetime (in flight cycles) of the coating. Other advantages of EB-PVD ceramic coatings include excellent adherence to both rough and smooth surfaces. The final coating is also smooth, requiring no surface finishing. Additionally, the vapor deposition process does not plug small air-cooling holes in turbine blades during deposition.3 Figure 3 shows an X-ray pattern of a tetragonal zirconia EB-PVD coating.
In the APS powder application method, the ceramic material is in the form of a flowable powder that is fed into a plasma torch and sprayed molten onto the surface of the metallic substrate. Droplets of molten material form “splats” on the metallic substrate. Sprayed coatings have half the thermal conductivity of the EB-PVD coatings and are therefore better insulators (for the same 8% YSZ composition). The “splats” form a lamellar structure consisting of fissures with a non-uniform density and pore size (see Figure 4).
In contrast to EB-PVD coatings, APS coatings require a rough depositing surface for good adhesion. In addition, thermal-sprayed coatings are more prone to spalling, reducing the performance lifetime of the coating relative to EB-PVD coatings. Thermal-sprayed parts are also not as recyclable as parts coated by EB-PVD because the extensive spalling and extrinsic cracking cause the APS-coated parts to be damaged beyond repair. However, the equipment portability and lower production cost of APS often makes the process more commercially attractive than EB-PVD.
Considering the SourceAn under-appreciated facet to the thermal barrier coatings business is how the quality of the source material (ingot) relates to the quality of the final coating. For example, ingots for EB-PVD must have a high purity (>99.5%) and a consistent and uniform density and pore structure. If the ingots are too dense, they will undergo severe thermal shock when they encounter the electron beam.
It is also essential that the mechanical integrity of the ingot be maintained during the electron beam (EB) melting process. If this integrity is damaged, the surface of the molten pool will be disrupted, and molten liquid will seep into newly created cracks beneath the pool. Freshly exposed zirconia in contact with the molten liquid will evolve gaseous oxygen, which will eject molten material from the surface in a process called “spitting.” The presence of impurities with a high vapor pressure or the reaction of impurities to form high vapor pressure species could also cause spitting.
In an ingot of non-uniform density or porosity, closed porosity may exist. In this case, the release of trapped gas may also cause spitting or eruptions. Molten spits, when trapped in the coating, will cause defects and potential failure sites.3 The optimum density for an EB-PVD barrier coating ingot is usually in the range of 60-70% of theoretical density. If the density is below 60%, the material throughput is lessened. The photo below shows a variety of YSZ ingots for EB-PVD.
Arc-plasma sprayable powder must have a particle size large enough to flow through the plasma torch but not so large that the entire particle is not melted coming out of the plasma gun. In addition to the composition, the particle size, particle size distribution and flowability are important considerations for APS thermal spray powder.
Although YSZ has been the industry standard first-generation coating material, it has a number of drawbacks that hinder the improvement of thermal barrier coatings. One problem is its lack of phase stability at high temperatures. Three commonly formed phases exist in the zirconia-rich section of the zirconia-yttria binary system: cubic, tetragonal and monoclinic. Under operating or forming conditions, phase transformations can occur that cause mechanical stresses and promote spalling or bond coat failure.
In addition, while YSZ has a low thermal conductivity (2.3 W/m K),4 a refractory ceramic material with a lower thermal conductivity than YSZ would be desirable. If the coating progressively sinters and densifies while in service, the thermal conductivity will increase along with the thermal shock sensitivity. Therefore, materials at least as refractory as YSZ are required. It can also be difficult to match the thermal expansion of YSZ-containing coatings to the bond coat layer and the metal substrate. A great deal of research is currently under way to find improved materials for thermal barrier coatings.
A class of lanthanide zirconate pyrochlores (Ln2Zr2O7)4-6 might provide one solution. These materials have lower thermal conductivity than YSZ (1.5-1.8 W/m K),4 as well as improved phase stability over a wide range of compositions and temperatures. In addition, they are less susceptible than YSZ to sintering during operation, while showing a thermal expansion match to the bond-coat layer as good as or better than YSZ. The decreased thermal conductivity of the coating made with these materials would allow the turbine to run at a higher temperature and therefore increase the Carnot efficiency. It could also allow the turbine blade to remain cooler, retarding those thermal processes that lead to coating failure and increasing the useful lifetime of the turbine.
Research on other potential ingot materials is also ongoing. As the palette of available material choices increases, it might also become possible to create multiple-layer coatings using several different materials. Thermal expansion match and adhesion considerations will also continue to drive the selection of materials for thermal barrier coating applications.
Increasing OpportunitiesThermal barrier coatings are a low-cost, effective way to increase the operating temperature of metallic alloys used for turbine blades. As research on new materials and tailored microstructures comes to fruition, the operating temperature for gas turbines will increase, along with the performance lifetime. This will translate into an improved performance for all applications involving gas turbines, such as jet engines, turbines for power generation and a host of niche applications.
For more information:For more information about ingot materials for EB-PVD applications, as well as other advanced materials, contact Trans-Tech Inc., a subsidiary of Alpha Industries, 5520 Adamstown Rd., Adamstown, MD 21710; (301) 695-9400; fax (301) 695-7065; e-mail firstname.lastname@example.org or email@example.com; or visit www.trans-techinc.com.
References:1.Miller, R.A., “Thermal Barrier Coatings for Aircraft Engines: History and Directions,” Journal of Thermal Spray Technology, 6 , 1997, pp. 35-42.
2. Padture, N.P.; Gell, M.; and Jordan, E.H., “Thermal Barrier Coatings for Gas-Turbine Engine Applications,” Science, 296, 2002, pp. 280-284.
3. Lammermann, H. and Kienel, G., “PVD Coatings for Aircraft Turbine Blades,” Advanced Materials and Processes, 140 , 1991, pp. 18-23.
4. Wu, J.; Wei, X.; Padture, N.P.; Klemens, P.G.; Gell, M.; Garcia, E.; Miranzo, P.; and Osendi, M., “Low-Thermal-Conductivity Rare-Earth Zirconates for Potential Thermal-Barrier-Coating Applications,” Journal of the American Ceramic Society, 85 , 2002, pp. 3031-3035.
5. Maloney, M.J., “Thermal Barrier Coating Systems and Materials,” U.S. Patent No. 6117560, 2000.
6. Subramanian, R., “Thermal Barrier Coating Having High Phase Stability,” U.S. Patent No. 6258467 [B1], 2001.