DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention is generally applicable to components subjected to high temperatures, and particularly to components such as the high and low pressure turbine nozzles and blades, shrouds, combustor liners and augmentor hardware of gas turbine engines. An example of a high pressure turbine blade 10 is shown in FIG. 1 . The blade 10 generally includes an airfoil 12 against which hot combustion gases are directed during operation of the gas turbine engine, and whose surface is therefore subjected to hot combustion gases as well as attack by oxidation, corrosion and erosion. The airfoil 12 is protected from its hostile operating environment by a thermal barrier coating (TBC) system 20 schematically depicted in FIG. 2 . The airfoil 12 is anchored to a turbine disk (not shown) with a dovetail 14 formed on a root section 16 of the blade 10 . Cooling passages 18 are present in the airfoil 12 through which bleed air is forced to transfer heat from the blade 10 . While the advantages of this invention will be described with reference to the high pressure turbine blade 10 shown in FIG. 1 , the teachings of this invention are generally applicable to any component on which a thermal barrier coating may be used to protect the component from a high temperature environment.

[0016] The TBC system 20 is represented in FIG. 2 as including a metallic bond coat 24 that overlies the surface of a substrate 22 , the latter of which is typically a superalloy and the base material of the blade 10 . As is typical with TBC systems for components of gas turbine engines, the bond coat 24 is an aluminum-rich composition, such as an overlay coating of an MCrAlX alloy or a diffusion coating such as a diffusion aluminide or a diffusion platinum aluminide of a type known in the art. Aluminum-rich bond coats of this type develop an aluminum oxide (alumina) scale 28 , which is grown by oxidation of the bond coat 24 . The alumina scale 28 chemically bonds a thermal-insulating ceramic layer, or TBC 26 , to the bond coat 24 and substrate 22 . The TBC 26 of FIG. 2 is represented as having a strain-tolerant microstructure of columnar grains 30 . As known in the art, such columnar microstructures can be achieved by depositing the TBC 26 using a physical vapor deposition technique, such as EBPVD. The present invention is particular directed to yttria-stabilized zirconia (YSZ) as the material for the TBC 26 . A suitable composition for the YSZ is about 2 to about 20 weight percent yttria, more preferably about 3 to about 8 weight percent yttria. However, the invention is believed to be generally applicable to zirconia-based TBC, which encompasses zirconia partially or fully stabilized by magnesia, ceria, calcia, scandia or other oxides. The TBC 26 is deposited to a thickness that is sufficient to provide the required thermal protection for the underlying substrate 22 and blade 10 , generally on the order of about 75 to about 300 micrometers.

[0017] While much of the following discussion will focus on columnar TBC of the type shown in FIG. 2 , the invention is also believed to be applicable to noncolumnar TBC deposited by such methods as plasma spraying, including air plasma spraying (APS). The microstructure of this type of TBC is represented in FIG. 3 , in which the same reference numbers used in FIG. 2 to identify the columnar TBC 26 on a substrate 22 and bond coat 24 are now used to identify a similar substrate 22 and bond coat 24 on which a noncolumnar TBC 26 was deposited by plasma spraying. In the plasma spraying process, TBC material is deposited in the form of molten “splats,” resulting in the plasma-sprayed TBC 26 of FIG. 3 having a microstructure characterized by splat-shaped (i.e., irregular and flattened) grains 30 and a degree of inhomogeneity and porosity.

[0018] As a result of the process by which the TBC 26 of either FIG. 2 or 3 is deposited, the individual grains 30 of the TBC's 26 are characterized by a uniform dispersion of alumina particles and/or precipitates 32 (hereinafter, particles) within the grains 30 and at and between the grain boundaries. According to the invention, the alumina particles 32 perform the function of improving the fracture toughness of YSZ, which is believed to promote the overall impact and erosion resistance of the TBC 26 if present in sufficient amounts in the form of a fine limited dispersion within the TBC 26 , without discrete and homogeneous layers of alumina, and without creating abrupt compositional interfaces that would promote spallation attributable to weak (low-toughness) interfaces between the dissimilar TBC materials (YSZ and alumina). More particularly, the alumina particles 32 are believed to increase the hardness, bend strength, elastic modulus and fracture toughness of the TBC 26 . Improved impact resistance of the TBC 26 is believed to result from increased fracture toughness, while improved erosion resistance is believed to occur as a result of increased fracture toughness, fracture strength, bend strength, hardness and elastic modulus of the TBC 26 . Additional potential benefits include thermal stabilization of the YSZ, which retards the gradual increase in thermal conductivity observed with YSZ TBC and associated with densification and/or sintering of YSZ at high temperatures, e.g., above 1000EC. In addition to having hardness, strength (bend, compressive and tensile) and an elastic modulus greater than that of YSZ, the alumina particles 32 are insoluble in YSZ and remain thermodynamically stable with YSZ at elevated temperatures to which the TBC 26 will be subjected within the environment of a gas turbine engine.

[0019] The alumina particles 32 are preferably present in an amount of at least 5 volume percent of the TBC 26 in order to contribute to the erosion and impact resistance of the TBC 26 . A suitable upper limit is about 40 volume percent so as not to unacceptably embrittle the TBC 26 . In a preferred embodiment, the alumina particles 32 are present in a range of about 15 to about 35 volume percent. The particles 32 preferably have diameters on the order of about 100 to about 5000 nanometers, more preferably about 1000 to about 5000 nanometers to promote the erosion and impact resistance of the TBC 26 .

[0020] A suitable process for depositing the columnar TBC 26 of FIG. 2 is a physical vapor deposition process, alone or assisted by chemical vapor deposition (CVD). A preferred process is believed to be EBPVD, which generally entails loading a component (such as the blade 10 of FIG. 1 ) to be coated into a coating chamber, evacuating the chamber, and then backfilling the chamber with oxygen and an inert gas such as argon to achieve a subatmospheric chamber pressure. The component is then supported in proximity to one or more ingots of the desired coating material, and one or more electron beams are projected onto the ingot(s) so as to evaporate the ingots and produce a vapor that deposits (condenses) on the component surface. While similar in many respects to conventional EBPVD, the process for depositing the columnar TBC 26 of this invention requires that each TBC coating material (YSZ and alumina) is present within one or more of the ingots. For example, the TBC 26 can be deposited by simultaneously evaporating separate ingots of YSZ and alumina. Alternatively, a single ingot containing YSZ and alumina regions or a dispersion of alumina can be evaporated to produce the TBC 26 . Another alternative is to evaporate a single ingot of YSZ using a chemical vapor deposition (CVD)-assisted process in which a source of aluminum vapors is continuously introduced into the coating chamber, causing oxidation of the aluminum and deposition of the resulting alumina vapors along with YSZ. Another alternative method is to use an ion beam source of aluminum (cathodic arc source) while evaporating a YSZ ingot to create the dispersion of alumina particles 32 .

[0021] A suitable process for depositing the noncolumnar TBC 26 of FIG. 3 is a plasma spraying technique, such as air plasma spraying (APS). Plasma spraying generally entails loading a component (e.g., the blade 10 ) to be coated into a coating chamber, and then melting a mixture of YSZ and alumina powders in the desired proportion with a plasma as it leaves a spray gun. Alternatively, the powder may be pre-alloyed to contain a mixture of YSZ and alumina. The molten powder particles impact the surface of the component, yielding grains 30 in the form of “splats” as represented in FIG. 3 .

[0022] For each of the above deposition processes, other process variables or fixturing, such as rotation and masking of a component, can be used to selectively deposit the TBC 26 of this invention on particular surface regions of the component that are relatively more prone to erosion or impact damage. For example, the TBC 26 could be selectively deposited on regions of the leading edge of the blade 10 , while conventional YSZ TBC could be selectively deposited on other surface regions of the blade 10 .

[0023] The deposition processes of this invention are all carried out so that alumina condenses to form the discrete and fine particles 32 represented in FIGS. 2 and 3 . Because alumina is not soluble in YSZ, the particles 32 remain as discrete particles that will not alloy with YSZ within the TBC 26 . Accordingly, the present invention differs from prior TBC materials sequentially deposited as discrete homogeneous layers or codeposited to form discrete layers surrounding YSZ grains. Finally, the TBC 26 of this invention is characterized by improved resistance to both erosion and impact, yet can be present as a relatively thin coating (e.g., less than 125 micrometers) to improve engine performance, fuel efficiency and component life.

[0024] While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. For example, instead of depositing the TBC 26 by EBPVD or CVD-assisted PVD, other atomic and molecular vapor deposition processes could be used, such as sputtering, ion plasma deposition, and all forms of melting and evaporation deposition processes. Accordingly, the scope of the invention is to be limited only by the following claims.