Patent Application
20070141367
This invention relates to coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a thermal barrier coating (TBC) that exhibits improved impact and erosion resistance as a result of being a composite material containing a ceramic reinforcement material embedded in a ceramic matrix material.
Components within the hot gas path of a gas turbine engine are often protected by a thermal barrier coating (TBC) system. TBC systems include a thermal-insulating ceramic topcoat, referred to as the TBC, typically bonded to the component with an environmentally-protective bond coat. Bond coat materials widely used in TBC systems include overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth or reactive element such as hafnium, zirconium, etc.), and diffusion coatings such as diffusion aluminides, notable examples of which are NiAl and NiAl(Pt). Ceramic materials and particularly binary yttria-stabilized zirconia (YSZ) are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques.
TBC's employed in the highest temperature regions of gas turbine engines are often deposited by electron beam physical vapor deposition (EBPVD), which yields a columnar, strain-tolerant grain structure that is able to expand and contract without causing damaging stresses that lead to spallation of the TBC. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., cathodic arc, laser melting, etc.). In contrast, plasma spraying techniques such as air plasma spraying (APS) deposit TBC material in the form of molten “splats,” resulting in a TBC characterized by flat (noncolumnar) grains and a degree of inhomogeneity and porosity that reduces heat transfer through the TBC.
While YSZ TBC's are widely employed for their desirable thermal and adhesion characteristics, they are susceptible to damage within the hot gas path of a gas turbine engine. For example, YSZ coatings are known to be susceptible to thinning from damage by particles of varying sizes that are generated upstream in the engine or enter the high velocity gas stream through the air intake of a gas turbine engine. The damage can be in the form of erosive wear (generally from smaller particles, lower particle velocities, and/or lower impingement angles) and impact spallation (generally from larger particles, greater particle velocities, and/or greater impingement angles). Impact damage and the resulting loss of TBC particularly occur along the leading edges of components such as turbine blades, while erosion is more prevalent on the concave and convex surfaces of the blades, depending on the particular blade design. Both forms of damage not only shorten component life, but also lead to reduced engine performance and fuel efficiency.
In commonly-assigned U.S. Pat. No. 5,683,825 to Bruce et al., an erosion-resistant TBC is disclosed in which alumina (Al2O3) or silicon carbide (SiC) is deposited as a protective coating on a TBC, or co-deposited with the TBC material to form a dispersion of particles in the TBC. Other examples of strengthening a TBC material through precipitate or particle dispersions include commonly-assigned U.S. Pat. Nos. 6,617,049 to Darolia et al. and U.S. Pat. No. 6,663,983 to Darolia et al., which disclose the inclusion of fine precipitates or particles on the order of up to five micrometers in diameter to provide a dispersion-hardening effect. Another use for fine precipitates in a TBC is taught in commonly-assigned U.S. Pat. No. 6,544,665 to Rigney et al., who disclose a TBC containing small amounts of alumina precipitates dispersed throughout its grain boundaries and pores to inhibit grain sintering and coarsening and pore coarsening that would lead to increased thermal conductivity.
Notwithstanding the above advances, there is still an ongoing need for TBC's that exhibit improved resistance to impact spallation and erosion for more demanding engine designs.
The present invention generally provides a thermal barrier coating (TBC) for a component intended for use in a hostile environment, such as the superalloy turbine, combustor and augmentor components of a gas turbine engine. The TBC of this invention exhibits improved impact and erosion resistance as a result of being a composite material consisting essentially of particles of a ceramic reinforcement material dispersed in a ceramic matrix material. The ceramic reinforcement material has a yield strength greater than the ceramic matrix material at about 1100° C., and the particles of the ceramic reinforcement material have an average maximum dimension of greater than five micrometers.
The particles of the ceramic reinforcement material are preferably of a sufficient size and present in a sufficient amount to structurally reinforce the ceramic matrix material. Improved impact and erosion resistance is believed to be attributable at least in part to the particles providing crack blunting and crack deflection that inhibit crack propagation through the ceramic matrix material. As such, the invention is directed to relatively large particles of ceramic reinforcement material that are intentionally larger than the fine particle dispersions previously proposed for dispersion strengthening TBC materials to improve impact and erosion resistance. The ceramic reinforcement material can be co-deposited with the ceramic matrix material through various processes, including physical vapor deposition and plasma spraying.
Other objects and advantages of this invention will be better appreciated from the following detailed description.
While the present invention is applicable to a variety of components subjected to high temperatures, such as the high and low pressure turbine nozzles and blades, shrouds, centerbodies, combustor liners, and deflectors of gas turbine engines, the invention will be discussed in reference to a high pressure turbine (HPT) blade 10 shown in
To protect the airfoil 12 from its hostile operating environment, at least the surfaces of the airfoil 12 are proved with a thermal barrier coating (TBC) system 20, which is schematically depicted in
The TBC 26 is represented in
As previously noted, the leading edge 16 and the pressure surface 18 of the blade airfoil 12 are susceptible to damage by particles in the high velocity gas stream of a gas turbine engine. Damage to the leading edge 16 is generally from impact with large particles and/or particles at greater velocities and/or greater impingement angles, while damage to the pressure surface 18 is generally in the form of erosive wear from smaller particles, lower particle velocities, and/or lower impingement angles. Impacting particles generate stress waves in the outer surface regions of the impacted columnar grains 30. The stress waves travel downward through the impacted grains 30, arriving at the interface between the TBC 26 and bond coat 24 as reflected stress waves. The stresses generated by the stress waves are compressive in the first few grains 30, but become tensile in succeeding grains 30. When these tensile stresses reach the interface between the TBC 26 and bond coat 24, separation of the TBC 26 at the bond coat interface can occur depending on the magnitude of the tensile stresses. In such an event, the TBC 26 completely separates (spalls) from the bond coat 24 from a mechanism referred to herein as impact spallation. Alternatively, particle impact can cause cracking within the TBC grains 30 as a result of the stresses exceeding the cracking threshold of the TBC material, causing erosion damage.
To increase erosion resistance and particularly impact spallation resistance, the TBC 26 of this invention is depicted as having been deposited so that its individual columnar grains 30 are made up of a ceramic matrix material in which relatively large particles 32 of an insoluble ceramic reinforcement material are uniformly dispersed. As such, the TBC 26 is a composite material consisting essentially of the particles 32 of sufficient size to reinforce the ceramic matrix material making up the balance of the TBC 26. According to the invention, the ceramic reinforcement material of the particles 32 is also characterized by a greater yield strength than the ceramic matrix material in which they are dispersed, with the result that the particles 32 promote the mechanical properties of the TBC 26. In particular, the ceramic reinforcement material exhibits greater yield strength than the ceramic matrix material at temperatures sustained by the TBC 26 during engine operation, generally in the range of about 1800° F. to about 2200° F. (about 980° C. to about 1315° C.).
A variety of ceramic matrix materials can be employed with the present invention. A preferred ceramic matrix material is yttria-stabilized zirconia (YSZ), with suitable compositions containing about 2 to about 20 weight percent yttria (2-20% YSZ), more preferably about 3 to about 8 weight percent yttria (3-8% YSZ). Other ceramic materials are also suitable for the ceramic matrix material, such as YSZ modified with additional oxides to reduce thermal conductivity, and zirconia stabilized by other oxides such as magnesia, ceria, calcia, scandia, etc. Other notable materials suitable for the ceramic matrix material include those formulated to have lower coefficients of thermal conductivity than 7% YSZ, examples of which are disclosed in commonly-assigned U.S. Pat. Nos. 6,586,115 to Rigney et al., U.S. Pat. No. 6,686,060 to Bruce et al., U.S. Pat. No. 6,808,799 to Darolia et al., and U.S. Pat. No. 6,890,668 to Bruce et al., commonly-assigned U.S. patent application Ser. No. 10/063,962 to Bruce, and U.S. Pat. No. 6,025,078 to Rickerby.
In view of the variety of materials suitable for the ceramic matrix material, there are potentially a wide variety of ceramic materials suitable for use as the reinforcement material. A fundamental requirement of suitable reinforcement materials is insolubility in the ceramic matrix material, so that the particles 32 will remain as discrete particles that will not alloy with the ceramic matrix material. As such, preferred reinforcement materials for a particular application will depend in part on the ceramic matrix material being reinforced with the reinforcement particles 32. If the ceramic matrix material of the TBC 26 is YSZ, particularly suitable reinforcement materials are believed to be alumina and chromia, both of which are insoluble in YSZ and have yield strengths that exceed the yield strength of 6-8% YSZ at 1100° C. Notably, an additional benefit of using alumina as the reinforcement material is enhanced resistance to spallation from contamination by compounds such as CMAS (a low-melting compound of calcia, magnesia, alumina and silica). As reported in commonly-assigned U.S. Pat. Nos. 5,660,885, 5,683,825, 5,871,820, 5,914,189, alumina is capable of interacting with molten CMAS to form a compound with a melting temperature that is significantly higher than CMAS, so that the reaction product of CMAS and alumina does not melt and infiltrate the TBC 26.
The particles 32 of the ceramic reinforcement material must be of sufficient size and present in a sufficient amount to structurally reinforce the ceramic matrix material and thereby contribute significantly to the mechanical properties of the TBC 26. In particular, relative large particles 32 are believed to be able to improve impact and erosion resistance by providing crack blunting and crack deflection that inhibit crack propagation through the ceramic matrix material. For this reason, the particles 32 preferably have an average maximum dimension of at least five micrometers, preferably up to about 10 micrometers. The reinforcement particles 32 are preferably present in an amount of at least 0.1 weight percent of the TBC 26 in order to contribute to the mechanical properties of the TBC 26. A suitable upper limit is about 20 weight percent, with a preferred range being about 2 to about 10 weight percent.
The ceramic reinforcement material can be co-deposited with the ceramic matrix material through various processes, including physical vapor deposition (PVD) and plasma spraying. A suitable process for depositing the columnar TBC 26 of
In investigations leading to this invention, YSZ TBC's having nominal yttria contents of about seven weight percent were deposited by EBPVD to have thicknesses of about 125 micrometers. Each of the TBC's were deposited on pin specimens formed of René N5 (nominal composition of, by weight, about 7.5% Co, 7.0% Cr, 6.5% Ta, 6.2% Al, 5.0% W, 3.0% Re, 1.5% Mo, 0.15% Hf, 0.05% C, 0.004% B, 0.01% Y, the balance nickel and incidental impurities), on which a platinum aluminide (PtAl) bond coat had been previously deposited. The microstructures of the TBC's differed from each other as a result of a control group of the specimens being deposited to consist entirely of 7% YSZ, while other (experimental) specimens were deposited to contain up to about 10 weight percent of either alumina or chromia. Processing difficulties were encountered when depositing the alumina specimens, resulting in a layered structure instead of the composite reinforcement of this invention. For this reason, the alumina specimens are not further discussed below.
The impact performance of the chromia-containing test specimens was assessed by cycling the coated pins in and out of a jet stream into which was injected alumina particulate having an average particle size of about 560 micrometers. Coating loss was then correlated to the mass of the particulate required to spall the TBC. The results were normalized to the coating thickness and recorded in grams of particulate per one mil (25 micrometers) of coating thickness (g/mil) to permit comparison between coatings of different thicknesses. The results are summarized in Table I below for the chromia-containing specimens.
Examination of the 9.43% chromia specimens showed heavy layering of chromia instead of a dispersion of reinforcement particles, and the poor results of these specimens were attributed to this layered structure. Otherwise, the above results demonstrated that significant improvements in impact spallation resistance of about 100% can be achieved with 7% YSZ through additions of chromia particles.
Erosion resistance of additional specimens was assessed under similar conditions as the impact test, but with the use of a finer alumina particulate having an average particle size of about 50 micrometers. After normalizing, the results evidenced that the erosion resistance of the experimental TBC's containing alumina and chromia reinforcement material were generally the same as the control specimens.
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. Accordingly, the scope of the invention is to be limited only by the following claims.
