Lanthanum molybdate abradable coatings, their methods of formation and use

TECHNICAL FIELD

Embodiments of the present invention relate generally to turbines. More specifically, embodiments of the invention generally relate to abradable coatings for metal shrouds, particularly those metal shrouds in gas turbine engines.

BACKGROUND

The turbine section of a gas turbine engine contains a rotor shaft and one or more turbine stages, each having a turbine disk (or rotor) mounted or otherwise carried by the shaft and turbine blades mounted to and radially extending from the periphery of the disk. A turbine assembly typically generates rotating shaft power by expanding hot compressed gas produced by combustion of a fuel. Gas turbine buckets or blades generally have an airfoil shape designed to convert the thermal and kinetic energy of the flow path gases into mechanical rotation of the rotor.

Turbine performance and efficiency may be enhanced by reducing the space between the tip of the rotating blade and the stationary shroud to limit the flow of air over or around the top of the blade that would otherwise bypass the blade. For example, a blade may be configured so that its tip fits close to the shroud during engine operation. Thus, generating and maintaining an efficient tip clearance may be particularly desired for efficiency purposes.

Although turbine blades may be made of a number of superalloys (e.g., nickel-based superalloys), ceramic matrix composites (CMCs)) are an attractive alternative to nickel-based superalloys for turbine applications because of their high temperature capability and light weight. However, CMC components must be protected with an environmental barrier coating (EBC) in turbine engine environments to avoid severe oxidation and recession in the presence of high temperature steam.

Thus, in certain components, regions of the EBC may be susceptible to wear due to rub events with adjacent components. For example, for the CMC blade, the EBC at the blade tip is susceptible to rub against metal shroud components. If the EBC coating wears away, the CMC blade is then open to recessive attack from high temperature steam that will open up the clearance between the CMC blade tip and the metal shroud, thereby reducing the efficiency of the engine.

Thus, it may be desirable in the art to provide materials and methods for reducing EBC wear on a CMC blade tip caused by a rub event during operation of a turbine.

BRIEF DESCRIPTION

Aspects and advantages of embodiments of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of embodiments of the invention.

Another embodiment includes a substrate having the formula: La2-xYxMo2-y-y′WyBy′O9-δ, forming a crystalline structure, where about 0.05≤x≤about 0.15; 0≤y≤about 1.5; about 0.01≤y′≤about 0.2; and 0≤δ≤about 0.2.

Yet another embodiment includes a substrate having the formula: La2-xYxMo2-yWyO9-δ, forming a crystalline structure, where about 0.05≤x≤about 0.15; 0≤y≤about 1.5; about 0.01≤y′≤about 0.2; and 0≤δ≤about 0.2.

A gas turbine is also provided that includes the coated substrate described above. For example, the coated substrate can defines a metal shroud positioned adjacent to CMC blade tips, and wherein upon contact with a CMC blade tip, a portion of the abradable coating is removed from the metal shroud.

DETAILED DESCRIPTION

Abradable coatings are generally provided for a substrate, particularly those substrates in a turbine that are positioned in close contact to a CMC component (e.g., a CMC turbine blade).FIG. 1shows is an illustration of a cross-section of a coated substrate100that includes a substrate102having a coating108on surface103. The coating108generally includes an abradable coating106and an optional bond coating104. The substrate102and coatings104,106are discussed in greater detail below.

FIG. 2is a schematic illustration of an exemplary turbofan engine assembly10having a central rotational axis12. In the exemplary embodiment, turbofan engine assembly10includes an air intake side14and an exhaust side16. Turbofan engine assembly10also includes a core gas turbine engine18that includes a high-pressure compressor20, a combustor22, and a high-pressure turbine24. Moreover, turbofan engine assembly10includes a low-pressure turbine26that is disposed axially downstream from core gas turbine engine18, and a fan assembly28that is disposed axially upstream from core gas turbine engine22. Fan assembly28includes an array of fan blades30extending radially outward from a rotor hub32. Furthermore, turbofan engine assembly10includes a first rotor shaft34disposed between fan assembly28and the low-pressure turbine26, and a second rotor shaft36disposed between high-pressure compressor20and high-pressure turbine24such that fan assembly28, high-pressure compressor20, high-pressure turbine24, and low-pressure turbine26are in serial flow communication and co-axially aligned with respect to central rotational axis12of turbofan engine assembly10.

During operation, air enters through intake side14and flows through fan assembly28to high-pressure compressor20. Compressed air is delivered to combustor22. Airflow from combustor22drives high-pressure turbine24and low-pressure turbine26prior to exiting turbofan engine assembly10through exhaust side16.

High-pressure compressor20, combustor22, high-pressure turbine24, and low-pressure turbine26each include at least one rotor assembly. Rotary or rotor assemblies are generally subjected to different temperatures depending on their relative axial position within turbofan engine assembly10. For example, in the exemplary embodiment, turbofan engine assembly10has generally cooler operating temperatures towards forward fan assembly28and hotter operating temperatures towards aft high-pressure compressor20. As such, rotor components within high-pressure compressor20are generally fabricated from materials that are capable of withstanding higher temperatures as compared to fabrication materials for rotor components of fan assembly28.

The turbine assembly10comprises a plurality of rotor blades40and an outer shroud42concentrically disposed about rotor blades40, as shown inFIG. 3. Rotor blade40comprises an inner root46, an airfoil48and an outer tip44. As best shown inFIG. 3, outer shroud42is spaced apart from blade tip44so as to define a clearance gap43therebetween. As generally discussed in the above background section, the performance and efficiency of the turbine is critically affected by clearance gap43. The greater the amount of leakage flow through clearance gap43, the greater the inefficiency of the turbine10, as the leakage flow is not exerting motive forces on the blade surfaces and accordingly is not providing work. Thus, the blade tip44is positioned in close working proximity to the stationary shroud42, such that rub or impact events are possible during operation of the turbine10.

Although the present embodiments are described herein in connection with turbine assembly10, the present embodiments are not limited to practice in turbine assembly10. The present embodiments can be implemented and utilized in connection with many other configurations. Therefore, it should be understood that turbine assembly10is an exemplary assembly in which the present embodiments can be implemented and utilized.

In one particular embodiment, the coated substrate100forms the shroud42, such the abradable coating106(as shown inFIG. 1) is facing the blade tip44. For example, the abradable coating106may be on a metal shroud of a turbine that is in close operating proximity to a blade tip of a turbine blade. In a rub event between the blade tip44(e.g., a EBC coated CMC blade tip) and the coated substrate100(e.g., a coated metal shroud42), the abradable coating106is configured to be softer than the EBC coating such that the abradable coating is removed from the substrate102instead of the EBC from the blade tip. The abradable coating is relatively dense, and generally mechanically resistant to spall in turbine engine environments. As discussed in greater detail below, the abradable coating generally includes a lanthanum molybdate-based material.

Although discussed hereinafter with respect to a metal shroud, the substrate coated with the abradable coating can be any component within the turbine, particularly metal components. When the substrate100is a metal component such as a metal shroud, a transitional layer104is, in particular embodiments, positioned between the metallic substrate100and the abradable coating106. For example, the transitional layer104can be a passive aluminum oxide-based scale layer formed when the substrate material (e.g., the metal alloy itself or a bond coat deposited on the substrate) thermally oxidizes. The transitional layer104may be a bond coat, for example a diffusion coating. Suitable bond coatings would include, for example, nickel aluminide, platinum aluminide, aluminum, and aluminum oxide, or a combination thereof. Additionally, a bond coat with the formula MCrAlY; where M is Ni, Co, Fe, or mixtures thereof may be used.

As stated, the abradable coating106generally includes a lanthanum molybdate-based material. The lanthanum molybdate-based material provides the functionality of the shroud coating in terms of providing a surface that will give way on a rub or impact event. For example, when applied on a metal shroud, the lanthanum molybdate-based material provides the abradable functionality for a rub or impact event with a CMC blade tip without imposing severe wear on the EBC coating on the CMC blade tip.

When on a metal shroud, the high temperature cubic phase of this lanthanum molybdate-based material is the preferred form since it has a thermal expansion very similar to that of nickel- and cobalt-based superalloys. The cubic phase can be stabilized to room temperature or below (i.e., such that there is no phase transition during the temperature range of operation) over a wide range of lanthanum molybdate based compositions where the lanthanum and/or molybdenum is substituted by another element.

Generally, the lanthanum molybdate-based material is based on a parent structure of the formula: La2Mo2O9. This parent structure has a with low temperature monoclinic structure, and thus is substituted by various other elements to form a cubic material over a broad range of temperatures (below room temperature to 1350° C. or higher). As such, the abradable coating comprises La2-xAxMo2-y-y′WyBy′O9-δforming a crystalline structure, where A comprises Li, Na, K, Rb, Cs, Sc, Y, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Th, Be, Mg, Ca, Sr, Ba, Cu, Bi, Cd, Zn, Ag, Au, Pt, Ir, Rh, Ru, Pd, or combinations thereof; 0<x≤about 0.2 (i.e., x is greater than zero to about 0.2); 0≤y≤about 1.5 (i.e., y is zero to about 1.5); B comprises Ta, Nb, V, Fe, Cr, Mn, Co, Ni, Sn, Ga, Al, Re, In, S, or combinations thereof 0≤y′≤about 0.2 (i.e., y′ is zero to about 0.2), wherein the sum of y and y′ is about 0.01 to about 1.6; and 0≤δ≤about 0.2 (i.e., δ is zero to about 0.2). In particular embodiments, x is about 0.1 to about 0.15 (i.e., about 0.1≤x≤about 0.15).

As stated, A is generally comprises a rare earth element or a mixture of rare earth elements to combine with La in a similar site of the crystalline structure. In particular embodiments, A can include Y, Gd, Ce, Ca, Sr, Ba, or combinations thereof. For example, in one particular embodiment, A is Y such that the abradable coating comprises La2-xYxMo2-y-y′WyFey′O9-δforming the crystalline structure, where about 0.05≤x≤about 0.15 (i.e., x is about 0.05 to about 0.15); 0≤y≤about 1.5 (i.e., y is zero to about 1.5); about 0.01≤y′≤about 0.2 (i.e., y′ is about 0.01 to about 0.2); and 0≤δ≤about 0.2 (i.e., δ is zero to about 0.2).

When present, tungsten (W) can serve to stabilize the beta phase of the crystal structure in the abradable coating, and/or stabilize the cubic phase to lower temperatures. In certain embodiments, y is about 0.01 to about 1.5 (i.e., about 0.01≤y≤about 1.5) such that some amount of W is present in the crystal structure of the abradable coating. In such an embodiment, y′ may be 0 such that no B element is present in the crystal structure of the abradable coating. Alternatively, y′ may be greater than zero to about 0.15 (i.e., 0<y′≤about 0.15) such that both W and at least one B element is present in the crystal structure of the abradable coating. The level of W assists in adjusting the softness of the abradable coating.

As stated, B includes Ta, Nb, V, Fe, Cr, Mn, Co, Ni, Sn, Ga, Al, Re, In, S, or combinations thereof. B can be present with W (as stated above) or without W present in the crystal structure of the abradable coating (i.e., y is 0). In certain embodiments, B can help stabilize stabilize the cubic phase to a low temperature, particularly when B includes Ta, Nb, V, or combinations thereof. Additionally, B can help stabilize Mo in the crystal structure of the abradable coating by inhibiting Mo from changing its oxidation state (e.g., reducing), lower the processing temperature of the material, and/or reduce the interaction of the material with the metal substrate or bond coat. Such properties may be particularly achieved when B includes Fe, Cr, Mn, Co, Ni, Sn, Ga, Al, In, or combinations thereof.

In any case, the cubic phase of the crystalline material of these lanthanum molybdate materials provides an abradable coating with very low thermal expansion mismatch with the substrate (particularly a metallic substrate), and thus can be deposited as a dense, uncracked layer that is robust in terms of thermal cycling behavior in the engine, yet is soft enough to rub without rapidly removing EBC coating from a blade in case of an incursion event where a CMC blade tip contacts the cubic phase of these lanthanum molybdate materials.

For further durability, as shown inFIG. 4, an optional TBC110may be positioned between bond coat104and abradable coating106. This TBC provides additional erosion resistance and serves as an additional, relatively hard layer, to mitigate risk of exposing bare substrate after a rub event or prolonged erosion. Options for TBC110would include, for example, rare earth zirconates and hafnates, such as scandium zirconate, yttrium zirconate, lanthanum zirconate, cerium zirconate, praseodymium zirconate, neodymium zirconate, promethium zirconate, samarium zirconate, europium zirconate, gadolinium zirconate, terbium zirconate, dysprosium zirconate, holmium zirconate, erbium zirconate, thulium zirconate, ytterbium zirconate, and lutetium zirconate, as well as scandium hafnate, yttrium hafnate, lanthanum hafnate, cerium hafnate, praseodymium hafnate, neodymium hafnate, promethium hafnate, samarium hafnate, europium hafnate, gadolinium hafnate, terbium hafnate, dysprosium hafnate, holmium hafnate, erbium hafnate, thulium hafnate, ytterbium hafnate, and lutetium hafnate, rare earth-doped zirconia with a cubic or tetragonal phase, rare earth-doped hafnia with a cubic or tetragonal phase, alkaline earth doped zirconia with a cubic or tetragonal phase, alkaline earth doped hafnia with a cubic or tetragonal phase, monoclinic hafnia, or combinations thereof. Application methods and thickness of the TBC110would range from about 0.003 inches to about 0.030 inches, and could be higher based on a component's particular need. Other descriptions of TBC are found in U.S. Provisional Patent Application Ser. No. 62/069,346 titled “Thermal and Environmental Barrier Coating Compositions and Methods of Deposition” filed on Oct. 28, 2014, and U.S. Provisional Patent Application Ser. No. 62/018,983 titled “Thermal and Environmental Barrier Coating Compositions and Methods of Deposition” filed on Jun. 30, 2014, the disclosures of which are incorporated by reference herein.

While embodiments of the invention have been described in terms of one or more particular embodiments, it is apparent that other forms could be adopted by one skilled in the art. It is to be understood that the use of “comprising” in conjunction with the coating compositions described herein specifically discloses and includes the embodiments wherein the coating compositions “consist essentially of” the named components (i.e., contain the named components and no other components that significantly adversely affect the basic and novel features disclosed), and embodiments wherein the coating compositions “consist of” the named components (i.e., contain only the named components except for contaminants which are naturally and inevitably present in each of the named components).