Patent Description:
The disclosure relates gas turbine engines. More particularly, the disclosure relates to thermal barrier coatings for gas turbine engines.

Gas turbine engine gaspath components are exposed to extreme heat and thermal gradients during various phases of engine operation. Thermal-mechanical stresses and resulting fatigue contribute to component failure. Significant efforts are made to cool such components and provide thermal barrier coatings to improve durability.

Exemplary thermal barrier coating systems include two-layer thermal barrier coating systems. An exemplary system includes NiCoCrAlY bondcoat (e.g., air plasma sprayed (APS), low pressure plasma sprayed (LPPS), or cathodic arc deposited) and yttria-stabilized zirconia (YSZ) (or gadolinia-stabilized zirconia (GSZ)) thermal barrier coating (TBC) (e.g., air plasma sprayed (APS) or electron beam physical vapor deposited (EBPVD)). Prior to and while the barrier coat layer is being deposited, a thermally grown oxide (TGO) layer (e.g., alumina) forms atop the bondcoat layer. As time-at-temperature and the number of cycles increase, this TGO interface layer grows in thickness. An exemplary YSZ is <NUM> weight percent yttria-stabilized zirconia (7YSZ). <CIT> suggests mischmetal oxides in combination with at least one other rare earth oxide for similar uses.

Exemplary TBCs are applied to thicknesses of <NUM>-<NUM> mils (<NUM>-<NUM>) and can contribute to a temperature reduction of up to <NUM>°F (<NUM>) at the base metal. This temperature reduction translates into improved part durability, higher turbine operating temperatures, and improved turbine efficiency.

Separately, the material known as didymium is used as safety glasses in the glassblowing and blacksmithing industries due to advantageous selective light-filtering properties. Didymium oxide is comprised of refined of praseodymium and neodymium oxides that have been extracted from rare earth containing minerals (e.g., separately and then mixed). Didymium is commonly extracted from Monazite and Bastnasite. Didymium ore may be formed by extracting cerium from monazite. Praseodymium and neodymium are, for example, extracted from the ore and oxidize in the process. These oxides are combined to form didymium oxide.

Discovery of the rare earth elements took approximately <NUM> years beginning in <NUM> with Gadolinite by C. In <NUM> C. Mosander discovered the element 'Didymium' upon extracting it from Cerite. The element's name was derived from the Greek 'didymos' meaning twins. In Mosander's experiments the behavior of didymium was peculiar, tracking lanthanum in some experiments wile tracking cerium in others. Scientists in the latter half of the <NUM>th century widely suspected that didymium was actually two elements, but did not have a method of separating the compound. Von Welsbach successfully separated Didymium into its individual components in <NUM>. The elements were named by A. Bettendorf: Praseodymium the "green twin" and Neodymium the "new twin".

In modern mining techniques, separation of the rare earth bearing heavy mineral deposits is accomplished through a series of processing sequences that exploit the small differences in mass, magnetic susceptibility, and electrochemical properties. The number and order of operations is predicated on the source of the deposit and purity of the heavy mineral deposits.

Recovery of REO from either mineral involves a complex series of operations by which the rare earths are separated from the radioactive components of the mineral. Monazite is dissolved into solution using caustic soda (NaOH). This mixture is then washed and filtered. Two byproducts are evolved from this step are a mixed rare earth (RE)-thorium-uranium hydroxide and a filtrate containing sodium phosphate. Hydrochloric acid is then added to the RE-Th-U hydroxide solution. The solution is subsequently filtered and washed to separate the radioactive constituents, uranium and thorium, from the desired RE components. The filtrate from this process is neutralized by chemical processing to yield a RE chloride mixture. The remaining liquid fraction is treated either with caustic soda and/or sodium bicarbonate to form additional RE hydroxide or RE carbonate. Similar processing is also used to digest bastnasite RE minerals; however different processing steps are utilized desired RE elements.

Cerium is the first RE to be removed from the mixture of lanthanide elements. This can be accomplished by drying the rare earth hydroxide mixture and then oxidizing cerium (III) to cerium (IV) in the presence of ozone. The mixture is then dissolved in nitric acid. The subsequent mixture is filtered leaving a cerium-free RE solution and cerium (IV) dioxide filtrate. The remaining mixture of rare earths can be further processed by a series complex ion exchange and digestion methods to separate the mixture into each elemental constituent. Praseodymium and Neodymium oxides are then blended together in exact ratios to form the mixtures as described in Table III below. See, e.g., Extractive Metallurgy of Rare Earths, C. Gupta and N. Krishnamurthy, CRC Press <NUM>.

<CIT> identifies use of a lanthanum monazite phosphate in a thermal barrier coating.

The invention provides an article having a metallic substrate, a bondcoat atop the substrate, and a thermal barrier coating atop the bondcoat. The thermal barrier coating or a layer thereof comprises didymium oxide and zirconia. The didymium oxide is a combination of neodymium oxide and praseodymium oxide. The combination of praseodymium oxide and neodymium oxide in said thermal barrier coating or layer thereof is at a concentration of <NUM>-<NUM> weight percent; and <NUM>% or more by weight of the remainder is zirconia.

In one or more embodiments of any of the foregoing embodiments, the thermal barrier coating or layer has improved resistance to molten silicate, CMAS, salts (e.g., relative to a pure 7YSZ baseline).

In one or more embodiments of any of the foregoing embodiments, the combination of praseodymium oxide and neodymium oxide in said thermal barrier coating or layer thereof is at a concentration of at least one of <NUM>-<NUM> mole percent and <NUM>-<NUM> weight percent and the zirconia in said thermal barrier coating or layer thereof is at a concentration of at least one of <NUM>-<NUM> mole and <NUM>-<NUM> weight percent.

In one or more embodiments of any of the foregoing embodiments, the thermal barrier coating or layer thereof has a characteristic thickness of at least <NUM> micrometers and the bondcoat has a characteristic thickness of at least <NUM> micrometers.

In one or more embodiments of any of the foregoing embodiments, the substrate comprises a nickel-based superalloy.

In one or more embodiments of any of the foregoing embodiments, the article is a gas turbine engine component.

In one or more embodiments of any of the foregoing embodiments, the thermal conductivity of the resulting ceramic system is between <NUM> and <NUM> (W m-<NUM>-l).

In one or more embodiments of any of the foregoing embodiments, the thermal barrier coating possesses resistance to molten silicate, CMAS, salts.

In one or more embodiments of any of the foregoing embodiments, the thermal barrier coating comprises said layer and at least one YSZ layer between the bondcoat and said layer.

In one or more embodiments of any of the foregoing embodiments, the thermal barrier coating further comprises a mixed phase field of fluorite and pyrochlore crystal structure-rare earth zirconate A<NUM>B<NUM><NUM><NUM> between said layer said at least one YSZ layer.

In one or more embodiments of any of the foregoing embodiments, the thermal barrier coating comprises a plurality of said layers alternating with a plurality of said at least one YSZ layer.

In one or more embodiments of any of the foregoing embodiments, a method for manufacturing the article comprises: applying the bondcoat; and applying the didymium oxide and the zirconia forming didymium zirconate.

In one or more embodiments of any of the foregoing embodiments, the applying the combination of praseodymium oxide and neodymium oxide and the zirconia comprises mixing and forming an ingot and vaporizing the ingot.

In one or more embodiments of any of the foregoing embodiments, the applying the combination of praseodymium oxide and neodymium oxide and the zirconia comprises EB-PVD.

Another aspect of the disclosure involves a method for coating an article, the method comprising: applying a bondcoat; and applying a combination of praseodymium oxide, neodymium oxide and zirconia forming the zirconate.

In one or more embodiments of any of the foregoing embodiments, the combination consists essentially of praseodymium oxide, neodymium oxide and zirconia.

<FIG> shows a thermal barrier coating system <NUM> atop a metallic substrate <NUM>. In an exemplary embodiment, the substrate is a nickel-based superalloy or a cobalt-based superalloy such as a cast component (e.g., a single crystal casting) of a gas turbine engine. Exemplary components are hot section components such as combustor panels, turbine blades, turbine vanes, and air seals.

Exemplary substrate compositional ranges are shown in Table I:.

In some embodiments of the materials in Table I (and Tables II and III below), the materials may consist essentially of the listed elements (e.g., with at most trace amounts of other elements). In some embodiments, other elements may be present in individual quantities less than <NUM> weight percent and/or aggregate quantities less than <NUM> weight percent, more narrowly <NUM> weight percent individually and <NUM> weight percent aggregate.

The coating system <NUM> may include a bondcoat <NUM> atop a surface <NUM> of the substrate <NUM> and a thermal barrier coating (TBC) system <NUM> atop the bondcoat. A thermally grown oxide (TGO) layer <NUM> may form at the interface of the bondcoat to the TBC. The exemplary bondcoat is a single-layer bondcoat. Alternatives may have two or more layers. In the exemplary system, the bondcoat consists of or consists essentially of the single layer (e.g., subject to relatively small gradation/transition with each other and with the TBC as noted above).

The exemplary TBC is also a single-layer TBC. Alternatives may involve a multi-layer TBC with at least two layers. In the exemplary system, the TBC consists of or consists essentially of the single layer (e.g., subject to relatively small gradation/transition the bondcoat as noted above).

<FIG> shows a vane <NUM> comprising the cast metallic substrate <NUM>. The vane includes an airfoil <NUM> having a surface comprising a leading edge <NUM>, a trailing edge <NUM>, a pressure side <NUM>, and a suction side <NUM>. The airfoil extends from an inboard end at a platform or band segment <NUM> to an outboard end and an outboard shroud or band segment <NUM>. The segments <NUM> and <NUM> have respective gaspath surfaces <NUM> and <NUM>. These are essentially normal to the airfoil surfaces. The TBC system extends at least along the surface of the airfoil and the surfaces <NUM> and <NUM>.

<FIG> shows a blade <NUM> having an airfoil <NUM> extending outward from a platform <NUM>. The blade includes an attachment root <NUM> inboard of the platform. The platform <NUM> has an outboard gaspath surface <NUM>.

The exemplary bondcoat <NUM> is an overlay MCrAlY bondcoat. An exemplary MCrAlY overlay bondcoat is a NiCoCrAlYHfSi. Alternative bondcoats are diffusion aluminides or platinum aluminides. Table II provides exemplary bondcoat compositions:.

Exemplary bondcoat thicknesses are <NUM>-<NUM> micrometers, more narrowly, <NUM>-<NUM> micrometers or <NUM>-<NUM> micrometers on average.

The exemplary TBC provides a mixture didymium oxide zirconate. Table III shows two nominal commercially recorded compositions for didymium oxides:.

Two ranges of different breadths are also given. The broad range in Table III encompasses the chemical variations in monazite deposits in placer deposits from around the world. The narrow range is typical of what is extracted from active mining facilities. The two nominal commercial compositions are derived from the products of heavy mineral sands from two different monazite deposits. While similar in nominal composition, there are enough differences in the derived typical nominal chemistry that they can be considered to be different products and one or the other may have more beneficial properties in a given situation. Thus, the ranges are given to encompass both. In a first example, the <FIG> TBC comprises <NUM>-<NUM> molar percent didymium oxide. A mass fraction of <NUM> % or more of the remainder being zirconia. A more particular range composition is <NUM>-<NUM> molar percent didymium oxide with a similar balance zirconia. This may be applied EB-PVD or other appropriate process resulting in the formation of a rare earth zirconate. A rare earth zirconate the term used to describe the alloyed mixture of the rare earth oxide and zirconia. It is commonly used in reference to the line compound of the composition M<NUM>Zr<NUM>O7 (i.e. Gd<NUM>Zr<NUM>O7 - gadolinium zirconate). More broadly it can be used to describe the solid solution of the desired metal oxide, M<NUM>O<NUM> in conjunction with zirconia, (ZrO<NUM>). Exemplary TBC layer thickness is in excess of <NUM> micrometers, more particularly, in excess of <NUM> micrometers or <NUM> - <NUM> micrometers.

Exemplary deposition methods include plasma spray deposition and electron beam physical vapor deposition (EB-PVD). <FIG> shows an exemplary process with several branches showing several variations including three exemplary plasma spray variations and one exemplary EB-PVD variation. In the EB-PVD variation <NUM>, an appropriate amount of didymium oxide is weighed <NUM>. It is then mixed <NUM> with zirconium oxides. The mixed powder is then ground <NUM>. The ground powders are then mixed <NUM> with binder. The mixed powder/binder is then pressed <NUM> into an ingot. The ingot is then sintered <NUM>. The sintered ingot is then used in EB-PVD coating <NUM> in a conventional manner.

In a first plasma spray variation <NUM>, the weighing, mixing, and grinding may be performed but then the mixed/ground powders are then mixed <NUM> with binder and water to form a slurry. The slurry is spray dried <NUM> to form powders. The dried powders are screened <NUM>. The screened powders are sintered <NUM>. The sintered powders may be plasma sprayed <NUM> in conventional form. A second plasma spray variation <NUM> branches off earlier after the weighing of didymium oxide ore. The didymium oxide ore is added <NUM> with concentrated nitric acid to a reflux reactor. The reactor is heated <NUM> (e.g., to <NUM> to <NUM>) to dissolve the didymium oxide ore.

The reacted product is then distilled <NUM> to form didymium nitrate. Amounts of didymium nitrate and a soluble zirconium salt powders are then weighed <NUM>. The powders are dissolved <NUM> (e.g., in deionized water). Ammonium hydroxide is then titrated <NUM> into solution. The resulting precipitate is filtered <NUM> into a cake. The cake is dried and calcined <NUM>. The dried and calcined cake forms a powder which is mixed <NUM> with solvent and dispersant which may then be plasma sprayed.

In a further variation <NUM>, after the dissolving <NUM> the solution is filtered <NUM> and the filtered solution may directly be plasma sprayed.

<FIG> shows a two-layer thermal barrier coating system. An underlayer <NUM> comprises or consists essentially of yttria-stabilized zirconia (e.g., <NUM>-<NUM> weight percent yttria, or more particularly, <NUM>-<NUM> percent or a nominal <NUM> percent). A second layer <NUM> (e.g., an outer layer) is deposited atop the underlayer and has the composition noted for the <FIG> example. Exemplary YSZ layer thickness is at least <NUM> micrometer. Exemplary didymium oxide zirconate layer thickness is at least <NUM> micrometers.

<FIG> shows a system otherwise similar to the <FIG> system but wherein a third layer <NUM> intervenes between the YSZ underlayer <NUM> and the didymium oxide - zirconia layer <NUM>. The exemplary intervening layer <NUM> is a flourite/pyrochlore zirconate A<NUM>B<NUM>O<NUM>. This layer may be characterized as a stabilized zirconia coating comprising an oxide of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Mg, Ca, Hf, Sr, <NUM>-<NUM> mol%, more narrowly, <NUM>-<NUM>%. Exemplary thickness of the intervening layer is at least <NUM> micrometers, more particularly <NUM>-<NUM> micrometers.

<FIG> shows an alternative embodiment where there are alternating layers: YSZ layers <NUM>-<NUM>, <NUM>-<NUM>; and the didymium oxide-zirconia layers <NUM>-<NUM>, <NUM>-<NUM> (two pairs being shown but more being possible). Exemplary thicknesses of each layer are in excess of <NUM> micrometers, more particularly <NUM>-<NUM> micrometers. Exemplary overall thickness may be in the overall range above.

Where a measure is given in English units followed by a parenthetical containing SI or other units, the parenthetical's units are a conversion and should not imply a degree of precision not found in the English units.

Claim 1:
An article (<NUM>; <NUM>) comprising:
a metallic substrate (<NUM>);
a bondcoat (<NUM>) atop the substrate; and
a thermal barrier coating (<NUM>; <NUM>, <NUM>) atop the bondcoat,
wherein the thermal barrier coating or a layer thereof comprises:
a combination of praseodymium oxide and neodymium oxide in said thermal barrier coating or layer thereof with a mass fraction of <NUM> % to <NUM> %; wherein a mass fraction of <NUM>% or more of the remainder is zirconia.