Low density oxidation resistant superalloy materials capable of thermal barrier coating retention without a bond coat

Abstract: A low density superalloy and no bond coat thermal barrier coating system is described. The combination of alloy and TBC system can be used to fabricate light weight turbine blades with reduced blade pull. The present invention concerns nickel base superalloy articles which include additions of yttrium and hafnium that causes them to develop a durable, adherent aluminium oxide coating which will adhere to both the substrate and a ceramic thermal barrier coating thereby eliminating the need for an intermediate bond coat.

Description: As gas turbine engines have evolved, the requirements placed on the superalloys which form the operating components of such turbines have increased. Early gas turbine engines used polycrystalline cast turbine airfoils without protective coatings. The turbine art discovered that improved mechanical properties could be obtained by casting superalloy articles in a columnar grain form comprising elongated grains whose direction of elongation is in the direction of the major stress axis. This technique reduces the number of transverse grain boundaries and improves the mechanical properties of the components. Also, starting at about this time it became common to use protective coatings to protect the components from oxidation and corrosion. The next step in the evolution of gas turbine components was the development of single crystals. Single crystals are free from internal grain boundaries and offer an increased level of mechanical properties. Single crystal alloys were developed for use at higher temperatures and required effective protective coatings. Starting in the 1980s it became common to use ceramic thermal barrier coatings to protect superalloy components in the hottest portions of the engine, to provide thermal insulation and to permit operation at higher temperatures. U.S. Patents 4,248,940, and 4,321,311 describe thermal barrier coatings which use a bond coat which develops an adherent alumina layer to which a ceramic thermal insulating coating will adhere. While thermal barrier coatings with bond coats are very effective as thermal insulators, the weight of the bond coat contributes to the tensile stress on rotating components, especially in modern engines operating at high rotational speeds. Bond coats are also known to be generally brittle at intermediate temperatures; this lack of ductility contributes to premature thermal fatigue cracking in engine service. For these reasons, and because of cost, airfoil durability can be improved by eliminating bond coats. U.S. Patent 5,262,245 describes a ceramic thermal barrier coating system comprising a superalloy which develops an adherent alumina scale to which the ceramic thermal barrier coating will adhere without an intermediate bond coat. U.S. Patents 4,209,348, 4,459,160 and 4,643,782 describe superalloy compositions adapted to be used as single crystals. Additional superalloy compositions are described in EP-A-1038983 and US-A-5783318. The present invention comprises a nickel base superalloy substrate, and a combination of this nickel base superalloy and a thermal barrier coating system which includes a durable adherent alumina scale formed in situ on the substrate and a ceramic thermal barrier layer applied directly to the alumina scale without an intermediate bond coat. According to a first aspect, the present invention provides a nickel base superalloy article comprising: a) a directionally cast substrate having a single crystal or columnar grain microstructure and a composition consisting of: from 7% to 13% Cr; from 4.5% to 7% Al; from 1% to 2% Ti; from 3% to 11% W; up to 12.5% Ta; up to 15% Co; from 0.05% to 1.5% Hf from 0.003% to 0.040% Y; up to 3.5% Mo; up to 1% Re; up to 0.05% C; up to 0.005% B; up to 0.1% Zr; up to 2% Nb; up to 2% V; balance Ni and unavoidable impurities, wherein the substrate composition has a P parameter as calculated from the equation P = -200 Cr + 80 Mo 2 - 250 Ti 2 - 50 (Ti x Ta) + 15 Nb + 200 W - 14 W 2 + 30 Ta - 1.5 Ta 2 + 2.5 Co + 1200 Al - 100 Al 2 + 100 Re + 1000 Hf - 2000 Hf 2 + 700 Hf 3 - 2000 V - 500 C - 15000 B - 500 Zr, which does not exceed 2500; b) a durable adherent alumina scale adhered to at least a portion of the substrate, and c) a ceramic thermal barrier coating adhering to said alumina scale. Viewed from a second aspect the present invention provides a method for producing a gas turbine engine component having a bond coat free thermal coating comprising the steps of: a) directionally casting a superalloy having a composition consisting of: from 7% to 13% Cr; from 4.5% to 7% Al; from 1% to 2% Ti; from 3% to 11% W; up to 12.5% Ta; up to 15% Co; from 0.05% to 1.5% Hf from 0.003% to 0.040% Y; up to 3.5% Mo; up to 1% Re; up to 0.05% C; up to 0.005% B; up to 0.1% Zr; up to 2% Nb; up to 2% V; balance Ni and unavoidable impurities, the casting having a single crystal or columnar grain microstructure, wherein the substrate composition has a P parameter as calculated from the equation P = -200 Cr + 80 Mo 2 - 250 Ti 2 - 50 (Ti x Ta) + 15 Nb + 200 W - 14 W 2 + 30 Ta - 1.5 Ta 2 + 2.5 Co + 1200 Al - 100 Al 2 + 100 Re + 1000 Hf - 2000 Hf 2 + 700 Hf 3 - 2000 V - 500 C - 15000 B - 500 Zr, which does not exceed 2500; b) cleaning the surface of said casting; c) developing a durable adherent alumina scale on the clean surface of the casting by heating said casting to an elevated temperature in a low oxygen potential environment; and d) applying a ceramic thermal barrier coating to said durable adherent alumina scale. The superalloy is a relatively low density alloy with excellent low cycle fatigue capability, and the thermal barrier layer adheres to the alumina scale, formed on the substrate, without requiring a bond coat. The combination of a low density superalloy and a no bond coat thermal barrier system reduces centrifugal stresses in rotating applications by reducing component weight. This is critical for applications where components are operating at high rotational speeds. The invention has utility in gas turbine applications, particularly gas turbine blades. Such blades generally comprise an airfoil portion and a root or attachment portion. Turbine airfoils operate in high temperature environments, temperatures in excess of about 1500° F, and are usually internally cooled. Engine performance, durability, and efficiency can be enhanced by thermally insulating the airfoil portions of cooled airfoils. Certain preferred embodiments will now be described by way of example only. Unless otherwise noted, compositions are given in weight percent. Advanced superalloy compositions have been developed which exhibit improved strength and high temperature capabilities. However, many of these advanced compositions contain heavy elements such as rhenium, molybdenum and tungsten which raises their density. Higher density alloys combined with higher rotating speeds typical of modem turbine designs increases the tensile stress on moving airfoils. The increase in stress is a particular problem in the root or attachment portion of turbine blades. 3 A major aspect of the present invention is the discovery that a less dense class of superalloys can be rendered surprisingly more oxidation resistant through minor compositional modifications, without compromising other critical properties. Alloys so modified have been found to develop an alumina scale of greatly improved adherence and durability and to be suitable for use as a substrate for thermal barrier coatings without requiring a bond coat. This discovery has positive implications. The invention superalloys are substantially less dense than many superalloys which have been developed more recently. Also, because there is no bond coat required with the superalloys of the present invention, finished blade weight is reduced even further, thereby reducing the tensile forces resulting from engine rotation. Other advantages are that TMF (thermal mechanical fatigue) cracking is greatly delayed, reduced, or eliminated. We have also found that TBC spallation resistance is increased when the TBC is applied directly to the invention superalloy (with a thin intermediate alumina scale) rather than to an intermediate bond coat. The present invention arises of the discovery that the addition of yttrium and hafnium to certain superalloys causes them to develop a durable, adherent aluminum oxide coating which will adhere to both to the substrate and to a ceramic thermal barrier coating thereby eliminating the need for a intermediate bond coat. 4 Invention Alloy Composition Range in wt. % Broad Range Pref. A Pref. B. Pref. C . Cr 7.0-13 8-12 7.5-8.2 7-13 Al 4.5-7 4.5-5.5 5.45-5.75 4.5-6.7 Ti 1-2.0 1-2 1-1.2 1-2 W 3.0-11 3-5 7.6-8.4 3-11 Ta 0-12.5 10-12.5 5.8-6.2 0-5 Co 0-15 3-7 4.3-4.9 0-15 Hf 0.05-1.5 0.25-0.45 .15-.5 0.15-0.5 Y 0.003-.040 0.003-.040 0.003-0.040 0.003-0.040 Ni Bal Bal Bal Bal Mo 0-3.5 0-1 .3-.7 0-3.5 Re 0-1 0-1 0-1 0-1 C* 0-0.05 0-0.05 0-0.05 0-0.05 B* 0-0.05 0-0.005 0-0.005 0-0.005 Zr* 0-0.1 0-0.05 0-0.05 0-0.05 Nb 0-2 0-1 0-1 0-2 V 0-2 0-1 0-1 0-1.5 Table I shows broad range for the invention, and three preferred ranges. The broad range includes compositions suited for producing columnar grain and single crystal articles. The three preferred ranges are optimized for single crystal applications by keeping C less than 0.05 %, B less than 0.005 % and Zr less than 0.1%. Preferably the ranges in Table I are subject to the constraint that the (Al + Ti + .2 Ta) value is from about 6.5 to about 11.5 and most preferably from about 7.0 to about 10.5; while the value for (W + .8 Ta) is from about 9.5 to about 17.5 and most preferably from about 10.5 to about 16.5. An essential aspect of the present invention is the discovery that adding small, carefully controlled amounts of hafnium and yttrium to these alloys substantially improves their oxidation resistance by improving the durability and adherence of the alumina scale which forms upon exposure to oxidizing conditions. We use controlled thermal oxidation under conditions of low oxygen partial pressure since we have found that this produces an alumina scale with superior adherence and durability. The enhanced durability and adherence of the scale permits the commonly used metallic bond coat to be eliminated. The increased alumina scale durability and adherence observed which results from the practice of the invention is surprising and unexpected in view of the relatively low aluminum content in the invention alloy and the small amounts of Y and Hf utilized. U.S. Patent 5,221,336 describes casting techniques to control the amount of Y in castings. U.S. Patent 4,719,080 defines broad ranges for nickel base superalloys and describes a quantity called the P parameter calculated using an equation, which defines a desired relationship between various elements to produce an optimum combination of properties with a focus on high creep strength . The P parameter equation from US 4,719,080 is repeated below: Equation I - P Parameter P = -200 Cr + 80 Mo 2 - 250 Ti 2 - 50 (Ti x Ta) + 15 Cb + 200 W - 14 W 2 + 30 Ta - 1.5 Ta 2 + 2.5 Co + 1200 Al - 100 Al 2 + 100 Re + 1000 Hf - 2000 Hf 2 + 700 Hf 3 - 2000 V - 500 C - 15000 B - 500 Zr While the P parameter is a good indicator/predictor of superalloy creep-rupture properties, achieving a high P parameter generally requires that heavy alloying elements be utilized. The resultant increase in alloy density leads to increased centrifugal forces during operation, without a concurrent improvement in LCF capability thereby effectively negating some of the improved creep properties which result from a high P parameter. The invention alloys have low to moderate levels of heavy alloying elements compared to current high strength alloys and therefore are less dense and develop lower centrifugal stresses than alloys with higher P parameters. Also, since the present invention alloy does not require a bond coat for TBC adherence, the effective density of a TBC coated component is further reduced; since it will also be appreciated that bond coats add to component weight. The minimum P parameter disclosed in U.S. Patent 4,719,080 for an alloy as having high strength capability is 3360 and the maximum P parameter disclosed in that patent is 4700. When the composition ranges in Table I above are inserted in the P parameter equation, the highest value for the broad range is about 2130 and the lowest value of the broad range is minus 807. Thus the compositions which are the focus of the present invention are distinguishable from those in U.S. Patent 4,719,080 by the P parameter. Broadly to achieve the desired combination of high LCF strength and reduced density, the P parameter should be less than about 2500, and preferably be less than about 1800. The enhanced adherence between the alumina scale and the thermal barrier coating which results from the additions of yttrium and hafnium is surprising and unpredictable for these alloys. The invention alloys produce alumina scales which are durable and adherent. These adherent scales ensure good bonding of a subsequently applied ceramic coating, and also enhance uncoated oxidation resistance. The alumina scale is preferably developed by thermal oxidation of the invention alloy surface prior to the application of the ceramic TBC layer. Oxidation is preferably performed in an atmosphere of low oxygen potential. A hydrogen atmosphere with a dewpoint of from about -30°F (-34°C) to about -100°F (-23°C) at temperatures of 1800-2100F (982-1149°C). for times of 1-10 hrs. is preferred. A particularly preferred heat treatment is about 1975F (1080°C). for about 4 hours at dewpoint of about -40F (-40°C). USSN 09/274,127 describes details of a preferred surface preparation process. The thickness of the resultant alumina scale will be from about .2 to about 2 microns and preferably from about .5 to about 1.5 microns. For purposes of this disclosure, an alumina scale is defined as being durable and adherent if it can withstand ten, and preferably 100 burner rig cycles without scale spallation, where each cycle comprises four minutes in a 2100° F (1149°C) flame and two minutes of forced air cooling. The ceramic coatings which may be employed as thermal barrier coatings with the present invention comprise oxide ceramics and mixtures of oxide ceramics. Specifically, fully or partially stabilized zirconia may be used where additions of an oxide selected from the group consisting of Y 2 O 3 , Yb 2 O 3 , CaO and MgO and mixtures thereof may be employed as stabilizers. Zirconia stabilized with 5-20 wt% Y 2 O 3 is an industry standard. Other ceramics based on ceria may be used as may pyrochlore ceramics and near pyrochlore ceramics where the pyrochlore compound A 2 B 2 O 7 is employed where A is selected from the group consisting of La, Gd, Y and mixtures thereof, and B is selected from the group consisting of Ti, Zr, Hf and mixtures thereof. The TBC may be applied by EBPVD (electron beam physical vapor deposition) or by plasma or flame spray techniques. EBPVD application techniques are preferred for use on rotating parts. U.S. patents 4,321,311 and 5,262,245 illustrate such techniques. As described in US 4,321,311, ceramic coatings applied by EBPVD techniques possess a beneficial strain tolerant columnar microstructure that promotes good adhesion. A ceramic coating thickness of 3-10 mils (76-254µm) is typical. The invention alloy-coating system provides improved thermal barrier spallation life. Example I Three sets of coated samples were tested in a burner rig in a cycle which comprised four minutes in a 2200F (1205°C) and two minutes in a forced air cooling jet. The three sets of samples were as follows: 1. Single crystal alloy PWA 1484 (described in U.S. Patent 4,719,080) with a 5 mil (127µm) coating of a metallic overlay coating (described in U.S. Patent 4,321,311) with a 10 mil (254µm) TBC layer comprising 7% Y stabilized ZrO 2 applied by EBPVD. 2. Single crystal alloy, Pref A in Table I supra containing .1% Hf and 100 ppm Y with a 10 mil TBC layer comprising 7% Y stabilized ZrO 2 applied by EBPVD. 3. Single crystal alloy, Pref A in Table I supra containing .35% Hf and 100 ppm Y with a 10 mil TBC layer comprising 7% Y stabilized ZrO 2 applied by EBPVD. The results were as follows (average of 4 specimens): 1. 100% relative spallation life 2. 136% relative spallation life 3. 224% relative spallation life It can be seen that the present invention provides improved TBC spallation lives relative to the prior art. The invention alloy is less dense than recently developed alloys with higher creep strength such as PWA 1484 described in U.S. Patent 4,719,080. The reduced density of the invention alloy is particularly significant for rotating turbine components such as turbine blades. In some designs, turbine blades are limited by the LCF (low cycle fatigue life) properties in the root area where the blade is held in the turbine disk. Taking density in account, the invention alloy (Pref. A) has 12.5 % greater LCF strength capability than the U.S. Patent. 4,719,080 alloy, when tested in a notched LCF test at 1200F. The reduced density of the invention alloy (Pref. A) also reduces the stresses imposed on the supporting turbine disk. In engine operation, the blades exert a significant centrifugal force on the disk, an effect commonly known as blade pull. While blade pull varies with engine design and operating conditions, in a typical modern engine, the Pref. A produces a beneficial reduction of 9% in relative blade pull as shown below. PWA 1484 + metallic bond coat + stabilized zirconia thermal barrier coating = 100% relative blade pull PWA 1480 + metallic bond coat + stabilized zirconia thermal barrier coating = 97.5% relative blade pull Invention alloy, no metallic bond coat + stabilized zirconia thermal barrier coating = 91% relative blade pull Since the density of the PWA 1480 is the same as the density of the Pref. A invention alloy, it can be seen that by eliminating the metallic bond coat (which is an invention benefit), blade pull can be reduced by almost 7%. It can also be seen that (taking the bond coat weight into account) the lower density of the invention alloy reduces blade pull by about 2.5%. Thus the invention results in a substantial reduction in blade pull, a critical engine design factor. The reduced blade pull increases LCF life and permits the designer to reduce the size and weight of the turbine disk. Yet another advantage of the invention alloy is improved resistance of a TBC coated blade to thermal mechanical fatigue cracking during service operation. Thermal mechanical fatigue cracking comprises cracks which originate at the substrate surface of a cooled airfoil as a result of thermal cycling. Thermal mechanical cracking is also exacerbated by the temperature difference between the surface and interior of the cooled blade. Modern high turbine blades are air cooled, the outer surface temperature can range from 1600 to >2000F (871 to >1093°C) while the inner surface temperature may exceed 800F (427°C). In a test of thermal mechanical fatigue cracking on simulated blade samples conducted at 1900F (1038°C) cycling both temperature and imposed stress (where the stress was controlled to produce .25% strain), the previously described PWA 1484 alloy, with a metallic bond coat, had a crack initiation life which was only one third the life of the invention alloy (Pref. A) tested under the same conditions. It can be seen therefore that the elimination of the bond coat conventionally used with EBPVD coatings has yet another substantial benefit. The present invention alloy may be used without a thermal barrier coating, and when so used displays substantial uncoated oxidation resistance. Example II Several sets of samples were tested in a burner rig cyclic oxidation test, four minutes in a 2100F (1149°C) flame followed by two minutes of forced air cooling. The samples were as follows: 1. Single crystal samples of PWA 1480 (U.S. Patent 4,209,348) 2. Single crystal samples of PWA 1484 (U.S. Patent 4,719,080) 3. Single crystal samples of PWA 1487 (U.S. Patent 5,262,245) 4. Single crystal samples of Pref A composition, Table I, Supra, with .1% Hf and 100 ppm Y 5. Single crystal samples of Pref A composition, Table I, Supra, with .35% Hf and 100 ppm Y The test results were as follows: 1. 100% relative oxidation life 2. 490% relative oxidation life 3. 2,600% relative oxidation life 4. 2,080% relative oxidation life 5. 2,140% relative oxidation life It can be seen that the uncoated oxidation life of the invention is markedly superior to the PWA 1480 and 1484, and only slightly less than that of PWA 1487 which is an alloy with inferior mechanical properties to that of the invention alloy. Thus example I shows substantially enhanced thermal barrier coating spallation life and Example II shows substantially improved uncoated oxidation resistance for the invention alloy through the simple and subtle additions of yttrium and hafnium which produce dramatic results.

Claims: A nickel base superalloy article comprising: a) a directionally cast substrate having a single crystal or columnar grain microstructure and a composition consisting of: from 7% to 13% Cr; from 4.5% to 7% Al; from 1% to 2% Ti; from 3% to 11% W; up to 12.5% Ta; up to 15% Co; from 0.05% to 1.5% Hf from 0.003% to 0.040% Y; up to 3.5% Mo; up to 1% Re; up to 0.05% C; up to 0.005% B; up to 0.1% Zr; up to 2% Nb; up to 2% V; balance Ni and unavoidable impurities, wherein the substrate composition has a P parameter as calculated from the equation P = -200 Cr + 80 Mo 2 - 250 Ti 2 - 50 (Ti x Ta) + 15 Nb + 200 W - 14 W 2 + 30 Ta - 1.5 Ta 2 + 2.5 Co + 1200 Al - 100 Al 2 + 100 Re + 1000 Hf - 2000 Hf 2 + 700 Hf 3 - 2000 V - 500 C - 15000 B - 500 Zr, which does not exceed 2500; b) a durable adherent alumina scale adhered to at least a portion of the substrate; and c) a ceramic thermal barrier coating adhering to said alumina scale. A superalloy article as claimed in claim 1, wherein the quantity Al + Ti + .2Ta (in wt %) ranges from 7-10, and the quantity W + .8Ta (in wt. %) ranges from 12-18. A superalloy article as claimed in claim 1 or 2, wherein the substrate composition consists of (by wt. %) : from 8 % to 12% Cr; from 4.5% to 5.5% Al; from 1 % to 2% Ti; from 3% to 5% W; from 10% to 12.5% Ta; from 3% to 7% Co; from 0.25% to 0.45% Hf from 0.003% to 0.040% Y; up to 1% Mo; up to 1% Re up to 0.05% C; up to 0.005% B; up to 0.05% Zr; up to 1% Nb; up to 1% V balance Ni and unavoidable impurities. A superalloy article as claimed composition as claimed in claim 1 or 2, wherein the substrate composition consists of: from 7.5% to 8.2% Cr; from 5.45% to 5.75% Al; from 1% to 1.2% Ti; from 7.6% to 8.4% W; from 5.8% to 6.2% Ta; from 4.3% to 4.9% Co; from .15% to .5% Hf; from 0.003% to 0.040% Y; up to .3% to .7% Mo; up to 1% Re; up to 0.05% C; up to 0.005% B; up to 0.05% Zr; up to 1% Nb; up to 1% V balance Ni and unavoidable impurities. A superalloy article as claimed in claim 1 or 2, wherein the substrate composition consists of (by wt. %) : from 7 % to 13% Cr; from 4.5% to 6.7% Al; from 1% to 2% Ti; from 3% to 11% W; up to 5% Ta; up to 15% Co; from 0.15% to 0.5% Hf from 0.003% to 0.040% Y; up to 3.5% Mo; up to 1% Re up to 0.05% C; up to 0.005% B; up to 0.05% Zr; up to 2% Nb; up to 1.5% V balance Ni and unavoidable impurities. A superalloy article as claimed in any preceding claim, wherein the substrate has a single crystal microstructure. A superalloy article as claimed in any preceding claim, wherein the substrate is a gas turbine engine blade. A method for producing a gas turbine engine component having a bond coat free thermal coating comprising the steps of: a) directionally casting a superalloy having a composition consisting of: from 7% to 13% Cr; from 4.5% to 7% Al; from 1% to 2% Ti; from 3% to 11% W; up to 12.5% Ta; up to 15% Co; from 0.05% to 1.5% Hf from 0.003% to 0.040% Y; up to 3.5% Mo; up to 1% Re; up to 0.05% C; up to 0.005% B; up to 0.1% Zr; up to 2% Nb; up to 2% V; balance Ni and unavoidable impurities, the casting having a single crystal or columnar grain microstructure, wherein the substrate composition has a P parameter as calculated from the equation P = -200 Cr + 80 Mo 2 - 250 Ti 2 - 50 (Ti x Ta) + 15 Nb + 200 W - 14 W 2 + 30 Ta - 1.5 Ta 2 + 2.5 Co + 1200 Al - 100 Al 2 + 100 Re + 1000 Hf - 2000 Hf 2 + 700 Hf 3 - 2000 V - 500 C - 15000 B - 500 Zr, which does not exceed 2500; b) cleaning the surface of said casting; c) developing a durable adherent alumina scale on the clean surface of the casting by heating said casting to an elevated temperature in a low oxygen potential environment; and d) applying a ceramic thermal barrier coating to said durable adherent alumina scale. A method as claimed in claim 8, wherein said casting is a single crystal casting. A method as claimed in claim 8 or 9, wherein said thermal barrier coating is applied by electron beam physical vapour deposition (EBPVD) and has a columnar structure.

Inventors: CETEL ALAN D, GUPTA DINESH K, ZELESKY MARK F, CETEL, ALAN D., GUPTA, DINESH K., ZELESKY, MARK F.
Applicants: UNITED TECHNOLOGIES CORP, UNITED TECHNOLOGIES CORPORATION
Application year: 2001
Main classification: C22C19
IPC codes: C22C19, C23C8, C23C28, F01D5