CA2146503A1

CA2146503A1 - High temperature coating for combustion turbines and aeroengines

Info

Publication number: CA2146503A1
Application number: CA002146503A
Authority: CA
Inventors: Vasudevan Srinivasan
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1994-04-07
Filing date: 1995-04-06
Publication date: 1996-01-05
Also published as: JPH07278721A; KR950032712A

Abstract

Improved overlay compositions are provided for protecting turbine component metallic substrates, preferably nickel and cobalt based super alloys, from oxidation, corrosion, or both. The improved overlay compositions are designed to improve the oxidation and corrosion of the base substrate alloy which is exposed to gaseous environments containing sulfur. The improved performance achieved by incorporating minor amounts of aluminum oxide, preferably Al2O3 to the overlay coating composition. Advantageous properties are also obtained by the incorporation of silicon and hafnium. The improved overlay coatings are advantageously applied to turbine combustors, blades, and vanes.

Description

j - 21~6503 - 1 - 57,650 IMPROVI~D SUPERALLOY COATING COMPOSITIONS
~JD METHODS FOR USING THE SAME
FIELD OF THE INVENTION
The present invention relates to overlay coating compositions used to protect metal substrates from oxidation, corrosion, or both. Specifically, the invention relates to the incorporation of aluminum oxide particles, preferably Al2O3 particles, into the overlay composition.
BACKGROUND OF THE INVENTION
Protective coatings are commonly employed to extend the operational life of metallic substrates used in combustion sections of turbines. The metallic substrates are conventionally superalloy materials that are either nickel, cobalt, or iron based alloys, or combinations thereof, and usually contain other elements in significant quantities such as chromium, aluminum, titanium, and the refractory metals.
Various superalloys are shown in U.S. Pat. Nos. 4,933,239 and 3,754,902.
The superalloy substrates are exposed to oxidative and corrosive environments during use in such applications as gas turbine combustors, transitions, blades and vanes. This harsh environment leads to shortened useful life of the component as the structure, dimension and geometry of the substrate is deteriorated over time.
Various coatings have been developed to protect the surface of the superalloy substrate. One type of such coatings are referred to as the "overlay" coatings. These coatings are generally denoted as MCrAlY coatings where the M represents such elements as Ni, Co, Fe, and combinations - 2 - 57,650 thereof. These coatings derive their protective capability from their ability to form a thin layer of alumina scale on the outer exposed surface. This alumina layer has been found to be quite beneficial in oxidation resistance. However, the alumina scale has a tendency to spall and must be reformed during use of the substrate. Additives such as yttrium, hafnium, and silicon have been incorporated into such overlay coatings to imp~ove coating overall performance, and the alumina adherence to the substrate and to aid in the regeneration of the alumina scale. The overlay coatings are typically applied to the substrate surface through such processes as low pressure plasma spraying, physical vapor deposition, ion plating, and sputtering or slurry sintering.
Examples of such overlaying coatings are set forth in U.S.
Pat. Nos. 4,615,865; 4,585,481; 4,198,442; 4,101,715; and 3,754,903.
Another class of coatings for the oxidation and corrosion protection of the substrate is the "aluminide"
coatings. These coatings are generated by an aluminizing technique such as pack diffusion or chemical vapor diffusion.
These coatings are formed by interactions between an aluminum source and the substrate surface. The aluminum forms cobalt and nickel aluminide at the surface of the cobalt and nickel based superalloy substrates, respectively. The coating characteristics are largely affected by the substrate chemistry and deposition process parameters. Examples of such coatings are shown in U.S. Pat. No. 5,000,782.
Combinations of the two classes of coatings have also been used to formulate protective coatings by aluminizing an overlay coating as shown in U.S. Pat. Nos. 4,933,239;

4,910,092; and 4,897,315.
The dsscribed overlay and aluminide coatings preferably contain yttrium as an additive element to aid in the aluminide scale formation and retention. However, when a substrate is exposed to harsh corrosive environments, such as when fuel containing sulfur and other salt impurities is oxidized in the turbine, the yttrium is essentially .

- 3 - 57,650 deactivated. Thus, there currently exists a need to develop a protective coating that contains an additive or additive combination that can extend the oxidative and corrosive properties of the substrate when fuels containing sulfur and other salt impur:ities are oxidized with ingested impure air.
SUMMARY OF THE INVENTION
The present invention provides an improved overlay coating composition which provides superior oxidation and corrosion resistance when exposed to gaseous environments containing sulfur compounds. The overlay coating composition contains nickel, from 8-50% wt. chromium, from 6-40% wt.
aluminum, from 10-40% wt. cobalt, and from 0.1-10~ wt.
particulate aluminum oxide. The aluminum oxide is uniformly distributed throughout the -particulate overlay coating composition, and has been found to improve the protective characteristics of the coating. The overlay coating composition is deposited to a bare substrate, conventionally comprised of a nickel or cobalt super alloy by conventional techniques.
The overlay coating has been found to provide improved protective resistance to oxidation and corrosion by incorporating haenium, silicon or mixtures thereof into the composition. The combination of Al203 with silicon is particularly preferred.
The overlay coating is useful in protecting a metallic substrate. The coated metallic substrate is useful as a part within a turbine or engine. Preferred uses of the overlay coating are to extend the life of combustion or gas turbine metallic substrate surfaces.
BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 is a graph of the experimental test results of Example 1 showing the improved coating protection afforded by the inclusi~n of Al2O3 particles into the coating composition.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides improved coating compositions fol use in extending the life of metallic - 4 - 57,650 substrates used, for instance, in the hot sections of gas turbines, combustion turbines, and aero-jet engines. Examples of tubine components to which the coating compositions can be applied include, for instance, gas turbine combustors, transitions, blades and vanes. The coating compositions are advantageously employed to coat and protect substrates that are exposed to the combustion products of fuels containing sulfur and other salt impurities. The coatings of the present invention employ the use of aluminum oxide within the matrix of the coating t~ enhance aluminum oxide scale formation on the exposed surface of the coating and the adherence of that formed scale to the substrate. Other inert metal oxides such as, for example cerium oxides, scandium oxides, indium oxides, and thorium oxides, can also be used in place of Al2O3 with similar beneficial effects, with Al2O3 being preferred.
The base metallic substrate upon which the coating composition is deposited can be any metallic substrate, however the substrate is preferably a superalloy that is a base alloy of nickel, cobalt, or combination thereof. The nickel or cobalt based alloys can be in either the cast or forged/wrought form. The nickel based superalloys derive their high temperature mechanical strength primarily from precipitation hardening processes. The major precipitates are gamma prime with a composition of Ni3Al or Ni3AlTi type. The nickel based superalloys are commonly used as substrates in rotating blades used in turbines. The cobalt based alloys are strengthened by solid solution hardening. The cobalt based superalloys are commonly used as substrates in stationary blades used in turbines. Other solution-strengthened Ni-based or Fe-based al~oys can also be used. The superalloys generally contain elements such as iron, boron, carbon, and zirconium along with refractory elements such as tungsten, tantalum, molybdenum and niobium for solid solution strengthening. Examples of nickel-based superalloys are shown in U.S. Pat. No. 3,754,902 which is herein incorporated by reference in its entirety.

-_ 5 _ 57,650 The superalloy substrate is then coated with an overlay coating alloy. The coating alloy composition contains sufficient amounts of chromium and aluminum to form protective chromia and alumina scales during exposure to an oxidative environment. The presence of the chromium reduces the amount of aluminum necessary to form the alumina scale by the "gettering" effect. The upper limit of the chromium in the coating alloy composition is functionally restricted by reduced oxidation resistance and the lower limit is restricted by reduced corrosion resistance. Excess aluminum is deleterious to the ductility of the coating and it also reduces corrosion resistance, whiie a sufficient amount of aluminum is necessary to enhance the oxidation performance.
The balance of these factors generally determines the appropriate level of the chromium and aluminum in the coating alloy composition. The chromium is present in an amount of from about 8-50% wt., preferably from 15-3S% wt., and more preferably from 20-30% wt. and the aluminum is present in an amount of from about 6-40% wt., preferably 6-20% wt., and more preferably from 8-12% wt. of the coating alloy composition.
The coating alloy composition also contains cobalt and nickel. The content of the cobalt and nickel will depend upon whether the substrate base alloy is a nickel or cobalt based alloy. If the overlay coating is to be used with a nickel based substrate, then the cobalt is present in an amount of from about 10-40% wt., preferably from 15-35% wt., and more prefer~bly from 20-30% wt. of the coating alloy composition. The nickel would then constitute essentially the balance of the nickel based overlay coating, excluding the stated additive elements or compounds set forth herein. If the overlay coating is to be used with a cobalt based substrate, then the nickel is present in an amount of from about 10-40% wt., preferably from 15-35% wt., and more preferably from 20-30% wt. of the coating alloy composition.
The cobalt would then constitute essentially the balance of the cobalt based overlay coating, excluding the stated additive elements or compounds set forth herein.

~146503 - 6 - 57,650 The way by which overlay coatings such as the NiCoCrAl type protect alloy substrates is by thermally forming a tenacious, uniform and slowly forming thin protective oxide scale, usually an alumina scale. The alumina scale acts as s a barrier for further diffusion of Al and 2~ thereby slowing down the formation of alumina scale to a low steady terminal rate. The thickness of this scale increases over time upon exposure to process conditions. The scale is not very ductile, and ultimately cracking and spallation occurs during the thermal cycling process. Reformation of the scale occurs at the spalled locations at higher rates thus contributing to fast depletion of Al from the coating. When the Al is depleted from the coating, non-protective oxide scales of the base metal form and scale penetration occurs resulting in a lack of protection for the base alloy substrate.
The coating alloy composition of the present invention further includes aluminum oxide for the enhancement of the oxidation and corrosion resistance, especially wherein the substrate is to be used in an atmosphere containing the combustion products of a fuel and air containing sulfur and other salts or where gaseous SO2 or S03 (SOx) compounds are present. The inert oxide is present in the coating alloy composition in an amount of from about 0.1-10% wt., preferably 0.1-5% wt., and more preferably from 0.5-3% wt. The preferred oxide is aluminum oxide, Al2O3.
The coating alloy composition can also contain other additives useful in adhering the alumina scale to the surface of the overlay ~oating. Representative additives include hafnium and silicon. The hafnium can be present in an amount of from 0.01-4% wt., preferably from 0.1-2% wt.; the silicon can be present in an amount of from 0.01-5% wt., preferably from 0.1-2.5% wt.; all basea upon the total overlay coating alloy composition. Another element, cerium, can also be added in the coating alloy composition in an amount of from 0.01-10%
wt., preferably from about 0.1-5% wt., and more preferably from 0.1-2.5% wt. Further, rhenium can be added to improve oxidation -resist:ance in amounts of about 0.01-40% wt., - ~14650~

- 7 - 57,650 preferably from about 0.01-8~ wt., and more preferably from 0.01-4% wt.
Variou; other additives can be incorporated into the overlay coating alloy composition such as Sc, La, Gd, and combinations thereof. These additives can be present in an amount of from 0.1-10% wt. individually, however the total of these additives is preferably below about 20% wt., more preferably below about 15% wt., of the coating composition.
Yttrium can also be added to the coating composition in an amount of from about 0.01-10% wt., preferably from 0.1-4% wt., however it is not desired for uses where sulfur containing fuels are employed. It is believed that if the yttrium is oxidized, then it is not resistant to sulfation and the aluminum oxide scale adhesion is correspondingly decreased.
The thickness of the overlay coating is generally at least about 0.002 inches (0.005 cm), preferably at least about 0.003 inches (0.0076 cm), more preferably from about 0.003 inches (0.0076 cm) to about 0.015 inches (0.038 cm), and even more preferably from about 0.004 inches (0.01 cm) to about 0.01 inches (0.025 cm) in thickness. The aluminum oxide is present throughout the thickness of the coating as disperoids, and not just on the surface, and is preferably present in a substantially uniform amount, even more preferably present in a uniform homogeneous amount, throughout the thickness of the overlay coating.
The coating alloy composition is prepared by first making an alloy melt of the elements and compounds in the coating alloy composition except for the aluminum oxide. This melt portion of the coating alloy is then spray atomized to form a particulate alloy using conventional techniques such as argon spray atomization.
The particulate alloy is then blended with particulate aluminum oxide to form a homogeneous particulate coating alloy composition. The average particle size of the atomized alloy portion is preferably from about S to about S0 microns, and the average particle size of the aluminum oxide ~146503 -- 8 - 57,650 is from about 0.5 to about 3 microns. This particle size distribution may vary according to the selection of application method and process parameters.
The particulate coating alloy is then deposited onto the substrate surface. The deposition process is preferably a low pressure plasma spraying (LPPS) process. Other thermal spray methods including high velocity oxy fuel (HVOF) and sputtering can also be used to apply the overlay composition coating of the present invention.
The present inventive coatings can also be used as a bond coat or base coat below any top thermal barrier coating with and without any tie coat or intermediate coating.

Example Cyclic hot corro:3ion/oxidation studies Tests were performed to analyze the behavior of NiCoCrAl coatings containing additions of Y or Hf with Alz03 and Al203/Si. The CoNiCrAlY coating was used as a base reference coating. The base substrate alloy was Inconel IN738LC which has a composition of, in weight percent, 16% Cr, 8.5% Co, 3.4~ Al, 3.4% Ti, 1.6% Mo, 1.6% Ta, 2.5% W, 1% Nb, and 0.1~ C with the balance Ni. The three overlay coatings applied to the base alloy are set forth in Table 1.1 in a weight percent basis except Al203 which is given in volume percent.

~ 9 ~ 57,650 Table l.l Overlay Coating Composition~
Constituent A B C
Co(wt.%) 38 23 23 5 Cr(wt.%) ~l 20 20 Al(wt.%) 8 ll ll Y(wt.%) ().4 Hf(wt-%) ~ 0.6 0.6 Al2O3(vol-%) ~ 2.0 2.0 lO Si(wt.%) - - 0.7 Ni(wt.%) Bal Bal Bal The IN738LC substrate was prepared free from dirt and other contaminants. The three different coatings were applied using a l;ow pressure plasma spray process (LPPS). The apparatus used for the coating process was a Model EPI03C8 supplied by Elec1:ro Plasma Inc.; the substrate was coated in a closed chamber maintained at a pressure of about 35 torr of argon. A mixture of argon and helium was used to generate plasma. Powder of NiCoCrAlHf and NiCoCrAlSiHf was homogeneously pre-mixed with Al2O3 powder by ball milling.
The particle size distribution of the base coating powders and the Al2O3 powder was from 10-40 ~m and from 0.6-2.5 ~m, respectively. The coating thickness applied to the substrate was uniformly 6 mils + l mil.
Cyclic hot corrosion testing was performed using a laboratory electric furnace maintained at about 1850F. The coated substra~s had a layer of about l mg/cm2 Na2SO4 deposited upon 1:hem by dip-dry process. The thus coated samples were then exposed to the furnace conditions. The samples were thermally cycled three times a day - they were removed from the furnace, fan cooled to about 350F, and reintroduced into the furnace. Periodically, about once every ~1~6503 - - 10 - 57,650 168 hours, the s~mples were removed from the furnace, cooled to ambient, weighed, coated with fresh Na2S04 and placed back into the furnace The results of the testing are shown in Fig. 1 where the x-axis is the hours of exposure with the thermal cycling and the y-axis is the weight change of the sample. The weight change represent-; the difference between the weight gain due to alumina oxide scale formation and the weight loss due to scale cracking, ~nd scale/coating spallation. Lower weight changes represent:less of a reaction rate and scale spallation indicative of a longer coating life and higher degree of protection afforded by the coating. As can be seen from the Fig. 1, the weight change was marked improved by the replacement of the Y and the addition of the Al203 either with or without the Si, however the addition of the Si further improved the coating life characteristics.

Claims

1. A particulate alloy composition useful for coating a metallic substrate within a turbine or engine to impart improved oxidation resistance upon exposure of the substrate to the combustion products of sulfur containing fuels, comprising nickel, cobalt, from 8-50% wt. chromium, from 6-40% wt. aluminum, and from 0.1-10% wt. particulate inert metallic oxide; wherein the aluminum oxide is uniformly distributed within the composition.

2. The particulate alloy composition of claim 1 wherein the nickel, chromium, aluminum, and cobalt are present as a base atomized alloy composition in particulate form and wherein the inert metallic oxide comprises aluminum oxide and is admixed with said base alloy composition.

3. The particulate alloy composition of claim 2 further comprising 0.01-5% wt. silicon in the alloy composition.

4. The particulate alloy composition of claim 2 further comprising 0.01-4% wt. hafnium in the alloy composition.

5. The particulate alloy composition of claim 2 further comprising 0.01-10% wt. yttrium in the alloy composition.

6. The particulate alloy composition of claim 2 wherein the aluminum oxide is present in an amount of from 0.5-3% wt in the alloy composition.

7. The particulate composition of claim 6 further comprising 0.01-5% wt. silicon in the alloy composition.

8. The particulate alloy composition of claim 7 further comprising 0.01-4% wt. hafnium in the alloy composition.

9. The particulate alloy composition of claim 6 further comprising 0.01-4% wt. hafnium in the alloy composition.

10. The particulate alloy composition of claim 9 wherein the cobalt is present in an amount of from 10-40% wt.
in the alloy composition and the balance is essentially nickel.

11. The particulate alloy composition of claim 9 wherein the nickel is present in an amount of from 10-40% wt.
in the alloy composition and the balance is essentially cobalt.

12. A turbine containing a metallic substrate having an improved alloy coating for inhibiting oxidation upon exposure to sulfur containing fuels, comprising:
(a) a tubine component metallic substrate having an exposed face;
(b) an alloy coating deposited upon said substrate exposed face, said alloy coating comprising nickel, cobalt, from 8-50% wt. chromium, from 6-40% wt. aluminum, and from 0.1-10% wt. aluminum oxide; wherein the aluminum oxide is present throughout said coating.

13. The coated substrate of claim 12 wherein said coating has a first surface bound to said exposed substrate face and a second surface opposite said first surface, the distance between the first and second surfaces defining the thickness of the coating, said thickness being from about 0.003 inches (0.0076 cm) to about 0.01 inches (0.025 cm), wherein the concentration of the aluminum oxide is uniform throughout the thickness of said coating.

14. The coated substrate of claim 13 wherein the alloy coating further comprises 0.01-5% wt. silicon.

15. The coated substrate of claim 13 wherein the alloy coating further comprises 0.01-4% wt. hafnium.

16. The coated substrate of claim 13 wherein the alloy coating further comprises 0.01-10% wt. yttrium.

17. The coated substrate of claim 13 wherein the aluminum oxide is present in an amount of from 0.5-3% wt of the alloy coating.

18. The coated substrate of claim 17 wherein the alloy coating further comprises 0.01-5% silicon.

19. The coated substrate of claim 18 wherein the alloy coating further comprises 0.01-4% wt. hafnium.

20. The coated substrate of claim 17 wherein the alloy coating further comprises 0.01-4% wt. hafnium.

21. A method of coating a tubine metallic substrate to improve its resistance to oxidation upon exposure to combustion products of sulfur containing fuels, comprising the steps of:
(a) providing a turbine component metallic substrate comprising an exposed face;

(b) coating said substrate on its exposed face with an alloy, said alloy coating comprising nickel, cobalt, from 8-50% wt. chromium, from 6-40% wt. aluminum, and from 0.1-10%
wt. aluminum oxide; wherein the concentration of the aluminum oxide is substantially uniform throughout the thickness of said coating, said thickness being from about 0.003 inches (0.0076 cm) to about 0.01 inches (0.025 cm).

22. The method of claim 21 wherein the alloy coating further comprises 0.01-5% silicon, and the aluminum oxide is present in an amount of from 0.5-3% wt. of the coating alloy composition.