Superalloy component and slurry composition

The present invention relates to a slurry composition for aluminising a superalloy component and a superalloy component comprising an aluminide coating.

Components that are exposed to hot corrosive gases, like for example gas turbine components which are exposed to the hot and corrosive combustion gas, are typically made of superal- loys which offer high strength at elevated temperatures. Such superalloys are, in particular, nickel based super alloys, iron based superalloys and cobalt based superalloys.

Although superalloys show high strength at temperatures up to 1,200 ⁰ C and more they need to be protected from oxidation and/or corrosion. This protection is typically done by alumina forming coatings such as so called MCrAlY-overlay coat- ings, in which M stands for iron (Fe) , cobalt (Co) or nickel (Ni) and Y represents yttrium (Y), hafnium (Hf) or a rare earth element. During the preparation of the overlay coating the aluminium is oxidized and forms a tightly adherent scale of alumina which protects the components from oxidation and corrosion. Such coatings, for example, are known from EP 0 486 489 Bl, EP 0 786 017 Bl or EP 1 306 456 Al.

Another method of forming corrosion and/or oxidation resistant scale of alumina is the so called slurry aluminising. In a slurry aluminising process an aluminium containing slurry is coated onto the surface of the component. Then, the component is subjected to a heat treatment which leads to melting of the aluminium. The melted aluminium then reacts with the material of the component and forms an aluminide coating with a scale of alumina protecting the component from oxidation and/or corrosion. In addition to aluminium, the slurry can also contain silicon leading to an aluminide-silicide coating which shows better oxidation resistance than the aluminide coating alone. Forming aluminide coatings and aluminide- silicide coatings by a slurry process is, for example, disclosed in US 5,547,770, US 6,080,246, EP 1 820 875 A2 , US 2007/0298269 Al and US 6,126,758.

Besides, it is known from US patent US 3,720,537 that a slurry technique may be used for coating upon alloy substrates. Finely divided alloys of cobald-aluminum, nickel- aluminum and/or iron-aluminum are mixed with chromium- aluminum alloy. Yttrium may be added as an alloy.

It is an objective of the present invention to provide an advantageous slurry composition for aluminising a superalloy component, in particular a nickel (Ni) based, cobalt (Co) or iron (Fe) based superalloy component. It is a further objective of the present invention to provide an advantageous superalloy component comprising an aluminide coating.

The first objective is solved by a slurry composition for aluminising a superalloy component, as claimed in claim 1. The second objective is solved by a superalloy component comprising an aluminide coating, as claimed in claim 7. The depending claims contain further developments of the invention.

An inventive slurry composition for aluminising a superalloy component comprises an organic binder, i.e. a solvent like water with binder salts, and a solid content. The solid content includes aluminium. According to the invention, the slurry composition further comprises hafnium (Hf) and/or yttrium (Y) , specifically hafnium and yttrium. The hafnium and/or the yttrium may either be added as powder, in form of flakes, swarfs or the like to the solid content of the slurry or in form of salts to the organic binder. As organic binders used in the inventive slurry chromate and/or phosphate organic binders are of particular use in the context of the present invention.

It has been found that the addition of hafnium and/or yttrium to the slurry leads to an improvement in oxidation and corrosion protection provided by an aluminide coating which is formed after heat treating a superalloy component provided with the inventive slurry. An advantageous hafnium content is up to about 2.5% by weight of the slurry, preferably up to about 1% by weight of the slurry. An advantageous yttrium content is up to about 0.1% by weight of the slurry in particular up to about 0.05% (5ppm) by weight of the slurry.

In addition to aluminium, the solid content may include silicon between 1% and 40% by weight of the total solid content. Like hafnium and/or yttrium the silicon improves the oxidation and corrosion resistance of a coated superalloy component by forming an aluminide-silicide coating rather than an aluminide coating alone.

In a further development of the inventive slurry composition the slurry comprises at most a sulphur (S) content of 5ppm by weight of the slurry. Since sulphur is corrosive for the su- peralloy material the presence of sulphur in the slurry would weaken the superalloy component when the alumina scale is formed during the heat treatment of the slurry coated component. By reducing the sulphur content in the slurry as much as possible weakening of the component during the heated treatment due to sulphur can be avoided.

An inventive superalloy component comprises an aluminide coating. The coating material comprises at least one layer which comprises hafnium and/or yttrium - specifically hafnium and yttrium - in addition to aluminium. It has been found that corrosion resistance and/or oxidation resistance of the aluminide coating can be improved if hafnium and/or yttrium is/are present in addition to the aluminium.

In a special embodiment of the inventive superalloy component the coating comprises at least two layers wherein the coating material of one of the layers comprises aluminium without hafnium and without yttrium. The coating material of the other layer comprises hafnium and/or yttrium. In a further development, the coating comprises at least three layers wherein the coating material of two of the layers comprises aluminium without hafnium and without yttrium. The coating material of the third layer comprises hafnium and/or yttrium in addition to aluminium. This third layer can, in particular, be sandwiched between layers without hafnium and/or yttrium. It shall be noted, however, that although layers with yttrium and/or hafnium and layers without yttrium and/or hafnium are present in the described development of the inven- tion, the invention is not restricted to cases in which layers without hafnium and without yttrium are present. In particular, both layers or, in case of three layers, two layers or all three layers may contain hafnium and/or yttrium in the context of the present invention.

If a layer of the coating of the inventive superalloy component comprises hafnium in addition to aluminium the hafnium content is preferably up to about 3% by weight of the layer components. Such a layer can, for example, be achieved by a slurry comprising 70% binder and up to 1% hafnium by weight of the slurry.

If the layer comprises yttrium in addition to aluminium the yttrium content is preferably up to about 0.17% by weight of the layer components. Such a layer can be achieved by using a slurry comprising up to 70% organic binder and 0.05% (5ppm) yttrium by weight of the slurry. According to a further development of the invention, the coating material comprises at least one layer. Coating material of that at least one layer comprises silicon in addition to aluminium. The silicon content of this layer may be between 1% and 40% by weight of the layer components. Note that it is possible that all aluminium containing layers also contain silicon.

Further features, properties and advantages of the present invention will become clear from the following description of an embodiment of the invention in conjunction with the accompanying drawings .

Figure 1 shows a first embodiment of an inventive component.

Figure 2 shows a second embodiment of an inventive component .

Figure 3 shows a third embodiment of an inventive component.

A first embodiment of an inventive superalloy component is shown in Figure 1. The Figure shows the component, which may be a gas turbine component like a turbine blade or vane, or a liner element of a hot gas flow path, in a highly schematic sectional view. The view shows the superalloy 1 with a coating 3 applied thereon. The superalloy 1 may, in particular, be one of the superalloys disclosed in EP 1 204 776 Bl, EP 1 306 454 Bl, EP 1 319 729 Al, WO 99/67435 Al or WO 00/44949 Al.

Specifically according to EP 1 204 776 Bl a superalloy 1 may be made from a nickel-base superalloy, the composition of which comprises the following elements, in percent by weight: 11-13 % chromium, 3-5 % tungsten, 0.5-2.5 % molybdenum, 3-5 % aluminium, 3-5 % titanium, 3-7 % tantalum, 0-12 % cobalt, 0-1 % niobium, 0-2 % hafnium, 0-1 % zirconium, 0-0.05 % boron, 0- 0.2 % carbon, 1-5 % rhenium, 0-5 % ruthenium, and a remainder Ni and impurities.

From EP 1 306 454 Bl a protective layer for protecting a com- ponent against corrosion and oxidation at high temperatures is known, which is composed of the following elements (details in per cent by weight) : 0.5 to 2% of rhenium, 15 to 21% of chromium, 9 to 11.5% of aluminium, 0.05 to 0.7% of yttrium and/or at least one equivalent metal from the group consist- ing of scandium and the rare earths, 0 to 1% of ruthenium, and a remainder cobalt and/or nickel and production-related impurities .

According to EP 1 319 729 Al, the superalloy 1 may be made from a nickel-base superalloy, the composition of which comprises the following elements, in percent by weight: 9 - 10.75% of chromium, 3 - 5% of tungsten, 0.5 - 2.5% of molybdenum, 3 - 5% of aluminium, 3 - 5% of titanium, 3 - 7% of tantalum, 0 - 12% of cobalt, 0 - 1% of niobium, 0 - 2% of hafnium, 0 - 1% of zirconium, 0 - 0.05% of boron, 0 - 0.2% of carbon, at least one element selected from the group consisting, of rhenium, ruthenium, in which 1 - 5% rhenium, and/or 0.1 to 5% ruthenium is used, and a remainder Ni and impurities .

Nickel Aluminide coatings are also provided by the company

Sermatech (R) International under the brand names "SermaLoy(R) J" and "SermaLoy (R) 1515". "SermaLoy(R) J" is a slurry applied aluminide diffusion coating designed for protection of gas tur- bine hot section components and industrial components suffering from hot corrosion. It is applied to the component surface, then diffused. This results in a coating that adheres tightly to the part surface. "SermaLoy 1515" is designed to protect components made of high-strength, low-chromium nickel-base alloys extremely resistant to both high and low temperature hot corrosion, as well as oxidation. It is de- signed specifically to protect the inside cooling passages of turbine blades.

The coating 3 is an aluminide-silicide coating which com- prises hafnium and yttrium. The hafnium content is about 3% by weight of the layer components while the yttrium content is about 0.17% by weight of the layer components.

The coating 3 - corresponding to the "first layer" according to the claims - is formed by a slurry aluminising process. In this process, a solid content comprising aluminium and, optionally, silicon wherein the silicon content is between 1% and 40% of the total weight of the solid content is mixed with an organic binder comprising chromate and phosphate to form a slurry which can be applied to the surface of the su- peralloy 1. Hafnium powder is added to the slurry with an amount of 1% by weight of the slurry. Furthermore, yttrium powder is added to the slurry to an amount of 0.05% by weight of the slurry. When the slurry is formed the sulphur amount of the slurry is carefully controlled to a very low trace amount of less then 5ppm.

The slurry is then applied to the surface of the superalloy component, for example by spraying it onto the receiving part. After applying it onto the surface of the component the slurry is cured by heat treatment of about 300 ⁰ C. The aluminium, the optional silicon, the hafnium and the yttrium contained in the cured slurry are then diffused into the surface of the superalloy 1 by a diffusion heat treatment at tempera- tures between 700 ⁰ C and 1,100 ⁰ C. By use of the diffusion heat treatment the aluminium, the optional silicon, the yttrium and the hafnium are incorporated into a surface layer of the superalloy 1 while the solvent of the organic binder is vola- tised. The remaining components of the binder that have not been volatised can be easily removed by mechanical means like, for example, by lightly blasting the surface. A second embodiment of the inventive component is shown in Figure 2. This embodiment differs from the first embodiment in that a second coating layer 5 is applied on the coating layer 3 of the first embodiment. Thus the coating 3 will be arranged between the superalloy 1 and the second coating layer 5. Coating layer 3 will also be called first coating layer 3 in respect to the second embodiment. The second coating layer 5 can, in principle, be a coating layer as it has been described with respect to the first coating layer 3 of the first embodiment. However, it is also possible to provide a coating layer neither containing yttrium nor hafnium as the second coating layer 5.

In a modification of the second embodiment, the first coating layer 3 only contains one of the elements hafnium and yttrium while the second coating layer contains the other one of these elements. For example, the first coating layer 3 may contain hafnium while the second coating layer 5 contains yttrium. Such layers could be produced with the process de- scribed with respect to the first embodiment for producing the coating layer 3, if only hafnium or yttrium is added to the respective slurry.

A third embodiment of the inventive superalloy component is shown in Figure 3. The coating of the third embodiment is a three layer coating with, as seen from the superalloy substrate 1, a first coating layer 7 - corresponding to the "second layer" according to the claims - neither containing yttrium nor hafnium, a second coating layer 9 - corresponding to the "first layer" according to the claims - containing yttrium and hafnium, and a third coating layer 11 - corresponding to the "third layer" according to the claims - containing neither yttrium nor hafnium. All layers can be produced according to the process described above by just adding or not adding yttrium and/or hafnium to the respective slurry. The embodiment shown in Figure 3 can be modified in that either of the first coating layer 7, the second coating layer 9 and the third coating layer 11 may contain at least one of the elements hafnium and yttrium. In particular, all three layers my contain yttrium and/or hafnium. On the other hand it would be possible, for example, that the first coating layer 7 contains neither yttrium nor hafnium, the second coating layer 9 contains yttrium and the third coating layer 11 contains hafnium.

Note that for each coating layer a heat treatment or a laser treatment as mentioned above will be performed when the coating comprises more than one coating layer.

In all three embodiments the hafnium content is preferably up to about 3% by weight of the layer components and/or the yttrium content is up to about 0.17% by weight of the layer components, which can, for example, be achieved by using a slurry containing, by weight of the slurry, 70% organic binder, 1% by weight hafnium and/or 0.05% by weight yttrium, rest solid content, wherein the solid content comprises aluminium and may further comprise silicon in the range of 1% to 40% by weight of the solid content, in particular 1% to about 20% by weight of the solid content and, in special embodi- ments, about 10% to 15% by weight of the solid content.