The present invention relates generally to oxidation and corrosion resistant coatings, and relates more particularly to aluminide coatings containing one or more active elements and improved oxidation and corrosion resistance.

Overlay coatings are widely used in high temperature and/or corrosive environments, for example in gas turbine engines, as a stand alone coating, e.g., to provide high temperature corrosion and oxidation resistance to the underlying substrate, and also as an adherent bond coat for a subsequently-applied ceramic thermal barrier coatings. A typical overlay coating is an MCr, MCrAl or MCrAlY coating, such as a coating disclosed in commonly-owned U.S. Pat. No. 4,585,481 and Reissue No. 32,121, both to Gupta et al. The M is selected from the group including nickel, cobalt and iron or combinations of these elements. The Y typically indicates yttrium but may also include silicon and/or other active elements such as hafnium. Overlay coatings are generally, although not necessarily, applied by plasma spraying. See, e.g., U.S. Pat. Nos. 4,321,311 and 4,585,481 and Reissue No. 32,121. Application of overlay coatings by other applications, including but not limited to, electron-beam physical vapor deposition, chemical vapor deposition, cathodic arc and electroplating are also possible. While the bond coat thickness may vary depending upon the particular component and application, the illustrated bond coat typically has a thickness of less than about 5 mils (125 µm), although thicker or thinner coatings are also used.

Aluminide coatings are also used in high temperature and/or corrosive environments, for example in gas turbine engines, as a stand alone coating, e.g., to provide high temperature corrosion and oxidation resistance to the underlying substrate, and as an adherent bond coat for a subsequently-applied ceramic thermal barrier coating. Some aluminide coatings also include one or more noble metals, which enhance erosion and/or corrosion resistance. See, e.g., U.S. Pat. No. 5,856,027 to Murphy. Aluminide coatings, including those containing noble metal(s), are traditionally applied by a pack process or by chemical vapor deposition (CVD). In a typical "in pack" process, the article to be coated is usually initially electroplated with a noble metal, and is then placed in a pack containing a source of aluminum, an activator, e.g., halide, and inert materials, e.g., alumina. The pack and article are then heated, forming vapors of the aluminum, which reacts with the nickel or cobalt in the article to form the aluminide. The coatings may be further heat treated to obtain desired coating properties. In a typical CVD process, individual generators produce aluminum vapors, and the vapors are conveyed into a chamber to a heated article to be coated where the vapors condense and react with the nickel or cobalt in the article to form the aluminide.

It is generally accepted that it is difficult to produce active element containing aluminides of consistent quality. It is also generally accepted that it is at least as difficult to consistently produce aluminide coatings containing more than one active element.

Numerous patents describe various overlay and aluminide coating compositions and processes. Exemplary patents are identified below.

U.S. Pat. No. Re 32,121 describes forming an MCrAIY (M including nickel, cobalt or a combination) bond coat by plasma spraying, with the MCrAIY composition including about 0.1 - 0.7 % silicon, and 0.1 - 2 % hafnium.

U.S. Pat. No. 4,897,315 describes a plasma sprayed NiCoCrAlY overlay, which is then aluminized to form an aluminide coating on the substrate.

U.S. Pat. No. 5,658,614 describes a platinum aluminide (without an MCrAlY) coating formed by electroplating the platinum onto the substrate, and then aluminizing by CVD.

It is a general object of the present invention to provide a coating having improved properties. It is another object to provide an aluminide coating, and a process for applying an aluminide, having improved durability.

It is still another object to provide a repeatable process for forming active element(s) containing aluminide coatings that produces high quality coatings more consistently.

According to the invention, a method is disclosed for improving the corrosion and oxidation resistance of a substrate. The method includes providing a superalloy substrate, and an overlay coating including at least one oxygen active element which is applied onto the substrate by an overlay step. Platinum is applied onto the overlay coating, by electroplating. The overlay coating and metal is then aluminized by chemical vapor deposition. A ceramic thermal barrier coating may also be applied. A coated article is also disclosed.

Certain preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which:

• FIG. 1 is a photomicrograph of a coating of a preferred embodiment, including a ceramic coating.
• FIG. 2 is a flow diagram illustrating a preferred process for fabricating the coating of the present invention.

Turning now to FIG. 1, a substrate having an aluminide coating in accordance with the present invention is illustrated by the reference numeral 10. The coating may serve as a stand alone coating, e.g., for high temperature oxidation resistance, or as an adherent bond coat for a subsequently-applied thermal barrier such as a layer of ceramic material, e.g., stabilized zirconia. In the embodiment illustrated in FIG. 1, a ceramic layer is applied over the aluminide to form a thermal barrier. The ceramic layer may be a stabilized zirconia, such as is disclosed in commonly owned U.S. Pat. 4,321,311 to Strangman or Ser. No. 09/164,700 to Maloney both of which are commonly owned with the present invention and are expressly incorporated by reference herein.

A substrate 12 is typically composed of nickel, cobalt and /or iron base superalloy material. As described further below with reference to FIG. 2, an overlay coating 14 such as an MCrAI type coating is first applied to the substrate, by low pressure plasma spray and together with one or more oxygen active elements such as hafnium, yttrium and silicon or other oxygen active element. Platinum is then deposited by electroplating, and is aluminized to form an adherent alumina layer 16. The article is also heat treated to provide the coating with desired properties, e.g., improved mechanical properties. As indicated in FIG. 3, the present invention provides coatings having improved properties, e.g., corrosion and oxidation resistance and durability relative to prior coatings. While the invention illustrated below is used with a nickel base, cobalt base or iron base superalloy material, the invention is not limited to use with these materials.

Typical compositions of such alloys are shown in Table 1. Exemplary U.S. Patents describing columnar and single crystal and directionally solidified alloys include 4,209,348; 4,643,782; 4,717,432; 4,719,080 and 5,068,084, each of which is expressly incorporated by reference herein. Cooling holes, which may be positioned on one or more portions of a turbine blade, may be provided for flowing cooling air over the specific portions of the airfoil during operation, as is known generally in the art. Alloy Type Ni Co Cr Al Mo Ta W Re Hf Ti Nb PWA 1422 DS Bal. 10 9 5 - - 12 - 1.6 2 1 DS R80H DS Bal. 9.5 14 3 4 - 4 - 0.75 4.8 - CM247LC DS Bal. 9.2 8.1 5.6 0.5 3.2 9.5 - 1.4 0.7 - PWA 1480 SC Bal. 5 10 5 - 12 4 - - 1.5 - PWA 1484 SC Bal. 10 5 5.65 1.9 8.7 5.9 3 0.1 - - Rene' N5 SC Bal. 7.5 7 6.2 1.5 6.5 5 3 0.15 - - CMSX-4 SC Bal. 9 6.5 5.6 0.6 6.5 6 3 0.1 1 -

Other alloys include, for example, Rene N4 and CMSX-2, which are described in the prior art.

Generally, the active element(s) is applied by a conventional overlay process, and may or may not contain other elements, e.g., as part of an MCr or MCrAI overlay coating. In accordance with the present invention, an overlay coating such as an MCrAI is applied to the substrate surface by low pressure plasma spray. As is known, M indicates nickel, cobalt, iron and mixtures thereof. The bond coat also includes at least one oxygen active element, e.g., yttrium, hafnium, silicon or others. As applied, the present invention includes an overlay coating having a thickness of between about 1 - 5 mils (0.001 - 0.005 inches, (25.4-127 µm).

The overlay coating (described further in an example below) according to the invention which we have used successfully is a NiCoCrAl coating with added Y, Hf and/or Si. In broad terms, the coating is composed in weight percentage of about 5 - 40 Cr, 8 - 35 Al, up to 2 Y, 0.1 - 7 Si, 0.1 - 5.5 Hf, balance Ni and/or Co.

Platinum is then deposited on the MCrAI coating. We believe that the final coating should contain in weight percent 10 - 11 Pt as described below we have obtained good results. Platinum is deposited by electroplating, in a known manner. Plating the platinum to a thickness of about 0.05 - 0.15 mils (1.3-3.8 µm) shood provide a final coating with the above-desired transition metal content. Plating processes are known generally and are not described here in detail.

Aluminum is then applied to the part by chemical vapor deposition to aluminize the part. While CVD aluminizing processes are known generally and are not described here in detail, a coating gas has a composition including some amount, in vol. % of a carrier gas, such as hydrogen, and an amount, in vol. %, of an aluminum containing gas. The coating gas may be formed by passing a carrier gas over a source of aluminum. The above process is adjusted as desired to provide a given quantity of aluminum on the part surface. The coating gas is then impinged upon the heated substrate, at a given delivery rate, with the substrate typically being heated to between about 1800 - 2200 F (982 - 1204°C) and preferably about 1950 - 2000F (1065 - 1093°C)

After aluminizing, the coated part is diffusion heat treated. The diffusion heat treatment includes heating the component to a temperature of about 1975 F (1079°C) for a sufficient time, of about 3 hours, followed by a precipitation heat treatment, at about 1600 °F (871°C) for about 16 hours. A resulting coating is illustrated in FIG. 1, which includes a subsequently applied, columnar grain, ceramic thermal barrier layer.

As noted above, coatings in accordance with the present invention may be employed to provide stand alone coatings or bond coats for subsequently-applied ceramic thermal barrier coating. Typical ceramics are zirconia based, and may be partially or fully stabilized with additions of yttria or other appropriate stabilizer. Exemplary ceramic coatings composed of yttria stabilized zirconia (YSZ) are described, for example, in commonly-owned U.S. Pat. Nos. 4,321,311, 5,262,245. The ceramic may be applied by EB-PVD, by plasma spray or by another suitable method.

Samples were prepared using superalloy substrates in accordance with preferred embodiments and some were also coated with a standard, zirconia based columnar ceramic thermal barrier coating. The overlay coating as described above was applied by low pressure plasma spray, and then plated with platinum in a known manner. The samples were aluminized using a coating gas included about 80 vol. % of a carrier gas, such as hydrogen, and about 20 vol. %, of an aluminum containing gas, in this case AlCl₃. The coating gas was formed by passing HCI over a source of aluminum at about 600 C. The coating gas was impinged upon the heated substrate, at a delivery rate of about 224 standard cubic feet per minute, with the substrate heated to a nominal temperature of between about 1950-2000 F (1065-1093°C). Some of the samples were then coated with a ceramic thermal barrier coating composed of yttria stabilized zirconia, as taught for example in the above referenced '311 patent to Strangman, while other samples were tested without such a ceramic coating.

Coated articles in accordance with the present invention have been tested in a burner rig apparatus. Testing indicated that the present invention coatings, which tests included samples having a subsequently applied thermal insulating layer, are about 2 - 3 times more durable than current TBC's. The inventive coated articles were also tested as stand alone coatings, e.g., no overlying ceramic layer, and also demonstrate improved protection and durability.

The samples were tested in high temperature burner rigs. The test cycles comprised 117 minute exposure at 2150 degree F (1176°C) followed by 3 minute air cooling per cycle together with standard platinum aluminides. The samples prepared in accordance with the present method exhibited improved lives over the samples including the standard aluminides by a factor of about 2.5.

As a result of the testing, we believe that the composition of the final coating is, in weight percent, 10 - 11 Pt, 2.6 - 4.2 Si, 13.4 - 13.6 Al, 3.9 - 5.3 Hf, remainder Ni, Co, and Cr.

The present invention provides significant advantages over prior processes. The improved process enables the production of active element containing aluminide coatings having significantly more consistent compositions, and thus significantly more consistent properties and improved durability. The quality of the resulting coatings are thus similarly improved.