Technical Field

This invention has to do with the production of interdiffusion alloy coatings such as are employed on high performance metal parts, including turbine vanes and blades, pump parts, and other parts subject to unusual demands for erosion, corrosion and wear resistance. More particularly, the invention relates to improvements in methods of producing such coatings on a wide variety of parts, which enables the rapid, repeatable obtention of superior performance parts of individually tailored properties through the selective inclusion, in a controlled interdispersion, of refractory oxide or hard carbide material and/or elemental metal additives, which are interdispersed, i.e. exist as discrete particulate material in the interdiffusion layer, with a high degree of control as to location, concentration, and, thereby., as to properties of the final product.

Background Art

It is known to form intermetallic compounds and alloys on high performance parts such as turbine blades and blades by subjecting the surface of the part to a diffusion of one or more constituents of a diffusion pack to form an interdiffusion layer, sometimes referred to as a diffusion coating. Typically, the pack comprises the one or several metals to be diffused, frequently aluminum, aluminum oxide, a halogen and possibly other materials depending on the particular objective of the diffusion. The pack is heated for long periods at very high temperatures, and an intermetallic compound is formed typically both outwardly and inwardly of the part original surface. This kind of coating can lend extreme corrosion resistance properties to

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««ore easily formed, less costly metals, is renewable, and otherwise provides improved properties at lower overall cost.

The following patents have been considered in connection with the preparation of this application:

U.S.P. 3,345,197 U.S.P. 3,573,963 U.S.P. 3,677,789 U.S.P. 3,961,910 U.S.P. 4,070,507 U.S.P. 4,105,512

U.S. P. 3,345,197, to Martini et al , teaches the incorporation by codiffusion with aluminum, of undissolved materials, or interdispersed phases ^' such as aluminum oxide, for particular purposes involved with improvements in the properties of the final coating. To accomplish this Martini finely grinds the aluminum oxide, e.g. to a size as fine as 5 microns, and mixes the resultant ^• colloidal" material with aluminum to form a powder, and then heats the part in the powder to achieve an aluminide coating with interdispersed aluminum oxide, some of which has migrated out of the powder into the surface of the part. Martini is limited in his pack composition to the extremely finely ground aluminum oxide, and to the use of aluminum as the diffusing metal. No halide is used to assist in aluminum diffusion. Thus limited, the Martini method is of limited utility, although the product realized has potential.

Important among the disadvantages of Martini is the alteration of conventional pack compositions, as by requiring quantities of finely ground aluminum oxide as a necessary part of the pack, at the risk of not achieving the desired incorporation of refractory oxide material in

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e final coating, where the particle size is excessive. The flexibility of result in Martini is quite limited, since incorporation of the oxide from within the pack is restricted to but one value of refractory material density in the diffusion coating and that value is dependent on the original pack composition.

Description of the Invention

It is accordingly an object of the present invention to provide improvements in methods of diffusion coating whereby the desirable results of interdispersed phase are realized and without the processing disadvantages heretofore necessarily suffered. It is a further object to provide method and product which has the benefits of a wide variety of incorporated materials, with essentially conventional pack formulation, and with no critical limits on particle sizes of pack constituents. Additionally, more active packs can be used, and multiple material incorporations effected simultaneously.

These, and other objects of the invention to become apparent hereinafter, are realized in the method of forming diffusion coatings for improved corrosion, erosion or wear properties in high performance alloy structures wherein the coatings are characterized by the intimate presence of an inter ispersed phase material in desired amount to enhance one or more of the properties, which includes interdif fusing under non-oxidizing conditions a portion of the structure surface and an alloying element disposed therewith in a ditfusion pack of predetermined cc position desirable for the interdiffusion, through the improvement comprising maintaining the desired amount of interdispersed phase material relatively richly concentrated within a selected pack zone correspon ing to the locus of

interdiffusion for intimate diffusion coating interdispersal from the pack zone in interdiffusion responsive relation and without varying the predetermined composition of the pack..

It will be noted that the method thus provides for obtaining special phase intermixing in conjunction with conventional pack diffusion, locally at the part surface, and without need of accommodating special composition or component size dictates which have heretofore been thought necessary to achieve like results.

In particular aspects, the invention includes: maintaining the interdispersed phase material as an adherent film at the locus at the inception of interdiffusion; maintaining a refractory oxide separate from the pack constituents and within the interdiffusion locus to form the interdispersed phase; maintaining a hard carbide separate from the pack constituents and within the interdiffusion locus to form the interdispersed phase; maintaining a metal having an atomic number of 24 or higher separate from the pack constituents and within the interdiffusion locus to form the interdispersed phase; supporting the interdispersed phase material on a substrate within the pack in relative rich concentration defining rela ^' tion; having the surface define the interdispersed phase material substrate; forming a coating of the interdispersed phase material on the structure surface as the locus of interdiffusion concentration of the interdispersed phase; supporting the interdispersed phase material on the structure surface with an organic binder; forming an adherent layer of interdispersed phase material in organic liquid binder on the structure surface, and drying the binder; adhering to the structure surface at the

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_ocus of interdiffusion an interdispersed phase material selected from refractory oxides comprising zirconia, alumina, titania, magnesia, yttria, and hafnia, hard carbides comprising tungsten carbide, tantalum carbide, zirconium carbide, titanium carbide, silicon carbide, and chromium carbide, and/or adhering to the structure surface at the locus of interdiffusion an interdispersed phase material selected from the group consisting of tungsten; tantalum, molybdenum, zirconium, titanium, hafnium, yttrium, and chromium.

Preferred Modes

"Interdispersed" and its cognitives herein refer to a discrete partiσulate higher atomic number metal, i.e. having an atomic number of 24 or above, of a refractory oxide material, or of a hard carbide material, incorporated in a diffusion-formed alloy or intermetallic compound.

EXAMPLE I A conventional diffusion pack comprising per 100 parts by weight, 6 parts of aluminum powder, -200 mesh, and 94 parts of aluminum oxide powder, 100 to 325 mesh, is prepared. A jet engine blade comprised of nickel superalloy of the percent composition:

0.12 Carbon 12.5 Chromium 4.2 Molybdenum

2.0 Columbium ø.8 Titanium

6.1 Aluminum 0.012 Boron 0.1 Zirconium

Bal. Nickel is coated with a adherent layer of 10 micron powder

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aluminum oxide, by painting onto the part the oxide suspended in a lacquer binder. After the binder dries, the aluminum oxide is firmly bound to the part surfaces and the part, thus coated, is immersed in the mentioned pack. The pack is heated to 1050-1100O Centigrade, in a hydrogen atmosphere, for 4-5 hours. A second engine blade of identical composition, also placed in the pack during the heat, is also diffused but without first precoating with aluminum oxide as a Control. Microseσtions of the Example and Control parts are compared by icroscopic examination. Comparison of the Example and Control parts reveals that each has a diffusion coating which extends both inwardly and outwardly from the nominal surface of the part before diffusion. The Example part exhibits an oxide dispersion characterized by uniform distribution of an interdispersed phase of aluminum oxide particulate, which will greatly enhance erosion resistance of the part. The Control part, on the other hand, exhibits no significant aluminum oxide interdispersed phase, and thus is unimproved in erosion resistance by the described diffusion procedure.

EXAMPLE II Example I is duplicated using as the pack composition, in lieu of the aluminum powder in the pack, titanium powder, -200 mesh (Example la) ; boron powder, -325 mesh (Example lib); and silicon powder, -200 mesh (Example l e) . The interdispersed material is also varied using tungsten carbide (Example lid) , tantalum carbide (Example lie) , zirconium carbide (Example Ilf ) ; titanium carbide (Example Ilg) ; silicon carbide (Example Ilh) ; chromium carbide (Example Hi) . The interdispersed material is also varied using tungsten (Example IIj) , tantalum (Example Ilk) , molybedenum (Example III) ; zirconium (Example Urn) ; titanium (Example Iln) ; hafnium (Example

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IIo); yttrium (Example Up); all powdered and held on with organic binder, and plated-on chromium (Example Ilq) as the metal coating on the blade part; and in lieu of the aluminum oxide refractory coating on the part, zirσonia (Example Hr) ; titania (Example IIs) ; magnesia (Example l t) ; yttria (Example IIu) ; and hafnia (Example IIv) .

The products of Examples Ila-v are improved in corrosion, erosion and/or wear performance over controls produced at the same time, but without maintaining the particular interdispersed material at the interdiffusion locus, by the expedient of coating the part surface with the interdispersed material, in advance of subjecting the part to pack diffusion. Also, it will be noted that the pack compositions need not be specially formulated, either as to constituents or particle size of constituents. The disclosed method therefore affords a product of variously improved properties with only the coating on the part to be diffused being varied, for a given pack composition.

The reason for the improvement in properties of a part diffused with an interdispersed phase over a part without such a phase appears to be that the dispersed phase can stabilize a protective oxide phase formed in part use, provide some protection against impinging particles, or change the macroscopic hardness of the coating.

The interdispersed phase may be a refractory oxide as indicated above whereby the property improvements are typically improved erosion resistance, a hard carbide whereby the property improvements are typically wear resistance, or a metal whereby the property improvements are typically in corrosion resistance.

Typical pack diffusion conditions for carrying out

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the present method are temperatures between 760o C. to lløøo C. and heating for 2 to 20 hours.

Suitable binders, if used, are synthetic organic polymeric materials suitably soluble in organic vehicles which will evaporate at a satisfactory rate for formation of adherent coating on a part to be diffused, or a portion of such part, where specifically varied properties are desired.