Vapor phase co-deposition coating for superalloy applications

Details Vapor phase co-deposition coating for superalloy applications Vapor phase co-deposition coating for superalloy applications

2001-02-07

2003-09-16

Jones, Deborah (Department: 1775)

Stock material or miscellaneous articles

Composite

Of silicon containing

C428S446000, C428S610000, C428S650000, C420S548000

Reexamination Certificate

active

06620518

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to an oxidation and corrosion resistant coating. More particularly, the present invention relates to a coating composition that is produced by a process for co-depositing transition metals on metallic components. This coating is particularly useful in protecting nickel and cobalt and iron-based superalloys from heat corrosion and oxidation attack, especially during high temperature operation. Such coating includes aluminum and silicon and the coated substrate may comprise precious metal, nickel, cobalt or MCrALY. Such coated substrates are particularly useful in gas turbine and jet engine hot zones.
BRIEF DESCRIPTION OF THE PRIOR ART
There are numerous applications in which metal components are exposed to elevated temperatures for prolonged periods of time. In such applications, it is important that the metal components retain their solid strength and mechanical properties after repeated exposures to high temperatures. High temperature operation is often found in turbomachinery blading members such as turbine blades, vanes, nozzles etc. used in aerospace and land-based machinery wherein the temperature of the component, or portion of the component, may rise to well above 1500° F. (815° C.). For example, modern gas turbine engines, commonly known as “jet engines,” frequently operate in high temperature environments in excess of 2000° F.
Components manufactured from what has become known in the art as “superalloy materials” are recognized as generally providing for a higher degree of shape retention, and significantly more strength retention, at a wider variety of temperatures than non-alloy materials. Superalloys include metals containing high nickel, high cobalt and high nickel-cobalt-base. While often exhibiting more desirable mechanical properties at high temperatures, superalloys frequently suffer, as many other metals and alloys, from oxidation, sulfidation and corrosion degradation reactions (as for example when such component is exposed to salt spray and sulfur compounds), all of which are accelerated at high temperatures. While the efficiency of a gas turbine engine generally increases with increasing nominal operating temperature, the ability of turbine blades and vanes made from superalloys to operate at increasingly great temperatures is limited by the ability of the turbine blades and vanes to withstand the heat, oxidation and corrosion effects of the impinging hot gas stream.
Superalloy components are frequently coated with materials that are less prone to such degradation reactions or which form an adherent oxide scale which protects the superalloy material from such reactions. Such degradation-resistant coatings often incorporate elements such as aluminum, silicon, chromium, and platinum group metals, and may comprise composite alloys such as MCrAlY, where M is selected from the group consisting of iron, nickel, cobalt, and various mixtures thereof. A thermal barrier coating, such as a ceramic, may also be bonded to a degradation-resistant coating to further insulate the component from the high temperature, as such ceramic materials often do not directly adhere to the oxidized superalloys themselves. Degradation-resistant coatings and thermal barrier coatings can markedly extend the service life of gas turbine engine blades, vanes, and the like.
Degradation-resistant coatings are often chosen to provide high resistance to oxidation or hot corrosion, with little regard to the mechanical properties of the coating. Degradation-resistant coatings are typically applied in a thickness of 0.001-0.010 inches. Components may be coated differentially depending on whether one or more areas of the component is subjected to more or less degradative environments. Preferably, the degradation-resistant coatings should not crack when subjected to mechanically or thermally-induced strain. If the degradation-resistant coating is designed to form a protective oxide scale on the component, such scale preferably should not be dissolvable in liquids which may come in contact with the coated component.
A wide variety of techniques and processes are known for applying a degradation-resistant coating or layer to the surface of metal articles. Such techniques include diffusion coating, physical vapor deposition, plasma spray, and slurry coating.
Diffusion Coating
In diffusion coating, elements such as Al, Cr, Si, and/or Ti are reacted with halogenated activator at elevated temperatures to form gaseous species of Al, Cr, Si, and/or Ti which condense on the substrate and form a coating. Pack cementation is one of the most commonly employed diffusion coating techniques wherein the parts to be coated are placed in surface contact with the coating source material.
An early example of aluminum-silicon co-deposition by pack cementation is set forth in U.S. Pat. No. 3,779,719 to Clark et al. The Clark et al. reference discloses that mixtures of aluminum, silicon and chromium heated at about 1750° F. for 8 to 12 hours to a maximum coating temperature of 1900° F. produce, by diffusion of such materials into the substrate, corrosion resistant coatings and that performance is enhanced when the silicon content is at least 5% by weight and the Si/Cr weight ratio is within the range of 0.6 to 1.4. A silicon pack cementation process is also described in U.S. Pat. No. 4,369,233 to van Schaik, wherein a silicon containing coating is produced by overcoating surfaces previously treated with active metal species, such as Y or Ti. Preferably, the active metal is said to be ion plated, diffused in a vacuum, and mechanically treated prior to application of the silicon. The van Schaik reference suggests that a protective coating of ternary silicides, such as Ti
6
Ni
16
Si
7
and Ni
49
Ti
14
Si
37
, is formed. Likewise, U.S. Pat. No. 5,492,727 to Rapp et al. describes a pack cementation process wherein chromium and silicon are co-deposited onto ferrous substrates utilizing a dual activator system in a two-step heating cycle.
Physical Vapor Deposition
In physical vapor deposition techniques (“PVD”), metallic components which are to be incorporated into a coating are applied by means of vaporization. Numerous physical vapor deposition techniques have been described in the literature and include above-the-pack (“ATP”), chemical vapor deposition (“CVD”), and electron beam physical vapor deposition (“EB-PVD”). ATP processes are accomplished in a manner similar to pack cementation, however, the substrate is held out-of-contact with the metal containing and activator containing source materials, and a coating forms by physical vapor deposition and diffusion of metal onto and into the substrate. The metal source may be present in powdered form or as metallic chunks. CVD, a specialized form of vapor coating, is usually accomplished using a starting gas. The gas can either be the source of the deposited metals or can be the reactant used to generate the metallic vapor done by passing it over or through a bed of metallic source. CVD processing typically requires more stringent processing controls and cleaner source materials. EB-PVD functions by creating a molten pool of metal from which material evaporates and then deposits on the substrate in a line-of-sight path. ATP, CVD, and EB-PVD processes typically result in coatings that are smoother, cleaner and cosmetically improved compared to parts coated by pack cementation. Numerous examples of ATP, CVD, and EB-PVD and other types of physical vapor deposition processes can be found in the art. The list of reactant sources, materials and substrates used in such processes is long and varied.
U.S. Pat. No. 3,486,927 to Gauje (SNECMA Corp.) discloses a method for vapor depositing aluminum to make a coating that protects metal articles subject to high temperature. At the Third International Conference on Chemical Vapor Deposition (Salt Lake City, Utah 1972), Felix and Beutler demonstrated coated nickel superalloys by CVD methods in a stream of silicon tetrachloride and hydrogen at 980° C. to 1080° C. Resultant coat

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