Stock material or miscellaneous articles – All metal or with adjacent metals – Having composition – density – or hardness gradient
Patent
1994-11-04
1998-08-18
Zimmerman, John
Stock material or miscellaneous articles
All metal or with adjacent metals
Having composition, density, or hardness gradient
428652, 428678, 416241R, B32B 1500, F01D 528
Patent
active
057956596
DESCRIPTION:
BRIEF SUMMARY
This application was filed under 35 U.S.C 371 from PCT/US93/08413, filed Sep. 7, 1993.
TECHNICAL FIELD
This invention relates to aluminide coatings for aluminizing of heat resistant alloy substrates, to the slurry coating compositions for the coatings and to a process for applying such coatings, more specifically to composite coatings containing aluminum-rich and silicon-rich phases to improve their resistance to hot corrosion and oxidation.
During operation, components in the turbine section of a gas turbine are exposed to combustion gas temperatures that can reach 1200.degree. C. (2200.degree. F.). These components are typically made of nickel and cobalt base superalloys specially formulated for strength at these temperatures. However, upon exposure to oxygen in the combustion gases at such high temperatures, these heat resistant materials begin to revert to their natural metal oxide form. The nickel and cobalt oxide scales that form on the surfaces of these alloys are not tightly adherent. During thermal cycling, they crack and spall off the surface exposing more unreacted substrate to the environment. in this manner, oxidation roughens and eventually consumes unprotected parts made of these alloys. See FIG. 1. Adding sodium and contaminants containing chlorine and sulphur to the combustion gases speeds degradation. Above about 540.degree. C. (1004.degree. F.), sodium and sulphur react to form low melting point sulphate salts which not only dissolve the oxide films on nickel and cobalt, but also can directly attack the substrates. See FIG. 2.
BACKGROUND ART
One solution to the hot corrosion and oxidation problem which is widely applied in gas turbine engines, is to allow aluminum into the surface of a superalloy component, a process known as aluminizing. Aluminum forms stable intermetallic compounds with both nickel and cobalt. The oxide layer which forms on these compounds at high temperature is no longer a metal oxide of nickel or cobalt, but rather a tough, tightly adherent, protective layer of alumina, Al.sub.2 O.sub.3. See FIG. 3.
A variety of commercial coatings are based upon this protection scheme. Sometimes aluminum is deposited from a vapor phase in a process that has come to be known as pack aluminizing. In pack aluminizing, aluminum powder is reacted with halide activators to form gaseous compounds which condense on the metal surface and react producing aluminum metal. The aluminum atoms diffuse into the substrate, reacting to produce intermetallic aluminides. This process has been described in detail in a number of patents, including U.S. Pat. No. 3,257,230 (Wochtell et al). This patent is incorporated herein by reference.
State-of-the-art MCrAlY overlay coatings also rely upon alumina films for their hot corrosion resistance. Owing to the presence of chromium and yttrium in the film, aluminum contents in these coatings do not need to be as high as in pack aluminides; however, protection is still derived from a tightly adherent scale of alumina.
Slurry aluminizing is another alternative method of providing a protective, alumina forming intermetallic aluminide coating on a superalloy. In the slurry process, an aluminum-filled slurry coating is first deposited on the hardware. When the coated part is heated in a protective atmosphere, aluminum in the film melts and reacts with the substrate to form the desired intermetallic phases.
The demonstrable resistance of aluminide coatings to hot corrosion and oxidation is due to the thermodynamic stability of the alumina scale that forms on them. However, they do have some susceptibility to "low temperature" hot corrosion attack at about 700.degree.-800.degree. C. by alkali metal oxides (e.g. Na.sub.2 O) and acidic oxides of refractory metals (e.g. MoO.sub.3 and W.sub.2 O.sub.3).
Silicon dioxide (SiO.sub.2) is another very stable oxide. Like aluminum, silicon forms stable intermetallic compounds (silicides) with nickel and cobalt as well as chromium and other elements typically found in refractory alloys, such as molybdenum, tungsten and titani
REFERENCES:
patent: Re26001 (1966-04-01), Waontell et al.
patent: 2612442 (1952-09-01), Goetzel
patent: 3220876 (1965-11-01), Moeller
patent: 3494748 (1970-02-01), Todd
patent: 3620693 (1971-11-01), Soma
patent: 3656919 (1972-04-01), Lucas et al.
patent: 3779719 (1973-12-01), Clark et al.
patent: 3864093 (1975-02-01), Wolfina
patent: 4310574 (1982-01-01), Deadmore et al.
patent: 4326011 (1982-04-01), Goebel et al.
patent: 4500364 (1985-02-01), Kutenat
Jones Alan T.
McMordie Bruce G.
Meelu Mehar C.
International Inc.
Rolls-Royce, Inc, plc.
Zimmerman John
LandOfFree
Aluminide-silicide coatings coated products does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Aluminide-silicide coatings coated products, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Aluminide-silicide coatings coated products will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-1113134