Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Patent
1994-07-06
1996-08-20
Robinson, Ellis P.
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
428635, 428650, 428651, 428680, 427252, 4273768, 427399, 4274197, B32B 1501, C23C 1600
Patent
active
055477704
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to aluminide coatings for aluminizing of heat resistant alloy substrates and a process for applying such coatings, more specifically to composite coatings containing aluminium-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 fall 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 alloy aluminium into the surface of a superalloy component, a process known as aluminizing. Aluminium 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 (FIG. 3).
A variety of commercial coatings are based upon this protection scheme. Sometimes aluminium is deposited from a vapour phase in a process that has come to be known as pack aluminizing. In pack aluminizing, aluminium powder is reacted with halide activators to form gaseous compounds which condense on the metal surface and react producing aluminium metal. The aluminium 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).
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, aluminium 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 aluminium- filled slurry coating is first deposited on the hardware. When the coated part is heated in a protective atmosphere, aluminium 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 aluminium, 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 titanium. This reduces the segregation of these elements into the outer surface protective oxide layer, thus improving its protectiveness. Furthermore, unlike aluminium, silicon is unable to
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Jones Alan T.
McMordie Bruce G.
Meelu Mehar C.
Jones III Leonidas J.
Robinson Ellis P.
Rolls Royce Plc.
Sermatech International Inc.
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