Coating processes – Coating by vapor – gas – or smoke – Metal coating
Reexamination Certificate
1999-09-07
2001-09-18
Meeks, Timothy (Department: 1762)
Coating processes
Coating by vapor, gas, or smoke
Metal coating
C427S253000, C427S255390, C427S255393, C427S255395, C205S192000, C205S228000, C148S527000
Reexamination Certificate
active
06291014
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to active element modified platinum aluminide diffusion coatings and chemical vapor deposition (CVD) methods for their manufacture.
BACKGROUND OF THE INVENTION
At temperatures greater than about 1000° C. (1832° F.), high temperature oxidation is the most important form of environmental attack observed with aluminide diffusion coatings. High temperature oxidation is a chemical reaction whose rate controlling process for an aluminide coating is diffusion through a product (oxide) layer. Diffusion is a thermally activated process, and consequently, the diffusion coefficients are exponential functions of temperature. Since the oxidation of aluminide coatings is a diffusion controlled reaction and diffusion coefficients are exponential functions of temperature., the oxidation rate is also an exponential function of temperature. At low temperatures where diffusion coefficients are relatively small, the growth rate of a protective scale on any aluminide coating is also small. Thus, adequate oxidation resistance should be provided by any state of the art aluminide coatings, such as: chromium aluminide, aluminide or two phase [PtAl
2
+(Ni,Pt)Al] platinum aluminide, all inward grown coatings made by pack cementation. However, at high temperatures where the diffusion coefficients and consequently the oxidation rate increase rapidly with increasing temperature, only coatings which form high purity alumina (Al
2
O
3
) scales are likely to provide adequate resistance to environmental degradation.
The presence of platinum in nickel aluminide has been concluded to provide a number of thermodynamic and kinetic effects which promote the formation of a slow growing, high purity protective alumina scale. Consequently, the high temperature oxidation resistance of platinum modified aluminide diffusion coatings generally is better as compared to aluminide diffusion coatings not containing platinum.
In recent years, several limitations of the industry standard, two phase [PtAl
2
+(Ni,Pt)Al], inward grown platinum aluminide coatings have been identified. First, the two phase coatings have metastable phase assemblages and thicknesses, as demonstrated in engine tests at both General Electric and Rolls-Royce. Second, the two phase coatings are sensitive to TMF cracking in engine service, and the growth of these coatings in service only makes this problem worse. Third, the thick, inward grown platinum aluminides exhibit rumpling during both cyclic oxidation and engine testing. This phenomenon can have undesirable consequences when platinum aluminide coatings are used as the bond coat in thermal barrier coating systems. Fourth, the two phase platinum aluminide coatings are hard and brittle, and this can result in chipping problems during post coat handling and assembly operations.
Many of the problems encountered with the previous industry standard platinum aluminides can be attributed to the two phase, inward grown structure and can be overcome by using outwardly grown, single phase platinum aluminde coatings as described, for example, in the Conner et al. technical articles entitled “Evaluation of Simple Aluminide and Platinum Modified Aluminide Coatings on High Pressure Turbine Blades after Factory Engine testing”, Proc. AMSE Int. Conf. of Gas Turbines and Aero Engine Congress Jun. 3-6, 1991 and Jun. 1-4, 1992. For example, the outwardly grown, single phase coating microstructure on directionally solidified (DS) Hf-bearing nickel base superalloy substrates was relatively unchanged after factory engine service in contrast to the microstructure of the previous industry standard two phase coatings. Further, the growth of a CVD single phase platinum aluminide coating was relatively insignificant compared to two phase coatings during factory engine service. Moreover, the “high temperature low activity” outward grown platinum aluminde coatings were observed to be more ductile than inward grown “low temperature high activity” platinum aluminide coatings.
Copending application Ser. No. 08/330,694, now U.S. Pat. 5,658,614, of common assignee herewith described a CVD process for making a single phase outwardly grown platinum aluminide diffusion coating on a nickel base superalloy substrate, such as IN 738.
Copending applications Ser. Nos. 08/197,478 and 08/197,497 also of common assignee disclose CVD methods for codeposition of aluminum, silicon and one or more active elements, such as yttrium, hafnium and/or zirconium, on a substrate to form a platinum aluminide diffusion coating modified by the inclusion of the active element(s).
SUMMARY OF THE INVENTION
The present invention provides a CVD outwardly grown platinum aluminide diffusion coating on a nickel or cobalt base superalloy substrate wherein the platinum modified aluminide diffusion coating is modified to include silicon, hafnium, and optionally zirconium and/or other active elements (e.g. Ce, La, Y) each in a concentration of about 0.01 weight % to about 8 weight % of the outer additive layer of the coating. A preferred coating of the present invention includes about 0.01 weight % to less than 2 weight % of each of silicon, hafnium, and optionally zirconium and/or other active elements in the outer additive layer, preferably with a Hf/Si ratio less than about 1 and, when Zr also is present, a Hf+Zr/Si ratio of less than about 1. A coating microstructure is provided characterized by an inner diffusion zone or region adjacent the substrate and the outer additive layer having a single (Ni,Pt)(Al,Si) matrix phase including hafnium silicide and other silicide second phase particles or regions dispersed throughout the matrix phase of the additive layer. Carbide particles may also be present.
The present invention also provides a thermal barrier coating system including the aforementioned CVD outwardly grown active element modified platinum aluminide diffusion coating as an intermediate bond coat between a nickel or cobalt base superalloy substrate and an outer ceramic thermal barrier layer, such as an physical vapor deposited (PVD) columnar ceramic layer deposited on a thin alumina layer formed on the bond coat.
The present invention also provides a method of making such CVD active element modified platinum aluminide diffusion coatings on nickel or cobalt base superalloy substrates wherein in one embodiment a platinum layer first is deposited on the substrate followed by CVD codeposition of Al+Si+Hf+optionally Zr and/or other active elements to produce a diffusion coating useful, although not limited, for service applications in the hot turbine section of a gas turbine aircraft engine where shorter overhaul intervals are practiced.
A method in accordance with another embodiment of the present invention involves first CVD codepositing Al+Si+Hf+optionally Zr and/or other active elements on the substrate to form an initial active element bearing aluminide diffusion layer, then depositing platinum on the layer, and finally CVD aluminizing the Pt coated initial layer to produce a thicker overall aluminide diffusion coating useful, although not limited, for service applications where longer overhaul intervals can be tolerated, such as land based gas turbine engines.
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Basta William C.
Near Daniel L.
Punola David C.
Warnes Bruce M.
Howmet Research Corporation
Meeks Timothy
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