Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
1999-06-30
2001-08-28
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S678000, C428S680000, C420S442000, C420S444000, C420S445000, C420S456000, C420S460000
Reexamination Certificate
active
06280857
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to an improved class of protective coatings for superalloy structural parts, especially for gas turbine vanes and blades.
In the field of gas turbine engines, designers continually look toward raising the operating temperature of the engine to increase efficiency. In turn, the oxidation rate of materials increases dramatically with increasing temperature. Gas turbine components can also be subjected to hot corrosion, when corrosive species are ingested into the engine via intake air and/or impurities in the fuel. Modern structural superalloys are designed for the ultimate in mechanical properties thereby sacrificing oxidation, and, to an even larger extent corrosion resistance.
To increase the useful life of gas turbine components it is customary to use protective coatings, such as aluminide or MCrAlY coatings where M may be Ni, Co, Fe or mixtures thereof. Since a coated turbine blade undergoes complicated stress states during operation, i.e. during heating and cooling cycles, advanced high temperature coatings must not only provide environmental protection but must also have specifically tailored physical and mechanical properties.
If the protective coating is to be used as a bond coat for thermal barrier coatings (TBCs) there are additional requirements. For an overlay coating, i.e. no TBC, the thermally grown oxide can spall and regrow provided that the activity of Al in the coating remains sufficiently high. For a TBC bond coat, oxide growth rate and oxide scale adherence are the life controlling parameters since if the oxide spalls, the TBC will spall. In summary, advanced high temperature protective coatings must have: a high oxidation resistance; a slowly growing oxide scale (low kp value); a good oxide scale adherence; a hot corrosion resistance, superior to SX/DS superalloys; a low interdiffusion of Al and Cr into the substrate to prevent the precipitation of brittle needle-like phases under the coating; a creep resistance comparable to conventional superalloys; a high ductility at low temperatures and a low ductile brittle transition temperature; and a thermal expansion coefficient similar to the substrate over the entire temperature range.
U.S. Pat. Nos. 5,273,712 and 5,154,885 disclose coatings with significant additions of Re which simultaneously improve the creep and oxidation resistance at high temperatures. However, the combination of Re with high Cr levels, which is typical with traditional coatings, results in an undesirable chase structure of the coating and the interdiffusion layer. At intermediate temperatures (below 950-900° C.), &agr;-Cr phase is more stable in the coating than the &ggr;-matrix. This results in low toughness and low ductility. In addition, a significant excess of Cr in the coating compared to the substrate results in diffusion of Cr to the base alloy, which enhances precipitation of needle-like Cr—, W— and Rerich phases.
U.S. Pat. No. 4,758,480 discloses a class of protective coatings whose compositions are based on the compositions of the underlying substrate. The similarities in microstructure (gamma prime phase in gamma matrix) render the mechanical properties of the coating similar to the mechanical properties of the substrate, thereby reducing thermomechanically induced damage during service. However, the amount of Al (7.5-11 wt %) and Cr (9-16 wt %) in the coating may not provide sufficient oxidation and/or corrosion resistance for the long exposure times that are customary in stationary gas turbines.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a new coating for structural parts of gas turbines, especially for blades and vanes, which overcomes the above-mentioned disadvantages of the heretofore-known coatings, which exhibits improved mechanical behavior and which provides sufficient oxidation/corrosion resistance for the long exposure times that are customary in stationary gas turbines.
A nickel base alloy is provided that is particularly adapted for use as a coating for advanced gas turbine blading. The alloy is prepared with the elements in an amount to provide an alloy composition as shown in Table 1.
TABLE 1
Range of Coating Compositions of Present Invention
Elements of composition (% by weight)
Ni
Co
Cr
A
Re
Y
Si
Ta
Nb
La*
Mg
B
Coating
bal.
18-28
11-15
11.5-14
1-8
0.3-1.3
1-2.3
0.2-1.5
0.2-1.5
0-0.5
0-1.5
0-0.1
La* = La + elements from Lanthanide series
Y + La (+ La-series) ≦ 0.3 − 2.0
Si + Ta ≦ 2.5 wt %
Hf, C < 0.1 wt %
The alloy simultaneously provides optimum oxidation and corrosion resistance, phase stability during diffusion heat treatment and during service, and excellent mechanical behavior, especially high ductility, high creep resistance, and thermal expansion similar to the substrate.
This is achieved by a specific phase structure consisting of &bgr;-reservoir phase precipitates (45-60 vol %) in a ductile &ggr;-matrix (40-55 vol %).
Preferably, the alloy can be produced by a vacuum melt process in which powder particles are formed by inert gas atomization. The powder can then be deposited on a substrate using, for example, thermal spray methods. However, other methods of application may also be used. Heat treatment of the coating using appropriate times and temperatures is recommended to achieve a good bond to the substrate and a high sintered density of the coating.
A number of different alloys with compositions according to the present invention, which have been tested, are given in Table 2(a).
TABLE 2(a)
Preferred Coating Compositions
Elements in wt % of composition
Ni
Co
Cr
Al
Re
Y
Si
Ta
Nb
La
Mg
PC1
bal.
24.1
11.8
12.1
2.8
0.3
1
1
0.3
—
—
PC2
bal.
23.8
13
12
3
0.5
1.7
0.5
0.3
—
0.2
PC3
bal.
23.8
13
11.8
3
0.3
1
1
0.3
0.1
—
These preferred alloys exhibit the desired coating behavior with optimum oxidation and corrosion resistance, phase stability during diffusion heat treatment and during service, and excellent mechanical behavior, especially high ductility, high creep resistance, and thermal expansion similar to the CMSX4 substrate material.
In order to prove the advantage of the preferred compositions a number of additional alloys whose compositions are given in Table 2(b) have also been tested. Alloys EC1-EC8 were found to exhibit poor properties in comparison with the preferred compositions PCI, PC2, and PC3.
TABLE 2(b)
Additional Coating Compositions
Coating
Ni
Co
Cr
Al
Re
Y
Si
Ta
Nb
Hf
EC1
bal.
12
20.5
11.5
—
0.5
2.5
1
—
—
EC2
bal.
12
16
11.5
—
0.3
2.5
1
—
—
EC3
bal.
24
16
11
—
0.3
2
1
—
—
EC4
bal.
24
13
11
3
0.3
2
—
0.5
—
EC5
bal.
24
13
11.5
3
0.3
1.2
—
—
0.5
EC6
bal.
24
14
11
—
0.3
2
0.5
—
0.5
EC7
bal.
—
16
8
—
0.5
2
0.5
—
—
EC8
bal.
12
8.5
7
3
0.5
1
3
0.3
0.7
TABLE 2(b)
Additional Coating Compositions
Coating
Ni
Co
Cr
Al
Re
Y
Si
Ta
Nb
Hf
EC1
bal.
12
20.5
11.5
—
0.5
2.5
1
—
—
EC2
bal.
12
16
11.5
—
0.3
2.5
1
—
—
EC3
bal.
24
16
11
—
0.3
2
1
—
—
EC4
bal.
24
13
11
3
0.3
2
—
0.5
—
EC5
bal.
24
13
11.5
3
0.3
1.2
—
—
0.5
EC6
bal.
24
14
11
—
0.3
2
0.5
—
0.5
EC7
bal.
—
16
8
—
0.5
2
0.5
—
—
EC8
bal.
12
8.5
7
3
0.5
1
3
0.3
0.7
The beneficial phase structure of the preferred alloy compositions (&bgr;-phase in ductile &ggr; matrix) is reflected by the results of tensile tests at RT and 400° C. (Table 3). While tensile specimens coated with EC1 fail below 0.4% strain, specimens coated with the preferred compositions show tensile elongations of >4% and >9% at RT and 400° C., respectively.
TABLE 3
Strain to Failure of selected coatings at RT and 400° C.
Strain to failure at
coating
strain to failure at RT (%)
400° C.
EC1
<0.4
<0.4
EC2
0.8
1.9
EC3
2
4.5
EC4
2.2
4.8
1, PC2, PC3
>4
>9
In addition, experimental TMF data (Table 4) show that the improved coatings of this invention also have superior TMF behavior. In contrast to coating EC1 which cracks at the is first cycle and a conventional overlay coating which fails after 2000 cycles, the coatings according to the present invention have a TMF life of >3000 cycles, i.e. very similar to that of the un
Bossmann Hans-Peter
Holmes Peter
Konter Maxim
Schmutzler Hans Joachim
Sommer Marianne
Alstom
Jones Deborah
Young Bryant
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