Metal treatment – Process of modifying or maintaining internal physical... – Producing or treating layered – bonded – welded – or...
Reexamination Certificate
2000-04-28
2001-10-02
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Producing or treating layered, bonded, welded, or...
C427S454000, C427S456000
Reexamination Certificate
active
06296723
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to improved multilayered combustion system components, such as combustor liners or transition ducts of a gas turbine engine, wherein the inner surface comprises a protective thermal barrier coating (TBC), which includes a ceramic top coat and a metallic bond coat, and the outer surface consists of a structural layer bonded to the TBC through the bond coat. The improved qualities of the new components over current components include a superior thermal barrier coating, a better high-temperature structural material, a smoother inside surface, no irregularities (welds) within the component, and excellent reproducibility. This is accomplished by a vacuum plasma spray (VPS) process which is used to form the ceramic top coat layer on a suitable mold, followed by a metallic bond coat layer and ending with a structural superalloy layer. Thereafter, the mold is removed to form the multilayered component of the present invention.
2. Description of the Prior Art
It is accepted practice in the gas turbine industry to provide TBC's consisting of a ceramic top coat and a metallic bond coat (typically an MCrAlY) on the inner surface of preformed combustion system components. Two of the components protected by such coatings are combustor liners and transition ducts, which contain the combustion flame and channel the extremely hot gas (>1,300° C.) to the first stage vanes. The transition ducts in particular have a fairly complex geometry and the presently known technology does not allow for satisfactory coating of internal surfaces of components with such complex geometries.
The current fabrication process of combustion system components, such as combustor liners and transition ducts, consists of: (i) mechanically forming two or more individual sections of the component; (ii) plasma spraying by atmospheric plasma spray (APS) the inner surface of each section to form the thermal barrier coating system; (iii) welding the sections so coated; (iv) plasma spraying by APS the protective TBC coatings on the welds whenever possible; and, for transition ducts, (v) laser drilling cooling holes through the structural wall and the coating. There are several significant problems with components which have been fabricated in this fashion. One problem is the nonhomogeneity at the welds. Weld regions act as weak sites from which failure may initiate due to poor quality finish of both the top coat and the bond coat of the TBC. Also, due to the rough surface of the TBC inherent in the APS process and particularly of the weld regions, an undesirable change in flow pattern of the hot gas is often produced. Moreover, because the current fabricating process consists of mechanically forming sections of the component followed by welding and spraying inner surfaces of these sections, there is a limitation on the choice of suitable superalloys. Only superalloys with high elongation such as, nickel-chromium alloys known under trade names Haynes 230, IN-617, etc. are suitable. Superalloys which do not possess the required elongation or ductility cannot be used with the current fabrication process, even if they possess other superior properties, such as better high temperature strength and creep resistance, e.g. IN-738LC superalloy.
It should be noted that demand on engine performance has increased in recent years for both aero and industrial gas turbine engines. In 1984, the US Air Force created the High Performance Turbine Engine Initiative (HPTEI) in which increasing the combustor and turbine entry temperatures (TET) was a major goal. A similar program known as Advanced Turbine System (ATS) was initiated shortly thereafter by the US Department of Energy (DOE) which envisaged an increase in firing temperatures above 1427° C.
Gas turbine hot-section materials constitute an important limiting factor and are critical to achieving the higher firing temperatures. Current methods of producing closed combustion system components, e.g., combustor liners and transition ducts, to contain and guide the hot gas, have inherent limitations which are difficult to overcome, especially in more demanding conditions, such as higher temperatures and pressures.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to obviate the problems and disadvantages mentioned above and to provide improved multilayered combustion system components through VPS near net-shape forming thereof with a smooth TBC inner layer of predetermined thickness.
Another object is to provide combustion system components which resist high gas temperatures of the order of 800° C.-1600° C.
A still further object of the present invention is to form components with a protective inner TBC, which do not require welding as an integral part of the fabrication process.
Other objects and advantages of the invention will become apparent from the following description thereof.
Essentially the novel components of the present invention are near net-shape VPS formed multilayered combustion system components, such as combustor liners or transition ducts, which comprise:
(a) an inner ceramic top coat of uniform predetermined thickness which resists high gas temperatures and thermal shock during operation within the combustion system, such as a gas turbine engine, and has a smooth inside surface;
(b) an intermediate metallic bond coat of MCrAlY where M is Ni, Co, Fe or a combination thereof, adjacent to the ceramic top coat, which provides protection from high temperature corrosion and oxidation while ensuring good adhesion between the ceramic top coat and an outer structural superalloy; it has a predetermined thickness which is smaller than that of the top coat; and
(c) an outer structural superalloy layer formed by VPS on top of the bond coat without any weld regions or nonuniformities in the surface finish that may act as initiation sites for failure of the component, said structural superalloy layer having a predetermined thickness that may vary within the component depending on operating requirements, and is such as to be capable of withstanding temperatures in excess of 700° C.
The ceramic top coat is normally of a thickness greater than 250 &mgr;m and preferably greater than 1 mm. The preferred range of the top coat thicknesses is between 1 and 1.5 mm. It is formed of ceramic materials such as zirconia (ZrO
2
) and calcia-silica (Ca
2
SiO
4
). ZrO
2
may be partially stabilized with yttria (Y
2
O
3
) as is known in the art.
The metallic bond coat is made of MCrAlY where M is Ni, Co, Fe or a combination thereof. For example, CoNiCrAlY is an excellent bond coat material when sprayed to a thickness of between about 100-200 &mgr;m. Such material is already described, for example, in U.S. Pat. No. 5,384,200 of Jan. 24, 1995, where it is deposited as part of a TBC on the surface of combustion chamber components by plasma spray; the components themselves in that case are, however, not formed by plasma spray and furthermore no use of VPS is disclosed.
The near net-shape VPS formed outer structural superalloy layer is normally formed of a nickel-base or cobalt-base superalloy having good structural and thermal resistance properties, such as Inconel, Hastelloy or Haynes Alloy, however, unlike known technology where such alloys had to be mechanically preformed and, therefore, had to possess sufficient elongation and ductility for that purpose; in the present case, any desired superalloy may be employed, since the outer structure is also formed in accordance with the present invention by vacuum plasma spray unlike anything taught by the prior art for such multilayered applications. Thus, a superalloy, such as IN-738LC which has excellent high temperature resistance properties, but is too brittle to be mechanically formed, can now be used within the present invention.
The structural superalloy layer is usually between 1 and 5 mm thick, and should be capable of withstanding temperatures in excess of 700° C. Because it is formed by VPS, it has no seams or welds and it may be deposited to
Cavasin Alexander P.
Grenier Serge
Kim George E.
Tsantrizos Peter G.
Primak George J.
Pyrogenesis Inc.
Wyszomierski George
LandOfFree
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