Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
1999-09-10
2001-10-02
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of metal
C428S472000, C428S472200, C428S937000, C416S24100B
Reexamination Certificate
active
06296945
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the field of thermal barrier coatings, and more particularly to a thermal barrier coating for a very high temperature application such as a combustion turbine engine. In particular, this invention relates to the field of multiphase ceramic thermal barrier coatings for high temperature application for coating superalloy components of a combustion turbine.
2. Background Information
The demand for continued improvement in the efficiency of combustion turbine and combined cycle power plants has driven the designers of these systems to specify increasingly higher firing temperatures in the combustion portions of these systems. Although nickel and cobalt based “superalloy” materials are now used for components in the hot gas flow path, such as combustor transition pieces and turbine rotating and stationary blades, even these superalloy materials are not capable of surviving long term operation at temperatures sometimes exceeding 1,200° C.
Examples of cobalt or nickel based superalloys are, for example, Cr.Al.Co.Ta.Mo.W, which has been used for making SC turbine blades and vanes for gas turbines, as taught, for example, in U.S. Pat. No. 5,716,720 (Murphy). These turbine components are generally protected by a basecoat of MCrAlY, where M is selected from the group of Fe, Co, Ni, and their mixtures, as taught for example, by U.S. Pat. Nos. 5,763,107 and 5,846,605 (both Rickerby et al.) and by U.S. Pat. Nos. 4,916,022; 5,238,752; 5,562,998; and 5,683,825 (Solfest et al.; Duderstadt et al.; Strangman; and Bruce et al., respectively). These basecoats are usually covered by an aluminum oxide layer and a final thermal barrier coating (“TBC”). The standard thermal barrier coating, however, is made from yttria-stabilized zirconia, ceria-stabilized zirconia, scandia-stabilized zirconia or non-stabilized zirconia, as taught, for example, by U.S. Pat. No. 5,683,825 Bruce et al. patent. A particularly useful state of the art TBC is 8 wt. % yttria stabilized zirconia (“8YSZ”).
Many of the ceramic thermal barrier layers are deposited as a columnar structure in the direction of the coating layer thickness, as taught in U.S. Pat. Nos. 4,321,311 and 5,830,586 (Strangman and Gray et al., respectively). This structure can be formed by electron beam physical vapor deposition (“EBPVD”) as in Bruce et al. U.S. Pat. No. 5,683,825, or a combination of electron beam deposition and ion beam irradiation, or the like, such as the ZrO
2
thermal barrier layer taught in U.S. Pat. No. 5,630,314 (Kojima et al.). Strangman U.S. Pat. No. 5,562,998, additionally vapor infiltration or sol-gel coats the columnar grains with a submicron thick layer of unstabilized zirconia or unstabilized hafnia, functioning as a bond inhibitor between the discrete columns.
Modern gas turbine engines can achieve higher efficiencies by increasing the turbine inlet temperatures. This subjects the TBCs to high temperatures. TBC materials that are phase stable at high temperatures upon long term exposure will be required. The current state-of-the-art electron beam physical vapor deposited (“EBPVD”) 8 YSZ coatings destabilize above approximately 1200° C. In addition, the long term high temperature exposure leads to potential sintering and loss of strain compliance, and possible premature TBC failure. 8YSZ coatings are also susceptible to corrosion upon exposure to contaminants in the fuel and erosion due to foreign object damage. Therefore, some of the key requirements for new TBC candidates for high temperature applications are high temperature phase stability, a reduced tendency to sinter, good corrosion and erosion resistance, all of them to be maintained upon long term exposure. These requirements are in addition to the primary needs of a TBC, such as, a low thermal conductivity with minimal coefficient of thermal expansion mismatch with the superalloy substrate.
SUMMARY OF THE INVENTION
Therefore, it is a main object of this invention to provide improved thermal barrier coating layers for use on underlayers, such as alumina and MCrAly, protecting turbine components, such as superalloy turbine blade assemblies that can operate over 1000° C.
These and other objects of the invention are accomplished by providing a turbine component comprising a metal alloy substrate and a columnar thermal barrier coating on the substrate surface the coating having (a) a columnar-grained ceramic oxide structural material base and (b) a heat resistant ceramic oxide sheath material covering the columns of the base, where the sheath comprises the reaction product of a ceramic oxide precursor sheath material which consists essentially of the composition C
Z
O
W
and the ceramic oxide columnar structural material which consists essentially of the composition (A,B)
x
O
y
, where A and B are selected from stable oxides which will react with C
Z
O
W
, and C is selected from stable oxides that will react with (A,B)
x
O
y
. A, B and C can be, for example, at least one of Al
2
O
3
, CaO, Y
2
O
3
, Sc
2
O
3
, ZrO
2
, MgO, and the like.
The preferred precursor sheath material is a thin coating of alumina, Al
2
O
3
and the preferred base column material is yttria stabilized zirconia where yttria, Y
2
O
3
, content can range from dopant amounts of 10 wt %-20 wt % of the total up to 60 wt % of the total with zirconia ZrO
2
. The preferred structure is one of discrete columns disposed in the direction of the coating thickness, separated by a microcrack volume. The reaction product can be prompted upon heating to about 1200° C. to 1500° C. and has the composition, in this preferred case, of a material comprising Y
3
Al
5
O
12
. Another preferred material is the use of rare earth oxide-stabilized zirconia deposited as columns, and an oxide such as Al
2
O
3
deposited between the columns to initiate the reaction between the rare earth oxide (“ReO”) in the stabilized ZrO
2
and Al
2
O
3
to form a reaction product ReO and Al
2
O
3
, for example, Re
3
Al
5
O
12
The invention also resides in a method of making a turbine component having a coated, adherent columnar thermal barrier coating on its surface comprising the steps of: (a) providing a nickel or cobalt based superalloy substrate; (b) depositing a columnar-grained ceramic base thermal barrier coating comprising stabilized zirconia, where the thermal barrier coating comprises discrete columns with microcrack volumes between columns; (c) separately depositing a precursor sheath oxide material between the discrete columns of the base thermal barrier coating, which precursor sheath oxide is capable of reacting with the stabilized zirconia of the base thermal barrier coating; and (d) prompting a reaction between the precursor sheath oxide material and the thermal barrier coating to provide a heat resistant sheath material reaction product. Usually, the reaction is prompted by heating the substrate before service or when the component is in service. The precursor sheath oxide material consists essentially of the composition C
Z
O
w
where C is selected from stable oxides that will react with the stabilized zirconia of the thermal barrier coating. Also, one can apply the TBC and overlay coating precursor onto a hot substrate operating as a heat sink with enough heat to prompt formation of the overlay coating reaction product. The reaction product exterior sheath coatings of the invention can be a continuous layer completely covering the columns or a discontinued layer partly covering the columns. These multiphase TBCs are phase stable and strain tolerant up to temperatures higher than 1400° C. for very long term exposures and provide additional benefits of erosion and corrosion resistance. Also, the invention described here can readily be adapted to current production sequences with an additional step of chemical vapor deposition or other infiltration processes of the overlay coating by a non-line-of-sight process. The coating process is economically feasible and thus has an opportunity for ease of transfer of the technology to production.
U
Jones Deborah
Siemens Westinghouse Power Corporation
Young Bryant
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