Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2002-07-08
2004-05-11
Zimmerman, John J. (Department: 1775)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S469000, C428S336000, C428S446000, C428S448000, C428S641000, C428S699000, C428S701000, C428S697000, C416S24100B
Reexamination Certificate
active
06733908
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a multilayer system for protecting components exposed to severe environmental and thermal conditions such as the hostile environment present in gas turbine engines.
BACKGROUND OF THE INVENTION
A major limitation in the efficiency and emission of current gas turbines is the temperature capability (strength and durability) of metallic structural components (blades, nozzles and combustor liners) in the engine hot section. Although ceramic thermal barrier coatings are used to insulate metallic components, thereby allowing the use of higher gas temperatures, the metallic component remains a weak link. Such components must allow for the possibility of coating loss from spallation or erosion.
Silicon-containing ceramics are ideal materials for high temperature structural applications such as heat exchangers, advanced gas turbine engines, and advanced internal combustion engines. They have excellent oxidation resistance in clean oxidizing environments due to the formation of a slow-growing silica scale (SiO
2
). However, durability in high temperature environments containing molten salts, water vapor or a reducing atmosphere can limit their effectiveness. Molten salts react with silica scale to form liquid silicates. Oxygen readily diffuses through liquid silicates and rapidly oxidizes the substrate. High water vapor levels lead to hydrated silica species (Si(OH)
x
) and subsequent evaporation of protective scale. Complex combustion atmospheres containing oxidizing and reducing gases form SiO
2
and reduce it to SiO
(g)
. In situations with low partial pressure of oxidant, direct formation of SiO
(g)
occurs. All of these reactions can potentially limit the formation of a protective silica scale and thus lead to accelerated or catastrophic degradation.
Examples of silicon-containing ceramics are SiC fiber-reinforced SiC ceramic matrix composites (SiC/SiC CMC's), SiC fiber-reinforced Si
3
N
4
matrix composites (SiC/Si
3
N
4
CMCs), carbon reinforced SiC ceramic matrix composites (C/SiC CMCs), monolithic silicon carbide and monolithic silicon nitride. A primary problem Si-containing ceramics face is rapid recession in combustion environments due to the volatilization of silica scale via reaction with water vapor, a major product of combustion. Therefore, use of silicon-containing ceramic components in the hot section of advanced gas turbine engines requires development of a reliable method to protect the ceramic from environmental attack. One approach in overcoming these potential environmental limitations is to apply a barrier coating which is environmentally stable in molten salts, water vapor and/or reducing atmosphere.
An early environmental barrier coating system (EBC) consisted of two layers, a mullite (3Al
2
O
3
.2SiO
2
) coat and a yttria-stabilized zirconia (YSZ) top coat. The mullite coat provided bonding, while the YSZ coat provided protection from water vapor. Mullite has a good coefficient of thermal expansion match and chemical compatibility with Si-based ceramics. However, the relatively high silica activity of mullite and the resulting selective volatilization of silica cause its rapid recession in water vapor. This EBC provided protection from water vapor for a few hundred hours at 1300° C. During longer exposures, however, water vapor penetrated through cracks in the mullite and attacked the Si-containing substrate, leading to coating delamination.
Another EBC with improved performance was developed as part of a NASA High Speed Research-Enabling Propulsion Materials (HSR-EPM) Program in joint research by NASA, GE, and Pratt and Whitney. The EBC consisted of three layers: a silicon bond coat, an intermediate coat consisting of mullite or mullite and barium strontium aluminosilicate (BSAS), and a BSAS top coat. The mullite, mullite and BSAS, and BSAS layers were applied by a modified plasma spray process developed at the NASA Glenn Research Center as disclosed in U.S. Pat. No. 5,391,404, which is incorporated by reference herein in its entirety. The EBC was applied to SiC/SiC CMC combustor liners used in three Solar Turbine Centaur 50s gas turbine engines. The combined operation of the three engines resulted in the accumulation of tens of thousands of hours without failure at a maximum combustor lining temperature of about 1250° C. A drawback of this BSAS-top coat EBC is that when applied to the solar turbine SiC/SiC liners it suffered from substantial BSAS recession after engine testing.
FIG. 1
of EP 1142850 shows an EBC which employs a YSZ topcoat
18
, a YSZ-containing intermediate layer
24
between the topcoat and a Si-containing substrate
12
, and a BSAS layer
22
between the YSZ-containing intermediate layer
24
and the substrate. The inventor of the present application has found that when BSAS and YSZ react, an undesirable low melting glass results. This problem was not recognized in the EP '850 disclosure as is apparent by the close proximity of BSAS layer
22
and YSZ layer
24
. EP '850 discloses that intermediate YSZ-containing layer
24
can includes sublayers in which an inner sublayer in contact with the BSAS layer
22
contains one of BSAS, mullite or alumina and an outer sublayer in contact with the YSZ top coat consists essentially of YSZ. This again is disadvantageous in that it can position the outer YSZ sublayer in contact with an inner sublayer containing BSAS.
EP '850 also discloses compositionally grading layer
24
using YSZ and one of BSAS, mullite or alumina. The EBC of EP '850 will have BSAS-mullite contact when the mullite/YSZ graded layer is used. The inventor has found that mullite-BSAS reaction can become a serious durability issue in long-term exposures (over several hundred hours) as the reaction has the potential to produce a reaction product with a melting point as low as 1300° C. Thus, the inventor has found that it is desirable to avoid the mullite-BSAS contact especially at outer layers where the temperature is higher. Another disadvantage is that the BSAS layer
22
adds 125 to 500 &mgr;m of thickness to the EBC. A thick EBC has increased interlayer stress which may result in delamination. Therefore, the BSAS layer
22
at best presents a risk of delamination and at worst is deleterious to the EBC upon reaction of BSAS and YSZ or reaction of BSAS and mullite.
Current EBCs fail by delamination and spallation along a “weak link”. The stress caused by the YSZ layer accelerates the failure. A key source for the creation of this “weak link” is environmental/chemical degradation. Key material properties for long life EBCs should include environmental/chemical stability, low CTE, low modulus, sinter resistance, low thermal conductivity, and phase stability. Multilayer systems containing a YSZ outer layer and a mullite or mullite and alkaline earth metal aluminosilicate-containing intermediate layer, are potentially effective EBC systems, but there is a need to improve their performance by prolonging life or increasing the capacity to withstand higher operating temperatures.
SUMMARY OF THE INVENTION
The present invention is directed to a multilayer article which includes a substrate comprising a compound selected from the group consisting of a ceramic compound, a Si-containing metal alloy and combinations thereof. The multilayer article also includes an outer layer and a plurality of intermediate layers located between the outer layer and the substrate. The outer layer comprises fully or partially stabilized zirconia (ZrO
2
), preferably yttria stabilized zirconia, although rare earth elements besides yttria may be used as stabilizers. Intermediate layers are located between the outer layer and the substrate, one of which comprises a mullite (3Al
2
O
3
.2SiO
2
)-containing layer comprising 1) mullite or 2) mullite and an alkaline earth metal aluminosilicate. The mullite (no BSAS) intermediate layer is desirable when the multilayer article is used at temperatures of 1300° C. and above for extended periods of time. One intermediate layer is an (outer)
Bansal Narottam P.
Lee Kang N.
McNeil Jennifer
Stone Kent N.
The United States of America as represented by the Administrator
Zimmerman John J.
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