Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2002-10-24
2004-09-21
McNeil, Jennifer (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S632000, C428S701000, C428S702000, C428S697000, C428S699000, C428S336000, C427S435000, C427S453000, C416S24100B
Reexamination Certificate
active
06794059
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an article resistant to attack from environmental contaminants under high temperature conditions, the article being of the type comprising a metal alloy part, a thermal barrier coating deposited onto the metal alloy part, and a protective top coat of a material different from the material of the thermal barrier coating deposited onto the thermal barrier coating.
BACKGROUND OF THE INVENTION
Articles of this type are used, for example, as metal alloy parts in gas turbine engines operating under high temperature conditions. The thermal barrier coatings reduce the heat flow into the coated metal part during operation of the engine, and allow the metal part to run cooler than the gas stream, thereby extending part life and resulting in a higher combustion efficiency by permitting higher gas temperatures.
Conventional thermal barrier coatings are comprised of ceramic materials, for example chemically-stabilized zirconia, including yttria-stabilized zirconia (YSZ), scandia-stabilized zirconia, calcia-stabilized zirconia and magnesium zirconia, with YSZ being the thermal barrier coating of choice. These coatings are bonded to the surface of the metal part or to an intermediate metal bond coat.
A conventional thermal barrier coating is porous. It usually contains a degree of porosity ranging from 3-20%. The pores and any small micro-cracks also present in the material are not well-connected. Therefore, environmental contaminants do not have a ready path from the coating surface to the metal-ceramic interface. In use, these small micro-cracks lengthen and subsequently provide an easy, or more direct path for contaminants to reach the metal surface. The propagation and extension of micro-cracks is due to a combination of operating factors including, but not limited to, high temperature, high pressure, coating erosion by particulates, particle impact, chemical reactions and stress caused by differential thermal expansion. Some gases may also react with the thermal barrier coating to form molten salts that may effectively penetrate the micro-cracks and connected pores. Subsequent failure of the coating, which is also referred to as delamination or spalling, is a result of the corrosion of the metal at the metal-ceramic interface or within the ceramic layer adjacent to the metal.
The prior art describes certain techniques which aim to impede or reduce damage to the thermal barrier coatings, thus extending the service life of the protected metal parts.
There is a considerable body of information available regarding the numerous alternate methods of increasing thermal protection of the parent metal and/or extending the life of the ceramic coatings. This includes data in the open literature [Journal of Thermal Spray Technology, Journal of Engineering for Gas Turbines and Power, Engineered Materials Handbook Vol. 4 (Ceramics & Glasses) Section 11 (ASM International, 1991), etc.] and patent protected data. Coatings that consist of a metal bond layer with a single ceramic thermally insulating top coat have generally concentrated on improved oxidation resistant metal bond coats and more erosion, corrosion or thermal shock resistant ceramic coatings. The use of 8% yttria stabilized zirconia instead of 20% yttria stabilized zirconia and the development of ceria stabilized zirconia [Siemers et al., U.S. Pat. No. 4,328,285 (1982)] are typical examples. Bruce et al. U.S. Pat. No. 5,683,761 discloses the use (in certain applications) of pure alpha alumina, as a means of obtaining higher erosion resistance and lower density (than zirconia). Similarly, graded layers have been used, for improved thermal shock resistance. These approaches do not all have a single ceramic coating. Also, these coatings fail before failure of layers below them. They are “sacrificial” coatings.
Hasz el al. U.S. Pat. Nos. 5,871,820 issued on Feb. 16, 1999 and 5,851,678, issued on Dec. 22, 1998 describe a method for protecting a thermal barrier coating from environmental contaminants and a coating protected by the method. A top coat in the form of an impermeable, non-porous barrier coating is deposited onto the surface of the thermal barrier coating. This non-porous, impermeable barrier coating is intended to prevent the environmental contaminants from coming into contact with the ceramic thermal barrier coating.
U.S. Pat. No. 5,773,141 issued to Hasz et al. on Jun. 30, 1998 describes the provision of a top coat in the form of a single protective layer of a sacrificial or reactive oxide that overlays the outer surface of the thermal barrier coating. The sacrificial layer reacts with liquid contaminants to increase the viscosity or melting temperature of these contaminants. This inhibits chemical attack on the underlying thermal barrier. Since this protective layer reacts with the environment and is progressively depleted with continued use, it is said to be sacrificial, and will protect the thermal barrier coating for a limited period of time.
Bruce et al. U.S. Pat. No. 5,683,761 describes the use of pure alpha alumina as a top coat over zirconia ceramic coatings. The alpha alumina coating is used to increase erosion resistance and fails, by erosion or cracking, before the underlying zirconia coating is affected. That is, it is a sacrifical coating. Similarly, Voss et al. U.S. Pat. No. 6,006,516 describes the use of low porosity mullite over zirconia, to provide a chemically inert surface. This mullite coating must have lower porosity than the underlying zirconia, be smoother than the zirconia and fails before the underlying zirconia. Voss et al. describe the need for increased coating thickness for increased protection.
The present invention is concerned with the provision of thermal barrier coatings with increased resistance to attack by environmental contaminants at high temperature conditions.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, an article as described above is characterized in that:
the top coat is non-sacrificial;
the top coat is porous;
the top coat is substantially thinner than the thermal barrier coating;
the top coat is a material selected from the group consisting of ceria stabilized zirconia (CSZ); calcium-stabilized zirconia (CaSZ); zirconia toughened alumina (ZTA); a compound oxide other than mullite and modified mullite ceramics; and mixtures of two or more thereof; and
the top coat is selected from materials more resistant to the environmental contaminants than the base thermal barrier coating.
It has been shown that the use of a top coat with these characteristics provides good protection for the thermal barrier coating. The top coat protects the thermal barrier coating in several ways, depending on the environmental contaminants and conditions present:
1. The protective top coat covers and encases the thermal barrier coating, providing a physical barrier on the surface of the thermal barrier coating and thereby substantially reducing the rate of infiltration of molten environmental contaminants into the thermal barrier coating.
2. The protective top coat provides a chemical barrier minimizing the chemical interaction of the contaminant environment with the thermal barrier coating, thereby significantly slowing down the chemical dissolution of the thermal barrier coating at high operating temperatures.
3. Since the top coat is selected not to react with environmental contaminants, adherence of these contaminants to the surface is resisted and the migration of these contaminants into the thermal barrier coating is accordingly minimised. In addition, most of the deposits that may form on the surface of the thin protective top coat are only loosely adhered to the surface and are quickly removed by the high velocity gas flowing over the surface, or during cool down owing to the consequent differential thermal expansion.
4. In environments where particulate impact or erosion may occur, a higher resistance in the thin protective top layer of the present invention reduces the rate of microcracking of
Aikins Patent Company
McNeil Jennifer
Standard Aero Limited
Thrift Murray E.
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