Plasma sprayed thermal bond coat system

Details Plasma sprayed thermal bond coat system Plasma sprayed thermal bond coat system

2001-04-26

2003-08-19

Bareford, Katherine A. (Department: 1762)

Coating processes

Spray coating utilizing flame or plasma heat

Metal oxide containing coating

C427S456000, C427S405000, C427S419200, C427S419700

Reexamination Certificate

active

06607789

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to protective coatings for components exposed to high temperatures, such as components of a gas turbine engine. More particularly, this invention is directed to a process for forming a thermal barrier coating system utilizing a NiAl bond coat and a ceramic top coat using an air plasma spray method.
BACKGROUND OF THE INVENTION
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through formulation of nickel and cobalt-base superalloys, though such alloys alone are often inadequate to form components located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to thermally insulate such components from the hot gases of combustion in order to minimize their service temperatures and to provide environmental protection to prevent deterioration from these hot, corrosive, oxidative gases. For this purpose, thermal barrier coating (TBC) systems formed on the exposed surfaces of high temperature components have found widespread use.
To be effective, thermal barrier coating systems must have low thermal conductivity, strongly adhere to the article, and remain adherent through many heating and cooling cycles. They also must protect the underlying substrate from environmental damage. Adherence to a substrate is a technical challenge due to the different coefficients of thermal expansion between materials having low thermal conductivity such as the ceramic materials typically used for thermal barrier coatings, and superalloy materials typically used to form turbine engine components. Thermal barrier coating systems capable of satisfying the above have generally required a metallic bond coat deposited on the component surface to provide an intermediate layer that may have a coefficient of thermal expansion that lies between that of the substrate material and the ceramic materials used for thermal barriers, but primarily is formulated to provide environmental protection from the hot oxidative and corrosive gases of combustion found in the turbine environment. Such coatings produce an adherent thermally grown oxide (TGO) layer that aids in the adherence of the TBC deposited on top of it.
Various ceramic materials have been employed as the ceramic layer, particularly zirconia (ZrO
2
) stabilized by yttria (Y
2
O
3
), magnesia (MgO), ceria (CeO
2
), scandia (Sc
2
O
3
), or other oxides. These particular materials are widely employed in the art because they can be readily deposited by plasma spray, flame spray and physical vapor deposition techniques. In order to increase the resistance of the ceramic layer to spallation when subjected to thermal cycling, thermal barrier coating systems employed in higher temperature regions of a gas turbine engine are typically deposited by physical vapor deposition (PVD) techniques, particularly electron beam vapor deposition (EB-PVD), that yield a spall-resistant columnar grain structure in the ceramic layer that is considered to be strain tolerant. PVD processes are preferred for deposition of ceramic layers at these hot surface locations because of the need for smooth thickness transitions, cooling hole communication between internal cooling fluid supplies and external surfaces. Air plasma sprayed (APS) are used in regions not having a large number of cooling holes open to the surface, but requiring thermal protection using thicker coatings than can efficiently and economically be applied using PVD. APS ceramic coatings typically require bond coats with surface roughnesses sufficient to enhance the mechanical bond between the two layers.
The bond coat typically is formed from an oxidation resistant aluminum-containing alloy to promote adhesion of the ceramic layer to the component through the formation of a TGO at the interface. The bond coat is critical to promoting the spallation resistance of a thermal barrier coating system. Examples of prior art bond coatings include MCrAlY (where M is iron, cobalt, and/or nickel), diffusion coatings such as nickel aluminide or platinum aluminide bond coats, and beta-phase NiAl, which are oxidation-resistant aluminum based intermetallics. The MCrAlY bond coats typically are deposited by air plasma spray (APS), while beta-phase NiAl is typically deposited by low pressure plasma spray (LPPS) techniques or high velocity oxyfuel (HVOF) techniques. The LPPS bond coats are smooth and grow a smooth, strongly adherent and continuous TGO layer that chemically bonds the ceramic layer to the bond coat, and protects the bond coat and the underlying substrate from oxidation and hot corrosion.
Bond coat materials are particularly alloyed to be oxidation and corrosion resistant through the formation of the thin, adherent alumina scale which may be further doped with chromia or other reactive oxides or elements. However, when used solely as an environmental coating, that is, without a ceramic topcoat, the thin alumina or chromia-doped alumina scale is adversely affected by the hot, corrosive environment, but quickly reforms. However, the reforming of a replacement scale gradually depletes aluminum from the environmental coating. When used as an environmental coating or bond coat for TBC applications, aluminum is lost from the bond coat as a result of interdiffusion into the superalloy substrate. Eventually, the level of aluminum within the bond coat is sufficiently depleted to prevent further growth of the protective alumina scale and/or stresses in the TGO have risen significantly, at which time spallation may occur at the interface between the bond coat and the ceramic layer.
In addition to the depletion of aluminum, the ability of the bond coat to form the desired alumina scale on the bond coat surface can be hampered by the diffusion of elements from the superalloy into the bond coat, such as during formation of a diffusion aluminide coating or during high temperature exposure. Oxidation of such elements within the bond coat can become thermodynamically favored as the aluminum within the bond coat is depleted through oxidation and interdiffusion. High levels of elements such as nickel, chromium, titanium, tantalum, tungsten and molybdenum incorporated into the TGO can increase the growth rate of oxide scales and form non-adherent scales on the bond coat surface that may be deleterious to adhesion of the ceramic layer. One of the ways in which such problems have been addressed is the addition of a monolithic beta-phase NiAl layer to the surface of a superalloy component using methods such as LPPS, e.g., U.S. Pat. No. 5,975,852 Nagaraj et al., with an oxide layer formed directly on top of the &bgr;-NiAl substrate. LPPS using relatively fine powders produces a relatively smooth surface, and after application of the &bgr;-NiAl layer, the coated surface is treated to have a surface finish not greater than about 50 microinches (about 1.2 micrometer) R
a
, such as by electropolishing, vapor honing, polishing or light abrasive blasting. Such layers are required to be thick in order to exhibit an enhanced service life for the component. A ceramic topcoat having columnar grains is then applied by a physical vapor deposition (PVD) process. Frequently, however, the bond coat is intentionally sprayed to provide a rough surface finish to enable the formation of a better mechanical bond between the bond coat and an APS ceramic topcoat.
In contrast to LPPS, because APS bond coats that include aluminum are deposited at an elevated temperature in the presence of air, they inherently form entrapped oxides and the scale that forms during disclosure may not be smooth and continuous. As a result, thermal barrier coating systems employing APS bond coats have not had the high temperature (e.g. above 1000° C.) oxidation resistance of systems employing LPPS bond coats. Fur

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