Coating processes – Spray coating utilizing flame or plasma heat – Metal oxide containing coating
Reexamination Certificate
2000-04-25
2001-08-14
Jones, Deborah (Department: 1775)
Coating processes
Spray coating utilizing flame or plasma heat
Metal oxide containing coating
C427S456000
Reexamination Certificate
active
06274201
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to protective coatings applied to metals. More specifically, it is directed to a thermal barrier coating system for a metal substrate which is used in a high temperature environment.
Specially-formulated coatings are often used to protect metal parts which are exposed to high temperatures. Aircraft engines are made from such parts, as are land-based gas turbines. The combustion gas temperatures present in the turbine engine of an aircraft are maintained as high as possible for operating efficiency. Turbine blades and other elements of the engine are usually made of alloys which can resist the high temperature environment, e.g., superalloys, which have an operating temperature limit of about 1000° C.-1150° C. Operation above these temperatures may cause the various turbine elements to fail and damage the engine.
The protective coatings, often referred to as thermal barrier coatings or “TBC”s, effectively increase the operating temperature of the turbine engine by maintaining or reducing the surface temperature of the alloys used to form the various engine components. Most TBC's are ceramic-based, e.g., based on a material like zirconia (zirconium oxide), which is usually chemically stabilized with another material such as yttria. For a jet engine, the coatings are applied to various surfaces, such as turbine blades and vanes, combustor liners, and combustor nozzles.
Usually, the TBC ceramics are applied to an intervening bond layer (“bond coat”) which has been applied directly to the surface of the metal part. The bond layer is often critical for improving the adhesion between the metal substrate and the TBC. Bond layers are typically formed from a material like “MCrAlY”, where “M” represents a metal like iron, nickel, or cobalt. The bond layer may be applied by a variety of conventional techniques, such as PVD; plasma spray, or other thermal spray deposition methods such as HVOF (high velocity oxy-fuel). In the past, those skilled in the protective coating arts have generally concluded that the bond layers should be as dense and rough as possible. However, the deposition processes still undesirably resulted in coatings which were not dense, i.e., which could be characterized as “spongy”.
The effectiveness of a TBC coating is often measured by the number of thermal cycles it can withstand before it delaminates from the substrate which it is protecting. In general, coating effectiveness decreases as the exposure temperature is increased. The failure of a TBC is often attributed to weaknesses or defects related in some way to the bond coat, e.g., the microstructure of the bond coat, or deficiencies at the bond coat-substrate interface or the bond coat-TBC interface.
It should be apparent from this discussion that new protective coating systems of increased quality are very desirable—especially for high performance applications, such as superalloy parts exposed to high temperatures and frequent thermal cycles. In addition to maintaining their integrity over a large number of thermal cycles, the coating systems should be compatible with conventional application equipment, e.g., various plasma spray techniques.
SUMMARY OF THE INVENTION
This invention is based on the discovery of new coatings which satisfy the needs discussed above. In one aspect, this invention embraces an article, comprising:
(i) a metal-based substrate, such as a superalloy;
(ii) a dense, primary bond layer over the substrate;
(iii) a secondary bond layer over the dense layer, having a microstructure which comprises an open network of interconnected pores; and
(iv) a thermal barrier coating over the secondary bond layer.
The dense, primary bond layer is often applied by a vacuum plasma spray technique or by a high velocity oxy-fuel (HVOF) technique. The secondary bond layer is usually applied by a plasma spray process, although alternative techniques may be used.
As described below, the secondary bond layer has a spongy structure. This bond layer is usually formed by plasma-spraying particles of the bond layer material, wherein the particles usually have a specified size range. The microstructure of the bond layer has at least about 225 continuous strings of oxide greater than 25 microns in length, per square millimeter of sample, viewed in cross-section.
Another aspect of the invention relates to a method for providing a protective coating on a metal-based substrate. The method comprises the steps of:
(a) applying a metallic bond layer over the substrate, wherein the bond layer has a microstructure which comprises an open network of interconnected pores; and then
(b) applying a thermal barrier coating over the bond layer.
In some instances, a dense, primary bond layer is applied over the substrate before the application of the metallic bond layer in step (a), as described previously.
Other details regarding the various embodiments of this invention are provided below.
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Borom Marcus Preston
Gray Dennis Michael
Lau Yuk-Chiu
Pabla Surinder Singh
General Electric Company
Ingraham Donald S.
Jones Deborah
Stoner Douglas E.
Young Bryant
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