Metal fusion bonding – Process – Repairing – restoring – or renewing product for reuse
Reexamination Certificate
2002-02-04
2003-10-28
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Repairing, restoring, or renewing product for reuse
C228S245000, C228S254000, C427S124000, C427S142000, C427S156000, C029S017200, C029S402130, C029S889100
Reexamination Certificate
active
06637643
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention generally relates to bond coatings and thermal barrier coatings applied to metals, e.g., metal components used in turbine engines. In some specific embodiments, it relates to improved techniques for applying such coatings to surfaces where access is difficult.
Metal components are used in a wide variety of industrial applications, under a diverse set of operating conditions. In many cases, the components are provided with coatings which impart various characteristics, such as corrosion resistance, heat resistance, oxidation resistance, and wear resistance. As an example, the various components of turbine engines, which typically can withstand in-service temperatures in the range of about 1100° C.-1150° C., are often coated with thermal barrier coatings (TBC's), to effectively increase the temperature at which they can operate.
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 superalloy surfaces, such as turbine blades and vanes, combustor liners, and combustor nozzles. Usually, the TBC ceramics are applied to an intervening bond coating (sometimes referred to as a “bond layer”, “bond coat”, or “bond coat layer”) which has been applied directly to the surface of the metal part. The bond coating is often critical for improving the adhesion between the metal substrate and the TBC.
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 coating, e.g., the microstructure of the bond coating. TBC failure can also result from deficiencies at the bond coat-substrate interface or the bond coat-TBC interface.
The microstructure of the bond coating is often determined by its method of deposition. The deposition technique is in turn often determined by the requirements for the overlying protective coating. For example, many TBC's usually require a very rough bond coat surface for effective adhesion to the substrate. An air plasma spray (APS) technique is often used to provide such a surface.
While the APS process has several advantages, it also results in a porous coating microstructure. Such a microstructure allows significant internal oxidation of the bond coating. The oxidation of regions of the bond coating often reduces the concentration of aluminum in other bond coat regions. This phenomenon can in turn result in the diffusion of aluminum from an adjacent, aluminum-containing substrate, e.g., a superalloy. The depletion of aluminum from a superalloy substrate is especially severe when the component is used at the elevated temperatures described above. The loss of aluminum can be detrimental to the integrity of superalloy components.
In a pending U.S. patent application of M. Borom, et al., Ser. No. 09/385,544, now U.S. Pat. No. 6,165,628 problems associated with the microstructure of porous bond coats are addressed. In one embodiment of the reference, a bi-layer is used to bond a TBC to a metal substrate. The bi-layer includes a dense, primary bond coating over the substrate, and a “spongy” secondary bond coating over the dense coating. The primary bond coating is usually applied by a vacuum plasma spray (VPS) or high velocity oxy-fuel (HVOF) technique. The spongy, secondary bond coating is usually applied by an air plasma spray technique. The primary bond coating helps to protect the substrate from excessive oxidation. The secondary bond coating promotes adhesion between the primary coating and the subsequently-applied TBC, while also acting as a strain-reliever between these two other coatings. The resulting TBC system exhibits high integrity during exposure to high temperatures and frequent thermal cycles.
Clearly, the various thermal spray techniques mentioned above are quite suitable for applying bond coatings to many substrates. However, they are sometimes not effective for applying the coatings to regions of a substrate which are somewhat inaccessible, since the spray equipment may be too large and cumbersome for such regions. For example, it can be very difficult to thermally spray a bond coating on a flange or other surface of a turbine engine part. Moreover, the spray process, which may include one or more masking steps, is sometimes very time-consuming. It is often very difficult to carry out local repairs using this process.
Thus, new methods for efficiently applying bond coatings and TBC's to inaccessible regions of a substrate would be welcome in the art. The methods should also be capable of providing a bond coating microstructure which protects the substrate from excessive oxidation. The methods should result in bond coats which provide a desirable level of adhesion between the substrate and a subsequently-applied TBC. The overall TBC should be effective in protecting components used in high performance applications, e.g., superalloy parts exposed to high temperatures and frequent thermal cycles. It would also be desirable if the methods were generally compatible with conventional application equipment, e.g., various plasma spray techniques.
SUMMARY OF THE INVENTION
One embodiment of this invention is a method for applying at least one bond coating to a surface of a metal-based substrate, comprising the following steps:
(a) attaching a foil which comprises the bond coating to the substrate surface, and then
(b) fusing the foil to the substrate surface, so that the bond coating adheres to the substrate.
The foil is often prepared by thermally spraying the bond coating material onto a removable support sheet. Exemplary thermal spray techniques are vacuum plasma deposition (VPS), high velocity oxy-fuel (HVOF), and air plasma spray (APS). When the support sheet is removed, the free-standing foil of bond coating material remains.
The free-standing foil is typically fused to the substrate surface by a brazing or welding process. Various brazing techniques are possible. As an example, a slurry of the braze composition can be applied to a surface of the foil, which is then attached to the substrate surface, with the braze composition contacting the substrate. The braze composition is then exposed to a suitable brazing temperature. An alternative technique involves applying the braze slurry to the substrate surface first. The foil is then attached to the slurry-coated substrate, followed by brazing. As still another alternative, a green braze tape cart be used to attach the foil to the substrate surface, followed by brazing.
The bond coating usually comprises an alloy of the formula MCrAlY, where M is selected from the group consisting of Fe, Ni, Co, and mixtures of any of the foregoing. In some embodiments of the invention, the foil is made from at least two bond coatings. For example, it can be based on a dense, primary bond coating and a “spongy” secondary bond coating over the dense coating, as further described below.
Moreover, other embodiments of this invention include the application of a thermal barrier coating applied over the bond coating on the removable support sheet. The TBC is usually zirconia-based, and can be applied over the bond coating by various techniques, such as a plasma spray process. Thus, the free-standing foil in this embodiment would include both a bond coating (or multiple bond coatings) and the TBC.
Yet another embodiment of this invention includes a method for repairing a worn or damaged thermal barrier coating system applied over a metal-based substrate. The method includes the step of removing the worn or damaged system (i.e., including at least one bond coating and the TBC), followed by replacement of the coating system, using the free-standing foil mentioned above. As described previously, the foil is usually cut to the
Borom Marcus Preston
Hasz Wayne Charles
Nelson Warren Arthur
Viggiani James Edward
Zanneti John
DiConza Paul J.
General Electric Company
Patnode Patrick K.
Stoner Kiley
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