Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
1999-06-16
2001-04-03
Speer, Timothy M. (Department: 1775)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S469000, C428S472000, C428S447000, C428S448000, C428S450000, C428S701000, C428S702000, C106S286500, C106S287100, C106S287120, C106S287170, C427S427000, C427S376100, C427S376600, C427S419200
Reexamination Certificate
active
06210791
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to coatings for metal or ceramic substrates, and more particularly relates to coatings for metal or ceramic articles for use at elevated temperatures.
BACKGROUND OF THE INVENTION
In some applications, a structural part is exposed to high surface temperatures on a heated surface of the part. An oppositely disposed cooled surface of the part is cooled with a flow of cooling air. The maximum temperature reached by the part is determined by a balance between the amount of heat that enters the part from the heated surface, and the amount of heat removed by the cooling air flow over the cooled surface. Examples of such applications include combustors, nozzles, liners, and turbines in aircraft gas turbine engines.
It has been known to apply a thermal barrier coating system to the heated surface of the part to serve as an insulation that reduces the heat flow into the part and allows it to operate in a hotter external environment. The thermal barrier coating system typically includes a metallic bond coat overlying the metal or ceramic substrate that forms the part, and a ceramic layer overlying the bond coat. The bond coat improves the adherence of the ceramic layer to the substrate. The ceramic layer, which is typically a zirconium-based ceramic, reduces the heat flow into the substrate from the hot surface.
Coated articles having a metal or ceramic substrate, an intermediate thermal barrier coating overlying the substrate, and a low-emissivity metallic top coat over the thermal barrier coating are known. The available thermal barrier coatings typically utilize a ceramic layer overlying a metallic bond coat. The ceramic layer insulates the substrate, and the metallic bond coat improves the adherence of the ceramic layer to the substrate. The low-emissivity top coat reflects some of the thermal energy incident upon the coated article, so that the metal or ceramic part below is exposed to less heat input. While operable, the available systems with metallic top coats have not been practical for use in high-temperature environments such as gas turbines, because the reflective metal degrades and/or volatilizes after only a few minutes of exposure to the high-temperature, corrosive environment.
U.S. Pat. No. 5,851,679 discloses the use of a low-emissivity coating in the form of a ceramic multilayer optical reflector overlying the thermal barrier coating, to further reduce the heat flow into the coated part. This type of low-emissivity top coating is an improvement over metallic low-emissivity coatings, because it is more stable in the gas turbine environment.
However, the inventors have recognized that the available thermal barrier coatings have shortcomings in respect to their performance in conjunction with the ceramic multilayer or metallic low-emissivity top coats. Consequently, there is a need for a thermal barrier coating system which promotes the formation and functioning of low-emissivity top coats. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides an article having a substrate optionally coated with a thermal barrier coating system, a diffuse reflective barrier coating overlying the substrate or, where present, the thermal barrier coating system, and a low-emissivity top coating overlying the diffuse reflective barrier coating. The diffuse reflective barrier coating and the low-emissivity, heat rejecting top coating cooperate to reduce heat flow into the article from a heating source on the same side of the article as the low-emissivity top coating. This combination achieves improved performance as compared with the article having a thermal barrier coating system alone, or the thermal barrier coating system and the low-emissivity top coating, but without any diffuse reflective barrier coating present.
A coated article suitable for use at elevated temperature comprises a metal or ceramic substrate, a diffuse reflective barrier coating overlying the substrate, and a low-emissivity top coat covering the diffuse reflective barrier coating. The diffuse reflective barrier coating comprises a transmissive medium having a transmissive-medium index of refraction, and a plurality of radiation-scattering centers distributed in the transmissive medium. The radiation-scattering centers have a scattering-center index of refraction different from the transmissive-medium index of refraction. The indices of refraction are preferably those measured for radiation in the range of from about 0.8 to about 5 micrometers wavelength. The diffuse reflective barrier coating desirably has a surface roughness of less than about 150 microinches RMS.
The transmissive medium may be a first plurality of first ceramic particles bonded together with a binder. Such first ceramic particles may have a single size range. Preferably, however, they are of two distinct size distributions, with a second set of the ceramic particles having a second size range different from a first size range of a first set of ceramic particles. The average of the second size range is preferably much smaller than the average of the first size range, with the average of the second size range most preferably being {fraction (1/7+L )} or less of the average of the first size range. The two sets of ceramic particles produce the desired smooth surface of the diffuse reflective barrier layer, with the smaller particles filling the spaces between the larger particles. The two sets of ceramic particles also produce a controllable array of voids within the transmissive medium. The radiation-scattering centers may be, for example, such an array of voids or an array of second ceramic particles dispersed through the transmissive medium, each of which has an index of refraction different from that of the transmissive medium.
In one embodiment, the diffuse reflective barrier coating is prepared from a diffuse reflective barrier coating composition comprising first ceramic particles having a number average diameter between 3.0 and 6.0 micrometers, second ceramic particles having a number average diameter of between 0.05 and 0.8 micrometers, a binder precursor, and a carrier liquid. The first ceramic particles and the second ceramic particles are preferably aluminum oxide, the binder precursor is preferably a silicone, and the carrier liquid is preferably an anhydrous alcohol.
The diffuse reflective barrier coating composition is applied to a metal substrate which has a thermal barrier coating and a bond coat already applied thereto. The thermal barrier coating may serve as the diffuse reflective barrier coating, if the thermal barrier coating is prepared according to the requirements set forth herein for the diffuse reflective barrier coating. The diffuse reflective barrier coating composition may also be applied to a ceramic substrate. In any event, the diffuse reflective barrier coating composition is applied overlying (but not necessarily contacting) the substrate, preferably by wet spray, is dried, and is thereafter fired to produce the desired diffuse reflective barrier coating. A low-emissivity top coating composition is applied over the diffuse reflective barrier coating to produce the desired coated articles.
The resulting article has a metallic substrate (optionally with a thermal barrier coating thereon) or a ceramic substrate, a diffuse reflective barrier coating comprising the transmissive medium formed of the first ceramic particles bonded together by the binder, and the scattering medium, and a low-emissivity coating layer overlying the diffuse reflective barrier coating. As noted, the ceramic thermal barrier coating may serve as the diffuse reflective barrier coating if it is structured as set forth herein. The diffuse reflective barrier coating has a surface roughness of less than about 150 microinches RMS.
The diffuse reflective barrier coating of the invention cooperates with the low-emissivity top coating to improve the heat-reflectance characteristics of the coated article in at least two ways. First,
Murphy Jane A.
Skoog Andrew J.
Stowell William R.
General Electric Company
Hess Andrew C.
Narciso David L.
Speer Timothy M.
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