Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
1998-11-17
2002-06-25
Jenkins, Daniel J. (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
Reexamination Certificate
active
06409795
ABSTRACT:
TECHNICAL FIELD
This invention relates to coating methods, products used in coating methods, and articles coated by such methods and products. In a particular aspect the invention is concerned with methods for the abradable coating of turbine engine shroud segments, products for forming coatings thereon, and coated articles, particularly turbine engine shroud segments with abradable coatings. More specifically, the invention relates to improved methods and products for effecting such coatings, and better coated articles. The invention improved coating products include flexibly formable biscuits for coating turbine engine shroud segments, which are laminar in form to provide different thicknesses of coating material for different applications, from shrouds, to turbine blade leading edges, to blade tips, and to other applications. The biscuits of the invention will conform generally to the curvature of any part. The invention biscuits are readily produced from mixtures of alloy powders of wider compositional variety than previously known biscuits for the purpose, and contain stripping agents or stripping agent precursors effective to strip oxides, such as aluminum oxide, or yttrium oxides from the substrate surface and from the coating composition during formation of the coating and thereby ensure maximum efficiency in bonding development between the substrate and coating.
In another aspect, the invention provides novel brazing compositions and methods for fabrication of the coating biscuit products. In this aspect the invention utilizes diffusion alloy processes, such as cementitious pack diffusions, to controllably add, in optimum distribution, fusion temperature lowering amounts of silicon to MCrAl and MCrAlY alloy powders to be subsequently converted into biscuits and formed into coatings on metal parts. In a further aspect, the invention provides methods of fabricating MCrAl, MCrAlY and other alloy powders into useful forms, suitably laminar and separable into layers of different thicknesses for different depths of coating on a selected metal part by the use of resinous processing aids such as polytetrafluorethylene.
BACKGROUND OF THE INVENTION
In the aspect of the invention concerned with production of coated turbine engine shrouds. The old technology improved upon by the present invention is set forth in U.S. Pat. Nos. 4,842,953 and 4,937,042, both to Perkins and Smith, referred to herein as the Perkins patents. In these patents the patentees describe the need in gas turbine engines to achieve as great a reduction in leakage as possible between stationary and rotating gas turbine members and the use of abradable seals on turbine engine shroud segments to effect control of leakage. These patents describe the formation of suitable abradable seals on turbine engine shrouds by a biscuit process in which two specific, composition-limited powder alloys are mixed and compacted to a preform exhibiting self-supporting “green strength”. The preform is suitably pre-sintered by taking its temperature to a temperature at which the lower, but not the higher melting alloy melts, i.e. above the liquidus temperature of the lower melting alloy but not as high as the liquidus temperature of the higher melting alloy. The resulting preform is a “biscuit” of fixed shape; thus fixed in shape the biscuit must of necessity be closely congruent with the article to be coated, e.g. a shroud surface, since the preform biscuit by the Perkins patent process is brittle and cannot subsequently be flexed to conform to any surface but the intended one. Once securely placed on the shroud surface, the biscuit preform is heated to a temperature at which the lower melting alloy powder liquid phase sinters the powder composition, and, then further heated to temperatures at which the interdiffused powered alloys fuse together and bond to the substrate. The patentees require defined levels of silicon and yttrium in the overall composition, the presence of one and not the other in the respective alloy powders, which with other criteria imposed severely limits the number of alloys useful in the method and the variety of products to be obtained. Alloys containing substantial amounts of aluminum, yttrium, or titanium, or other metals prone to form surface oxides, such as Inco 738 containing about 4% by weight of each of aluminum and titanium, are advantageously coated with the compositions and methods of this invention. By the use of the invention compositions and methods such oxides are stripped from the coating composition and from the surface of the alloy articles being coated, as well, allowing bonding where the presence of these oxides had been thought to preclude it. For yttrium containing alloys such as CO-159, increased amounts of yttrium can be used in coatings in accordance with the invention beyond the 0.1% by weight taught as the maximum in the Perkins patents. This is because, in contradistinction to the Perkins patents, the present invention provides for stripping the oxides generated in the coatings or on the article surface before coating. Oxide presence is inimical to effective bonding of the biscuit to the substrate.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved method of coating turbine engine shrouds. It is another object to provide such a coating method using biscuits which are flexible to be conformed to virtually any shape of substrate surface. It is a further object to provide a method of making biscuits for such coating methods which is not limited in which alloys it can use by narrow constraints on the presence and quantities of silicon and/or yttrium, aluminum or titanium and novel biscuits by such method. Yet another object is to provide biscuit compositions which are self-stripping of oxides, and coatings and coated articles made therefrom which are stripped of oxides in the course of coating formation as a result of the biscuit composition. Still another object is to provide a repair composition, suitably but not necessarily in the form of a biscuit, for repair of metal parts, including gas turbine components such as shrouds, blade surfaces and blade tips, and components of all types, e.g. where an abradable surface coating is desirable. Another object is to provide improvements in the processing of superalloy powders into coating products such as biscuits. Other objects include provision of coating preparation methods which enable the use of a wide number of superalloys, and not just those having ab initio the desired amount of silicon present for fusion, and fabrication methods which simplify biscuit formation, provide flexible biscuits, and as well incorporate oxide-stripping components in situ in the coating composition.
These and other objects of the invention to become apparent hereinafter are realized by provision of a novel biscuit of a widely variable composition, which, when coated onto a shroud or other substrate, forms an effective coating, such as an abradable coating. More particularly, the invention objects are realized by modifying in a cementitious diffusion pack any of numerous alloys to incorporate silicon into the alloys without change in the basic composition. These diffusion-modified alloys are then used to distribute silicon highly uniformly through the biscuit and thus through the coating composition which comprises the diffusion-modified alloy, now a lower melting alloy, and a second, higher melting alloy; their combination forming a braze coating composition in which the lower melting alloy is the low temperature fusing, sintering alloy in the blend with a higher melting suitable alloy.
In another aspect of the invention, these objects are further realized by adding a source of hydrofluoric acid to the alloy blend to act as an oxide stripper upon final melt formation of the abradable coating or repair. The HF source is suitably in the form of a high molecular weight fluorinated polymer which will facilitate biscuit formation and biscuit flexibility before volatilizing as HF and other products in the final steps of the
Clark Eugene V.
Sievers George K.
Garmong Gregory O.
General Electric Company
Jenkins Daniel J.
Narciso David L.
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