WO1995021319A1

WO1995021319A1 - Honeycomb abradable seals

Info

Publication number: WO1995021319A1
Application number: PCT/US1995/001246
Authority: WO
Inventors: Roland Benoit; Eugene M. Beverly; Charles M. Love
Original assignee: United Technologies Corporation
Priority date: 1994-02-01
Filing date: 1995-01-31
Publication date: 1995-08-10

Abstract

A method is taught for the preparation of abradable seals for gas turbines. The honeycomb of a suitably abradable nickel alloy composition is brazed to the turbine component, and the open cells thereof are smeared closed, such as by grinding, to provide a smooth, dense, and impermeable abradable surface. The surface may be abraded by a cubic boron nitride abrasively tipped blade to form an inexpensive but highly efficient seal.

Description

HONEYCOMB ABRADABLE SEALS

DESCRIPTION

Technical Field of the Invention

This invention relates to abradable seals for gas turbine engines, designed so as to provide an effective, smooth, impermeable, closed honeycomb abradable surface, equivalent to a smooth, hard abradable surface, with great aerospace compatibility and tolerance to rubbing by a blade tip incorporating an abrasive such as cubic boron nitride-

Background of the Invention Large gas turbine engines are widely used for aircraft propulsion and for ground based power generation. Such large gas turbine engines are generally of the axial type, and include a compressor section, a combustor section, and a turbine section, with the compressor section normally preceded by a fan section. Each of the fan, compressor, and turbine sections comprises a plurality of disks mounted on a shaft, with a plurality of airfoil shaped blades projecting radially therefrom. A hollow case surrounds the various engine sections. Located between the disks and projecting inward from the case assembly which surrounds the disks are a plurality of stationary vanes. During operation of the fan, compressor, and turbine sections, axially flowing gases alternately contact moving blades and the stationary vanes. In the fan and compressor sections, air is compressed and the compressed air is combined with fuel and burned in the combustion section to provide high pressure, high temperature gases which flow through the turbine section, where energy is extracted by causing the bladed turbine disks to rotate. A portion of this energy is used to operate the compressor section and the fan section. Engine efficiency depends to a significant extent upon minimizing leakage by control of the gas flow in such a manner as to maximize interaction between the gas stream and the moving and stationary airfoils. A major source of inefficiency is leakage of gas around the tips of the compressor and fan blades, between the blade tips and the engine case.

Accordingly, means to improve efficiency by reduction of leakage are increasingly important. Although a close tolerance fit may be obtained by fabricating the mating parts to a very close tolerance range, this fabrica¬ tion process is extremely costly and time consuming. Further, when the mated assembly is exposed to a high temperature environment and high stress, as when in use, the coefficients of expansion of the mating parts may differ, thus causing the clearance space to either increase or decrease. The latter condition would result in a frictional contact between blades and housing, causing elevation of temperatures and possible damage to one or both members. On the other hand, increased clearance space would permit gas to escape between the compressor blade and housing, thus decreasing efficiency.

One means to increase efficiency is to apply a coating of suitable material to the interior surface of the housing, to reduce leakage between the blade tips and the housing. Various coating techniques have been employed to coat the inside diameter (tip shroud) of the compressor housing with an abradable coating that can be worn away by the frictional contact of the compressor blade, to provide a close fitting channel in which the blade tip may travel. Thus, when subjecting the coated assembly to a high temperature and stress environment, the blade and the case may expand or contract without resulting in significant gas leakage between the blade tip and the housing. This abradable coating technique has been employed to not only increase the efficiency of the compressor, but to also provide a relatively speedy and inexpensive method for restoring excessively worn turbine engine parts to service. As generally mentioned in U.S. Patents 3,879,831 to Rigney et al, and 3,084,064 to Cowden et al, abradable seals must have a peculiar combination of properties. They must be resistant to erosion from the high velocity, high temperature gas streams which at times may carry fine particulate matter with them. However, they must also be subject to removal (i.e. abrading) when contacted by the tip of a high speed blade, so that the tip of the blade is not degraded. It is critical that the housing coating abrade rather than wear the blade tip, since a decrease in blade tip size will increase clearance between the blade tip and the housing all around the circumference, resulting in a greater increase in gas leakage than would result from abrasion of only a small arc of the coating around the circumference of the housing. Conventionally, the tip of the blade is coated with a highly erosion resistant material, such as aluminum oxide or cubic boron nitride. One form of abradable seal is that prepared by the teachings of the aforementioned Rigney et al, U.S. Patent 3,879,831. This patent discloses an abradable material having a composition of 60 - 80 percent nickel, 2 - 12 percent chromium, 1 - 10 percent cobalt, 4 - 20 percent aluminum, and 3 - 15 percent inert material such as diatomaceous earth, boron nitride, silica glass, mica, etc. Up to 3 percent of a metal such as yttrium, hafnium, or lanthanum may also be present. The abradable materials produced by this reference are characterized by a high degree of porosity, oxidation resistance, low thermal conductivity, and the ability to be abraded away cleanly in a localized area. Similarly, U.S. Patent 3,084,064, of Cowden et al, deals with the preparation of abradable coatings on turbine surfaces by flame spraying nichrome and from 2 to 20 weight percent of a finely divided powder of a high melting material such as boron nitride, carbon, graphite, or magnesium oxide. The abradable characteristics of this coating are believed to be due to the dispersed material preventing formation of a solid, dense, strongly cohesive metal phase. In other words, the high melting powder permits the surface to easily flake off in relatively uniform particles when subjected to an abrading force.

Another form of abradable seal developed in the past was a porous structure, obtained by use of a fugitive material in the precursor article.

In the prior art, pressing and sintering and other metallurgical techniques have been used together with thermal spraying to produce porous structures. Metal deposits with densities as low as 75 - 85 percent may be applied by plasma spraying. However, to obtain densities lower than this, which were formerly believed to be desirable for abradable seals, it was necessary to incorporate non-metallic materials. Most preferably, a fugitive material such as a water soluble salt or a heat-decomposable polymer was sprayed with the metal, and then subsequently removed. For example, an abradable seal structure may be prepared in accordance with the teachings of Otfinoski et al, U.S. Patent 4,664,973, who teaches spraying a polymethylmethacrylate resin and a nichrome metal, and then removing the resin by heating the resultant structure to a temperature of about 315°C.

Among numerous other forms of abradable seals developed previously are those such as set forth by Shiembob, in U.S. Patent

4,566,700, comprising a composite structure wherein an abrasive layer is interposed between the abradable layer and the component substrate, and powder metal filled honeycomb structures, such as set forth by Davies, in U.S. Patent 3,844,011. Davies teaches the use of a honeycomb structure as a labyrinth seal, in which the honeycomb like structure provides a coherent and structurally continuous body of compacted metal powders in which a network of cavities is formed, of various geometric configura¬ tions. Such cavities are preferably filled with compacted metal powders, metallurgically compatible with the basic honeycomb structure. The desired degree of abradability of the seal may be achieved by controlling the density or composition of either the powder metal filler or the compacted metal powder forming the honeycomb structure. Another honeycomb abradable seal is set forth by Ryan, U.S. Patent 4,546,047, which teaches filling the cells of a honeycomb with an abradable by insertion of a flexible tape preform consisting of an abradable material and a braze material. Thus, both Davies and Ryan teach that the cavities, or cells, of a honeycomb structure are to be filled with an abradable material, so as to provide an abradable structure.

In addition, it is known to use open celled honeycomb structures for abradable seals in the inner air seal portion of a gas turbine engine, where the abradable honeycomb is cut, or grooved, by an abrasive knife edge having aluminum oxide, for example, on the surface thereof. In such instances, the cutting action of the knife edge leaves the honeycomb as an open cell surface, which a channel cut therein. When using a honeycomb having a 1/32 inch cell size, having a wall thickness of 3 mils, the amount of metal present precludes the use of a fan blade to cut a channel in such a seal, due to the high temperatures which would be generated by such cutting, and the roughness of the surfaces generated thereby. An open honeycomb cell structure results in leakage, which in turn negatively affects stall margin and efficiency. The roughness of the open cell structure also decreases efficiency.

Although these various methods produce abradable coatings usable for turbine applications and the like, they have disadvantages of either providing coatings which are hard to chip off in small discreet amounts by friction contact so as to provide a well defined blade tip channel having no large cavities through which gases may escape, or producing an interconnected porous surface layer which in itself permits the escape of gases, thus lowering efficiency. Further, such methods are relatively expensive, both in initial fabrication and in repair. Accordingly, it is an objective of the present invention to provide an improved seal system which contributes to engine efficiency by providing an outer air seal, which while abradable and smooth, is impermeable to gas flow. It is a further object of the present invention to provide a seal useful for both compressor and fan sections of a gas turbine engine which is inexpensive to manufacture and fabricate, and offers long life, high efficiency, and ease of abradability.

As previously indicated, the abradable surface must be structurally sound to resist failure at points other than where contacted by the blade tip, must resist the thermal and vibratory strains imposed upon it in use, and must be readily fabricated in a reproducible and cost efficient manner. Considerable effort has gone into the development of abradable seals having the desired combination of properties. The present invention is reflective of that continuing effort.

SUMMARY OF THE INVENTION

The present invention comprises a novel method for preparation of an abradable seal, wherein a metal honeycomb is directly affixed to the structure of the outer air seal of a gas turbine engine, such as by brazing. The cells of the honeycomb are then sealed, capped, or closed, such as by machining, grinding, or lapping, to form a very smooth, highly densified, impermeable, abradable surface.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to an abradable seal having particular utility in gas turbine engines, particularly those of the axial flow type. Such engines include alternate rows of stationary vanes and moving blades, with the blades being attached at the periphery of shaft mounted rotating disks. The seal of the present invention comprises a honeycomb structure affixed to the surface of an engine component where interaction occurs or is anticipated with another component, i.e. a fan or compressor blade. While the invention will be described herein as being applicable to the outer air seal of the compressor section, it is to be understood that the invention is also applicable to other sections of the engine, such as an inner air seal. The honeycomb structure consists of a multiplicity of open cells which are separated from each other by thin metal cell walls. The cells are initially open at both ends, and while preferably hexagonal in shape, may also be of various geometric configurations, such as dia¬ monds, squares, circles, sinusoidal patterns, crescent or straight lines parallel to a single axis, and the like. The cell size may vary from 1/8 inch to 1/64 inch, or smaller, but is preferably from 1/16 inch to 1/64 inch, and most preferably from 1/32 inch to 1/64 inch. The wall thickness separating the cells may vary from about 0.75 mil to about 1.2 mil, for the 1/64 inch cell honeycomb, and the height of the honeycomb may vary from 50 mils to 200 mils, dependent upon specific application, engine size and requirements, and blade with which the seal is to interact. The honeycomb may comprise any material suitable for use as an abradable seal, but is preferably a nickel-based alloy such as AMS 5536, an alloy comprising 22 percent chromium, 1.5 percent cobalt, 0.10 percent carbon, 18.5 percent iron, 9.0 percent molybdenum, 0.6 percent tungsten, the remainder nickel (all percentages in weight percent), and having a density of 0.299. Such honeycombs are generally available in sheet form or as foils, and may be cut or joined to the size necessary for the specific seal being made.

The metal honeycomb is firmly affixed and secured to the backing member or substrate, such as the tip shroud outer air seal, preferably by such means as brazing. Any conventional brazing technique suitable for joining one nickel based alloy structure to another may be used. Braze materials are a class of relatively low melting point materials, often based upon nickel or gold, with additions of various melting point depressants. The particular braze material used will depend in part on the compositions of the honeycomb and the substrate, which may comprise any metal suitable for use in the manufacture of gas turbine engines. It is to be noted that choice of the specific braze material, and the height to which the braze is allowed to wick up into the honeycomb cell, will directly influence the degree to which the cell structure may be closed, just as will the specific honeycomb material and cell wall thickness. It has been learned that the use of a gold based braze material results in a less brittle honeycomb than a nickel based braze. As the honeycomb is brazed to the substrate, a controlled amount of the braze material will rise up the sides of the cell walls, such as approximately 1/4 the height of the honeycomb, by wicking, and will influence the metallurgical and physical properties of the honeycomb.

After the honeycomb is brazed to the substrate, the open ends of the honeycomb cells are closed, by smearing. As used in reference to this invention, the term "smearing" is meant to refer to any means by which the open cell structure may be closed, such as by wiping with high pressure grinding, polishing, lapping, machining, etc. Alternatively, the terms "capping" or "closing" may be used to express the concept of closing the cells of the honeycomb by displacement of materials from the cell walls. The smearing operation is directed to totally closing all cells of the honeycomb so as to achieve a density of at least 90 percent of the theoretical density of the metal from which the honeycomb is fabricated, and preferably 95 percent. The porosity of the smeared honeycomb should be less than about 5 percent, and preferably less than about 2 percent, with a permeability of less than about 10 percent, preferably less than about 2 percent, and most preferably approaching zero. The surface roughness of the smeared honeycomb surface should be less than about 600 micro-inches, preferably from about 50 to 250 micro-inches, and most preferably below about 200 micro-inches. Smearing, for example, may be conducted with a diamond grinding wheel, preferably positioned so as to displace the cell walls in the same direction as the abradable seal material will be wiped by the blade tip in subsequent use.

After the abradable surface has been smeared to the smoothness set forth above, the seal may be subjected to abrasion by the blade tip, or to "cutting in" of the channel in which the blade tip will travel. The specific density, hardness, and porosity of the abradable seal should of course reflect the specific blade tip with which it will be used. For best results, using a seal of about 100 mils thickness, comprising the braze and the smeared honeycomb, the blade should cut a channel from about 5 mils to about 50 mils deep in the seal material, preferably from about 25 mils to about 45 mils deep. It is of course recognized that to achieve a seal thickness of any specific dimension, the height of the honeycomb must be selected so as to provide the desired dimension after smearing, and the compression which occurs during smearing.

Gas turbine engine seals prepared in accordance with the present invention offer advantages such as lower cost than the conventional plasma sprayed seal coatings, improved efficiency retention, reduced weight, and improved erosion resistance. Accompanying these are such secondary advantages as improved blade tip clearance and blade durabili¬ ty. Further, the seals of the present invention are easily repaired, highly reproducible, and easily retrofit into current engines in the field.

EXAMPLE I

A 1/50 inch cell size honeycomb and a 1/64 inch cell size honeycomb, each having a wall thickness of 1.2 mils, and a height of 100 mils, and comprising Hastelloy X nickel alloy, were brazed to a tip shroud outer air seal, using a conventional nickel braze. When these brazed structures were subjected to rubbing with a knife edge coated with aluminum oxide, channels were cut into the honeycombs, said channels having open celled surfaces at the bottom.

EXAMPLE II A 1/64 inch cell size honeycomb, having a wall thickness of 1.2 mils, and a height of 100 mils, and comprising AMS 5536 nickel alloy, was brazed to a tip shroud outer air seal, using a conventional nickel braze. When this brazed structure was subjected to grinding using a cubic boron nitride grinding wheel, the grinding wheel itself was worn down before a smooth surface could be obtained on the smeared honeycomb.

When a gold braze was used in place of the nickel braze, the honeycomb was easily smeared to a surface roughness of less than 200 micro-inches.

EXAMPLE III

A series of abradable seals were prepared in accordance with the present invention, and tested to determine properties thereof. In each of the examples below, a honeycomb comprising AMS 5536 nickel alloy, having a 1/32 inch cell size and 2 mils wall thickness was brazed to a nickel fiat plate to simulate a tip shroud outer air seal, using a gold braze comprising 82 percent gold, 18 percent nickel. The air seals were then mounted in a full-scale rub rig to simulate use in a compressor, rubbing each with a blade tipped with CBN (cubic boron nitride) having a particle size between 100 and 120 mesh, at an incursion rate of 4.6 mps and a tip speed of 800 fps. The results of these tests are shown in Tables I and II. TABLE I

SEAL CELL BLADE SEAL BLADE SEAL CELLS WALL WEAR WEAR TEMP. TEMP.

OPEN? Mils Mils Mils °F °F

OPEN 2.0 2.7 41 < 1300 < 1300

CLOSED 2.0 8.8 50 1300 < 1300

OPEN 3.0 10.4 42 1360 1300

TABLE II

SEAL CELL LOADING CELLS WALL TANG. AXIAL RADIAL

OPEN? Mils lbs. lbs. lbs.

OPEN 2.0 80 55 150

CLOSED 2.0 80.4 96.7 138

OPEN 3.0 200 140 440

As illustrated by Tables I and II, substantial increases in wear and loading result from an increase in wall thickness of the honeycomb cell from 2 mils to 3 mils. While loading and wear also increase when one goes from open cells to smeared or closed cells, the debit is not as great as results from increasing wall thickness. Further, the increase of temperature resulting at both the blade and the seal is not affected as significantly by the state of cell closure as it is by increasing wall thickness. EXAMPLE IV

Abradable seals made in accordance with the present invention were compared to a smooth, hard seal made in accordance with industrial practice. The smooth hard seal comprised a plasma sprayed oxidation resistant superalloy encompassing hexagonal boron nitride, with less than about 15 percent porosity, a permeability of less than 20 percent, a surface roughness of less than 600 micro-inches, and a density from about 4.4 to 5.2. The abradable seal made in accordance with the present invention, comprised a nickel superalloy as set forth above, smeared to a surface roughness of less than 250 micro-inches, and having a density after smearing of about 0.27, a porosity of less than 5 percent, and a permeability approaching zero.

The two seals were mounted in a test rig and subjected to rubbing with a cubic boron nitride tipped fan blade. Both seals were cut by the blade tips, and grooves were established for passage of the blade in each.

The honeycomb was essentially 100 percent closed, with a surface roughness below about 250 micro-inches. Blade wear was negligible in each case, but the honeycomb seal was more easily cut.

EXAMPLE V In addition, comparative seals were prepared using 1/64 inch cell honeycomb and the hard smooth seal of Example IV. These seals were subjected to rubbing with abrasive tipped blades, and the surface roughnesses thereof were measured. The 1/64 inch honeycomb, without smearing to a smooth surface, was abraded by cubic boron nitride tipped blade, resulting in a surface roughness of 1258 micro-inches. The same honeycomb, after being smeared, was abraded by an aluminum oxide tipped blade, resulting in a surface roughness of 227 micro-inches. The hard smooth seal, prepared by plasma spraying, was abraded by an aluminum oxide tipped blade to a surface roughness of 295 micro-inches. These tests demonstrate the advantageous smooth surface obtained with the smeared honeycomb of the present invention.

There are several obvious advantages of the seal made in accordance with the present invention. First, the densities of the seals are an order of magnitude different. The material of the conventional hard smooth seal will weigh approximately 247 pounds per cubic foot, while the material of the smeared honeycomb will weigh about 18 pounds per cubic foot, a significant weight difference in engine applications. Secondly, the hard smooth seal requires a plasma spray application, incorporating a plasma sprayed bond coat. The brazing and smearing steps of the present invention represent a substantial cost reduction in application and also in refurbishment. Testing has shown that there was no deterioration of stall margin using the present invention, and that the compressor efficiency is essentially the same. Although this invention has been shown and described with respect to detailed and preferred embodiments thereof, it will be understood by those skilled in the art that various changes and modifications therein may be made without departing from the spirit and scope of the claimed invention.

Claims

WHAT IS CLAIMED IS:

1. A method for preparation of an abradable seal, said method comprising affixing a nickel alloy based honeycomb to a seal substrate, and smearing the surface of said honeycomb to a surface roughness of less than about 600 micro-inches, a density of greater than about 90 percent of the theoretical density of said nickel alloy, a porosity of less than about 5 percent, and a permeability of less than about 10 percent.

2. A method as set forth in claim 1, wherein said honeycomb is affixed to said substrate by brazing.

3. A method as set forth in claim 2, wherein said smearing is conducted by grinding.

4. A method as set forth in claim 2, wherein the cell size of said honeycomb is from about 1/16 inch to 1/64 inch.

5. A method as set forth in claim 4, wherein the wall thickness of said honeycomb is from about 0.75 mils to 1.2 mils.

6. A method as set forth in claim 5, wherein said surface roughness after smearing is from about 50 to 250 micro-inches.

7. A method as set forth in claim 6, wherein said porosity is less than about 2 percent.

8. A method as set forth in claim 7, wherein said permeability is less than about 2 percent.

9. A method as set forth in claim 2, wherein said brazing is conducted with a gold braze.

10. An abradable seal comprising a substrate, and a nickel based alloy abradable material adhered to said substrate and forming an abradable surface layer, said abradable surface layer having a surface roughness of less than about 600 micro-inches, a density of greater than about 90 percent of the theoretical density of said nickel based alloy, a porosity of less than about 5 percent, and a permeability of less than about 10 percent, said abradable surface layer having been formed by brazing a honeycomb of said alloy to said substrate, and smearing said honeycomb to a closed, dense structure.

11. An abradable seal as set forth in claim 10, wherein said surface layer has a surface roughness of from about 50 to 250 micro-inches.

12. An abradable seal as set forth in claim 11 , wherein said surface layer has a porosity of less than about 2 percent, and a porosity of less than about 2 percent.

13. An abradable seal as set forth in claim 12, wherein the cell size of said honeycomb prior to smearing is from about 1/16 inch to about 1/64 inch.

14. An abradable seal as set forth in claim 13, wherein said the wall thickness of said honeycomb prior to smearing is from about 0.75 mils to 1.2 mils.

15. An abradable seal as set forth in claim 14, wherein said alloy comprises 22 percent chromium, 1.5 percent cobalt, 0.10 percent carbon, 18.5 percent iron, 9.0 percent molybdenum, 0.6 percent tungsten, the remainder nickel.

16. An abradable seal as set forth in claim 15, wherein the cell size of said honeycomb prior to smearing is about 1/32 inch.

17. An abradable seal as set forth in claim 15, wherein the cell size of said honeycomb prior to smearing is about 1/50 inch.

18. An abradable seal as set forth in claim 15, wherein the cell size of said honeycomb prior to smearing is about 1/64 inch.