Abstract:
A turbine engine having an interior housing and one or more annular abradable seals for the turbine blades. The abradable seal comprises a resin having fractured hollow inorganic non-metallic microspheres forming nooks, crannies and undercuts in the resin and a solid lubricant in the resin and in the nooks and crannies and undercuts formed by the fractured hollow non-metallic inorganic microspheres.

Description:
This application is a divisional of application Ser. No. 07/733,559, filed Jul. 22, 1991, now U.S. Pat. No. 5,304,032. 
    
    
     BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION 
     Efficiency of rotating aircraft turbine engines (as opposed to reciprocating) depends partially on the efficient air compressor section which, in turn, depends on the clearance control between blade tips and the adjacent surfaces. The analogy would be the fit up and clearance of a piston ring and cylinder wall for an efficient compression stroke. 
     Because of the close assembly tolerances and the thermal expansion of the parts, a close clearance is maintained by having an abradable material opposite the tips of the blades. The blades cut their own groove or path, thereby minimizing leakage of air around the tips of the blades. Each engine has rows of moving blades mounted on rotating disks with rows of stationary blades in between each rotation section to redirect the flow into the next rotating stage. The stationary blade tips cut their groove into the rotating seal attached to the rotating compressor disk. 
     The performance of the abradable seal material is such that while it must be abradable and not abrade the tips Of the blades, it cannot stick to the blades. It must also be able to stand the elevated temperature due to compression as well as the erosion of abrasive particles injected into the intake air. The dynamic seal material must also have excellent bond strength to withstand the centrifugal forces induced at 30 to 40,000 rpm. Moreover, when the unit is stopped and parts are in contact, a lower coefficient of friction would facilitate starting by reducing drag forces at the interfaces of the seal material and blade tips. 
     In titanium bladed auxiliary power units (A.P.U.) a problem exists that when the unit remains stationary, particularly in humid and/or salty atmospheres, the titanium blades tend to seize against the abradable seal. This is because the abradable seal material is about 85% aluminum, or aluminum bronze, which are very much apart on the anodic chart with titanium and hence there is accelerated galvanic-type erosion of the aluminum alloy causing a seizing at the seal interface. The solution appeared to be in a non-metallic abradable seal material. 
     The present commercial system that has been widely accepted by tile aerospace industry is a system which calls for thermal spraying a mixture of aluminum powder and resin (85% aluminum and 15% resin). The process includes grit blasting the surface, applying a bond coat by thermal spray for better adhesion, then thermal (plasma) spraying the aluminum or aluminum bronze powder, machining this deposit to dimensional requirements, then a seal coat of a resin is applied to impregnate the sprayed deposit. Because the spray process must spray the aluminum powder and resin separately into the plasma, there at at least 15 variables and Taguchi Statistical Process Control methods are constantly applied every time some of these variables are changed. 
     In other similar systems, the material is also an aluminum or aluminum bronze spray powder with a polyimide powder rather than a polyester powder. None of the versions address the galvanic problem. 
     A polytetrafluoroethylene (a synthetic resin polymer, solid lubricant) coating is placed over a high temperature resin microsphere mix after the substrate has been machined to allow a coating of 5 mils to be added to the surface. This type of abradable seal solved many of the above problems. However, if the polytetrafluoroethylene particles on the surface were fully nucleated or melted together rather than just sintered, the coating in the thicker areas would tend to peel rather than abrade. Moreover, due to the dimensional tolerances, and difficulty and economics of applying a thick (0.003&#34; or over) coating of polytetrafluoroethylene, it was more realistic to assume that the wear of the blade tips would penetrate into substrate. 
     In a preferred embodiment of the invention, the tetrafluoroethylene coating is eliminated and solid lubricant polytetrafluoroethylene is incorporated into the microsphere and high temperature resin mix. 
     From wear tests and metallographic examinations of the surfaces, it appears that some of the polytetrafluoroethyiene or solid lubricant particles embedded in the resin break loose and are trapped in the cavities, nooks and crannies of the microspheres on the surface, thus producing a lower friction force. The friability and the frangibility of this system enables the wear to take place in the abradable seal material without wear or adherence to the titanium blade tips. This system has about 1/3 the variables that are in the present system and does not require the use of an expensive plasma or high velocity thermal spray system and associated equipment. 
     In a further preferred embodiment, ceramic fibers are incorporated into the resin, microspheres, polytetrafluoroethylene mix. 
     The basic objectives of the invention are to provide an improved abradable seal for rotating turbine engines and a method of producing same. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The above and other objects, advantages and features of the invention will become more apparent when considered with the following specification and attached drawings wherein: 
     FIG. 1 is a section through one stage of a turbine engine, 
     FIG. 2 is a section through the abradable seal on the stationary ring, and 
     FIG. 3 is a section through the abradable seal on the rotating compressor disc. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A diagrammatic illustration of the abradable seals of a rotating aircraft turbine engine is shown in FIG. 1. The efficiency depends partially on an efficient air compressor section which, in turn, depends on the clearance control between rotating blade tips BT and adjacent stationary surfaces SC. Because of close assembly tolerances and thermal expansion of parts, a close clearance is maintained by having an abradable material opposite the tips of the blades on the adjacent surfaces so that the blades cut their own groove or path to thereby minimize leakage of air around the blade tips and thus enhance the efficiency. 
     Referring collectively to FIGS. 1, 2, and 3, a stage portion 10 of a air compressor section of a multistage turbine engine has a fixed housing 11, a shaft 12 carrying rotatable compressor rotor discs 13 having outwardly extending blades (which may be titanium or other compressor blade alloy), and an abradable seal ring 15 on rotating compressor disc ring 16. The tips 17 of rotor blades 14 engage stationary abradable seal ring 18 formed on the interior of housing 11 adjacent stationary blades 19 which are secured to housing 11 and have tips 20 which engage rotating abradable seal ring 15. 
     According to this invention, the abradable seal surfaces 15 and 18 are provided with a coating comprised of a mixture of polytetrafluoroethylene, hollow-microspheres of ceramic or glass and a high temperature resin (RTM). To form the seals 15 and 18, some of the polytetrafluoroethylene particles embedded in the resin break loose and are trapped in the cavities, nooks and undercuts of fractured microspheres on the surface to produce a lower friction non-galvanic surface. The friability and frangibility of this system enables the wear to take place in the seal material without wear or adherence to the blade tips, which may be titanium. 
     The high temperature resin has the character of gray viscous plaste polytetrafluoroethylene is a white powder and the microspheres are free flowing. 
     In a further preferred embodiment, ceramic fibers, which a needle-like pieces of ceramic 1/8&#34; to 100 microns long, are incorporated into the high temperature resin, microsphere and polytetrafluoroethylene mix for additional strength especially in order to withstand the hoop stresses induced by the high rotational speeds of the compressor disks. 
     The use of a thermo-set resin, using a hardener, makes use of epoxy and polyamide-imide materials that have a high glass transition temperature (Tg) and a high heat deflection temperature (HDT). These properties are enhanced with the addition of the ceramic fibers, microspheres and polytetrafluoroethylene powders. 
     The resin, polytetrafluoroethylene and microspheres (RTM) formulation is preferably applied to the turbine&#39;s surfaces which have been heated to over 100 degrees Farenheit (125-150 degrees) and kept at that temperature by a hot air blower. The RTM mix should be applied in a manner to prevent folding in or entrapment of air, and an excess of material is added to allow for machining to final dimensions. However, excessive thickness may cause sagging which should be avoided. The parts are preferably rotated to allow major voids to surface and be eliminated, and then cured in an oven (3 hrs. @100 degrees C. and 180 degrees C. for 1 hr.) while the part or parts are rotated at about 6 rpm. At the end of the curing cycle the parts are cooled slowly to at least 70 degrees C. The RTM mix call be thermally sprayed while rotating the part on which the abradable seal is to be formed. 
     The following ratios of resin to fillers are percentages by weight of a preferred embodiment: 
     50/35 ratio of resin to microspheres to this is added 45 to 50% of polytetrafluoroethylene plus 12-13% ceramic fibers, example: 
     100 gms of resin +35% by weight of microspheres, i.e. 
     65 gms resin 
     35 gms microspheres 
     40-45 gms Teflon™ 
     b 12-13 gms ceramic fibers 
     The Resin and Fillers are blended uniformly for 5 minutes. Slight warming will lower viscosity for easier handling, the resin mix is then vacuum degased for 3 minutes at 28&#34; Hg. 
     Appropriate amounts are screed onto the surface to provide a minimum of about 0.060 thickness coating on the test samples described below. 
     Parts are placed in a curing oven for 3 hours at 100 degrees C. and 1 hour at 180 degrees C. and allowed to cool to 150 degrees before removing from oven. 
     Surface of samples are machined to a 120 micron finish and the surface is air blasted clean. The surface is inspected at 30× to 50× to insure removal of particles from the fractured microsphere cavities and undercuts. 
     TENSILE TESTING 
     Tensile tests are performed on each of the two sample pieces per conventional ASTM tensile tests. 
     Minimum tensile test results should average 2000 psi with no value lower than 1750 psi. 
     Five samples of abradable coatings were tested for tensile strength. The samples were as shown below: 
     
         ______________________________________SAMPLE NO.   RESIN TO TEFLON ™______________________________________1            2 to 12            3 to 13            4 to 14            4 to 15            45 to 23______________________________________ 
    
     MICROEXAMINATION 
     The samples were cross-sectioned and prepared for examination. All five coatings showed good adhesion to the substrate. Porosity varied from approximately 15 to 20% on Samples No. 1 and 2 to approximately 50% on Sample 5. No cracking was present in any of the samples. 
     COEFFICIENT OF FRICTION 
     The coefficient of friction for the five samples was determined as shown below: 
     
         ______________________________________   KINETIC      STATICSAMPLE  COEFFICIENT  COEFFICIENT  RESIN TONO.     OF FRICTION  OF FRICTION  TEFLON______________________________________1       .173         .360         2 TO 12       .168         .312         3 TO 13       .190         .325         4 TO 14       .203         .302         4 TO 15       .190         .472         45 TO 23______________________________________ 
    
     MECHANICAL TESTS 
     The samples were adhesively bonded with epoxy as shown in FIGS. 1-3 and mechanically tested to determine tensile properties and location of failure. The test results are shown below: 
     
         ______________________________________      TENSILE STRENGTH                      LOCATION OFSAMPLE NO. PSI             FAILURE______________________________________1          2700            COATING2          2550            COATING3          2700            COATING4          2750            COATING5          1150            COATING______________________________________ 
    
     CONCLUSION 
     From the preceding examination we may make the following observations: 
     a. No disadhesion from the substrate or cracking was present on any of the samples. 
     b. The amount of porosity varied from approximately 15% to 50% between the samples. 
     c. The tensile strengths were consistent, ranging from 2550 to 2750 PSI, except on Sample 5, where the tensile strength was 1150 PSI. 
     While there has been shown and described a preferred embodiment of the invention, it will be appreciated that other embodiments will be readily apparent to those skilled in the art and it is desired to encompass such obvious modifications and variations within the spirit and scope of the claims appended hereto.