IMPROVED METHOD FOR ADHESION OF GRIT TO BLADE TIPS

Abstract: Metal coated ceramic particles are bonded to a metallic substrate (12) in a high temperature sintering process. A low viscosity binder solution containing fine metallic particulates (16) is first applied to the substrate surface. Then, the coated ceramic particles (18) are disposed upon the substrate surface, and the binder solution and the metal particulates (16) therein are attracted by capillarity into regions of point contact between the ceramic particles (18) and the substrate surface. During a subsequent high temperature sintering operation, the metal coating on the ceramic particles (18) diffuses into the metal substrate (12), and the metallic particulates (16) melt and solidify to bridge the gap between the substrate (12) and ceramic particles (18).

Description: The present invention concerns a method for joining a plurality of cramic particles to the surface of a metallic article used at elivated temperature according to the precharacterizing portion of claim 1. More specifically, it relates to a method for joining a single layer of closely spaced silicon carbide particles to the metallic surface of a turbine engine component. Gas turbine engines and other turbomachines have rows of blades which rotate within a generally cylindrical case. As the blades rotate, their tips move in close proximity to the internal wall surface of the case. To maximize engine operating efficiency, the leakage of the gas or other working fluid between the blade tips and the case should be minimized. As has been known for some time, this may be achieved by blade and seal systems in which the blade tips rub against a seal attached to the interior of the case. Generally, the blade tip is made to be harder and more abrasive than the seal so that the tips cut into the seal during those portions of the engine operating cycle when they contact each other. One type of blade tip which is particularly useful in the high temperature section of a gas turbine engine is described in US-A-4,249,9l3 to Johnson et al, entitled "Alumina Coated Silicon Carbide Abrasive", of common ownership herewith. In the Johnson et al invention, silicon carbide abrasive particles of about 0.20-0.75 mm average diameter are coated with a metal oxide such as alumina and incorporated by powder metal techniques in nickel or cobalt base matrix alloys. A powder metal compact containing up to about 45 volume percent of these ceramic particles may be made which is then bonded to the tip of the blade. The resulting abrasive blade tip is particularly well suited for rubbing metal as well as ceramic seals. As described in greater detail in the EP-A-0166676 improved techniques for the fabrication of blade tips useful at high temperatures are desired. Specifically, the blade tip should be as thin as possible, and the amount of abrasive particles minimized. In order for the tip to provide the required abrasive characteristics, it is essential that the abrasive particles be securely bonded to the blade tip surface. When an abrasive layer is provided on a superalloy turbine blade tip, its method of application must be metallurgically compatible with the superalloy substrate so that the properties of the substrate are not degraded. Such considerations place restraints on the kinds of materials and processing techniques which are useful in the fabrication of such abrasive layers. The method for joining a plurality of ceramic particles to the surface of metallic article used at elevated temperature, according to the present invention is characterized in using as said binder solution a solution containing in addition metal particulates which are smaller than said ceramic particles and which are capable of diffusing into the article surface and into the metal layer on the ceramic particles and characterized in that during the disposition of said single layer of the ceramic particles a plurality of the metal particulates are attracted by capillarity into the joint between the ceramic particles and the article surface so that during the subsequent heating of the article the metal particulates diffuse in said joint into said metal layer and into the article surface. The invention is particularly useful in the fabrication of an abrasive layer on the tip surface of a rotor blade used in a gas turbine engine. For desired operating characteristics, the particle density per unit area of blade tip surface should be maximized, while at the same time the interparticle contact should be minimized. Most importantly, the particles must be securely bonded to the blade tip to withstand the stresses of engine operation, particularly rubbing with air seals. In a preferred embodiment, the ceramic particles are silicon carbide which are coated with aluminum oxide and then overcoated with a nickel-boron alloy. The aluminum oxide prevents diffusion or dissolution of the silicon carbide at elevated temperatures, and the nickel-boron readily diffuses into the blade tip. The binder solution contains a mixture of toluene and diglyme as carrier liquids, polystyrene as an adhesive resin, and nickel flake or powder (both referred to as particulates) as a sintering aid. During the sintering operation, there is simultaneous diffusion of the nickel-boron coating as well as the nickel particulate into the blade tip surface at regions of point contact between each ceramic particle and the tip. Also, some of the nickel particulate diffuses into the nickel-boron coating which remains on each ceramic particle. After the sintering operation, a matrix alloy is deposited onto the tip surface to cover the silicon carbide particles sintered thereto and to fill in the spaces between the particles. The matrix is then simultaneously heated and pressed to eliminate any voids which may be present, and to securely bond the matrix to the substrate and, by interdiffusion, to the metal coating on each particle. The abrasive layer is then machined to a relatively flat surface, and then part of the matrix is chemically removed to cause portions of the particles to project into space. When blades having such an abrasvie layer are installed in an engine, these exposed particles can effectively rub an airseal during engine operation, and minimize the leakage of working fluids around the blade tips, thus improving engine operating efficiency. The foregoing and other objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments and accompanying drawings. Figure l generally shows the radially outer portion of a typical gas turbine blade having an abrasive layer made according to the invention; Figure 2 shows in cross section the appearance of an abrasive layer produced according to the teachings of the present invention; Figure 3 shows in cross section the appearance of a prior art abrasive layer; Figure 4 shows in cross section coated ceramic particles useful in the invention; Figure 5 shows in side view the metal particulates attracted towards the ceramic particles; and Figure 6 shows in side view the abrasive layer after application of a metal matrix. The invention is described with reference to the fabrication of an abrasive layer on a gas turbine blade tip. However, those skilled in the art will recognize that the invention is useful in other applications where small ceramic particles or components need to be securely bonded to a metal substrate. In an example of the practice of the invention, and referring to Figures l and 2, an abrasive layer l0 is formed on the tip surface ll of the airfoil portion l2 of a gas turbine blade l4. The blade l4 is preferably made of a nickel base superalloy (such as the alloy described in U.S. Patent No. 4,209,348), while the abrasive layer l0 contains ceramic silicon carbide particles l8 in a nickel base superalloy matrix l6. As is described below, an important feature of the abrasive layer l0 fabricated according to this invention is that each of the particles l8 is securely bonded to the blade tip surface ll. The abrasive layer l0 is subject to high stresses during engine operation, and therefore it is important that the layer l0 have a certain configuration and properties so as to perform its function. In particular, the particles l8 must be disposed on and secured to the tip surface ll in a certain manner to obtain optimum performance. In a prior art abrasive layer 20 for a turbine blade 2l shown in Figure 3 and discussed in the Background section above, the abrasive particles 22 are randomly dispersed within a matrix metal 24. The abrasive particles 22 are not individually bonded to the blade tip surface 23 as in the invention; rather, the particles 22 are preferably spaced from the surface 23 for reasons related to the bonding process used to join the layer 20 to the surface 23. Referring again to Figures l and 2, the abrasive layer l0 made according to the invention is characterized by a single layer of closely spaced abrasive particles l8 surrounded by matrix material l6. The matrix metal l6 has a thickness W which is preferably less than the overall thickness T of the particles l8. As a result, a portion of each particle l8 projects into space, thereby enabling favorable rubbing interaction with air seals during engine operation. For optimum performance, the particles l8 as well as matrix metal l6 must be securely bonded to the blade tip ll. Furthermore, the unexposed portion of the particles l8 must be surrounded by matrix metal l6, and the particles l8 must be closely spaced apart from each other. Use of a single layer of abrasive l8 minimizes the mass of the entire layer l0, thus reducing the centripetal force on the blade l4 as it rotates during engine operation. Also, it allows each particle l8 to be enveloped in matrix material l6 (except the outermost region on each particle l8), thereby enhancing the integrity and strength of the layer l0. In the blade tip made in accordance with the invention, each abrasive particle l8 is sinter bonded to the blade tip ll, and the majority (preferably at least about 80-90%) of the particle surface area (excluding that surface area exposed at the blade tip) is surrounded by matrix metal l6 rather than being in contact with another particle l8. Thus, the particles l8 are all securely joined to the tip ll. Also, the particles l8 are, in general, evenly and densely spaced apart on the blade tip ll. Densities of about 35-ll0 particles per cm² of tip surface ll are preferred, with about 50 particles per cm² being most preferred. As shown in Figure 2, the particles l8 have a thickness (length), T, and the matrix thickness W is about 50-90 percent of the particle thickness T. Silicon carbide particles (nominally about 0.20-0.75 mm in size) have been found to be particularly useful in the practice of the invention, although other sizes might be useful. Summarizing the fabrication of the abrasive layer l0 according to this invention, the particles l8 are placed in a single layer on the surface of the blade tip ll which has previously been coated with a low viscosity binder solution which contains fine metal particulates. The blade l4 is then heated to an elevated temperature which sinter bonds each particle l8 to the tip surface ll, and the metal particulates to both the particles l8 and tip surface ll. Each of the particles l8 is coated with a multiple layer coating. The first layer 30 (Figure 4) is an oxide coating which is stable at elevated temperatures; this coating prevents the particles l8 from dissolving or diffusing into the blade tip ll during the elevated sintering (bonding) operation and during service operation. The preferred oxide coating 30 for silicon carbide particles is aluminum oxide, 0.005-0.025 mm thick, applied in accordance with the aforementioned Johnson et al patent. As shown in Figure 4, the aluminum oxide coating 30 substantially encapsulates the silicon carbide particle l8. This is necessary to best prevent dissolution and/or diffusion of the particles l8 at high temperatures. The second layer 32 is metallic and capable of diffusing into the tip surface ll during the high temperature sintering operation. The metallic layer 32 must be compatible with the substrate, i.e., not form any phases or compounds which would degrade the properties of the blade. In general, the layer 32 is selected from the transition elements of the periodic table, or any alloy thereof, when nickel, cobalt, or iron base matrix and blade alloys are used. The metallic layer 32 substantially encapsulates the oxide layer 30. As discussed above, the sinter bond which forms between each particle l8 and the blade tip ll must have high strength in order for the abrasive layer l0 to have the required characteristics. When an abrasive layer l0 containing silicon carbide particles l8 is made on a nickel base superalloy, these properties are achieved by using a nickel-boron alloy as the metal layer 32. The boron content should be, by weight percent, about 2 to 4%, preferably about 3%. The thickness of the layer 32 should be about 0.005 to 0.0l5 mm, preferably about 0.008 mm. Formation of the sinter bond is further improved by coating the blade tip surface ll with a binder solution before the ceramic particles l8 are placed on the tip ll. The solution contains a thermoplastic resin and fine metallic particulates in a low viscosity carrier liquid. When the particles l8 are placed on the tip surface ll which has been coated with the binder solution, the resin adhesively bonds each particle l8 to the tip ll. Then, with the passage of time and due to the low viscosity of the carrier liquid, the carrier liquid and particulates 34 are drawn by capillarity into the region of point contact between each ceramic particle l8 and tip surface ll. See Figure 5. During the high temperature sintering operation, in addition to the aforementioned nickel-boron layer 32 diffusion, the particulates 34 diffuse into the tip surface ll and into the metal layer 32 on each ceramic particle l8, bridging the gap between each particle l8 and the surface ll, thereby resulting in an even higher strength bond. The particles l8 may be placed on the blade tip surface ll in any convenient fashion. The preferred practice is discussed in more detail in the copending and commonly assigned application "Method for Depositing a Layer of Abrasive Material on a Substrate", US-A-46080 199 to Vontell et al, filed March 2l, l986. In this method a vacuum (suction) is drawn through a transfer tool which has spaced apart perforations therein. The tool is then placed over a container of loose particles l8, and the suction draws and holds one particle l8 over each perforation. (Of course, the perforations are smaller than the nominal size of each particle l8.) The tool is then positioned over the blade tip ll and the suction level is adjusted so that the particles l8 drop onto the tip surface ll. While the particles l8 lie on the surface of the binder solution coated blade tip ll, the carrier liquid and the metal particulates 34 are drawn into the areas of point contact 36 (i.e., into the joint) between the particles l8 and the tip surface ll. For this movement to take place, the viscosity of the carrier liquid must be low and the particulates 34 must be small. The preferred carrier liquid consists essentially of about l00 cc of toluene and l cc of diglyme. It also contains an adhesive resin, preferably about 5 g of polystyrene. The particular choice of this thermoplastic resin is important in the practice of the invention. As has been noted above, the composition of the blade alloy reflects a highly refined metallurgical design which results in the achievement of particular properties. Fabrication of the blade tip must not detrimentally affect these properties. Polystyrene is chosen as the resin binder because when it volatilizes, it depolymerizes and leaves behind no carbon residue. Thermosetting resins are not useful since they do not depolymerize, but rather crosslink, and leave a carbon residue when they volatilize. The presence of carbon on the blade tip surface would result in carburization of the tip during the high temperature sintering operation, which would likely degrade mechanical properties. Pure nickel flake particulates, nominally about 0.5 to l.0 microns in size, preferably 0.8 microns, are useful in the invention when the blade is a nickel base superalloy and the ceramic particles are nickel-boron coated silicon carbide. Nickel is preferred since it does not appreciably change the composition of the blade alloy when it diffuses into the tip surface ll. During the time required for the nickel particulates 34 to coalesce around the abrasive particles l8 (usually about l5-20 minutes for the particular binder solution described above), the diglyme-toluene carrier liquid evaporates, and the particles l8 and particulates 34 become adhesively bonded to the blade tip surface ll. Then, the blade l4 is heated to a temperature sufficient to volatilize the resin and cause the metal layer 32 on the particles l8 to diffuse into the tip surface ll at regions of point contact. The preferred sintering conditions are about l,080°C for l-6 hours in a non-oxidizing atmosphere. Some of the particulates 34 in the joint diffuse into the tip surface ll and others diffuse into the metal layer remaining on the particles l8, thereby bridging the joint gap and improving the particle-surface sinter bond. This may be appreciated by examining the bond which forms in the absence of the particulate: since the particles l8 tend to be irregularly shaped, the sinter bond which forms as a result of diffusion of only the Ni-B alloy into the tip surface exists only at the regions of point contact of each particle and the tip. Increasing the Ni-B coating thickness, in an attempt to increase the amount of alloy available for diffusion, does not appreciably increase the bond strength; it only adds unnecessary Ni-B coating thickness to each particle l8. However, when nickel particulates 34 are added to the binder solution, a significantly greater amount of diffusable metal is available to form the sinter bond. Accordingly, each particle l8 is securely bonded to the tip surface ll and is thereby capable of providing the required abrasive characteristics to the blade tip l0. Also, few, if any, of the particles l8 are dislodged from the tip surface ll during a subsequent matrix application step, described below. Following the sintering operation, the particles l8 are oversprayed with a layer of matrix material l6 deposited by plasma arc spraying or physical vapor deposition to a thickness T′ as shown in Figure 6. A nickel base superalloy of the type generally described in the aforementioned Johnson et al patent may be used. The preferred matrix composition is, by weight percent, about 25 Cr, 8 W, 4 Ta, 6 Al, l.0 Hf, 0.l Y, 0.23 C, balance Ni. Of course, other matrix alloys may be equally useful, such as Hastelloy X, Haynes l88, INl00, or other similar materials. Although the sprayed layer of matrix material l6 will have about 95 percent theoretical density, it may contain some porosity or voids, which could reduce the mechanical properties of the overall abrasive layer l0. To eliminate such voids, the blade l4 is then subjected to a hot isostatic pressing (HIP) procedure. The HIP treatment also enhances the bond between the matrix l6, particles l8, and blade tip ll. For the specific superalloy matrix material described above, a temperature of about l,l00°C and a gas pressure of about l38 MPa applied for two hours is sufficient. Other hot pressing procedures may be used to consolidate the matrix material l6 and achieve the object of densification and bonding. Next, the surface of the abrasive layer l0 is machined using a conventional procedure such as grinding to produce a smooth, planar surface. Finally, the surface of the abrasive layer l0 is contacted with a chemical etchant or other substance which will attack and remove some of the matrix material l6, causing a portion of each of the particles l8 to project into space. For example, electrochemical machining can be used, as is described in US-A- 4,522,692 to Joslin. This step reduces the matrix thickness to a dimension W, which is about 50-90 percent of the dimension T, and results in an abrasive layer l0 having the shape schematically shown in Figure 2. When the particles are long and thin (i.e., have a high aspect ratio), they tend to lie on their sides either when first placed on the blade tip surface ll or in the interval between the volatilization of the adhesive agent and the attainment of a metallic bond. Such laying-at-length causes undue interparticle contact, and reduces the abrasive nature of the layer l0. Thus, according to a preferred embodiment the invention is best practiced when the aspect ratio of the particles is less than about 1.9 to 1 and preferably is about 1.5 to 1 or less. The aspect ratio is defined herein as the average ratio of the longest particle dimension to the cross sectional dimension, as such is measured on a Quantimet Surface Analyzer (Cambridge Instruments, Cambridge, England). The sintering aspect of the invention will be better understood by referring to the following example which is intended to be illustrative, and not limiting in scope. Nominal 0.30 mm silicon carbide particles were coated with a dual layer coating of 0.00l5 cm aluminum oxide and 0.0008 cm nickel-3% boron. A binder solution containing nickel flake having a size range of about 0.5-l.0 microns, polystyrene, toluene and diglyme was prepared and applied to the tip surface of nickel base superalloy test blades. Binder solutions containing varying amounts of nickel flake were evaluated to examine the effect of such variation on the strength of the sinter bond which formed. The binder solutions were brushed on the blade tip surface and then the ceramic particles placed thereon. The particle density was ranged between about 50-l00 per square cm². After about 20 minutes, the blade was heated at 1080°C (l,975°F) in argon and held for l-6 hours, as shown in Table I. To examine the strength of sinter bonds, impact tests were conducted. In these tests, a 4.5kg (ten pound) weight was dropped onto each test blade from a height of about 28 cm. The strength of the bond was characterized by comparing the number of particles bonded to the tip before the test with the number bonded to the tip after the test. This ratio is presented in Table I as "% Grit Retention". Of course, high percentages are indicative of good bonds. As is seen, when no nickel flake was added to the binder solution, many particles were dislodged during the impact test, indicating a poor bond. Improved results were achieved as the amount of nickel flake increased. The maximum amount of nickel flake which will be useful is dictated by two factors: (l) how the diffusion of such flake alters the composition of the blade tip, and (2) how the viscosity of the binder solution is affected by addition of the flake. Between about 0.5 and l5 grams of the 0.5-l.0µm (micron) nickel powder or nickel flake will be useful when the diffusion heat treatment is about 1080°C (1,975°F) for 1-6 hours. The preferred range is 0.5-4 grams for l-2 hours at 1080°C (1,975°F). Carrier liquids other than the preferred toluene-diglyme mixture may be used, such as, e.g., xylene or mixtures of xylene and toluene. They must have a low viscosity, for reasons discussed above, of no greater than about l-l0 centistokes. Also, they should have the same evaporation characteristics as the preferred mixture. Thermoplastic resins such as polymethylstyrene may be used rather than polystyrene.

Claims: A method for joining a plurality of ceramic particles (18) to the surface of a metallic article used at elevated temperature comprising the steps of: a) depositing on each particle (18) a multiple layer coating (30,32) comprising a first oxide layer (30) stable at elevated temperatures and a second metal layer (32) compatible with and capable of diffusing into the article; b) coating the article surface with a binder solution comprising a low viscosity carrier liquid and a thermoplastic resin wherein the resin leaves no carbon residue on the article surface after volatilization. c) disposing a single layer of the ceramic particles (18) in closely spaced relation on the article surface and d) heating the article to diffuse at least a portion of the metal layer (32) on each ceramic particle into the article surface characterized in using as said binder solution a solution containing in addition metal particulates (34) which are smaller than said ceramic particles (18) and which are capable of diffusing into the article surface and into the metal layer on the ceramic particles (18) and characterized in that during the disposition of said single layer of the ceramic particles (18) a plurality of the metal particulates (39) are attracted by capillarity into the joint between the ceramic particles (18) and the article surface so that during the subsequent heating of the article the metal particulates (34) diffuse in said joint into said metal layer and into the article surface. A method for joining a plurality of silicon carbide particles (18) the tip surface(11) of a nickel base superalloy article comprising the steps of: a) depositing on each particle a two layer coating wherein the first layer (30) is an aluminum oxide and the second layer (32) is a metal alloy, wherein the aluminum oxide encapsulates the silicon carbide particle (18) and the metal alloy coating encapsulates the aluminumoxide coating; b) coating the blade tip surface(11) with a binder solution; c) disposing a single layer of the silicon carbide particles (18) in spaced apart relation on the tip surface (11) and d) heating the article to volatilize the binder solution and to diffuse a portion of the metal alloy layer (32) on each silicon carbide particle (18) into the tip surface(11) characterized in using as said metal alloy layer (32) a nickel-boron alloy and in using as said binder solution a solution containing toluene, diglyme, polystyrene and nickel particulates (34) wherein the particulates(34) are much smaller than the silicon carbide particles (18) and characterized in that during the disposition of said single layer of the silicon carbide particles (18) on the tip surface (11) a plurality of the nickel particulates (34) are attracted by capillarity into the joint between each silicon carbide particle (18) and the tip surface(11) so that during the subsequent heating of the article the toluene diglyme and polystyrene volatilize and the nickel particulates (34) diffuse in said joint into the nickel-boron coating and into the tip surface (11). The method of claim 2, further comprising the steps of: a) depositing on the tip surface (11) a metallic matrix material (16) to fill in the spaces between the silicon carbide particles; and b) removing a portion of the matrix material (16) to cause a portion of each of the particles (18) to project into space. The method of claim 3, wherein the matrix material (16) is plasma sprayed. The method of claim 3, wherein the superalloy article is a gas turbine engine blade. The method of claim 2, wherein the silicon carbide particles are about 0.2-0.5 mm and the nickel particulates are about 0.5-1.0 microns.

Inventors: PIKE, ROSCOE ADAMS
Applicants: UNITED TECHNOLOGIES CORPORATION
Application year: 1987
Main classification: C23C4
IPC codes: C23C24, B24D18, F02C7, F01D5