Method of making a functionally graded material

[0002] This application is a continuation-in-part application of U.S. patent application Ser. No. 09/453,774 filed on Dec. 3, 1999 now U.S. Pat. No. 6,248,286, entitled “Method of Making a Functionally Graded Material” the entire disclosure of which is incorporated herein by reference.

[0004] The present invention relates to functionally graded materials (FGM) and methods of making same, and more particularly to making functionally graded materials via slurry processing methods.

[0005] The goal of most ceramic forming processes is to create a green body that is as homogeneous as possible, so that the structure will be very uniform after sintering. It is well known that even minor inhomogeneities in the green body can lead to stresses, distortions, and poor mechanical properties (even fracture) of the object after sintering. For example, during slip casting the movement of water from the liquid slurry into the porous mold creates small nonuniformities in the green density of the component and can lead to residual stresses after firing. One of the major benefits of the ceramic gelcasting process is that a uniform slurry is created and poured into a mold, where gelation of the liquid vehicle locks the constituent powder particles into place before settling, floatation, flocculation, or other source of inhomogeneities can operate.

[0006] On the other hand, there are many applications in which it is desirable to have a dense ceramic and/or metal component in which the properties vary in some way throughout the article. It may be desirable to produce an article characterized by high hardness on one surface and high toughness in the interior, for example. Particularly for uses involving microwave energy, it is often desirable to have the loss tangent and dielectric constant of an article vary so that microwave energy can be effectively absorbed by the article while minimizing reflections.

[0007] Accordingly, objects of the present invention include provision of methods of making metallic, ceramic, and/or composite articles having compositions that are graded spatially within the articles so that one or more physical properties of the finished articles vary in some desired way therethroughout. Such methods include provisions for minimizing the effects of variable properties of the constituents, such as sintering characteristics. Further and other objects of the present invention will become apparent from the description contained herein.

[0008] In accordance with one aspect of the present invention, the foregoing and other objects are achieved by a gelcasting method of making an internally graded article which includes the steps of:

[0009] a. preparing a slurry including a least two different phases suspended in a gelcasting solution, the phases characterized by having different settling characteristics;

[0010] b. casting the slurry into a mold having a selected shape;

[0011] c. allowing the slurry to stand for a sufficient period of time to permit desired differential settling of said different phases in order to achieve a vertical compositional gradient in the molded slurry;

[0012] d. gelling the slurry to form a solid gel while preserving the vertical compositional gradient in the molded slurry;

[0013] d. drying the gel to form a dried green body; and

[0014] e. sintering the dry green body to form a solid object, at least one property thereof varying along one direction because of the vertical compositional gradient in the molded slurry.

[0015] FIG. 1 is a schematic view of a 5-layered functionally graded material (FGM) in accordance with the present invention.

[0016] FIG. 2 is a schematic view of an 11-layered functionally graded material in accordance with the present invention.

[0017] FIG. 3 is a schematic view of a continuous functionally graded material in accordance with the present invention.

[0018] FIG. 4 shows the distribution of properties along the length of a functionally graded ceramic component made in accordance with the present Invention.

[0019] FIG. 5 is a schematic cross-sectional view of a continuous, radially oriented, functionally graded material in accordance with the present invention.

[0020] For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is prepared to the following disclosure and appended claims in connection with the above-described drawings.

[0021] Applicants have discovered, surprisingly, that the gelcasting process can be modified to create such Functionally Graded Materials (FGM). The invention can be carried out in several ways. First, two components (phases) which are at least partially miscible in one another can be dispersed into two slurries. These two slurries can then be poured into a mold in layers or continuously blended as the mold is filled, thereby creating a desired compositional gradation within the object being molded. The slurries are then gelled to lock the constituent particles into the desired nonuniform distribution. Alternatively, several phases can be dispersed into a common gelcasting slurry, which is initially homogeneous. This slurry is poured into a mold and allowed to settle for some period of time sufficient to create a compositional gradient in the vertical direction through the action of buoyancy forces. After the desired compositional gradient is established, gelation of the vehicle locks it into place, preventing further settling of the individual constituents.

[0022] As will be shown in the Examples, a key aspect of the inventive method is that the individual slurries must have comparable sinterability so that the sintering shrinkage is substantially uniform throughout the body even though the composition is not. By matching the sintering shrinkage, distortion of the body during sintering is thereby minimized or avoided.

[0023] The invention makes use of the fact that gelcasting slurries are generally highly fluid with well-dispersed particulate phases suspended in such a way that two such slurries can, in principle, be blended to create a composition intermediate between the two. The mixed slurries could therefore be layered into a mold or blended (through a static mixer, for example) to provide a continuously graded composition as the mold is filled.

[0024] Although two slurries may be suitable for gelcasting and sintering individually, blended articles having varied compositions can be subject to warpage, cracking, and other problems that arise because of differences in the amount of shrinkage which each composition undergoes during sintering. The present invention provides a method of making a functionally graded material which eliminates the aforementioned problem by tailoring the individual slurries so that they have comparable sinterability even though they have different compositions.

[0025] Methods of making FGMs in accordance with the present invention include, but are not limited to, layered casting, continuously graded casting, and casting with controllled settling.

[0026] Experiments to fabricate Al₂O₃/SiC FGM samples by a layered gelcasting method were conducted from three arrays of slurry compositions as described in Table I. Referring to FIG. 1, rectangular, bar shaped specimens 10 of approximately 0.25″×0.50″×3.00″ were fabricated by pouring equal aliquots of each slurry composition into a mold in successive layers, followed by gel polymerization to lock in the structure and preserve the layers.

[0027] In a first array of 55 v. % solids slurries, compositionally graded specimens of Al₂O₃and SiC powders were made by layering five slurries (Arrays 1-1 to 1-5, Table 1) with incrementally increasing proportions of SiC. The first layer 12 of each of the specimens comprised essentially 100% Al₂O₃. SiC replaced Al₂O₃in each successive layer in relative amounts of 0% 12, 10% 14, 20% 16, 30% 18, and 40% 20, respectively. Although the gelcasting (forming) phase of this array of slurries was successful, excessive warpage and anisotropic shrinkage occurred in specimens 10 during the sintering stage due to different sintering kinetics for the two materials. For example, on specimen No. 1-5 for the pure Al₂O₃layer,˜14.2% shrinkage occurred (from beginning wet to post-sintering). On the opposing end containing 40% SiC, shrinkage was only˜4.0%. (Reference specimens 1-1-1-5)

[0028] In a second array of experiments similar to that above, the total solids loading was increased to 60 v. % (in efforts to minimize differential shrinkage caused by the changing compositions). The first layer of each of the specimens comprised essentially 100% Al₂O₃. SiC replaced Al₂O₃in each successive layer in relative amounts of 0%, 10%, 20%, 30%, 40%, and 50% additions respectively (for six layers total).

[0029] Referring to FIG. 2, in another array of experiments, the objective was to produce specimens 30 having bilateral symmetry, thus producing a continuously graded material that would yield a graded dielectric loss over its length. Eleven layers containing 0% 32, 10% 34, 20% 36, 30% 38, 40% 40, 50% 42, 40% 44, 30% 46, 20% 48, 10% 50, 0% 52 additions of SiC were poured into molds and gelled to yield specimens 30 with opposite ends composed of Al₂O₃and graded symmetrically toward the center (increasing the amounts of SiC with each successive layer) to a center composition of 50/50 Al₂O₃/SiC (for eleven layers total). The gelcasting forming for this array was also successful, however, once again, excessive warpage and shrinkage still occurred during the sintering stage due to anisotropic sintering kinetics. For example, on specimen No. 2-1 for the pure Al₂O₃end layer, approximately 13% shrinkage occurred from the beginning, wet state to the post-sintering state. On the center layer containing 50% SiC, shrinkage was only about 2.5% (Reference specimens 2-1-2-6).

[0030] (Table I follows on next page.)

[0031] After considering the principles which were causing the anisotropic sintering, the amount of shrinkage in each successive layer was tailored by adding fully dense Al₂O₃grit (in increasing proportions and also increasing particle sizes) toward the high Al₂O₃ends. In this manner, as the Al₂O₃v. % increased, the grit size and the grit v. 1% was also increased, as shown in Table I. This reduced the shrinkage because the dense Al₂O₃grit served as refractory filler material. For example, on specimen No. 3-2 for the pure Al₂O₃layer, only about 6.25% shrinkage occurred (from beginning wet to post-sintering). On the layer containing 50% SiC, shrinkage was about 5.6%. Again the objective was to grade some of the specimens to have bilateral symmetry; (0%, 10%, 20%, 30%, 40%, 50%, 40%, 30%, 20%, 10%, 0% additions of SiC) thus opposite ends would be Al₂O₃only, graded symmetrically toward the center (increasing the amounts of SiC with each successive layer) to a center composition of 50/50 Al₂O₃/SiC (for eleven layers total). Thus, careful selection of material particle sizing as well as fractional percent has demonstrated a method of tailoring shrinkage for ceramic functionally graded material composites (Reference specimens 3-1-3-2).

[0032] The specimens fabricated in Example 2, designed to yield a stepwise grading of SiC content in Al₂O₃between 0% and 50%, are used to attenuate microwaves in various applications. As shown in FIG. 4, a sample prepared according to the method above had a gradient of its dielectric properties along its length corresponding to the compositional gradient established by the inventive technique. Skilled artisans will appreciate that an insulating rod with such tapering of its dielectric properties will be useful in applications such as insulator rods in a helix travelling wave tube.

[0033] An alternate method of inducing a particular amount of grading is to prepare a gelcasting slurry having at least two different dispersed phases having different settling characteristics. All materials are added to the same slurry, introduced into a mold, and allowed to stand for a sufficient period of time to permit differential settling, flotation, or flocculation to effect fractionation in order to achieve a vertical compositional gradient in the molded slurry. Once the desired gradient is attained, the structure is “locked-in” using thermally activated gelation. By carefully timed initiation and polymerization, the amount of settling and thus the amount of compositional grading can be controlled. Settling characteristics of the materials, for example, particle size, density, and viscosity of the surrounding medium all contribute to the distribution of settling fractions of the settling metal powder. Time becomes the controllable variable following selection of materials.

[0034] Graded specimens of Al₂O₃and Ni metal powders were fabricated by gelcasting methods. A 55 v. % slurry of Al₂O₃(RCLS DBM) was first prepared by an ordinary mixing procedure. Ni metal powder was then added at 10 v. % (of the solids) to the slurry and thoroughly mixed (Table 2). The initiator and catalyst were added, and the slurries were poured into molds to gel. Since the density of the Ni powder is 8.9 g/cc, and the density of the Al₂O₃slurry was only 2.63 g/cc, the Ni metal powder settled through the Al₂O₃slurry. In these experiments, prepared mixtures containing the metal powder were allowed to stand (vertically) in cylindrical molds for varying amounts of time, for example, 0 min., 5 min., 10 min., followed by thermal gelation. By increasing the initiator and catalyst, gelation was achieved in less than 5 minutes, thus locking in the graded structure. The specimens so produced exhibited distinctive grading in a continuous manner from top to bottom. As an example, FIG. 3 shows a specimen 60 having a lower concentration of Ni in a top region 62, graduating to an intermediate concentration of Ni in a middle region 62, graduating to a higher concentration of Ni in a bottom region 66.

[0035] Specimens which were allowed to settle only during the polymerization stage (0 min.) exhibited the most gradual grading over the distance (3″ depth). By placing a relatively strong ceramic magnet on the as-gelled samples, the degree of grading can easily be qualitatively measured. Another way to carry out this invention is to place magnets along some direction of the forming mold (prior to initiating polymerization), to induce a desired type of compositional grading via the applied magnetic field as opposed to simple gravimetric settling.

[0036] It will be appreciated that the term differential settling also encompasses differential buoyancy. For example, the second component in the slurry might be small particles (or droplets) of a polymer intended to act as fugitive pore formers during sintering. If these particles are less dense than the liquid slurry, they will preferentially float upward, creating a vertical distribution of pores in the final sintered product. This method could be used to replicate the variable density of bone, for example, for a bone repair or prosthesis material.

[0037] It will be further appreciated that the term “vertical” as used herein relates broadly to the direction of buoyant forces, which can be due to normal gravity or can be driven by centrifugation or other conventional means for causing fractionation based on density and/or settling velocity.

[0038] Variable properties of FGM include, but are not limited to, dielectric constant, magnetic permeability, hardness, strength, fracture toughness, electrical conductivity, thermal expansion, dielectric loss, color, and chemical composition.

[0039] A graded transition material has been developed that, among other uses, minimizes thermal fatigue in multi-chip power modules by having a low coefficient of thermal expansion on one surface (near that of silicon devices) and a high coefficient of expansion on the other (near that of an aluminum heat sink). The material also exhibits high thermal conductivity. An AlN preform that is graded in porosity levels through the thickness is fabricated and is subsequently infiltrated with molten aluminum.

[0040] Cylindrical AlN foam specimens were gel cast in three 20 wt % aqueous monomer systems: (1) N-(hydroxymethyl)acrylamide (HMAM) with 4% glycerine, (2) 3:1 methacrylamide (MAM) and polyethylene glycol 1000 dimethacrylate (PEG-DMA) with 4% glycerin, and (3) 3:3:1 MAM, 1-vinyl-2-pyrrolidone (NVP), and N,N′-methylenebisacrylamide (MBAM). Suspensions containing about 40 vol % AlN were prepared using 1 wt % Emphos CS-1361 as a dispersing agent. The effect of adding Y₂O₃as an sintering aid was studied by incorporating 1, 2.5 or 5 wt % (based on the weight of AlN) Y₂O₃powder to the suspension. The suspensions were foamed by rapid mechanical stirring with a paddle stirrer operating between 750 and 2000 rpm. After foaming, the suspensions were cast into glass molds coated with Polyester Parfilm mold release agent (Price Driscoll Corp., 17 Industrial Drive, Waterford Conn. 06385) and allowed to stand for 5 minutes to allow fractionation of the phases, i.e., a concentration of bubbles toward the top and a concentration of solid material toward the bottom of the molds. Gelation was initiated at ˜45° C. by ammonium persulfate and was accelerated by N,N,N′N′-Tetramethyl-ethylenediamine.

[0041] The green ceramic bodies were dried, and the polymer system was removed by heating in air at 600° C. The samples were then sintered in a BN crucible by heating for 2 hr in a graphite element furnace at 1800° C. or 1900° C. with a N₂overpressure of 0.6 MPa. The densities of the sintered foams were measured in absolute ethanol by the Archimedes technique. Table III shows the effect of processing conditions on the density and open porosity in foamed AlN.

[0042] In these samples, air bubbles comprised a fairly high fraction of the total volume of the slurry. Buoyancy forces created a porosity gradient visible to the naked eye in the samples sectioned after sintering. It will be readily appreciated that the degree of fractionation will be controlled by the size and volume fraction of bubbles. Larger bubbles will fractionate faster than small bubbles owing to the competing forces of buoyancy and viscous drag. Also, a sample with a low volume of bubbles can potentially have a greater density gradient than one with a very high volume fraction of bubbles, owing to the physical crowding of the bubbles and the resistance to (downward) fluid flow as the spacing between adjacent bubbles decreases. The skilled artisan can therefore manipulate the interplay between these well-understood effects in order to create virtually any desired density distribution, before initiating the gelation process to lock in the desired final microstructure.

[0043] (Table III follows on next page.)

[0044] The bulk densities ranged from 30 to 60 % of the theoretical density (TD) of AlN indicating that the samples were foamed by entrapped air. The closed density was close to TD except in the 44 Vol % loaded MAM/NVP/MBAM system sintered at 1800° C. This foam exhibited closed density that was less than TD. This suggests that in this system there may be some closed-cell foam that collapses when sintered at 1900° C.

[0045] The first three rows in the table show the effect on the sintered foam density of using different monomer systems when the ceramic solids loading in the slurry was kept constant. The HMAM gel resulted in the highest density foam with MAM/PEG being only slightly lower. The MAM/NVP/MBAM foam was significantly less dense. The lower density foam with the latter system should allow greater processing control; therefore, subsequent tests were conducted with this gel casting system.

[0046] Increasing the ceramic loading in the slurry increased the sintered bulk density as shown by comparing the results in row three with rows four and five. Furthermore, the stirring speeds of greater than 750 rpm were not necessary since the open porosity did not change much when the speed was increased.

[0047] Sintering at 1900° C. usually resulted in about a 5% increase in the bulk density. The addition of Y₂O₃increased the density of the sintered foams. There is a positive correlation between the foam density and the concentration of Y₂O₃. However, increasing the concentration of Y₂O₃from 1 wt % to 5 wt % did not increase the density as much as the first 1 wt % addition.

[0048] As described above, one useful application of the present invention is to make a preform having graded interconnected porosity, which is then infiltrated with another phase, typically a metal.

[0049] The graded porous material described above in Example V is pressure infiltrated with, for example, molten aluminum. The resulting composite has a microstructure grading from nearly solid AlN on one surface to nearly solid Al on the opposite surface. It will be appreciated that this structure is useful as a heat sink for a power semiconductor device, because the side comprising AlN will have a coefficient of thermal expansion similar to that of the Si component, whereas the CTE will grade toward that of Al at the opposite side. This arrangement minimizes thermal expansion mismatch (and thereby minimizes the resulting stresses imposed on the Si component) during service.

[0050] In the previous examples, fractionation of phases in order to obtain the desired gradient in the finished material was generally achieved by allowing the cast multiphase slurry to stand for a sufficient period of time to achieve a desired fractionation before gelling the mixture to lock in the desired gradient. This results in a fractionation of the phases to form a gradient in a substantially vertical direction. Because gelcasting slurries are thixotropic to some degree, the density fractionation process may be assisted by agitating or vibrating the slurry to decrease its apparent viscosity. It will be appreciated that in cases where the densities of the two phases are not very different, centrifugation may be used to impose greater effective gravity or buoyancy forces. In this case, the sample would be centrifuged in an orientation where the “vertical” axis of the sample (i.e., the direction in which the density gradient is desired) lies radially in the centrifuge.

[0051] It will also be appreciated that situations might arise in which a radially symmetrical density gradient is desired, for example, in cylindrical articles where it is desirable to have a gradient from the exterior toward the interior thereof FIG. 5 shows a cross section of a functionally graded material 70 that has been subjected to centrifugation about an axis 72 in order to achieve fractionation. The specimen 70 has a lower concentration of the heavier component in an inner region 74, graduating to an intermediate concentration of the heavier component in a middle region 76, graduating to a higher concentration of the heavier component in a bottom region 78. The region closest to the axis of rotation 72 may be void (hollow). It is well known that the optimal properties of living tissues frequently depend on gradations in density. For example, bone typically has a denser outer region and a more porous inner region.

[0052] Following the procedures described above, a slurry containing a desired volume fraction of air bubbles is placed in a cylindrical mold and rotated about the axis of the cylinder. The rotational speed is selected to provide the desired degree of radial buoyancy forces, thereby creating a dense outer region and a porous inner region. Furthermore, the size of the air bubbles, or fugitive pore formers if desired, is selected to be of the correct size range to encourage vascularization, tissue in-growth, and/or other desired physiological responses.

[0053] While there has been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications can be prepared therein without departing from the scope of the inventions defined by the appended claims.