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Definitions

• the present invention relates to improved composite materials and the melt infiltration methods for producing the same. Specifically, the present invention relates to silicon carbide composites wherein preferably at least a portion of the silicon carbide is produced by reactive infiltration.
• Silicon carbide composites have been produced by reactive infiltration techniques for more than thirty- five years.
• a reactive infiltration process comprises contacting molten silicon with a porous mass containing silicon carbide plus carbon in a vacuum or an inert atmosphere environment.
• a wetting condition is created, with the result that the molten silicon is pulled by capillary action into the mass, where it reacts with the carbon to form additional silicon carbide.
• This in-situ silicon carbide typically is interconnected.
• a dense body usually is desired, so the process typically occurs in the presence of excess silicon.
• the resulting composite body thus comprises silicon carbide and unreacted silicon (which also is interconnected), and may be referred to in shorthand notation as Si/SiC.
• the process used to produce such composite bodies is interchangeably referred to as "reaction forming", “reaction bonding" or "reactive infiltration”.
• Popper U.S. Patent No. 3,275,722
• Popper produced a self-bonded silicon carbide body by infiltrating silicon into a porous mass of silicon carbide particulates and powdered graphite in vacuo at a temperature in the range of 1800 to 2300 C.
• Taylor also produced dense silicon carbide bodies by reactively infiltrating silicon into a porous body containing silicon carbide and free carbon.
• Taylor first made a preform consisting essentially of granular silicon carbide, and then he introduced a controlled amount of carbon into the shaped mass.
• Taylor added the carbon in the form of a carbonizable resin, and then heated the mass containing the silicon carbide and infiltrated resin to decompose (carbonize) the resin.
• the shaped mass was then heated to a temperature of at least 2000 C in the presence of silicon to cause the silicon to enter the pores of the shaped mass and react with the introduced carbon to form silicon carbide.
• Chiang et al. U.S. Patent No. 5, 509,555 discloses the production of silicon carbide composite bodies through the use of a silicon alloy infiltrant.
• the preform to be infiltrated by the alloy can consist of carbon or can consist essentially of carbon combined with at least one other material such as a metal like Mo, W, or Nb; a carbide like SiC, TiC, or ZrC; a nitride like Si 3 N 4 , TiN or AJN; an oxide like ZrO 2 or
• the liquid infiltrant includes sihcon and a metal such as aluminum, copper, zinc, nickel, cobalt, iron, manganese, chromium, titanium, silver, gold, platinum and mixtures thereof.
• the preform can be a porous carbon preform
• the liquid infiltrant alloy can be a silicon-aluminum alloy containing in the range of from about 90 at% to about 40 at% silicon and in the range of from about 10 at% to about 60 at% aluminum
• the carbon preform can be contacted with the silicon-aluminum alloy at a temperature in the range of from about 900 C to about 1800 C for a time sufficient so that at least some of the porous carbon is reacted to form silicon carbide.
• the dense composite formed thereby can be characterized by a phase assemblage comprising silicon carbide and at least one phase such as silicon-aluminum alloy, a mixture of silicon and aluminum, substantially pure aluminum or mixtures thereof.
• Pressure infiltrations require much more complex and expensive equipment than do pressureless infiltration techniques, and usually are more limited in the size and shape of the parts that can be produced thereby.
• the present invention is not limited to pressureless systems, unless otherwise noted, the infiltrations of the present invention refer to those not requiring the application of pressure.
• Chiang et al. state that their method allows production of composites very near net-shape without a need for additional machining steps. They describe a number of non- machining techniques for removing the residual, unreacted liquid infiltrant alloy remaining on the reacted preform surface. Specifically, Chiang et al. state that following infiltration, the composite body may be heated to a temperature sufficient to vaporize or volatilize the excess liquid alloy on the surface. Alternatively, the reacted preform may be immersed in an etchant in which the excess unreacted liquid infiltrant is dissolved while the reacted preform is left intact. Still further, the reacted preform may be contacted with a powder that is chemically reactive with the unreacted liquid infiltrant alloy such as carbon, or a metal like Ti, Zr, Mo or W.
• a powder that is chemically reactive with the unreacted liquid infiltrant alloy such as carbon, or a metal like Ti, Zr, Mo or W.
• Milivoj Brun et al. also are concerned with removing excess infiltrant following production of silicon carbide bodies by an infiltration process.
• Brun et al. contact the reaction formed body with an infiltrant "wicking means" such as carbon felt.
• the wicking means may comprise porous bodies of infiltrant wettable materials that are solid at the temperature at which the infiltrant is molten.
• the wicking means has capillaries that are at least as large or larger than the capillaries remaining in the reaction formed body.
• the infiltrant material comprises at least two constituents, and at least one of the constituents comprises silicon.
• silicon undergoes a net volume expansion of about 9 percent upon solidification.
• a sihcon carbide composite body having a residual infiltrant component that undergoes substantially no net volume change upon solidification.
• the amount of free carbon in the permeable mass is kept as low as necessary to accomplish complete infiltration in a reliable manner but without unduly compromising the binder qualities of the carbon when preforms (e.g., self-supporting permeable masses) are used.
• preforms e.g., self-supporting permeable masses
• large bodies can be infiltrated with minimal changes in the infiltrant alloy composition, thereby resulting in a silicon carbide composite body having a dispersed alloy phase of relatively uniform composition throughout the body.
• the alloying of sihcon infiltrant with one or more different elemental constituents can substantially depress the melting point of the infiltrant.
• Desirable alloying elements in this regard include alurninum, beryllium, copper, cobalt, iron, manganese, nickel, tin, zinc, silver and gold.
• the lowered melting or liquidus temperatures permit the infiltration to be conducted at lower temperatures.
• the infiltrant comprises a silicon- alumi um alloy, it is possible to infiltrate a porous mass comprising some elemental carbon at a temperature in the range of about 1100 to about 1300 C.
• the infiltrant when the infiltrant consists essentially of silicon, the temperature must be maintained at least above the silicon melting point of about 1412 C, and often substantially above the melting point so that the melt is sufficiently fluid.
• One ofthe most important consequences of being able to operate at lower temperatures is the discovery that at the lower temperatures, the infiltration is more reliably terminated at the boundaries ofthe permeable mass.
• cheaper materials such as a loose mass of ceramic particulate may be used.
• the ability to conduct infiltrations at lower temperatures gives operators more control over the process, not to mention saving time and energy.
• Figures 1A and IB are side and top schematic views, respectively, of an arrangement of materials used to produce a silicon carbide composite "U-channel" in accordance with Example 4.
• Figure 2 is a photograph of a sihcon carbide composite air bearing support frame produced in accordance with Example 6.
• a permeable mass containing at least some carbon is infiltrated with a molten, multi-constituent alloy comprising silicon.
• the silicon component ofthe infiltrant alloy chemically reacts with at least a portion ofthe carbon in the permeable mass to form silicon carbide.
• typically some alloy material remains in the infiltrated body, and distributed throughout. The body thus formed containing in-situ silicon carbide and residual alloy is therefore a composite body.
• the starting amount of silicon alloy is insufficient to fill the interstices ofthe permeable mass, at least some ofthe residual, unreacted alloy in the body may be distributed as discrete, isolated pockets. Usually an excess of infiltrant material is supplied to the permeable mass, and the residual alloy in the composite body is interconnected.
• the present invention encompasses placing one, several or all ofthe constituents ofthe multi-component infiltrant within the permeable mass to be infiltrated, or at an interface between the mass and an adjacent body ofthe infiltrant material.
• the constituents ofthe infiltrant material are provided as an alloy, possibly in ingot or other bulk form, that is then brought into contact with the permeable mass to be infiltrated.
• the infiltrant material may be placed into direct contact with the permeable mass to be infiltrated, or the infiltrant material may remain substantially isolated from the permeable mass, with a wicking means interposed between the two to create a pathway or conduit for the molten infiltrant material to migrate toward and into the permeable mass.
• the wicking means could be most any material that is wet by molten infiltrant material, with silicon carbide being preferred.
• the present invention contemplates producing in-situ silicon carbide. Accordingly, at least one constituent ofthe multi-constituent infiltrant material comprises silicon.
• the other constituent(s) may be any that are capable of producing some desirable effect during processing or on the final character or properties ofthe resulting composite body.
• the non-silicon constituent(s) may give rise to an alloy having a lower liquidus temperature than the melting point of pure silicon. A reduced liquidus temperature might then permit the infiltration to be conducted at a lower temperature, thereby saving energy and time, as well as reducing the tendency for the infiltrant to over-infiltrate the boundaries ofthe preform or permeable mass into the supporting materials.
• a non-silicon constituent infiltrated into the permeable mass along with the reactive sihcon constituent may produce superior properties ofthe resulting composite body— enhanced strength or toughness, for instance.
• a non-silicon constituent so infiltrated may also counterbalance the expansion ofthe silicon phase upon solidification, a desirable result from a number of standpoints, as will be discussed in more detail later.
• Elemental non-silicon constituents that fulfill one or more ofthe advantageous attributes include aluminum, beryllium, copper, cobalt, iron, manganese, nickel, tin, zinc, silver, gold, boron, magnesium, calcium, barium, strontium, germanium, lead, titanium, vanadium, molybdenum, chromium, yttrium and zirconium.
• Preferred constituents include aluminum, copper, iron, nickel, cobalt and titanium. Particularly preferred are aluminum and copper.
• One such alloying element that has been identified as fulfilling all three desirable attributes is aluminum.
• the present inventors have observed that a silicon carbide composite body that also contains some aluminum phase is substantially tougher than a silicon carbide composite containing residual, unreacted silicon. Still further, the present inventors have discovered that when the residual infiltrant component ofthe composite body comprises about 40 to 60 percent by weight silicon and 60 to 40 percent aluminum, the volume change ofthe residual infiltrant phase is practically zero.
• a preform comprising silicon carbide particulate and about one to several percent by weight of carbon may be readily infiltrated in a rough vacuum at about 1100 C with an infiltrant alloy comprising roughly equal weight fractions of sihcon and alurriinum to produce a composite body comprising silicon carbide plus residual alloy having a composition of about 40 to 45 percent by weight silicon, balance aluminum.
• the present inventors have discovered that at this lower infiltration temperature of about 1100C, a loose mass of silicon carbide particulate can be used to support the permeable mass or preform to be infiltrated without itself being infiltrated by the molten infiltrant. This discovery greatly simplifies the furnacing operation and obviates the need for expensive graphite fixturing and tooling.
• the temperature at which the infiltration is conducted is the lowest at which infiltration occurs quickly and reliably. Also, in general, the higher the temperature, the more robust is the infiltration. Unnecessarily high infiltration temperatures are not only wasteful in terms of energy costs and the extra heating and cooling time required, but the more likely it is that undesired reactions can occur.
• a number of ceramic materials that are usually thought of as being inert and uninfiltratable at moderate temperatures e.g., aluminum oxide, boron nitride, silicon nitride
• the atmosphere in which the infiltration of a silicon-containing alloy is conducted is usually one that is inert or mildly reducing. Accordingly, argon, helium, forming gas and carbon monoxide may be used.
• a vacuum environment is preferred, however, at least from the standpoint of facilitating the reliability or robustness of infiltration.
• the mass or preform to be infiltrated by the silicon-containing infiltrant must be one that is permeable to the infiltrant under the local processing conditions. Given sufficient temperature, e.g., about 2150C, pure silicon carbide can be infiltrated by silicon in a pressureless manner (see for example, U.S. Patent No.
• the permeable mass contains some elemental or free carbon to facilitate the process.
• the more carbon that is present the more silicon carbide that is produced in-situ. While it is possible to reactively infiltrate a permeable mass containing large amounts of carbon, such is generally undesirable in the context ofthe present invention because the infiltrant alloy will change too much from one zone in the preform to the next. Large compositional changes are usually undesirable for at least two reasons: First, the altered alloy composition may be such that it no longer wets the permeable mass to be infiltrated.
• the balance ofthe permeable mass may comprise one or more materials that are substantially inert under the process conditions, e.g., "filler materials".
• suitable materials for use in the present invention would include the carbides such as SiC, B 4 C, TiC and WC; the nitrides such as Si 3 N , TiN and A1N; the borides such as SiB 4 , TiB 2 , and A1B 2 ; and oxides such as Al 2 O 3 and MgO.
• the form ofthe filler material may be any that can be produced, for example, particulate, fiber, platelet, flake, hollow spheres, etc.
• the filler material bodies may range in size from submicron to several millimeters, with sizes ranging from several microns to tens of microns being common. Filler material bodies having different sizes may be blended together, for example, to increase particle packing.
• Permeable masses comprising one or more filler materials may range appreciably in terms of their packing or theoretical density.
• a permeable mass comprising flakes or a reticulated structure may be only 5 to 10 percent dense.
• a sintered preform may be 90 to 95 percent dense. As long as the preform is capable of being wetted by the infiltrant material and contains interconnected porosity, it should be capable of being infiltrated to form the composite bodies ofthe present invention.
• the form ofthe carbon component is significant, especially when attempting to infiltrate filler materials that are normally difficult to infiltrate, e.g., the oxides. While carbon in particulate form may be satisfactory for infiltrating a mass of silicon carbide, other fillers may necessitate that the carbon be reticulated or forming a network or skeletal structure. Especially preferred is carbon in the form of a coating on the filler material bodies. Such a form of carbon can be achieved by introducing the carbon into the permeable mass in liquid form, as for example, a resin.
• the permeable mass containing such a carbonaceous resin is then thermally processed to decompose or pyrolyze the resin to solid carbon, which may be graphite, amorphous carbon or some combination thereof.
• solid carbon which may be graphite, amorphous carbon or some combination thereof.
• carbonaceous resins are available including epoxy resins, phenolic resins and furfuryl alcohol. Preferred however, are sugar-based resins. These resins can be water-based and as such, are much more "friendly" in terms of environmental and human health. Particularly preferred are aqueous resins made from fructose.
• a binder In addition to assisting in the infiltration process, another important role played by the carbonaceous resin is that of a binder.
• a binder preferably here a carbonaceous binder, and then pressed or cast or molded to a desired shape using techniques known in the art. Curing the binder then renders the formed body self-supporting.
• the present inventors have noted the general conditions (or trends in changing conditions) whereby infiltration tends to occur or is enhanced, and those conditions under which infiltration tends not to occur, or tends to be inhibited.
• the inventors have observed that reactive infiltration of an infiltrant comprising sihcon into a permeable mass comprising carbon occurs more robustly when the carbon is present in elemental form rather than chemically combined with other elements.
• the infiltration is more robust when the elemental carbon is present in three-dimensionally interconnected form, as opposed to discrete particle form.
• the permeable mass comprises a component other than elemental carbon, for example, aluminum nitride
• the three-dimensionally interconnected elemental carbon phase could be present as, for example, a coating on at least some ofthe aluminum nitride bodies.
• the infiltration is more robust when the temperature of infiltration is increased, both in terms of absolute temperature as well as in terms ofthe homologous temperature (e.g., percentage or fraction ofthe melting temperature). Still further, infiltration is more robust when conducted under vacuum as opposed to inert gas atmosphere such as argon.
• silicon carbide for example, is infiltratable by silicon melts to produce a composite body.
• silicon carbide is reliably infiltrated by silicon (e.g., "siliconizing") only at temperatures well above the melting point of silicon. At temperatures just slightly above the sihcon melting point, infiltration becomes rather difficult. If a metal like aluminum is alloyed with the silicon, the melting point or liquidus temperature is depressed, and the processing temperature similarly can be decreased, which further reduces the propensity for infiltration. Under these conditions, such silicon carbide material can be used as a bedding or barrier material.
• silicon carbide as a bedding material
• the same source of silicon carbide can be used as a bedding material as is used as a permeable mass to be infiltrated without exposing the resulting silicon carbide composite body to alien or additional contaminants.
• a silicon-containing alloy may infiltrate a silicon carbide mass containing free carbon at the relatively low processing temperatures, particularly if the elemental carbon is three-dimensionally interconnected in a reticulated structure. Such a structure may result when carbon is added to a permeable mass as a resin and the resin is subsequently pyrolyzed.
• the present invention permits the graphite trays or boats to be used to support the bedding material, which in turn supports the permeable mass to be infiltrated and/or the infiltrant material. This advance in the art permits these graphite structures to be reused in subsequent infiltration runs, rather than having to be discarded as scrap.
• silicon undergoes a net volume expansion of about 9 percent upon solidification.
• a constituent such as a metal that undergoes a net volume shrinkage upon solidification
• the particularly preferred alloying element of aluminum by itself exhibits a solidification shrinkage of some 6.6 percent by volume.
• infiltration can be achieved using infiltrants ranging from about 10 percent by weight silicon up to substantially 100 percent sihcon.
• the residual infiltrant component ofthe formed sihcon carbide body may range from nearly 100 percent aluminum to substantially 100 percent silicon.
• the volumetric change ofthe residual infiltrant material upon solidification can be tailored with infinite variability between negative 6.6 percent (for pure aluminum) and positive 9 percent.
• solidification shrinkage say for example to negative 2 or negative 1 percent
• sohdification swelling from positive 9 percent to perhaps positive 7, positive 5 or positive 3 percent, or less.
• solidification porosity may be the lesser concern.
• solidification porosity With thoughtful lay-up design and excess infiltrant material or a reservoir of infiltrant supplying the mass to be infiltrated, solidification porosity largely can be avoided if the last region to freeze in the composite body can be supplied with molten infiltrant material from outside the body. Sometimes directional solidification ofthe composite body is employed to accomplish this desired result.
• the ability to reduce or even eliminate this solidification expansion ofthe silicon constituent ofthe infiltrant material by alloying the silicon with a material that shrinks upon solidification represents an important advance in the field of silicon- containing composite materials. Not only may such composite bodies be made more dimensionally accurate in the as-infiltrated condition, but may be produced without requiring an extra process step to remove the exuded silicon. Additionally, larger bodies may now be produced with less risk of cracking due to expansion ofthe silicon phase within the composite during cooling through its sohdification temperature.
• Example 1 This example demonstrates the production of a reaction bonded Si/SiC composite body. More specifically, this Example demonstrates the infiltration of substantially pure silicon into a silicon carbide preform containing an interconnected carbon phase derived from a resinous precursor.
• a preform was prepared as follows. One hundred parts by weight of
• CRYSTOLON blocky (regular), green sihcon carbide particulate (St. Gobain/Norton Industrial Ceramics, Worchester, MA) was combined with fifteen parts of Karo corn syrup (CPC International Inc., Englewood Cliffs, NJ) by mixing.
• the silicon carbide particulate content consisted of about 70 percent having a median particle size of about 44 microns (Grade F 240), and the balance having a median particle size of about 13 microns (Grade F 500).
• the mixing was conducted in a Model RV02 Eirich high shear mixer as follows: First the SiC particulates were mixed for 2 minutes on the "low” speed. Then half the corn syrup was added and mixing continued on "low” for an additional 1 minute.
• the admixture was pushed through a 16 mesh screen (average opening size of about 1180 microns) to break up agglomerates.
• coupons measuring about 51 mm square by about 10 mm thick were uniaxially pressed in a steel die under an applied pressure of about 28 MPa.
• the preform comprising the SiC particulate and the corn syrup was ejected from the die and placed into a controlled atmosphere furnace.
• the preform was heated to a temperature of about 800 C at a rate of about 100 C per hour. After mamtaining this temperature for about 2 hours, the com syrup had been substantially completely pyrolyzed to carbon.
• the furnace and its contents were cooled at a rate of about 200 C per hour. After cooling back substantially to ambient temperature, the preform was recovered from the furnace and calculated (based upon bulk density measurements) as having a volumetric loading of about 61 percent of theoretical. Carbon analysis via oxidation, performed on a similar sample, showed the presence of about 2.5 percent by weight of free carbon in the preform.
• a lay-up to confine the infiltration process was prepared. Specifically, the interior surfaces of a Grade ATJ graphite boat (Union Carbide Corp., Carbon Products Div., Cleveland, OH) measuring about 375 mm by about 298 mm by about 51 mm deep was coated with a boron nitride slurry or paint at a rate or thickness of about 3.1 mg per square centimeter.
• the boron nitride paint was prepared by mixing four parts by weight of LUBRICOAT boron nitride paste (ZYP Coatings, Oak Ridge, TN) with three parts water and spray coating using a Model 95 Binks spray gun.
• the 46 gram preform was placed into the coated graphite boat. About 23 grams of silicon in lump form (Elkem Metals Co., Pittsburgh, PA) and comprising by weight about 0.5 percent Fe (max) and the balance Si, was placed on top ofthe preform. The top ofthe boat was covered with a loose-fitting (non-hermetic) BN coated graphite hd. The completed lay-up was then placed into a vacuum furnace at about ambient temperature (e.g., about 20 C). The air was evacuated using a mechanical roughing pump, and a rough vacuum of about 25 millitorr residual pressure was thereafter maintained. The lay-up was then heated from ambient temperature to a temperature of about 1350 C at a rate of about 200 C per hour.
• ambient temperature e.g., about 20 C
• the air was evacuated using a mechanical roughing pump, and a rough vacuum of about 25 millitorr residual pressure was thereafter maintained.
• the lay-up was then heated from ambient temperature to a temperature of about 1350 C at a
• the temperature was further increased to a temperature of about 1550 C at a rate of about 200 C per hour.
• the temperature was decreased to a temperature of about 1450 C at a rate of about 100 C per hour. Without holding at this temperature, the lay-up temperature was further decreased to a temperature of about 1300 C at a rate of about 25 C per hour, which was immediately followed by a cooling at a rate of about 200 C per hour to approximately ambient temperature.
• the furnace atmosphere was brought back to ambient pressure and the lay-up was removed from the furnace. Disassembly ofthe lay-up revealed that sihcon had fully infiltrated the preform to form a composite body comprising silicon carbide and silicon. After sandblasting off the residual silicon at the surface where infiltration commenced, a density ofthe composite body of about 2.89 g/cc was measured by the water immersion technique. Using the theoretical densities of SiC and Si, the body was calculated as being about 64 percent by volume of SiC and 36 percent. Si.
• Example 1 The technique of Example 1 was repeated, with a major change being that the infiltrant featured aluminum substituted for about half of the mass of silicon. Thus, excluding impurities, the infiltrant was about 50 percent by weight silicon and about 50 percent aluminum. Also, in this Example the infiltration was conducted at a lower temperature than in Example 1. The masses ofthe preform and infiltrant were slightly different from those of Example 1, at 44.8 grams and 20 grams, respectively. As for the differences in the heating schedule, the lay-up ofthe present Example was heated in vacuo from about ambient temperature to a temperature of about 1000 C at a rate of about 200 C per hour.
• the temperature was further increased to a temperature of about 1150 C at a rate of about 150 C per hour.
• the lay-up was cooled to near-ambient temperature at a rate of about 200 C per hour.
• Disassembly ofthe lay-up revealed that the infiltrant had again fully infiltrated the preform to form a composite body. Significantly, there was no excess infiltrant oozing out ofthe surface ofthe formed body. The density ofthe composite body was again about 2.89 g/cc.
• This example demonstrates the effect of infiltrant chemistry on some selected physical properties of a silicon carbide composite body.
• each preform comprised by volume about 70 percent SiC and by weight about 3 percent elemental carbon.
• Sample A One ofthe preforms, Sample A, was infiltrated according to the infiltrant composition and thermal processing schedule detailed in Example 1.
• Sample B was infiltrated according to the infiltrant composition and thermal processing schedule described in Example 2.
• Sample A was infiltrated with nominally pure silicon
• Sample B was infiltrated with nominally Si-50A1 alloy.
• the resulting SiC composite bodies were characterized. Selected properties are provided in Table I.
• This example demonstrates the fabrication of a silicon carbide composite "U channel" featuring a multi-constituent infiltrant phase.
• Preforms were prepared by a sedimentation casting process. Specifically, about 25 parts of liquid were added to 100 parts of CRYSTOLON blocky (regular), green silicon carbide (St. Gobain/Norton Industrial Ceramics, Worchester, MA) and 8 to 12 parts of KRYSTAR 300 crystalline fructose (A.E. Staley Manufacturing Co., Decatur, IL) to make a slurry.
• the silicon carbide particulate consisted of about 70 parts by weight of Grade F 240 (median particle size of about 44 microns) and the balance Grade F 500 (median particle size of about 13 microns).
• the solids and liquids were added to a plastic jar and roll mixed for about 40 hours. The slurry was de-aired in about 760 mm of vacuum for about 5 minutes. About 15 minutes prior to casting, the slurry was re-roll mixed to suspend any settled particulates.
• a graphite support plate was placed onto a vibration table.
• a rubber mold having a cavity ofthe desired shape to be cast was wetted with a surfactant consisting of a 10 weight percent aqueous solution of JOY dishwashing detergent (Proctor and Gamble, Cincinnati, OH).
• the wetted rubber mold was then placed onto the graphite plate and allowed to dry. The slurry was poured into the cavity. Vibration was commenced.
• the frozen casting was demolded and placed onto a graphite setter tray for drying and bisque firing.
• the graphite tray and preform were then placed into a nitrogen atmosphere furnace at ambient temperature.
• the fumace was energized and programmed to heat to a temperature of about 40 C over a period of about one-half hour, to hold at about 40 C for about 2 hours, then to heat to a temperature of about 650 C over a period of about 5 hours, to hold at about 650 C for about 2 hours, then to cool down to about ambient temperature over a period of about 5 hours.
• the bisque fired preform was removed from the fumace and stored until the infiltration step. This firing operation pyrolyzes the fructose, yielding a well bonded preform containing about 2 to 3 percent by weight carbon.
• the above-mentioned steps were employed to produce a "beam” preform and a "U-channel” preform.
• the U-channel preform had a mass of about 182 g and had overall dimensions of about 76 mm long by about 64 mm wide by about 38 mm high.
• This preform consisted of a flat base and two flat walls parallel to one another and at right angles with respect to the base. The base and walls were each about 10 mm thick.
• the beam preform was in the shape of a rectangular prism and measured about 89 mm long by about 11 mm wide by about 3 mm thick. During infiltration, this beam preform would serve as a conduit for conducting molten infiltrant toward and into the U-channel preform.
• a lay-up for infiltration was then prepared.
• CRYSTOLON blocky, green silicon carbide particulate 11 having a median particle size of about 216 microns was poured into a graphite tray 13 measuring about 400 mm square by about 50 mm in height.
• This silicon carbide particulate bedding material was arranged within the graphite tray so as to be slightly higher in elevation out towards the wall ofthe tray than towards the center ofthe tray.
• the U-channel preform (and specifically the base portion thereof) was placed into contact with the beam. More specifically, the U-channel preform was cemented to one end ofthe beam preform with a slurry comprising by weight about 67 percent CRYSTOLON regular green silicon carbide particulate (Grade F 500, St. Gobain/Norton Industrial Ceramics) having a median particle size of about 13 microns, and the balance being about equal weight fractions of water and KRYSTAR 300 fructose (A.E. Staley Manufacturing Co.).
• the bonded preforms were placed onto the SiC particulate bedding material with the U-channel preform 12 at the higher elevation, and the opposite end ofthe beam preform 15 extending down towards the lower elevations.
• a number of fragments 17 of an infiltrant material comprising by weight about 68 percent silicon, balance substantially pure aluminum and having a total mass of about 62 g were arranged at the foot ofthe beam preform, at the lower elevation. Additional Grade F 90 SiC particulate was arranged in a ring 19 around the pile of infiltrant material 17 to help confine the latter once it was made molten.
• the graphite tray and its contents were then placed into a larger graphite container (e.g., a "boat") having a non- hermetically sealing graphite lid, thereby completing the lay-up.
• the lay-up was placed into a vacuum fumace.
• the heating chamber was evacuated to a pressure below 100 millitorr with a mechanical roughing pump.
• the chamber and its contents were then heated from a temperature of about 40°C to about 1100°C over a period of about 5 hours, then held at about 1100°C for about 1 hour, then heated to about 1270°C in about 1 hour, then held at about 1270°C for about 4 hours, then cooled to about 40°C in about 6 hours.
• the boat and its contents was recovered from the vacuum furnace.
• the silicon-aluminum alloy had melted, infiltrated through the beam preform and into the U-channel preform to form a dense, silicon carbide composite body.
• the beam was bonded to the U-channel and had to be removed by cutting with a diamond saw, the infiltration of alloy into the preforms was well controlled. Specifically, there was no infiltration into the SiC particulate bedding material, nor was there exuding of excess alloy (as droplets or otherwise) from the surfaces ofthe infiltrated preforms.
• This example demonstrates the fabrication of a silicon carbide composite air bearing support frame featuring a multi-constituent infiltrant phase. This example also demonstrates the fabrication of a relatively large composite body.
• An air bearing support frame preform was fabricated in two longitudinal sections using substantially the same sediment casting slurry as was described in Example 4. Following sedimentation casting and freezing, the preform halves were dried to a temperature of about 150C, with a carefully controlled heating up to this temperature to avoid cracking the parts due to the potential for excessive water vapor generation. The preform halves were then additionally thermally processed in a nitrogen atmosphere substantially in accordance with the heating described in Example 4 to pyrolyze the fructose binder to carbon. The preforms could then be green machined.
• the sections were cemented together with a slurry comprising by weight about 67 percent CRYSTOLON regular green silicon carbide particulate (Grade F 500, St. Gobain/Norton Industrial Ceramics) having a median particle size of about 13 microns, and the balance being about equal weight fractions of water and KRYSTAR 300 crystalline fructose (A.E. Staley Manufacturing Co.).
• This slurry was roll mixed for about 4 hours, then de-aired.
• the mating surfaces of the preform were spray coated with KRYLON lacquer (Borden, Inc., Columbus, OH) to retard the water absorption somewhat during the gluing operation.
• the air bearing preform had approximate dimensions of about 511 mm long by about 35 mm wide by about 70 mm in height, and had a mass of about 2145 g.
• a lay-up was next prepared. Specifically, CRYSTOLON regular, green silicon carbide particulate (Grade F 90, St. Gobain/Norton Industrial Ceramics) having a median particle size of about 216 microns was poured into a graphite tray measuring about 790 mm long by about 230 mm wide by about 50 mm deep and leveled to form a bedding material. The air bearing preform was placed on the bedding material. About 836 g of an infiltrant alloy comprising by weight about 68 percent silicon, balance substantially pure aluminum was placed nearby. The graphite tray was then placed into a larger graphite vessel having a non-hermetically sealing graphite hd to complete the lay-up.
• CRYSTOLON regular, green silicon carbide particulate having a median particle size of about 216 microns was poured into a graphite tray measuring about 790 mm long by about 230 mm wide by about 50 mm deep and leveled to form a bedding material.
• the air bearing preform
• This lay-up which measured about 850 mm long by about 290 mm wide by about 240 mm high, was then placed into a vacuum furnace and thermally processed in substantially the same manner as that for Example 4, except that the temperature was maintained at about 1270C for about 6 hours instead of about 4 hours.
• An air bearing support frame measuring about 120 mm square and about 19 mm in height was fabricated substantially along the lines described in Example 5 except that bonding ofthe two pieces was not carried out until after each piece had been infiltrated with sihcon-aluminum alloy to form a reaction bonded silicon carbide composite part.
• Figure 2 shows the more complex-shaped piece ofthe two in the as-infiltrated condition.
• Examples 4 through 6 thus demonstrate that a shaped silicon carbide composite part, even one having a complex geometry, can be produced by the present reactive infiltration technique with the final composite body accurately replicating the shape and surfaces ofthe starting preform. Comparative Example 1
• Example 2 The materials and techniques of Example 2 were substantially repeated, except that instead of adding com syrup and pressing a preform, the permeable mass consisted only of a loose mass ofthe silicon carbide particulates. While the Si-50A1 alloy melted and covered the surface ofthe silicon carbide particulate, no infiltration occurred.
• Example 2 was substantially repeated, except that instead of adding about 15 percent by weight of com syrup to the silicon carbide particulate filler material, about 1 percent by weight of ELVACITE acrylic resin (The DuPont Co., Wilmington, DE) was added. About 99.1 percent ofthe resin was removed during the preform heating step prior to the infiltration step. Again, no infiltration occurred.
• ELVACITE acrylic resin The DuPont Co., Wilmington, DE
• Example 2 was substantially repeated, except that instead of adding 15 percent by weight of com syrup to the silicon carbide particulate and pressing a preform, about 3 percent by weight of Grade KS-6 graphite powder (Lonza, Inc., Fairlawn, NJ) was mixed into the particulates. While infiltration might have resulted at higher temperatures, no infiltration of Si-50A1 alloy into a loose mass of this admixture occurred at the present operating temperature of about 1150C. Comparative Example 4
• Example 1 was substantially repeated, except that instead of a vacuum environment, an atmosphere of commercially pure, flowing argon gas was used. No infiltration occurred.
• Example 2 was substantially repeated, except that instead of a vacuum environment, an atmosphere of commercially pure, flowing argon gas was used. Some regions ofthe preform were not infiltrated with alloy material. Moreover, those regions that were infiltrated were porous and non-uniform.
• the methods and compositions ofthe present invention find utility in applications requiring high specific stiffness, low thermal expansion coefficient, high hardness, high thermal conductivity and/or high wear resistance. Accordingly, the silicon carbide composite materials ofthe present invention are of interest in the precision equipment, robotics, tooling, armor, electromc packaging and thermal management, and semiconductor fabrication industries, among others.
• the present silicon carbide composite materials are candidate materials for wear critical components. Specific articles of manufacture contemplated by the present invention include semiconductor wafer handling devices, vacuum chucks, electrostatic chucks, air bearing housings or support frames, electronic packages and substrates, machine tool bridges and bases, mirror substrates, mirror stages and flat panel display setters.

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