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  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/01—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics
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  • C04B35/71—Ceramic products containing macroscopic reinforcing agents
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  • C04B2235/32—Metal oxides, mixed metal oxides, or oxide-forming salts thereof, e.g. carbonates, nitrates, (oxy)hydroxides, chlorides
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  • C04B2235/30—Constituents and secondary phases not being of a fibrous nature
  • C04B2235/32—Metal oxides, mixed metal oxides, or oxide-forming salts thereof, e.g. carbonates, nitrates, (oxy)hydroxides, chlorides
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  • C04B2235/32—Metal oxides, mixed metal oxides, or oxide-forming salts thereof, e.g. carbonates, nitrates, (oxy)hydroxides, chlorides
  • C04B2235/3224—Rare earth oxide or oxide forming salts thereof, e.g. scandium oxide
  • C04B2235/3225—Yttrium oxide or oxide-forming salts thereof
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  • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
  • C04B2235/50—Constituents or additives of the starting mixture chosen for their shape or used because of their shape or their physical appearance
  • C04B2235/52—Constituents or additives characterised by their shapes
  • C04B2235/5208—Fibers
  • C04B2235/5216—Inorganic
  • C04B2235/524—Non-oxidic, e.g. borides, carbides, silicides or nitrides
  • C04B2235/5244—Silicon carbide
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  • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
  • C04B2235/50—Constituents or additives of the starting mixture chosen for their shape or used because of their shape or their physical appearance
  • C04B2235/52—Constituents or additives characterised by their shapes
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  • C04B2235/526—Fibers characterised by the length of the fibers
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  • C04B2235/50—Constituents or additives of the starting mixture chosen for their shape or used because of their shape or their physical appearance
  • C04B2235/52—Constituents or additives characterised by their shapes
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  • C04B2235/5264—Fibers characterised by the diameter of the fibers
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  • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
  • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
  • C04B2235/50—Constituents or additives of the starting mixture chosen for their shape or used because of their shape or their physical appearance
  • C04B2235/52—Constituents or additives characterised by their shapes
  • C04B2235/5276—Whiskers, spindles, needles or pins
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  • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
  • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
  • C04B2235/50—Constituents or additives of the starting mixture chosen for their shape or used because of their shape or their physical appearance
  • C04B2235/54—Particle size related information
  • C04B2235/5418—Particle size related information expressed by the size of the particles or aggregates thereof
  • C04B2235/5445—Particle size related information expressed by the size of the particles or aggregates thereof submicron sized, i.e. from 0,1 to 1 micron
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  • C04B2235/608—Green bodies or pre-forms with well-defined density
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  • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
  • C04B2235/65—Aspects relating to heat treatments of ceramic bodies such as green ceramics or pre-sintered ceramics, e.g. burning, sintering or melting processes
  • C04B2235/66—Specific sintering techniques, e.g. centrifugal sintering
  • C04B2235/668—Pressureless sintering
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  • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
  • C04B2235/70—Aspects relating to sintered or melt-casted ceramic products
  • C04B2235/74—Physical characteristics
  • C04B2235/77—Density

Definitions

• the invention herein relates to ceramic bodies. More particularly it relates to ceramic bodies reinforced by single crystal or monocrystal- line silicon carbide whiskers, BACKGROUND ART
• the desirable properties of ceramic bodies including strength, low porosity and heat resistance, have been known for some time to make ceramics highly valuable in many industrial applications. These pro ⁇ perties should serve to make the ceramics directly competitive with metals in many such applications. It has been found, however, that the ceramics' lack of "toughness" often prevents them from successfully competing with metals.
• Toughness refers to the ability of a body to " resist crack propagation through the body; the tougher a materia*!, the more it is able to slow the rate of crack propagation through it. Materials which are brittle possess little toughness, and cracks which are started in such materials propagate rapidly so that such materials can fracture catastro- phically.
• the pres ⁇ sureless sintering operation is much more desirable since it eliminates the necessity of maintaining high temperatures and high pressures simultaneously.
• the operator can use common low temperature molding equipment and the heating can be conducted in open- ended continuous furnaces.
• this invention is a method for the formation of high density reinforced ceramic bodies which comprises (a) forming* a mixture containing 0.5-21.0 volume percent silicon carbide whiskers' and the balance ceramic matrix powder; (b) compacting the mixture to form a shaped body having a density of 55-70% of the theoretical maximum density of the body; and (c) heating the body at a sintering temperature of 70-90% of the melting temperature of the ceramic matrix for a period of time sufficient to sinter the powder and whisker mixture into a high-strength, high-toughness, monolithic, shaped ceramic composite having a density of at least 85% of the theoretical maximum density of the body.
• the invention also comprises the high den ⁇ sity, tough reinforced ceramic composite bodies formed by the method of this invention. MODES FOR CARRYING OUT THE INVENTION
• the ceramic matrix materials of the present invention can be any type of ceramic which is capable of being sintered to form coherent bodies. These may include oxides, carbides, borides and nitrides. Typi ⁇ cal examples include alumina, silica, aluminum sili ⁇ cates, silicon nitride, aluminum nitride, titanium diboride, zirconia and titanium carbide. Of these the ceramic of most interest because of its wide variety of applications is alumina. The ceramic mixture may also contain small amounts of other ceramic or doping materials to modify or enhance sin- terability or physical properties; typical of such additive materials are yttria and magnesia.
• the ceramic matrix material is used in finely divided powdered form.
• the particle sizes involved will be dependent on the specific ceramic being used and the desired density of the ultimate product, as well as on the degree of reinforcement to be obtained from the Whiskers. Normally, the higher-density finished products are obtained from ceramic powders having the smaller particles sizes. Particle sizes may range from as high as approxi- mately 3 mm (6 mesh) down to approximately 0.01 urn in size. Most frequently the particle sizes will be in the range of 0.1-100 um.
• the silicon carbide whiskers used as rein ⁇ forcement in the present invention are high strength materials which are usually formed by the high temperature reaction of silica and a carbonaceous material.
• the whiskers commonly have lengths in the range from about 10-100 um and average diameters on the order of 1 um or less.
• the crystalline structure is normally alpha or beta silicon carbide.
• the fibers are 98-99% silicon carbide whisker with the impurities being a variety of metals, primarily calcium, manganese and aluminum.
• Silicon carbide whiskers and methods for their formation are shown in Handbook of Fillers and Reinforcements for Plastics, Chapter 25, pages 446-464 ("Whiskers", by J. V. Milewski and H. S. Katz) (Von Nostrand Reinhold Co., N.Y. 1978). Particularly preferred are the whiskers manufactured by the Advanced
• silicon carbide whiskers are single crystal or monocrystalline materials manufactured from rice hulls.
• the silicon carbide whiskers typically have average diameters on the order 0.6 um and aspect ratios on the order of 15-150. Strengths are typically on the order of 1 million psi (70,000 ' kg/cn_2) and tensile moduli .on the order of 60-100. million psi (4-7 million kg/c ⁇ .2).
• the silicon carbide whiskers are thermally stable to 3200°F (1760°C).
• Short fiber materials of the polycrystal- line type are to be distinguished from the single crystal whiskers used in this invention.
• the poly- crystalline filaments or chopped fibers are much larger in diameter e.g., 10 microns or larger.
• the polycrystalline fibers are much larger in diameter e.g., 10 microns or larger.
• the ceramic powder and the silicon carbide whiskers are mechanically blended to produce a thorough mixture of the fibrous and particulate components.
• conventional organic forming binders may be added to the mixture.
• the whiskers component of the mixture will comprise 0.5-21%, preferably 2-18% of the blend.
• Composites containing less than 0.5% of fibers do not have a sufficient whisker content to provide significant reinforcement and improvement of proper ⁇ ties over the unreinforced ceramics. Composities containing more than 21% whiskers cannot be suffi- ciently densified upon pressureless sintering to provide high strength composites.
• the blended mass of particles and whiskers is densified (consolidated) in a mold or by isostatic pressing to form a shaped body having a density in the range of about 55-70%, preferably about 55-65%, and more preferably about 58-62%, of the theoretical density of the final sintered body.
• This densification is critical to the success of the subsequent sintering operation. If the components are not densified to this level prior to sintering the high density rein ⁇ forced product cannot be obtained from the subsequent sintering operation. It is preferred to densify to less than the 70% upper limit in order to avoid practical problems of binder outgasing which may be encountered at the higher levels.
• the densification is performed by conven ⁇ tional techniques such as extrusion, injection- molding, slip casting, cold pressing or cold isostatic techniques. Mold pressures are generally in the range of from about 10-50 tons/in 2 (1400-7000 kg/cm 2 ), although pressures may be greater or lesser depending on the particular molding technique used and the desired shape of the end product. Generally speaking, ambient temperatures are appropriate and preferred for carrying out this initial densification. In cases where procedures such as extrusion and injec ⁇ tion molding are used, mild heating sufficient to soften organic binder materials, e.g. to about 300°C, can be employed.
• the green body After consolidating the components to the 55-70% of theoretical density and obtaining the "green" shape, the green body is sintered at a temperature in the range of 70-90% of the melting temperature of the matrix materials. This will normally be in the range of 1500-3200°F (800-1750°C) . More refractory materials will require sintering at higher temperatures.
• the green bodies are usually maintained at sintering temperature for a period of from 15 minutes to 2 hours.
• the sintering is conducted in an atmosphere which will not adversely react with the component particles and whiskers. Because of the small particle and whisker size, the green bodies contain very large surface areas. Such large surface areas make oxidizable components highly reactive to oxidation in the presence of an oxidizing atmosphere at high temperatures. Consequently, the atmosphere in the sintering furnace must be of an inert or nonoxidizing - gas such as hydrogen, carbon monoxide, nitrogen or the noble gases such as argon and helium.
• the atmos ⁇ phere will normally be maintained at approximately ambient pressure. Alternatively, one can conduct the sintering under vacuum or moderate positive pressure. Vacuum sintering is normally performed at pressures on the order of 0.5 inch (12mm) of of mercury. Positive pressure sintering (distinguished from hot isostatic pressing) is normally performed at a very slight positive pressure. The pressureless sintering is carried out until the shaped body has a density which is at least about 85% of the theoretical density.
• the pressure-less sintering is carried out until the shaped body has reached closed porosity, a state usually corresponding to a density which is at least 94% of the theoretical density.
• Closed porosity bodies are characterized by the substantial absence of pores or void spaces communicating with the surface - i.e., such pores or void spaces which occur in the shaped bodies do not communicate with the surface of the body but rather are contained within the interior volume of the body. It is of considerable importance that the shaped bodies which have been brought to the condi ⁇ tion of closed porosity as above indicated, can by containerless hot isostatic pressing procedures be brought to nearly 100% of the theoretical density thus resulting in a product which has greatly enhanced properties of strength, hardness, etc.
• the closed porosity sintered articles are then subjected to containerless hot isostatic pressing to bring the density to at least about 98% of the theoretical maximum. In this way products having outstanding properties of toughness and strength are formed while avoiding costly procedures which were heretofore deemed necessary.
• the hot isostatic pressing to which the closed porosity sintered composite can be subjected is preferably carried out in a gas autoclave employ ⁇ ing nitrogen or argon atmosphere. Since the sintered composites are characterized by closed porosity, the hot isostatic pressing is containerless - i.e., the sintered composites need not be placed in a container to accomplish the isostatic pressing. In this pres ⁇ sing, pressures generally ranging from about 10,000 psi up to 30,000 psi or higher are appropriate. Temperatures generally ranging from about 1500°C to about 1800°C are suitable for pressing times of about 15 minutes to about 2 hours.
• Shape A had a density in the range 58-62% of maximum theoretical while shapes B and C had densities in the range 58-60% of maximum theoretical.
• the green shape composites were subjected to pressureless sintering at ambient pressure under a nitrogen atmosphere " . Sintering was accomplished by heating the green shapes to 1595°C, maintaining the temperature for 1 hour, cooling to 1250°C, maintain- ing this temperature for 1 hour and then cooling to room temperature. In each case the density of the pressureless sintered composite was in excess of 94% of the theoretical maximum and each composite was characterized by closed porosity.
• the closed porosity sintered composites were subjected to containerless hot isostatic pres ⁇ sing in a argon gas autoclave. Over a period of 4 hours the composites were heated to 1575°C and then maintained at that temperature for 1 hour, the pres- sure being 20,000 psi. Thereafter the pressed com ⁇ posites were cooled over a 4 hour period.
• Shape A 97.96 98. .99 Shape B 96.1 98. .9

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• 1986
  • 1986-03-13 WO PCT/US1986/000528 patent/WO1986005480A1/en unknown
  • 1986-03-13 EP EP86902963A patent/EP0217946A1/de not_active Withdrawn
  • 1986-03-13 AU AU58683/86A patent/AU5868386A/en not_active Abandoned

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