Classifications

• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
  • C04B35/58—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides
  • C04B35/581—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides based on aluminium nitride
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
  • C04B35/56—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides
  • C04B35/565—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides based on silicon carbide
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/622—Forming processes; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/64—Burning or sintering processes
  • C04B35/65—Reaction sintering of free metal- or free silicon-containing compositions

Definitions

• This invention relates to the field of ceramics and particularly to ceramics containing solid solutions containing the elements Si, C, Al, 0, and N (referred to by the acronym SiCAlON) , and a method for densif ing such ceramics.
• SiC and Si3N4 ceramics are leading candidates for applications in high temperature environments including energy conversion devices due to their high strength to temperatures on the order of 1500°C. SiC and Si3N4 ceramics also find use. in applications, over a wide temperature range, where wear and/or chemical resistance is required.
• Pressureless sintering of SiC has been accom ⁇ plished using either B and C (or B4C) or Al (or I2O3) as sintering aids to obtain nearly single phase SiC with densities greater than 97% of theoretical.
• Very active powders having high surface area are required to provide the driving force for sintering.
• Very little densification occurs by pressureless sintering with SiC particle sizes greater than 1 micrometer. Since pressureless sintering allows fabrication of complex shapes economically, it would therefore be an improvement in the art if SiC particles in the 1-10 micrometer range could be densified without pressure.
• a process for pressureless sintering i.e., sintering in the absence of applied pressure
• SiCAlON ceramics has been invented.
• this process allows mixtures of the solid solution with other materials to be formed.
• mixtures of the solid solution with SiC and AlN allow improved thermal conductivity as compared to the complete solid solutions, while retaining the desirable aspects of pressureless sintering and engineering of properties.
• a method for densifying solid solutions of at least aluminum oxycarbide and silicon carbide and/or aluminum nitride i.e., SiCAlON ceramics.
• Materials can be made which consist of an intimate mixture of SiCAlON solid solution with a distinct second phase of either SiC or AlN.
• a technique for pressureless sintering of oxides, carbides and nitrides of silicon and aluminum in the presence of aluminum and carbon or aluminum carbide or aluminum oxycarbide to form a substantially dense polycrystalline body of virtually any shape has been discovered.
• These ceramic bodies can comprise about 1 to 99% by volume of a solid solution consisting of aluminum oxycarbide and silicon carbide and/or aluminum nitride and at least one refractory phase of SiC and/or AlN.
• FIG. 1 is an x-ray diffraction pattern of a pressureless sintered SiCAlON ceramic described in Example 2.
• FIG. 2 is an optical micrograph of the SiCAlON ceramic in FIG. 1, taken at 1500 magnifications, showing the existence of two distinct phases. The solid solution is therefore inho ogeneous.
• FIG. 3 is an x-ray diffraction pattern illus- trating peaks due to the solid solution and SiC.
• FIG. 4 is a graphical representation of the strength data from pressureless sintered bars of SiCAlON (containing a refractory phase of SiC) in comparison to pressureless sintered SiC (without the solid solution).
• FIG. 5 is a graphical representation of the fracture toughness data from SiCAlON (containing a refractory phase of SiC) in comparison to SiC without the solid solution.
• FIG. 6 is a graphical representation of the sintering behavior of SiC*Al2 ⁇ C without containment when sintered at 2000°C in 2 for 5 minutes.
• FIG. 7 is a graphical representation of the sintering behavior of SiC and SiCAlON (containing a refractory phase of SiC) as a function of the starting SiC particle size.
• Dense polycrystalline SiCAlON ceramic bodies can be made according to the instant invention by mixing certain compounds containing the elements Si, C, Al, 0, and N in the proper proportions and manner, forming shaped bodies (including complex shapes) by conventional pressing techniques, slip casting, injection molding, and the like, and sintering without applied pressure in a furnace.
• shaped bodies including complex shapes
• the bracket [ ] is meant to imply that a solid solution of the indicated chemical composition can be formed. However, a complete solid solution is not necessarily formed in the densified ceramic. Several types of solid solutions may exist as discrete particles within the ceramic body that might be rich or deficient in certain elements. However, the overall or averaged composition of the resulting body would be essentially that of the bracketed compound.
• Cutler and Miller U.S. Patent 4,141,740 claimed that a complete solid solution exists between AI2OC, SiC and AIN indicating that the variables a, b, n, x, and z vary over the entire stoichiometry range.
• Rafaniello and Virkar found that the solid solution between SiC and AlN varied between 5 and 100 weight percent aluminum nitride at 2100°C. Since the formation of the solid solution is diffusion limited, the wide range for the solid solution is dependent on particle size, sintering temperature and time. Since the present invention does not rely on the formation of a complete solid solution, the only limitation on x and z is that there be enough liquid phase to allow sintering.
• the ratio of x to z can be as great as 99:1.
• SiC, AlN or other compounds i.e., BeSiN2, MgSiN2, beta Al4SiC4, beta AI5C3N, Si3Al4 4C3, and the like
• ratios of x to z are preferably not less than 2:98.
• the unique technique of the present invention comprises performing the sintering in such a manner as to substantially limit decomposition or volatilization of the powder compact prior to densification. Densification occurs rapidly over a narrow temperature range (between 1800 and 2000°C).
• a liquid phase is known to be present at temperatures in excess of 1840°C in the AI2O3-AI4C3 system due to a eutectic reaction between AI2O3 and AI4O4C.
• Microstructural evidence of solution-precipitation confirms that a liquid phase is present during the reaction. Liquid phase sintering therefore competes with decomposition of some of the reaction constituents due to their high vapor pressure.
• Decomposition may be limited by a number of different techniques including 1) using a closed crucible containing the green body (i.e., a graphite or boron nitride crucible); 2 ) by embedment of the green body in a loosely packed mass of ceramic particles of a substan ⁇ tially similar chemical composition; 3 ) by controlling the heating rate and sintering time to limit decomposi ⁇ tion and promote sintering; and 4) by controlling the sintering atmosphere so as to suppress the decomposition and subsequent volatization of reaction components. By suppressing decomposition, sintering to high densities is possible. There is a minimum temperature at which the above reactions take place.
• SiC (0.615 grams) made by the carbother al reduc ⁇ tion of Si ⁇ 2, 0.255 grams ⁇ I2O3 (Meller 0.3 micrometers) and 0.360 grams AI4C3 made by the carbothermic reduction of I2O3, were mixed in an agate mortar and pestle for 15 minutes.
• the binder was pyrol zed by slowly heating to 900°C under 2.
• the compacted powder was then placed in a 20 mm diameter by 20 mm deep cavity within a dense graphite (Poco graphite) crucible 9 cm in diameter and 10 cm high.
• the crucible was closed using a graphite foil seal which mated the crucible to a threaded graphite lid.
• the crucible was then placed in a graphite resis ⁇ tance heated furnace and heated at a rate of approxi ⁇ mately 75°C/minute to 2015°C under flowing N2 and held for 15 minutes.
• micro- structure indicated the presence of a single phase when viewed optically at 1500X magnification.
• X-ray diffrac ⁇ tion also indicated that a homogeneous solid solution had formed (see Table 1) and confirmed that the samples had reacted to form a solid solution consisting of 70 mole percent SiC and 30 mole percent I2OC. Since sintering was done in a nitrogen environment there is no doubt that the solid solution contains some AlN.
• EXAMPLE 2 METHOD FOR DENSIFYING AN INHOMOGENEOUS SOLID SOLUTION SiC (150 grams of Starck BD-10 beta SiC, 17 m 2 /g containing B and C additions), AI2O3 (58.19 grams of Biakowski CR-30), and AI4C3 (87.81 grams, Cerac) were milled for 10 hours in a polyethylene ball mill with 425 ml of 2-propanol and 1 kg. of high purity alumina milling media to make a uniform mixture of the powders. After air drying the 5 gram disks of the powders was formed by uniaxial pressing at 34.5 MPa, followed by isostatic pressing at 207 MPa.
• the pressed disk was loaded into the graphite cylinder described in example 1 and heated in 2 at a rate of approximately 60°C per minute to 2000°C and held there for 1 hour. Upon cooling, it was determined that the linear shrinkage was 13.6% and the density was 2.93 g/cc or 95% of theoretical.
• X-ray diffraction showed that the SiC*30 mole % AI2OC material was a complete solid solution (FIG. 1).
• Optical micro ⁇ scopy showed two distinct phases (FIG. 2), which were apparently Si and Al rich SiCAlON solid solutions.
• SiC made by carbothermal reduction of silica, 3.0 grams
• Al -15SiC made by Cutler process (U.S. Pat. 4,141,740), 3.0 grams
• AI2O3 Meller, 0.8293 grams
• C carbon black, 0.2928 grams
• Al -325 mesh, 0.8778 grams
• SHEET EXAMPLE 9 METHOD FOR PRESSURELESS SINTERING SiCAlON VTA EMBEDMENT SiC (Stark AD-10, 90 grams), AI2O3 (Reynolds HP-DBM, 51 grams), Al (Cerac, 54 grams), and C (Cabot Mogul L, 18 grams) were ball milled with 1500 grams of high purity alumina media in a plastic mill with 500 ml isopropanol for 12 hours. The powder was pressed into a pellet as in Example 2 and subsequently embedded in its own powder. The embedded sample was heated to 2000°C in 10 minutes and held for 5 minutes. The embedded disk sintered to greater than 95% of theoretical density and had an x-ray diffraction pattern of a mixture of SiCAlON and SiC.
• the pressed pellets prepared as in Example 2 were sintered uncontained in 2 by heating from 1000°C in less than 5 minutes (FIG. 6).
• EXAMPLE 11 SINTERING 1-5 MICRON Si C USING SiCAlON SiC ( Carborundum 1500 grit alpha SiC without B or C additions , 152 .04 grams ) , AI2O3 (Reynold ' s HP-DBM,
• the present invention is unique in the following respects:
• Starting materials may be conventional ceramic powders in terms of composition (e.g., SiC, AI2O3, and the like), which are of a conventional particle size
• the starting particle sizes are preferably 1-5 microns if inhomoge- neous solid solutions or mixtures of the solid solution and a refractory phase are desired, or preferably less than 0.5 microns if a complete solid solution is desired.
• Complex shapes may be formed in the green state (using conventional binders) and sintered without the application of external pressure to form a dense, strong ceramic body having properties equivalent to those of SiCAlON ceramics heretofore only attainable using applied pressure while hot pressing.
• the compositions of the ceramic body can be controlled while still allowing densification to occur.
• the processing technique allows for the densification of complete solid solutions or mixtures of the solid solution with another refractory phase.
• the solid solution can therefore be used as a sintering aid to promote the densification of ceramics which are otherwise difficult to sinter.

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• 1986
  • 1986-09-19 WO PCT/US1986/001957 patent/WO1987001693A1/fr not_active Application Discontinuation
  • 1986-09-19 CA CA000518683A patent/CA1256126A/fr not_active Expired
  • 1986-09-19 EP EP19860906120 patent/EP0241514A4/fr not_active Ceased

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