Compositions: ceramic – Ceramic compositions – Refractory
Utility Patent
1998-12-04
2001-01-02
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C501S089000, C501S128000, C264S641000, C264S669000, C264S681000, C264S682000
Utility Patent
active
06169048
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of manufacturing whisker-reinforced ceramic cutting tool inserts with improved structure homogeneity.
It is well-known in the art that the fracture toughness of a ceramic material can be increased by addition of monocrystalline hair crystals (whiskers) and/or platelets. SiC-whiskers in an alumina matrix are disclosed in U.S. Pat. No. 4,543,345. U.S. Pat. No. 4,867,761 discloses the use of carbides, nitrides or borides of Ti and/or Zr in an alumina matrix. U.S. Pat. No. 4,849,381 discloses a cutting tool insert comprising a mixture of whiskers and platelets.
Cutting tool inserts made of SiC-whisker-reinforced alumina are an established product on the cutting tool market mainly used for the machining of heat resistant materials and to some extent for machining of cast iron.
Whisker-reinforced ceramic inserts are generally manufactured by uniaxial pressure sintering. Another possible way to manufacture such inserts is by tool pressing and glass encapsulated hot isostatic pressing. This latter manufacturing method is mainly used for inserts with more complex geometries. In both cases, the powder that is filled into the pressing tool has to be granulated in order to obtain necessary flow properties. The most common way of granulating such powders is by spray-drying technique. Spray drying means that a slurry containing the powdery components of the final composition is dried and granulated in hot gas in the same process step. However, during spray drying, the whiskers are drawn into the middle of the granules and the granule borders are depleted of whiskers. As a result the granules have a core armored by whiskers and cannot therefore be completely crushed during the subsequent compaction process. This leads to an inhomogeneous structure with visible granule borders in the sintered material.
An alternative to conventional spray drying is disclosed in U.S. Ser. No. 08/827,878 (our reference: 024000-965). In this case, the granulation is performed by freeze granulation followed by freeze drying. As a result, a ceramic body with homogeneous structure with essentially no granule borders is obtained.
GB 2,214,178 discloses the manufacture of whisker-reinforced ceramics by freeze drying a whisker/powder suspension. The resulting ‘cake’ is then shaken to break up the agglomerates which are sifted through a 1 mm sieve. The powder is then compacted and sintered. The powder obtained after sifting has very poor flow properties and the method can only be used in the manufacture of bodies of simple shape such as cylinders, circular discs, etc., which are subsequently ground to final shape and dimension.
A method of obtaining a whisker-reinforced ceramic body with a structure essentially free of granule borders and an essentially one-dimensional whisker orientation by injection molding is disclosed in U.S. Pat. No. 5,648,029.
In U.S. Pat. No. 5,340,532, a method of forming ceramic bodies by temperature induced flocculation is disclosed which, if applied to whisker-reinforced ceramic bodies, gives a structure essentially free of granule borders and with an isotropic whisker orientation.
Pressure sintering techniques as hot pressing or hot isostatic pressing are normally required for whisker-reinforced alumina materials if the whisker content exceeds 10-15 weight %. Hot pressing is the preferred performance for articles of simple geometrical shapes. HIPping requires canning (gas-impermeable envelope) since closed porosity cannot be obtained using pressureless sintering. Glass encapsulation is the preferred method for complicated shapes.
Whisker-reinforced ceramics are produced using dispersions of the powder and whisker constituents in a liquid. Further steps in the processing of the slurry involve normally a granulation step using spray-drying, freeze-drying or simply granulation in a mixer or similar methods. The powder obtained contains roughly spherical agglomerates with sizes ranging from 25 &mgr;m to 200 &mgr;m and offers better flow properties than ungranulated powders which is essential for uniform filling of the die.
The powder to be densified is normally precompacted in a graphite die in order to minimize the stack height and to increase the number of parts that can be produced in one hot pressing run. Too loose a powder can lead to nonuniform density in the sintered part since powder flow in the hot press is restricted due to die wall friction and especially for whisker-reinforced materials also due to a large flow resistance in the powder itself because of the whiskers.
An alternative to produce granulated powder to fill the graphite dies could be to preform the desired shape (i.e., a disc) using consolidation in an impermeable mold.
Several techniques are known and are in principle possible, for example, consolidation by a polymerization reaction (Gel Casting), destabilization (Direct Coagulation Consolidation, DCC), polycondensation reactions (Hydrolysis Assisted Solidification, HAS) or freezing.
Still, each of the existing methods has disadvantages, such as the use of toxic initiators in water-based gel casting, expansion and ice crystals in water-based systems, or contraction when using organic solvents in freeze forming or low wet-body strength in DCC and HAS.
Starch particles have previously been used as a filler in various ceramic materials to produce materials with controlled porosity. Potential applications include supports for ceramic membranes, filters, gas burners and bioceramics. The pore size of the ceramic material is related to the original size of the starch particles.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of this invention to avoid or alleviate the problems of the prior art.
It is further an object of this invention to provide a method of manufacturing whisker-reinforced ceramic cutting tool inserts with improved structure homogeneity.
In one aspect of the invention there is provided a method of making a whisker-reinforced ceramic body preform by:
dispersing 10%-60% by volume of a ceramic powder mixture containing conventional sintering aids and/or grain growth inhibitors in water or an organic solvent;
adding 1-15 wt-% starch to the dispersion;
pouring the dispersion into a mold of a desired shape;
heating the suspension to 50°-100° C. for 2-4 hours while covering the mold with a lid;
further heating without the lid to complete drying;
removing the preform form from the mold; and
presintering the preform in air for 10 h at about 600° C.
In another aspect of the invention there is provided a ceramic body comprising 2-50 volume-% SiC-whiskers of which the SiC-phase is present as >95% cubic &bgr;-SiC in a ceramic matrix in which body the structure and composition is uniform, free from granule borders and with a uniform distribution of whiskers and in which the orientation of the whiskers is three-dimensional, the degree of orientation (texture) of the whiskers as determined by X-ray diffraction is 1.0<T<2.5, where T is
T=[(SiC
1
/SiC
2
)
p
/(SiC
1
/SiC
2
)
c]
0.5
where SiC
1
=the intensity from the {1010}-peak of the hexagonal &agr;-SiC
SiC
2
=the sum of the intensity from the {111}-peak of the cubic &bgr;-SiC and {0002} peak of the hexagonal &agr;-SiC
p is from a plane perpendicular to the direction of the compaction
c is from a plane parallel to the direction of the compaction.
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patent: 4849381 (1989-07-01), Brandt et al.
patent: 4867761 (1989-09-01), Brandt et al.
patent: 5030600 (1991-07-01), Hida et al.
patent: 5340532 (1994-08-01), Bergström
patent: 5618757 (1997-04-01), Benker
patent: 5648029 (1997-07-01), Collin
patent: 579 587 (1993-07-01), None
patent: 2214178 (1989-01-01), None
patent: 2-289473 (1989-04-01), None
patent: 02 199076 (1990-08-01), None
patent: WO 89/01920 (1989-03-01), None
Brandt Gunnar
Lyckfeldt Ola
Sjögren Clas
Burns Doane Swecker & Mathis L.L.P.
Group Karl
Sandvik AB
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