Plastic and nonmetallic article shaping or treating: processes – Formation of solid particulate material directly from molten... – Coated particles
Reexamination Certificate
1999-10-13
2002-10-15
Derrington, James (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Formation of solid particulate material directly from molten...
Coated particles
C264S005000, C264S008000, C264S009000, C264S015000, C264S678000, C264S683000
Reexamination Certificate
active
06464906
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of making balls or spherical bodies of high density from powder by way of rotation, and particularly to a method for making or rather manufacturing small spherical bodies made of inorganic material such as silicon nitride ceramic balls in a low cost mass-production by way of rotation. This invention also relates to spherical bodies of high density and particularly to ceramic balls for use in various ball-using applications, made by the method of the invention. This invention further relates to an adjusted powder composition that is suitably used for the method of the present invention to make the spherical bodies.
BACKGROUND OF THE INVENTION
Ceramic balls (or spherical ceramic bodies), particularly silicon nitride ceramic balls, can be incorporated into bearings requiring high duty, high durability, high speed and/or high precision. This is because ceramic balls are advantageous in improving the performance of a bearing in view of their hardness and light weight, as compared to metal balls. On the other hand, because ceramic balls for use as bearings are expensive, they have not yet been widely used nor popularly accepted across the total industry employing a very large number of high-performance balls.
One reason for the expense of the ceramic balls is that a low-cost method for forming ball-shaped or spherical sintered bodies having a high density from a ceramic powder has not yet been established, which requires a performance at least similar to or comparable with the metal balls for bearings.
Conventionally, a molding method has been employed for manufacturing relatively uniform and less defective spherical ceramic bodies so long as their diameter is relatively large. However, due to its low molding efficiency, this conventional molding process is not suited for inexpensively manufacturing a large number of small balls, for example, those for use in most bearings. For example, computer hard disk drive motors or other spindling devices require a large number of small bearing balls having a diameter of, e.g., 2 mm or the like, mostly less than 4.5 mm.
A die-pressing method using press punches requires two hemispheric cavities formed in their end faces. Such a die-pressing method unavoidably causes the formation of an unnecessary circular flange or brim portion around a press-formed spherical body due to unilateral movement of the punches, and tends to form a low density unfired spherical ball that possibly leaves pores or areas of non-uniform density inside the ball after sintering. The unnecessary portion has to be removed before firing an unfired ball or before polishing the fired body, adding more cost and inefficiency in manufacturing the ceramic balls.
Other known methods or processes for manufacturing spherical bodies from powder include a tumbling granulation process, a stirring granulation process and a squeezing granulation process. These processes require a material powder having large agglomerates ranging from 50-200 &mgr;m with a high powder fluidity, normally made by spray-drying a slurry containing inorganic powder. On the other hand, a high performance ball bearing requires balls of fine surface. Namely, a fine ceramic powder having small agglomerates with a low fluidity is desirably employed for making ceramic bearing balls with uniform density and without defects such as discontinuous boundaries, holes and pores inside or on the ball surface, which increases reliability of the bearings.
The spray-dried powder material having such a large agglomerate size of from 50-200 &mgr;m, which tends to induce non-uniform shrinkage, deformation, cracks or chipping during or after sintering unfired ceramic bodies, has been used in manufacturing relatively large-sized balls such as ball-media, e.g., for use in a cement-powdering process that does not require the high reliability or durability.
SUMMARY OF THE PRESENT INVENTION
It is therefore an object of the present invention to provide a low-cost method for making spherical bodies from a powder, suitable for mass-production of spherical bodies. One of the features of the method of the present invention is directed to the use of a nucleus which forms and grows a granular body around the nucleus by rotating the nucleus. Another feature of the present method is directed to the use of an adjusted powder suitably adjusted for forming and growing a granular body having the nucleus inside the body during the rotation.
Specifically, the method of making spherical bodies from powder according to one aspect of the invention, comprises:
(1) preparing an adjusted powder having at least one powder characteristic selected from the group consisting of an average particle size, a powder particle size distribution and a BET specific surface area;
(2) preparing nuclei having a particle size larger than that of the adjusted powder;
(3) rotating the nuclei; and
(4) adding the adjusted powder and a solvent to the rotating nuclei so that particles of the adjusted powder accumulate on the nuclei to form granular bodies. Preferably, each granular body has the nuclei therein. The rotation of the nuclei may include a meaning of rolling. or tumbling the nuclei, i.e., rotation of the nuclei with some vibration added to the nuclei.
An advantage of the method of the present invention is that a large quantity of spherical inorganic bodies (e.g., bearing balls) of ceramic such as nitrides including silicon nitride, aluminum nitride or boron nitride, oxides including zirconia or alumina, carbides including tungsten carbide or titanium carbide, and the like, are inexpensively and efficiently manufactured.
Preparing the adjusted powder in the above method can advantageously comprise mixing at least an inorganic powder and a sintering aid powder uniformly to form a powder mixture having powder characteristics comprising an average particle size of 0.1-2 &mgr;m, a 90% powder particle size distribution falling within a range of from 0.7-3.5 &mgr;m, and a BET specific surface area of from 5-20 m
2
/g.
An advantage of this additional step is that a large quantity production of spherical green bodies, namely, spherical unfired balls, is facilitated. Another advantage is that the spherical bodies unfired or sintered have a high uniform density and are produced uniformly with less defects in a large quantity by virtue of using the adjusted powder in the method according to the invention.
An alternative of the above for preparing the adjusted powder comprises steps of
(1) forming a slurry by uniformly mixing at least a ceramic powder, a sintering aid powder and a solvent;
(2) drying the slurry into agglomerates; and
(3) pulverizing the agglomerates into a fine powder having an average particle size of 0.1-2 &mgr;m, a 90% powder particle size distribution falling within a range of from 0.7-3.54 &mgr;m determined according to a cumulative relative frequency distribution of powder particle sizes, and a BET specific surface area of 5-20 m
2
/g.
The advantage of these steps is directed to the slurry. This is because the slurry forms a uniform distribution of the ceramic powder and sintering aid powder mixed in the slurry and further forms aggregates comprised of the ceramic powder particles uniformly distributed with the sintering aid powder particles.
Various kinds of solvents may be used as long as pulverization of the aggregates can be accomplished. Water is recommended for the solvent, because water does not cause an environmental issue.
The method of making spherical bodies by rotation or tumbling according to the invention may further comprise:
(1) firing spherical granular bodies to thereby produce sintered spherical bodies; and
(2) grinding surfaces of the sintered spherical bodies to produce polished spherical bodies. This is because spherical bodies generally need a certain hardness and a surface smoothness in actual use.
In one aspect of the embodiments according to the invention, a method is provided for manufacturing spherical sintered ceramic bodies from ceramic powder of silicon nitride. T
Niwa Tomonori
Yogo Tetsuji
Derrington James
NGK Spark Plug Co. Ltd.
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