Method of manufacturing spherical bodies

Abstract: There is provided a method for manufacturing spherical sintered bodies, especially of silicon nitride of small diameter at high efficiency with fewer defects. Such spherical bodies find use as bearings for computer hard disk drives.

Description: The present invention relates to a method of making balls or spherical bodies of high density from powder by way of rotation, 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. Ceramic balls (or spherical ceramic bodies), particularly silicon nitride ceramic balls can be incorporated in bearings requiring high duty, high durability, high speed and/or high precision, since ceramic balls are advantageous in improving performances of a bearing in views of hardness and light weight, compared to metal balls. On the other hand, since the ceramic balls in use for the bearing have been expensive, they have not been widely used nor popularly accepted yet across the total industry employing a very large number of high-performance balls. One reason for the expensiveness of the ceramic balls is that there has not established a low-cost method for forming ball-shaped or spherical sintered bodies with high density from a ceramic powder yet, which require performance at least similar to or comparable with the metal balls for bearings. Conventionally, a molding method have 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 of bearings, the inventors believe. For example, computer hard disk drive motors or other spindling devices demand for a large number of small bearing balls having a diameter of e.g. 2 mm or so, mostly of 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 formation of an unnecessary circular flange or brim portion around a press-formed spherical body due to a unilateral movement of the punches and tends to form a low density unfired spherical ball that possibly leave pores or non uniformity in density inside the ball after sintering. The unnecessary portion has to be removed before firing a unfired ball or before polishing the fired body, adding more cost and inefficiency in manufacturing ceramic balls. Other known methods or process 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 50-200 µm with a high powder fluidity, normally made by spray-drying a slurry containing inorganic powder. However on the other hand, a high performance ball bearing requires balls of fine surface, i.e. 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 bearings. The spray-dried powder material having such a large agglomerate size of 50-200 µm, 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. in use for a cement-powdering process that does not require the high reliability or durability. DD 300288A describes the manufacture of abrasion-proof ceramic grinding balls based on silicon nitride and silicon carbide powders having a broad particle size range using conventional tumbling/granulation and sintering techniques. Similar procedures are described also in DGK 69(1992) 90-95. In contrast, EP 0300543A describes the manufacture of hollow metallic or ceramic spheres by pyrolising a foamed polymer particle bearing a solid metallic or ceramic layer. One of the features in the method of the present invention is directed to use of a nucleus which forms and grows a granular body around the nucleus by rotation of the nucleus. The other feature in the present method is directed to use of a powder suitably adjusted for forming and growing a granular body having the nucleus inside the body during rotation. In accordance with the invention, there is now provided a method of making spherical bodies from powder, the method comprising the steps of: (1) providing a powder having an average particle size of 0.1-2 µm, a 90% cumulative relative frequency particle size distribution of 0.7-3.5 µm and a BET specific surface area of 5-20 m 2 /g; (2) providing nuclei having a particle size larger than that of the powder; (3) rotating the nuclei; and (4) adding the powder and a solvent to the rotating nuclei whereby the particles of the powder accumulate on the nuclei to form granular bodies containing the nuclei. An advantage in 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 so on are inexpensively and efficiently manufactured. The step of preparing the adjusted powder in the above method can comprise advantageously a step of: mixing at least an inorganic powder and a sintering aid powder uniformly to form a powder mixture having the powder characteristics comprising the average particle size of 0.1-2 µm, the 90 % powder particle size falling into 0.7-3.5 µm, and the BET specific surface area of 5-20 m 2 /g. An advantage of this additional step is that a large quantity production of spherical green bodies i.e. spherical unfired balls are facilitated. The other advantage is that the spherical bodies unfired or sintered have a high uniform density and 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 additional step of preparing the adjusted powder comprises steps of: (1) forming a slurry by uniformly mixing at least a ceramic powder, an sintering aid powder and a solvent, (2) drying the slurry into agglomerates, and (3) pulverizing the agglomerate into a fine powder having the average particle size of 0.1-2 µm, 90 % of particle sizes of the fine powder falling into 0.7-3.5 µm determined according to the cumulative relative frequency distribution of powder particle sizes and the BET specific surface area of 5-20 m 2 /g. The advantage of these steps is directed to the slurry, 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 the water does not cause an environmental issue. The method of making spherical bodies by rotation or tumbling according to the invention may further comprise the steps of: (1) firing spherical granular bodies thereby producing sintered spherical bodies, and (2) grinding surfaces of the sintered spherical bodies to produce polished spherical bodies, simply because most of the spherical bodies need a hardness and a smoothness of the surfaces of the spherical bodies in the actual use. In one of aspects of the embodiments according to the invention, there is provided a method for manufacturing spherical sintered ceramic bodies from ceramic powder of silicon nitride. The method comprises the steps of: preparing an adjusted powder which predominantly comprises silicon nitride powder and a suitable amount of sintering aid powder, wherein the adjusted material powder has at least, an average particle size of 0.1-2 µm, more preferably 0.3-2 µm, and a 90% particle size of 0.7-3.5 µm, both as preferably measured by use of a laser diffraction granulometer, as well as a BET specific surface area of 5-20 m 2 /g, more preferably 5-13 m 2 /g; forming the adjusted powder into spherical green bodies by rotating nuclei adhering the adjusted powder; and sintering the spherical green bodies to yield the spherical sintered bodies of silicon nitride. Herein, the 90 % particle size corresponds to the particle size value when Nrc (i.e. relative cumulative frequency) becomes 90 % as understood by referring to Fig.12(b), or to the particle size value when a Nrc value of 90% is attained or when the number of Nc (i.e. cumulative frequency of particle size classes counted from the smallest particle size class) is divided by NO (i.e. total frequency counted from the smallest particle size to the maximum particle size) as may be more clearly understood by referring to Fig.12(a). The average particle size can be expressed as a 50% particle size herein and defined as the Nrc value of 50% that is attained with a way similar to the above, i.e. the average particle size means the particle size value when Nrc = 50%. A feature of the adjusted powder is directed to adjustment of an original powder material, i.e. the adjusted powder adjusted from the original powder material shall have at least one of three powder characteristics including an average powder particle size, a relative cumulative frequency of powder particle sizes and a BET specific surface area of powder. An advantage of using the adjusted powder is facilitation in adhering powder particles around the nucleus to form a spherical granular body and further adhering the powder particles to the granular body to grow the granular body further in size during rotation of the nucleus, with addition of a solvent such as water to the nucleus in rotation. In one aspect of the invention, the best result of making spherical bodies is attained by use of the adjusted powder having all the powder characteristics of the average powder particle size, the relative cumulative frequency and the BET specific surface area, in the method of the present invention. These powder characteristics are important factors in forming the spherical granular body by rotation or tumbling according to the invention. Specifically, When the average power particle size, relative cumulative frequency of powder sizes and BET specific surface area are in the range of 0.1-2 µm, 0.7-3.5 µm at 90 % relative cumulative frequency, and 5-20 m 2 /g, respectively, a good result is attained in forming the spherical granular bodies unfired and resultant spherical sintered bodies made by the method according to the invention. The averaged powder particle size and/or relative cumulative frequency of powder sizes of the adjusted powder are determined by a laser granulometer, and the BET specific surface area of the adjusted powder is determined by a Brunauer-Emmett-Teller method. In one aspect, spherical balls made according to the method of the invention has a high relative density, because the adjusted powder uniformly and firmly adheres around the rotating nucleus and around the rotationally growing granular body having the nucleus in the center due to a radial directional force being applied to the nucleus as well as the granular body by rotation. The relative density of the spherical body in a unfired state becomes no less than 61 % and as high as 75%, rendering more than 98 % or rather very close to 100 % relative density to the resultant spherical sintered body when the unfired spherical granular body is subjected under a HIP ( hot isostatic pressing) or a GP( gas pressure) sintering condition, in the case of e.g. a ceramic spherical body, particularly a silicon nitride ceramic spherical body. In one aspect, the density of the spherical body becomes high and uniform in a circular direction inside the spherical body made by the method of the invention as well as in a radial direction, because fine particles of the adjusted powder are forced into gaps among the particles adhered on the granular body surface by a force in the radial direction of the spherical granular body. A layer of the adjusted powder adhered on the granular body surface is formed and packed uniformly in the circular direction as well as in the radial direction during rotation of the granular body that has the nucleus therein. This circular uniformity in addition to the radial uniformity of the density of the spherical body is advantageously important because the spherical body unfired or fired is protected from forming defects such as pores and a hole inside. The resultant sintered spherical body is protected from cracking in a radial direction when it is actually used in an application such as a bearing. If there is such a defect inside or particularly close to or on the ball surface, high reliability and/or high performance of the bearing are not attained because the ball in actual use for the bearing receives a high load in the radial direction possibly causing breakage or crack leading to a serious matter. A feature of the spherical bodies (or balls) according to the invention is as follows. A lightness and/or a chromaticness observed on an inside surface that is made by cutting the spherical ball across the ball center into hemispherical halves and polishing the inside surface of the half are less varied in the circular direction but more varied in the radial direction of the spherical body between the center and the surface. The lightness and chromaticness are clearly identified as circular stripes in the case of unfired green spherical bodies. The variation of the lightness and/or chromaticness in the radial direction means uniformity of such lightness and/or chromaticness in the circular direction. The variation of lightness and/or chromaticness in a radial direction and the uniformity of lightness and/or chromaticness in a circular direction are often observable more or less on the polished surface on the cut surface of the sintered spherical body, although the clear stripes as seen inside the unfired spherical body are disappeared in the sintered body. In still another aspect, the spherical body made from the adjusted powder and then sintered in the HIP or GP, according to the invention, can be made as having substantially no pores inside between the nucleus and on the surface of the spherical sintered body, because of combination of the adjusted powder to be used, the method of the invention and a pressure-applied sintering process including the HIP or GP. In the accompanying drawings: Fig. 1 is a schematic view showing a cylindrical container that rotates into which, nuclei and/or adjusted powder are poured. Fig. 2 is a schematic view showing the cylindrical container in rotation, incorporating the nuclei with addition of the adjusted powder and a solvent Figs. 3(a), 3(b), 3(c), 3(d) and 3(e) show a variety of shapes of the nuclei that can be rotated in the method of the invention. Figs. 4(a), 4(b), 4(c), 4(d) and 4(e) schematically illustrate various nuclei that can be used in the method of the invention can be made by a variety of methods. Fig.5(a) schematically shows that a nucleus is rotating and a granular body is rotationally growing in size by adhering powder particles around the nucleus. Fig.5(b) schematically illustrates that a granular body is growing in size by adding layers of aggregate particles. Fig.5(c) schematically illustrates that a powder particle forces into particles layered on a granular body surface. Fig.5(d) schematically illustrates that a deformed granular body made without a nucleus is rotating. Fig.5(e) schematically illustrate that particles are arrayed accompanied with a filtration of a solvent (w). Fig.6 is a schematic view showing a cross section of a spherical sintered body having a nucleus in the center. Fig.7 (a) is a schematic perspective view showing a rotating cylinder rotationally growing granular green bodies inside the rotating cylinder. Fig.7(b) is a schematic view showing an opening of the rotating cylinder of Fig.7(a). Fig.8 is a schematic longitudinal sectional view showing a apparatus for making an adjusted powder for use in a method of present invention for making spherical bodies. Fig.9 (a) is a schematic view of a ball media placed on a media holder having holes in the apparatus in Fig.8. Fig.9 (b) is a schematic view of the ball media through which a hot air blows and dries the ball media. Fig.10 is a schematic view of the ball media colliding balls each other so that aggregate layers formed on the balls are pulverized into a fine powder and blown by the hot air. Fig.11 is a schematic view of a secondary particle aggregated of solitary primary particles. Fig.12(a) shows a particle size frequency as a function of particle size distribution, used for a explanatory purpose of defining a cumulative frequency (Nc) and a total number (No) of particles in a powder particle size distribution. Fig.12(b) shows a relative cumulative frequency of powder particle size as a function of powder particle size, used only for a explanatory purpose in defining a 90 % particle size and a 50 % particle size (i.e. average particle size) of an adjusted powder. Fig.13 is an enlarged SEM photo showing a broken section of a spherical granular green body made by the method of the invention, showing a nucleus or core inside the spherical body. Fig.14(a) is an enlarged photo showing a cross section of a spherical body fired at 1400°C made by the method according to the invention, showing a nucleus in the center of the body. Fig.14(b) is a further enlarged photo of Fig.(a), showing the nucleus and a circular arc inside the body. Fig.14(c) is a still further enlarged photo of Fig.14 (b), showing the nucleus having a density different from a uniform density of the rest. Fig.14(d) is a still further enlarged photo of Fig.14(b), showing the circular arc having a density different from the uniform density of the rest. Figs.15(a),15(b) and 15(c) are some enlarged photos taken on cross sections of three different spherical sintered bodies of silicon nitride ceramic, according to the invention, each showing a lightness and/or cromaticness varying in a radial direction but not varying in a circular direction. The present inventors have found that, by adjusting a material powder into an adjusted powder having powder characteristics of the average particle size and the 90% particle size, as measured by use of a laser diffraction granulometer, falling within the above-mentioned ranges and a BET specific surface area falling within the above-mentioned range, spherical bodies formed by any of the above or later described granulation processes ― which are more efficient than, for example, a die pressing process ― are less suffering non-uniform density or discontinuous boundaries which would otherwise result from biased distribution of powder particles, thereby drastically reducing fraction defective in terms of defects caused by non-uniform shrinkage of sintered bodies, or in terms of deformation of, cracking in, or chipping of sintered bodies. The present invention can accelerate implementation of ceramic bearing balls for use under special working conditions, such as those for use in bearings of hard disk drives or semiconductor apparatus for computer use requiring high reliability. A laser diffraction granulometer is publicly known and useful. Briefly, a sample powder of interest is irradiated with a laser beam. A beam diffracted by the sample powder particles is detected by means of a photodetector. A scattering angle and an intensity of the diffracted beam are obtained from the data obtained by the photodetector. Powder characteristics such as particle size distribution including the averaged particle size (i.e. 50 % particle size of a cumulative particle size distribution) and the 90 % particle size can be obtained from the scattering angle and the intensity. Particles of a material powder to be formed and sintered ― which powder is normally composed of fine primary particles and sintering aid particles ― often assume a form of secondary aggregate particles as shown schematically in FIG. 11 referring as a solitary primary particle size (d) and a secondary aggregate particle size(D). Various factors such as influence of an added organic binder and/or static electricity are considered to cause the fine primary particles to aggregate to form the secondary aggregate particle comparatively large in size. When the material powder is subjected to measurement by the laser diffraction granulometer, the secondary aggregate particle and the solitary primary particle do not result in much difference in the diffracting behavior of an incident laser beam. Accordingly, whether a measured particle size is of a solitary primary particle or of a secondary aggregate particle is not definitely known. That is, the thus-measured particle size reflects secondary particle size D shown in FIG. 7 and in this case, the solitary primary particle is also considered to be a secondary particle as defined in a broad sense. The average particle size and the 90% particle size computed from the measured particle sizes with the laser diffraction granulometer reflects an average particle size and a 90% particle size of secondary particles including solitary particles of the powder of interest A BET specific surface area is the surface area of particles per unit weight. This BET value is obtained from the amount of gas adsorbed by aggregates of particles. The gas molecules penetrate into the secondary particle to thereby cover the surfaces of constituent solitary primary particles. As a result, an obtained BET specific surface area reflects the specific surface area of primary particles of the powder; i.e. the average of primary-particle sizes(d) as shown in FIG. 11. One of the best material for the adjusted powder used in the method of the present invention for manufacturing a large quantity of spherical sintered bodies in use for the bearings may be silicon nitride (Si 3 N 4 ), because the silicon nitride ceramic is light in weight, high in toughness (or strength) and high in hardness. A second best material for the adjusted powder for making spherical bodies for bearings may be ziconia, because the zirconia ceramic has a thermal expansion coefficient closer to metals constituting bearing parts that incorporate the spherical bodies, although it is heavier than silicon nitride ceramic. A third one may be alumina, because it is hard and enexpensive. Other material candidates for the adjusted powder material are carbide such as TiC, WC and SiC, nitride such as TiN and TaN, cermet, because these materials give some electrical conductivity that prevents a static electricity. Diamond or BN (like cubic boron nitride ) can be a good candidate for the adjusted powder material, because they are hardest or second hardest among all materials. All other materials including other ceramic, metal, glass and /or a composite material, which require a firing or sintering process that follows for making spherical bodies, can be used for the adjusted powder usable in the method of the present invention. In order to obtain sintered bodies of silicon nitride which have sufficiently high density and strength and a low fraction defective by the present method of the invention, an important point is as follows. The powder material of interest is prepared and adjusted so as to have powder characteristics showing the average particle size or 90% particle size (as measured by use of the laser diffraction granulometer) as small as about 1/10 of that of the conventional powder material that is normally obtained by means of the spray-drying process. Specifically, the adjusted powder according to the invention is adjusted from a powder material of interest, so as to have an average particle size of 0.1-2 µm and/or a 90% particle size of 0.7-3.5 µm, both of which reflect the secondary-particle size. The adjusted powder with these two range is less likely forming non-uniform aggregation as the secondary particles, and facilitates to make a density of particles uniformly distributed in the aggregates. As a result, spherical green bodies formed through granulation process (i.e. growing granular green bodies by rotation of a nucleus) according to the invention, is less likely suffering biased distribution of particles in the green body and less likely causing defects such as pores or holes in resultant sintered spherical bodies. When the average particle size of the adjusted powder is in excess of 2 µm and/or the 90% particle size is in excess of 3.5 µm, the spherical granular bodies unfired become likely to suffer biased distribution of particles, or potentially causing nonuniform shrinkage of the body during firing or sintering, which, in turn, causes deformation, cracking, or chipping of the resultant sintered body. By contrast, the adjusted powder having the average particle size of less than 0.1 µm and/or 90% particle size of less than 0.7 µm requires a considerably long preparation time (i.e. pulverization time), resulting in increased manufacturing cost due to low manufacturing capability. More preferably, the average particle size of the adjusted powder is 0.3-1 µm and the 90% particle size is 0.7-2 µm for the adjusted powder. When the BET specific surface area of the adjusted material powder is less than 5 m 2 /g, the primary-particle size would be excessively large, causing nonuniform sintering of the granular spherical body. As a result, an obtained spherical sintered body suffers defects with a deterioration in strength or toughness. By contrast, the material powder having the BET specific surface area in excess of 20 m 2 /g requires a considerably long preparation time ( including pulverization time), resulting in increased manufacturing cost due to low manufacturing capability. More preferably, the BET specific surface area of the adjusted powder is 5-13 m 2 /g and more preferably, 5-10 m 2 /g. Any publicly known material may be used as sintering aid powder for the case of ceramic powder. Examples of such material include alumina, yttria and other rare-earth oxides, magnesia, spinel, zirconia, or silica. The sintering aid powder to be mixed with ceramic powder e.g. silicon nitride powder may be in an amount of 1-10 parts by weight per 100 parts by weight of silicon nitride. When the above amount of sintering aid powder added to the silicon nitride is less than 1 part by weight, the spherical sintered body is less likely to become denser in density, potentially resulting in poor strength of the spherical silicon nitride sintered body, and may not be suitable in use for bearings. When the above amount of the sintering aid powder is in excess of 10 parts by weight, silicon nitride particles contained in the sintered body become coarser, again potentially resulting in poor strength of the sintered body. However, other powder material to be adjusted may need a different range of a sintering aid amount, depending on its material characteristics and conditions in an actual use. Processing conditions for the method of making spherical granular bodies by rotation or tumbling of the nucleus according to the invention may require that the spherical granular body unfired yet assumes a relative density of at least 61%, when water is selected as the solvent . The spherical body having a relative density of less than 61% becomes difficult for handling, due to poor strength of the granular body and the yield of resultant sintered spherical bodies of ceramic, specifically, of silicon nitride is impaired accordingly. The adjusted powder according to the invention forms a granular body having more than 61 % relative density when it is made by the method of the present invention. Spherical granular bodies are preferably sintered under the HIP or GP sintering process. The GP (:gas-pressure) sintering process may employ a nitrogen gas atmosphere, a gas pressure of about 2-100 atm, and a firing temperature of about 1500-1850°C in sintering the spherical bodies of silicon nitride ceramic. The HIP (:hot isostatic press) sintering process may employ a nitrogen gas atmosphere, a pressure of about 100-2000 atm, and a firing temperature of 1500-1850°C in sintering the same. An embodiment of the present invention will next be described with more focussing to the method of making spherical sintered bodies of silicon nitride ceramic, with referring to the accompanying drawings. FIG. 1 schematically shows that a powder of nuclei(50) and an adjusted powder (10) according to the invention are being poured inside a container (132) that is a part of a spherical granular bodies-growing device 30 as in Fig.2, in which the container (132) rotates along an axis (O) of the rotor rotating or rather rotationally tumbling the nuclei 50 along an layer of the adjusted powder (10k) distributed inside the container (132). In this process, the nuclei 50 may be firstly tumbled or rotated by the rotating container 132 and then the adjusted powder and a solvent (w) may be poured in through a tube 51 controlling an amount of the adjusted powder and solvent. A particle shape of the nuclei 50 may take a different one such as shown by reference numerals (50a), (50b), (50c), (50d) and 50(e) in Figs. 3(a), 3(b), 3(c), 3(d) and 3(e), respectively. A particles size of the nucleus is selected from the size of 50-500 µm which is far bigger or at least ten times bigger than the particle size of the adjusted powder and which is preferably 20-200 times bigger than the average particle size of the adjusted powder that will be described later. However, the particle size of the nucleus is selected as not exceeding 1/2 of the diameter of the spherical granular body in most cases, because the spherical sintered body is subjected to grinding out a surface of the sintered body. A 100-300 µm particle size of the nucleus performs best with the adjusted powder used with the method of the invention. The material of the nucleus can be selected from various kind of materials such as ceramic , glass, metal and resin such as wax, as long as a the nucleus materials does not cause a bad effect to the spherical body unfired or sintered. However, it is preferable to use the nuclei having the same material as the adjusted powder material that is formed into the granular body. When the wax ball is used as the nucleus, the resultant spherical sintered body may have a hollow inside the body or no nucleus substance remaining inside the ball because the wax ball is lost during firing and the granular body shrinks during sintering. In this case, the spherical sintered body made according to the invention is defined as having the nucleus even if the nucleus is not observed. This may be identified by the difference of lightness and/or chromaticness observed on the cross section between the center and the rest surrounding the center. The nuclei to be used in the method of the invention to form granular green bodies around the nuclei by rotation can be made various ways. One way may be, as shown in Fig.4(a), to form a nucleus (60) by pressing a powder with a die(51a) and press punches(51b,51b). The other way is selecting large size particles of aggregates by classifying the conventional spray-dried particles of aggregates into the nucleus sizes. The third way is, as shown in Fig.4(b), blowing a fused compound (63) contained in a first nozzle (61) by jetting gas or liquid from a second coaxial nozzle (62) formed circularly on the first nozzle(61) to form solid nucleus particles (50). The fourth way may be forming nucleus particles by leading the fused compound(63) into a cavity (70), as shown in Fig.4(c). The fifth way may be crushing a formed granular body unfired or sintered and classifying the crushed powder. The sixth way may be forming a spherical nucleus droplet by dropping a resin material (74) from a nozzle 74. The adjusted powder to be utilized in the method of the present invention is prepared by the following method and apparatus which will be next explained and described by referring to Fig. 8. A hot air passage (1) has a vertically disposed hot air duct (4). The hot air duct (4) includes a holder plate (5) having air passages holes, supported at an intermediate position of the hot air duct (4). A drying ball media 2 composed of alumina ceramic balls are gathered forming a ball media layer (3) on the holder (2) having the passages as shown in Fig.9(a). As schematically illustrated in Fig. 4 and Fig.9(b), a hot air is blown in from a lower position of the duct (4) through the passage of the holder plate (5), drying and moving the ceramic balls colliding each other. A slurry (6) comprising powder material of interest and a solvent is fed from a slurry tank(20) through a flow control pump (P) to the drying ball media (2), and at the same time, the hot air is caused to flow through the drying media 2 while agitating the drying media 2. The slurry poured on the ball media layer(3) is dried forming a layer of aggregates of the powder materials. The Fig.10 schematically illustrates that the layer(6)of aggregates (PL) is formed on the ball media(2) and the layer of aggregate(PL) formed on the balls is crushed or rather pulverized into a fine powder (9) by the balls rubbing and/or colliding each other. The fine powder (9) of thus pulverized particles is flown out from the ball media by the hot air blowing through the ball media (2) as shown in Fig.10. As schematically illustrated in Fig.8 The fine powder of the pulverized particles is carried with the hot to some distance, classified by a cyclone separator (S) into an adjusted powder 10 having a desired powder characteristics which is gathered in a reservoir 21. The powder characteristics can be modified by various factors such as a flow rate and a temperature of the hot air and a amount of the slurry and a mount of the ball media (2).. In the case of ceramic powder, the adjusted powder (10) yielding the powder characteristics having an average particle size of 0.1-2 µm and a 90% particle size of 0.7-3.5µm, both as measured by use of a laser diffraction granulometer, as well as a BET specific surface area of 5-20 m 2 /g is best. The flow of hot air causes repeated agitation and fall of the drying ball media 2. Thus, the drying media 2 collide and rub against one another, whereby the powder aggregate layers PL are broken (pulverized) into fine particles (9). Some of the fine particles (9) assume the form of the solitary primary particle, but most of the material particles assume the form of the secondary aggregate particle that is the aggregation of primary particles. The fine powder particles (9) having a particle size not greater than a certain value are conveyed downstream by the hot air(FIG. 8). The fine particles (9) having a particle size greater than a certain value are not blown by the hot air, but again fall onto the drying media 2, thereby undergoing further pulverization effected by the drying media 2. Preferably, the drying media 2 has a substantially uniform diameter so as to leave an appropriate space thereamong, whereby the motion of the drying media 2 is accelerated during flow of hot air. A thickness t1 of the drying media layer 3 is determined such that the drying media 2 move appropriately according to the velocity of hot air. If the thickness t1 is excessively large, the drying media 2 will have difficulty in moving, causing poor impact force. If the thickness t1 is excessively small, the drying media 2 will collide less frequently, resulting in impaired processing efficiency. The temperature of the hot air is determined such that the slurry 6 is dried sufficiently and the material powder 10 does not suffer any problem, such as thermal deterioration. The hot air having a temperature lower than 100°C fails to sufficiently dry the fed slurry 6. Now, another embodiment of the method of making spherical bodies from powder by rotation of nuclei, will be explained with referring to Figs.7 (a) and 7(b). A tumbling granulation apparatus (30) includes a relatively flat, cylindrical granulation container (32), which is open at a top end, and a rotating shaft 31, one end of which is connected substantially perpendicularly to a central portion of the bottom of the granulation container 32. The granulation container 32 is rotated by a rotational drive unit through the rotating shaft 32. The adjusted powder 10 and water are charged (e.g. by spraying) into the granulation container 32 rotating at a constant peripheral speed, wherein nuclei are rotated or rotationally tumbled. As shown in FIG. 7(b), the adjusted powder 10 accumulated around the nucleus rolls down on an inclined powder layer formed within the granulation container 32 to thereby agglomerate into a spherical granular body 80 having the nucleus therein. Operating conditions of the tumbling granulation apparatus 30 are adjusted such that the formed spherical granular body 80 has a diameter of not greater than 5 mm and a relative density of at least 61%. Specifically, the rotational speed of the granulation container 32 is adjusted to 10-200 rpm. Water feed rate is adjusted such that the final spherical granular green body 80 has a water content of 10-20 wt%. It is preferable that the thus-obtained spherical granular bodies 80 are sintered through gas-pressure sintering or hot isostatic-press sintering, in the case that the adjusted powder is ceramic such as silicon nitride. The resultant sintered bodies undergo polishing for adjustment of, for example, diameter, sphericity, or surface roughness, thereby yielding ceramic bearing balls of e.g. silicon nitride. Notably, when the spherical bodies 80 are sintered in bulk, burrs may be formed thereon due to fusion. However, these burrs are removed by polishing. The following Examples illustrate the invention. Silicon nitride powder (average particle size: 0.3-4.0 µm; BET specific surface area: 4-13 m 2 /g; 94 parts by weight), alumina powder (average particle size: 0.4 µm; BET specific surface area: 10 m 2 /g; 3 parts by weight) serving as sintering aid, yttria powder (average particle size: 1.5 µm; BET specific surface area: 10 m 2 /g; 3 parts by weight), and pure water (50 parts by weight) serving as solvent were mixed to prepare various kinds of slurry of powder material to be adjusted. The average particle size and the 90% particle size were measured by use of a laser diffraction granulometer (model LA-500, product of Horiba, Ltd.). The BET specific surface area was measured by use of a BET-specific-area measuring device (MULTISOPE 12 made by Yuasa lonix Corp.). Various kinds of adjusted powders (P) were made from the various kinds of the above slurry by use of the apparatus of FIG. 8. Specifically, alumina balls having a diameter of 2 mm were used as drying media 2 , and the apparatus was set to the following conditions: Inner diameter R2 of hot air duct 4 equipped with drying-media holder 5: Approx. 200 mm; Depth of layer of drying media 2: approx. 150 mm; Temperature of hot air: 160°C; and Velocity of hot air: 3 m/s. These various kinds of adjusted powders were measured for average particle size, 90% particle size, and BET specific surface area in a manner similar to that described previously. As a result, the average particle size was 0.3-2.0 µm; the 90% particle size was 0.7-3.5 µm; and the BET specific surface area was 5-13 m 2 /g (sample Nos. 1-6). Various kinds of comparative powder materials serving as comparative examples were prepared in the following manner. Silicon nitride powder (average particle size: 1.0 µm; BET specific surface area: 10 m 2 /g; 94 parts by weight), alumina powder (average particle size: 0.4 µm}m; BET specific surface area: 10 m 2 /g; 3 parts by weight) serving as sintering aid, yttria powder (average particle size: 1.5 µm; BET specific surface area: 10 m 2 /g; 3 parts by weight), water (40 parts by weight) serving as a solvent, and an organic binder (6 parts by weight) were mixed to prepare slurry. The slurry was spray-dried, yielding two kinds of material powder having an average particle size of 70 and 100 µm, a 90% particle size of 150 and 200 µm, and a specific surface area of about 11 m 2 /g (sample Nos. 8 and 9). The spray-dried powder of sample No. 8 was subjected to degreasing of the organic binder. The resultant powder was ball-milled, yielding material powder having a 90% particle size of 70 µm (sample No. 7). Further, slurry that does not contain the organic binder was placed in a container and then dried in a drier at 200°C. The dried substance was pulverized in a mortar, yielding material powder having an average particle size of 50 µm, a 90% particle size of 200µm, and a specific surface area of 11 m 2 /g (sample No. 10). The thus-prepared various kinds of powder materials (including the adjusted powder and comparative powder) were formed into spherical granular bodies each having a nucleus in the center by use of the tumbling granulation apparatus as shown in FIG. 7. Conditions of tumbling granulation were as follows: Inner diameter R1 of granulation container 32: 400 mm; Angle 1 of inclination of rotating shaft 31: 15°; Rotational speed: 30 rpm (rotational peripheral speed: 37.7 m/min); and Water content of final spherical green bodies effected as a result of supply of water: 12 wt%. The relative density of the spherical green(i.e. unfired) body was obtained in the following manner. The volume of the spherical green body was obtained from the diameter of the spherical body while the spherical body was considered as a sphere. The weight of the spherical body was divided by the obtained volume. The spherical green bodies were subjected under a gas-pressure sintering process (pressure: 75 atm; temperature: 1750°C; atmospheric gas: nitrogen) or a hot isostatic pressing-sintering process, or an HIP process, (pressure: 1000 atm; temperature: 1750°C; atmospheric gas: nitrogen), yielding spherical sintered bodies of silicon nitride. The spherical sintered bodies of silicon nitride were observed for the condition of cross section by use of an optical microscope and were evaluated according to the following criteria: good: the cross section is free of any defect; poor: the cross section has a defect. Further, the spherical sintered bodies were polished so as to attain a target sphericity of 0.08 µm, a surface roughness (average roughness along centerline) of 0.012 µm, and a diameter variation of 0.08 µm, thereby yielding ceramic bearing balls. The ceramic bearing balls were tested to see whether they are applicable to a bearing. The results are shown in Table 1. FIG. 13 shows an enlarged photo taken by a SEM on a cross section of a spherical granular green body made according to the method of the invention, showing a core(i.e. nucleus) in the center of the body. This green body showed a 71 % relative density. FIGS. 14(a)-14(d) shows enlarged photos showing a cross section of the spherical body of silicon nitride, made according to the method of the invention, but fired at 1400°C in a nitrogen atmosphere. The relative density of this pre-fired spherical body was about 74%. FiG.15(a) shows an enlarged photo taken on an cross section of a spherical sintered body of silicon nitride sintered at 1750°C in the nitrogen atmosphere under the HIP sintering, made according to the method of the invention. FIG.15(b) and Fig(c) show enlarged photos taken on cross sections of silicon nitride sintered at 1750°C in the nitrogen atmosphere under the GP sintering. The relative density of any of the spherical green bodies having more than 60%, made according to the method of the invention using the adjusted powder disclosed herewith, becomes more than 99.9 %, i.e. substantially 100% under the HIP or GP, showing uniformity of the density in the sintered body and incurring substantially no pores in the sintered body. The sintered bodies of the comparative examples were found to suffer many internal defects, such as nonuniform density conceivably caused by biased distribution of powder particles and voids caused by discontinuous boundaries. Therefore, they could not be polished to a target accuracy and thus failed to be applied to bearing balls. By contrast, the sintered bodies of the examples were free of any defect and were polished into bearing balls. These bearing balls raised no problem when used in actual bearings.

Claims: A method of making spherical bodies from powder, the method comprising the steps of: (1) providing a powder having an average particle size of 0.1-2 µm, a 90 % cumulative relative frequency particle size distribution of 0.7-3.5 µm, and a BET specific surface area of 5-20 m 2 /g; (2) providing nuclei having a particle size larger than that of the powder; (3) rotating the nuclei; and (4) adding the powder and a solvent to the rotating nuclei whereby the particles of the powder accumulate on the nuclei to form granular bodies containing the nuclei. A method according to claim 1, wherein the nuclei have a particle size of 1/100 - 1/2 of that of the granular body. A method according to claim 1 or claim 2, wherein the nuclei have a particle size 20 to 200 times greater than that of the powder. A method according to any one of claim 1 to 3, wherein the nuclei have a particle size of 50 - 500 µm. A method according to any one of claims 1 to 4, wherein the powder comprises a ceramic material. A method according to claim 5, wherein the ceramic powder comprises Si 3 N 4 . A method according to claim 6, wherein the ceramic powder comprises also Y 2 O 3 and/or MgO as a sintering aid for Si 3 N 4 . A method according to claim 6 or claim 7, wherein the ceramic powder comprises Al 2 O 3 . A method according to any one of claims 1 to 4, wherein the powder has been prepared by combining an inorganic powder and a sintering aid to form a uniform mixture having the specified average particle size, 90 % cumulative relative frequency particle size distribution and BET specific surface area. A method according to any one of claim 1 to 8, wherein the powder has been prepared by the steps of: (1) mixing a ceramic powder, a sintering aid and a solvent thereby forming a slurry; (2) drying the slurry thereby forming an agglomerate; and (3) pulverizing the agglomerate into a powder having the specified average particle size, 90 % particle size and BET specific surface area. A method according to any one of claims 1 to 8, wherein the powder has been prepared by the steps of: (1) uniformly mixing an inorganic powder, an inorganic sintering aid powder and a solvent thereby forming a slurry; (2) blowing hot air through a medium comprising inorganic and/or organic balls so that the balls are heated and caused to vibrate/collide; (3) pouring the slurry onto the heated medium so that the solvent vaporizes thereby forming a layer of aggregated material on the surface of the balls; (4) crushing the layer of aggregated material by virtue of the balls colliding with each other thereby forming a pulverized powder from the aggregated material; (5) drying and blowing the pulverized powder from the medium with the hot air; and (6) classifying/sorting the pulverized powder. A method according to any one of claims 1 to 11, comprising the further steps of: (1) firing the granular bodies thereby producing sintered bodies, and (2) grinding the sintered bodies to produce finished spherical bodies. A method according to any one of claims 1 to 11, comprising the further steps of subjecting the granular bodies to gas pressure sintering or to hot isostatic pressing thereby forming high density spherical sintered bodies. A method according to any one of claims 1 to 13 for manufacturing spherical sintered bodies of silicon nitride, comprising the steps of: preparing material powder which comprises predominantly silicon nitride powder together with a predetermined amount of sintering aid and has an average particle size of 0.3-2 µm, a 90 % particle size of 0.7-3.5 µm, and a BET specific surface area of 5-13 m 2 /g; forming the powder into spherical bodies; and sintering the spherical bodies.

Inventors: NIWA TOMONORI, YOGO TETSUJI, NIWA, TOMONORI, YOGO, TETSUJI
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1999
Main classification: C04B35
IPC codes: C04B35, B01J2, F16C33, C01B21