JP2001294478A - Ceramic ball, method of producing ceramic ball and ceramic ball bearing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a silicon nitride sintered compact having improved wear resistance and effectively suppressed dispersion. SOLUTION: The level of glass peak is controlled so that when scattering intensity at 1,200 cm-1 in a spectrum profile obtained by Raman spectrosopic analysis of a sintered compact is defined as X1 and the standard scattering intensity level is defined as X0, the increased scattering intensity, Y1=X1-X0, becomes not less than 1,500 counts. Thereby, the wear resistance of the silicon nitride sintered compact can be improved and further the dispersion of the sintered compact can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a ceramic ball, a method for manufacturing a ceramic ball, and a ceramic ball bearing.

[0002]

2. Description of the Related Art Balls used for bearings (hereinafter referred to as "balls")
Bearing balls) are generally made of metal such as bearing steel, but those using ceramic bearing balls have also begun to spread in order to impart more wear resistance. . By the way, although the surface of a conventional ceramic bearing ball used for a general machine tool main spindle or the like often has minute defects, even if the cumulative area ratio is relatively large, the bearing performance is large. There was no problem, and little attention was paid to these flaws.

[0003]

However, since ceramic balls of bearings used as bearings for precision electronic equipment, for example, computer hard disk drives, are used at high speed rotation, their mechanical properties such as wear resistance are low. Needless to say, strict requirements are also placed on dimensional accuracy so that noise and vibration do not occur even at high rotations, and a minute defect often causes abnormal noise and vibration.

[0004] An object of the present invention is to provide a ceramic ball which can prevent or suppress the occurrence of troubles such as abnormal noise and vibration even when applied to a high-speed rotating bearing and the like, a method of manufacturing the same, and a ball bearing using the ceramic ball. Another object of the present invention is to provide a hard disk drive mechanism using the ball bearing.

[0005]

Means for Solving the Problems and Actions / Effects In order to solve the above-mentioned problems, a first structure of the ceramic ball of the present invention has a structure in which at least the surface layer portion is made of ceramic and the surface has a ground surface. And the sphericity is 0.08 μm
The following and the dimension 1 μm observed on the polished surface
The cumulative area ratio of the above defects is 1% or less, and the arithmetic mean roughness Ra of the polished surface is 0.012 μm or less.

In a second aspect of the present invention, the amount of voids formed in the polished surface is grasped from the viewpoint of a numerical formation density. Is a polished surface and the sphericity is 0.08 μm
The following and the dimension 1 μm observed on the polished surface
The average number of these defects per mm ² is 500
And the arithmetic mean roughness Ra of the polished surface is 0.012 μm or less.

The present invention also provides a ball bearing in which a plurality of the above-mentioned ceramic balls are incorporated as bearing rolling elements. Such ball bearings
For example, it can be suitably used as a bearing component of a rotating spindle of a hard disk as a magnetic storage medium. Further, the present invention provides a member (hereinafter, referred to as a rotating member) on the above-mentioned ball bearing, and one of an outer ring and an inner ring of the ball bearing being a fixed side and the other being a rotating side.
A hard disk drive mechanism comprising: a drive unit for rotating the drive; and a hard disk that rotates integrally with the rotating member.

In the present specification, the arithmetic average roughness Ra
Refers to the arithmetic average roughness measured by the method specified in JIS-B0601 (1994). According to this rule, when the arithmetic average roughness Ra is in the range of 0.012 μm or less, the cutoff value is 0.08 mm, and the evaluation length is 0.1 mm.
4 mm is a standard value.

Further, in this specification, as shown in FIG. 12, the relative cumulative frequency of the particles from the small particle size side is such that the particles to be evaluated are arranged in the order of the particle size, and When counting the frequency of particles from the particle size side, assuming that the cumulative frequency up to the particle size of interest is Nc and the total frequency of the particles to be evaluated is N0, nrc = (Nc / N0) × 100 (%) Means the relative frequency nrc represented by The X% particle size refers to a particle size corresponding to nrc = X (%) in the above-described arrangement. For example, a 90% particle size is defined as nrc = 90
(%).

On the other hand, the size of a crystal grain or a defect is
As shown in FIG. 18, when various circumscribed parallel lines not crossing the inside of crystal grains or defects are drawn on the polished surface structure by SEM or the like while changing the positional relationship with the crystal grains or defects, , The average of the minimum spacing dmin and the maximum spacing dmax of the parallel lines (ie,
d = (dmin + dmax) / 2).

Further, the sphericity refers to a maximum value of a radial distance between each point on the surface of the smallest spherical ceramic ball circumscribing the surface of the ceramic ball. Further, the difference in diameter means a difference between the maximum value and the minimum value of the diameter of one ceramic ball.

As a result of extensive studies by the present inventors, ceramic balls used for ball bearings have a sphericity of not more than 0.08 μm and have a dimension of not less than 1 μm observed on a polished surface. Keep the cumulative area ratio of defects at 1% or less, or 1 mm
By keeping the average existence number per ² to 500 or less, when used in precision electronic equipment such as a computer hard disk drive, for example, high-speed rotation (for example, 5
Even when used at a speed of 400 to 12000 rpm, generation of noise and vibration is extremely effectively prevented or suppressed, and the life thereof can be ensured for a long period of time. Further, by ensuring the arithmetic average roughness Ra of the polished surface in the range of 0.012 μm or less, the sphericity or the cumulative area ratio of defects and / or the value of the average number of existing particles can be combined with the above-mentioned value. The problem of occurrence of vibration or abnormal noise can be more effectively prevented.

When the sphericity exceeds 0.08 μm or the arithmetic average roughness Ra of the polished surface exceeds 0.012 μm, when the ceramic ball is incorporated into the ball bearing and used, vibration and abnormal noise may occur. Is more likely to occur. The sphericity is more desirably 0.03 μm or less, and the arithmetic average roughness Ra of the polished surface is more desirably 0.1 mm.
It is preferable that the thickness be not more than 01 μm. In addition, for the same reason as the sphericity, the value of the unequal ball diameter is 0.10 μm or less,
Desirably, the thickness is 0.07 μm or less.

Further, the dimension 1 μm observed on the polished surface
If the cumulative area ratio of defects of m or more exceeds 1% or the average number of existing defects per 1 mm ² exceeds 500,
Even if the surface roughness Ra of the polished surface is maintained at 0.012 μm or less, vibration and abnormal noise are likely to occur when ceramic balls are used in a ball bearing.

In the polishing process, for example, a method in which ceramic crystal grains on the polished surface are dropped by fixed abrasive grains of diamond to gradually increase the processing accuracy of the polished surface can be adopted. Therefore, the size of the crystal grains of the ceramic affects the limit of the processing accuracy of the polished surface. In the bearing ball, particularly precise surface polishing is performed, and as evident from the above-described ranges of sphericity and uneven diameter, extremely strict processing accuracy is required for the polished surface. At this time, depending on the distribution of crystal particle diameters of the silicon nitride-based ceramic material, the falling ceramic crystal particles may damage the polished surface and deteriorate the processing accuracy of the polished surface. When the processing accuracy of the polished surface is deteriorated, as described above, it causes abnormal vibration and abnormal noise in bearings and the like.

In this case, attention is paid particularly to 90% particle diameter d90% in the ceramic sintered body structure as a raw material.
By setting the 0% particles 90 in the range of 2.4 μm or less, it becomes easy to secure the arithmetic average roughness Ra of the polished surface in the range of 0.012 μm or less, and it is possible to achieve extremely high polishing processing accuracy. it can. That is, by forming the crystal grain diameter by sintering as a whole small, specifically, by forming 90% of the crystal grains in a range of 2.4 μm or less, the ceramic crystal grains on the polished surface by the abrasive grains are formed. Is relatively reduced, and the size of the dropped particles is also reduced. Therefore, there is almost no fear that the dropped ceramic crystal particles further damage the polished surface and deteriorate the processing accuracy of the polished surface, so that a ceramic ball with higher surface accuracy can be obtained.

In the particle size distribution of the crystal particles, the particle size at which the relative cumulative frequency from the small particle size side becomes 50% (hereinafter, referred to as the particle size).
D50% is 1.3 μm.
It can be adjusted to the following range. 90% particle size d90
By defining the 50% particle diameter d50% in the above range in addition to the%, the dropping of particles during polishing becomes more difficult to occur,
It becomes easier to maintain the arithmetic mean roughness Ra of the polished surface in a range of 0.012 μm or less.

Here, when the particle size distribution of the crystal particles is formed symmetrically about the average particle size, for example, as a normal distribution, the 50% particle size d50% is equal to the average particle size. Generally, the 50% particle diameter d50% is given as a numerical value approximating the average particle diameter, and can be replaced by the average particle diameter. However, there is a slight difference between the two in the meaning of the particle diameter distribution. . That is, 50% particle diameter d50%
Is determined by the relative cumulative frequency from the small particle size side, so that it is adjusted within a more severe range without being affected by sporadically appearing large crystal grains. On the other hand, since the average particle diameter is the average of all the target crystal grains, it is adjusted in a relatively large range under the influence of sporadic large-diameter particles.

The maximum size of a defect observed on the polished surface is preferably 5 μm or less. Of the defects such as voids formed on the polished surface of the ceramic ball, the largest one is kept at 5 μm or less, so that sudden vibration and abnormal noise can be made more difficult to occur.

As a specific material of the ceramic to which the present invention is applied, for example, a silicon nitride ceramic having high strength and excellent wear resistance can be used. The silicon nitride-based ceramic is mainly composed of silicon nitride, and the remaining component is a sintering aid component, such as 3A, 4A, 5A, 3B (for example, Al (alumina, etc.)) in the periodic table. And 4B (for example, Si (silica or the like)) and at least one selected from the group consisting of Mg and 1 to 10% by weight in terms of oxide. These exist mainly in the oxide state in the sintered body.

When the sintering aid component is less than 1% by weight, it is difficult to obtain a dense sintered body, and defects such as voids tend to remain on the surface of the polished surface. As a result, the cumulative area ratio of defects having a dimension of 1 μm or more observed on the polished surface is set to 1% or less;
Alternatively, it is difficult to reduce the average number of defects observed per 1 mm ² to 500 or less, or to reduce the maximum size of defects to 5 μm or less. On the other hand, if the content exceeds 10% by weight, the strength, toughness or heat resistance will be insufficient, and in the case of a sliding part, the wear resistance will be reduced.
The content of the sintering aid component is desirably 2 to 8% by weight. In the present invention, “main component” (also synonymous with “subject” or “mainly”) means that the content of the component in the substance of interest is 5% unless otherwise specified.
0% by weight or more.

The sintering aid component of group 3A includes S
c, Y, La, Ce, Pr, Nd, Sm, Eu, Gd,
Tb, Dy, Ho, Er, Tm, Yb, and Lu are generally used. The content of these elements R is RO only for Ce.
₂ and others are calculated using R ₂ O ₃ type oxides. Of these, oxides of heavy rare earth elements of Y, Tb, Dy, Ho, Er, Tm, and Yb are preferably used because they have the effect of improving the strength, toughness, and wear resistance of the silicon nitride sintered body. used. In addition, magnesia spinel, zirconia and the like can also be used as a sintering aid.

The structure of the silicon nitride-based sintered member has a form in which main phase crystal grains containing silicon nitride as a main component are bonded by a vitreous and / or crystalline binder phase. The main phase has a β conversion of 70% by volume or more (preferably 90% by volume).
(% By volume or more) of Si ₃ N ₄ phase. In this case, the Si ₃ N ₄ phase is a phase in which a part of Si or N is substituted by Al or oxygen, and a phase in which metal atoms such as Li, Ca, Mg, and Y are solid-dissolved. There may be. For example, it can be exemplified Sialon which is expressed by the following general formula; _{beta-sialon: Si 6-z Al z O} z N 8-z (z =
0-4.2) α-sialon: M _x (Si, Al) ₁₂ (O, N)
₁₆ (x = 0 to 2) M: Li, Mg, Ca, Y, R (R is a rare earth element excluding La and Ce).

The above-mentioned sintering aid component mainly constitutes a binder phase, but a part thereof may be taken into the main phase. The binder phase may contain unavoidable impurities, for example, silicon oxide contained in the silicon nitride raw material powder, in addition to components intentionally added as a sintering aid.

It is desirable that the silicon nitride powder used as a raw material has an α conversion (a ratio of α silicon nitride in the total silicon nitride) of 70% or more, and a rare earth element, At least one element selected from the group consisting of elements of groups 3A, 4A, 5A, 3B and 4B is 1 to 10 in terms of oxide.
% By weight, preferably 2 to 8% by weight. In addition, at the time of compounding the raw materials, in addition to oxides of these elements, compounds which can be converted into oxides by sintering, for example, carbonates and hydroxides may be added.

In addition to the silicon nitride ceramics, ceramics such as alumina (aluminum oxide), zirconia (zirconium oxide), and silicon carbide can be used. The metal cation component is Ti, Zr, Nb, Ta in alumina or zirconia ceramic.
And a composite ceramic material containing a conductive inorganic compound phase of at least one of W and W.
Such a composite ceramic material is formed by mixing a powder that is a source of a conductive inorganic compound phase with a green body powder for forming an alumina ceramic or a zirconia ceramic, and molding the same by the same rolling granulation method as described above. Can be obtained by firing. By containing the conductive inorganic compound phase, conductivity can be imparted to the alumina or zirconia ceramic material, and the ceramic material can be subjected to electric discharge machining such as wire cutting. Further, by providing conductivity, an antistatic effect can be achieved.

The conductive inorganic compounds include Ti, Zr, Nb,
Metal nitride, metal carbide, metal boride, metal carbonitride, and / or tungsten carbide containing at least one of Ta as a metal cation component can be used. Specifically, titanium nitride, titanium carbide, Examples include titanium boride, tungsten carbide, zirconium nitride, titanium carbonitride, and niobium carbide. Note that the content of the conductive inorganic compound phase is 20% in order to sufficiently improve the conductivity while securing the strength and the fracture toughness value of the composite ceramic material.
It is preferable to set to 60% by volume. Further, in order to further impart toughness to the alumina ceramic, a composite ceramic material containing zirconia ceramic may be used. Such a composite ceramic material uses a ceramic powder in which the highest content ceramic component is one of alumina and zirconia, and the second highest content ceramic component is the other of alumina and zirconia.
It can be obtained by molding and firing by the same rolling granulation method as described above. The amount of the zirconia ceramic mixed with the alumina ceramic is preferably 5 to 60% by volume.

Next, the method for producing a ceramic ball of the present invention will be described in more detail. In the present invention, it is essential that the arithmetic average roughness Ra of the polished surface be 0.012 μm or less, and a defect having a dimension of 1 μm or more observed on the polished surface has a cumulative area ratio of 1% or less, Alternatively, it is necessary to keep the average number of defects observed per 1 mm ² to 500 or less. For that purpose, it is necessary to increase the density of the powder compact as much as possible, specifically, to secure a relative density value of 61% or more. However, for example, the diameter is less than 8 mm, especially 5 m
m or smaller diameter ceramic balls,
It is difficult to achieve the above-mentioned high density only by the die pressing method, and it is not sufficient to use the cold isostatic pressing (CIP) method together.

As a result of intensive studies, the present inventors have found that it is effective to employ a rolling granulation method in order to increase the density of a compact having a small diameter. Specifically, the method includes coexisting a molding base powder including a ceramic raw material powder and a molding core in a granulation container, and rolling the molding core in that state, while surrounding the molding core. The base powder for molding is adhered and agglomerated in a spherical shape to give a relative density of 6
A rolling granulation forming step of obtaining a spherical shaped body of 1% or more, a step of firing the spherical shaped body, and a step of polishing the surface of the fired body to obtain a ceramic ball.
In the rolling granulation method, it is difficult to obtain a compact having a high sphericity by conventional press molding. A compact having a diameter of 8 mm or less, particularly a small diameter of 5 mm or less can be easily formed at a high density. Can be manufactured. In addition, the production efficiency for obtaining a spherical molded product is high, and in addition, since a strip-shaped unnecessary portion is not generated in the molded product as in press molding, the problem of an increase in polishing allowance can be avoided.

In this case, it is necessary to employ a method in which a liquid mainly composed of a liquid molding medium is supplied to the molding core, and a molding powder is adhered thereto to obtain a spherical molding. It is effective in increasing body density. The liquid molding medium may be, for example, an aqueous solvent such as water or an aqueous solution in which an additive is appropriately added to water, but is not limited thereto.For example, an organic solvent may be used. . According to the method, when the liquid molding medium and the molding base powder adhere to the uneven portions existing on the surface of the molded body, the powder particles adhere while closely rearranging due to the osmotic pressure of the liquid molding medium. Therefore, it is considered that the density of the molded body can be increased. In order to enhance such an effect, it is desirable to spray the liquid molding medium directly on the molded body. In addition, the step of spraying the liquid forming medium may be performed over the entire period of the forming step (for example, the rolling granulation step), or may be performed only during a part of the forming step (for example, only the final stage). Is also good. The liquid forming medium may be supplied continuously or intermittently.

When the tumbling granulation method is adopted, it is more effective to use the following as the molding base powder. That is, the average particle diameter measured by a laser diffraction type particle size analyzer is 0.3 to 2 μm, and 90%
A molding base powder having a particle size of 0.7 to 3.5 μm and a BET specific surface area of 5 to 13 m ² / g is used.

The average particle size and the 90% particle size measured by a laser diffraction type particle size meter belong to the above range, and the BET
By using a molding base powder having a specific surface area value within the above range, defects such as uneven density and discontinuous boundaries due to unevenness of the powder are less likely to occur, and as a result, deformation and deformation due to uneven shrinkage of the sintered body are reduced. The occurrence rate of defects due to cracking or chipping can be greatly reduced. The measurement principle of a laser diffraction type particle sizer is well known, but in brief, a sample powder is irradiated with laser light, the diffracted light by the powder particles is detected by a photodetector, and the diffracted light obtained from the detected information is obtained. The particle size can be known from the scattering angle and the intensity of the particles.

Here, as shown schematically in FIG. 11, a plurality of primary particles of the molding base powder made of a ceramic raw material are formed due to various factors such as the action of an added organic binder and the action of electrostatic force. Often agglomerated to form secondary particles. In this case, the measurement by the laser diffraction particle sizer does not cause a large difference between the diffraction behavior of the incident laser light by the aggregated particles and the diffraction behavior by the isolated primary particles. It cannot be distinguished from each other whether it is the particle size of the particles or the particle size of the aggregated secondary particles. That is, the particle diameter measured by this method is a value reflecting the secondary particle diameter D in FIG. 11 (in this case, isolated primary particles that do not cause aggregation are also regarded as secondary particles in a broad sense). In addition, the average particle diameter or 90% particle diameter calculated based on this reflects the average particle diameter or 90% particle diameter of the secondary particles.

On the other hand, the specific surface area of the molding base powder is measured by an adsorption method. Specifically, the specific surface area can be determined from the amount of gas adsorbed on the powder surface. In general, an adsorption curve showing the relationship between the pressure of a measurement gas and the amount of adsorption is measured, and a known BET formula (collection of the inventors, Brunauer, Emmett, Teller) for multi-molecule adsorption is obtained. By applying, the adsorption amount vm when the monomolecular layer is completed is obtained, and the BET specific surface area value calculated from the adsorption amount vm is used. However, when approximately equivalent results are obtained, a simple method that does not use the BET equation,
For example, a method of directly reading the adsorption amount vm of the monolayer from the adsorption curve may be adopted. For example, when a section where the adsorption amount is substantially proportional to the gas pressure appears in the adsorption curve, there is a method of reading the adsorption amount corresponding to the end point on the low pressure side of that section as vm (The Journal of American Chemical Society, 57
Volume (1935), p. 1754, Brunauer and Emet
t). In any case, in the specific surface area value measurement by the adsorption method, the gas molecules to be adsorbed also penetrate into the secondary particles and cover the surface of the individual primary particles constituting the secondary particles, and as a result, the specific surface area value is This reflects the specific surface area of the primary particles, and thus the average value of the primary particle diameter d in FIG.

The BET ratio reflecting the primary particle diameter is set so that the compacted ceramic powder can sufficiently promote the densification of the ceramic sintered body and obtain a sintered body having few defects and sufficient strength. The surface area value is set to be as small as 5 to 13 m ² / g. The important point is that the average particle diameter or 90% particle diameter by a laser diffraction type particle size analyzer reflecting the secondary particle diameter is 0.3 to 2 μm or 0.7 to 3.5 μm, respectively, and the spray drying method is used. Is set to a small value of about 1/10 or less as compared with the molding base powder obtained by the above method. This means that the agglomeration state as secondary particles in the green body powder for molding, and thus the local coarseness of the particle filling is eliminated as much as possible, and by adopting such a range of the particle diameter, the molded article finally obtained is obtained. This makes it difficult for the powder to be biased.

The above-mentioned average particle diameter of the base powder for molding exceeds 2 μm, or the 90% particle diameter is 3.5 μm.
If it exceeds, the powder tends to be biased in the molded body,
Failure such as deformation, cracking or chipping of the sintered body due to uneven shrinkage is likely to occur. On the other hand, the average particle diameter is less than 0.3 μm, or the 90% particle diameter is 0.7 μm.
Fine powders of less than a considerable amount of time are required for preparation (for example, pulverization time), which leads to an increase in cost due to a reduction in production capacity.
The average particle diameter of the green body powder for molding is preferably 0.1.
The diameter is preferably 3 to 1 μm, and the 90% particle diameter is desirably 0.7 to 2 μm.

On the other hand, when the BET specific surface area value of the green body powder for molding is less than 5 m ² / g, the primary particle diameter becomes too large, the uniformity of sintering is impaired, and defects occur in the obtained spherical sintered body. The strength decreases. On the other hand, a green body powder for molding having a BET specific surface area value of more than 13 m ² / g requires a considerably long time for preparation (for example, pulverization time), which leads to an increase in cost due to a reduction in production capacity. In addition, the BET of the base powder for molding
The specific surface area value is desirably 5 to 10 m ² / g.

The step of preparing the molding base powder as described above includes, for example, a step of mixing a ceramic powder and a sintering aid powder together with a solvent to prepare a powder, and a step of preparing a powder in the middle of the hot air flow passage. An aggregate of dry media formed in a granular or lump shape of ceramic or metal is arranged in a flowable or vibrable state within a predetermined space range, and the dried media aggregate is passed through hot air. By flowing or vibrating in a spatial range and supplying the fluid to the flowing or vibrating dry media assembly,
The method includes a drying step of evaporating the solvent while mixing the plasma with a drying medium, and a collecting step of guiding the forming base powder obtained by the drying together with hot air to a downstream side of the drying medium assembly to collect the powder. be able to.

In the above method, the slurry is supplied to the aggregate of the drying media, and the slurry is dried by hot air to become a powder and adhere to the surface of the media to form a powder aggregation layer. Then, the dried media on which the powder agglomeration layer is formed by the flow of the hot air vibrate or flow, and collide with each other or cause friction. At this time, the powder agglomeration layer adhering to the media surface is crushed, and is blown away and collected while alleviating the aggregation state. Thereby, the base powder for molding having the above-mentioned particle size range can be obtained easily and with high efficiency. As the drying medium, it is preferable to use a ceramic medium that is hard to wear as much as possible. For example, if a medium mainly composed of alumina, zirconia, or a mixed ceramic thereof is used, the medium is temporarily worn and the molding base material is used. Even if it is mixed in the powder, since it functions as a sintering aid component, the influence of the mixing can be reduced.

In the drying step in the step of preparing the green body powder for molding, the hot air flow path can be formed to include a vertically arranged hot air duct, and hot air is allowed to pass through the middle of the hot air duct, and the drying medium is dried. Can be formed as a media holding portion composed of a gas circulating body such as a net that does not allow the passage of the medium. In this case, the slurry can be supplied from above to the dry media aggregate held on the media holding unit. Further, the hot air can be circulated in the hot air duct so as to move the drying medium upward from the lower side of the drying media assembly while moving the drying medium upward, and the powder after drying passes through the hot air duct together with the hot air to the downstream side. Can be collected by the collection unit arranged in the storage area.

According to this method, a cycle is repeated in which the dry medium is blown up by the hot air blown from below, moves up, and is dropped on the integrated body. And can be added relatively uniformly. In addition, coarse particles among the crushed aggregated particles are not blown off by the hot air, but are returned to the dry media aggregate again and continuously crushed, so that coarse particles that cause powder unevenness or the like in the subsequent molding process are obtained. The generation of secondary particles can be further reduced.

In the tumbling granulation, the green body powder for molding and the molding nucleus are put into a granulation container, and the molding nucleus is rolled in the granulation container, while being rolled around the molding nucleus. It is desirable to obtain a spherical molded body by adhering and aggregating the molding base powder in a spherical shape. That is, in the granulation container, for example, while rolling the molding core on the molding base powder layer,
By adhering and aggregating the molding base powder in a spherical shape around the molding core to obtain a spherical molded body, the density of the aggregated layer of the molding base powder growing around the molding core is remarkably increased. In addition, defects such as pores and cracks due to bridging of the powder particles are reduced in the formed aggregated layer. As a method of rolling the molded core (or growing compact) in the granulation container, a method of rotating the granulation container is simple. For example, the method is based on a principle similar to that of the vibration type barrel polishing apparatus. Vibration may be applied to the grain container, and the molded core may be rolled based on the vibration.

In this case, the ceramic ball obtained by firing has a cross section substantially passing through the center, and
A core portion that can be distinguished from the outer layer portion is formed. The term “identifiable” as used herein does not only mean that it is visually identifiable, but also a specific physical property value (for example, density or hardness) that causes a difference between the core and the outer layer. , Etc.) is included.

By adopting a sintered body structure having such a structure, a spherical ceramic sintered body having a high density and a high strength can be realized with a small defect formation rate in the surface layer, which is a key to improving the performance of bearings and the like. Specifically, if the above-mentioned spherical molded body produced by the method of the present invention is fired, the resulting spherical ceramic sintered body is, for example, when a cross-section passing substantially through the center is polished and this is magnified and observed, At the center thereof, a core portion derived from the molded core is formed so as to be distinguishable from an outer layer portion derived from an aggregated layer having high density and few defects.

Next, when the ceramic ball is made of silicon nitride ceramic, the ratio A / W between the surface area A (unit: mm ² ) of the ball and the weight W (unit: g) is particularly 30.
When it is formed into a small diameter sphere of 0 or more, its density is 3.2 g.
/ Cm ³ or more to reduce the pores of the polished surface or reduce the arithmetic average roughness, and to improve the value of sphericity and uneven diameter. Indispensable. In order to manufacture such a silicon nitride ceramic ball, a molding base powder mainly composed of a silicon nitride powder and a sintering aid powder is mixed with a surface area A ′ (unit: mm ² ) and weight. The ratio A ′ / W ′ to W ′ (unit: g) is 350 or more, and the density is 2.0 to
It is effective to form the sphere into a shape of 2.5 g / cm ³ .

In particular, in the case of small-diameter, high-grade ceramic balls premised on application to bearings and the like for electronic precision equipment, the HIP method (for example, a sintering pressure of 200 atm or more, which is effective for increasing the density of the sintered body, usually 1000-200
0 atm) can be employed, but since the surface of the sintered body is easily hardened, polishing becomes somewhat difficult, which may affect securing high precision sphericity or irregular diameter. Therefore, when considering the production of silicon nitride ceramic balls by normal-pressure or gas-pressure firing, the surface of which is hard to harden, the bearing finished ball in which the density decreases due to the inhibition of sinterability and the resulting deterioration in performance such as wear resistance is remarkable. Is the surface area A (unit: mm ² ) and the weight W (unit:
g) and the ratio A / W was found to be 230 or more. At the stage of the molded body before firing, the density is 2.
Considering that the green body powder for molding is formed to be 0 to 2.5 g / cm ³ , the surface area A ′ (unit: mm ² )
Ratio A ′ / W ′ between weight and weight W ′ (unit: g) is approximately 350
This corresponds to the above.

Further, as a result of further study, it was found that the green body powder was molded so that the density (density) of the green body before firing was within the range of 2.0 to 2.5 g / cm ^3. It has been found that even a completed ball having a small diameter such that / W is 300 or more can be sintered at a sufficiently high density by normal pressure or gas pressure firing. The surface of the pre-polished bearing element ball obtained by the firing is hardly hardened. In addition, the formation of the auxiliary component gradient layer described above makes it possible to perform precise polishing extremely easily and efficiently. . The A / W is 230 or more, and the density is increased to 3.2 g / cm ³ or more. For example, the sphericity is 0.08 μm or less, and the diameter difference is 0.10 μm or less. It is possible to easily realize a high-performance and high-accuracy small-diameter silicon nitride-based ceramic ball, which was practically impossible with the conventional method.

In the present specification, the gas pressure firing is performed in the following manner.
The calcination is performed in an atmosphere containing at least nitrogen of at least 200 atm or more, and the normal pressure calcination is performed in an atmosphere of at least nitrogen of 1 atm or less. By setting the upper limit of the atmospheric pressure of the gas pressure firing to 200 atm, excessive hardening of the surface of the sintered body can be effectively prevented, and furthermore, the sphericity or the diameter difference of the polished silicon nitride ceramic ball is further improved. It becomes easy to secure.

The above effects are more effectively exhibited when the A / W exceeds 500. On the other hand, A / W is 500
If it exceeds 0, it becomes difficult to produce a high-density spherical molded body. Therefore, it is desirable to target a finished ball having an A / W of less than that, more preferably 2,000 or less. The above range of A / W corresponds to the range of the ball diameter of 0.5 to 6 mm (preferably 1 to 6 mm) in consideration of the density of the silicon nitride ceramic.

When the density of a bearing element ball or a completed bearing ball (hereinafter, collectively referred to as a bearing ball) is less than 3.2 g / cm ^3, wear resistance and strength are insufficient, and especially high-speed rotation is insufficient. When applied to the required bearings for precision electronic devices such as computer hard disks, sufficient performance and lifetime cannot be ensured. On the other hand, the upper limit of the density of the silicon nitride-based ceramic is a theoretical density value determined for each ceramic composition. The closer to the theoretical density value as much as possible by selecting firing conditions, etc., the more advantageous in improving wear resistance or strength. It is.

When the theoretical density value can be measured or estimated, the density of the bearing ball can be represented by the ratio of the apparent density to the theoretical density, that is, the relative density. In this case, the relative density is preferably 99.0% or more, more preferably 99.5% or more. Further, that the relative density is less than 100% means that fine voids remain in the structure of the sintered body and the density of the sintered body is reduced. Therefore, the existence ratio of the voids in the sintered body can be a parameter reflecting the relative density of the sintered body.

More specifically, it is desirable that the average number of voids having a dimension of 1 μm or more per mm ^{2 of the} cross section observed in the structure of the cross section passing substantially through the center of the ball satisfy 500 or less. If the number exceeds 500, wear resistance and strength when applied to bearing balls and the like are insufficient, and particularly when applied to bearings for precision electronic devices such as computer hard disks requiring high-speed rotation, Sufficient performance and lifetime cannot be secured.
Also, when the voids having a dimension of 1 μm or more increase, these open pores (open pores) are formed on the surface of the polished ceramic ball.
In some cases, causing abnormal noise and vibration.

The surface area A ′ (unit: mm ² ) and the weight W ′ (unit:
g) is less than 350, it is impossible to make the A / W value of the finally obtained bearing ball 230 or more (strictly speaking, the bearing element ball and the bearing ball). Although there is a difference in size between the finished ball and the polishing allowance, the polishing allowance is sufficiently smaller than the diameter of the sphere and does not significantly affect the value of A / W. Note that the upper limit of A ′ / W ′ is about 2560 (preferably 1650) corresponding to the upper limit of A / W of the obtained ceramic ball of 5000 (preferably 2000).

The spherical molded body has a density of 2.0 g / cm.
^If it is less than ³ , densification does not proceed during sintering by gas pressure sintering or normal pressure sintering, resulting in deterioration of performance such as abrasion resistance, or a true sphere of 0.10 μm or less due to residual voids (pores). The degree and diameter cannot be achieved. On the other hand, if the density of the molded body exceeds 2.5 g / cm ³ , the stress remaining in the molded body after molding becomes too high, which may cause the molded body to collapse or promote cracks and cracks during firing. This may cause a problem that lowers the yield.
In addition, the density of the spherical molded body is more preferably 2.15 to 15.
It is preferably 2.38 g / cm ³ .

[0055]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention will be described below with reference to a case where a silicon nitride ceramic is used as a material. First, it is desirable to use a silicon nitride powder as a raw material having an α ratio of 70% or more, and a rare earth element, 3A, 4A, 5A, 3B, or an element group of 4B as a sintering aid. At least one of them is converted to oxide
-10% by weight, preferably 2-8% by weight. In addition, at the time of compounding the raw materials, in addition to oxides of these elements, compounds which can be converted into oxides by sintering, for example, carbonates and hydroxides may be added.

Hereinafter, an example of a method of preparing a molding base powder and a molding method will be described, but as described above, the invention is not limited thereto. FIG. 8 shows an embodiment of an apparatus used in the step of preparing a green body powder for molding. In the apparatus, the hot air flow passage 1 is formed to include a vertically arranged hot air duct 4, and a gas flow member that allows hot air to pass therethrough and does not allow the drying medium 2 to pass therethrough, in the middle of the hot air duct 4. For example, a media holding unit 5 formed of a net or a perforated plate is formed. On the medium holding portion 5, a dry medium 2 composed of ceramic spheres mainly composed of alumina, zirconia, or a mixed ceramic thereof is used.
Are stacked to form a layered dry media assembly 3.

On the other hand, the raw material is in the form of a slurry 6 obtained by adding a water-based solvent to a blend of silicon nitride powder and sintering aid powder and wet-mixing (or wet-mixing / grinding) with a ball mill or an attritor. Be prepared in In this case, the size of the primary particles is such that the BET specific surface area value is 5 to 13 m ² / g.
It is adjusted so that

As shown in FIG. 9, the dry media assembly 3
On the other hand, the hot air is circulated in the hot air duct 4 so as to move upward from the lower side of the medium holding portion 5 while moving the drying medium 2 up. On the other hand, as shown in FIG.
Is pumped up from the slurry tank 20 by the pump P, and is dropped and supplied to the dry media assembly 3 from above. Thereby, as shown in FIG. 10, the slurry is dried by the hot air and adheres to the surface of the drying medium 2 in the form of the powder aggregation layer PL.

Then, due to the circulation of hot air, the drying medium 2 repeatedly vibrates and falls and strikes each other, and the powder agglomeration layer PL is pulverized into the base powder particles 9 for molding by the friction caused by the impact. . The crushed base powder particles 9 for molding include those in the form of isolated primary particles, but are mostly secondary particles in which the primary particles are aggregated. The molding base powder particles 9 having a particle size of a certain size or less flow downstream along with the hot air (FIG. 8). On the other hand, the crushed particles larger than a certain size are not blown off by the hot air but fall again to the dry media assembly 3 and are further crushed between the media.

In this way, the molding base powder particles 9 which have flowed to the downstream side together with the hot air pass through the cyclone S to the collecting section 2.
1 is collected as a molding base powder 10. The recovered base material powder for molding 10 has an average particle size of 0.3 to 2 μm measured by a laser diffraction type granulometer, and 90%
The particle diameter is 0.7 to 3.5 μm, and the BET specific surface area is 5 to 13 m ² / g. Since the definition of the 90% particle size is as described above, a detailed description is omitted here.

In FIG. 8, the diameter of the drying medium 2 is
It is set appropriately according to the flow cross-sectional area of the hot air duct 4. If the diameter is insufficient, the impact force on the powder agglomeration layer formed on the medium is insufficient, and a base powder for molding having an intended particle diameter may not be obtained. On the other hand, if the diameter is too large, even if hot air is circulated, it is difficult for the dry medium 2 to move, so similarly the striking force is insufficient, and there is a case where a molding base powder having an intended particle diameter cannot be obtained. . Note that it is desirable to use a dry medium 2 having a uniform size as much as possible in order to form an appropriate gap between the media and promote the movement of the medium during hot air circulation.

The filling depth t 1 of the drying medium 2 in the drying medium assembly 3 is appropriately set in accordance with the flow rate of the hot air and within a range where the flow of the medium 2 occurs without excess or deficiency. If the filling depth t1 is too large, the flow of the drying medium 2 becomes difficult, and the impact force is insufficient, so that there is a case where a base powder having a desired particle diameter cannot be obtained. On the other hand, if the filling depth t1 is too small, the amount of the dry medium 2 is too small, and the frequency of impact is reduced, leading to a reduction in processing efficiency.

Next, the temperature of the hot air is appropriately set within a range in which the drying of the slurry proceeds sufficiently and no problems such as thermal deterioration of the powder occur. For example, when the solvent of the slurry is mainly water, if the hot air temperature is less than 100 ° C., the supplied slurry does not sufficiently dry, and the moisture content of the obtained molding base powder becomes too high. Aggregation tends to occur, and a powder having an intended particle size may not be obtained.

Further, the flow velocity of the hot air is appropriately set within a range in which the drying medium 3 is not blown to the collecting section 21. If the flow velocity is too low, the flow of the drying medium 2 becomes difficult, and the impact force is insufficient, so that there is a case where a molding base powder having an intended particle diameter cannot be obtained. On the other hand, if the flow velocity is too high, the drying medium 2 rises too high and the collision frequency is rather reduced, leading to a reduction in processing efficiency.

The molding base powder 10 thus obtained is
It can be formed into a spherical shape by the rolling granulation molding method. That is, as shown in FIG. 1, the green body powder for molding 10 is charged into a granulation container 132, and as shown in FIG. 2, the granulation container 132 is driven to rotate at a constant peripheral speed. In addition, the water W is supplied to the molding base powder 10 in the granulation container 132 by, for example, spraying or the like. As shown in FIG. 5, the injected molding base powder agglomerates in a spherical shape while rolling on an inclined powder layer 10 k formed in a rotating granulation container to form a compact 80. The operating conditions of the tumbling granulator 30 are adjusted so that the relative density of the obtained compact G is 61% or more. Specifically, the rotation speed of the granulation container 32 is adjusted at 10 to 200 rpm, and the water supply amount is
Moisture content in the finally obtained molded body is 10 to 20% by weight
It is adjusted so that If a green body powder for molding in which the above-mentioned kind of sintering aid powder is blended in a range of 1 to 10% by weight is used, the density of the green body is set to 2.
It can be secured to about 0 to 2.5 g / cm ³ . As for such attained density, for example, the ratio A ′ / W ′ of the surface area A ′ to the weight W ′ of the obtained molded body G is 350 or more (for example, the diameter is 6.7).
3 mm or less) can be sufficiently achieved, and when this is fired, the ratio A / A of the surface area A to the weight W can be obtained.
W is 300 or more (diameter is 6.35 mm or less, for example, 5 mm
The following bearing element balls can be obtained.

At the time of tumbling granulation, it is desirable to put the molded core 50 into a granulation container 132 as shown in FIG. In this way, as shown in FIG. 5 (a), while the molding core 50 is rolling on the molding base powder layer 10k, the molding core 50 is formed around the molding core 50 as shown in FIG. 5 (b). The base powder 10 adheres and agglomerates in a spherical shape to form a spherical compact 80 (rolling granulation step). By sintering the molded body 80, a bearing element ball 90 is obtained as shown in FIG.

The molding core 50 is made mainly of a ceramic powder like a molding core 50a shown in FIG. 3A, and is made of a material having a composition similar to that of the molding base powder 10, for example. (However, a ceramic powder (an inorganic material powder) different from the ceramic powder constituting the main body of the molding base powder may be used), but the core acts as an impurity source on the finally obtained ceramic ball 90. It is desirable because it is difficult to do. However, if there is no concern that the core component can be diffused to the surface layer of the ceramic ball 90, the core is made of a ceramic material different from the ceramic powder (inorganic material powder) that forms the main body of the molding base powder. It is also possible to use a powder or a metal nucleus 50d or a glass nucleus 50e as shown in FIGS. 3 (d) and 3 (e). It is also possible to form the nucleus with a material that disappears by thermal decomposition or evaporation during firing, for example, a polymer material such as wax or resin. The molded core may have a shape other than a spherical shape as shown in FIG. 3B or 3C, for example, but as shown in FIG. Needless to say, it is desirable to increase the sphericity.

Although there is no particular limitation on the method of manufacturing the molded core, various methods such as those shown in FIG. First, (a)
Is a method in which the ceramic powder 60 is compression-molded with a die 51a and press punches 51b, 51b (of course, another compression method may be used) to obtain a core body 50. (B) shows a method in which a powder is dispersed in a molten thermoplastic binder to form a molten compound 63, which is spray-solidified to obtain a spherical core 50. (C)
Is a method of injecting a molten compound 63 into a spherical cavity of an injection mold to form a spherical core 50.
Further, (e) shows a method in which the molten compound 63 is dropped freely from a nozzle to form a sphere by surface tension, and is cooled and solidified in air to obtain the core 50. Also, a slurry comprising the raw material powder, the monomer (or prepolymer) and the dispersion solvent is dispersed as droplets in a liquid immiscible with the slurry, and the monomer or prepolymer is polymerized in that state to obtain a spherical molded body. On the other hand, there is also a method of using this as a core, while in FIG.
An agglomerate of powder is generated by rotating the container at a lower speed than at the time of growth of the molded body, and when an agglomerate of a sufficient amount and size is generated, the rotation speed of the container 132 is then increased,
The compact 80 may be grown using the aggregate as the core 50. In this case, it is not necessary to put the core body manufactured in a separate process as described above into the container 132 together with the green body powder 10 for molding.

The molded core body 50 obtained as described above
Can stably maintain its shape without collapse even when some external force acts. As a result, as shown in FIG. 5A, even when rolling on the green body powder layer for molding 10k, a reaction due to its own weight can be reliably received. FIG.
As shown in (e), it is considered that the powder particles rolled up when rolled can be pressed firmly against the surface, so that the powder is appropriately compressed and a dense layer 10a can be grown. In addition, it is also possible to perform rolling granulation without using a core. In this case, as shown in FIG. 5D, the agglomerate 100 corresponding to the nucleus has a slightly low agglomeration degree at the initial stage of molding and is weak. It is advisable to drop a little.

It is preferable that the size of the core body 50 be at least about 40 μm (preferably about 80 μm). If the core 50 is too small, the growth of the aggregated layer 10a may be incomplete. Further, if the core is too large, the thickness of the formed cohesive layer may be insufficient, and the sintered body may be liable to have a defect or the like. Therefore, the size is preferably set to, for example, 1 mm or less.

The molding core is made of a ceramic powder and a bulk density (for example, JIS-Z2504 (197)
It is desirable to use an aggregate agglomerated at a higher density than the apparent density specified in 9) in order to reliably receive the pressing force of the powder particles and promote the growth of the aggregate layer 10a. Specifically, it is preferable to use a material that has been agglomerated to 1.5 times or more the bulk density of the molding base powder. in this case,
It is sufficient that the particles are agglomerated to such an extent that they do not collapse due to rolling impact on the molding base powder layer 10k.

In order to achieve more stable growth of the compact, it is desirable to set the size of the core body 50 as follows according to the size of the compact to be obtained. That is, as shown in FIG. 5B, the size of the molded core 50 is represented by a diameter dc of a sphere having the same volume, while (of course, when the core 50 is spherical, Is equivalent to the dimension itself here), and dc is dc and dc / dg is 1/100 to 1 where dg is the diameter of the finally obtained spherical molded body.
/ 2 is set. dc / dg is 1/100
If it is less than 10%, there is a concern that the core is too small, the growth of the cohesive layer 10a is incomplete, or only the one having many defects can be obtained. On the other hand, if it exceeds １ ／, for example, if the density of the core body 50 is not so high, the strength of the obtained sintered body may be insufficient. Note that dc / dg is
Preferably 1/50 to 1/5, more preferably 1/20
It is preferable to adjust in the range of up to 1/10. The size dc of the molding core is preferably set to 20 to 200 times the average particle size when the average particle size of the molding base powder is viewed as a scale. The absolute value of the dimension dc is, for example, 50
It is preferable to adjust the thickness to 500 μm.

For example, if the molded body 80 is fired by the method described later, a silicon nitride ceramic elementary ball (hereinafter, also simply referred to as elementary ball) can be obtained. The sintering is performed by gas pressure sintering performed in an atmosphere containing at least nitrogen of 1 atm or more and 200 atm or less, or normal pressure sintering performed in an atmosphere of 1 atm or less containing at least nitrogen. The firing temperature is, for example, 1500 to 1800 ° C.
It is better to set within the range. If the firing temperature is lower than 1500 ° C., defects such as pores cannot be eliminated and the strength decreases, while if the temperature exceeds 1800 ° C.,
It is not preferable because the strength of the sintered body is reduced by the grain growth. This firing can also be performed by two-stage firing of primary firing and secondary firing. For example, the primary firing is performed under a normal pressure or a gas pressure of 10 atm or less containing nitrogen.
It is carried out in a non-oxidizing atmosphere at a temperature of 1800 ° C. or less, and it is desirable to carry out the treatment so that the relative density of the sintered body after the primary firing becomes 78% or more, preferably 90% or more. If the relative density of the sintered body after the primary firing is less than 78%, many defects such as pores tend to remain after the secondary firing, which is not preferable. Also,
The secondary firing is performed at a normal pressure or a gas pressure of 200 atm or less containing nitrogen in a non-oxidizing atmosphere at 1500 to 1800.
C. If the firing pressure exceeds 200 atm, the surface hardness of the resulting sintered elementary balls increases, making it difficult to perform polishing or the like, making it impossible to ensure the dimensional accuracy of the product balls.

Since the relative density of the elementary sphere obtained by sintering has been increased to 61% or more by the above-mentioned tumbling granulation method, densification is remarkably progressed, and voids and the like are also formed on the surface layer of the sphere. Defects are less likely to remain. The base ball is subjected to coarse polishing for dimensional adjustment and then precisely polished using fixed abrasive grains to obtain a silicon nitride ceramic ball as the ceramic ball of the present invention. In the ceramic ball, the cumulative area ratio of defects having a size of 1 μm or more observed on the polished surface is 1% or less, and the average number of formed defects per 1 mm ² is 500 or less. In addition, the arithmetic mean roughness Ra of the polished surface can be set to 0.012 μm or less, and the sphericity can be ensured to 0.08 μm or less. Further, the diameter difference can be secured to 0.10 μm or less.

As shown in FIG. 6, the elementary sphere 90 obtained by firing the spherical compact 80 obtained by the tumbling granulation method was polished at a cross section substantially passing through the center, and this was enlarged and observed. Sometimes, at its center, a core 91 derived from the molded core is
It is formed so as to be identifiable between the high-density outer layer portion 92 having few defects and originating from the aggregation layer. In the polished cross section, the nucleus portion 91 often exhibits a visually recognizable contrast with the outer portion in at least one of brightness and color. It is presumed that this is because the density ρe of the ceramic constituting the outer layer portion 92 is different from the density ρc of the ceramic constituting the core portion 91.
For example, when the molding core 50 (FIG. 5) has a lower density than the aggregated layer 10a, the density ρe of the ceramic forming the outer layer portion 92 becomes higher than the density ρc of the ceramic forming the core portion 91. In many cases, the outer layer portion 92 appears in a lighter tone than the core portion 91. The relative density of the outer layer portion 92 is
From the viewpoint of ensuring the strength and durability of the ceramic, the content is preferably 99% or more, and more preferably 99.5% or more.
In any case, by forming a sintered body structure in which the above structure appears in the polished cross section, the defect formation ratio of the outer layer portion 92 which is the key to the performance improvement of the bearing and the like is small (for example, to the extent that pores are not confirmed). A high-density and high-strength spherical ceramic sintered body is realized. However, when the firing proceeds uniformly, the sintered body may have a substantially uniform density in the radial direction from the surface layer to the center. Also, even if there is a difference in color tone and brightness between the core and the outer layer, there is almost no difference in density.
It may be possible. Further, when the sintering proceeds more uniformly, it may be difficult to visually confirm the concentric contrast in the core portion 91 or the outer layer portion 92.

As shown in FIG. 5 (b), the diameter of the molded core 50 is dc, and the diameter of the elementary sphere obtained by firing is dg.
Dc / dg is 1/100 to 1/2 (preferably 1
/ 50 to 1/5, more preferably 1/20 to 1/5), the cross section of the sintered body 90 in FIG. Alternatively, when a material that disappears due to evaporation, for example, a material composed of wax, resin, or a polymer material, the core portion 9 is used.
1 is a void portion) with the diameter D of a circle having the same area as
On the other hand, when the diameter of the ceramic sintered body is Dg, Dc / Dg is 1/100 to 1/2 (preferably 1 /
A tissue satisfying 50 to 1/5, more preferably 1/20 to 1/10) is exhibited. Dc / Dg is 1/50
If it is less than 10, the cohesive layer 10 a serving as the base of the outer layer portion 92 (FIG.
Defects are likely to occur in 1), which may lead to insufficient strength and the like. On the other hand, if it exceeds 1/5, the strength of the sintered body may be insufficient if the density of the core body 50 is not so high, for example. Dc / Dg is more preferably 1
It is preferable to adjust in the range of / 20 to 1/10.

As a state in which a visually recognizable contrast occurs between the core portion 91 and the outer layer portion 92 in the elementary sphere 90, for example, a difference in brightness or color tone is formed in the radial direction of the sphere and formed in the circumferential direction. A state that has not been performed can be exemplified. As a specific mode, a layered pattern 93 surrounding the core 91 may be formed concentrically on the outer side of the polished cross section. This is one of the characteristic structures observed when the rolling granulation method is adopted (naturally, it is also carried over to the polished ceramic balls).
The cause of formation can be estimated as follows. That is, FIG.
As shown in (a), the molded body 80 is made of the green body powder layer 1 for molding.
While the agglomeration layer 10a grows while rolling on 0k, the compact 80 does not always exist on the green compact powder layer 10k during the continuation of the tumbling granulation. That is, as shown in FIG. 7, due to the avalanche flow of the powder accompanying the rotation of the granulation container 132, when the powder reaches the lower side of the molding base powder layer 10 k, it falls into the molding base powder layer 10 k, and Of the base material powder layer for molding 10k
And rolls down again on the green body powder layer for molding 10k. When sunk into the molding base powder layer 10k, the surroundings are pressed down by the powder, and the impact due to rolling and falling is relatively unlikely to be applied, and the powder particles adhere relatively loosely. On the other hand, when rolling on the molding base powder layer 10k, an impact due to rolling and falling is applied, and the liquid spray medium W such as moisture is easily sprayed, so that the powder is easily tightened. Then, since the rolling on the molding base powder layer 10k and the dipping into the molding base powder layer 10k are periodically repeated, the adhesion form of the powder is also periodically changed. It is considered that the layer 10a has radial tightness, which appears as a slight difference in density or the like even after firing, resulting in the formation of a layered pattern 93 (when the difference in hydrophobicity is very small, In fact, it may not be possible to confirm the actual occurrence of the density in the accuracy level of the ordinary density measurement.) For example, it is considered that the above-mentioned layered pattern 93 is formed by alternately laminating concentric arc-shaped portions and remaining portions having a higher density than the concentric arc portions in the radial direction.

FIG. 13 shows a fractured surface of a spherical molded body 50 in which a spherical molded nucleus 50 was formed by spray drying using silicon nitride powder, and an agglomeration layer was formed therearound by a tumbling granulation method. It is a SEM observation image (100 times magnification).
The average relative density of this spherical compact is about 71%. It is clearly observed that a spherical molded nucleus (portion indicated as Core) is present in the center. On the other hand, FIG.
(A) to (d) show the cross-sections of the molded body calcined in a nitrogen atmosphere at 1200 ° C. (average relative density is about 74%) by optical microscope observation at various magnifications. (The magnification is indicated by the scale in the image. The observation surface is colored with iron oxide). At the center, a circular region (about 1/10 of the diameter of the calcined body, about 200 μm) corresponding to the molded core 50 is clearly observed. A concentric layered pattern is observed in the region corresponding to the aggregation layer 10a. FIGS. 15A to 15C are optical micrographs of the polished surface of the sintered body (the magnification is indicated by a scale in the image). The core portion 91 (the darkest portion in the center) schematically shown in FIG. 6 and the outer layer portion 92 (the layered pattern is observed concentrically or in a ring shape) are clearly distinguished by the color contrast. We can see that we can do it. The relative density of this sintered body is 99.9% or more.

As shown in FIG. 16, if the ceramic ball 43 obtained as described above is assembled between an inner ring 42 and an outer ring 41 made of metal or ceramic, for example,
A radial type ball bearing 40 is obtained. If the shaft SH is fixed to the inner surface of the inner ring 42 of the ball bearing 40,
The ceramic ball 43 is held rotatably or slidably with respect to the outer ring 41 or the inner ring 42. The sphericity of the ceramic ball 43, the arithmetic average roughness Ra of the polished surface, the cumulative area ratio of defects having a size of 1 μm or more observed on the polished surface, and 1 m
By average formation number of m ² per defect is adjusted to as described above, it is possible to significantly suppress the generation of vibration and noise even during high-speed rotation.

FIG. 17 is a longitudinal sectional view showing an example of the configuration of a hard disk drive mechanism using the ball bearing. The hard disk drive mechanism 100 includes a main body case 1
In the center of the inner surface of the bottom of 02, a cylindrical shaft holding portion 108 is formed so as to rise vertically, and a cylindrical bearing holding bush 112 is coaxially fitted inside the shaft holding portion 108. The bearing holding bush 112 has a bush fixing flange 110 formed on the outer peripheral surface thereof, and is positioned in the axial direction such that the bush fixing flange 110 comes into contact with one end of the shaft holding portion 108.
Also, at both inner ends of the bearing holding bush 112,
Each of the ceramic balls 144 of the present invention is attached to the inner ring 140.
A plurality of ball bearings 116 and 118 having the same structure as that shown in FIG. 16 are coaxially fitted between the outer ring 136 and both ends of a bearing fixing flange 132 formed to protrude from the inner peripheral surface of the bearing holding bush 112. Are respectively abutted and positioned.

A disk rotating shaft 146 is inserted and fixed in each of the inner rings 140, 140 of the ball bearings 116, 118, and is rotatably supported by the bearings 116, 118 with respect to the bearing holding bush 112 and the main body case 102. A flat cylindrical disk fixing member (rotating member) 152 is integrated with one end of the disk rotating shaft 146, and a wall portion 154 is formed so as to extend downward along the outer peripheral edge thereof. While an exciting permanent magnet 126 is attached to the inner peripheral surface of the wall 154,
A magnetic field coil 124 fixed to the outer peripheral surface of the bearing holding bush 112 has an exciting permanent magnet 12
6 are arranged opposite to each other. Field coil 124
The excitation permanent magnet 126 constitutes the DC motor 122 for driving the rotation of the disk. Also, the disk fixing member 15
A disk fixing flange 156 protrudes from the outer peripheral surface of the second wall portion 154, and the inner peripheral edge of the recording hard disk 106 is held and fixed by being sandwiched between the pressing plate 121 and the recording disk hard disk 106. . A fixing bolt 151 is screwed into the disk rotation shaft 146 so as to penetrate the holding plate 121.

The motor 122 operates by energizing the field coil 124 to generate a rotational driving force using the disk fixing member 152 as a rotor. As a result, the hard disk 106 fixed to the disk fixing member 152 is moved to the disk rotating shaft 14 supported by the bearings 116 and 118.
6 will be driven to rotate around the axis.

[0083]

[Experimental Examples] In order to confirm the effects of the present invention, the following experiments were conducted. Silicon nitride powder (purity of silicon nitride: 98% by weight, average particle diameter: 0.5 μm, 90% particle diameter: 1.0 μm, BET specific surface area: 10 m ² / g) as a raw material powder, and yttria powder as a sintering aid component (Average particle size 0.6μ
m, 90% particle diameter 1.0 μm, BET specific surface area 10 m
² / g), alumina powder (average particle diameter 0.4 μm, 90
% Particle diameter 1.0 μm, BET specific surface area 10 m ² / g)
Was prepared. The average particle diameter was measured using a laser diffraction type particle size analyzer (manufactured by Horiba, Ltd., product number: LA-500), and BE
The T specific surface area was measured by a BET specific surface area measuring device (Multi Soap 12, manufactured by Yuasa Ionics Co., Ltd.).

The above material powders were blended so that the composition ratio was 100 parts by weight of silicon nitride powder, 5 parts by weight of yttria powder, and 5 parts by weight of alumina powder.
50 parts by weight of pure water as a solvent and an appropriate amount of an organic binder were added to parts by weight, and the mixture was mixed by an attritor mill for 30 hours to obtain a slurry of a molding base powder. The slurry was made into a green body powder for molding by the apparatus shown in FIG. Specifically, alumina spheres having a diameter of 2 mm were used as the drying medium, and other conditions were set as follows: A hot air duct 4 in which the drying medium holding portion 5 was formed.
Inner diameter R2: about 200 mm; filling depth t1 of the drying medium 2: about 150 mm; temperature of hot air: 160 ° C .; flow rate of hot air: 3 m / s. The 50% particle size of the obtained green body powder for molding was 0.6.
The 90% particle diameter was 1.0 μm, and the BET specific surface area value was 10 m ² / g.

Next, this molding base powder was subjected to tumbling granulation (abbreviated as “Method A” in Table 1) to obtain a diameter of about 4 mm.
Was produced. The conditions of the tumbling granulation are as follows: the inner diameter R1 of the granulation vessel 32: 400 mm; the inclination angle θ1 of the rotating shaft 31: 15 °; the rotating speed: for the first 300 minutes for nucleation. 10r
pm, and then adjusted in the range of 5-30 rpm in order to set the size of the molded body to various values. In addition, 30r
The rotational peripheral speed at pm is 37.7 m / min. -Moisture content in the molded body 80 finally obtained by supplying water: 12 wt%. In addition, the density of each obtained molded body was measured by a mercury intrusion method, and the relative density (%) of the molded body was calculated with the true density being 3.23 g / cm ³ . In this measurement method, the weight M of the molded body is measured in advance, and then the molded body is placed in a container and mercury is poured at normal pressure. Then, the molded body volume V is determined from the change in the height of the mercury liquid level when the molded body is taken out of mercury, and M
/ V is used to determine the density of the compact. It has been confirmed that mercury hardly penetrates into the molded body under normal pressure.

On the other hand, a slurry obtained by drying a slurry with a usual spray dryer was prepared as a comparative powder. In addition, the 50% particle diameter of the obtained powder was 80 μm, the 90% particle diameter was 100 μm, and the BET specific surface area value was 5 m ² / g. Then, the comparative powder is replaced with a rolling granulation by a normal die press (however, a pressing pressure of 500 kg / cm ² ).
, And then a cold isostatic press (CIP) was performed to form a compact having a diameter of about 4 mm, and its relative density was measured in the same manner.

The obtained spherical molded body was primarily fired at 1550 ° C. to 1600 ° C. for 2 hours in a nitrogen atmosphere at normal pressure.
1650-1 under a pressurized nitrogen atmosphere of 50-100 atm
Secondary baking was performed at 700 ° C. for 2 hours, and rough polishing was performed as needed to roughly adjust the shape. Then, wet-precision mechanical polishing was performed using a grooved surface grindstone (count: # 20000) to obtain a ceramic ball having a diameter of 3 mm. Tables 1 and 2 show the conditions for each test sample.

The sphericity and unequal diameter of the obtained silicon nitride ceramic balls were measured by using a well-known shape measuring machine, Taylor.
It was measured by Hobson Tarilon 73P. Also, the arithmetic average roughness Ra of the polished surface of each ceramic ball is
It was measured by the above-mentioned shape measuring machine according to JIS-B0601. Furthermore, the surface area A (unit: m
m ² ) and the volume while calculating its weight W (unit: g)
Was obtained by dividing by the volume, and the relative density was obtained by dividing the above by the true density. Further, the structure of the polished spherical surface was examined with a scanning electron microscope (SEM: magnification 100).
(× 0), and the observed image was subjected to image analysis to determine the cumulative area ratio of voids having a dimension of 1 μm or more according to the above definition and the average number of voids per 1 mm ² . Specifically, the area and the number of voids of 1 μm or more are measured in a visual field of 50 μm × 50 μm, converted into the cumulative area ratio and the number per 1 mm ^2, and the same measurement is performed for the five visual fields arbitrarily extracted. The average value is used to calculate the final void cumulative area ratio and the number of existing per 1 mm ² .

Further, in the obtained ceramic balls, polished bearing balls were arranged between an outer race and an inner race made of metal to form a ceramic bearing as shown in FIG. Then, a microphone (pickup sensor) is attached to the outer ring, the outer ring is fixed, and the inner ring is
The presence or absence of sound when rotated at 00 rpm was measured. The determination was made as abnormal noise generation (x) when the sensor output exceeded 30 dB, slight abnormal noise generation (発 生) when it was 30 to 25 dB, and normal (○) when it was less than 25 dB. On the other hand, each bearing was
Perform a life test by continuously rotating for 000 hours,
10% or more fluctuation in rotational vibration is observed, and / or a ceramic ball having an abnormal appearance after the test is unacceptable (x), and fluctuation of rotational vibration is in the range of 10 to 5%. And / or if no abnormalities were observed except for the very slight appearance of the ceramic ball after the test (△), the fluctuation of the rotational vibration was less than 5%, and the appearance of the ceramic ball after the test was A sample in which no abnormality was observed was judged as good (○). Table 3 shows the above results.
And Table 4 below.

[0090]

[Table 1]

[0091]

[Table 2]

[0092]

[Table 3]

[0093]

[Table 4]

That is, the sphericity is 0.08 μm or less, the cumulative area ratio of defects having a size of 1 μm or more observed on the polished surface is 1% or less, and the arithmetic average roughness Ra of the polished surface is It can be seen that a bearing using a ceramic ball having a diameter of 0.012 μm or less has no abnormal noise and has a good life test result.

[Brief description of the drawings]

FIG. 1 is an explanatory view of a process of rolling granulation.

FIG. 2 is a process explanatory view following FIG. 2;

FIG. 3 is an explanatory view illustrating some molded cores.

FIG. 4 is an explanatory view illustrating some examples of a method for producing a molded core.

FIG. 5 is a view for explaining the progress of a rolling granulation forming step.

FIG. 6 is a schematic diagram showing a cross-sectional structure of a spherical ceramic sintered body manufactured by a rolling granulation method.

FIG. 7 is an explanatory diagram showing the concept of a relative cumulative frequency.

FIG. 8 is a longitudinal sectional view conceptually showing an example of an apparatus for producing a green body powder for molding.

FIG. 9 is a diagram for explaining the operation of the apparatus of FIG. 1;

FIG. 10 is an operation explanatory view following FIG. 2;

FIG. 11 is a diagram illustrating the concept of a primary particle diameter and a secondary particle diameter.

FIG. 12 is an explanatory diagram showing definitions of dimensions of crystal grains or defects.

FIG. 13 is an SEM observation image showing a fractured surface of an example of a spherical molded body manufactured by the rolling granulation method.

14 is an optical microscope observation image showing the cross section of the calcined body of the spherical molded body shown in FIG. 13 at various magnifications.

FIG. 15 is an optical microscope observation image showing a cross-sectional structure of some silicon nitride ceramic balls manufactured by the rolling granulation method.

FIG. 16 is a schematic view of a ball bearing using the ceramic ball of the present invention.

17 is a longitudinal sectional view showing an example of a computer hard disk drive mechanism using the ball bearing of FIG.

FIG. 18 is an explanatory diagram showing definitions of dimensions of voids or crystal particles.

[Explanation of symbols]

 40, 116, 118 Ball bearing 43, 144 Ceramic ball 100 Hard disk drive

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Tetsuji Ago 14-18 Takatsuji-cho, Mizuho-ku, Nagoya-shi, Aichi F-term (reference) 3J101 AA02 AA42 AA62 BA10 EA44 EA75 FA01 FA31 GA53 4G030 AA07 AA17 AA36 AA45 AA47 AA49 AA52 BA12 BA19 5D109 BB05 BB16 BB21 BB31

Claims (11)

[Claims] At least the surface layer is made of ceramic, the surface is polished, and the sphericity is 0.08.
μm or less, the cumulative area ratio of defects having a size of 1 μm or more observed on the polished surface is 1% or less, and the arithmetic average roughness Ra of the polished surface is 0.012 μm or less. Ceramic ball. 2. At least the surface layer is made of ceramic, the surface is polished, and the sphericity is 0.08.
μm or less, and the number of defects having a size of 1 μm or more observed on the polished surface is 500 or less per 1 mm ² , and the arithmetic average roughness Ra of the polished surface is 0.012 μm or less. A ceramic ball, wherein 3. The size 1 μ observed on the polished surface.
3. The ceramic ball according to claim 2, wherein the cumulative area ratio of the defects having a length of m or more is 1% or less. 4. The particle diameter distribution of crystal grains present on the polished surface, wherein the particle diameter d90% at which the relative cumulative frequency from the small particle diameter side becomes 90% is adjusted within a range of 2.4 μm or less. Item 4. The ceramic ball according to any one of Items 1 to 3. 5. The ceramic ball according to claim 1, wherein a maximum size of a defect observed on the polished surface is 5 μm or less. 6. The ceramic ball according to claim 1, wherein the ceramic is a silicon nitride ceramic. 7. A molding base powder containing a ceramic raw material powder is placed in a granulation container, and an agglomerate of the molding base powder is rolled and grown into a spherical shape in the container to have a relative density of 61%. %, And a step of firing the spherical molded body; and polishing the surface of the fired body to obtain the ceramic ball according to any one of claims 1 to 6. Method of manufacturing ceramic balls to obtain. 8. In the rolling granulation molding step, a molding base powder containing a ceramic raw material powder and a molding nucleus coexist in the granulation vessel, and the molding nucleus is rolled in this state. 8. The method for producing a ceramic ball according to claim 7, wherein the green compact is obtained by adhering and aggregating the green body powder in a spherical shape around the molded core. 9. A ball bearing comprising a plurality of ceramic balls according to claim 1 incorporated as a bearing rolling element. 10. The ball bearing according to claim 9, wherein the ball bearing is used as a bearing component of a rotating main shaft portion of a hard disk as a magnetic storage medium. 11. A ball bearing according to claim 10, wherein one of an outer ring and an inner ring of the ball bearing is a fixed side and the other is a rotating side, and a member on the rotating side (hereinafter referred to as a rotating member) is rotationally driven. And a hard disk that rotates integrally with the rotating member.