Ceramic spherical body

Abstract

Ceramic spheres that include zirconia as a primary component and are 80–95 volume% tetragonal crystals and no more than 5 volume% monoclinic crystals, the ceramic spheres being characterized in that, when average particle diameter is X (μm), the maximum height of waviness Wz (μm) of the line of intersection between a cross-section and the surface of spheres that have a diameter of X / 2 (μm) is 0.5%–1.2% of the average particle diameter X (μm).　The present invention provides ceramic spheres that can be used as a medium, such as balls or beads, that is for use in a grinder and do not readily break, even when grinding and dispersion are performed at normal temperature and a high water temperature.

Description

Technical Field

[0001] The present invention relates to ceramic spherical bodies.

Background Art

[0002] When pulverizing powders used for electronic material applications or dispersing pigments in ink applications, pulverizers such as ball mills, vibration mills, sand mills, and bead mills that pulverize using grinding media are widely used. As grinding media such as balls and beads used for such pulverizers (hereinafter sometimes simply referred to as "media"), ceramic sintered bodies mainly composed of zirconia, which are excellent in terms of wear resistance and impact resistance, are used.

[0003] As a ceramic sintered body mainly composed of zirconia, media having improved durability and wear resistance are disclosed by limiting the composition ratio of ZrO2 and Y2O3 and controlling the amounts of Al2O3 and SiO2 (for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In recent years, particularly for the purpose of improving the performance of materials to be pulverized, further refinement of particles has been demanded. Along with this, the use of small-diameter grinding media of 300 μm or less for pulverization has been expanding. Small-diameter media are generally manufactured by methods such as rolling granulation molding, in-liquid granulation molding, and plasma melting molding. However, in any granulation method, due to the influence of factors such as the growth history, heat history, and surface tension of the particles during the molding process, wavy shapes exist on the surface of the media.

[0006] Such undulations on the media surface are locations with a locally small radius of curvature. As a result of investigations by the present inventors, during the collision between media, between media and the material to be pulverized, and between media and the device wall surface, at the undulated locations on the media surface, the contact area is small, and as a result of high pressure being applied, it has been found that this is a factor that easily causes damage to the media. In particular, when the material to be pulverized and the media are mixed in a pulverizer or the like in water and pulverization / dispersion is performed for a long time, the water temperature rises, the deterioration of the ceramic sintered body progresses, and damage easily occurs.

[0007] An object of the present invention is to provide a ceramic spherical body that can be used as media such as balls and beads used in a pulverizer and is less likely to be damaged even when pulverization / dispersion is performed at normal temperature and at a high water temperature.

Means for Solving the Problems

[0008] That is, the present invention for solving the above problems is a ceramic spherical body having zirconia as a main component, with a ratio of tetragonal crystal being 80% by volume or more and 95% by volume or less, and a ratio of monoclinic crystal being 5% by volume or less. When the average particle size is X (μm), the maximum height undulation Wz (μm) at the intersection of the cross-section of the spherical body having a diameter of X / 2 (μm) and the surface of the spherical body is 0.5% or more and 1.2% or less of the average particle size X (μm). It is a ceramic spherical body characterized by this.

Effects of the Invention

[0009] The ceramic spherical body of the present invention has the effect that even when pulverization / dispersion of the material to be pulverized is performed at normal temperature and at a high water temperature using this, damage to the ceramic spherical body is suppressed.

Brief Description of the Drawings

[0010]

Figure 1

Embodiments for Carrying Out the Invention

[0011] The ceramic spherical body of the present invention is composed of a ceramic sintered body mainly composed of zirconia. In the following description of this specification, the ceramic sintered body as the final product, that is, the ceramic sintered body as an intermediate obtained through sintering one or more times in the manufacturing process other than the grinding media, is collectively referred to as the "intermediate sintered body". Also, both the ceramic sintered body as the final product and the intermediate sintered body are collectively simply referred to as the "sintered body".

[0012] The ceramic spherical body of the present invention can be obtained by forming a ceramic raw material powder mainly composed of zirconia (hereinafter sometimes simply referred to as "raw material powder") into a spherical shape. Here, in this specification, the phrase "mainly composed of zirconia" means that the ratio of zirconia is 90% by weight or more. However, if the ratio of zirconia is 93% by weight or more of all components, it is preferable because particularly high strength can be obtained.

[0013] The content of each component in the ceramics can be determined as follows. First, a sample of the ceramics is crushed using a universal testing machine, and about 0.3 g of the crushed pieces are placed in a platinum crucible and melted with potassium hydrogen sulfate. This is dissolved in dilute nitric acid and made up to a fixed volume, and each metal element is quantified using ICP emission spectrometry, and further converted into oxides to determine the content. Hereinafter, the components in the ceramic spherical body of the present invention may be expressed in terms of metal elements or in terms of oxides.

[0014] In addition to the main components as described above, the ceramic spherical body of the present invention preferably contains yttria (Y2O3), ceria (CeO2), alumina (Al2O3), magnesia (MgO), calcia (CaO), etc. in terms of oxide conversion. These function as stabilizers and can improve the strength and toughness of the ceramic spherical body. Among them, it is particularly preferable to contain yttria. The content of yttria is preferably 4.6 / 95.4 or more and 5.6 / 94.4 or less, more preferably 4.8 / 95.2 or more and 5.5 / 94.5 or less, in terms of the molar ratio of yttria / zirconia in the ceramic spherical body.

[0015] The ceramic spherical body of the present invention has a tetragonal ratio of 80% by volume or more and 95% by volume or less, and a monoclinic ratio of 5% by volume or less. When the content of the tetragonal phase is 80% by volume or more, when stress is applied, the tetragonal phase transforms into the monoclinic phase and expands in volume, and cracks in the media can be suppressed. However, if it is less than 80% by volume, the effect may be reduced. On the other hand, when the content of the tetragonal phase is greater than 95% by volume, it is likely to deteriorate in high-temperature water. Therefore, when the water temperature rises due to long-term pulverization and dispersion, the ceramic spherical body may be easily damaged. Also, from the perspective of preventing damage, the lower the ratio of the monoclinic phase, the better, and it is 5% by volume or less. Preferably it is 3% by volume or less, more preferably 1% by volume or less. However, in the manufacturing process of the ceramic spherical body, it is common to perform wet polishing and post-washing as described later to smooth the surface shape. In the process of rising water temperature during wet polishing, washing after polishing, and drying, at least 0.1% or more of the monoclinic phase is generally formed, so it generally does not become completely zero. The ratio of each crystal phase of the ceramic spherical body can be measured by the powder X-ray diffraction method.

[0016] In the ceramic spherical body of the present invention, when the average particle size is X (μm), the maximum height undulation Wz (μm) at the intersection of the cross-section of the spherical body and the surface of the spherical body, where the diameter is X / 2 (μm), is 0.5% or more and 1.2% or less of the average particle size X (μm), that is, (Wz / X)×100 is 0.5 or more and 1.2 or less. Generally, the maximum height undulation increases with the particle size of the particles. Therefore, in the present invention, the value obtained by dividing the maximum height undulation by the average particle size is evaluated. If (Wz / X)×100 is greater than 1.2, local pressure concentration occurs in the ceramic spherical bodies during the collision between the ceramic spherical bodies during pulverization or between the ceramic spherical bodies and the material to be pulverized, etc., resulting in easy breakage. It is more preferable that (Wz / X)×100 is 1.0 or less. Also, if (Wz / X)×100 is less than 0.5, the productivity as an industrial product is poor.

[0017] Here, the average particle size X can be measured using image analysis and measurement software after photographing the ceramic spherical bodies. Specifically, it is a value measured as follows. An aggregate of ceramic spherical bodies is photographed with a digital microscope at a magnification of 10 to 200 times. Using image analysis and measurement software, the photographed image is binarized based on the brightness of the measurement image. The binarized image is separated into circular shapes by the least-squares mean, and the diameter of each separated circle is calculated as the diameter of each ceramic spherical body. The number average value of the diameters of 1000 ceramic spherical bodies is taken as the average particle size X.

[0018] Also, the "maximum height undulation Wz" can be obtained by observing the ceramic spherical body from above 4 with a laser microscope for the intersection 3 of the cross-section of the spherical body and the surface of the spherical body, where the diameter is X / 2, which is smaller than the diameter 1 of the ceramic spherical body, as shown in FIG. 1, based on JIS B 0601:2013. As a method for reducing the maximum height undulation Wz, for example, long-term rolling can be performed while adding only water in a rolling granulator described later.

[0019] The internal defect rate of the ceramic spherical body of the present invention is preferably 0.5% or less. Here, "internal defect" refers to cracks and pores inside the ceramic spherical body. By grinding the ceramic spherical body and having an internal defect rate of 0.5% or less, breakage of the ceramic spherical body can be further suppressed. As a method for setting the internal defect rate to 0.5% or less, for example, subjecting the obtained ceramic spherical body to a hot isostatic pressing treatment described later, or performing a surface undulation reduction step of a green body described later, can be mentioned.

[0020] The ceramic spherical body of the present invention preferably has an average particle size X of 30 μm or more and 300 μm or less. When the average particle size X is 30 μm or more, separation of the material to be pulverized and the ceramic spherical body becomes easy, and mixing of the ceramic spherical body can be prevented. When the average particle size X is 300 μm or less, the material to be pulverized can be pulverized and dispersed uniformly and finely. The average particle size X can be set within the above range by means of sieve classification described later.

[0021] The ceramic spherical body of the present invention preferably has a minimum particle size of 0.7X (μm) or more and a maximum particle size of 1.3X (μm) or less. When the minimum particle size is 0.7X or more, separation of the material to be pulverized and the ceramic spherical body becomes easy, and mixing of the ceramic spherical body can be prevented. Also, when the maximum particle size is 1.3X (μm) or less, the material to be pulverized after pulverization can have a uniform particle size distribution. The minimum particle size and the maximum particle size can be measured by using image analysis and measurement software in the same manner as the measurement of the average particle size X described above, and taking the minimum value of the diameter of each circle separated into circular figures as the minimum particle size and the maximum value as the maximum particle size. The minimum particle size and the maximum particle size can be set within the above range by means of sieve classification described later.

[0022] In addition, due to the non-uniformity in the manufacturing process of the ceramic spherical bodies, it is difficult to make all the particles spherical, and it is common for 1 to several percent of particles with poor sphericity to exist. In particular, for elliptical objects, there is a possibility that they may pass through a classification screen with an aperture width smaller than the equivalent circle diameter along the minor axis of the ellipse, or conversely, may be captured by a classification screen with an aperture width larger than the equivalent circle diameter along the major axis of the ellipse, and they may exist as outliers outside the particle size distribution of the ceramic spherical bodies. Therefore, in order to exclude the influence of particles with such special shapes in the particle size evaluation by sampling, it is more preferable to define by the 1% particle size (D1) and the 99% particle size (D99) rather than the minimum particle size and the maximum particle size for more accurately grasping the particle size range of the ceramic spherical bodies. Therefore, for the ceramic spherical bodies in the present invention, it is preferable that D1 is 0.7X (μm) or more and D99 is 1.3X (μm) or less. D1 and D99 can be evaluated by the same method as the minimum particle size and the maximum particle size.

[0023] The ceramic spherical bodies of the present invention can be manufactured by various methods. Hereinafter, as an example, the details of an example manufactured by the rolling granulation molding method will be described.

[0024] First, the raw material powder is formed into a spherical shape using the rolling granulation molding method. The rolling granulation molding method is a method of forming spherical fine particles by alternately adding a ceramic raw material powder and a liquid binder containing a binder and moisture into a rotating drum, and then growing the particles by imparting the linkage of rotation to the fine particles and the powder to produce a spherical molded body.

[0025] Next, as a step for reducing the surface undulation of the obtained formed body, at least a formed body weighing 100 kg or more is further tumbled in a rolling granulator while adding only water for at least 10 hours or more, preferably about 20 hours, more preferably 30 hours or more. As a result, the surface of the formed body becomes flattened and the surface undulation becomes smaller. It is desirable to set the moisture content in the rolling granulator during this step 2 to 5% higher than that during granule growth. By making the surface layer of the ceramic spherical body contain a large amount of moisture in this way, particle movement (plastic deformation) when receiving rolling pressure becomes easy, and as a result, the convex portions are flattened, and a smooth ceramic spherical body with a small maximum height undulation Wz can be obtained. In addition, in order to prevent aggregation between particles due to excessive moisture, quality control is required during tumbling, such as visual observation of the appearance to grasp the humidification state of the particles during rolling granulation at each elapsed time, microscopic observation of the particle state by taking a small amount of samples, or grasping physical quantities indicating the humidification state such as moisture content and bulk density.

[0026] Further, the above-described surface undulation reduction step also has an effect of promoting densification of the formed body and is also effective in reducing the internal defect rate.

[0027] Since the formed body obtained in this way contains moisture, if it is directly subjected to the sintering step described below, the moisture inside the formed body may rapidly evaporate, causing the formed body to crack. Therefore, the formed body is subjected to a drying step of gradually reducing the moisture inside the formed body using a dryer or the like before being subjected to the sintering step.

[0028] In this way, by performing a sintering step of putting the formed body that has been formed and passed through the drying step into a crucible or the like and firing it in a firing furnace, the binder is removed and the powder particles are bonded to obtain a ceramic sintered body. In the sintering step, it is preferable to fire at 1350 to 1450 °C for 1 to 3 hours.

[0029] The sintered body obtained through the sintering process can be used as a grinding medium as it is or after further grinding described later. However, in order to further reduce the defects of the grinding medium, it is preferable to perform a hot isostatic pressing process described later. Hereinafter, the case of performing the hot isostatic pressing process after the sintering process will be described. Note that when the hot isostatic pressing process is not performed, the sintered body after the aforementioned sintering process is the final product and not an "intermediate sintered body", but in the following description when the hot isostatic pressing process is performed, it will be described as an "intermediate sintered body".

[0030] As described above, the intermediate sintered body obtained in the sintering process is preferably subjected to a hot isostatic pressing process in which a hot isostatic pressing (Hot Isostatic Pressing) treatment (hereinafter referred to as "HIP treatment") is performed next. The HIP treatment is a treatment in which high temperature and isotropic pressure are simultaneously applied to the object to be treated. By performing the HIP treatment on the intermediate sintered body, defects such as voids and cracks remaining inside the intermediate sintered body can be removed without changing the shape.

[0031] The HIP treatment is preferably performed at a temperature 0°C to 50°C lower than the sintering temperature in the sintering process. If the temperature is lower than that, the diffusion of ceramic powders such as zirconia during the HIP treatment becomes insufficient, and defects may remain. On the other hand, if the temperature of the HIP treatment is higher than the sintering temperature, the intermediate sintered body may undergo grain growth, resulting in a decrease in strength and a larger variation in strength. The temperature of the HIP treatment is more preferably 0°C to 40°C lower than the sintering temperature in the sintering process, and even more preferably 0°C to 30°C lower.

[0032] The pressure of the HIP treatment only needs to be sufficient to remove defects, and it can be processed without problems if it is processed at a pressure of 100 MPa or more. To create a high-pressure state, it is preferable to process in an Ar gas atmosphere.

[0033] The sintered body obtained as described above can be used as a grinding medium as it is. However, a higher-quality grinding medium can be obtained by further polishing the surface using equipment such as a barrel polishing device, a ball mill, or a bead mill.

[0034] Furthermore, it is preferable to classify the sintered body by a classification process. By the classification process, a desired average particle size, minimum particle size, and maximum particle size can be achieved. Examples of classification methods include sieve classification using a mesh-like sieve. The sieve classification may be performed by stacking two sieves and separating relatively coarse powder with a larger particle size and relatively fine powder with a smaller particle size in a single operation.

[0035] Regarding the above surface polishing process, as a result of investigations by the inventors, it has been found that particularly good surface smoothness can be obtained by performing wet polishing using a bead mill device having high stirring energy. Surface smoothness is a factor that greatly affects the wear amount of ceramic spherical bodies in a wet dispersion process. When the surface smoothness is poor, that is, when the surface irregularities are large or numerous, during collisions that occur between ceramic spherical bodies or between a ceramic spherical body and the material to be dispersed, the convex-shaped portions are easily scraped off, resulting in an increase in the wear amount of zirconia, which is the main component of the ceramic spherical bodies, and this can have a significant impact on the quality of the material to be dispersed. In particular, in the process of wet-dispersing barium titanate powder, which is used as a high-dielectric raw material for manufacturing multilayer ceramic capacitors, which is a main application of the ceramic spherical body media of the present invention, the zirconia component resulting from the wear of the ceramic spherical bodies mixes into the barium titanate, which has an effect of inhibiting the sintering reaction of the barium titanate, and it is known that the uniformity of the primary particle size of the barium titanate after sintering is impaired. Such non-uniformity of the primary particle size can deteriorate the electrical characteristics (capacitance, dielectric loss, etc.) of the capacitor, or promote surface irregularities in the formation of a dielectric layer with a thickness of less than even 1 μm per layer, and can be an inhibitory factor in the process of forming a flat laminated structure. Therefore, in the wet dispersion process of barium titanate, the zirconia wear amount mixed into the material to be pulverized is managed with high precision, and a ceramic spherical body media with a small wear amount and stability is desired. To achieve this, it is necessary to ensure the smoothness of the surface of the ceramic spherical bodies.

[0036] In the polishing process using the above bead mill device, the important process factors for obtaining good surface smoothness are the type of abrasive (material, particle size), its slurry concentration, stirring speed (peripheral speed), and processing time. Since ceramic spheroids with a smaller size have a lighter self-weight, higher polishing energy is required to polish the surface. As the abrasive, it is desirable to use silicon carbide (SiC) or alumina (Al2O3) with high cutting power. The larger the particle size, the higher the cutting power, while the smaller the particle size, the easier it is to reduce cutting scratches and thus obtain a smooth surface. Therefore, from the perspective of production efficiency, it is useful to first perform rough polishing with an abrasive of a large particle size and then use an abrasive of a small particle size for finish polishing. For rough polishing, a particle size of about 3 to 10 μm is desirable, and for finish polishing, a particle size of about 0.5 to 2 μm is desirable. Also, the slurry concentration of the abrasive is desirably about 1 to 5% by weight from the viewpoint of preventing aggregation of the abrasive itself during the polishing process. Similarly, from the viewpoint of preventing aggregation, it is desirable to add a dispersant corresponding to the type of abrasive in an amount of about 0.3 to 3% by weight. The stirring speed (peripheral speed), which is an operating condition of the device, is desirably high from the perspective of production capacity. However, if it is too high, residues of the abrasive are likely to adhere to the surface of the ceramic spheroids. Therefore, from the perspective of balance, a range of 8 to 14 m / s is desirable. The processing time varies depending on the device specifications, the size of the ceramic spheroids, the type of abrasive, etc., but it is desirable to perform it for at least 2 hours or more, preferably 4 hours or more. Also, after the polishing process is completed, it is possible to remove the residues adhering to the surface of the ceramic spheroids by treating them with only water without the abrasive or only water and the dispersant, and it is desirable to perform this for about 0.5 to 2 hours.

[0037] As a result of performing wet grinding using the bead mill as described above under appropriate conditions, for example, in the barrel grinding method, it becomes possible to obtain smoothness with a surface roughness Ra of 2 to 10 nm, compared to smoothness with a surface roughness Ra of about 20 to 40 nm. To make it less than 2 nm, it is necessary to perform grinding for a long time or at a high peripheral speed using an abrasive with a smaller particle size. However, since aggregation of the abrasive is likely to occur, there is concern about contamination of the product. Therefore, the above surface roughness range is appropriate as the manufacturing method according to the present invention. The surface roughness Ra can be evaluated using an atomic force microscope (AFM). In the present invention, 10 ceramic spheres are extracted and evaluated, and the average value is used as the surface roughness value Ra.

[0038] As a result of evaluating the wear amount in the wet dispersion of barium titanate of the ceramic spheres with the above surface roughness Ra of 2 to 10 nm, it was found that the wear amount decreases as the surface becomes smoother from Ra = 10 nm to 5 nm, but becomes almost constant at 5 nm or less. This is presumably because the zirconia spheres are subjected to a cutting action from barium titanate, so even if the initial smoothness is about 2 nm, the smoothness deteriorates to around 5 nm due to cutting scratches after use. From the above results, regarding the wet dispersion application of barium titanate in the present invention, to reduce and stabilize the zirconia wear amount caused by the ceramic spheres, a surface roughness Ra of the ceramic spheres in the range of 2 to 5 nm is desirable.

Examples

[0039] Hereinafter, the present invention will be specifically described based on examples, but the present invention is not limited to these examples.

[0040] (Measurement method) (Average particle size, minimum particle size, maximum particle size, 1% particle size (D1), 99% particle size (D99) The particle size was measured by the following method. An aggregate of ceramic spherical bodies was photographed with a digital microscope VHX-2000 (manufactured by Keyence) at magnifications of 10 to 200 times. Using image analysis and measurement software WinROOF (registered trademark: manufactured by Mitani Shoji Co., Ltd.), the brightness of the measurement image was binarized as a reference. The binarized image was separated into circular shapes by the least squares mean, and the diameter of each separated circle was calculated as the diameter of each ceramic spherical body. Also, the number average value of the diameters of 1000 ceramic spherical bodies was defined as the average particle size X. In addition, the minimum value of the diameter of each separated circle was defined as the minimum particle size, and the maximum value was defined as the maximum particle size. Furthermore, the equivalent diameter at a cumulative number of 1% counted from the minimum side by the number ratio was defined as the 1% particle size (D1), and the equivalent diameter at a cumulative number of 99% was defined as the 99% particle size (D99).

[0041] (Ratio of crystal phase) The sample was embedded in resin, sectioned, and mirror-polished to obtain a measurement sample. It was attached to a sample holder and measured by wide-angle X-ray diffraction (micro X-ray diffraction). The measurement conditions are as follows. X-ray source: CuK line (using a multilayer mirror) Output: 50 kV, 22 mA Slit system: 100 μmφ pinhole Measurement range: 2θ = 23° to 33°, 70° to 77° Integration time: 3600 seconds / frame.

[0042] From the measurement results, the content rates of each crystal layer of zirconia were calculated using the following equations. Content rate of monoclinic crystal (%) = [{I m (111) + I m (1 - 1 - 1)} / {I m (111) + I m (1 - 1 - 1) + I t+c (111)}] × 100 Content rate of cubic crystal (%) = [I t+c (111) / {I m (111) + I m (1 - 1 - 1) + I t+c (111)}] × [{I c (400) / {I c (400) + It (400)+I t (004)}×100 Content ratio of tetragonal crystal (%) = 100 - content ratio of monoclinic crystal - content ratio of cubic crystal Here, I represents the diffraction intensity. The subscripts m, t, and c represent monoclinic crystal, cubic crystal, and tetragonal crystal, respectively. The values inside the parentheses of the diffraction intensity indicate the indices of each crystal.

[0043] (Maximum height waviness Wz) The maximum height waviness Wz (μm) is based on JIS B 0601:2013. Using a laser microscope VK-X-150 (manufactured by Keyence) for ceramic spherical bodies, from the measurement direction shown as 4 in Fig. 1 (the z-axis direction in Fig. 1), for the intersection part of the cross-section of the spherical body (on the xy plane perpendicular to the z-axis in Fig. 1) where the diameter becomes X / 2 (μm) shown as 2 in Fig. 1 and the surface of the spherical body, that is, the part shown as 3 in Fig. 1, without contact, for 10 spherical bodies, the measurement length = average particle diameter X / 2 × 3.141 (μm), the cut-off value λs for removing high-frequency components = 2.5 (μm), and the low-frequency component removal λc = none, the maximum height waviness Wz in the z-axis direction was measured, and the average value of the maximum height waviness Wz was calculated. Here, 5 in Fig. 1 is an example of the measurement profile of the maximum height waviness Wz in the present invention.

[0044] (Surface roughness Ra) The surface roughness Ra (nm) is based on JIS B 0601:2013. Arbitrarily extract 10 particles from an aggregate of ceramic spherical bodies, and using an atomic force microscope (Bruker, NanoScopeV), scan the vicinity of the center of the spherical body at a scanning speed = 0.3 Hz and a resolution of 256 × 256 in a measurement area of 1 / 10 of the average particle diameter X of the ceramic spherical body in terms of the square size. For the obtained image, perform Flatten 1st order and PlaneFit - x3rd order processing to obtain an image with the curved surface fitted and corrected to a plane. Evaluate the surface roughness Ra for the image corrected to a plane. Evaluate each particle 3 times, and the average value of Ra for 10 particles × 3 times = a total of 30 points is taken as the surface roughness value Ra of this ceramic spherical body.

[0045] (Internal defect rate) The internal defect rate was measured by the following method. After grinding the ceramic spherical bodies with a grinding machine to a size of 40 to 60% of the spherical body diameter, they were further polished with a diamond slurry with a particle size of 6 μm for 10 minutes or more to obtain a substantially cross-sectional view. The obtained samples were observed with a digital microscope VHX-2000 (manufactured by Keyence) at magnifications of 10 to 200 times, and the number of observable cracks was counted. 200 ceramic spherical bodies were observed, and the ratio of the ceramic spherical bodies with cracks or point defects among them was calculated and taken as the internal defect rate.

[0046] (Crushing load value) The crushing load value was measured by the following method. The ceramic spherical bodies were sandwiched with a cylindrical jig made of zirconia with a diameter of 20 mm, and a compression load was applied at a speed of 0.5 mm / min with an electronic universal testing machine CATY-2000YD (manufactured by Yonekura Seisakusho), and the load value at the time of fracture was measured. The measurement was performed on 30 ceramic spherical bodies, and the average value was adopted as the value. Also, as a strength test when the ceramic spherical bodies were exposed to high-temperature water, the obtained ceramic spherical bodies were left standing in water at a temperature of 90 °C for 50 hours, and the crushing load value of the ceramic spherical bodies after that was measured as the "crushing load value after hydrothermal test". The measurement was performed on 30 spherical bodies, and the average value was adopted as the value. Furthermore, it was calculated as the reduction rate of the crushing load after hydrothermal test by {(crushing load value before hydrothermal test) - (crushing load value after hydrothermal test)} / (crushing load value before hydrothermal test) × 100.

[0047] (Crack test) The crack test was performed by the following method. 220 g of the obtained aggregate of ceramic spherical bodies was filled into a bead mill device (manufactured by Hiroshima Metal & Machinery Co., Ltd., model UAM-015), 300 g of pure water at 20 °C was circulated, and stirring was performed at a peripheral speed of 12 m / s for 24 hours. After stirring, the ceramic spherical bodies were taken out and observed with a digital microscope VHX-2000 (Keyence) at magnifications of 10 to 200 times to confirm the presence or absence of cracks. 1000 ceramic spherical bodies were confirmed, and the number of cracked ceramic spherical bodies was taken as the number of cracks.

[0048] (Evaluation of wear amount and crack in wet dispersion of barium titanate) The wear amount of the ceramic spherical bodies in the wet dispersion of barium titanate was evaluated by the following method. 220 g of the obtained aggregate of ceramic spherical bodies was filled into a bead mill apparatus (manufactured by Hiroshima Metal & Machinery Co., Ltd., model UAM-015), and a slurry prepared by mixing 30 g of barium titanate (Sigma-Aldrich, barium titanium(IV) oxide) and 3 g of a dispersant (Tokyo Chemical Industry Co., Ltd., sodium dodecylbenzenesulfonate) in 300 g of pure water at 20°C was circulated, and wet dispersion was carried out at a peripheral speed of 12 m / s for 4 hours. The obtained slurry was dried in a hot air dryer at 90°C for 24 hours, and the dried barium titanate powder was finely pulverized using a mortar. Then, the ratio of the intensity peak area of zirconium to the intensity peak areas of titanium and barium was determined using a fluorescence X-ray analyzer (ZSX PrimusII manufactured by Rigaku Corporation), and the amount of zirconia (wear amount of ceramic spherical bodies) in the barium titanate powder was calculated. Also, after the implementation of this test, the number of cracked ceramic spherical bodies was confirmed using a digital microscope by the same method as the aforementioned cracking test.

[0049] [Example 1] Yttrium chloride was added to zirconium oxychloride so that the ratio shown in the yttria / zirconia molar ratio in Table 1 in terms of oxide in the obtained ceramic spherical bodies was obtained, and the raw material powder was prepared by a coprecipitation method.

[0050] Next, using the above raw material powder, a compact was granulated and formed to a size such that the average particle size X after sintering was around 50 μm by a rolling granulation forming method.

[0051] Next, as a surface undulation reduction step of the obtained compact, the surface undulation was reduced by performing rolling for about 40 hours while adding only water to keep the moisture content constant in a rolling granulator.

[0052] After drying the molded body obtained as described above, it was fired at 1400 °C for 2 hours to obtain an intermediate sintered body (sintering step). Thereafter, the intermediate sintered body was subjected to HIP treatment at 1380 °C and 120 MPa for 1.5 hours (hot isostatic pressing step). After surface polishing the obtained sintered body using a barrel polishing device, sieving classification was performed to produce the spherical media for grinding shown in Table 1.

[0053] [Example 2] Using the raw material powder of Example 1, the molded body was granulated to a size with an average particle size X of around 100 μm after sintering by the rolling granulation molding method in the same manner as in Example 1, and a surface undulation reduction step was carried out. After drying the obtained molded body to remove moisture, firing and HIP treatment were performed. The obtained sintered body was surface polished with a barrel polishing device, and then sieving classification was performed to produce the spherical media for grinding shown in Table 1.

[0054] [Example 3] Using the raw material powder of Example 1, the molded body was granulated to a size with an average particle size X of around 200 μm after sintering by the rolling granulation molding method in the same manner as in Example 1, and a surface undulation reduction step was carried out. After drying the obtained molded body to remove moisture, firing and HIP treatment were performed. The obtained sintered body was surface polished with a barrel polishing device, and then sieving classification was performed to produce the spherical media for grinding shown in Table 1.

[0055] [Example 4] In the same manner as in Example 1, the molded body was granulated to a size with an average particle size X of around 50 μm after sintering by the rolling granulation molding method. Except that the time of the surface undulation reduction step was shortened to 10 hours, it was carried out in the same manner as in Example 1 to produce the spherical media for grinding shown in Table 1.

[0056] [Example 5] In the same manner as in Example 2, the molded body was granulated to a size with an average particle size X of around 100 μm after sintering by the rolling granulation molding method. Except that the time of the surface undulation reduction step was shortened to 10 hours, it was carried out in the same manner as in Example 2 to produce the spherical media for grinding shown in Table 1.

[0057] [Example 6] In the same manner as in Example 3, the compacts were granulated by the rolling granulation method to a size in which the average particle size X after sintering was about 200 μm. The same procedure was carried out as in Example 3, except that the time for the surface waviness reduction step was shortened to 10 hours, to produce the spherical grinding media shown in Table 1.

[0058] [Example 7] As in Example 2, the compacts were granulated by the rolling granulation method to a size in which the average particle size X after sintering was about 100 μm. The same procedure was carried out as in Example 2, except that the processing temperature in the HIP process was changed from 1380° C. to 1300° C., and the spherical grinding media shown in Table 1 were produced.

[0059] [Example 8] The same manufacturing process as in Example 1 was carried out up to the HIP treatment, and the polishing process was carried out using a bead mill as follows. Using a polishing slurry containing 3.0 wt% alumina with a particle size of 3 μm (Tipton Co., Ltd., Light 1A) as an abrasive and 0.5 wt% sodium polycarboxylate (Cerna D-305, Chukyo Yushi Co., Ltd.) as a dispersant, polishing was carried out for a total of 6 hours at a bead mill stirring peripheral speed of 12 m / s, and then polishing was carried out for 4 hours using a polishing slurry containing 1.0 wt% alumina with a particle size of 1 μm (Tipton Co., Ltd., Light 1A) and 0.5 wt% D-305 as a dispersant. Finally, by performing co-rubbing for 2 hours with a slurry containing only 0.5 wt% D-305, the surface residue of the ceramic sphere was removed, and a ceramic sphere with a surface roughness Ra = 2 nm was obtained. Subsequent sieve classification was carried out in the same manner as in Example 1.

[0060] [Example 9] The same manufacturing process as in Example 1 was carried out up to HIP treatment, and the polishing process was carried out using a bead mill as follows. Using a polishing slurry prepared by mixing 3.0 wt% of alumina with a particle size of 3 μm (Chipton Co., Ltd., Light 1A) as an abrasive and 0.5 wt% of sodium polycarboxylate (Chukyo Yushi Co., Ltd., Cerna D-305) as a dispersant, the ceramic spherical bodies were polished for a total of 6 hours at a bead mill agitation peripheral speed of 12 m / s, and then co-polished for 2 hours with a slurry containing only 0.5 wt% of D-305 to remove surface residues of the ceramic spherical bodies, obtaining ceramic spherical bodies with a surface roughness Ra = 5 nm. The subsequent sieve classification was carried out in the same manner as in Example 1.

[0061] [Example 10] The same manufacturing process as in Example 1 was carried out up to HIP treatment, and the polishing process was carried out using a bead mill as follows. Using a polishing slurry prepared by mixing 3.0 wt% of alumina with a particle size of 3 μm (Chipton Co., Ltd., Light 1A) as an abrasive and 0.5 wt% of sodium polycarboxylate (Chukyo Yushi Co., Ltd., Cerna D-305) as a dispersant, the ceramic spherical bodies were polished for a total of 3 hours at a bead mill agitation peripheral speed of 12 m / s, and then co-polished for 2 hours with a slurry containing only 0.5 wt% of D-305 to remove surface residues of the ceramic spherical bodies, obtaining ceramic spherical bodies with a surface roughness Ra = 10 nm. The subsequent sieve classification was carried out in the same manner as in Example 1.

[0062] [Comparative Example 1] In the same manner as in Example 1, the green body was granulated to a size with an average particle size X of around 50 μm after sintering by the rolling granulation molding method. Except for omitting the surface undulation reduction process, it was carried out in the same manner as in Example 1 to produce the spherical media for grinding shown in Table 1.

[0063] [Comparative Example 2] In the same manner as in Example 2, the green body was granulated to a size with an average particle size X of around 100 μm after sintering by the rolling granulation molding method. Except for omitting the surface undulation reduction process, it was carried out in the same manner as in Example 2 to produce the spherical media for grinding shown in Table 1.

[0064] [Comparative Example 3] Similar to Example 3, the green body was granulated to a size such that the average particle size X after sintering was around 200 μm by the rolling granulation method. Except for omitting the surface undulation reduction process, the procedure was the same as in Example 3, and spherical media for grinding shown in Table 1 were produced.

[0065] [Comparative Examples 4 - 6] Yttrium chloride was added to zirconium oxychloride in a ratio shown by the yttria / zirconia molar ratio in Table 1 in terms of oxide conversion in the resulting ceramic spherical bodies, and raw material powders were produced by the coprecipitation method.

[0066] Next, using the above raw material powders, in the same manner as in Example 2, the green body was granulated to a size such that the average particle size after sintering was 100 μm by the rolling granulation method, and the surface undulation reduction process was carried out. After the obtained green body was dried to remove moisture, sintering and HIP treatment were performed. After the surface of the obtained sintered body was polished with a barrel polishing device, sieving classification was carried out to produce spherical media for grinding shown in Table 1.

[0067] [Comparative Example 7] The same manufacturing method as in Comparative Example 1 was carried out up to the HIP process, and the polishing process was carried out under the same polishing conditions using the same bead mill as in Example 8 to obtain a surface smoothness with a surface roughness Ra = 3 nm. Sieving classification was also carried out in the same manner as in Example 8.

[0068] The evaluation results are shown in Tables 1 - 2.

[0069] As shown in Examples 1 - 6, by reducing the surface undulation, ceramic spherical bodies that are difficult to break were obtained. In Example 7, the internal defect rate increased due to lowering the HIP temperature, and the number of cracks slightly increased, but it was within the allowable range.

[0070] In Comparative Examples 1 to 3, since the surface undulation was large, the ceramic spherical bodies were easily damaged. In Comparative Example 4, since the proportion of monoclinic crystals was large, the ceramic spherical bodies were easily damaged. In Comparative Example 5, since the proportion of tetragonal crystals was large, the reduction rate of the crushing load value after the hydrothermal test was large, and the ceramic spherical bodies had a high possibility of being damaged when the water temperature increased. In Comparative Example 6, since the proportion of tetragonal crystals was small, the ceramic spherical bodies were easily damaged.

[0071] Also, as shown in Example 1 and Examples 8 to 10, in the range where the surface roughness Ra = 5 to 20 nm, the wear amount of zirconia in the wet dispersion of barium titanate decreased with the reduction of Ra, but it was the same at 5 nm and 2 nm. Also, the number of cracked ceramic spherical bodies at that time was zero in all of Examples 1, 8 to 10. Also, the crack resistance in the crack test was zero in Examples 8 to 10 as well as in Example 1.

[0072] In Comparative Example 7, the wear amount of zirconia in the wet dispersion of barium titanate was less than that in Example 10 but higher than those in Examples 8 to 9. Cracks were observed in the ceramic spherical bodies, and it is possible that there was an influence of minute crack fragments being mixed into the barium titanate dispersion. Regarding the number of cracks in the crack test, there was no significant difference from Comparative Example 1, and cracks were observed.

[0073]

Table 1

[0074]

Table 2

Explanation of Signs

[0075] 1: Diameter of ceramic spherical body 2: Diameter at X / 2 (μm) 3: Intersection part between the cross-section of the ceramic spherical body with a diameter of X / 2 (μm) and the surface of the spherical body 4: Measurement direction of the maximum height wave Wz 5: Example of the measurement profile of the maximum height wave Wz

Claims

1. A ceramic spherical body having zirconia as a main component, wherein the proportion of tetragonal crystal is 80% by volume or more and 95% by volume or less, and the proportion of monoclinic crystal is 5% by volume or less, wherein when the average particle size is X (μm), the maximum height undulation Wz (μm) at the intersection of the cross-section of the spherical body having a diameter of X / 2 (μm) and the surface of the spherical body is 0.5% or more and 1.2% or less of the average particle size X (μm). The ceramic spherical body is characterized by this.

2. The ceramic spherical body according to Claim 1, wherein the proportion of monoclinic crystal is 0.1% by volume or more.

3. The ceramic spherical body according to Claim 1 or 2, wherein the 1% particle size (D1) in the particle size distribution is 0.7X (μm) or more, and the 99% particle size (D99) is 1.3X (μm) or less.

4. The ceramic spherical body according to any one of Claims 1 to 3, wherein the minimum particle size is 0.7X (μm) or more and the maximum particle size is 1.3X (μm) or less.

5. The ceramic spherical body according to any one of Claims 1 to 4, wherein the internal defect rate of the ceramic spherical body is 0.5% or less.

6. The ceramic spherical body according to any one of Claims 1 to 5, wherein the average particle size of the ceramic spherical body is 30 μm or more and 300 μm or less.

7. The ceramic spherical body according to any one of Claims 1 to 6, wherein the surface roughness Ra is 2.0 nm or more and 5.0 nm or less, and it is used for wet grinding of barium titanate powder.

8. A method for manufacturing a ceramic spherical body according to any one of Claims 1 to 7, comprising a surface undulation reduction step of forming a raw material powder into a spherical shape using a rolling granulation molding method, and then rolling the obtained molded body in a rolling granulator for 10 hours or more while adding only water.

9. The method for manufacturing a ceramic spherical body according to Claim 8, wherein the moisture rate in the rolling granulator during the surface undulation reduction step is 2 to 5% higher than that during granulation growth.

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