Abstract: A ceramic dynamic-pressure bearing having dynamic-pressure grooves. At least a portion including a surface (dynamic-pressure gap definition surface) facing a dynamic-pressure gap is formed of ceramic, and dynamic-pressure grooves GV are formed, by electrolytic etching, on the ceramic dynamic-pressure gap definition surface such that, on a cross section of the groove GV taken perpendicular to the longitudinal direction of the groove GV, a curvature portion R having a radius of 3-7 &mgr;m is formed at a meeting position where a groove sidewall surface SG and a groove bottom surface BG meet. In a preferred embodiment, a large number of pores K are formed on one or more of the rotary surfaces M of the dynamic-pressure bearing formed of ceramic such that the surface porosity falls in the range of 10 to 60%.

Abstract: A zirconia containing ceramic ball is formed of a zirconia containing ceramic material which contains in an amount of not less than 10% by volume the zirconia ceramic phase composed predominantly of zirconium oxide. The size of the largest pore among pores present in a region extending radially from the surface of the ball to a depth of 50 &mgr;m as observed on a polished cross section of the ball taken substantially across the center of the ball is not greater than 3 &mgr;m. The ceramic ball can be manufactured through firing a green body whose relative density is enhanced to not lower than 61% by use of rolling granulation.

Abstract: A ceramic dynamic pressure bearing, includes: a first member having an outer periphery formed with a gap forming surface causing a radial dynamic pressure, and a second member having an inner periphery defining a through hole formed with a gap forming surface causing the radial dynamic pressure. The first member is inserted into the through hole as to form a radial gap. The first member and the second member make rotation relative to each other to cause a fluid dynamic pressure at the radial gap. The first member and the second member are composed of an alumina ceramic including: an aluminum 90% to 99.5% by weight, and an oxide sintering assistant 0.5% to 10% by weight. The outer periphery has cylindricity not larger than 1.0 &mgr;m, and roundness not larger than 0.5 &mgr;m. The inner periphery has cylindricity not larger than 1.5 &mgr;m, and roundness not larger than 1.0 &mgr;m.

Abstract: A ceramic dynamic pressure bearing comprising a first member and a second member, wherein the first member and the second member define a dynamic pressure gap therebetween, the first member and the second member relatively rotate around an axis of rotation, the relative rotation generates a fluid dynamic pressure in the dynamic pressure gap, the first member has a first surface directing to the dynamic pressure gap and the second member has a second surface directing to the dynamic pressure gap, and at least one of the first and second surfaces comprises ceramic, the ceramic comprises: a ceramic matrix that is selected from alumina ceramic, zirconia ceramic and silicon nitride ceramic; and 15 vol % to 70 vol % of an electrically-conductive inorganic compound phase based on the total amount of the ceramic, the phase is dispersed in the ceramic matrix, the phase comprises at least one compound selected from tungsten carbide, silicon carbide, conductive oxide, metallic nitride, metallic carbide, metallic boride,

Abstract: A ceramic dynamic-pressure bearing which is not prone to wear or a like problem associated with starting, stopping, or a like operation mode and which can realize smooth rotation. A dynamic-pressure gap 17 is formed between a first member 14 and a second member 15, which undergo relative rotation about a predetermined axis of rotation. The relative rotation of the first member 14 and the second member 15 generate fluid dynamic-pressure in the dynamic-pressure gap 17. At least a portion of one or both of the first member 14 and the second member 15 includes a polished surface (a dynamic-pressure gap definition surface) formed of alumina ceramic facing the dynamic-pressure gap 17, and the alumina ceramic, which forms the dynamic-pressure gap definition surface finished by polishing, is adjusted to have an apparent density of 3.5-3.9 g/cm3. Also disclosed is a motor, hard disk drive and polygon scanner containing the bearing.

Abstract: A ceramic bearing ball in which at least a portion of a constituent ceramic is formed of an electrically conductive inorganic compound phase, whereby a proper electrical conductivity is imparted to the ceramic. Thus, electrifying of a bearing ball is prevented or effectively suppressed. This prevents the problem involved in production of balls of small diameter wherein such balls adhere to an apparatus (e.g., a container) during production thereof, thus hindering smooth progress of the production process. In addition, when ceramic balls are used in precision electronic equipment, such as a hard disk drive of a computer, which is operated at high rotational speed, adhesion of foreign substance due to electrification of the balls, and resultant generation of abnormal noise or vibration can be prevented or effectively suppressed.

Abstract: A conductive ceramic bearing ball made of composite ceramic and having a mixed microstructure of a silicon nitride phase and a titanium nitride phase. The content ratio of the silicon nitride phase to the titanium nitride phase is such that the conductive ceramic bearing ball has an average coefficient of linear expansion within a temperature range of 20° C.-100° C. of from 2×10−6/K to 5×10−6/K.

Abstract: A high-purity alumina sintered body having an alumina purity of not less than 99.9% by mass and a relative density of not less than 97% and exhibiting a weight loss of not greater than 100×10−4 kg/m2 when immersed in boiling 6N H2SO4 or 6N NaOH aqueous solution for 24 hours as measured according to JIS R1614 (1993). The high-purity alumina sintered body is obtained by firing a green body formed from an alumina powder having an alumina purity of not less than 99.9% by mass and containing, as impurities, Si, an Mg, Fe and alkali metals including Na, K and Li in a total amount of less than 100 ppm.

Abstract: A ceramic dynamic-pressure bearing having dynamic-pressure grooves. At least a portion including a surface (dynamic-pressure gap definition surface) facing a dynamic-pressure gap is formed of ceramic, and dynamic-pressure grooves GV are formed, by electrolytic etching, on the ceramic dynamic-pressure gap definition surface such that, on a cross section of the groove GV taken perpendicular to the longitudinal direction of the groove GV, a curvature portion R having a radius of 3-7 &mgr;m is formed at a meeting position where a groove sidewall surface SG and a groove bottom surface BG meet. In a preferred embodiment, a large number of pores K are formed on one or more of the rotary surfaces M of the dynamic-pressure bearing formed of ceramic such that the surface porosity falls in the range of 10 to 60%.

Abstract: A ceramic dynamic pressure bearing comprising a first member and a second member, wherein the first member and the second member define a dynamic pressure gap therebetween, the first member and the second member relatively rotate around an axis of rotation, the relative rotation generates a fluid dynamic pressure in the dynamic pressure gap, the first member has a first surface directing to the dynamic pressure gap and the second member has a second surface directing to the dynamic pressure gap, and at least one of the first and second surfaces comprises ceramic, the ceramic comprises: a ceramic matrix that is selected from alumina ceramic, zirconia ceramic and silicon nitride ceramic; and 15 vol % to 70 vol % of an electrically-conductive inorganic compound phase based on the total amount of the ceramic, the phase is dispersed in the ceramic matrix, the phase comprises at least one compound selected from tungsten carbide, silicon carbide, conductive oxide, metallic nitride, metallic carbide, metallic boride,

Abstract: A ceramic dynamic pressure bearing, includes: a first member having an outer periphery formed with a gap forming surface causing a radial dynamic pressure, and a second member having an inner periphery defining a through hole formed with a gap forming surface causing the radial dynamic pressure. The first member is inserted into the through hole as to form a radial gap. The first member and the second member make rotation relative to each other to cause a fluid dynamic pressure at the radial gap. The first member and the second member are composed of an alumina ceramic including: an aluminum 90% to 99.5% by weight, and an oxide sintering assistant 0.5% to 10% by weight. The outer periphery has cylindricity not larger than 1.0 &mgr;m, and roundness not larger than 0.5 &mgr;m. The inner periphery has cylindricity not larger than 1.5 &mgr;m, and roundness not larger than 1.0 &mgr;m.

Abstract: A method of making spherical bodies from powder, which comprises (1) preparing an adjusted powder so as to have at least one powder characteristic selected from the group consisting of an average particle size, a powder particle size distribution and a BET specific surface area, (2) preparing nuclei having a particle size larger than that of the adjusted powder, (3) rotating the nuclei, and (4) adding the adjusted powder and a solvent to the rotating nuclei so that particles of the adjusted powder accumulate on the nuclei to form granular bodies. Also disclosed is a spherical body having a core or nucleus formed in the spherical body, an adjusted powder composition for forming a spherical body, and a method for manufacturing spherical sintered bodies of silicon nitride.

Abstract: A conductive ceramic bearing ball made of composite ceramic and having a mixed microstructure of a silicon nitride phase and a titanium nitride phase. The content ratio of the silicon nitride phase to the titanium nitride phase is such that the conductive ceramic bearing ball has an average coefficient of linear expansion within a temperature range of 20° C.-100° C. of from 2×10−6/K to 5×10−6/K.

Abstract: A ceramic ball is described wherein no magnetic inclusion is observed on a surface of said ceramic ball, or when a magnetic inclusion is observed on the surface, dmax of the observed magnetic inclusion is not greater than 20 &mgr;m, wherein dmax is the distance between parallel lines circumscribing the observed magnetic inclusion and whose distance is the greatest among such circumscribing parallel lines.

Abstract: A silicon nitride sintered body, e.g., a ball, exhibiting excellent machinability is disclosed, having an X-ray diffraction profile measured on a cross section of the silicon nitride sintered body by means of a diffractometer such that X&bgr;/(X&agr;+X&bgr;) is 0.9−1, wherein X&agr; is the highest peak intensity associated with a silicon nitride phase and X&bgr; is the highest peak intensity associated with &bgr; silicon nitride phase, and silicon nitride grains of said body have an average grain size dav of 0.5-5.0 &mgr;m and a grain aspect ratio &agr; of 1-5, and are present in an average number of not less than 2×104 per square millimeter in a predominant crystalline phase observed on the cross section.

Abstract: A ceramic bearing ball in which at least a portion of a constituent ceramic is formed of an electrically conductive inorganic compound phase, whereby a proper electrical conductivity is imparted to the ceramic. Thus, electrifying of a bearing ball is prevented or effectively suppressed. This prevents the problem involved in production of balls of small diameter wherein such balls adhere to an apparatus (e.g., a container) during production thereof, thus hindering smooth progress of the production process. In addition, when ceramic balls are used in precision electronic equipment, such as a hard disk drive of a computer, which is operated at high rotational speed, adhesion of foreign substance due to electrification of the balls, and resultant generation of abnormal noise or vibration can be prevented or effectively suppressed.

Abstract: A high-purity alumina sintered body having an alumina purity of not less than 99.9% by mass and a relative density of not less than 97% and exhibiting a weight loss of not greater than 100×10-−4 kg/m2 when immersed in boiling 6N H2SO4 or 6N NaOH aqueous solution for 24 hours as measured according to JIS R1614 (1993). The high-purity alumina sintered body is obtained by firing a green body formed from an alumina powder having an alumina purity of not less than 99.9% by mass and containing, as impurities, Si, an Mg, Fe and alkali metals including Na, K and Li in a total amount of less than 100 ppm.

Abstract: A zirconia containing ceramic ball is formed of a zirconia containing ceramic material which contains in an amount of not less than 10% by volume the zirconia ceramic phase composed predominantly of zirconium oxide. The size of the largest pore among pores present in a region extending radially from the surface of the ball to a depth of 50 &mgr;m as observed on a polished cross section of the ball taken substantially across the center of the ball is not greater than 3 &mgr;m. The ceramic ball can be manufactured through firing a green body whose relative density is enhanced to not lower than 61% by use of rolling granulation.

Abstract: A sintered ceramic body which can be worked by electrical discharge machining, has high mechanical strength, and can be worked into sintered ceramic tools which have long life is disclosed. Further, sintered ceramic tools which have long life and scarcely cause flaws in work pieces are provided. The sintered ceramic body comprises ceramic particles having specific particle diameter and particle distribution.

Abstract: A sintered ceramic body which can be worked by electrical discharge machining, has high mechanical strength, and can be worked into sintered ceramic tools which have long life is disclosed. Further, sintered ceramic tools which have long life and scarcely cause flaws in work pieces are provided. The sintered ceramic body comprises ceramic particles having specific particle diameter and particle distribution.