Ceramic dynamic pressure bearing, motor with bearing, hard disk device, and polygon scanner

A ceramic dynamic pressure bearing, comprising:

• a first member having a substantially cylindrical outer periphery which is formed with a gap forming surface for causing a radial dynamic pressure, the first member being composed of an alumina ceramic comprising:
• an aluminum in a range from 90% to 99.5% by weight, the figures in % being an Al2O3 conversion, and
• an oxide sintering assistant in a range from 0.5% to 10% by weight, the figures in % being an oxide conversion,
• the outer periphery of the first member having a cylindricity not larger than 1.0 μm, and a roundness not larger than 0.5 μm which is measured in an arbitrary cross section perpendicular to an axis of the first member;
• and
• a second member having an inner periphery defining a substantially cylindrical through hole which is formed with a gap forming surface for causing the radial dynamic pressure, the first member being inserted into the through hole of the second member in such a manner as to form a radial gap between the gap forming surface of the first member and the gap forming surface of the second member, the first member and the second member making a rotation relative to each other so as to cause a fluid dynamic pressure at the radial gap, the second member being composed of the alumina ceramic comprising:
• the aluminum in the range from 90% to 99.5% by weight, the figures in % being the Al2O3 conversion, and
• the oxide sintering assistant in the range from 0.5% to 10% by weight, the figures in % being the oxide conversion,
• the inner periphery defining the through hole of the second member having a cylindricity not larger than 1.5 μm, and a roundness not larger than 1.0 μm which is measured in an arbitrary cross section perpendicular to an axis of the second member.