Although a sintered ceramic requires higher manufacturing cost than metal such as steel, it has high mechanical strength and excellent wear resistance and rigidity, has a lighter specific weight than steel and has an insulating property and high corrosion resistance.
By utilizing these characteristics, and being used as a wear resistant member for a sliding device such as a bearing or a ball nut, or a valve and the like of a fluidic valve which controls a high pressure fluid, it is possible to reduce the weight, prevent damages due to a load and repetitions of sliding or damages such as wear, corrosion and electrical corrosion, maintain performance for a long period of time, increase the life of components and reduce maintenance labor.
For bearings and the like which are used in an environment particularly in the vicinity of wind power generators, compressors of air-conditioners, vehicles and the like of electrical vehicles and hybrid vehicles and electrical systems, and in which the temperature and humidity significantly change, ceramic spheres are frequently employed instead of metals such as steel which is significantly influenced by damages and the like by corrosion or electric corrosion and requires lower manufacturing cost, since the ceramic spheres require lower maintenance cost than metals such as steel.
Further, a rigid, light weight and long-life valve body is required for a fluidic valve in particular which is opened and closed at a high speed under a high pressure, and therefore employing a ceramic sphere for the valve body brings a significant advantage.
A general ceramic is sintered using a plurality of raw materials and sintering agents and, when, for example, a silicon nitride sintered compact is sintered, silicon nitride (Si3N4) which is a raw material hardly causes solid-state sintering itself and cannot provide a dense sintered compact, and therefore a rare-earth oxide such as Y2O3 and an oxide such as Al2O3 are mixed and molded as sintering agents and are densified by liquid-phase sintering to obtain a silicon nitride sintered compact.
Thus, when a ceramic is sintered using a plurality of raw materials and sintering agents, fine flaws are produced on a surface or in an inner part depending on conditions and these flaws cause surface peeling due to fatigue resulting from repetitions of loading.
For example, known Patent Literature 1 discloses defining the porosity of a sintered compact and the maximum air hole diameter in a grain boundary phase, and obtaining a wear resistant member made of a silicon nitride sintered compact which has an excellent rolling life because flaws such as scratches and cracks on a rolling element surface lead to deterioration of reliability of quality.
However, the technique disclosed in Patent Literature 1 only targets at flaws such as scratches, the cracks and the air holes, and does not provide any countermeasure for white spots (snow flakes) which influence more on wear resistance and durability than on the scratches, cracks and air holes.
Further, known Patent Literature 2 discloses focusing on the composition which is observed as white branches formed with an aggregate of micropores (fine air holes corresponding to fine flaws) in a range of from a surface to the depth of 1 mm, and, if the aggregate of the micropores has a given size or less, causing peeling due to rolling fatigue which causes a trouble when a ball is used for a bearing material, irrespective of the rate of an area which occupies in the entire area of the ball.
However, although the aggregate of micropores has a small size, if the amount of micropores is great, a crushing load decreases, thereby causing damages and causing surface peeling due to fatigue resulting from repetitions of loading.
Further, this white branch portion has different characteristics such as the rigidity and the density from other portions, and therefore, even a small white branch portion blocks improvement in dimensional accuracy such as the sphericity and the surface roughness when a silicon nitride sphere or a silicon nitride roller which is a wear resistant member is polished and processed, resulting in causing surface peeling due to fatigue resulting from repetitions of loading.
Furthermore, there are problems that flaws or remaining internal distortion inside a silicon nitride sphere or a silicon nitride roller which is a wear resistant member make an internal stress state uneven and become starting points of destruction, or cause wear or vibration because dimensional accuracy does not improve as described above.
Still further, when a ceramic sphere is inspected as a product, a means is required for observing and inspecting that there are no flaws such as scratches, cracks and air holes near a surface nor snow flakes without destroying the ceramic sphere.
As a device which inspects a ceramic sphere without destroying the ceramic sphere, a device which optically observes the surface of the ceramic sphere as disclosed in, for example, Patent Literature 3 and a device which observes the surface of the ceramic sphere and the inner part of the surface layer by means of an ultrasonic wave as disclosed in Patent Literature 4 are known.