Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2002-01-11
2004-08-31
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C501S097200, C501S097300, C384S492000, C384S907100
Reexamination Certificate
active
06784131
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to a wear resistant member mainly composed of silicon nitride and a method of manufacturing the member, and more particularly to a silicon nitride wear resistant member and a method of manufacturing the member capable of exhibiting excellent wear resistance, particularly rolling life characteristics when the wear resistant member is used as rolling bearing member, and is suitable as a material for constituting a rolling bearing member requiring an excellent durability.
2. Description of the Related Art
Wear resistant member (abrasion resistant member) has been applied to various fields such as, for example, bearing member, various roller members for rolling operation, compressor vane, gas-turbine blade, engine parts such as cam roller or the like. As a material for constituting the wear resistant member, various ceramic materials have been conventionally used. In particular, silicon nitride sintered body is excellent in mechanical strength and wear resistance property. For this reason, the silicon nitride sintered body has been applied to various technical fields.
Various sintering compositions for the silicon nitride sintered bodies are well known: such as silicon nitride/yttrium oxide/aluminum oxide system; silicon nitride/yttrium oxide/aluminum oxide/aluminum nitride system; and silicon nitride/yttrium oxide/aluminum oxide/titanium oxide system or the like. Sintering assistant agents composed of the oxides of rare earth elements, such as yttrium oxide (Y
2
O
3
) in the sintering compositions listed above, have a function of generating grain boundary phase (liquid phase) composed of Si-rare earth element-Al—O—N or the like during the sintering operation. Therefore, the sintering assistant agents are added to a material composition for enhancing the sintering characteristics of sintering materials, and achieve high density and high strength of the sintered bodies.
According to the conventional art, silicon nitride sintered bodies are generally mass-produced as follows. After a sintering assistant agent as mentioned above is added to the material powder of silicon nitride, the material mixture is molded to form a compact. Thus obtained compact is then sintered in a sintering furnace at about 1,650-1,900° C. for a predetermined period of time followed by naturally cooling (self-cooling) the resultant sintered body in the furnace at a high cooling rate.
Among the various applications to the wear resistant members using the above silicon nitride sintered body, the bearing member has been generally recognized to be useful material. Such bearing member has been used to various applications, and also started to be reviewed to use as an important protection safety parts. For this reason, the bearing member composed of silicon nitride sintered body i.e. rolling bodies such as ball and roller or the like has been required to further improve reliability.
For example, defects such as flaw and crack or the like formed on a surface of the rolling body will result to a breakage of not only the bearing member per se but also an entire system using the bearing member. Therefore, there is adopted a manufacturing process for excluding or eliminating such defects as completely as possible. In a similar way, a pore existing close to the surface of the rolling body would also be a cause of deteriorating the reliability of the bearing member, so that the pore is removed at a process when the member is worked into a product having a final shape.
Although the silicon nitride sintered body produced by the conventional method achieves an improved bending strength, fracture toughness and wear resistance, however, the improvement is insufficient. A durability as a rolling bearing member requiring a particularly excellent sliding property is insufficient, so that a further improvement has been demanded.
In these days, a demand of ceramic material as precision device members has increased. In these applications, advantages such as high hardness and light weight together with high corrosion resistance and low thermal expansion property of the ceramic are utilized. In particular, in view of the high-hardness and high wear resistance, application as a wear resistant member for constituting a sliding portion of the bearing or the like has been rapidly extended.
However, in a case where rolling balls of a bearing or the like were constituted by the wear resistant member composed of ceramic, when the rolling balls were rolled while being repeatedly contacted with counterpart at a high stress level, the rolling life of the wear resistant member was not sufficient yet. Therefore, a surface of the wear resistant member is peeled off and the member causes cracks, so that the defective member was liable to causes vibration and damage to a device equipped with the bearing. At any rate, there had been posed a problem that the durability and reliability as a material for constituting the parts of the device was low.
The silicon nitride sintered body produced by the conventional manufacturing method was inevitably formed with defects such as flaws and cracks at a stage after sintering operation, the defects were formed at portions not only the surface of the sintered body but also a deep portion so that the defects extend to a relatively inner portion of the sintered body. These defects lead to defective products, or even if the defects do not lead to the defective products, a manufacturing manpower for obtaining a surface having a high reliability by removing the cracks or the like i.e. a manpower required for a process of a surface grinding operation till the cracks or the like were substantially eliminated was disadvantageously increased, thus leads to an increase of the manufacturing cost of the rolling bodies.
That is, when a silicon nitride molded body is sintered, a part of impurity oxygen contained in the silicon nitride powder (material powder) and a part of oxygen contained in the sintering assistant agent are evaporated thereby to generate a gas component. The gas component is simultaneously generated at an almost the same time when the silicon nitride molded body starts to shrink at the sintering operation. In ordinary sintering method, since a densification starts at a surface portion of the sintered body in accordance with the start of the shrinkage of the molded body, it is difficult to remove the gas component contained in the inner portion of the sintered body.
When the above gas component remains in the sintered body, pores are formed in the silicon nitride sintered body, and oxygen is combined with Si thereby to remain as SiO
2
. In the conventional manufacturing method, since the gas component could not be sufficiently removed, the pores and SiO
2
resulting from the gas component remain in a relatively broad region, and these pores and SiO
2
caused a crack or the like extending towards inner portion of the sintered body. In order to remove the cracks, it was required to remove the surface of the sintered body to some depth extent, so that these removal operations invites the occurrence of defects and an increase of the manufacturing cost of the sintered body.
Further, for example, when the sintered body is subjected to HIP treatment thereby to fill the pores with a liquid phase formed by the sintering assistant agent, the resultant silicon nitride sintered body can be highly densified. However, a segregation of the liquid phase component are disadvantageously formed to a portion where the pores were existing. The liquid phase components have strength and hardness lower than those of silicon nitride crystal grains, so that the segregation of the liquid phase component becomes a starting point of a breakage when the silicon nitride sintered body is used as the wear resistant member. Accordingly, such an aggregated substance is also required to be removed.
At any rate, in the conventional method of manufacturing the silicon nitride sintered body, there has been posed a problem that pores, cracks, segregations
Komatsu Michiyasu
Komorita Hiroshi
Tonai Hiroki
Group Karl
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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