Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2001-08-13
2003-11-04
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C501S097100, C501S087000, C501S092000, C384S492000, C384S907100, C264S683000
Reexamination Certificate
active
06642165
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to wear resistant member for electronic equipment in which malfunction due to static electricity is lessened, and a bearing and spindle motor therewith.
2. Related Art
In recent years, development of magnetic recorders such as hard disk drives (HDDS) and floppy disk drives (FDDs), optical disk drives such as CD-ROMs and DVDs, and various kinds of game machines is remarkable. In the electronic equipment, usually, a rotation actuator such as a spindle motor or the like makes a rotating shaft rotate at a high speed, thereby making various kinds of disks attached to the rotating shaft function.
So far, in bearing member supporting such rotating shaft, in particular in bearing balls, metal such as bearing steels or the like is mainly used. However, metal such as the bearing steels is insufficient in wear resistance. Accordingly, in a field where a high-speed rotation of 4,000 rpm or more is required like in the electronic equipment for instance, an increase of dispersion of life results in incapability of providing reliable rotation drive.
To overcome such malfunction, recently ceramic material such as a silicon nitride sintered body is in use for the bearing ball (cf., for instance, Japanese Patent Laid-open Publication No. JP-A 2000-314426). Among the ceramic materials, the silicon nitride sintered body is excellent in sliding properties and has excellent wear resistance. Accordingly, even when rotating at high speeds, rotation drive with mechanical reliability may be provided.
However, the silicon nitride bearing ball is electrically an insulator. Accordingly, there is a problem that when rotating at high speeds, generated static electricity cannot be skillfully liberated to the bearing member other than the rotating shaft or a bearing ball receiver made of metal such as bearing steels. Thus, when the static electricity cannot be skillfully dispersed and is built up unnecessarily on the bearing or peripherals, in a recorder that employs magnetic signals like the HDD for instance, a recording medium is adversely affected. As a result, there are worries that memory in the HDD may be lost, and furthermore the electronic equipment such as the HDD may be destroyed.
In addition, portable personal computers, electronic notebooks and various kinds of mobile products are yearly miniaturized, and to the HDDs or the like used therein demand for higher capacity and more miniaturization is yearly stronger. To cope with such demand, in the HDD for instance, further higher speed rotation is under study, and in the future it is expected to realize a high-speed rotation of 10,000 rpm or more. It is the rotating shaft and the bearing comprising the bearing balls and the bearing ball receiver that supports such high-speed rotation, excessive pressure thereof being substantially concentrated on the bearing balls.
In addition, when rotating at high speeds, on the basis of sliding of the bearing balls, an amount of generated heat (frictional heat) increases. The bearing ball made of the existing silicon nitride sintered body is such low in thermal conductivity as approximately 20 W/m·K. Accordingly, the frictional heat cannot be effectively dissipated. AS the rotation speed of the bearing becomes higher, lower heat dissipation capability causes more problems. From these circumstances also, the existing silicon nitride bearing ball is said cannot sufficiently cope with the high speed rotation of a long time.
On the other hand, a conductive silicon nitride sintered body of which electrical resistivity is approximately 10
−5
&OHgr;·m is known (cf. Japanese Patent Publication No. HEI 2-43699 JP-B). Such conductive silicon nitride sintered bodies are used in material for manufacturing a blade or nozzle of a turbine engine by means of electric discharge technique. Therein, a lot of conductivity enhancer such as metal carbides or metal nitrides is added to realize conductive property (lower electrical resistance). In such existing conductive silicon nitride sintered body, though electrical resistance itself may be lowered, much added conductivity enhancer agglomerates with ease, agglomerated particles of the conductivity enhancer causing to lower mechanical strength or the sliding properties of the silicon nitride sintered body.
For instance, in the use where compressive and/or tensile stress is repeatedly exerted to cause fatigue like in the case of the bearing balls, a number of agglomerated particles tend to cause cracks therefrom, resulting in largely deteriorating sliding properties. Accordingly, the existing conductive silicon nitride sintered body as disclosed in the above publication is very difficult to apply in the wear resistant member such as the bearing ball. The above publication does not assume to apply the conductive silicon nitride sintered body in the sliding member, but only assume to enhance electrical conductivity to make use of electric discharge machining.
Japanese Patent Publication No. HEI 7-29855 JP-B, Japanese Patent No. 2566580 and Japanese Patent Laid-open Publication No. HEI 6-227870 JP-A also disclose the silicon nitride sintered body including conductive compounds such as metal carbides or metal nitrides. In Japanese Patent Publication No. HEI 7-29855 JP-B, a silicon nitride sintered body is disclosed in which SiC in the range from 0.1 to 11% by mass is added to a concoction of silicon nitride and rare earth compound. In Japanese Patent No. 2566580 publication, a composite sintered body of silicon nitride and silicon carbide is disclosed. Furthermore, in Japanese Patent Laid-open Publication No. HEI 6-227870 JP-A, a composite sintered body is disclosed in which a dispersant such as silicon carbide or titanium nitride is dispersed in a silicon nitride matrix in three-dimensional network. However, the technologies disclosed in the publications cannot necessarily give the composite sintered body in which appropriate electrical conductive properties and excellent sliding properties are present at the same time.
Furthermore, in Japanese Patent Laid-open Publication No. HEI 8-296649 JP-A, ceramic material having volume resistivity of 10
4
&OHgr;·m or less is disclosed to apply in a dynamic pressure bearing having a spiral groove generating a dynamic pressure effect. However, ceramic materials used there are sintered bodies mainly composed of conductive ceramics such as SiC, TiC, TiN and TiB
2
, the composite sintered body of Al
2
O
3
and TiC, and the composite sintered body of ZrO
2
and NiO. That is, the ceramic materials in which excellent sliding properties and appropriate electrical conductive properties are present at the same time are not disclosed.
BRIEF SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide wear resistant member for electronic equipment that when applying in bearing balls or the like, may realize a high speed rotation with stability, and in addition suppress unnecessary build-up of static electricity. Furthermore, another object is to provide a bearing and a spindle motor that by the use of such wear resistant member for electronic equipment, enable to realize higher performance and higher reliability of the electronic equipment such as the magnetic recorders like the HDDs and the optical disk drives like the DVDs.
The present wear resistant member for electronic equipment is one that comprises a silicon nitride sintered body containing particles of conductivity enhancer and having electrical resistivity in the range from 1 to 10
5
&OHgr;·m. The silicon nitride sintered body comprises agglomerations of the particles of conductivity enhancer in which distances between the respective particles of conductivity enhancer are less than 1 &mgr;m, the agglomerations of the particles of conductivity enhancer being present in the range of 30% or less by area ratio per unit area in the silicon nitride sintered body.
In the present wear resistant member for electronic equipment, particles of conductivity enhancer to be contained in the silicon
Fukuda Yoshiyuki
Komatsu Michiyasu
Miyashita Kimiya
Shinosawa Katsuhiro
Takao Minoru
Foley & Lardner
Group Karl
Kabushiki Kaisha Toshiba
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