Method of cutting grooves in hydrodynamic bearing made of ceramic material

A ceramic hydrodynamic bearing used as a radial or a thrust bearing in a spindle motor for a hard disk driver. The bearing includes a ceramic bearing base having hydrodynamic pressure generating grooves. A sliding surface of the bearing base is irradiated with an energy beam with a high energy density to remove said material, thereby forming hydrodynamic pressure generating groove, and at the same time the grooves are coated with a modification layer that is formed by melting the ceramic material that constitutes said ceramic bearing base with the energy beam and by solidifying said ceramic bearing base.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of cutting grooves in a ceramic 
hydrodynamic bearing which is suitably used as radial and thrust bearings 
for a rotating member that rotates at high speed. 
Recently, with the increase in the speed of spindle motors for HDDs (Hard 
Disk Drivers), radial and thrust bearings of high performance have been 
demanded, and employment of ceramic hydrodynamic bearings has been 
proposed as one technique to meet the demand. In the ceramic hydrodynamic 
bearings, it is essential to minimize the absolute value of the number of 
fine particles generated from the sliding surface. 
Hitherto, shot blasting process has been mainly employed as a method of 
cutting hydrodynamic pressure generating grooves in the sliding surface of 
such a ceramic hydrodynamic bearing. 
However, if groove cutting is effected by shot blasting process, cuticles 
(pineal) 2b are formed on the surface of the machined portion, that is, 
the portion formed with a hydrodynamic pressure generating groove 2, as 
shown in FIG. 3, so that the abrasive or cuttings are caught between the 
cuticles 2b and it is not easy to wash them off. The abrasive or cuttings 
that are left unremoved by washing constitute a large source of generation 
of fine particles. Meantime, cuticles themselves, which are likely to 
separate, constitute a source of generation of fine particles. In 
addition, a large number of microcracks are generated under some shot 
blasting conditions, and these microcracks invite an increase in the 
amount of fine particles. 
JP. A, 58-179588 discloses a method of forming grooves in a ceramic 
hydrodynamic bearing wherein a laser beam is irradiated only to the 
required place to form the grooves. The burrs at circumferential edges of 
each groove are removed by finishing after forming grooves. 
SUMMARY OF THE INVENTION 
In view of these circumstances, it is an object of the present invention to 
solve the above-described problems of the prior art and provide a method 
of cutting grooves in a ceramic hydrodynamic bearing which is designed so 
that it is possible to reduce the amount of fine particles generated from 
the surface that is formed with hydrodynamic pressure generating grooves 
or it is possible to eliminate the generation of such fine particles. The 
other object of the present invention is to provide a method of cutting 
grooves in the above-described ceramic hydrodynamic bearing without 
generating burrs at the edges of the grooves. 
To attain the above-described object, the present invention provides a 
method of cutting grooves in a ceramic hydrodynamic bearing including a 
ceramic bearing base having hydrodynamic pressure generating grooves with 
a predetermined configuration formed in a sliding surface thereof The 
inner surfaces of the hydrodynamic pressure generating grooves have a 
cross-sectional configuration that the depth is from 3 to 20 .mu.m and the 
ratio of the depth to the width is from 10.sup.-4 to 10.sup.-2. The inner 
surfaces are coated with a modification layer that is formed by melting 
and solidifying a ceramic material that constitutes the ceramic bearing 
base. 
In the method of cutting grooves in a ceramic hydrodynamic bearing, a 
sliding surface of a bearing base made of a ceramic material is irradiated 
with an energy beam with a high energy density from an energy beam 
irradiation device to remove the ceramic material, thereby forming 
hydrodynamic pressure generating grooves with a predetermined 
configuration, and at the same time, coating the inner surfaces of the 
hydrodynamic pressure generating grooves with a modification layer that is 
formed by melting and solidifying the ceramic material of the bearing 
base. 
A Q-switch YAG laser device may be employed as the energy beam irradiation 
device to apply a YAG pulse laser beam with a mean power of 100 Watt and 
downward, a pulse repetition of 3 to 10 kHz, and an energy density of not 
higher than 300 J/cm.sup.2 for a ceramic material of SiC or Si.sub.3 
N.sub.4, and an energy density of 200 to 600 J/cm.sup.2 for a ceramic 
material of Al.sub.2 O.sub.3, thereby forming the hydrodynamic pressure 
generating grooves and, at the same time, forming the modification layer 
on the surfaces thereof without generating burrs at the edges of the 
grooves. 
An excimer laser device may be employed as the energy beam irradiation 
device to apply an excimer laser beam with an appropriate energy density, 
thereby forming the hydrodynamic pressure generating grooves and, at the 
same time, forming the modification layer on the surfaces thereof. 
Since the inner surfaces of the hydrodynamic pressure generating grooves 
are coated with a modification layer that is formed by melting and 
solidifying the ceramic material that constitutes the bearing base of the 
ceramic hydrodynamic bearing, the surface of the modification layer is 
extremely smooth, so that there is no possibility that cuticles (pineal) 
will be formed on the surface of the machined portion as in the prior art 
wherein groove cutting is effected by shot blasting process which is 
attended with the problem that the cuticles themselves, or the abrasive or 
cuttings that are caught between the cuticles, constitute a large source 
of generation of fine particles. 
In addition, a Q-switch YAG laser device is employed to apply a YAG pulse 
laser beam with a mean power of 100 Watt and downward, a pulse repetition 
of 3 to 10 kHz, and an energy density of not higher than 300 J/cm.sup.2 
for a ceramic material of SiC or Si.sub.3 N.sub.4, and an energy density 
of 200 to 600 J/cm.sup.2 for a ceramic material of Al.sub.2 O.sub.3, 
thereby removing the ceramic base material in the irradiated portion, and 
thus forming hydrodynamic pressure generating grooves and, at the same 
time, coating the surfaces of the grooves with a modification layer which 
is formed by melting and solidifying the ceramic base material. 
Accordingly, it is possible to produce a ceramic hydrodynamic bearing with 
no or minimal generation of fine particles extremely easily.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
One embodiment of the present invention will be described below with 
reference to the accompanying drawings. 
FIG. 2 is a plan view of a ceramic hydrodynamic thrust bearing produced by 
the method of cutting grooves in a ceramic hydrodynamic bearing according 
to the present invention, and FIG. 1 is a sectional view of a hydrodynamic 
pressure generating groove portion of the thrust bearing. As shown in 
these figures, a sliding surface 1a of a ceramic bearing base 1 is formed 
with hydrodynamic pressure generating grooves 2. The hydrodynamic pressure 
generating grooves 2 are spiral grooves disposed at predetermined spacings 
in the circumferential direction. The inner surface of each hydrodynamic 
pressure generating groove 2 is coated with a thin modification layer 2a 
that is formed by melting and solidifying a ceramic material that 
constitutes the ceramic bearing base 1. The hydrodynamic pressure 
generating grooves 2a and the modification layer 2a are formed on the 
sliding surface 1a of the ceramic bearing base 1 by applying a Q-switch 
YAG pulse laser beam to the surface of the ceramic bearing base 1 made of 
SiC in the shape of the hydrodynamic pressure generating grooves 2 to 
thereby remove the base material in the portions irradiated with the laser 
beam. In this case, the power density of the Q-switch YAG pulse laser beam 
is set to be not higher than 300 J/cm.sup.2, whereby at the same time as 
the hydrodynamic pressure generating grooves 2 are formed in the sliding 
surface 1a of the bearing base 1, the modification layer 2a is formed on 
the surfaces of the grooves 2 from the ceramic base material that is 
melted and then solidified. If the energy density of the laser beam 
exceeds 300 J/cm.sup.2, microcracks 2c are generated in the solidified 
modification layer 2, as shown in FIG. 5, and the solidified layer becomes 
extremely thick. For this reason, it is necessary in order to form a 
modification layer 2a with a smooth surface, as shown in FIG. 4, to set 
the energy density at a level which is not higher than 300 J/cm.sup.2 and 
at which the base material can be removed. 
FIG. 6 shows a cross-section of a groove 2 formed by a YAG pulse laser 
under the following conditions. 
Material of sliding surface 1: SiC 
Mean power of the pulse laser: 36 Watt 
Pulse repetition of the pulse laser: 10 kHz 
Energy density of the pulse laser: 250 J/cm.sup.2 
The groove shown in FIG. 6 has a cross-sectional configuration that the 
depth is about 18 .mu.m and the width is about 2 mm, and no burrs along 
the groove. 
FIG. 7 shows a cross-section of a groove 2 formed by a YAG pulse laser 
under the following conditions. 
Material of sliding surface 1: SiC 
Mean power of the pulse laser: 27 Watt 
Pulse repetition of the pulse laser: 5 kHz 
Energy density of the pulse laser: 320 J/cm.sup.2 
The groove shown in FIG. 7 has a cross-sectional configuration that the 
depth is about 16 .mu.m and the width is about 1.8 mm, and has burrs B 
along the edges of the groove. 
FIG. 8 shows a cross-section of a groove 2 formed by a YAG pulse laser 
under the following conditions. 
Material of sliding surface 1: Al.sub.2 O.sub.3 
Mean power of the pulse laser: 60 Watt 
Pulse repetition of the pulse laser: 10 kHz 
Energy density of the pulse laser: 400 J/cm.sup.2 
The groove shown in FIG. 8 has a cross-sectional configuration that the 
depth is about 14 .mu.m and the width is about 1.3 mm, and no burrs along 
the groove. 
FIG. 9 shows a cross-section of a groove 2 formed by a YAG pulse laser 
under the following conditions. 
Material of sliding surface 1:Al.sub.2 O.sub.3 
Mean power of the pulse laser: 60 Watt 
Pulse repetition of the pulse laser: 10 kHz 
Energy density of the pulse laser: 800 J/cm.sup.2 
The groove shown in FIG. 9 has a cross-sectional configuration that the 
depth is about 15 .mu.m and the width is about 1.3 mm, and has burrs B 
along the edges of the groove. 
FIG. 10 shows a cross-section of a groove 2 formed by a YAG pulse laser 
under the following conditions. 
Material of sliding surface 1:Al.sub.2 O.sub.3 
Mean power of the pulse laser: 18 Watt 
Pulse repetition of the pulse laser: 10 kHz 
Energy density of the pulse laser: 180 J/cm.sup.2 
The groove shown in FIG. 10 has a cross-sectional configuration that the 
depth is about 2 .mu.m and the width is about 1.5 mm, and no burrs along 
the groove. 
The energy density is not sufficient to form a groove in a good shape in 
the condition shown in FIG. 10. 
There are following relationships with respect to an energy density, mean 
power and a width of pulse. 
EQU mean power (w)=peak power (kw).times.width of pulse (ns).times.pulse 
repetition (kHz) 
EQU energy density (J/cm.sup.2)=mean power (w)/working area (cm.sup.2) 
In order to avoid generating burrs at the edges of the grooves, it is found 
that the YAG pulse laser beam employed in the present invention should 
have a dimension that the mean power is 100 Watt and below, the pulse 
repetition is 3 to 10 kHz, and the energy density is 300 J/cm.sup.2 and 
below for SiC or Si.sub.3 N.sub.4, 200 to 600 J/cm.sup.2 for Al.sub.2 
O.sub.3. 
It is found that there is the following relation between generation of 
burrs at the edges of the grooves and the energy density: 
______________________________________ 
Energy density (J/cm.sup.2) 
Generation of burrs 
______________________________________ 
150 NO 
210 NO 
270 NO 
300 NO 
330 YES 
390 YES 
______________________________________ 
Although in the foregoing embodiment a Q-switch YAG laser beam is employed 
as an energy beam with a high energy density, it should be noted that the 
present invention is not necessarily limitative thereto and that an 
excimer laser beam may also be applied by use of an excimer laser 
irradiation device. In this case, the energy density of the excimer laser 
beam is set to be not higher than 20 J/cm.sup.2. If the energy density of 
the excimer laser beam exceeds 20 J/cm.sup.2, microcracks are generated in 
the modification layer 2a. Therefore, it is necessary to set the energy 
density at a level which is not higher than 20 J/cm.sup.2 and at which the 
base material can be removed. 
In the above-described embodiment, an XY table is employed to scan the 
surface of the bearing base with the laser beam at a rate of 5 mm/s. 
However, the laser beam scanning means is not limited to the XY table. It 
is possible to employ either a galvanometer type optical scanner system 
wherein a laser beam is transmitted in a scanning motion by use of a 
galvanomirror, or an optical fiber system wherein the distal end of an 
optical fiber that transmits a laser beam is moved in a scanning motion. 
In addition, a mask may be used jointly. 
EFFECTS OF THE INVENTION 
As has been described above, the present invention provides the following 
advantageous effects: 
(1) Since the machined surface that is formed with hydrodynamic pressure 
generating grooves is coated with a modification layer with a smooth 
surface which is formed by melting and solidifying the ceramic base 
material, it is possible to minimize or eliminate the generation of fine 
particles. 
(2) Since it is possible to form a modification layer with a smooth surface 
by melting and solidifying the ceramic base material at the same time as 
groove cutting process is carried out, hydrodynamic pressure generating 
grooves with no or minimal generation of fine particles can be formed 
extremely easily. 
(3) Since the modification layer that is formed on the surfaces of the 
hydrodynamic pressure generating grooves is attended with substantially no 
microcracks or no cracks that are contiguous with each other, it is 
possible to prevent separation of the modification layer and development 
of microcracks. 
(4) The advantageous effects (2) and (3) enhance the reliability of the 
product. 
(5) Since washing is not needed to remove the abrasive or cuttings attached 
to the machined portion as in the conventional groove cutting process by 
shot blasting, the load in the washing process can be reduced. 
(6) Since cutting of hydrodynamic pressure generating grooves can be 
carried out in the atmosphere, the machining equipment can be simplified. 
(7) Since no burrs are formed at the circumferential edges of the grooves, 
no finishing process after forming the grooves are necessary in the method 
according to the present invention.