U.S. patent application number 09/260768 was filed with the patent office on 2002-03-14 for glass-ceramic substrate for an information storage medium.
Invention is credited to GOTO, NAOYUKI, ISHIOKA, JUNKO, KAWASHIMA, YASUYUKI.
Application Number | 20020031670 09/260768 |
Document ID | / |
Family ID | 27307457 |
Filed Date | 2002-03-14 |
United States Patent
Application |
20020031670 |
Kind Code |
A1 |
GOTO, NAOYUKI ; et
al. |
March 14, 2002 |
GLASS-CERAMIC SUBSTRATE FOR AN INFORMATION STORAGE MEDIUM
Abstract
There is provided a glass-ceramic substrate for an information
storage medium usable in an information storage device of the ramp
loading system. The glass-ceramic substrate has Young's modulus
(GPa)/specific gravity of 37 or over and includes 0 to less than 10
weight percent (expressed on oxide basis) of Al.sub.2O.sub.3. The
glass-ceramic substrate has, as its predominant crystal phases,
lithium disilicate (Li.sub.2O.2SiO.sub.2) and .alpha.-quartz
(.alpha.-SiO.sub.2), has a coefficient of thermal expansion within
a range from 65.times.10.sup.-7/.degree. C. to
130.times.10.sup.-7/.degree. C. wtihin a temperature range from
-50.degree. C. to +70.degree. C. and has a surface roughness (Ra)
(arithmetic mean roughness) of 9 .ANG. or below.
Inventors: |
GOTO, NAOYUKI;
(SAGAMIHARA-SHI, JP) ; ISHIOKA, JUNKO;
(SAGAMIHARA-SHI, JP) ; KAWASHIMA, YASUYUKI;
(SAGAMIHARA-SHI, JP) |
Correspondence
Address: |
JAMES V COSTIGAN
HEDMAN GIBSON & COSTIGAN
1185 AVENUE OF THE AMERICAS
NEW YORK
NY
100362601
|
Family ID: |
27307457 |
Appl. No.: |
09/260768 |
Filed: |
March 2, 1999 |
Current U.S.
Class: |
428/426 ; 501/4;
501/6; G9B/5.288 |
Current CPC
Class: |
C12N 2510/00 20130101;
C12N 2517/04 20130101; G11B 5/73921 20190501; Y10S 428/90 20130101;
C03C 10/0027 20130101 |
Class at
Publication: |
428/426 ; 501/4;
501/6 |
International
Class: |
B32B 017/06 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 23, 1998 |
JP |
94020/1998 |
Apr 20, 1998 |
JP |
125316/1998 |
Dec 10, 1998 |
JP |
351682/1998 |
Claims
What is claimed is:
1. A glass-ceramic substrate for an information storage medium
having Young's modulus (GPa)/specific gravity of 37 or over and
comprising 0 to less than 10 weight percent (expressed on oxide
basis) of A.sub.2O.sub.3.
2. A glass-ceramic substrate as defined in claim 1 wherein the
Young's modulus is within a range from 95 GPa to 120 GPa and the
specific gravity is within a range from 2.4 to 2.6.
3. A glass-ceramic substrate as defined in claim 1 wherein a
coefficient of thermal expansion is within a range from
65.times.10.sup.-7/.degree. C. to 130.times.10.sup.-7/.degree. C.
within a temperature range from -50.degree. C. to +70.degree.
C.
4. A glass-ceramic substrate as defined in claim 1 wherein a
surface roughness Ra (arithmetic mean roughness) after polishing is
9 .ANG. or below.
5. A glass-ceramic substrate as defined in claim 1 wherein
predominant crystal phases are lithium disilicate
(Li.sub.2O.2SiO.sub.2) and .alpha.-quartz (.alpha.-SiO.sub.2).
6. A glass-ceramic substrate as defined in claim 1 which is
substantially free of Na.sub.2O and PbO.
7. A glass-ceramic substrate as defined in claim 1 wherein crystal
grains of the crystal phases are fine globular grains.
8. A glass-ceramic substrate as defined in claim 1 wherein an
average diameter of crystal grains of crystal phases is 0.30 .mu.m
or below.
9. A glass-ceramic substrate as defined in claim 1 comprising 0.3
weight percent or over (expressed on the basis of composition of
the oxide) of MgO.
10. A glass-ceramic substrate as defined in claim 1 having a
composition which consists in weight percent expressed on the basis
of composition of oxides of:
9 SiO.sub.2 71-81% Li.sub.2O 8-11% K.sub.2O 0-3% MgO 0.3-2% ZnO
0-1% P.sub.2O.sub.5 1-3% ZrO.sub.2 0.5-5% TiO.sub.2 0-3%
Al.sub.2O.sub.3 4-8% Sb.sub.2O.sub.3 0.1-0.50% SnO.sub.2 0-5%
MoO.sub.3 0-3% NiO 0-2% CoO 0-3% Cr.sub.2O.sub.3 0-3%
and having, as predominant crystal phases, lithium disilicate
(Li.sub.2O.2SiO.sub.2) and .alpha.-quartz (.alpha.-SiO.sub.2).
11. A glass-ceramic substrate for an information storage medium
having, as its predominant crystal phases, lithium disilicate
(Li.sub.2O.2SiO.sub.2) and .alpha.-quartz (.alpha.-SiO.sub.2) which
have fine globular crystal grains and having a surface roughness Ra
(arithmetic mean roughness) after polishing of 9 .ANG. or
below.
12. A method for manufacturing a glass-ceramic substrate for an
information storage medium which comprises steps of: melting a base
glass having a composition which consists in weight percent
expressed on the basis of composition of oxides of:
10 SiO.sub.2 71-81% Li.sub.2O 8-11% K.sub.2O 0-3% MgO 0.3-2% ZnO
0-1% P.sub.2O.sub.5 1-3% ZrO.sub.2 0.5-5% TiO.sub.2 0-3%
Al.sub.2O.sub.3 4-8% Sb.sub.2O.sub.3 0.1-0.50% SnO.sub.2 0-5%
MoO.sub.3 0-3% NiO 0-2% CoO 0-3% Cr.sub.2O.sub.3 0-3%
forming molten glass, annealing formed glass and then heat treating
the formed glass for nucleation under nucleation temperature within
a range from 550.degree. C. to 650.degree. C. for one to twelve
hours and further heat treating the formed glass for
crystallization under cyrstallization temperature within a range
from 680.degree. C. to 800.degree. C. for one to twelve hours and
polishing the glass-ceramic to a surface roughness (Ra) of 9 .ANG.
or below.
13. An information storage medium provided by forming a magnetic
film and, if necessary, other layers including an undercoat layer,
a protective layer and a lubricating layer, on a glass-ceramic
substrate as defined in claim 1.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to a glass-ceramic substrate for an
information storage medium and, more particularly, to a
glass-ceramic substrate for an information storage medium such as a
magnetic disk made of a glass-ceramic having improved super
flatness of a surface of the substrate, a high Young's modulus and
a low specific gravity capable of coping properly with a high speed
rotation, and a range of coefficient of thermal expansion matching
with coefficients of thermal expansion of constituent elements of
the information storage medium. The invention relates also to a
method for manufacturing the same and also to an information
storage medium using this glass-ceramic substrate. In this
specification, the term "information storage medium" means an
information storage medium in the form of a disk and includes fixed
type hard disks, removable type hard disks and card type hard disks
used respectively for so-called "hard disks" for personal computers
and storage of information in a network and other information
storage medium in the form of a disk which can be used for storage
of data in, e.g., digital video cameras and digital cameras.
[0002] Recent development of personal computers for multi-media
purposes and digital video cameras and digital cameras which
requires handling of a large amount of data has necessitated a
magnetic information storage device of a large recording capacity.
As a result, for increasing the recording density, there is a
growing tendency in a magnetic information storage medium toward
increasing in bit and track density and reducing the size of a bit
cell. In conformity with the reduction in the size of the bit cell,
a magnetic head performs its operation in closer proximity to the
surface of a disk. As the magnetic head performs its operation in a
near-contact state or contact state against the disk surface,
technical development of a landing zone system has become important
as a technique for starting and stopping a magnetic head. According
to this system, a sticking prevention processing such as texturing
is made in a specific zone of a disk (e.g., a radially inward or
outward unrecorded portion of a disk) and starting and stopping of
the magnetic head are performed in this zone which is called
"landing zone".
[0003] In the current magnetic information storage device, the CSS
(contact start stop) system is generally employed according to
which a magnetic head is in contact with a surface of a magnetic
information storage medium before starting and is lifted from the
surface of the medium when the head has started its operation. If
the surface of the medium on which the magnetic head contacts is
exceedingly of a mirror surface, stiction takes place between the
surface of the medium and the magnetic head with resulting
difficulty in smooth starting of rotation of the medium due to
increased friction and occurrence of damage to the surface of the
medium. Thus, a substrate for a magnetic information storage medium
must satisfy two conflicting requirements for a lower glide height
of a magnetic head accompanying increased storage capacity and
prevention of sticking of the magnetic head on the surface of the
medium. For satisfying these conflicting requirements, the landing
zone system has been developed and, aside from the landing zone
system, development of a ramp loading system is under way according
to which a magnetic head is completely in contact with the surface
of a medium except for starting and stopping of the magnetic head
when the magnetic haed is moved away from the surface of the
medium. Accordingly, a current requirement for a substrate for a
magnetic information storage medium is a smoother surface.
[0004] A technical development is under way for a higher speed
transfer of information by a higher speed rotation of a magnetic
information storage medium used for a magnetic information storage
device. As the number of revolution of a medium increases,
deflection and deformation of the medium occur and this gives rises
to a requirement for a higher Young's modulus. Further, in addition
to the conventional fixed type hard disks, information storage
media such as a removable type hard disks and card type hard disks
have been proposed and put into practice and application of digital
video cameras and digital cameras for various uses have been
started.
[0005] Known in the art of magnetic disk substrate materials is
aluminum alloy. The aluminum alloy substrate, however, has
projections or spot-like projections and depressions on the
substrate surface during polishing due to various defects of the
material and, therefore, is not sufficient as a substrate for a
high recording density storage medium in flatness and smoothness.
Besides, since aluminum alloy is a soft material and has a low
Young's modulus and surface hardness, vibration of the substrate
takes place during a high speed rotation of the medium with
resulting deformation of the medium. Difficulty also arises in
making the information storage medium thinner. Further, damage of
the medium by contact with a head is liable to occur. Thus, the
aluminum alloy substrate cannot sufficiently cope with the
requirements for a high speed recording.
[0006] As materials for overcoming the above problems of the
aluminum alloy substrate, known in the art are chemically tempered
glasses such as soda-lime glass (SiO.sub.2--CaO--Na.sub.2O) and
alumino-silicate glass (SiO.sub.2--Al.sub.2O.sub.3--Na.sub.2O).
These materials, however, have the following disadvantages: (1)
Since polishing is made after the chemical tempering process, the
chemically tempered layer is seriously instable in making the disk
thinner. (2) Since the glass contains Na.sub.2O as an essential
ingredient, the glass has the problem that the film forming
characteristics of the medium is deteriorated and, for preventing
diffusion of Na.sub.2O, it becomes necessary to apply a barrier
coating over the entire surface of the substrate. This prevents
stable production of the product at a competitive cost.
[0007] Aside from the aluminum alloy substrate and chemically
tempered glass substrate, known in the art are som glass-ceramic
substrates. For example, the glass-ceramics of a
SiO.sub.2--Li.sub.2O--MgO--P.sub.2O.sub.- 5 system disclosed in
U.S. Pat. No. 5,626,935 containing lithium disilicate
(Li.sub.2O.2SiO.sub.2) and .alpha.-quartz (.alpha.-SiO.sub.2) as
main crystal phases is an excellent material as a material textured
over the entire surface in which, by controlling the grain diameter
of globular crystal grains of .alpha.-quartz, the conventional
mechanical texturing or chemical texturing can be omitted and the
surface roughness after polishing (Ra) can be controlled within a
range from 15 .ANG. to 50 .ANG.. This glass-ceramic, however,
cannot sufficiently cope with the requirement for the low glide
height necessitated by the rapidly increasing recording density
which requires the surface roughness (Ra) of 9 .ANG. or below,
preferably 6 .ANG. or below. Further, no discussion or suggestion
about a coefficient for thermal expansion has been made in this
patent.
[0008] Japanese Patent Application Laid-open Publication No. Hei
9-35234 discloses a magnetic disk substrate made of a glass-ceramic
of a SiO.sub.2--Al.sub.2O.sub.3--Li.sub.2O system having
predominant crystal phases of lithium dislicate
(Li.sub.2O.2SiO.sub.2) and .beta.-spodumene
(Li.sub.2O.Al.sub.2O.sub.3.4SiO.sub.2). This glass-ceramic has a
composition which contains a relatively large amount of
Al.sub.2O.sub.3 ingredient and in which growth of SiO.sub.2
crystals such as .alpha.-quartz (.alpha.--SiO.sub.2) and
.alpha.--cristobalite (.alpha.-SiO.sub.2) is extremely restricted.
The center line mean surface roughness of this glass-ceramic after
polishing is defined as 20 .ANG. or below but the center line mean
surface roughness disclosed in examples is a rough one of 12
.ANG.-17 .ANG. which fails to reach the above described desired
surface roughness and, therefore, this glass-ceramic cannot cope
sufficiently with the requirement for the low glide height of a
magnetic head. Further, since this glass-ceramic requires a high
temperature of 820.degree. C. to 920.degree. C. for crystallization
which prevents a large scale production of the product at a
competitive cost.
[0009] International Publication WO97/01164 which includes the
above described Japanese Patent Application Laid-open Publication
No. Hei 9-35234 discloses a glass-ceramic for a magnetic disk in
which the lower limit of the Al.sub.2O.sub.3 ingredient is lowered
and temperature for crystallization is reduced (680.degree.
C.-770.degree. C.). A sufficient improvement however cannot be
achieved by merely lowering the lower limit of the Al.sub.2O.sub.3
ingredient. Besides, crystals grown in all examples disclosed are
.beta.-eucriptite (Li.sub.2O.Al.sub.2O.sub.3.2SiO.- sub.2).
[0010] It is, therefore, an object of the invention to eliminate
the above described disadvantages of the prior art and provide a
glass-ceramic substrate for an information storage medium having an
excellent sufrace characteristic capable of coping with the ramp
loading system (i.e., contact recording of a magnetic head) for a
high density recording, having an improved relation between Young's
modulus and specific gravity by which the medium can stand a high
speed rotation without causing vibration, and having a coefficient
of thermal expansion which matches with coefficients of thermal
expansion of component parts of the medium.
[0011] It is another object of the invention to provide a method
for manufacturing the glass-ceramic substrate.
[0012] It is another object of the invention to provide an
information storage medium employing this glass-ceramic
substrate.
SUMMARY OF THE INVENTION
[0013] Accumulated studies and experiments made by the inventors of
the present invention for achieving the above described objects of
the invention have resulted in the finding, which has led to the
present invention, that, in glass-ceramics having, as their
predominant crystal phases, lithium disilicate
(Li.sub.2O.2SiO.sub.2) and .alpha.-quartz (.alpha.-SiO.sub.2), a
glass-ceramic can be obtained which is advantageous over the prior
art glass-ceramics for an information storage medium in that it has
fine globular crystal grains and therefore has an excellent
processability, has a smoother surface after polishing, has a
coefficient of thermal expansion matching with coefficients of
thermal expansion of component parts of the medium and has a high
Young's modulus and a low specific gravity capable of coping with a
high speed rotation of the medium. It has been found that the
glass-ceramic substrate for an information storage medium achieving
the object of the invention is particularly useful for the ramp
loading system owing to its super-flatness.
[0014] For achieving the above described object of the invention,
there is provided a glass-ceramic substrate for an information
storage medium having Young's modulus (GPa)/specific gravity of 37
or over and comprising 0 to less than 10 weight percent (expressed
on oxide basis) of Al.sub.2O.sub.3.
[0015] In one aspect of the invention, the Young's modulus is
within a range from 95GPa to 120GPa and the specific gravity is
within a range from 2.4 to 2.6.
[0016] In another aspect of the invention, a coefficient of thermal
expansion is within a range from 65.times.10.sup.-7/.degree. C. to
130.times.10.sup.-7/.degree. C. within a temperature range from
-50.degree. C. to +70.degree. C.
[0017] In another aspect of the invention, a surface roughness Ra
(arithmetic mean roughness) after polishing is 9 .ANG. or
below.
[0018] In another aspect of the invention, predominant crystal
phases are lithium disilicate (Li.sub.2O.2SiO.sub.2) and
.alpha.-quartz (.alpha.-SiO.sub.2).
[0019] In another aspect of the invention, the glass-ceramic
substrate is substantially free of Na.sub.2O and PbO.
[0020] In another aspect of the invention, crystal grains of the
crystal phases are fine globular grains.
[0021] In another aspect of the invention, an average diameter of
crystal grains of crystal phases is 0.30 .mu.m or below.
[0022] In another aspect of the invention, the glass-ceramic
substrate comprises 0.3 weight percent or over (expressed on the
basis of composition of the oxide) of MgO.
[0023] In another aspect of the invention, the glass-ceramic
substrate has a composition which consists in weight percent
expressed on the basis of composition of oxides of:
1 SiO.sub.2 71-81% Li.sub.2O 8-11% K.sub.2O 0-3% MgO 0.3-2% ZnO
0-1% P.sub.2O.sub.5 1-3% ZrO.sub.2 0.5-5% TiO.sub.2 0-3%
Al.sub.2O.sub.3 4-8% Sb.sub.2O.sub.3 0.1-0.5% SnO.sub.2 0-5%
MoO.sub.3 0-3% NiO 0-2% CoO 0-3% Cr.sub.2O.sub.3 0-3%
[0024] and having, as predominant crystal phases, lithium
disilicate (Li.sub.2O.2SiO.sub.2) and .alpha.-quartz
(.alpha.-SiO.sub.2).
[0025] In another aspect of the invention, there is provided a
glass-ceramic substrate for an information storage medium having,
as its predominant crystal phases, lithium disilicate
(Li.sub.2O.2SiO.sub.2) and .alpha.-quartz (.alpha.-SiO.sub.2) which
have fine globular crystal grains and having a surface roughness Ra
(arithmetic mean roughness) after polishing of 9 .ANG. or
below.
[0026] In another aspect of the invention, there is provided a
method for manufacturing the glass-ceramic substrate for an
information storage medium which comprises steps of melting glass
materials, forming molten glass, annealing formed glass and then
heat treating the formed glass for nucleation under nucleation
temperature within a range from 550.degree. C. to 650.degree. C.
for one to twelve hours and further heat treating the formed glass
for crystallization under cyrstallization temperature within a
range from 680.degree. C. to 800.degree. C. for one to twelve hours
and polishing the glass-ceramic to a surface roughness (Ra) of 9
.ANG. or below.
[0027] In still another aspect of the invention, there is provided
an information storage medium provided by forming a magnetic film
and, if necessary, other layers including an undercoat layer, a
protective layer and a lubricating layer, on the above described
glass-ceramic substrate.
DETAILED DESCRIPTION OF THE INVENTION
[0028] Reasons for limiting the physical properties, surface
characteristics, predominant crystal phases and crystal grain
diameter, and composition will now be described. The composition of
the glass-ceramic is expressed on the basis of composition of
oxides as in their base glass.
[0029] Description will be made first about Young's modulus and
specific gravity.
[0030] As described previously, there is a growing tendency toward
a high speed rotation of an information storage medium for
improving the recording density and data transfer speed. For coping
with this tendency, a substrate material must have high rigidity
and low specific gravity for preventing vibration of a disk caused
by deflection during a high speed rotation. Further, in the case
where the medium is used for uses where a magnetic head comes in
contact with the medium or where the medium is used for a portable
type device such as a removable type storage device, the substrate
material must have sufficient mechanical strength, Young's modulus
and surface hardness to be adapted for such uses.
[0031] It has been found that, if a substrate has a high rigidity
but a large specific gravity, deflection of the disk occurs during
a high speed rotation due to its large weight with the result that
vibration of the disk occurs. Conversely, if the substrate has a
low specific gravity but a low rigidity, vibration of the disk
likewise occurs. Accordingly, there must be a balance between
apparently conflicting properties of a high rigidity and a low
specific gravity. It has been found that a proper range of Young's
modulus (GPa)/specific gravity is 37 or over, preferably, 39 or
over, more preferably 41 or over and, most preferably, 43 or over.
It has also been found that there is a preferred range of rigidity.
Even if the above ratio is satisfied with a low specific gravity,
Young's modulus of at least 95 GPa is preferable from the
standpoint of preventing vibration of the disk. Having regard to
processability of the substrate and increase in the weight of the
substrate, the upper limit of Young's modulus of the substrate
preferably is 120 GPa. As to specific gravity, having regard to
prevention of vibration, the substrate should preferably have
specific gravity of 2.6 or below even if the substrate has a high
rigidity. If specific gravity is below 2.4, a substrate having a
desired rigidity cannot be substantially obtained in glass-ceramics
of this glass system. Accordingly, Young's modulus (GPa)/specific
gravity preferably is 50 or below.
[0032] Description will now be made about a coefficient of thermal
expansion. As the recording density increases, positioning of the
magnetic head relative to the information storage medium requires a
high precision and, therefore, a high precision size is required
for the substrate and respective component parts for the medium.
Therefore, an influence of difference in the coefficient of thermal
expansion between the substrate and the component parts for the
medium cannot be ignored and difference in the coefficient of
thermal expansion must be reduced to the maximum extent possible.
As component parts for a small size magnetic information storage
medium, ones having a coefficient of thermal expansion in a range
from +90.times.10.sup.-7/.degree. C. to
+100.times.10.sup.-7/.degree. C. are frequently used so that the
substrate needs to have a coefficient of thermal expansion of the
same order. There is a case, however, where a drive maker employs a
component part made of a material which has a coefficient of
thermal expansion which is out of the above described range, i.e.,
a coefficient of thermal expansion within a range from about
+70.times.10.sup.-7/.degree. C. to about
+125.times.10.sup.-7/.degree. C. For this reason, in the crystal
system of the present invention, a range of coefficient of thermal
expansion has been determined so that the substrate will be
applicable to as wide a variety of materials of component parts as
possible while having sufficient regard to the strength of the
substrate. It has been found that the coefficient of thermal
expansion should preferably be within a range from
+65.times.10.sup.-7/.degree. C. to +130.times.10.sup.-7/.degree. C.
within a temperature range from -50.degree. C. to +70.degree. C. A
more preferable range of the coefficient of thermal expansion
within the same temperature range is from
+95.times.10.sup.-7/.degree. C. to +110.times.10.sup.-7/.degree.
C.
[0033] Description will now be made about the crystal grain
diameter of the predominant crystal phases and the surface
characteristics of the substrate.
[0034] As described previously, as the recording density of the
information storage medium increases, the glide height of the
magnetic head is extremely reduced to 0.025.mu.m or below and the
near contact recording system or the contact recording system has
been developed. For coping with such tendency, the medium must have
a more flat surface than the prior art disks. If one attempts to
perform high recording density inputting and outputting of
information on a magnetic information storage medium having a
surface of the prior art flatness, proper inputting and outputting
of a magnetic signal cannot be achieved because distance between
the magnetic head and the medium is too large. If this distance is
reduced, collision of the magnetic head against the surface of the
medium occurs with resulting damage to the head or medium. For
preventing occurrence of damages to the head and medium
notwithstanding the extremely low glide height or the contact
recording, the surface roughness (Ra) of the substrate should
preferably be 9 .ANG. or below, and more preferably 6 .ANG. or
below. For the same reason, a maximum surface roughness (Rmax) of
the substrate should preferably be 100 .ANG. or below, and more
preferably 72 .ANG. or below.
[0035] For obtaining a glass-ceramic substrate having such
flatness, the shape and diameter of grown crystal grains become
important factors. For processability and surface roughness of the
substrate, the grown crystal grains should preferably be fine
globular grains. More specifically, the crystal grains should
preferably have an average diameter of 0.30 .mu.m or below, or more
preferably 0.2 .mu.m or below, for achieving the desired surface
roughness. For obtaining the desired Young's module, the crystal
grains should preferably have an average diameter of 0.05 .mu.m or
over.
[0036] For realizing the above described physical properties,
coefficient of thermal expansion and surface roughness, it has been
found that the combination of lithium disilicate
(Li.sub.2O.SiO.sub.2) and .alpha.-quartz (.alpha.--SiO.sub.2) as
predominant crystal phases is the best combination.
[0037] As regards Na.sub.2O, if the substrate contains this
ingredient, diffusion of Na ion into the magnetic film occurs
during the film forming process and this makes the magnetic film
grains to become coarse and deteriorates orientation. The substrate
must therefore be substantially free of this ingredient. The
substrate should also be free of PbO which is undesirable from the
standpoint of environment protection.
[0038] Additionally, a substrate for an information storage medium
is required to be free from defects such as crystal unisotropy,
foreign matters and impurities and have a fine and uniform texture.
Such requirements are satisfied by providing the predominant
crystal phases (lithium disilicate and .alpha.-quartz) having the
above described crystal shape and diameter.
[0039] Reasons for limiting the composition range of the base glass
as defined in the claims will now be described.
[0040] The SiO.sub.2 ingredient is a very important ingredient for
growing lithium disilicate (Li.sub.2O.2SiO.sub.2) and
.alpha.-quartz (.alpha.-SiO.sub.2) as predominant crystal phases by
heat treating the base glass. If the amount of this ingredient is
below 71%, grown crystals of the glass-ceramic becomes instable and
its texture tends to become coarse. If the amount of this
ingredient exceeds 81%, difficulty arises in melting and forming of
the glass.
[0041] The Li.sub.2O ingredient is a very important ingredient for
growing lithium disilicate (Li.sub.2O.2SiO.sub.2) as a predominant
crystal phase by heat treating the base glass. If the amount of
this ingredient is below 8%, difficulty arises in growing of this
crystal phase and also in melting of the base glass. If the amount
of this ingredient exceeds 11%, the grown crystal is instable and
its texture tends to become coarse and its chemical durability is
deteriorated.
[0042] The K.sub.2O ingredient improves the melting property of the
glass and prevents the grown crystal from becoming too coarse. The
amount of up to 3% of this ingredient will suffice.
[0043] The MgO and ZnO ingredients are effective for stabilizing
the process of growth of the lithium disilicate (Li.sub.2O.2SiO)
crystal growing as a predominant crystal phase and preventing
growth of .alpha.-cristobalite (.alpha.-SiO.sub.2) crystal which
adeversely affects mechanical and thermal characteristics of the
glass-ceramic of the present invention. If the amount of the MgO
ingredient is below 0.3%, these effects cannot be achieved. If the
amount of the MgO ingredient exceeds 2% or the amount of the ZnO
ingredient exceeds 1%, the product obtained will be instable and
its texture will become too coarse.
[0044] The P.sub.2O.sub.5 ingredient is indispensable as a
nucleating agent. If the amount of this ingredient is below 1%,
growth of nucleus will become insufficient with resulting abnormal
growth of crystals. If the amount of this ingredient exceeds 3%,
opaque devitrification will take place in the base glass.
[0045] The ZrO.sub.2 and TiO.sub.2 ingredients are important
ingredients which, in addition to the function, like the
P.sub.2O.sub.5 ingredient, as nucleating agents, are effective for
making the grown crystals fine, improving the mechanical strength
and improving chemical durability. If the amount of the ZrO.sub.2
ingredient is below 0.5%, these effects cannot be achieved. If the
amount of the ZrO.sub.2 ingredient exceeds 5% or the amount of the
TiO2 ingredient exceeds 3%, difficulty arises in melting of the
base glass and ZrSiO.sub.4 and the like slug are left unmelted.
[0046] The Al.sub.2O.sub.3 ingredient is effective for improving
chemcial durability and mechanical strength of the glass-ceramic.
The type of grown crystal differs depending upon conditions of heat
treatment. Having regard to various conditions of heat treatment,
the amount of this ingredient should be below 10% for growing
lithium disilicate (Li.sub.2O.2SiO.sub.2) and .alpha.-quartz. A
preferable range of this ingredient is 4-8%.
[0047] The Sb.sub.2O.sub.3 ingredient is added as a refining agent
in melting the base glass. If the amount of this ingredient is
below 0.1%, this effect cannot be achieved. The addition of this
ingredient up to 0.5% will suffice.
[0048] The SnO.sub.2 and MoO.sub.3 ingredients are effective as
coloring agents of the glass-ceramic. These ingredients are
particularly effective for detecting surface defects of the
products. These ingredients may also be added for facilitating
absorption of LD excited laser (Nd:YAG and other) used for
texturing of a landing zone on a disk. These ingredients have an
excellent translucency in the glass state and therefore addition of
these ingredients facilitate examination of materials before
crystallization. The ingredients also colorize the glass-ceramic in
its crystallization process. It will suffice if the amount of the
SnO.sub.2 ingredient up to 5% is added and the amount of the
MoO.sub.3 ingredient up to 3% is added.
[0049] The NiO, CoO, Cr.sub.2O.sub.3 ingredients are effective,
like the SnO.sub.2 and MoO.sub.3 ingredients, for improving
absorption of LD excited laser (Nd:YAG and other) used for
texturing of a landing zone on a disk. These ingredients, however,
have no translucency in the glass state as the SnO.sub.2 and
MoO.sub.3 ingredients. It will suffice if the amount of the NiO
ingredient up to 2%, the amount of the CoO ingredient up to 3% and
the amount of the Cr.sub.2O.sub.3 ingredient up to 3% are added
respectively.
[0050] For manufacturing the glass-ceramic substrate for an
information storage medium according to the invention, glass
materials of the above described composition are melted and is
subjected to a hot or cold forming process. The formed glass is
subjected to heat treatment under a temperature within a range from
550.degree. C. to 650.degree. C. for one to twelve hours for
nucleation and then is subjected to further heat treatment under a
temperature within a range from 680.degree. C. to 800.degree. C.
for one to twelve hours for crystallization.
[0051] Predominant crystal phases of the glass-ceramic obtained by
the heat treatments are lithium disilicate (Li.sub.2O.2SiO.sub.2)
and .alpha.-quartz (.alpha.-SiO.sub.2) having globular crystal
grains with a grain diameter of 0.05 .mu.m or over and 0.30 .mu.m
or below.
[0052] The glass-ceramic then is lapped and polished in a
conventional manner and the glass-ceramic substrate for an
information storage medium having a surface roughness (Ra) of 3
.ANG.-9 .ANG. and Rmax of 100 .ANG. or below is obtained.
EXAMPLES
[0053] Examples of the present invention will now be described.
[0054] Tables 1 to 6 show examples (No. 1 to No.30) of compositions
of the glass-ceramic substrate for an information storage medium
made according to the invention together with the temperature of
nucleation, temperature of crystallization, predominant crystal
phases, crystal grain diameter (average), surface roughness (Ra)
after polishing, Rmax, Young's modulus, specific gravity, Young's
modulus (Gpa)/specific gravity and coefficient of thermal
expansion. Table 7 shows compositions and the above properties of
the prior art SiO.sub.2--Li.sub.2O--MgO--P.sub.2O.sub.5 system
glass-ceramic disclosed in U.S. Pat. No. 5,626,935 (Comparative
Example 1) and the prior art SiO.sub.2--Al.sub.2O.sub.3--Li.sub.2O
system glass-ceramics disclosed in Japanese Patent Application
Laid-open Publication No.Hei 9-35234 (Comparative Example 2) and
International Publication No. WO97/01164 (Comparative Example
3).
2 TABLE 1 Examples 1 2 3 4 5 SiO.sub.2 75.3 75.5 77.2 77.5 74.3
Li.sub.2O 9.9 9.9 10.4 9.9 9.5 K.sub.2O 2.0 2.0 2.0 MgO 0.8 1.0 0.5
0.5 0.5 ZnO 0.5 0.5 0.5 0.5 0.5 P.sub.2O.sub.5 2.0 2.0 2.0 1.8 2.0
ZrO.sub.2 2.3 2.3 2.6 2.6 2.0 TiO.sub.2 Al.sub.2O.sub.3 7.0 6.6 6.6
7.0 6.0 Sb.sub.2O.sub.3 0.2 0.2 0.2 0.2 0.2 As.sub.2O.sub.3
SnO.sub.2 1.5 MoO.sub.3 1.5 NiO CoO Cr.sub.2O.sub.3 Nucleation
temperature(.degree. C.) 550 550 550 550 560 Crytallization
temperature(.degree. C.) 780 770 780 780 780 Crystal phases and LD
LD LD LD LD grain diameter (average)(.mu.m) 0.10 0.10 0.10 0.10
0.10 .alpha.-q .alpha.-q .alpha.-q .alpha.-q .alpha.-q 0.20 0.20
0.20 0.20 0.20 Young's modulus(GPa) 100 105 113 120 105 Specific
gravity 2.47 2.48 2.50 2.52 2.48 Young's modulus(GPa)/specific 40
42 45 48 42 gravity Surface roughness (Ra) 7.0 8.0 7.5 6.0 7.3
Maximum surface roughness 79.0 83.0 80.4 72.0 81.2 (Rmax)
Coefficient of thermal expansion 110 100 119 123 115
(10.sup.-7/.degree. C.) (-50.degree. C. - +70.degree. C.)
[0055]
3 TABLE 2 Examples 6 7 8 9 10 SiO.sub.2 75.3 71.3 71.3 71.0 81.0
Li.sub.2O 10.0 10.0 10.0 11.0 9.0 K.sub.2O 1.5 1.5 1.5 1.5 1.0 MgO
0.5 1.0 1.0 1.0 0.5 ZnO 0.5 0.5 0.5 0.5 0.2 P.sub.2O.sub.5 2.0 2.0
2.0 2.0 1.5 ZrO.sub.2 1.5 2.0 2.0 2.0 0.7 TiO.sub.2 1.5 1.5 1.5 1.0
0.7 Al.sub.2O.sub.3 6.0 7.0 7.0 6.8 5.0 Sb.sub.2O.sub.3 0.2 0.2 0.2
0.2 0.2 As.sub.2O.sub.3 0.1 SnO.sub.2 1.5 2.0 0.1 MoO.sub.3 1.5 1.0
NiO 0.5 0.5 CoO 1.8 2.0 Cr.sub.2O.sub.3 0.5 0.5 Nucleation
temperature(.degree. C.) 560 560 560 590 550 Crytallization
temperature(.degree. C.) 770 760 780 790 780 Crystal phases and LD
LD LD LD LD grain diameter (average)(.mu.m) 0.10 0.10 0.10 0.10
0.05 .alpha.-q .alpha.-q .alpha.-q .alpha.-q .alpha.-q 0.20 0.20
0.05 0.05 0.10 Young's modulus(GPa) 100 115 118 118 100 Specific
gravity 2.54 2.54 2.53 2.48 2.48 Young's modulus(GPa)/specific 39
45 47 48 40 gravity Surface roughness (Ra) 5.5 6.3 5.3 5.0 5.0
Maximum surface roughness 63.0 76.0 53.0 51.0 54.0 (Rmax)
Coefficient of thermal expansion 98 100 105 108 100
(10.sup.-7/.degree. C.) (-50.degree. C. - +70.degree. C.)
[0056]
4 TABLE 3 Examples 11 12 13 14 15 SiO.sub.2 73.8 79.0 76.0 74.0
75.0 Li.sub.2O 9.9 9.0 10.0 10.5 9.0 K.sub.2O 2.0 3.0 1.0 2.0 2.5
MgO 0.8 0.5 0.3 2.0 1.0 ZnO 0.5 0.8 P.sub.2O.sub.5 2.0 1.5 2.5 2.0
1.5 ZrO.sub.2 2.8 1.5 1.0 3.0 3.3 TiO.sub.2 1.0 2.0 Al.sub.2O.sub.3
7.0 4.5 4.0 4.0 7.4 Sb.sub.2O.sub.3 0.2 0.3 0.3 0.2 0.1
As.sub.2O.sub.3 0.2 0.1 SnO.sub.2 0.5 3.1 0.1 MoO.sub.3 NiO CoO 0.3
Cr.sub.2O.sub.3 Nucleation temperature(.degree. C.) 570 600 550 580
600 Crytallization temperature(.degree. C.) 740 800 740 760 750
Crystal phases and LD LD LD LD LD grain diameter (average)(.mu.m)
0.05 0.05 0.05 0.10 0.10 .alpha.-q .alpha.-q .alpha.-q .alpha.-q
.alpha.-q 0.05 0.05 0.05 0.30 0.10 Young's modulus(GPa) 100 120 115
120 110 Specific gravity 2.47 2.40 2.43 2.48 2.46 Young's
modulus(GPa)/specific 40 50 47 48 45 gravity Surface roughness (Ra)
3.0 3.0 4.0 9.0 5.0 Maximum surface roughness 32.0 38.0 48.0 100.0
56.0 (Rmax) Coefficient of thermal expansion 95 109 100 128 115
(10.sup.-7/.degree. C.) (-50.degree. C. - +70.degree. C.)
[0057]
5 TABLE 4 Examples 16 17 18 19 20 SiO.sub.2 77.8 72.0 75.8 73.1
74.0 Li.sub.2O 10.5 8.5 9.5 9.5 10.5 K.sub.2O 2.0 0.5 1.0 0.5 MgO
1.5 1.0 0.3 1.5 1.5 ZnO 1.0 0.5 P.sub.2O.sub.5 1.3 1.0 2.2 2.5 1.5
ZrO.sub.2 1.5 4.0 5.0 0.5 5.0 TiO.sub.2 2.5 0.5 Al.sub.2O.sub.3 4.2
5.0 6.0 7.0 5.0 Sb.sub.2O.sub.3 0.1 0.3 0.2 0.2 0.2 As.sub.2O.sub.3
0.1 0.3 0.2 0.2 SnO.sub.2 4.5 MoO.sub.3 2.1 NiO 1.7 CoO
Cr.sub.2O.sub.3 2.7 Nucleation temperature(.degree. C.) 630 560 550
570 550 Crytallization temperature(.degree. C.) 780 740 760 780 740
Crystal phases and LD LD LD LD LD grain diameter (average)(.mu.m)
0.10 0.10 0.05 0.10 0.07 .alpha.-q .alpha.-q .alpha.-q .alpha.-q
.alpha.-q 0.20 0.10 0.05 0.10 0.10 Young's modulus(GPa) 108 118 110
116 100 Specific gravity 2.51 2.55 2.46 2.56 2.49 Young's
modulus(GPa)/specific 43 46 45 45 40 gravity Surface roughness (Ra)
8.0 4.0 3.0 4.5 4.0 Maximum surface roughness 95.0 38.0 29.0 51.0
35.0 (Rmax) Coefficient of thermal expansion 108 105 95 100 105
(10.sup.-7/.degree. C.) (-50.degree. C. - +70.degree. C.)
[0058]
6 TABLE 5 Examples 21 22 23 24 25 SiO.sub.2 76.4 75.2 72.5 75.6
78.0 Li.sub.2O 8.8 8.5 8.5 9.5 8.5 K.sub.2O 2.5 0.5 MgO 1.5 1.5 1.7
0.5 1.0 ZnO 0.5 0.8 0.3 0.7 P.sub.2O.sub.5 1.3 2.5 1.5 2.5 1.5
ZrO.sub.2 4.2 0.7 1.0 4.5 1.0 TiO.sub.2 3.0 2.5 0.5 0.5
Al.sub.2O.sub.3 4.5 4.0 8.0 4.5 5.3 Sb.sub.2O.sub.3 0.3 0.3 0.4 0.1
0.5 As.sub.2O.sub.3 0.3 0.3 0.5 SnO.sub.2 4.0 MoO.sub.3 2.8 NiO 1.5
CoO 2.5 Cr.sub.2O.sub.3 Nucleation temperature(.degree. C.) 580 600
550 570 620 Crytallization temperature(.degree. C.) 750 800 740 760
780 Crystal phases and LD LD LD LD LD grain diameter
(average)(.mu.m) 0.05 0.10 0.10 0.20 0.20 .alpha.-q .alpha.-q
.alpha.-q .alpha.-q .alpha.-q 0.10 0.10 0.30 0.05 0.10 Young's
modulus(GPa) 118 100 118 115 106 Specific gravity 2.43 2.46 2.48
2.49 2.51 Young's modulus(GPa)/specific 49 41 48 46 42 gravity
Surface roughness (Ra) 4.8 4.2 7.0 3.0 3.5 Maximum surface
roughness 58.0 49.0 88.0 31.0 45.0 (Rmax) Coefficient of thermal
expansion 115 118 130 108 100 (10.sup.-7/.degree. C.) (-50.degree.
C. - +70.degree. C.)
[0059]
7 TABLE 6 Examples 26 27 28 29 30 SiO.sub.2 72.0 75.1 73.0 72.0
79.0 Li.sub.2O 8.5 8.5 10.5 10.8 8.5 K.sub.2O 2.5 1.0 1.0 2.7 1.5
MgO 0.9 0.5 0.5 1.0 0.5 ZnO 0.8 1.0 0.8 0.5 1.0 P.sub.2O.sub.5 2.5
1.5 2.0 1.5 1.2 ZrO.sub.2 0.5 1.5 4.0 3.5 3.0 TiO.sub.2 1.0 1.0
Al.sub.2O.sub.3 4.5 6.0 5.0 4.5 5.0 Sb.sub.2O.sub.3 0.2 0.3 0.2 0.2
0.2 As.sub.2O.sub.3 0.1 0.3 0.3 0.1 SnO.sub.2 5.0 0.3 MoO.sub.3 3.0
NiO 2.0 CoO 3.0 Cr.sub.2O.sub.3 2.5 3.0 Nucleation
temperature(.degree. C.) 550 560 580 550 580 Crytallization
temperature(.degree. C.) 720 740 780 750 740 Crystal phases and LD
LD LD LD LD grain diameter (average)(.mu.m) 0.30 0.30 0.20 0.10
0.05 .alpha.-q .alpha.-q .alpha.-q .alpha.-q .alpha.-q 0.05 0.05
0.05 0.05 0.05 Young's modulus(GPa) 100 115 108 100 105 Specific
gravity 2.58 2.49 2.46 2.47 2.44 Young's modulus(GPa)/specific 39
46 44 40 43 gravity Surface roughness (Ra) 3.0 3.0 3.0 3.2 3.8
Maximum surface roughness 28.0 25.0 36.0 33.0 42.0 (Rmax)
Coefficient of thermal expansion 98 96 97 98 96 (10.sup.-7/.degree.
C.) (-50.degree. C. - +70.degree. C.)
[0060]
8 TABLE 7 Comparative Examples 1 2 3 SiO.sub.2 69.0 76.1 76.0
Li.sub.2O 9.0 11.8 10.0 K.sub.2O 7.0 2.8 2.8 MgO 3.5 ZnO 0.5
P.sub.2O.sub.5 1.5 2.0 2.0 ZrO.sub.2 1.0 PbO 1.5 Al.sub.2O.sub.3
5.0 7.1 7.0 BaO 1.5 Sb.sub.2O.sub.3 0.2 0.2 As.sub.2O.sub.3 0.5
Nucleation temperature(.degree. C.) 450 500 450 Crytallization
temperature(.degree. C.) 760 850 750 Crystal phases and LD LD LD
grain diameter (average)(.mu.m) 0.10 0.10 0.10 .alpha.-q
.beta.-spodumene .beta.-cristobalite 0.60 0.80 0.50 Young's
modulus(GPa) 87 89 90 Specific gravity 2.43 2.53 2.48 Young's
modulus(GPa)/specific 36 35 36 gravity Surface roughness (Ra) 15 17
10 Maximum surface roughness 180 230 124 (Rmax) Coefficient of
thermal expansion 64 60 64 (10.sup.-7/.degree. C.) (-50.degree. C.
- +70.degree. C.)
[0061] For manufacturing the glass-ceramic substrate of the above
described examples, materials including oxides, carbonates and
nitrates are mixed and molten in conventional melting apparatus at
a temperature wtihin the range from about 1350.degree. C. to about
1450.degree. C. The molten glass is stirred to homogenize it and
thereafter formed into a disk shape and annealed to provide a
formed glass. Then, the formed glass is subjected to heat treatment
to produce the crystal necleus under a temperature within the range
from 550.degree. C. to 650.degree. C. for about one to twelve hours
and then is further subjected to heat treatment for crystallization
under a temperature within the range from 680.degree. C. to
800.degree. C. for about one to twelve hours to obtain a desired
glass-ceramic. Then, this glass-ceramic is lapped with lapping
grains having average grain diameter rangig from 5 .mu.m to 30
.mu.m for about 10 minutes to 60 minutes and then is finally
polished with cerium oxide having grain diameter ranging from 0.5
.mu.m to 2 .mu.m for about 30 minutes to 60 minutes.
[0062] As shown in Tables 1 to 7, the glass-ceramics of the present
invention are different from the comparative examples of the prior
art glass-ceramics in the predominant crystal phases and crystal
grain diameter (average). In the glass-ceramics of the present
invention, crystal grains of lithium disilicate
(Li.sub.2O.2SiO.sub.2) and .alpha.-quartz (.alpha.-SiO.sub.2) are
fine globular grains whereas the glass-ceramics of the Comparative
Examples 1, 2 and 3 have a large grain diameter (average) of 0.5
.mu.m or over. In view of the current tendency toward the super
flatness, the glass-ceramics of the comparative examples with this
grain diameter will cause difficulties resulting from the surface
roughness after polishing and falling off of crystal grains from
the surface of the medium.
[0063] As regards Young's modulus, specific gravity and Young's
modulus (Gpa)/specific gravity, the glass-ceramics of the present
invention have excellent Young's modulus (Gpa)/specific gravity of
39 or over whereas the glass-ceramics of Comparative Examples 1, 2
and 3 have Young's modulus (Gpa)/specific gravity of less than 37
and therefore cannot sufficiently cope with a drive of a high speed
rotation. Further, as regards the coefficient of thermal expansion,
the glass-ceramics of the present invention have a coefficient of
thermal expansion of 95.times.10.sup.-7/.degree. C. or over whereas
the glass-ceramics of the Comparative Examples 1, 2 and 3 have a
low coefficient of thermal expansion of 64.times.10.sup.-7/.degree.
C. or below. Particularly, the glass-ceramics of Comparative
Examples 2 and 3 contain .beta.-spodumene and .beta.-cristobalite
which are crystal phases having a negative thermal expansion
characteristic and, therefore, difference in the coefficient of
thermal expansion between these glass-ceramics and the component
parts of the drive device will become so great that these
glass-ceramics are not suitable for a substrate for an information
storage medium.
[0064] On the glass-ceramic substrates of the above described
examples are formed films of a Cr middle layer (80 nm), a Co--Cr
magnetic layer (50 nm) and a SiC protective layer (10 nm) by the DC
sputtering method. Then, a perfluoropolyether lubricant (5 nm) is
coated over the formed film to provide an information storage
medium. The information storage medium thus obtained can reduce the
glide height as compared to the prior art information storage
medium owing to its excellent super flatness. Further, the
information storage medium of the invention can be used for the
information storage device of the ramp loading system in which the
magnetic head performs inputting and outputting of signals in
contact with the surface of the information storage medium without
damaging the head or medium.
[0065] As descrobed above, according to the present invention,
there is provided a glass-ceramic substrate suitable for an
information storage medium which has eliminated the disadvantages
of the prior art substrates and has a flat surface characteristic
capable of coping with a high recording density, has an excellent
balance between a high Young's modulus and a low specific gravity
suitable for a high speed rotation and a thermal expansion
characteristic matching with one of an information storage medium
drive device. According to the invention, there are also provided a
method for manufacturing the glass-ceramic substrate and an
information storage medium using this substrate.
* * * * *