Disk substrate for magnetic disk

A magnetic disk substrate contains Ti as main constituent, and O and Al as additive constituents. A mounts of O and Al falls within the range which satisfies O.ltoreq.0.6, Al.ltoreq.4, and 2.5 O+Al/3.gtoreq.1 (wt. %). The balance consists of Ti and unavoidable impurities.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a magnetic disk substrate for use in a 
high-density recording/reproducing magnetic disk to be used in a computer 
or the like. 
2. Description of the Related Art 
A magnetic disk as a recording medium for a computer is constituted by a 
substrate and a magnetic film formed on the substrate, and the substrate 
is required to have the following characteristics. 
(1) The substrate must be precisely polished so that a magnetic head can 
stably move on a disk. 
(2) The substrate must be free from cracks or steps which may cause a 
defect in a magnetic film when the magnetic film is formed on its surface. 
(3) The substrate must withstand heating performed during formation of a 
magnetic film. 
(4) The substrate must have a satisfactory hardness which can prevent the 
substrate from being damaged or abraded when it is brought into contact 
with a magnetic head. 
(5) The substrate must be light in weight and non-magnetic. 
As a substrate material which satisfies the above requirements, an aluminum 
alloy such as an Al-Mg alloy has been conventionally used. In conventional 
techniques, Ni-P plating or the like is performed to harden the surface of 
a substrate and to cover an inclusion which causes a magnetic film defect 
Masahiro Saito et al., Jitsumu Hyomen Gijutsu (Practical Surface 
Techniques), Vol. 35 (1988), No. 6. 
In addition, glass (Hiroyoshi Ishizaki, "Industrial Material", Vol. 35, No. 
5), titanium (Published Unexamined Japanese Patent Application Nos. 
52-105804, 59-151335, and 1-112521), and the like are developed as the 
substrate material. 
Substrates consisting of the aluminum alloy, glass, and titanium described 
above, however, respectively have the following problems. 
(1) Aluminum Alloy 
Since an aluminum alloy is a soft material, the surface of a substrate 
consisting of the aluminum alloy must be hardened by Ni-P plating as 
described above. It is, however, difficult to uniformly perform such an 
electrochemical treatment throughout a wide region on the substrate. In 
particular, the Ni-P plating easily causes a plating failure. In addition, 
a magnetic film is generally formed by sputtering, and the substrate is 
heated during the sputtering. When the Ni-P plating layer is formed on the 
surface of the substrate, the Ni-P plating layer is crystallized to be 
magnetic or to easily cause peeling if the substrate temperature exceeds 
300.degree. C. upon sputtering. Therefore, the temperature during the 
sputtering must be limited to be 300.degree. C. or less. Furthermore, a 
demand has recently arisen for a smaller thickness of a magnetic disk and 
a higher rotational speed. Since the aluminum alloy essentially has low 
strength and stiffness, it cannot sufficiently satisfy these requirements. 
(2) Glass 
Although glass has no problem in heat resistance as a substrate material, 
it is essentially a brittle material and therefore is easily broken. In 
addition, when a temperature is increased during sputtering, glass 
releases gas components, and an impurity in glass is diffused into a 
magnetic film, thereby degrading the magnetic characteristics of the 
magnetic film. 
(3) Titanium 
Although titanium is free from the above drawbacks of the aluminum alloy 
and glass and therefore expected to be promising as a magnetic disk 
substrate, the techniques described in the patent applications cited above 
have the following problems. 
That is, Published Unexamined Japanese Patent Application No. 52-105804 
discloses a technique of oxidizing or nitriding the surface of Ti to 
increase its surface hardness, thereby improving polishing properties to 
obtain good surface conditions and a high abrasion resistance. However, 
since it is difficult to form such a film having a uniform thickness 
throughout a wide region, the manufacturing yield is decreased to increase 
the manufacturing cost. Published Unexamined Japanese Patent Application 
Nos. 59-151335 and 1-112521 disclose disk substrates consisting of 
Ti-5Al-2.5Sn, Ti-6Al-4V, and Ti-15V-3Cr-3Al-3Sn alloys. However, since 
these titanium alloys contain an expensive alloy element at a high 
concentration, the manufacturing cost is increased. In addition, any of 
these titanium alloys has poor cold rolling properties (Nishimura; "Kobe 
Steel Ltd. Technical Reports 32 (1982)", No. 129, page 44) and therefore 
causes edge cracking upon cold rolling. Since a thin plate such as a disk 
substrate is broken by cracks, it is practically impossible to manufacture 
a disk substrate consisting of the titanium alloy by cold rolling. For 
this reason, a thin plate such as a disk substrate consisting of an alloy 
of this type is manufactured by hot rolling in accordance with a pack 
rolling method (Suenaga; "NKK Technical Reports (1987)", No. 127, page 
37). A disk substrate manufactured by this method, however, is very 
expensive. 
As described above, no magnetic disk substrate which can satisfy the 
market's needs in terms of both performance and manufacturing cost has 
been developed yet. 
SUMMARY OF THE INVENTION 
The present invention has been made in consideration of the above situation 
and has as its object to provide a magnetic disk substrate which consists 
of an inexpensive material, can be manufactured by cold rolling, has a 
high abrasion resistance and a high surface flatness, and can be made 
thinner and withstand high-speed rotation. 
According to the present invention, there is provided a magnetic disk 
substrate containing O and Al in amounts falling within a range which 
satisfies O.ltoreq.0.6, Al.ltoreq.4, and 2.50+Al/3.gtoreq.1 (wt %), 
wherein the balance essentially consists of Ti. 
In this manner, a magnetic disk substrate having the following excellent 
characteristics can be obtained by adding small amounts of Al and 0 as 
inexpensive elements to titanium 
(1) The magnetic disk substrate has a sufficient hardness and a high 
abrasion resistance upon contact with a magnetic head. 
(2) Since the magnetic disk substrate can be subjected to cold rolling and 
has good rolling properties, a high flatness which is an essential 
property of a disk substrate can be obtained, and a manufacturing cost can 
be decreased. 
(3) Since formation of steps caused by deformation twins and crystal grain 
boundaries can be prevented upon polishing, good surface conditions can be 
obtained. 
(4) Since the magnetic disk substrate has a high strength, it can be 
subjected to high-speed rotation. Furthermore, since the magnetic disk 
substrate essentially has an excellent heat resistance in addition to the 
high strength, it is not deformed even if a temperature upon sputtering is 
high. 
Additional objects and advantages of the invention will be set forth in the 
description which follows, and in part will be obvious from the 
description, or may be learned by practice of the invention. The objects 
and advantages of the invention may be realized and obtained by mean of 
the instrumentalities and combinations particularly pointed out in the 
appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A magnetic disk substrate according to the present invention contains O and 
Al in amounts within a region (including lines) surrounded by coordinate 
points (indicated by wt %) A (0, 0.4), B (0, 0.6), C (4, 0.6), D (4, 0), 
and E (3, 0) and the balance essentially consisting of Ti. 
In the present invention, titanium which essentially has a high stiffness 
and a high heat resistance is used as a main component, and at least one 
of Al and O as inexpensive elements can be added within a predetermined 
range to titanium. As a result, a magnetic disk substrate which can 
achieve the above object can be provided. 
More specifically, the following effects are obtained: 
(1) Since these elements are not only inexpensive but also can increase a 
strength to improve an abrasion resistance, the strength and the abrasion 
resistance of a substrate can be increased at a low cost by adding these 
elements. Therefore, a substrate hardening treatment need not be 
performed, and the substrate can be made thinner. In addition, the 
stiffness of the substrate can be maintained due to these characteristics 
even if a temperature is increased upon sputtering, and a limitation as to 
the sputtering temperature can be eliminated. 
(2) Cold rolling properties and the flatness of a rolled plate can be 
improved by adding the elements. Therefore, a magnetic disk substrate can 
be manufactured by cold rolling at a low cost, and the surface roughness 
of the substrate obtained after mirror surface polishing can be easily 
controlled to be less than an upper limit of 0.02 .mu.m. 
(3) Since a crystal grain size obtained after cold rolling and annealing is 
controlled to be 30 .mu.m or less, steps are not easily formed between 
crystal grains upon mirror surface polishing of a substrate. In addition, 
since Al and O suppress formation of deformation twins upon mirror surface 
polishing, steps caused by the twins are rarely formed. By these effects, 
a smooth surface preferred as a substrate free from polishing damages and 
unevenness can be easily obtained. 
If O and Al fall within the range of 2.5 O+Al/3&lt;1 (a region surrounded by 
O, A, and E in FIG. 1), since the addition effect of O and Al is too 
insignificant, a Vickers hardness Hr is less than 250, i.e., the hardness 
is unsatisfactory. If O&gt;0.6 or Al &gt;4, cracks are easily formed during cold 
rolling. 
The contents of O and Al are therefore defined as described above. 
EXAMPLES 
The present invention will be described in more detail below by way of its 
examples. 
Titanium alloys having compositions shown in Table 1 were VAR-melted and 
hot-forged at 1,000.degree. C. to manufacture 16-mm thick slabs. 
Subsequently, these slabs were hot-rolled at 800.degree. C. to obtain 5-mm 
thick hot-rolled plates. The obtained plates were subjected to scale 
removal by coil grinding to form 3.4-mm thick plates, and the formed 
plates were subjected to 70% cold rolling to obtain 1.5-mm thick 
cold-rolled plates. Note that in Table 1, composition Nos. 1 to 16 
indicate examples falling within the range of the present invention, and 
composition Nos. 17 to 30 indicate comparative examples falling outside 
the range. 
Table 1 shows whether cracks are formed or not formed during the above cold 
rolling. 
Vacuum annealing was performed for the obtained cold-rolled plates at 
630.degree. C. for one hour, and the Vickers hardness of each plate was 
measured (load=1 kg, an average value of five-point measurement). 
Subsequently, disks each having an outer diameter of 95 mm and an inner 
diameter of 25 mm were punched out from the cold-rolled plates, and the 
surfaces of the punched disks were sequentially polished by using 
grindstones of #400, #800, #1,500, and #4,000 (# is JIS (Japanese 
Industrial Standard) mesh number) and finally polished by alumina grinding 
grains. A differential interfering microscope was used to check whether 
deformation twins were formed on the disk surfaces upon polishing. In 
addition, measurement of a surface roughness R.sub.max and evaluation of 
an abrasion resistance were performed. Note that the evaluation of an 
abrasion resistance was performed at a rotational speed of 500 rpm for a 
sliding time of 24 hours, and examples having high abrasion resistances 
are indicated by symbols "o" and those having low abrasion resistances are 
indicated by symbols "x". 
Table 1 also shows the Vickers hardness, the presence/absence of 
deformation twins, the surface roughness, and the abrasion resistance. 
TABLE 1 
__________________________________________________________________________ 
Compo- Chmical Composition 
Cracks Vickers 
Deforma- 
Surface 
Abrasion 
sition (wt %) During Cold 
hard- 
tion Rough- 
Resis- 
No. Al 
O C Ni Rolling 
ness Twins ness (mm) 
tance 
__________________________________________________________________________ 
Exam- 
1 3.0 
0.075 
0.04 
0.015 
Not Formed 
256 Not Formed 
0.02 .smallcircle. 
ple 2 3.8 
0.081 
0.05 
0.014 
Not Formed 
262 Not Formed 
0.02 .smallcircle. 
3 1.7 
0.24 
0.04 
0.015 
Not Formed 
259 Not Formed 
0.02 .smallcircle. 
4 2.7 
0.26 
0.04 
0.013 
Not Formed 
263 Not Formed 
0.02 .smallcircle. 
5 3.8 
0.20 
0.05 
0.017 
Not Formed 
289 Not Formed 
0.02 .smallcircle. 
6 1.0 
0.41 
0.05 
0.016 
Not Formed 
268 Not Formed 
0.02 .smallcircle. 
7 2.2 
0.44 
0.04 
0.015 
Not Formed 
287 Not Formed 
0.02 .smallcircle. 
8 3.5 
0.37 
0.06 
0.014 
Not Formed 
290 Not Formed 
0.02 .smallcircle. 
9 0 0.43 
0.05 
0.014 
Not Formed 
262 Not Formed 
0.02 .smallcircle. 
10 0.8 
0.52 
0.04 
0.017 
Not Formed 
278 Not Formed 
0.02 .smallcircle. 
11 1.7 
0.54 
0.04 
0.014 
Not Formed 
285 Not Formed 
0.02 .smallcircle. 
12 2.8 
0.52 
0.04 
0.017 
Not Formed 
291 Not Formed 
0.02 .smallcircle. 
13 3.8 
0.50 
0.04 
0.016 
Not Formed 
301 Not Formed 
0.02 .smallcircle. 
14 0 0.52 
0.04 
0.015 
Not Formed 
278 Not Formed 
0.02 .smallcircle. 
15 1.3 
0.59 
0.04 
0.015 
Not Formed 
281 Not Formed 
0.02 .smallcircle. 
16 3.4 
0.58 
0.04 
0.014 
Not Formed 
293 Not Formed 
0.02 .smallcircle. 
Com- 
17 0 0.77 
0.05 
0.014 
Not Formed 
130 Formed 0.06 x 
para- 
18 0 0.13 
0.04 
0.014 
Not Formed 
194 Formed 0.05 x 
tive 
19 0 0.24 
0.04 
0.014 
Not Formed 
224 Not Formed 
0.03 x 
Exam- 
20 0.7 
0.20 
0.06 
0.013 
Not Formed 
238 Not Formed 
0.03 x 
ple 21 1.9 
0.80 
0.04 
0.017 
Not Formed 
220 Not Formed 
0.03 x 
22 0 0.65 
0.05 
0.016 
Formed 289 Not Formed 
0.02 .smallcircle. 
23 1.2 
0.63 
0.04 
0.016 
Formed 292 Not Formed 
0.02 .smallcircle. 
24 3.1 
0.66 
0.05 
0.015 
Formed 297 Not Formed 
0.02 .smallcircle. 
25 4.7 
0.63 
0.05 
0.014 
Formed 304 Not Formed 
0.02 .smallcircle. 
26 4.4 
0.53 
0.04 
0.014 
Formed 297 Not Formed 
0.02 .smallcircle. 
27 4.7 
0.44 
0.04 
0.014 
Formed 301 Not Formed 
0.02 .smallcircle. 
28 4.5 
0.25 
0.04 
0.014 
Formed 311 Not Formed 
0.02 .smallcircle. 
29 5.5 
0.20 
0.04 
0.017 
Formed 308 Not Formed 
0.02 .smallcircle. 
30 4.7 
0.7 0.04 
0.017 
Formed 274 Not Formed 
0.02 .smallcircle. 
__________________________________________________________________________ 
In accordance with the results shown in Table 1, The FIGURE shows a 
relationship of the contents of Al and O with respect to the 
presence/absence of cracks during cold rolling and the Vickers hardness. 
Referring to The FIGURE, the upper numerals in each plot indicate a 
composition No. and its lower numerals indicate the Vickers hardness. In 
addition, a region surrounded by A (0, 0.4), B (0, 0.6), C (4, 0.6), D (4, 
0), and E (3, 0) of the Al-O coordinate system shown in FIG. 1 is the 
range of the present invention. 
As is apparent from Table 1 and FIG. 1, in each of composition Nos. 1 to 16 
falling within the range of the present invention, a value of the Vickers 
hardness Hv was higher than 250 which is required in consideration of an 
abrasion resistance, and a measured abrasion resistance was actually high. 
In addition, no cracks were formed during cold rolling and no deformed 
twined crystals were formed during polishing, resulting in excellent 
surface conditions having a surface roughness of 0.02 .mu.m or less. That 
is, good characteristics which satisfy the characteristics required as a 
disk substrate were obtained. 
In each of composition Nos. 17, 18, and 19 corresponding to pure Ti of the 
first, second, and third kinds of JIS (Japanese Industrial Standard), an 
abrasion resistance was low due to a low Vickers hardness, and deformation 
twins were locally formed during mirror surface polishing to increase a 
surface roughness, resulting in poor surface conditions. In each of 
composition Nos. 20 and 21 in which small amounts of Al were added to pure 
Ti, no satisfactory hardness could be obtained, resulting in 
unsatisfactory values in both an abrasion resistance and surface 
conditions. In each of composition Nos. 22 to 30 in which the content of O 
exceeded 0.6% or the content of Al exceeded 4%, although good results were 
obtained in any of a hardness, a surface roughness, and an abrasion 
resistance, since end face (edge) cracking was significant during cold 
rolling, no yield which would allow actual manufacture could be obtained. 
Additional advantages and modifications will readily occur to those skilled 
in the art. Therefore, the invention in its broader aspects is not limited 
to the specific details, and representative devices, shown and described 
herein. Accordingly, various modifications may be made without departing 
from the spirit or scope of the general inventive concept as defined by 
the appended claims and their equivalents.