Compositions: ceramic – Ceramic compositions – Devitrified glass-ceramics
Reexamination Certificate
1999-09-03
2002-12-17
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Devitrified glass-ceramics
C501S005000, C501S009000, C501S010000, C428S690000, C428S690000
Reexamination Certificate
active
06495480
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a glass-ceramic substrate for an information storage medium, a method for manufacturing the same and an information storage disk. In this specification, the term “information storage medium” includes stationary type hard disks, removable type hard disks and card type hard disks used respectively such as so-called hard disks for personal computers and other information storage medium in the form of a disk which can be used for storage of data and can be also used in digital video cameras and digital cameras.
Aluminum alloy has been widely used as a material of a magnetic disk substrate used in a stationary type magnetic information storage device such as a hard disk of a computer. In this case, a blank substrate made of an aluminum alloy having a surface roughness Ra of about 200 Å to 1000 Å is subjected to degreasing, etching, substitution of Zn or Sn and Pd, electroless Ni—P plating, annealing, wrapping and polishing, cleaning and testing and forming of a Cr undercoat layer, a magnetic film and a protective layer to thereby form a magnetic information storage disk. Since, however, the aluminum alloy is a soft material, its Young's modulus and surface hardness are low with the result that a significant degree of vibration takes place during rotation of the disk drive at a high speed rotation which leads to deformation of the disk and hence it is difficult to cope with the requirement for making the disk thinner. Thus, the aluminum alloy cannot cope sufficiently with requirement for high-speed driving with a high recording density.
As a material for overcoming the above problems of the aluminum alloy substrate, known in the art are glass-ceramics and chemically tempered glass such as alumino-silicate glasses. Currently used glass-ceramic substrates and chemically tempered glass substrates do not have Ni—P plating on their surface as in the aluminum alloy substrates but the Cr undercoat layer and magnetic film are directly formed on the surface of the substrates. This is because Ni—P plating having excellent adhesion cannot be achieved on the surface of the glass-ceramic substrates and the chemically tempered glass substrates.
However, in the case that the Cr undercoat layer and magnetic film are formed directly on the surface of a glass-ceramic substrate or a chemically tempered glass substrate without the Ni—P plating, magnetic substance grains of the magnetic film tend to have an excessively large grain diameter as compared with a case where the Cr undercoat layer and magnetic film are formed on the Ni—P plating and this will become an obstacle to the requirement for the high density recording in the future.
Examples in which the Ni—P plating is applied to a glass-ceramic substrate or a chemically tempered glass substrate are disclosed in, for example, Japanese Patent Application Laid-open Publication No. Sho 62-230651, Japanese Patent Application Laid-open Publication No. Sho 63-225915, Japanese Patent Application Laid-open Publication No. Hei 6-87680, Japanese Patent Application Laid-open Publication No. Hei 7-272263. In these examples, chemical etching is made on the substrate in an attempt to roughen its surface and thereafter the Ni—P plating is applied to the roughened surface. Despite these efforts, however, sufficient roughening of the surface of the substrate by etching could not be achieved in most of these glass-ceramic substrates and chemically tempered glass substrates which resulted in insufficient adhesion of the Ni—P layer. In some glass-ceramic substrates, adhesion of the Ni—P layer was improved at the sacrifice of flatness of the surface and these substrates are not proper as a high recording density information storage medium required today. Thus, there is no practicable technique today by which the Ni—P plating can be sufficiently applied to glass-ceramic substrates and chemically tempered glass substrates.
As a type of a chemically tempered glass, known in the art are alumino-silicate glasses (SiO
2
—Al
2
O
3
—Na
2
O) disclosed in Japanese Patent Application Laid-open Publication No. Hei 8-48537 and Japanese Patent Application Laid-open Publication No. Hei 5-32431. This material, however, has 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. Further, the chemically tempered layer causes change with time after use for a long period of time with resulting deterioration in the magnetic property of the disk.
(2) Since the chemically tempered glass contains Na
2
O and K
2
O as its essential ingredients, the film forming property of the glass is adversely affected, and a barrier coating over the entire surface of the glass becomes necessary for preventing elution of Na
2
O and K
2
O ingredients and this prevents stable production of the product at a competitive cost.
(3) The chemical tempering is made for improving mechanical strength of the glass but this basically utilizes strengthening stress within the surface layer and the inside layer of the glass and hence its Young's modulus is about 83 GPa or below which is about equivalent to ordinary amorphous glass. Therefore, use of the glass for a disk with a high speed rotation is limited.
(4) In applying Ni—P plating, etching is made by using an HF type solution. Since etching is made uniformly over the surface of the substrate in a chemically tempered glass, a microstructure for securing adhesion of the Ni—P plating cannot be obtained with the result that a sufficient substrate for the Ni—P plating cannot be obtained. Therefore, characteristics of the glass for a high recording density information storage medium are not sufficient.
Aside from the aluminum alloy substrate and chemically tempered glass substrate, known in the art are some glass-ceramic substrates. For example, Japanese Patent Application Laid-open No. Hei 9-35234 and EP0781731A1 disclose glass-ceramic substrates for a magnetic disk made of a Li
2
O—SiO
2
system composition and has crystal phases of lithium disilicate and &bgr;-spodumene, or crystal phases of lithium disilicate and &bgr;-cristobalite. In these publications, however, the relation between Young's modulus and specific gravity with respect to high speed rotation is not taken into consideration at all. Further, no suggestion is made about an HF etching technique suitable for Ni—P plating and conditions of a microstructure suitable for Ni—P plating.
Japanese Patent Application Laid-Open Publication No. Hei 9-77531 discloses a glass-ceramic of a SiO
2
—Al
2
O
3
—MgO—ZnO—TiO
2
system which has Young's modulus of 93.4 GPa to 160.11 GPa and volume density of 2.83 to 3.24 and also a substrate for a high rigidity magnetic information storage disk made of this glass-ceramic. This glass-ceramic contains a large amount of Spinel crystal ((Mg or Zn) Al
2
O
4
) as its predominant crystal phase and MgTi
2
O
5
or Zn
2
Ti
2
O
5
and several other crystals as its subordinate crystal phases. The glass-ceramic contains a large amount of Al
2
O
3
and has a high specific gravity and a high Young's modulus.
Addition of such a large amount of Al
2
O
3
, however, is undesirable from the standpoint of production because it causes deterioration of melting property of the base glass and also deterioration of resistance to devitrification. Further, in this publication, the relation of Young's modulus (GPa)/specific gravity and the value of specific gravity per se which are necessary for the high speed rotation are not taken into consideration at all. No suggestion is made either as to an HF technique suitable for Ni—P plating and conditions of microstructure for Ni—P plating. Particularly, the specific gravity in this publication is a high value of 2.83 or over. This publication therefore merely proposes a glass-ceramic substrate made of a rigid material. Moreover, the glass-ceramic of this system has the serious disadvantage that processability is poor and therefore is not suited for a lar
Group Karl
Hedman & Costigan ,P.C.
Kabushiki Kaisha Ohara
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