Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
2000-04-19
2002-04-23
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C501S067000, C501S077000, C501S079000, C501S004000, C501S005000, C501S007000, C501S009000, C428S690000
Reexamination Certificate
active
06376402
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to glasses and glass-ceramics with a high specific elasticity modulus and their uses or applications.
2. Prior Art
Glass is advantageous for use as a substrate for data carriers (hard disks) in contrast to metals, such as aluminum or metal alloys, because of its planarity and its reduced surface roughness. Glass as the most uniform material provides the possibility of polishing the surface of the glass body so that it is very smooth. The manufacturing processes for glass substrates are more rapid and less expensive than the corresponding methods of making aluminum substrates.
Substrate glass for hard disks must withstand increased chemical, thermal and mechanical stresses in use. It experiences high temperatures with high cooling rates during coating (for example by cathode sputtering). High mechanical stresses occur when hard disks are used, e.g. during manufacture clamping stresses at the rotation axis and in operation stresses due to centrifugal and precession forces at high rotation speeds of currently from 3500 to 10000 rpm. Above all these stresses can only be withstood by glasses that are 0.25 to 3.0 mm thick when they are pre-stressed. Since an increase in mechanical strength by thermal pre-stressing is only possible with a minimum thickness of 3 mm, glasses must be chemically pre-stressed for the above-described application, i.e. by ion exchange.
G-shock loads, e.g. by rough transport, which lead to tearing and cracking, can be countered by pre-stressing. Furthermore the breaking strength is increased.
Ion exchange in glasses, especially with alkali metal ions, can achieve various goals. The two most important are (a) changing the optical values or data, in order to produce graded materials and (b) production of compressive stresses in the surface region of the glass material in order to reduce the susceptibility of the glasses to bending forces.
In graded glasses (a) it is desirable to avoid difficult and/or expensive working processes by introducing a predetermined index of refraction profile. Stresses in the glass are to be avoided, in order to suppress the complication of stress birefringence (division of an optical beam into ordinary and extraordinary beam). Alkali metal ions, which avoid production of stress double refraction but only when coupled with Ag
+
ions as described below, are used for ion exchange in this application. For this application Na
+
ions are preferably exchanged with Ag
+
ions; no stresses arise because the radii of the two ions are nearly equal.
If in contrast surface compressive stresses should be produced by ion exchange, ions are preferably exchanged with each other whose ionic radii are greatly different. Pre-stressing is however only achieved if the glass has a basic three dimensional structure that does not change during the ion exchange so that for example ions can directly diffuse into the glass from a salt bath to the positions of the ions that diffuse out of the glass. Thus ion exchange occurs at temperatures below the glass transition temperature, T
g
, since otherwise the three dimensional structure would relax and the pre-stressing would not occur. Compressive stresses build up, when ions of larger radius diffuse into the glass than diffuse out. Typically Na
+
ions are exchanged with K
+
ions; chemical pre-stressing occurs however in the exchange of Li
+
ions by Na
+
ions or K
+
ions by Cs
+
ions.
It has been shown that alumino-silicate glasses are especially suitable for ion exchange. An open three-dimensional structure, in which the alkali ions are especially mobile and which is stable to relaxation, is prepared by insertion of Al into Si-tetrahedral positions and by the associated charge compensation by alkali ions.
The requirements regarding mechanical stability of glasses acting as hard disk substrates increase with future increases in the rotation speed of the hard is disks.
Developments in the hard disk market are directed toward data carriers with higher capacity and greater data transfer rate with the measurements of the data carrying means remaining the same or being reduced. Higher data transfer rates require higher rotation speeds for the hard disk mounted on the drive device and lower flight heights of the reading device. With disks of the same dimensions capacity can be increased only by higher track densities on the hard disks or by increasing the number of hard disks in the disk drive device. However an increase in the rotation speed causes a strong flutter or pulsation of the hard disk outer edges, which again makes the desired higher track density, also a reduced track spacing and also a narrow stacking of the hard disks, impossible. Because of this flutter motion also the flight height or glide height of the read-write head over the hard disk cannot be reduced, as is desired for an increasing the read/write speed and the information density.
The hard disk thus requires a high shape stability, i.e. it should have a time dependent disk flutter that is as small as possible at its outer edges. The maximum disk flutter W is given by the following formula I:
W={[&rgr;×r
A
4
]/[E×d
2
]}f(&ngr;) I,
wherein
&rgr;=density
r
A
=outer diameter of the fixed plate
E=elasticity modulus
d=thickness of the fixed plate
f(&ngr;)=geometry-specific parameters.
The chief specifications or requirements for hard disks can be derived from this formula. When the geometry remains the same (i.e. d, r
A
, constant) the maximum flutter W is reduced when the elasticity modulus E is higher and/or the density &rgr; is less. Usually the quotient of these parameters E/&rgr; is designated as the specific Young's modulus. It should take the highest possible value.
However the known ion exchangeable alkali alumino-silicate glasses do not have a particularly high Young's modulus, typically E<90 GPa. Particularly the new environmentally friendly optical glasses are currently known to be glasses of high Young's modulus. To obtain the Young's modulus they contain, for example, La
2
O, Ta
2
O
5
or high proportions of TiO
2
beneath the classic glass forming agents, which however scarcely have the required three dimensional structure capable of ion exchange and which are structure changing agents, so that the glasses are inclined toward early crystallization.
An additional requirement of glass suitable for hard disk substrates is its thermal expansion coefficient, which should not be too different from that of the clamping and spindle material used for the drive device (with thermal expansion coefficients &agr;
20/300
>12×10
−6
/K) in order to avoid stresses.
Glass-ceramic material is, above all, an interesting material for the above-described application because of its fracture toughness, without chemical pre-stressing. However in currently used glass-ceramics the crystallite size limits the surface roughness to a high value. There is a danger with insufficiently smooth surfaces, especially with the flight height or glide height of the read-write head, that, when the read-write head is placed on the disk, mechanical damage of the disk can occur and thus data loss.
The glasses and glass-ceramics known for this application are chiefly high SiO
2
-containing alumino-silicate glasses or lithium aluminum silicate glass-ceramics, which have poor fusion properties because of their high SiO
2
content and high Al
2
O
3
content but fairly low Young's modulus. The chemically improved glass composition for substrates for information recording media disclosed in DE 42 06 268 A1 having a content of from 62 to 75 percent by weight SiO
2
should be mentioned. Also the glass-ceramic for magnetic disk substrates disclosed in EP 626 353 A1 having a SiO
2
content of from 65 to 83% by weight, which contains &agr;-quartz and lithium silicate, should be mentioned in this connection.
The known glass
Pannhorst Wolfgang
Woelfel Ute
Wolff Silke
Group Karl
Schott Glas
Striker Michael J.
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