Process for production of chemically prestressed glass

The process for producing a chemically prestressed glass of very high surface quality includes subjecting an Li.sub.2 O-free starting glass from the SiO.sub.2 -Al.sub.2 O.sub.3 -M.sub.2 O-MO system which also contains TiO.sub.2, CeO.sub.2 and F.sub.2 to chemical ion exchange by immersing it in a potassium salt bath at a temperature between 350.degree. C. and 550.degree. C. and at a residence time of between 0.5 and 20 hours to form the chemically prestressed glass. In a preferred embodiment of the process, the potassium salt bath contains a potassium nitrate melt and the glass is polished after prestressing. The chemically prestressed glass is particularly useful for making hard disks.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for the production of a 
chemically prestressed glass of high breaking strength and high chemical 
resistance, and to a method of using this glass. 
Glass is advantageous for use as a substrate for data carriers (hard disks) 
compared with metals, such as aluminum, or metal alloys, because of its 
low surface roughness and flatness, among other things. Such substrate 
glasses must withstand increased chemical, thermal and mechanical loads 
during manufacture of hard disks and during use. Thus, they are subjected 
to high temperatures and short cooling times during coating (for example, 
by cathode sputtering). When used as hard disks, high mechanical loads 
occur, for example, at rotational speeds from 3500 to 10,000 rpm and 
clamping stresses on the axis of rotation of up to 300 N/mm. 
Glasses which have not been prestressed are very susceptible to breakage 
because of tensile stresses. In particular, thin glasses having a 
thickness of from 0.25 to 3.00 mm can only withstand the loads. mentioned 
if they have been prestressed. 
Compared with unprestressed glasses, prestressed glasses have a lower 
probability of breakage for a given load or the probability of breakage 
only becomes of equal magnitude at a higher load. 
Since increasing the mechanical strength by thermal prestressing is only 
possible from a minimum thickness of 3 mm, chemical prestressing by ion 
exchange in a salt bath is the method of choice here. 
In chemical prestressing at below the glass transition temperature Tg, 
alkali metal ions having a small ionic diameter from the glass are 
replaced by alkali metal ions of large diameter from the salt bath, for 
example Li+by Na+, Na+by K+. Thus, with compressive stress zones having a 
thickness from about 14 to 230 pm, which corresponds to about 2/3 of the 
ion exchange depth, flexural strengths of from 350 to 900 N/mm.sup.2 can 
be built up. 
Another important factor for successful production of compressive stress 
layers is the composition of the glass. The presence of Li ions in the 
glass makes it more difficult to carry out ion exchange processes, since 
two types of ion are exchanged, namely Li+by Na+and Na+by K+, and since a 
specific mixing ratio between Na and K salts and narrow temperature limits 
must generally be observed during the exchange process. 
Together with oxygen ions, the fluoride component in the glass forms the 
anion network of the glass, in which large ions can easily diffuse. This 
favors stress reduction. 
If the substrate glass contains relatively large amounts of fluoride, 
chemical prestressing can usually only be achieved for a short time or is 
predominantly lost through the coating, for example during heating. 
For this use, the quality of the glass with respect to the number and size 
of flaws, such as solid inclusions and bubbles, is also of importance, 
since bubbles at the surface of the substrate cause, when polished, holes, 
which result in impermissible unevenness of the surface. 
German Published Patent Application DE 42 06 268 Al describes a 
lithium-containing aluminosilicate glass for use as a hard disk substrate 
glass. Although the presence of Li.sub.2 O improves the ability to refine, 
it also makes chemical prestressing more difficult. 
Besides the surface flatness, the chemical resistance of the substrate 
glass is of very great importance for the functionality of a fixed disk: 
The read/write head of a computer is at a small distance of about 50 nm 
from the fast-rotating hard disk. This distance must be maintained 
precisely for fault-free functioning. However, it is reduced if the 
surface of the hard disk substrate is not resistant to the effects of the 
atmosphere and chemical attack even before the coating operation roughens 
the surface (for example through efflorescence) or if the surface loses 
its adhesion to the layer sequence applied owing to the effects of the 
atmosphere and detaches therefrom. 
It is a problem that the resistance of a glass surface to attack by water, 
caustic lye or acid is usually reduced by chemical prestressing, i.e. the 
increase in the concentration of relatively large alkali metal ions. This 
is because the chemical resistance not only drops with increasing alkali 
metal concentration, but these attacks also become more effective the 
larger the alkali metal ions. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a process for 
production of a chemically prestressed glass which not only has high 
flexural strength, but also high chemical resistance. 
It is another object of the present invention to provide a method of using 
the chemically prestressed glass and a product of that method. 
According to the invention, the process for producing a chemically 
prestressed glass of high breaking strength and high chemical resistance 
comprises subjecting a starting glass having the composition (in mol %, 
based on oxides): 
SiO.sub.2 from 63.0 to 67.5 mol %, 
Al.sub.2 O.sub.3 from 9.5 to 12.0 mol %, 
Na.sub.2 O from 8.5 to 15.5 mol %, 
K.sub.2 O from 2.5 to 4.0 mol %, 
MgO from 3.0 to 9.0 mol %, 
.SIGMA. CaO+SrO+BaO+ZnO from 0 to 2.5 mol %, 
TiO.sub.2 from 0.5 to 1.5 mol %, 
CeO.sub.2 from 0.02 to 0.5 mol %, 
As.sub.2 O.sub.3 from 0 to 0.35 mol %, 
SnO.sub.2 from 0 to 1.0 mol %, and 
F.sub.2 from 0.05 to 2.6 mol %, 
wherein the molar ratio of the SiO.sub.2 to the Al.sub.2 O.sub.3 in the 
starting glass is between 5.3 and 6.85, the molar ratio of the Na.sub.2 O 
to the K.sub.2 O is between 3.0 and 5.6, the molar ratio of the Al.sub.2 
O.sub.3 to K.sub.2 O is between 2.8 and 3.6 and the molar ratio of the 
Al.sub.2 O.sub.3 to a total amount of the TiO.sub.2 and the CeO.sub.2 is 
between 7.6 and 18.5; to chemical ion exchange in an ion exchange bath 
containing more than 90% by weight of at least one potassium salt at a 
temperature between 350.degree. C. and 550.degree. C. and with a residence 
time of between 0.5 and 20 hours to form the chemically prestressed glass. 
A prerequisite for providing the compressive stress zone to be generated in 
the prestressing process with the desired properties is the composition of 
the glass. 
In the aluminosilicate glasses used in accordance with the invention, the 
SiO.sub.2 content must not exceed 67.5 mol %, since otherwise the melting 
points increase excessively. On the other hand, the SiO.sub.2 content must 
not drop below 63.0 mol %, since otherwise the chemical resistance is 
impaired excessively. For the same reasons, the Al.sub.2 O.sub.3 content 
must not exceed 12.0 mol % or drop below 9.5 mol %. In order to ensure 
both good acid resistance and good ion exchangeability, SiO.sub.2 and 
Al.sub.2 O.sub.3 must be present in a balanced ratio to one another. Thus, 
the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio should be between 5.3 and 
6.85. 
An essential constituent of the glasses are the alkali metal oxides. 
Because of the effective combination of refining agents described below, 
Li.sub.2 O can be omitted completely, which makes it possible to achieve 
the desired properties with respect to surface quality and chemical 
prestressing simultaneously in a single glass. 
The Na.sub.2 O content should be between 8.5 and 15.5 mol %. If it is 
higher than 15.5 mol %, the chemical resistance is impaired, and if it is 
lower than 8.5 mol %, firstly the glass becomes more difficult to melt and 
secondly the increase in strength by Na+/K+ion exchange only becomes 
possible to a restricted extent. 
A constituent having a particular and surprising action is K.sub.2 O, which 
should be present in an amount of from 2.5 to 4.0 mol %. It further 
increases the meltability of the glass and accelerates an ion exchange 
process of the Na ion in the glass by the K ion in the salt bath. In 
addition, homogenization is simplified, which means an improvement with 
respect to the desired freedom from bubbles. This is because K.sub.2 O 
increases the basicity of the glass more than the same number of moles of 
Na.sub.2 O, thus simplifying refining without impairing the high chemical 
resistance or chemical prestressability. The simplified production of 
glass of high bubble quality with the same chemical and mechanical 
properties is only ensured in the stated range. If the K.sub.2 O content 
is lower than 2.5 mol % and if the Al.sub.2 O.sub.3 /K.sub.2 O ratio is 
less than 2.8, the bubble density and bubble size increase; if the K.sub.2 
O content is greater than 4.0 mol % and the Al.sub.2 O.sub.3 /K.sub.2 O 
ratio is greater than 3.6, the strength of the glasses cannot be increased 
sufficiently during prestressing. 
The two alkali metal oxides must also be present in a balanced ratio to one 
another. Thus, the Na.sub.2 O/K.sub.2 O molar ratio should be between 3.0 
and 5.6. 
A further necessary constituent is MgO, which should be present in a 
minimum amount of 3.0 mol %. This increases the basicity of the glasses 
and thus promotes homogenization. However, it also inhibits the ion 
exchange process, since the Na ions are bound more strongly in 
aluminosilicate glass structures in the presence of divalent ions. For 
this reason, a maximum content of 9.0 mol % of MgO should not be exceeded. 
The other alkaline earth metal oxides and ZnO also have the same effects. 
The glass can therefore also contain CaO, SrO, BaO and ZnO in a total 
amount of 0-2.5 mol % .SIGMA. CaO+SrO+BaO+ZnO, preferably from 0.1 to 2.5 
mol % .SIGMA. CaO+SrO+BaO+ZnO. The preference for MgO over the other 
alkaline earth metal oxides and ZnO is due to the fact that MgO improves 
the meltability in a similar way as BaO and CaO, but reduces the chemical 
resistance much less than these oxides. 
TiO.sub.2 is a further necessary component of the glass. It should be 
present in an amount of at least 0.5 mol %, but a content of 1.5 mol % 
should not be exceeded, since otherwise difficulties occur during melting 
of the batch. 
The other components, refining agents and refining aids must also be 
present in a balanced combination in order to achieve the best results 
regarding bubble quality. CeO.sub.2 should be present in the glass in an 
amount of between 0.02 and 0.5 mol %. It not only has a refining function, 
but also additionally provides the glass with sufficiently high absorption 
of UV radiation, which is medically questionable, thus also enabling use 
of the glass in the lighting industry. It has been found that the 
requisite amount of refining agents depends on the amount of Al.sub.2 
O.sub.3 present. The more Al.sub.2 O.sub.3 that is present in the glass, 
the more refining agents are necessary. In particular, an Al.sub.2 O.sub.3 
/(CeO.sub.2 +TiO.sub.2) molar ratio of between 7.6 and 18.5 should be 
observed. 
As further refining agents, As.sub.2 O.sub.3 in amounts of up to 0.35 mol % 
and SnO.sub.2 in amounts of up to 1.0 mol % may be appropriate. Of these 
two components, at least one is preferably present in an amount of at 
least 0.02 mol %. In order to achieve particularly high quality with 
respect to a low number and size of bubbles, the glass used should contain 
As.sub.2 O.sub.3 in an amount between 0.02 and 0.35 mol %. 
The glass must furthermore contain F.sub.2 in an amount between 0.05 and 
2.6 mol %. The lower limit is set by the requirements for bubble quality, 
and the upper limit results from the effect of the fluoride in the glass 
network that has already been described above. In the case of 
arsenic-containing glasses, the range from 0.05 to 0.7 mol % of F.sub.2 is 
preferred, and in the case of arsenic-free glasses, the range from &gt;0.5 to 
2.6 mol % of F.sub.2 is preferred. 
Surprisingly, a d eep c ompressive stress zone with long-lasting 
compressive stress can be built up in these glasses in a simple manner by 
chemical prestressing without impairing the good chemical resistance. The 
glass articles are left for from 0.5 to 20 hours in salt baths comprising 
from 100 to more than 90% by weight of at least one potassium salt at bath 
temperatures between 350.degree. C. and 550.degree. C. This type of 
treatment produces compressive stress zones with a thickness from about 14 
.mu.m to greater than 230 .mu.m, lower temperatures making longer 
residence times necessary. All customary potassium salts whose anions are 
stable in the stated temperature range can be used for the salt bath. The 
salt bath (in general commencing with 100% of potassium salt) is replaced 
when the potassium content has dropped, because of the exchange, to the 
extent that the desired exchange depth is no longer achieved. This is in 
general the case at 90% by weight of potassium salts. It is also possible 
to use from the very beginning up to 10% by weight of other salts to lower 
the melting point of the bath. This naturally means that the exchange 
capacity of the bath is then exhausted correspondingly earlier. 
The chemical prestressing of the glasses by ion exchange can in some cases 
result in undesired roughness on the surface. 
In an advantageous embodiment of the novel process, the glasses are 
polished after the prestressing. This sequence of the process steps, which 
differs from the prior art, is enabled here, without loss of the high 
breaking strength, by the thickness of the resultant compressive stress 
layer. The layer should advantageously have a thickness of .gtoreq.25 
.mu.m; this is achieved, for example, by ion exchange lasting about 1 hour 
at a temperature of 450.degree. C., but can also--be achieved by other 
times and temperatures as shown in Table 2. Polishing after the 
prestressing allows the mechanical changes in the surface which have 
occurred during chemical prestressing to be removed again, giving glasses 
having excellent surface quality. The good chemical resistance is also 
retained. In addition to the improvement in the surface quality, the 
procedure described here also means a simplification in the production 
process, since fewer purification steps are required. The glasses are 
polished with cerium oxide to a residual roughness corresponding to an RMS 
(root mean square) value of&lt;1 nm. 
The prestressed glasses produced by this process simultaneously have the 
following advantages, which are unique in this combination: 
high flexural strength, 
a deep compressive stress zone, 
virtually no relaxation of the compressive stress produced, 
good surface quality, 
high chemical resistance. 
Because of their particular properties, the prestressed glasses produced by 
the novel process are highly suitable for use as substrate glasses for 
hard disks.

WORKING EXAMPLES 
Table 1 shows examples of prestressed glasses produced by the novel 
process. The table contains their composition, data on the ion exchange 
conditions in a KNO.sub.3 bath, and relevant properties regarding chemical 
resistance and strength. 
The glasses were produced from conventional raw materials in 4 1 platinum 
crucibles. The raw materials were introduced over a period of 8 hours at 
melt temperatures of 1580.degree. C. The glass was subsequently kept at 
this temperature for 14 hours, then cooled to 1400.degree. C. over the 
course of 8 hours with stirring and poured into a graphite mold preheated 
to 500.degree. C. For visual preliminary inspection, the resultant cast 
block was converted into a tube with polished surfaces. 
Circular glass disks with the shape and dimensions of hard disk substrates, 
i.e. an external diameter of 65.0 mm and a thickness of 0.635 mm, with a 
central hole having a diameter of 20.0 mm were produced from this cast 
block in a conventional manner. The finely ground and polished glass disks 
were then chemically prestressed in a KNO.sub.3 bath under the respective 
conditions shown in the table. 
The properties and parameters shown in the table were determined using the 
following physical analysis methods. 
The flexural strength is determined by the following method, which is 
conventional in the glass industry and is aimed at the loads occurring in 
practice. 
In the so-called double-ring test, the chemically prestressed glass disk 
having the shape and dimensions of a hard disk substrate is laid, 
centered, on a metal supporting ring having an annular cutter of hardened 
steel with a diameter of 60 mm, i.e. somewhat smaller than the disk to be 
tested (.phi.=65 mm). A further metal supporting ring having a steel 
cutter is likewise laid, centered, on top of the glass disk. This cutter, 
with a diameter of 25 mm, is somewhat larger than the central hole 
(.phi.=20 mm) of the hard disk substrate. This upper ring presses with its 
cutter at a rate of 2 mm/min on the glass disk lying on the cutter of the 
lower ring and thus exerts a constantly increasing force on the hard disk 
substrate. The load at which the substrate breaks is given as the flexural 
strength N/mm.sup.2 !. The test is regarded as passed if the breakage 
only occurs at a load of &gt;100 N. 
The compressive stress is measured by a stress-optical method. If the glass 
plate is subjected to compressive stress, the refractive indices parallel 
and perpendicular to the direction of the stress change, and the glass 
plate becomes birefringent. The birefringence, the difference between 
these refractive indices, is proportional to the applied stress via the 
stress-optical constant of the glass in question and is determined from 
the path difference between perpendicular and parallel polarized light 
after reflection at the glass surface. 
The thickness of the compressive stress zone is measured by the following 
method. 
A glass sample is observed under a polarizing microscope at a wavelength of 
546 nm. The sample coated by ion exchange is under compressive stress over 
the entire surface and under tensile stress in its interior for 
equilibrium reasons. In order to measure the stress, the sample is 
introduced between 2 crossed polarizers. The stress exerted on the sample 
causes a brightening in the ray path of the microscope owing to stress 
birefringence. The transition from tensile stress to compressive stress 
(O-order neutral zone) is evident as a broad dark strip under the 
microscope. The distance of the O order from the edge of the sample is a 
measure of the thickness of the compressive stress zone. 
The glass disks described above are too thin for this measurement, so glass 
slips measuring 6 mm.times.50 mm with a thickness of 2 mm produced and 
hardened under the same conditions as the glass disks are used. 
The Knoop hardness is determined in accordance with DIN 52333. 
In order to test the chemical resistance, a fast and simple test method 
which can be carried out using uncomplicated equipment is expedient. There 
is to date no standardized test method specifically for glass data 
carriers. In the glass industry, various methods for testing the chemical 
resistance of glass products are known, but these are, for various 
reasons, unsuitable for determining the chemical resistance of hard disk 
substrates. 
So-called alkali leaching of the disk produced by the novel process is 
determined by a new method which is simple to carry out and gives very 
meaningful results. The term alkali leaching here is taken to mean the 
total amount of alkali metal ions which are dissolved out of the sample in 
the following test under the following conditions. The sample is a 
prestressed, circular glass disk having the shape and dimensions as 
described above an external diameter of 65.0 mm with a central hole having 
a diameter of 20.00 mm and a thickness of 0.635 mm, with finely ground and 
beveled edges and surfaces polished to an RMS roughness of &lt;1 nm using the 
cerium oxide. The sample after prestressing or, in the preferred 
embodiment, after prestressing and polishing, is washed in a final 
cleaning step for 1/4 hour in an ultrasound bath at room temperature in 
deionized water, placed, still damp, in 25 ml of deionized water at 
80.degree. C. and left there for 24 hours. The amount of alkali metal ions 
leached out is measured by means of atom absorption spectrometry. 
The amount of alkali metal leached out in .mu.g in each case relates to one 
sample. 
TABLE I 
__________________________________________________________________________ 
Composition (in mol %, based on oxides) of 
Prestressed Glasses Produced according to the invention, and 
their essential properties. 
1 2 3 4 5 6 7 
__________________________________________________________________________ 
SiO.sub.2 65.54 
67.32 
63.60 
63.67 
66.26 
66.83 
67.36 
Al.sub.2 O.sub.3 
9.60 
11.18 
11.91 
9.74 
10.91 
10.91 
11.28 
Na.sub.2 O 8.66 
13.58 
12.49 
16.02 
11.30 
11.30 
12.82 
K.sub.2 O 2.68 
3.17 
3.48 
2.89 
3.82 
3.82 
3.82 
MgO 8.62 
3.29 
6.51 
3.97 
3.25 
3.25 
3.25 
CaO + SrO + BaO + ZnO 
1.25 
0.24 
0.47 
0.14 
0.12 
0.12 
0.24 
SnO.sub.2 1.0 -- -- 0.15 -- 0.15 
TiO.sub.2 1.19 
0.54 
0.66 
0.64 
1.23 
0.66 
0.54 
CeO.sub.2 0.06 
0.46 
0.02 
0.15 
0.19 
0.19 
0.15 
F.sub.2 1.41 
0.09 
0.51 
2.53 
2.59 
2.59 
0.22 
Al.sub.2 O.sub.3 
-- 0.17 
0.35 
0.05 
0.33 
0.33 
0.17 
Ion exchange 
500 450 400 450 
480 480 520 
temperature, .degree.C. 
Ion exchange 
10 1.5 5 4 6 6 20 
time, h 
Thickness of the 
105 35 45 52 128 125 220 
compressive stress 
zone, .mu.m 
Amount of alkali 
21 20 24 22 18 23 17 
metal leached, .mu.g 
"Stress relaxa- 
950 &gt;1000 
&gt;1000 
700 
800 850 &gt;1000 
tion" at 300.degree. C., h 
Flexural strength, 
720 490 410 560 
640 620 900 
N/mm.sup.2 
Knoop hardness, KH 
585 590 600 545 
562 568 609 
Modulus of elas- 
70 71 72 66 68 68 72 
ticity E, kN/mm.sup.2 
Coefficient of 
8.2 8.9 9.1 9.6 
9.1 9.1 8.9 
thermal expansion 
.alpha..sub.20/300, 10.sup.-6 /K 
Glass transition 
595 632 618 565 
573 579 626 
temperature, T.sub.g 
__________________________________________________________________________ 
The amount of alkali metal leached out is taken to mean the total amount of 
alkali metal ions, given in .mu.g leached out per hard disk substrate in 
the above-described test under the stated conditions. 
The "stress relaxation h!" line shows the time in hours at constant 
temperature (300.degree. C.) at which a measurable reduction in the 
thickness of the compressive stress zone occurs, where thickness changes 
of 4 .mu.m and more can be measured. The optical method used has already 
been explained. Such a reduction in the thickness of the compressive 
stress zone is associated with a reduction in compressive stress and is 
easier to measure than the compressive stress reduction itself. 
During chemical hardening of a glass having the composition of Example 3 
from Table 1 in a KNO.sub.3 bath, the exchange time was varied between 1/2 
h and 15 h and the bath temperature between 350.degree. C. and 550.degree. 
C. The compressive stress zone thicknesses achieved are shown in .mu.m in 
Table 2. Thicknesses of the compressive stress zones of between 14 and 230 
.mu.m were achieved, with, as expected, lower temperatures making longer 
residence times necessary. 
TABLE 2 
______________________________________ 
Compressive Stress Zones .mu.m! as a function of Exchange 
Time and Salt-Bath Temperature for a Glass having the Composition 
of Example 3 from Table 1 
Bath temperature .degree.C.! 
Exchange time h! 
350 400 450 500 550 
______________________________________ 
0.5 14 20 38 46 
1 26 55 68 
1.5 20 30 60 90 
2.5 31 -- -- -- 
4 -- 42 -- 121 
5 45 -- 98 -- 
8 -- 75 -- -- 
10 63 90 132 182 
15 47 72 103 166 230 
______________________________________ 
The thickness of the compressive stress zone-is thus clearly dependent both 
on the exchange time (at constant exchange temperature) and on the 
exchange temperature (at constant exchange time). 
Table 3 shows that this dependence does not apply significantly to the 
compressive stress at the surface, but certainly does for the flexural 
strength. Again, the example glass selected was the glass having the 
composition of Example 3 from Table 
TABLE 3 
______________________________________ 
Thickness of the Compressive Stress Zone, Compressive 
Stress at the Surface and Flexural Strength as a function of the 
Exchange Time at Constant Salt-bath Temperature (450.degree. C.) for a 
Glass having the Composition of Example 3 from Table 1: 
Thickness of the 
Compressive 
Flexural 
Exchange compressive stress at the 
Strength, 
time, h stress zone, .mu.m 
surface, N/mm.sup.2 
N/mm.sup.2 
______________________________________ 
1.5 30 880 .+-. 30 
430 
15 103 775 .+-. 30 
800 
______________________________________ 
These very good flexural strength values make the prestressed glass 
produced by the novel process highly suitable for withstanding the 
mechanical loads to which hard disks are subjected or will be subjected 
(increasing speeds of future hard disks). 
The temperature dependence of the stress relaxation mentioned above in 
Table 1 is shown by Table 4. To this end, a hard disk substrate made from 
a glass having the composition of Example 7 from Table 1 was hardened for 
20 hours in a KNO.sub.3 bath at 520.degree. C. A 220 .mu.m compressive 
stress zone having a compressive stress of 800 N/mm.sup.2 formed. Table 3 
shows the time at various temperatures before a reduction in the thickness 
of the compressive stress zone is observed. 
TABLE 4 
______________________________________ 
Temperature and Time before a Mesurable Reduction (4 
.mu.m) in the thickness of the Compressive Stress Zone occurs, for 
a Glass having the Composition of Example 7 from Table 1: 
Temperature, .degree.C. 
Time, h 
______________________________________ 
300 &gt;1000 
350 500 
400 100 
500 30 
______________________________________ 
Extrapolation gives a time of more than 50,000 hours at a temperature of 
200.degree. C. before stress reduction occurs. For significantly lower 
temperatures, for example &lt;60.degree. C., the compressive stress can be 
regarded as being maintained virtually indefinitely along with the 
strength properties. 
In addition to the high flexural strength, the low leaching of alkali 
metals, i.e. the high chemical resistance, is characteristic of glasses 
produced by the novel process. In order to illustrate this property, 
prestressing times and temperatures were again varied during chemical 
prestressing by immersion of the glass of Example 3 in a KNO.sub.3 (Table 
1), and the alkali metal leaching was determined in each case. The results 
are shown in Table 5 (variation of the exchange time) and in Table 6 
(variation of the exchange temperature). In each case, three samples were 
leached and the amount leached out determined, the results mentioned being 
reproduced. 
TABLE 5 
______________________________________ 
Amount of Alkali Metals leached out in .mu.g per 
Hard Disk Substrate as a function of the Prestressing Time at 
Constant Salt-bath Temperature (450.degree. C.) for a Glass having the 
Composition of Example 3 from Table 1: 
Prestressing 
Leaching .mu.g/sample! 
time, h Na+ K+ .SIGMA. Na+ + K+ 
______________________________________ 
0 22 3 25 
0.5 2 20 22 
1.5 2 22 24 
2.0 2 22 24 
4.0 2 24 26 
10.0 3 16 19 
15.0 6 8 14 
______________________________________ 
In the first line, the unstressed glass with a prestressing time of 0 hours 
is shown as comparison. The variations in the distribution of the 
leached-out ions arises from the different elemental distribution at the 
surface of stressed and unstressed glasses. However, it is striking and 
significant that the total amount of alkali metal ions leached out in the 
case of the prestressed glasses is just as low as in the case of the glass 
which has not been chemically prestressed. The good chemical resistance is 
thus retained even after prestressing. Surprisingly, alkali metal leaching 
hardly increases at all with increasing prestressing time, and indeed even 
becomes less at long exchange 
TABLE 6 
______________________________________ 
Alkali Metal Leaching in .mu.g per Hard Disk 
Substrate as a function of the Prestressing Temperature 
at Constant Prestressing Time (2 h) for a Glass having the 
Composition of Example 3 from Table 1: 
Prestressing Leaching .mu.g/sample! 
temperature, .degree.C. 
Na+ K+ .SIGMA. Na+ + K+ 
______________________________________ 
350 6 22 28 
400 2 21 23 
450 2 22 24 
500 3 20 23 
550 3 21 24 
______________________________________ 
The alkali metal leaching is hardly affected by the prestressing 
temperature. 
The fact that glasses having the composition described which have been 
chemically prestressed under the stated conditions can be considerably 
reduced in thickness, for example polished, without their alkali metal 
leaching, i.e. their chemical resistance, being significantly changed is 
shown by Table 7, in which the alkali metal leaching of a hard disk 
substrate having the composition of Example 3 from Table 1 which has been 
polished with cerium oxide on both sides to a thickness reduction of 10 
.mu.m in each case not before, but after prestressing, is shown for two 
different prestressing times. The residual roughness was &lt;1 nm. The 
thickness of the compressive stress zone was 33 .mu.m (prestressing time 
2.5 h) or 40 .mu.m (prestressing time 3.0 h) before the polishing. The 
values given were again verified by a triple determination. 
TABLE 7 
______________________________________ 
Alkali Metal Leaching in .mu.g per Hard Disk Substrate for 
Prestressing Times at a Prestressing Temperature of 460.degree. C. on a 
Hard Disk Substrate having the Composition of Example 3 from 
Table 1 which has been polished on both sides with a drop in 
thickness of 10 .mu.m in each case after Prestressing. 
Prestressing 
Leaching .mu.g/sample! 
time, h Na+ K+ .SIGMA. Na+ + K+ 
______________________________________ 
2.5 5 10 15 
3.0 5 13 18 
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This clearly shows the advantage of the procedure in the specific 
embodiment of the invention described, namely polishing only after 
prestressing: the good chemical resistance is retained, and the surface 
quality, which was impaired in some cases by ion exchange, is permanently 
improved by this late polishing. 
By "in mol % based on oxides" in the appended claims we mean that the moles 
of the ingredient is divided by the total number of moles of oxides in a 
sample to obtain the mol %. 
The disclosure in German Patent Application DE 196 16 679.9-45 of Apr. 26, 
1996 is incorporated here by reference. This German Patent Application, at 
least in part, describes the invention described hereinabove and claimed 
in the claims appended herein in below and provides the basis for a claim 
of priority for the instant invention under 35 U.S.C. 119. 
While the invention has been illustrated and described as embodied in a 
process for producing a chemically prestressed glass and a method of use 
of the prestressed glass to make a hard disk, it is not intended to be 
limited to the details shown, since various modifications and changes may 
be made without departing in any way from the spirit of the present 
invention. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention. 
What is claimed is new and is set forth in the following appended claims.