METHOD FOR MANUFACTURING GLASS SUBSTRATE FOR DATA STORAGE MEDIUM AND GLASS SUBSTRATE
The present invention relates to a method for manufacturing a glass substrate for data storage mediums, the method including a chemical strengthening treatment step of dipping a glass for a substrate in a mixed molten salt to form a compressive layer on front and back surfaces of the glass for a substrate, in which the glass for a substrate contains a lithium ion as an alkali component, the mixed molten salt contains sodium nitrate, potassium nitrate and lithium nitrate, an amount of lithium nitrate being from 1 to 6% by mass, and the glass for a substrate is dipped in the mixed molten salt at a treatment temperature of 325° C. or more and 475° C. or less for a treatment time of 30 minutes or less, under the requirement satisfying the following formula: 1900≦T×log(t 2 )≦2900, in which T is a treatment temperature (unit: K) and t is a treatment time (unit: second).
1 . A method for manufacturing a glass substrate for data storage mediums, said method comprising a chemical strengthening treatment step of dipping a glass for a substrate in a mixed molten salt to form a compressive layer on front and back surfaces of the glass for a substrate,
wherein the glass for a substrate contains a lithium ion as an alkali component,
the mixed molten salt contains sodium nitrate, potassium nitrate and lithium nitrate, an amount of lithium nitrate being from 1 to 6% by mass, and
the glass for a substrate is dipped in the mixed molten salt at a treatment temperature of 325° C. or more and 475° C. or less for a treatment time of 30 minutes or less, under the requirement satisfying the following formula:
1900≦ T ×log( t 2 )≦2900
in which T is a treatment temperature (unit: K) and t is a treatment time (unit: second).
2 . The method for manufacturing a glass substrate for data storage mediums according to claim 1 , wherein the glass for a substrate subjected to said chemical strengthening treatment has a fracture toughness value K c measured by an IF method according to JIS R1607, of 1.2 MPa·m 1/2 or more.
3 . The method for manufacturing a glass substrate for data storage mediums according to claim 1 , wherein the mixed molten salt comprises, in terms of mass percent, from 28 to 55% of sodium nitrate and from 40 to 69% of potassium nitrate, and has a melting point of 250° C. or lower.
4 . The method for manufacturing a glass substrate for data storage mediums according to claim 1 , wherein, in the chemical strengthening treatment step, the step of dipping the glass for a substrate in the mixed molten salt is limited to one step.
5 . The method for manufacturing a glass substrate for data storage mediums according to claim 1 , wherein, in the chemical strengthening treatment step, the glass for a substrate to be dipped in the mixed molten salt has a temperature not lower than the melting point of the mixed molten salt.
6 . The method for manufacturing a glass substrate for data storage mediums according to claim 1 , wherein, in the chemical strengthening treatment step, after dipping the glass for a substrate in the mixed molten salt, the glass for a substrate is not annealed, and after reaching a temperature of the glass for a substrate to 300° C. or lower, the glass for a substrate is quenched by bringing into contact with a cooling medium at a cooling rate of 100° C./minute or more.
7 . The method for manufacturing a glass substrate for data storage mediums according to claim 1 , wherein, after the chemical strengthening treatment step, the glass for a substrate is not repolished.
8 . The method for manufacturing a glass substrate for data storage mediums according to claim 1 , wherein the glass for a substrate comprises, in terms of mol % on the basis of oxides, from 58 to 66% of SiO 2 , from 11 to 17% of Al 2 O 3 , from 0 to 4% of MgO, from 8 to 16% of Li 2 O, and from 2 to 9% of Na 2 O, provided that Li 2 O+Na 2 O is from 13 to 21%.
9 . The method for manufacturing a glass substrate for data storage mediums according to claim 1 , wherein the glass for a substrate has a value obtained by dividing a content of Li 2 O in terms of mol % on the basis of oxides by a total content (R 2 O) of Li 2 O, Na 2 O and K 2 O of 0.4 or more.
10 . The method for manufacturing a glass substrate for data storage mediums according to claim 1 , wherein a glass disk for storage medium molded from the glass for a substrate having been subjected to the chemical strengthening treatment has:
a flatness of 3 μm or less;
an arithmetic average waviness (Wa) with cut-off values of 0.4 mm and 5 mm on a surface between a radius of 16 mm and a radius of 28 mm from a center of a 2.5-inch disk of said glass disk, of 0.6 nm or less; and
an arithmetic average roughness (Ra) of 0.15 nm or less.
11 . A glass substrate for data storage mediums, produced by the manufacturing method according to claim 1 .
12 . A data storage medium comprising the glass substrate for data storage mediums according to claim 11 , and a magnetic recording layer formed on said glass substrate.