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 including, in terms of mol % on the basis of oxides, from 58 to 66% of SiO 2 , from 9 to 15% of Al 2 O 3 , from 7 to 15% 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%, in a mixed molten salt to form a compressive layer on front and back surfaces of the glass for a substrate, in which the mixed molten salt includes, in terms of mass percent, from 1 to 7.5% of lithium nitrate, from 28 to 55% of sodium nitrate and from 40 to 69% of potassium nitrate.
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 comprising, in terms of mol % on the basis of oxides, from 58 to 66% of SiO 2 , from 9 to 15% of Al 2 O 3 , from 7 to 15% 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%, in a mixed molten salt to form a compressive layer on front and back surfaces of said glass for a substrate,
wherein the mixed molten salt comprises, in terms of mass percent, from 1 to 7.5% of lithium nitrate, from 28 to 55% of sodium nitrate and from 40 to 69% of potassium nitrate.
2 . The method for manufacturing a glass substrate for data storage mediums according to claim 1 , wherein said glass for a substrate subjected to said chemical strengthening treatment has:
a fracture toughness value K c measured by an IF method in accordance with JIS R1607 of 1.2 MPa·m 1/2 or more;
a value K c /K bulk obtained by dividing the fracture toughness value K c by a fracture toughness value K bulk measured by the IF method before subjecting the chemical strengthening treatment of 1.2 or more;
a flatness of 3 μm or less; and
an arithmetic average waviness (Wa) with cutoff 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 for a substrate, of 0.6 nm or less.
3 . A glass substrate for data storage mediums, manufactured by the manufacturing method according to claim 1 .
4 . A data storage medium comprising the glass substrate for data storage mediums according to claim 3 and a magnetic recording layer formed on said glass substrate.
5 . A glass substrate for data storage mediums, manufactured by the manufacturing method according to claim 2 .
6 . A data storage medium comprising the glass substrate for data storage mediums according to claim 5 and a magnetic recording layer formed on said glass substrate.