Glass substrate for magnetic disk and manufacturing method of the same

1. A glass substrate for a magnetic disk, the glass substrate being a disk-shaped glass substrate for a magnetic disk, having a main surface and an end face, and being subjected to chemical reinforcement treatment,

wherein in the glass substrate, a penetration length in an uppermost-portion stress layer on the main surface is 49.1 μm or less, and assuming that an angle between the main surface and compressive stress in a stress profile by a Babinet compensator method is θ, the penetration length in the uppermost-portion stress layer is a value y of {12·t·1n(tan θ)+(49.1/t)} or less,

the penetration length in the uppermost-portion stress layer is a length of a longer diagonal in an indentation when a diamond quadrangular pyramid indenter of rhombic transverse section with vertex angles of 172° 30 ′ and 130° is pressed against the main surface with a pressing force of 100 g, the tan θ is a value {(L 1 +L 2 )/(P 1 +P 2 )} calculated from stress values and stress depths obtained by the Babinet compensator method, the t is a thickness of the substrate,

P 1 is a compressive stress value, P 2 is a tensile stress value, L 1 is a compressive stress depth, and L 2 is a tensile stress depth,

a compressive stress depth is from 100 μm to 150 μm, and

glass materials forming the glass substrate include aluminosilicate glass,

where SiO 2 is 57 percent to 75 percent by mass,

Al 2 O 3 is 5 percent up to 20 percent by mass, (where the total amount of SiO 2+ Al 2 O 3 is 74 percent by mass or more),

the total of ZrO 2 , HfO 2 , Nb 2 O 5 , Ta 2 O 5 , La 2 O 3 , Y 2 O 3 , and TiO 2 exceeds 0 percent and is up to 6 percent by mass or less,

Li 2 O exceeds 1 percent by mass and is up to 9 percent by mass or less,

Na 2 O is 5 percent to 18 percent by mass (where the mass ration Li 2 O/Na 2 O is 0.5 or less),

K 2 O is 0 to 6 percent by mass,

MgO is 0 to 4 percent by mass,

CaO exceeds 0 percent and is up to 5 percent by mass or less, where the total amount of MgO and CaO is 5 percent by mass or less, and the content of CaO is higher than the content of MgO, and content of SrO+BaO is 0 to 3 percent by mass, and

ZrO 2 exceeds 0 percent and is up to 5.5 percent by mass or less.

2. The glass substrate for a magnetic disk according to claim 1 , wherein the value of y is {16·t·ln(tan θ)+(49.1/t)}.

3. The glass substrate for a magnetic disk according to claim 1 , wherein surface roughness Ra of the main surface measured by an atomic force microscope is 0.3 nm or less.

4. The glass substrate for a magnetic disk according to claim 1 , wherein surface roughness Ra of the end face is 0.2 μm or less in a measurement length of 0.8 mm in the circumference direction of the glass substrate for a magnetic disk.

5. A magnetic disk comprising:

the glass substrate for a magnetic disk according to claim 1 ; and

a magnetic layer provided on the main surface of the glass substrate for a magnetic disk directly or via another layer.

6. The glass substrate for a magnetic disk according to claim 1 , wherein the glass substrate has impact resistance withstanding accelerations of 1500 G for one thousandth of a second.

7. The glass substrate for a magnetic disk according to claim 1 , wherein the ratio (Li 2 O/Na 2 O) of Li 2 O amount to Na 2 O amount is 0.38 or less.

8. The glass substrate for a magnetic disk according to claim 2 , wherein surface roughness Ra of the main surface measured by an atomic force microscope is 0.3 nm or less.

9. The glass substrate for a magnetic disk according to claim 2 , wherein surface roughness Ra of the end face is 0.2 μm or less in a measurement length of 0.8 mm in the circumference direction of the glass substrate for a magnetic disk.

10. The glass substrate for a magnetic disk according to claim 3 , wherein surface roughness Ra of the end face is 0.2 μm or less in a measurement length of 0.8 mm in the circumference direction of the glass substrate for a magnetic disk.

11. The glass substrate for a magnetic disk according to claim 8 , wherein surface roughness Ra of the end face is 0.2 μm or less in a measurement length of 0.8 mm in the circumference direction of the glass substrate for a magnetic disk.

12. A magnetic disk comprising:

the glass substrate for a magnetic disk according to claim 2 , and

a magnetic layer provided on the main surface of the glass substrate for a magnetic disk directly or via another layer.

13. A magnetic disk comprising: the glass substrate for a magnetic disk according to claim 3 , and a magnetic layer provided on the main surface of the glass substrate for a magnetic disk directly or via another layer.

14. A magnetic disk comprising:

the glass substrate for a magnetic disk according to claim 4 , and a magnetic layer provided on the main surface of the glass substrate for a magnetic disk directly or via another layer.

15. A magnetic disk comprising: the glass substrate for a magnetic disk according to claim 8 , and a magnetic layer provided on the main surface of the glass substrate for a magnetic disk directly or via another layer.

16. A magnetic disk comprising:

the glass substrate for a magnetic disk according to claim 9 , and

a magnetic layer provided on the main surface of the glass substrate for a magnetic disk directly or via another layer.

17. A magnetic disk comprising: the glass substrate for a magnetic disk according to claim 10 , and a magnetic layer provided on the main surface of the glass for a magnetic disk directly or via another layer.

18. A magnetic disk comprising: the glass substrate for a magnetic disk according to claim 11 , and a magnetic layer provided on the main surface of the glass substrate for a magnetic disk directly or via another layer.