Glass composition, glass susbstrate employing it for an information recording medium, and information recording medium employing it

1. A glass composition comprising the following glass ingredients:

60.2 to 70% by weight of SiO 2 ;

1 to 20% by weight of Al 2 O 3 ;

B 2 O 3 , wherein the B 2 O 3 is present in an amount up to 10%;

SiO 2 +Al 2 O 3 +B 2 O 3 accounting for 61.2 to 90% by weight;

a total of 3.0 to 15% by weight of R 2 O compounds, where R=Li, Na, and K, wherein the following condition is fulfilled: 0.1% ≦Na 2 O<6%;

a total of 2.0 to 15% by weight of R′O compounds, where R′=Mg, and Zn; and

a total of 1.0 to 20% by weight of MOx (TiO 2 +ZrO 2 +Ln x O y ), where Ln x O y represents at least one compound selected from the group consisting of lanthanoid metal oxides, Y 2 O 3 , Nb 2 O 5 , and Ta 2 O 5 ,

wherein the glass composition does not include CaO and the following condition is fulfilled:

0.070<(total content of R′O compounds)/(SiO 2 +Al 2 O 3 +B 2 O 3 )<0.200.

2. A glass composition as claimed in claim 1 ,

wherein the following condition is further fulfilled:

0.10<Al 2 O 3 /(SiO 2 +Al 2 O 3 +B 2 O 3 )<0.35.

3. A glass composition as claimed in claim 1 ,

wherein the following condition is further fulfilled:

Al 2 O 3 /B 2 O 3 >2.0.

4. A glass composition as claimed in claim 1 ,

wherein the following condition is further fulfilled:

0.50<(total content of R′O compounds)/MOx<5.00.

5. A glass substrate for an information recording medium, the glass substrate comprising the following glass ingredients:

60.2 to 70% by weight of SiO 2 ;

1 to 20% by weight of Al 2 O 3 ;

B 2 O 3 , wherein the B 2 O 3 is present in an amount up to 10%;

SiO 2 +Al 2 O 3 +B 2 O 3 accounting for 61.2 to 90% by weight;

a total of 3.0 to 15% by weight of R 2 O compounds, where R=Li, Na, and K, wherein the following condition is fulfilled: 0.1%≦Na 2 O<6%

a total of 2.0 to 15% by weight of R'O compounds, where R′=Mg, and Zn; and

a total of 1.0 to 20% by weight of MOx (TiO 2+ ZrO 2 +Ln x O y ), where Ln x O y represents at least one compound selected from the group consisting of lanthanoid metal oxides, Y 2 O 3 , Nb 2 O 5 , and Ta 2 O 5 ,

wherein the glass substrate does not include CaO and the following condition is fulfilled:

0.070<(total content of R′O compounds)/(SiO 2 +Al 2 O 3 +B 2 O 3 )<0.200.

6. A glass substrate as claimed in claim 5 ,

wherein the glass substrate has:

a specific elastic modulus E/ρ of 33 or higher;

a Vickers hardness Hv in a range of from 550 to 675;

alkali elution A of 200 ppb or lower per 2.5-inch disk;

Si elution S of 500 ppb or lower per 2.5-inch disk;

a fracture toughness Kc of 0.85 MPa/m 1/2 or greater; and

a glass transition temperature Tg of 550° C. or lower.

7. A glass substrate as claimed in claim 6 ,

wherein the following conditions are fulfilled:

Tg/Hv> 0.85;

Tg×Kc> 500,

Tg×Kc/Hv> 0.80.

8. An information recording medium having an information recording layer formed on a glass substrate as claimed in claim 5 .

9. A glass composition as claimed in claim 1 ,

wherein the glass composition comprises 0.5% to 10% by weight of B 2 O 3 .

10. A glass substrate as claimed in claim 5 ,

wherein the glass substrate comprises 0.5% to 10% by weight of B 2 O 3 .

11. A glass composition as claimed in claim 1 , wherein the glass composition comprises 0.1% to 3.9% by weight of Na 2 O.

12. A glass substrate as claimed in claim 5 , wherein the glass substrate comprises 0.1% to 3.9% by weight of Na 2 O.

13. A glass composition as claimed in claim 1 , wherein the following conditions are further fulfilled:

0.080<(total content of R′O compounds)/(SiO 2 +Al 2 O 3 +B 2 O 3 )<0.180;

0.15<Al 2 O 3 /(SiO 2 +Al 2 O 3 30 B 2 O 3 )<0.25.

14. A glass substrate as claimed in claim 5 , wherein the following conditions are further fulfilled:

0.080<(total content of R′O compounds)/(SiO 2 +Al 2 O 3 +B 2 O 3 )<0.180;

0.15<Al 2 O 3 /(SiO 2 +Al 2 O 3 +B 2 O 3 )<0.25.