Glass substrate for information recording medium and magnetic information recording medium to which the glass substrate is applied

Abstract

Disclosed are a glass substrate for an information recording medium, having excellent scratch resistance and a light weight and having high fracture toughness, the glass substrate having a fragility index value, measured in water, of 12 μm −1/2 or less or having a fragility index value, measured in an atmosphere having a dew point of −5° C. or lower, of 7 μm −1/2 or less, or the glass substrate comprising, by mol %, 40 to 75% of SiO 2 , 2 to 45% of B 2 O 3 and/or Al 2 O 3 and 0 to 40% of R′ 2 O in which R′ is at least one member selected from the group consisting of Li, Na and K), wherein the total content of SiO 2 , B 2 O 3 , Al 2 O 3 and R′ 2 O is at least 90 mol %, and a magnetic information recording medium comprising a magnetic recording layer formed on the glass substrate.

1 .- 14 . (canceled) 
   
   
       15 . A magnetic information recording medium comprising a magnetic recording layer formed on a glass substrate for an information recording medium, said glass substrate comprising, by mol %, 40 to 75% of SiO 2 , 2 to 15% of Al 2 O 3 , 0 to 25% of B 2 O 3 , the total content of Al 2 O 3  and B 2 O 3  being 2% or more, 12% or less of total RO in which R is at least one member selected from the group consisting of Mg, Ca, Zn, Sr and Ba, 10 to 25% of total R′ 2 O in which R′ is at least one member selected from the group consisting of Li, Na and K, 0 to 7% of TiO 2  and 0 to 7% of ZrO 2 , the glass substrate containing 10 to 18% of Li 2 O, 1 to 15% of Na 2 O and 0 to 8% of K 2 O as R′ 2 O, the total content of SiO 2 , Al 2 O 3 , B 2 O 3 , RO and R′ 2 O being 90% or more, the total content of RO and R′ 2 O being 10 to 25%. 
   
   
       16 . The magnetic information recording medium of  claim 15 , wherein the glass substrate has a fragility index value, measured in an atmosphere having a dew point of −5° C. or lower, of 7 μm −1/2  or less. 
   
   
       17 . The magnetic information recording medium of  claim 15 , wherein the glass substrate has a Young's modulus of at least 70 GPa. 
   
   
       18 . The magnetic information recording medium of  claim 15 , wherein the glass substrate has a modulus of rigidity of at least 20 GPa. 
   
   
       19 . The magnetic information recording medium of  claim 15 , wherein the glass substrate is made of a glass having a region wherein the glass has a viscosity of at least 1 Pa·s, in a range of temperatures equivalent to, and higher than, a liquidus temperature of the glass. 
   
   
       20 . The magnetic information recording medium of  claim 15 , wherein the glass substrate is made of a glass having a thermal expansion coefficient of 60×10 −7 /° C. or greater at a temperature of 100° C. to 300° C. 
   
   
       21 . The magnetic information recording medium of  claim 15 , wherein the glass substrate has no chemically strengthened layer. 
   
   
       22 . The magnetic information recording medium of  claim 15 , wherein the glass substrate has a chemically strengthened layer. 
   
   
       23 . The magnetic information recording medium of  claim 15 , wherein said glass substrate contains MgO and RO. 
   
   
       24 . The magnetic information recording medium of  claim 15  wherein said glass substrate contains 1 mol % or more of MgO as RO. 
   
   
       25 . The magnetic information recording medium of  claim 23  or  24 , wherein said glass substrate contains 5 mol % or more of RO. 
   
   
       26 . The magnetic information recording medium of  claim 25 , wherein said glass substrate contains 1 to 5 mol % of Na 2 O. 
   
   
       27 . The magnetic information recording medium of  claim 26 , wherein said glass substrate contains 60 to 75 mol % of SiO 2 .