Magnetic disk glass substrate

Abstract

A magnetic disk glass substrate including compressive stress layers at main surfaces and a tensile stress layer between the compressive stress layers formed by chemical strengthening. When the glass substrate has a thickness of less than 0.5 mm and the tensile stress layer has a thickness L and a tensile stress of Pt (kg/mm 2 ), the following relation holds: 0.4 (kg/mm)≦ L·Pt ≦2.0 (kg/mm)

1 . A magnetic disk glass substrate for use in a hard disk drive and having a disk thickness of less than 0.5 mm and mirror-finished main surfaces, the glass substrate comprising:
 compressive stress layers formed at the main surfaces; and   a tensile stress layer formed between the compressive stress layers,   wherein a product of a thickness of the tensile stress layer and a maximum tensile stress of the tensile stress layer is set at a predetermined value, so that the magnetic disk glass substrate has a predetermined impact resistance and the main surface of the magnetic disk glass substrate has a predetermined waviness (Wa), and   wherein the thickness of the tensile stress layer is measured by observing a longitudinal section of the magnetic disk glass substrate with a Babinet compensator.   
     
     
         2 . The magnetic disk glass substrate according to  claim 1 , wherein:
 the product of the thickness of the tensile stress layer and the maximum tensile stress of the tensile stress layer falls within a range of 0.4 to 2.0 kg/mm.   
     
     
         3 . The magnetic disk glass substrate according to  claim 1 , wherein:
 the impact resistance is 3000 G or more and,   the waviness (Wa) is 1.0 nm or less.   
     
     
         4 . The magnetic disk glass substrate according to  claim 1 , wherein:
 a predetermined disk thickness is set by lapping the main surfaces, and   the mirror-finished surfaces are formed with no cracks by polishing the main surfaces.   
     
     
         5 . The magnetic disk glass substrate according to  claim 1 , wherein:
 a thickness of the tensile stress layer is 0.4 mm or less, and   a maximum tensile stress of the tensile stress layer is 10 kg/mm 2  or less.   
     
     
         6 . The magnetic disk glass substrate according to  claim 1 , wherein:
 a total thickness of the compressive stress layer at one main surface and the compressive stress layer at the other main surface is at least 40% of the disk thickness.   
     
     
         7 . The magnetic disk glass substrate according to  claim 6 , wherein:
 a maximum tensile stress of the tensile stress layer is 10 kg/mm 2  or less.   
     
     
         8 . The magnetic disk glass substrate according to  claim 1 , wherein:
 a highest compressive stress in the compressive stress layers is 4 kg/mm 2  or more.   
     
     
         9 . The magnetic disk glass substrate according to  claim 1 , wherein:
 the magnetic disk glass substrate is used for a magnetic disk installed in a hard disk drive that starts and stops operation by a load/unload system.   
     
     
         10 . A magnetic disk comprising:
 the magnetic disk glass substrate according to  claim 1 , and   a magnetic layer formed on the magnetic disk glass substrate.