Glass substrates are frequently used in the fabrication of information storage media, such as magnetic recording media for hard disk drives. Glass substrates enjoy a number of advantages when compared with substrates formed from aluminum alloys, which may be prone to deformation.
Because the information storage media may be rotated at high speeds, the substrate on which it is disposed must have sufficient mechanical strength to withstand the forces generated by this rotation. One approach to strengthening glass substrates involves chemically strengthening the glass thereof, by exchanging sodium ions near the surface of the substrate with larger potassium ions in order to provide a compressive stress, which offsets tensile stress and improves the toughness of the glass.
This chemical strengthening, however, does not address a fundamental limitation to the mechanical strength of glass, which arises due to cracks formed in the glass when the substrate is manufactured (e.g., cracks formed by scribing the inner and outer diameter of an annuls-shaped substrate). When stressed, these cracks can initiate a breakage that can propagate through a large region of the substrate, causing catastrophic failure of the information storage medium formed thereon. Such cracks are depicted in the glass substrate 101 illustrated in FIG. 1. As can be seen with reference to FIG. 1, substrate 101 is an annulus with opposing substantially planar surfaces, an inner edge 102 and an outer edge 103. On both edges 102 and 103 are disposed a number of cracks 104. These cracks may have resulted, for example, from a process of forming substrate 101, including scribing, grinding, cutting or polishing. Left untreated, these cracks may initiate a breakage when substrate 101 is subjected to mechanical stresses such as may be caused by high speed rotation.