Cracking mechanism in an anodized alumite substrate for thin film magnetic disks.

The crack nucleation mechanism in an alumite substrate for thin film magnetic film disks is investigated by using the scratch technique. The alumite substrate consists of a hard-brittle anodized alumite layer of μm-order thickness and a soft-ductile Al-Mg alloy base. In the scratching of the alumite substrate even with a relatively blunt indenter, the so-called median crack is observed at a low load prior to the tensile bow crack. This crack is a typica nucleation process when bulk hard-brittle materials are scratched by very a sharp indenter such as a Vickers indenter. It is also observed that two types of median cracks exist. One is nucleated at the elastic/plastic boundary just below the indenter and is propagated downward. Another is nucleated at the alumite/Al-Mg alloy boundary and is propagated upward. These crack nucleation mechanisms are investigated by stress analysis using FEM as well.