Corrosion Inspection\nfor Hard Disk Media with Carbon-Based\nOvercoats by In-Liquid Open-Loop Electric Potential Microscopy

The hard disk medium (HDM) with a carbon overcoat (COC)\nis a fundamental\ncomponent of a hard disk drive. The conventional test for its corrosion\ndurability, known as the “HOT/WET test,” requires considerable\ntime and effort and does not provide any local information about the\ncorrosion. Here, we address this issue by employing open-loop electric\npotential microscopy (OL-EPM), a potential measurement technique based\non atomic force microscopy (AFM), for corrosion inspection. To explore\nthe applicability of OL-EPM, we observed the surface of the HDMs with\ndifferent COC thicknesses in a dilute HNO<sub>3</sub> solution. Through\ntime-dependent and high-resolution OL-EPM observations, we found that\nthis technique can be used for detecting nanoscale COC defects. This\nis because the HDM surface under a COC defect is exposed to the solution\nand undergoes anodic dissolution, increasing the local potential around\nthe defect. This is readily detected by OL-EPM even before corrosion\nproduct formation around the defects induces the topographic change.\nThis work demonstrates that OL-EPM is useful not only for understanding\nthe local corrosion mechanisms but also for detecting the COC defects\nin a much shorter time (∼3 h) than the HOT/WET test (3–4\ndays).