Electrochemical tests on the carbon protective layer of a hard disk

Air humidity as well as various gases present in the environment or evolved from the hard disk file has a detrimental effect on the lifetime of the hard disk. The corrosion properties of unprotected Co78Cr12Pt10 magnetic layer and covered by a carbon overcoat have been studied using a three-electrode electrochemical corrosion test. Potentiodynamic scanning and direct cut-rent (DC) polarization resistance techniques have been used with 0.1 M NaCl solution at different pH values. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) and scanning electron microscopy (SEM) imaging of the corrosion sites revealed development of corrosion products along the scratch lines originating from the previous surface polishing and texturing step. Capillary retention of water vapor along the scratch valleys enhances creation of corrosion sites. The SEM images showed the build-up of hillocks formed by migrated Co and its hydroxide and chloride compounds on the carbon surface. A lower solubility and decrease in corrosion potential of Co were observed when the pH of the electrolyte was increased from 5.5 to 11. The void structure of the carbon overcoat enables the penetration of water vapor and dissolution of Co in the magnetic layer. A driving force for the Co ion migration through the porous structure of a carbon film is a galvanic electrochemical reaction between the Co alloy and the carbon overcoat. The carbon overcoat slows down the lateral growth of the Co corrosion products. The provoked corrosion of the magnetic layer had not led to a macroscopic change in the film magnetic properties, measured by vibrating sample magnetometer. (C) 2002 Elsevier Science B.V. All rights reserved.