Study of Tribochemical Processes on Hard Disks Using Photoemission Electron Microscopy

The interface between hard disk and slider involves mechanical and tribochemical processes between the hard carbon overcoat of the disk, the lubricant, and the carbon coated or uncoated slider surface. These processes have been studied by two related X-ray techniques-Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy and Photoemission Electron Microscopy (PEEM) using X-rays. NEXAFS allows studying the elemental composition and chemical bonding in a material, whereas PEEM combines this ability with imaging of the sample. Lubricated and unlubricated disks were worn under various conditions using carbon coated and uncoated sliders. The wear tracks on the hard disks were investigated using PEEM to find chemical and elemental changes caused by the wear. Local NEXAFS spectra taken in wear tracks using the PEEM microscope show no chemical changes on unlubricated disks, just a reduction of the hard carbon overcoat thickness. On lubricated disks remarkable chemical modifications of the lubricants caused by the wear are observed if the disks failed the wear tests. The chemical changes are manifested in a formation of various new carbon-oxygen (mostly carboxylic) bonds in the wear tracks and in a strong reduction of the amount of fluorine and carbon. The chemical modifications were only found inside the wear tracks and are clearly caused by the wear. It was found that lubricant degradation is not solely a mechanical process of molecule scission but accompanied by oxidation reactions. The chemical changes were strongly correlated to the tribological behavior of the disks: the worse the disks performed in the wear tests, the stronger were the chemical modifications.