The Influence of Ultraviolet Irradiation on the Surface Chemistry of Ztetraol Magnetic Hard Disk Lubricant: a Combined Temperature Programed Desorption and X-Ray Photoelectron Spectroscopic Study

Abstract

Magnetic hard disks coated with ztetraol lubricant were characterized with temperature programed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Ztetraol was found to have two adsorbed states, with desorption temperatures of ~650 and ~950 K. The complete removal of the low temperature state by solvent extraction identified it as due to the mobile lubricant. Desorption of the CF3 mass fragment was observed only at high temperature, indicating that lubricant in this state was in contact with the surface, which allowed the assignment of this high temperature state to the bonded lubricant. UV irradiation was found not to alter the TPD of the unextracted lubricant film. In contrast, the TPD of the UV-bonded layer remaining after extraction was observed to have only one desorption state, stabilized at ~40 K higher temperature as compared with the naturally bonded layer. XPS of the mobile layer was accomplished using spectral subtraction of the C1s carbon overcoat peak (284.8 eV BE), and the perfluoroethylene and perfluoromethylene lubricant peaks (293.5 and 295.0 eV BE, respectively), as a function of UV exposure. No change in the mobile lubricant layer was found with increasing UV exposure. C1s XPS of the UV-bonded layer identified five surface species and assigned XPS peaks to each: the carbon overcoat peak at 284.8 eV BE, ether peak at 286.4 eV BE, organic acid peak at 288.7 eV BE, perfluoroethylene peak at 293.5 eV BE, and the perfluoromethylene peak at 295.0 eV BE. Changes in the relative intensity of the assigned peaks with increasing UV irradiation exposure time were observed. The integration of the assigned XPS peaks from the UV-bonded layer with increasing UV exposure was used to identify UV dependent changes in the bonded layer. A significant relative decrease in the perfluoromethylene lubricant component was observed with increasing exposure, with a simultaneous increase of both the ether and organic acid surface concentrations. Quantum chemical calculations using small molecular models of the ztetraol were used to elucidate the XPS and TPD observations. The calculations revealed that the lubricant is fragmented with irradiation, forming reactive end groups, a volatile CF2O, and a hydrolyzeable CFO terminated fragment all consistent with the XPS results. The mechanistic implications and the possible surface chemistry are discussed.