The interaction of short-chain model lubricants with the surfaces of hydrogenated amorphous carbon films

Abstract

The adsorption of water and small model perfluorinated lubricants on hydrogenated amorphous carbon (a∶C-H) films of varying hydrogen content was investigated using thermal desorption spectroscopy (TDS). Hydrogen content of the carbon films was measured by Rutherford back scattering (RBS) and elastic recoil spectroscopy (ERS) and correlated to changes in surface free energies measured by contact angle analysis. Hydrogenated carbon films exhibiting the highest surface free energy provided a greater attractive interaction for the model lubricants. All model lubricant species studied - water (D2O), perfluorodiethyl ether (CF3CF2OCF2CF3), perfluoropentane (CF3(CF2)3CF3), perfluorooctane (CF3(CF2)6CF3), 2,2,2-trifluoroethanol (CF3CH2OH), and 1,1,7-H-perfluoroheptanol (CF2H(CF2)5CH2OH)—reversibly adsorbed to the carbon surface with little chemical reaction. Increases in desorption energies with increasing chain length were observed among the adsorbates and are ascribed to increasing van der Waals interactions. Incorporation of alcoholic end groups provided an avenue of hydrogen bonding to the surface and produced an ~20 kJ/mol increase in desorption energy relative to a perfluorinated alkane of the same chain length. Ether linkages within the model lubricant provide little increase in desorption energy as fluorine substituents effectively screen the oxygen. Together these findings implicate a predominantly physisorbed state for perfluorinated lubricants on hydrogenated carbon surfaces.