Fundamentals on the friction mechanism of diamondlike carbon films

The present study proposes to discuss the friction mechanism of DLC films at the molecular scale, as a function of the nature of the films deposited by d.c. plasma enhanced chemical vapor deposition (PECVD). Two films with different hydrogen contents (34 and 42 at.%), relative fractions of hydrogen bounded to carbon (0.57 and 1.00), and ratios of sp(2)/sp(3) carbon bonds (1.27 and 2.33) have been studied. The film structures and compositions have been investigated by forward recoil elastic scattering (FRES), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (NMR). Reciprocating pin-on-flat friction tests have been performed in ultrahigh vacuum, at room temperature and at various mean contact pressures, ranging between 250 and 660 MPa. Chemical analyses have been performed by in situ X-ray photoelectron spectroscopy inside the contact tracks. DLC with the highest hydrogen contents exhibit ultralow friction (approximate to 0.02). DLC with the lowest hydrogen content exhibits high friction (0.5 - 0.7), depending on the contact pressure. These opposite friction behaviors are described in terms of shear strength with two contributions, the first one attributed to the adhesive pressures between the two contacting surfaces, the second one attributed to the applied external pressure. The high shear strength of the DLC film with the highest sp(2)C content is consistent with both external and adhesive pressure contributions, the adhesive one being attributed to the predominance of the sp(2)C pi-pi* orbital interactions. The weak van der Waals interaction energy of the hydrocarbon polymerlike topcoats is consistent with the ultralow shear strength related to the highly hydrogenated film.