Structural order in near-frictionless hydrogenated diamondlike carbon films probed at three length scales via transmission electron microscopy

A series of hydrogenated diamondlike carbon films grown using plasma-enhanced chemical-vapor deposition is systematically studied as a function of source gas composition using transmission electron microscopy. The structure of the films is examined at three distinct length scales. Both plan-view and cross-sectional studies are undertaken to reveal any large-scale inhomogeneities or anisotropy in the films. The degree of medium-range order in the films is measured by performing fluctuation electron microscopy on the plan-view and cross-sectional specimens. Electron-energy-loss spectroscopy is employed to measure the mass density and sp²:sp³ carbon bonding ratios of the samples. Thus, inhomogeneity as a function of depth in the film is revealed by the measurements of the short- and medium-range orders in the two different sample geometries. Soft, low-density diamondlike carbon films with low coefficients of friction are found to be more homogeneous as a function of depth in the film and possess reduced medium-range order in the surface layer. We find that these properties are promoted by employing a high hydrogen content methane and hydrogen admixture as the growth ambient. In contrast, harder, denser films with higher coefficients of friction possess a distinct surface layer with a relatively elevated level of carbon sp³ bonding and a higher degree of medium-range order. The structure of the films is examined in the light of the energetics of the growth process. It appears that a high flux of penetrating hydrogen ions modifies the surface layer containing the remnant damage from the carbon ions, homogenizing it and contributing to a lowering of the coefficient of friction.