Structural analysis of a-C:H and a-C:H:Si films under high-pressure and high-temperature by synchrotron X-ray diffraction

Highlight
- X-ray diffraction (XRD) is not so major for the amorphous carbon film analysis, but it is excellent for quantitative analysis of bonding structures because it enables the number of sp2 and sp3 bonds to be individually estimated in principle if the pair distribution functions can be obtained because the atomic distances in sp2 and sp3 bonding structures differ
- The use of synchrotron XRD clarified the sp2/sp3 ratios for amorphous carbon films under high pressure and high temperature.

Amorphous carbon films have several outstanding tribology characteristics, including high hardness, surface smoothness, and low friction. Under tribological conditions, their surface is generally exposed to high-temperature and pressure. Although the structure of amorphous carbon films is likely changed by high temperature and pressure, there have been no reports on such structural changes of the films. To obtain information about their structural changes, synchrotron X-ray diffraction was used to analyze two kinds of amorphous carbon films, a-C:H and a-C:H:Si, under high-temperature and high-hydrostatic pressure conditions. Synchrotron X-ray diffraction was applied to films pressurized by a multi-anvil press installed in the PF-AR NE5C beamline at KEK at room temperature and at a high-temperature around 200 °C. The pair distribution functions derived by Fourier transformation of the obtained scattering intensity profiles showed that the sp2/sp3 ratios for both films decreased as the pressure increased and that the sp2/sp3 ratio for the a-C:H film increased as the temperature increased. This indicates that high-pressure creates sp3 stabilization in a-C:H and a-C:H:Si films while high-temperature creates sp2 transition in a-C:H film. The sp2/sp3 ratio for the a-C:H:Si film did not change much even at high-temperature due to the high thermal-oxidative stability of a-C:H:Si.

sp2/sp3 ratios for a-C:H film (left) and a-C:H:Si film (right) at ~ 200 °C.206