Tribofilm formation in ultrananocrystalline diamond film

Friction and wear properties of ultrananocrystalline diamond (UNCD) films are found to be superior, and therefore it could be useful for various applications. However, understanding of the tribological properties with respect to boundary phase composition in this material is not yet well understood. Here, the grain boundary phases such as graphite and amorphous carbon (a-C) of UNCD films were tailored during the chemical vapor deposition process by altering the Argon and Nitrogen gases in CH4 plasma medium. The significance of these grain boundary phases in UNCD film was discussed to explain the tribological properties. In run-in, friction coefficient was high in UNCDAr film deposited in CH4 (1%)/Ar plasma condition and it was decreased to lower value after longer sliding distance. However, ultrahigh wear resistance of this film was observed. Here, graphite and a-C phases were insignificant in the grain boundary region as evident from high resolution transmission electron microscope (HRTEM). Further, chemical bonding of these phases was quantitatively described by electron energy loss spectroscopy (EELS). In contrast, ultralow value of friction coefficient with significantly shorter run-in high friction regime was observed in UNCDN film deposited in CH4 (6%)/N-2 plasma. Such a unique characteristic was described by the nanographite phase encasing the needle-like diamond grain of UNCDN film, forming core-shell granular structure. Atomic force microscopy (AFM) showed nucleation of two dimensional(2D) nanographite particles in the deformed wear track after run-in. This was possible due to the presence of core-shell granular structure in UNCDN film. Graphitic nature of the shell for needle-like diamond grains in the wear track was investigated by micro-Raman spectroscopy. Moreover, graphite and a-C tribofilm phase in the wear track was investigated by X-ray photoelectron spectroscopy (XPS) having spatial resolution micrometer scale. Needle-like diamond grains and graphite phase of tribofilm could be one of the primary reasons for the marked reduction in the friction coefficient.