Friction and wear properties of smooth diamond films grown in fullerene plus argon plasmas

In this study, we describe the nanocrystalline structure and ultralow friction and wear properties of smooth (20-50 nm root mean square (RMS)) diamond films grown in a microwave plasma consisting of Ar and fullerene (the carbon source). The sliding friction coefficients of these films against Si3N4 balls are 0.04 and 0.12 in dry N-2 and air respectively, comparable to that of natural diamond sliding against the same pin material, but are lower by factors of 5-10 than that afforded by rough diamond films grown in conventional H-2 + CH4 plasmas. Furthermore, the smooth diamond films produced in this work afforded wear rates to SiC pins and Si3N4 balls that were one to two orders of magnitude lower than those of H-2 + CH4-grown films. Mechanistically, the ultralow friction and wear properties of the fullerene-derived diamond films correlate well with their initially smooth surface finish and their ability to become polished even further during sliding. The wear tracks reach an ultrasmooth (3-6 nm RMS) surface finish that results in very little abrasion and ploughing. Depending on the test environment, micrographitization may also occur during sliding and dominate the frictional behavior of these diamond films. The nanocrystalline microstructure and exceptionally pure sp(3) bonding in these smooth diamond films were verified by numerous surface and structure analytical methods, including X-ray diffraction, high-resolution AES, EELS, NEXAFS, SEM and TEM. An AFM instrument was used to characterize the topography of the films and rubbing surfaces.