Investigation of the Mechanics, Composition, and Functional Behavior of Thick Tribofilms Formed from Silicon- and Oxygen-Containing Hydrogenated Amorphous Carbon

A custom-grown silicon and oxygen-containing hydrogenated amorphous carbon (a-C:H:Si:O) film is subjected to ball-on-flat tribometry under controlled sliding environments (ambient, dry air, and dry N₂) at room temperature using a 52100 steel ball. The resulting friction coefficient is below 0.2 in ambient air and below 0.1 in dry N₂. Tribofilms on the steel ball with thicknesses in excess of 500 nm are observed. The tribofilms are derived from the a-C:H:Si:O and grow on the steel ball, and display chemical and structural modifications relative to the original a-C:H:Si:O film. Sliding of the tribofilm-coated steel ball against bare silicon results in low friction, highlighting the inherent lubricity afforded by the tribofilm. Tribofilms grown through sliding against a-C:H:Si:O are characterized, post-sliding, with multiple spectroscopic and imaging techniques which collectively demonstrate that the composition and structure of the tribofilm is strongly dependent on the sliding environment. The unusually high tribofilm thickness allows for nanoindentation analysis, which demonstrates that the films are laterally heterogenous and softer than the original a-C:H:Si:O, with moduli and hardness values ranging over three orders of magnitude. Many regions of the tribofilms are extremely soft, with measured hardness values below 100 MPa and reduced Young’s moduli below 1 GPa, and also show a viscous mechanical response. Transmission electron microscopy and electron energy loss spectroscopy (TEM/EELS) characterization of the tribofilm demonstrates that the bulk structure is not graphitic, and indicates the tribofilms are enriched in C−H bonding. Additionally, there is a marked segregation within the tribofilm of Si/O and carbon. It is proposed that a primarily polymeric tribofilm structure can explain the observed mechanical properties.