Phase composition and tribomechanical properties of Ti-B-C nanocomposite coatings prepared by magnetron sputtering

Protective nanocomposite coatings based on hard ceramic phases (TiC, TiB ₂) combined with amorphous carbon (a-C) are of interest because of their adequate balance between mechanical and tribological performances. In this work, TiBC nanocomposite coatings were prepared by co-sputtering of graphite and TiB ₂ targets. Varying the discharge power ratio applied to the graphite and TiB ₂ targets from 0 to 2, the a-C content in the coatings could be tuned from 0 to 60%, as observed by means of Raman and x-ray photoelectron spectroscopy (XPS). The microstructural characterization demonstrated a progressive decrease in crystallinity from an initial nanocrystalline (nc) TiB ₂-like structure to a distorted TiB _xC _y ternary compound with increasing C concentration. X-ray absorption near-edge structure measurements on the B K-edge helped to determine a hexagonal arrangement around the B atoms in the ternary TiB _xC _y phase. A fitting analysis of the C 1s XPS peak allowed us to evaluate the relative amount of a-C and TiB _xC _y components. A drastic change in hardness (from 52 to 13GPa) and friction coefficient values (from 0.8 to 0.2) is noticed when moving from nc-TiB ₂ to TiBC/a-C nanocomposites. The fraction of a-C necessary to decrease the friction below 0.2 was found to be 45%. Raman observation of the wear tracks determined the presence of disordered sp ²-bonded carbon phase associated with the diminution of the friction level.