Sputtered Si-containing low-friction carbon coatings for elevated temperatures

This work presents a tribological study on three sputtered amorphous carbon-based coatings containing Si and Cr (a-C, a-C:Cr and a-C:Si). Molecular dynamics simulations predict tetrahedral bonds between C and Si in the a-C matrix. Ball-on-disk-tests against Al2O3 carried out at room temperature revealed a coefficient of friction of 0.08-0.1 for all films. Between 250 and 325 °C, Si decreases the COF and wear rate to <0.05 and <5×10−17m3/N×laps, respectively. The a-C reference shows a COF of 0.15±0.05 and a wear rate of 1×10−16m3/N×laps, whereas the a-C:Cr film failed. The improved tribological performance of a-C:Si expands its application temperature to 450 °C and is most probably related to formation of Si-compounds on the film surface, as evidenced by X-ray photoelectron spectroscopy.