Structure and tribo-mechanical properties of Si-containing ta-C thin films grown by cathodic arc evaporation

The incorporation of silicon (Si) into tetrahedral amorphous carbon (ta-C) thin films is an effective strategy for improving the properties and expanding their application field. However, a key challenge lies in achieving controlled incorporation of low Si concentrations without significantly compromising hardness. To address this, graphite compound cathodes with 2.5 and 5 at.-% Si, as well as pure graphite, were employed as cathodes in a vertically arranged array of cathodic arc evaporators. This configuration enabled the deposition of ta-C:Si thin films on AISI M2 steel substrates with Si contents ranging from 0.1 to 6.6 at.-%.

The hardness of the ta-C:Si films decreases with increasing Si content, yet remains high at H ≈ 45 GPa even at the highest Si concentration. A high hardness of H ≈ 70 GPa is achieved at the lowest Si contents below 1 at.-%. The reduction in hardness is attributed to structural changes in the amorphous network revealed by Raman spectroscopy, which shows both an increased sp² fraction and a transformation from isolated olefinic sp² sites to more ordered aromatic-like configurations with rising Si concentration. Tribological tests reveal that high Si-containing ta-C:Si exhibits a shorter run-in phase and reduced wear of the steel counterpart compared to thin films with lower Si content. Additionally, a Si concentration of at least 5 at.-% ensures stable low coefficients of friction with μ < 0.2 over extended sliding distances. These findings highlight the importance of carefully control the Si content to adjust the tribo-mechanical performance of ta-C:Si thin films.