Silicon- and tungsten-containing hydrogen-free and hydrogenated amorphous carbon films for friction-reducing applications

For tribological applications, adding Si or W to hydrogen-free a-C or hydrogenated a-C:H is highly beneficial to tailor the film properties. Hence, a direct comparison between Si- and W-containing a-C and a-C:H considerably enhances the understanding of both the interaction between Si or W and the hydrogenation state as well as its effect on the structure and tribo-mechanical properties of these films. Therefore, non-modified a-C(:H), Si-containing a-C(:H):Si, and W-containing a-C(:H):W films were systematically grown in a mid-frequency magnetron sputtering process. The formation of W-based nanocrystallites within a-C(:H):W is identified by x-ray diffraction, whereas a-C(:H):Si still possesses an amorphous character. Raman scattering spectra show higher I(D)/I(G) ratios for hydrogen-free a-C(:X) films compared to the respective a-C(:H):X, indicating a higher number and larger sizes of sp2 clusters in the carbon network. For the hydrogenated a-C:H:X films, the reduced number of sp2 clusters is related to the presence of terminating CH bonds, which were detected as stretching modes. Among the different films, a-C:W has the highest I(D)/I(G) ratio, while a-C:H and a-C:H:Si exhibit the lowest I(D)/I(G) values. While a-C:Si and a-C:H:Si are characterized by comparable hardness values of (18.7 ± 1.3) and (18.4 ± 1.1) GPa, a-C:W has a lower hardness of (13.8 ± 1.0) GPa compared to a-C:H:W with (17.5 ± 0.9) GPa. Among all modified a-C(:H):X films, a-C:Si and a-C:H:Si reveal the lowest coefficients of friction, but show highest wear rates in dry sliding against 100Cr6 steel. Contrarily, a-C:W has higher friction and wear than a-C:H:W. Consequently, the Si-containing a-C(:H):Si films demonstrate comparable tribo-mechanical properties, while the hydrogenation state leads to different tribo-mechanical properties of a-C(:H):W. [Display omitted] •a-C:W undergoes a structural change from amorphous to crystalline.•a-C:H:Si exhibits the highest I(D)/I(G) ratio, indicating the highest amount of sp3 bonds•A modification with tungsten promotes the formation of sp2 bonds•Silicon-modified films reveal the lowest coefficient of friction with μ = 0.086 ± 0.018 for a-C:Si against 100Cr6.