Nanoscale Run-In of Silicon Oxide-Doped Hydrogenated Amorphous Carbon: Dependence of Interfacial Shear Strength on Sliding Length and Humidity

We conducted atomic force microscopy (AFM) experiments by sliding hard tetrahedral amorphous carbon (ta-C)-coated and diamond AFM probes against silicon oxide-doped hydrogenated amorphous carbon (a-C:H:Si:O) films. We reproducibly observe a substantial reduction in friction with repeated sliding. This behavior qualitatively resembles the run-in effects generally seen in macroscale frictional sliding on diamond-like carbons (DLCs), including this a-C:H:Si:O film in particular. As the applied normal load is increased with repetitive sliding, the friction reduces in tandem. The lateral stiffness of the nanoscale contact is measured as a function of applied normal load, thus the real contact area and the interfacial shear strength are inferred throughout the sliding experiments. These measurements show that the friction reduction is caused by a reduction in the interfacial shear strength of the contact. We propose that this arises from sliding-induced structural modification of the a-C:H:Si:O film. The calculated shear strengths are more than an order of magnitude higher than estimates from macroscale friction experiments. Additionally, humidity-controlled experiments show no significant humidity dependence of the friction despite a very strong dependence at macroscale. Reasons for these contradictions with macroscale experiments are discussed.