Friction at single-asperity contacts between hydrogen-free diamond-like carbon thin film surfaces

• DLC thin films with sp2/sp3 ratio about 2 were deposited by HiPIMS.
• Frictional behavior of single-asperity contacts between a DLC covered AFM tip and a DLC covered flat sample has been investigated.
• Friction force microscopy investigations showed the lubricant role of water adsorbed at a single-asperity contact between DLC films.

This work investigates the friction and adhesive forces at the single-asperity contact between hydrogen-free diamond-like carbon (DLC) coated surfaces in humid air and in argon. High power impulse magnetron sputtering deposition has been used to produce hydrogen-free DLC thin films on commercial silicon AFM tips and silicon wafers. The structure and surface morphology of the deposited DLC films has been investigated by X-ray photoelectron spectroscopy and tapping AFM, investigations that showed very smooth DLC films with about 29% of sp3 C − C bonds. The as-deposited hydrogen-free DLC thin films were hydrophilic (water contact angle about 65°) due to incorporation of a small amount of oxygen (about 9% of C = O bonds) at the film surfaces. The friction force measurements show that at low normal loading force the contact friction is much lower in the humid air than in the argon, while at large values of the loading force the contact friction does not depend on the measurement medium. A qualitative friction model based on the concept of dry and wet friction regimes, when the layer of water adsorbed on DLC film surfaces plays the role of lubricator, is proposed to explain these findings.