Lubrication of aluminium versus diamond-like carbon contacts with hydrophobin proteins

Hydrogenated diamond-like carbon coatings (a-C:H) and silicon-doped diamond-like carbon coatings (a-C:H(Si)) of 1 μm thickness were deposited on stainless steel substrates by the inductively coupled plasma chemical vapour deposition technique, including a 300 nm-thick SiN_x interlayer. Tribological experiments for both types of coating under pure and hydrophobin-containing water lubrication were performed using a pin-on-disc tribometer. To better understand wear behaviour, studies of hardness, surface morphology, water contact angle and chemical bonding were carried out using a nano-indenter, atomic force microscopy, contact angle measurement and X-ray photoelectron spectroscopy, respectively. Silicon doping in the a-C:H coating was found to slightly increase the surface roughness and wettability. The lowest friction coefficient (0.09) was obtained for the a-C:H(Si) coating sliding against an aluminium counterpart in water. The addition of HFBII hydrophobins to water increased both friction and wear of the aluminium counter body, and oxide tribofilm formation was prevented on the aluminium surface.