Timo J. Hakala

Adhesion and tribological properties of hydrophobin proteins in aqueous lubrication on stainless steel surfaces

Macroscale tribological properties of hydrophobin layers bound on stainless steel surfaces were investigated in an aqueous environment. Emphasis was on boundary lubrication because water easily fails in hydrodynamic lubrication due to its low viscosity. We studied the affinities of two different proteins, HFBI and FpHYD5, on stainless steel and their ability to bind water at the surface by combining quartz crystal microbalance (QCM-D) and ellipsometry. Both proteins contained an adhesive hydrophobic domain, but FpHYD5 also had a very strongly hydrating carbohydrate structure attached to it. The lubrication properties of the proteins were studied with two different methods, pin-on-disc (POD) (stainless steel vs. stainless steel) and circular translation pin-on-disc (CTPOD) (UHMWPE vs. stainless steel). It was observed that both hydrophobins could adhere to the stainless steel surface and form highly hydrated layers. Both proteins reduced friction and wear of the sliding contact between two stainless steel surfaces. With UHMWPE against stainless steel, the hydrophobins prevented the polyethylene transfer to the counterface. The lowest coefficient of friction (COF) 0.13 was observed when FpHYD5 hydrophobins were employed in pure water. On the other hand, the lowest wear was observed when FpHYD5 proteins were added in a 50 mM sodium acetate buffer. Increasing the water content and loosening the hydrophobin film structure on the stainless steel surface led to a reduction in friction and wear.

Biomimetic approach to water lubrication with biomolecular additives

The aim of this study is to find a connection between mechanical engineering and biotechnology by utilizing biomimetics in lubrication. The objective is to improve boundary lubrication by biomolecules in water-based systems. Proteins were used because they can form films and multilayers on the surfaces and thus prevent direct contact between them. In this study, hydrophobin and albumin proteins are studied as additives to enable water lubrication.

Effect of operational conditions and environment on lubricity of hydrophobins in water based lubrication systems

Abstract In this study, the effect of operational conditions (normal load, sliding velocity) and environment (pH and ionic strength) on the lubrication properties of two different hydrophobin proteins were investigated using pin on disc tribometry and ellipsometry. The studied proteins were wild type HFBI and the glycosylated hydrophobin FpHYD5. It was observed that the friction of a stainless steel versus stainless steel contact lubricated with either of the hydrophobins did not depend on the normal load. However, increased sliding velocity occasionally led to a decrease in friction when the surfaces were lubricated with the glycosylated FpHYD5. The tribological behaviour of FpHYD5 was studied at pH values ranging from 3 to 9 and generally lowered friction by 31–38% and wear by 40–65% compared to the corresponding buffer solutions. An exception was pH 9, where FpHYD5 increased friction and wear compared to the buffer solution. Ionic strength affected both the amount of protein that was adsorbed and the lubrication properties of glycosylated hydrophobins.

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 SiNx 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.