Water-based boundary lubrication with biomolecule additives on diamond-like carbon and stainless steel surfaces

Friction and wear incur high economic costs globally. It has been estimated that approximately 30% of energy is used to overcome friction. Developing new solutions, such as coatings, surface texturing and lubricants, to reduce friction in the boundary lubrication regime can have great importance to global energy savings in the future. In this thesis, water-based lubricants with hydrophobin protein (HFBI, HFBII and FpHYD5) and quince mucilage additives were used to lubricate engineering materials such as diamond-like carbon (DLC) coatings, stainless steels and plastics. It was found that hydrophobins can form monolayers on stainless steel, diamond-like carbon (a-C:H) and PDMS surfaces. On stainless steel surfaces, HFBI and FpHYD5 layers contain 40-64% water. Increasing the water content in hydrophobin film reduced friction in hydrophobin-lubricated stainless steel vs stainless steel contacts. The same effect was seen in quince mucilage-lubricated UHMWPE vs stainless steel contact. The lowest friction coefficients (COF) were measured in FpHYD5 hydrophobin-lubricated contacts where COF as low as 0.03 was measured. Quince mucilage-lubricated UHMWPE vs stainless steel reduced the friction coefficient to as low as 0.02. Of all the tests, the lowest friction coefficients (close to 0.01) were measured with HFBI and FpHYD5 hydrophobins in PDMS vs PDMS contacts. Based on the results, it can be suggested that the requirements for water-based lubricationwith biomolecule additives in industrial applications are• A mild temperature range, T= 4 - 95°C• Low contact pressures, 0.1-5 MPa• Hydrophobic surfaces, contact angle of water > 90°• Stable conditions (pH, ionic strength) In the future, water-based lubricants could be used in, among others, the food and beverageindustry, the textile industry and biomedical applications.