Abstract
This article presents the results of experimental investigation of tribological properties of commercial mineral motor oil with chiral nematic (also known as twisted nematic or cholesteric) liquid crystal additives. Cholesteryl stearate and valerate (fatty acid cholesterol esters) liquid crystals were used as oil additives in investigation. Tribological experiments were performed using a block-on-disc-type tribo-test machine at constant experiment time and sliding velocity conditions. The load (contact pressure), concentration of liquid crystalline additive in oil and Rz roughness of steel–steel friction surfaces were taken as variable parameters. The mean coefficient of friction of steel–steel friction pair lubricated by oil with and without liquid crystal additives and near-contact temperature were taken as dependent variables. Regression equations were then derived for each lubricant and tribological efficiency of liquid crystals as oil additives was evaluated. It was established that coefficient of friction of steel–steel friction pair and near-contact temperature are reduced to about a half in the presence of liquid crystal additives. Results of tribological experiments show that tribological efficiency of liquid crystals as oil additives increases with increase in their molecular mass.
Highlights
Nematic liquid crystals are orientationally ordered, textured, anisotropic, and viscoelastic materials.[1]
The tribological effectiveness of cholesteric liquid crystals as oil additives closely depends on their molecular structure
The minimum near-contact temperature and coefficient of friction of steel–steel friction pair lubricated with motor oil with cholesteric liquid crystal additives were obtained at low contact pressure, roughness of friction surfaces and high concentration of liquid crystals in oil; 4
Summary
Nematic liquid crystals are orientationally ordered, textured, anisotropic, and viscoelastic materials.[1] They exhibit fluidity as liquid and elasticity of crystalline solid body in direction perpendicular to the flow.[2,3]. The ability of liquid crystalline materials to form ordered boundary layers with good loadcarrying capacity and low coefficient of friction, contributing to increase the component’s service life and save energy, explains the interest in their use as lubricants,[4–6] especially under boundary lubrication conditions. Under these conditions, the chemistry occurring within a few nanometers of the surface becomes a critical factor determining friction, wear and stick-slip.[7]. The molecules either adsorb onto the sliding surfaces or react with the surfaces under severe conditions such as high load and low speed giving rise to protective tribolayers of low shear modulus.[7]
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