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Effect of Mesomorphism on Lubricating Efficiency of Nanomaterials under Boundary Friction
Abstract
The effect of temperature on the friction coefficient under boundary lubrication by cholesteric liquid crystal nanomaterials, as well as by their solutions in industrial oil, is studied. It is established that the minimum friction coefficient for the investigated substances is reached in the mesomorphic state. In the isotropic state, an insignificant monotonic increase in the friction coefficient with increasing temperature is observed. The friction coefficient also decreases with the increasing molecular weight of the lubricant. Under friction in solutions, an abrupt increase in the friction coefficient occurs when a certain critical temperature is reached, which could be caused by the temperature destruction of the adsorbed layer.
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.
The processes of self-assembly by hydrogen-bonding in the system of 4-n-alkoxycinnamic acid molecules (n-AOCA) with short hydrocarbon fragments (n = 2, 3), 4,4′-bipyridine (BPy) and 1,2-bis (4-pyridyl) ethane (BPyE) have been considered. The formation of hydrogen bonded (HB) complexes n-AOCA···BPy/BPyE···n-AOCA was confirmed by IR spectroscopy and quantum-chemical calculations.
The liquid crystal properties of HB-complexes were studied by differential scanning calorimetry and polarization thermomicroscopy. The interrelation of mesomorphic properties with the structure of the obtained supramolecules n-AOCA···BP/BPE···n-AOCA was analyzed. It is shown that the nematic mesophase of HB-complexes has less thermal stability and is formed at lower temperatures than mesophase of individual 4-n-alkoxycinnamic acids. These trends are more pronounced in systems with 1,2-bis(4-pyridyl)ethane than in systems with 4,4′-bipyridine. While testing the boundary friction, it was found that saturated solutions of HB-complexes in Vaseline oil slightly reduce the friction coefficient and significantly increase the lubricating film durability (6–7 times) under conditions of depleted lubrication compared to a base solvent.
A physical model of the lubricity of cholesteric liquid-crystal nanomaterials has been proposed in accordance with which the transition from single-crystal layers of a cholesteric liquid-crystal nanomaterial to polymolecular structurally sensitive liquid-crystal films, which occurs with varying temperature and is governed by the effect of molecules of cholesteric liquid-crystal compounds contained in the lubricant, as well as the reverse transition, lead to a reversible change in the shear resistance. This change results from the formation of continuous planar-oriented and helically twisted boundary layers in the zone of dynamic contact, which consist of molecules of the cholesteric liquid-crystal nanomaterials. The pitch distance of the helix and, therefore, the thickness of cholesteric liquid-crystal films of these nanomaterials exhibit a structurally sensitive response to changes in the temperature in the friction zone, which varies the dissipation of the energy during friction.
Optimization principles are proposed for composing additives to mesogenic compounds of LCA used for metal working structural steels. The considered additives were found to positively affect the drilling process. The problems of their use in practice are discussed.
Tribofizika zhidkikh kristallov
- S F Ermakov