Micro and Nanotribology
6 Materials for Micro and Nanotribology: Liquid Crystals, Fluid Fims and Other Lubricants
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Liquid crystals have an ordered structure and are expected to have good lubricating performance in both EHL and boundary lubrication . It is well known that there are two types of liquid crystals: lyotropic and thermotropic states. Thermotropic liquid crystals can exist between their melting points MP and clearing points CP, and they become isotropic liquid above CP. The molecular structures of typical thermotropic liquid crystals are shown in Figure 6.1. There are two parts in the molecules, a flexible alkyl chain and a rigid cyano-phenyl group. In general, the viscosity of the liquid crystals is dependent on the carbon number of their alkyl chain as indicated in Figure 6.2.
Tribological properties are closely related to their molecular structure. Figure 6.3. shows the traction coefficient of liquid crystals measured by a two-roller friction tester under EHL conditions. Although their viscosity is dependent on the carbon number of alkyl chain, the traction coefficients are independent of the carbon number but dependent upon the rigid part of the molecules. Cyanobiphenyl group CB has a flat structure, while CBC has a boat form. And it exhibits a lower traction level. CBC is known to have a cyanophenyl cyclohexyl group. CB gives a lower friction coefficient than CBC. Therefore, the flat structure is important for obtaining low traction. Figure 6.4 shows the temperature dependence of the traction coefficient using liquid crystal of 5CB . It is noteworthy that the traction coefficients continuously decrease, with temperature even beyond the clearing point. This demonstrates that the liquid crystal state is not necessary but that the structure under shear is important for obtaining a low traction level. In situ observation of lubricating film by micro-FTIR revealed that the flat structure of biphenyl groups of 5CB were oriented in the shear plane between two surfaces, an experimental evidence of which is indicated in Figure 6.5. Therefore, it is clear that molecular orientation of flat structure under shear is important for better tribological performance.
Molecular orientation of liquid crystals is controllable by applying an electric field. Molecular axis is oriented along the electric field and the apparent viscosity of LC increases with the increasing electric field of several MVm−1. The higher effective strength of electric field is required to obtain the viscosity increase at higher shear rates. Therefore, tribological properties such as film thickness and friction can be controlled actively by providing the electric field between two sliding surfaces .