Rubbed surface observations

Another conclusion is that the order of TLCs had no effect on its boundary friction properties. This conclusion is based on the assumption that the phases of the TLCs observed in the bulk will be similar to that of the thin films on the rubbing surfaces, provided that the temperatures were the same. However, while the orders of LCs in the bulk can be verified easily, and are well-documented, there is no information on the orders of the thin films on various surfaces. Thus, there could be a dramatic shift in the temperature range of the various orders in thin film versus bulk. This shift could also be a function of the type of surface (i.e., ceramic, aluminum, steel), surface roughness and even the order of the liquid crystal (cholesteric, smectic, nematic, etc.). Recent work by Frommer, et al.(121 seem to support the above argument. Determination of the order of thin films of liquid cryst s on various substrates is an interesting area of research, but is beyond tiie scope of this work. 

To observe chemical changes in the rubbed surface, Lee and Paek measured the polarity of a rubbed surface by using water contact angle measurements, and discussed a correlation between the pretilt angle and the polarity of the rubbed surface [44,62]. The measurement method is illustrated in Fig. 2.2. 

A first difficulty results from the fact that the impedance data reflect the overall state of the tested surface integrating the contributions of non-rubbed and rubbed surfaces. Such data must thus be de-convoluted in order to obtain the specific impedances of these two types of surface states. A first approach to this problem might be to use models similar to those describing the impedance of a sample undergoing a localized corrosion (Oltra Keddam, 1990). In that specific case, the overall impedance can be considered as the result of two impedances in parallel, namely the impedance of the non-rubbed surface and the one of the rubbed surface. A strict interpretation requires further an evaluation of the areas of these surfaces, e.g. by using profilometry and surface observations by light optical or scanning electron microscopy. 

SEM analysis is used to observe the worn surface of the steel ball after the friction test under the lubrication of LP with the as-prepared metal nanoparticles these results are presented in Figure 5.8 [18, 19]. It can be observed that all wear scar surfaces are relatively smooth and the rubbed traces are shallow. The results imply that these metal nanoparticles can melt and form instantaneous metallic films between rubbing surfaces, which prevents direct contact of the rubbing surfaces, and they exhibit good lubricating properties. 

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