Isotropic crystals, friction

The effect of temperature on friction between aluminum sheet (3004-H19) and 52100 steel balls due to the liquid crystal cholestryl linoleate (CL) and stearic acid (S A) was measured under boundary conditions on a modified MOFISS friction tester. Temperatures were selected so that the order of CL was cholestric (25 C), smectic (30 C) or isotropic (40 C). The results showed no temperature dependence on COF of CL. It is speculated that this lack of dependence might be due to dramatic shifts in phase-transition temperatures of the thin film LC from that of the bulk. 

To test this hypothesis, we investigated the effect of temperature on the friction properties of cholestryl linoleate (CL) under boundary conditions, and compart the results to a similar investigation of a well-known boundary additive, stearic acid (S A) which is not a liquid crystal. As shown in Figure 2, CL is crystalline below 20 C, smectic at 20-26 C, cholesteric at 26-35"C and isotropic above 35"C. CL was selected because its isotropic order is accessible at a relatively low temperature. 

Isotropic carbon, on the other hand, has no preferred crystal orientation and hence possesses isotropic material properties. There ate three types of isotropic carbon pyrolytic, vitreous, and vapor-deposited carbon. Pyrolytic carbons are formed by the deposition of carbon from a fluidized bed onto a substrate. The fluidized bed is formed from pyrolysis of hydrocarbon gas at between 1000 and 2500°C (Hench and Ethridge, 1982). Low-temperature isotropic (LTI) carbons are formed at temperatures below 1500°C. LTI pyrolytic carbon possesses good frictional and wear properties, and incorporation of silicon can further increase hardness and wear resistance (Bokros, 1978). 

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