Friction and Transfer of Some Polymers in Unlubricated Sliding
The paper deals mainly with the friction of PTFE and high density polyethylene sliding over hard clean flat surfaces. The behavior of these polymers is somewhat anomalous and is contrasted with the behavior of ‘‘normal11 polymers; that is virtually all other polymers. The friction and transfer behavior falls into two groups. (i) At low speeds the friction is low (µ ≃ 0.05) a thin film of polymer is drawn out of the slider and adheres to the counter surface. Its thickness is of order 50–200A° and the molecular chains are highly oriented in the sliding direction. This behavior does not appear to depend on the crystal 1inity or band structure of P.T.F.E. or on the spherulite size of P.E. If, however, bulky or straggly side-groups are incorporated into the polymer backbone the friction and transfer increase and resemble those of “normal” polymers. The behavior appears to be connected with a smooth molecular profile. (ii) At high speeds or low temperatures these “low-friction” materials give a high friction (p — 0.3) and the transfer is much heavier consisting of lumps or smeared films of polymer several 1000 Â thick. The behavior in this regime may be determined by crystallite size, the band structure of the polymer or the molecular weight. Experiments on High Density P.E. containing a mixture of CuO and Pb3O4 show that the filler has no influence on the friction or wear behavior at low sliding speeds. But at high sliding speeds the wear against a smooth steel counterface is markedly reduced. The filler apparently functions by producing a strongly adhering polymer film on the steel surface.
KeywordsFrictional Behavior High Density Polythene Counter Surface Spherulite Size Friction Regime
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