The sliding wear of polymers against steel

Master Thesis


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University of Cape Town

A laboratory wear testing facility has been developed to generate wear rate data for polymeric materials sliding at constant velocity against a hardened stainless steel base. The polymers investigated were ultra high molecular weight polyethylene (UHMWPE), ultra high molecular weight polyethylene-with a friction reducing additive (UHMWPE/FILL), polyoxymethylene (POM), poly(ethylene terephthalate) (PETP), molybdenum disulphide filled polyamide 6 (PA6/MoS₂) and graphite filled poly(amide-imide) (P(A-I)/GR). Testing was carried out as a function of sliding velocity between 0.13 to 2.27 ms⁻¹, loads of 1, 3 and 5 MP a and counterface roughnesses which varied from 1 micrometre to 0.25 micrometres. An increase in the counterface roughness resulted in a variable increase in the wear rate of the individual polymers except for the filled UHMWPE. These changes in the wear rate have been explained in terms of the mechanism of material removal. A progressive increase in sliding velocity has been shown to result in an initial increase in the wear rate followed by a decrease and finally a rapid increase for all materials under the majority of applied conditions. Explanations for such behaviour have been advanced in terms of the viscoelastic response of the polymers to strain rate and temperature. Low modulus materials however showed a significant drop in wear rate under low loads above a critical velocity which is believed to be due to a transition from boundary to partial el astohydrodynamic lubrication. Generally an increase in load gave an increase in wear rate for all polymers except for UHMWPE and filled UHMWPE at a counterface roughness of 1 micrometre. These conditions have been discussed with reference to the materials response to thermal effects and counterface interactions.

Bibliography: pages 89-93.