Friction and wear of polymeric materials at 293, 77 and 4.2 K P.C. Michael, E. Rabinowicz* and Y. Iwasa Francis Bitter National Magnet Laboratory and *Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Received 28 November 1990; revised 6 February 1991 Experiments were performed to determine the friction and wear coefficients of various polymer pins slid against either AISI 304 stainless steel or oxygen-free high conductivity (OFHC) copper at temperatures of 293, 77 and 4.2 K. Three kinds of polymeric materials were tested: 1, unfilled; 2, solid-lubricant filled; and 3, fabric- and particle-reinforced plastics. The polymers' penetration hardnesses were measured at 293 and 77 K to facilitate the wear coefficient calculations. Although the polymers were much harder at the cryogenic temperatures, it was found that their non-dimensional wear coefficients remained relatively constant, generally in the range 5 × 10 7_ 5 × 10 6. Cross-plots of the polymers' wear coefficients versus the corresponding friction coefficients show good correlation and indicate a third- to fourth-power functional relationship between friction and wear for both the unfilled and the reinforced plastics. The magnitude of the wear coefficients and the relationship between friction and wear suggest that adhesive wear is the dominant wear mechanism for these polymers. Keywords: polymers; mechanical properties; low temperature studies In cryogenic devices, particularly those used in liquid cryogen production and transportation, provision must be made to permit smooth motion between the sliding elements. While externally mounted, grease lubricated bearings are sometimes used in cryogenic test equip- ment ~, it is generally desirable that the bearings in mass-produced items be contained internally to ensure reliable operation and to minimize heat conduction into the cryogen. Because all conventional liquid lubricants freeze at cryogenic temperatures, the use of fluid and grease lubricated bearings is not possible. Similarly, since most cryogens are non-polar, low molecular weight molecules they possess low inherent lubricity; hence lubrication by cryogens is not a viable alterna- tive. Under these circumstances, an approach often used is to conduct extensive screening tests to identify materials that exhibit the most favourable sliding characteristics in the desired operating environment ~. Metal-on-metal sliding pairs are seldom used in cryogenic applications because of the high friction and wear that occur under these conditions. Previous investigations indicate that surface welding occurs for metal-on-metal pairs slid in liquid nitrogen 2 and that continuous, small amplitude sliding in liquid helium produces high friction results similar to those observed in high vacuum at 293 K 3. Polymer-on-metal sliding pairs, on the other hand, are used extensively in 0011 - 2275/91/080695- 10 © 1991 Butterworth - Heinemann Ltd cryogenic applications because of their characteristically low wear rates and low to moderate friction coefficients in the absence of external lubrication. Several polymers, of which Teflon is a prime example, are particularly favoured because of their ability to act as solid lubricants; as sliding commences, a coherent, low shear strength film of transferred polymer usually forms on the opposing metal counterface, leading to a marked 124 6 reduction in the friction coefficient " - . A major drawback to the continuous operation of polymeric bearings at cryogenic temperatures is their very low thermal conductivities; these increase the bear- ings' tendency towards unacceptably large interface temperatures. In cryogenic pumps this problem is often circumvented by using the pumped liquid as a cooling agent for the bearing surfaces 1'2'4'7'8. An alternative approach is to increase the bearings' thermal conduc- tivities by using high conductivity fillers or thin walled, metal backed bearings 9. Fabric and particle rein- forcements can likewise be used to improve a bearing's dimensional stability, load bearing capacity and mech- anical toughness, given that most polymers contract con- siderably and become increasingly brittle on cooling to low temperatures. Because of the importance of tribology to the con- tinued advancement of cryogenic systems, this study has been undertaken to examine the basic mechanisms of Cryogenics 1991 Vol 31 August 695