Investigation of the Tribological Behavior of Polytetrafluoroethylene at Cryogenic Temperatures

Abstract

Space applications are very demanding and require that lubricants provide low friction and predictable operation over a wide range of temperatures, environments, and contact conditions. Polytetrafluoroethylene (PTFE) is an attractive candidate solid lubricant due to its notably low friction coefficient, wide thermal range, and chemical inertness, but its tribology at space-relevant conditions has not been adequately investigated. This study seeks to gain insight into the cryogenic tribological behavior of PTFE using a well-studied linear reciprocating tribometer. The tribometer was contained within a nitrogen backfilled glove-box and tests were conducted at a constant background temperature of 296 K. Sliding experiments were conducted at a sliding speed of 50 mm/s and a normal pressure of 6.9 MPa, and the temperature of the lapped 304 stainless steel counterface was varied at 2% and 6% RH. Wear rate decreased monotonically with decreased interface temperature below 273 K in the absence and in the presence of ice, presumably due to improved mechanical properties at lower temperatures. The friction coefficient increased monotonically with decreased temperature in a manner consistent with thermal activation over van der Waals–type barriers; it deviated from this trend only during the phase and the glass transitions in the PTFE and after ice deposition occurred at temperatures below the estimated frost point. The data collected here are strikingly consistent with the general PTFE tribology literature and suggest that the friction coefficient of PTFE can be expected to increase by a factor of five as the temperature is reduced from 400 K to 200 K in a space environment.

KEY WORDS:

• Cryogenic
• Friction
• Thermal Activation
• Seals
• PTFE: Space
• Solid Lubrication

ACKNOWLEDGMENTS

Financial support for this work was provided through an AFOSR-MURI grant FA9550-04-1-0367. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the Air Force Office of Scientific Research. The author wishes to thank Professors Sawyer, Perry, and Schadler for guidance during these studies and Matthew Hamilton and Luis Alvarez for experimental contributions and valuable discussions.

Review led by Thierry Blanchet