ELSEVIER Wear 193 (1996)218-225 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG WEAR Thermal activation in boundary lubricated friction P.C. Michael I, E. Rabinowicz, Y. Iwasa * Francis Bitter Nutionul Magnet Laboratory and Department ofMechunicu1 Engineering, Mussachu.setts Institute c$Technology, Cambridge, MA 02139, USA Received 3 February 1995; accepted 22 June 1995 Abstract The friction coefficients for copper pairs lubricated with fatty acids and fluorinated fatty acids have been measured over a wide range of sliding speeds and temperatures. Sliding speeds in the range 10-7-10-2 m s-r and temperatures in the range 4.2-300 K were used. The friction coefficients near 300 K are generally low and increase with sliding speed, while the friction coefficients at low temperatures are markedly higher and relatively independent of velocity. Each lubricant’s friction vs. velocity behavior over the temperature range 150-300 K can be described by a friction-velocity master curve derived from a thermal activation model for the lubricant’s shear strength. The activation energies deduced from this friction model are identical to those obtained in the same temperature range for a vibrational mode associated with low temperature mechanical relaxations in similarly structured polymers. These results suggest that thermally activated interfacial shear is responsible for the fatty acids’ positive-sloped friction vs. velocity characteristics at low sliding speeds near room temperature. Keywords: Low temperature; Boundary lubrication; Thermal activation; Fatty acids 1. Introduction Stick-slip instabilities have been identified as a principal source of thermal perturbation in high-performance super- conducting magnets [l-3]. Although these magnets are designed with no intentionally moving parts, the large elec- tromagnetic forces that develop as they are energized can cause localized slippage between adjacent conductors. Because the magnets’ heat capacities at 4.2 K, their usual operating temperatures, are only = 1/4000th of their room temperature values, even the small amount of frictional energy produced during an abrupt (0.1-l ms duration) 10 km wire displacement can produce a local temperature rise sufficient to “quench” the conductor, that is, to drive it from its superconducting to nonsuperconducting state [ 2,3]. Hence, the continued development both of superconducting magnets and other cryogenic equipment will require more reliable methods to achieve smooth low-temperature motion. Many researchers analyze stick-slip behavior in terms of the slope of the friction vs. velocity function. Dynamic sliding models indicate that stick-slip can be avoided if the friction- velocity curve has a positive slope up to the maximum antic- ipated speed range of the device [ 4-71. Rabinowicz’s creep * Corresponding author. I Present address: PlasmaFusion Center, Massachusetts Institute of Tech- nology, Cambridge, MA 02139, USA. Elsevier Science S.A. SSD10043-1648(95)06722-l theory of adhesive friction suggests one method to ensure this friction-velocity condition during low-speed sliding, that is, to use a soft material which shears in a creep mode as one of the sliding surfaces [ 5,8]. Several studies have sought to experimentally identify cryogenic-temperature friction mate- rials that possess this desired, positive-sloped friction-veloc- ity characteristic [ 9-121. Thus far no intrinsically favorable friction materials have been identified [ 10,131. In a previous paper we concluded that thermally activated interfacial shear creep is a necessary condition for positive-sloped character- istics; the lack of thermally activated shear creep near 4.2 K thus precludes the desired friction-velocity behavior [ 131. In the present work we examine the velocity- and temper- ature-dependent friction behavior of the alkanoic fatty acids as a means to assess the practical limits of material-based frictional stabilization. We have selected boundary lubricated surfaces, because they allow the interfacial junction shear stress to be determined independently of the real contact area. 2. Background 2.1. Adhesion friction theory The adhesive friction force, Ff( L,T,v) , between two sur- faces at normal load L and temperature T, sliding with relative velocity u can be expressed: