Transient slip behaviour of metal/insulator pairs at 4.2 K

Abstract

Experiments have been conducted to observe the sliding behaviour, at 4.2 K, of freshly prepared metal/insulator combinations during a set of sequential slip tests. Measurements of the tangential force versus displacement, as well as displacement and acoustic emission rate versus time, were recorded. A triboelectric effect was also monitored. Depending on the particular metal/insulator pair tested, completely stable (slow velocity), completely unstable (rapid velocity), or a transition from unstable to stable sliting behaviour was observed. This transition behaviour was noticed for several metal/insulator combinations commonly used in superconducting magnet windings. An asperity plowing model is proposed to account for this behaviour, and is based upon the abrasive action of rubbing materials poorly matched in hardness against each other. The training phenomenon in unimpregnated superconducting magnet windings may be attributable to this wearing-in of frictional surfaces. A simple epoxy coating technique was successful in eliminating initial rapid slip events, thereby completely stabilizing the sliding behaviour of a conductor/insulator combination.