AbstractFive different slip modes have been identified for bundles of copper fiberssliding on a smooth copper substrate in atmospheric air, argon and nitrogenat pressures from atmospheric to 0.01 Torr. These are stick-slip, variable sliding, intermittent stick sliding and two kinds of smooth sliding, one apparently a basic property of clean surfaces and the other due to contaminants. These forms of sliding are rather persistent once established, and they follow some trends. Specifically, low-pressure smooth sliding is coordinated with a value of the coefficient of friction (μ) near 0.15 and is seen when the surface film is exceptionally thin, while intermittent stick sliding appears to be due to “pads” on the substrate surface,and variable sliding to small particles caught in between the fibers and the copper substrate. However, the five slip modes are erratic in that under the same conditions one or another or yet a third may be observed, even though the electrical contact resistance (R) depends rather reproducibly on time, load, velocity, ambient atmosphere and pressure. That dependence indicates an equilibrium between film destruction through sliding and film formation, overwhelmingly through the presence of oxygen. In the stick-slip mode the difference between pst tic and ųK itic appears to be roughly proportional to ų 0.15, i.e. tfiee xcess of e average value of the friction coefficient above 0.15, being about 20% for ų 0.3 andvanishing near ų =0.15. During slip episodes, R spikes roughly in proportion to their magnitude. Some tentative interpretations are offered, based on the concept that ų consists of three additive components, namely due to the bulk (ųBulk), due to debris (ųDebris), and dueto scoring of surface films (ųFilm).At any rate, the conclusion that the results contradict all previous models of “adhesive” wearis inescapable.