TRANSITION FROM STATIC TO KINETIC FRICTION IN A MODEL LUBRICATED SYSTEM

Molecularly thin confined fluids were deformed in shear faster than st ructural relaxations in response to shear could be accomplished, such that with increasing deformation the systems passed from the rest stat e to sliding. The response of these systems-two atomically smooth mica sheets separated by a fluid comprised of globularly shaped molecules [octamethylcyclotetrasiloxane]-was studied as a function of film thick ness of the fluid (from 80 to 10 Angstrom, i.e, from similar to 8 to s imilar to 1 molecular dimensions), as a function of normal pressure, a nd as a function of deformation rate, using a modified surface forces apparatus. Whereas the linear response was always liquid-like provided that the deformation rate was sufficiently slow, a ''stick-slip'' tra nsition from the rest state to sliding was observed when the deformati on rate was large, provided that the oscillatory frequency sufficientl y exceeded the inverse intrinsic relaxation time of the confined fluid . This transition was monotonic and reversible without hysteresis for relatively thick films but for thinner films was discontinuous with hy steresis. For films thicker than 3 molecular layers (ML), two length s cales in deformation were observed; the films showed nonlinear force-d eformation response beginning at a deformation amplitude of 3 iq but i n general showed stick slip only when the deformation was larger than this. The critical deformation at the point of stick slip decreased fr om 9 to 3 Angstrom with increasing normal pressure, indicating diminis hed plasticity of the confined structures with increasing normal press ure. The critical film thickness of 3 ML correlates with the possibili ty of one rather than more slip planes. The thinnest films under the h ighest compressive pressures showed moderate increase of the viscous s hear force with increasing effective sliding velocity, but in general the viscous force reached a plateau in which force showed almost no de pendence on sliding rate. In interpreting the results in the context o f friction, static friction was identified with the elastic stress at rupture and kinetic friction was identified with the limiting maximum observed level of viscous force. After normalizing friction and normal forces by the contact area, the static friction coefficient was found to be 0.44 and the kinetic friction coefficient;to be 0.14, In other words, as the normal pressure increased, the elastic force needed to r upture the system increased more rapidly than the limiting shear stres s. The magnitude of the limiting shear stress increased exponentially with decreasing film thickness with a decay length of 1 molecular dime nsion, This decay length correlates well with the known exponential de cay of oscillations in the static force-distance profile, The critical shear amplitude of 3 Angstrom, relative to the molecular dimension of approximate to 9 Angstrom, is reminiscent of early estimates by Frenk el of the point of instability when planes of atoms slide over one ano ther. (C) 1998 American Institute of Physics. [S0021-9606(98)52540-8].