RELATIONSHIP BETWEEN ADHESION AND FRICTION FORCES

The Surface Forces Apparatus technique was used to measure the normal (perpendicular) and lateral forces between variously prepared surfaces under both dry and lubricated conditions. 'Normal' forces include the force vs distance functions, F(D), for surfaces separated by thin liq uid films as well as the adhesion forces and energies, y, for two surf aces in adhesional contact. 'Lateral' forces include the static and ki netic friction forces F of the surfaces as they slide past each other at a given separation, D. The results show that very thin liquid films confined between two solid surfaces can sustain both normal and shear forces or stresses. The results further indicate that the normal forc e, F(D) or y, may be directly related to the static friction force, F( s), and simple equations are proposed that relate these forces (by 'st atic' friction force is meant the lateral force that must be applied t o initiate motion, but not necessarily to maintain this motion). In co ntrast, the kinetic friction force, F(k), which is the force that must be continually applied to maintain motion at a given velocity, was fo und to be related, not to the equilibrium or reversible interaction bu t to the dissipative or irreversible part of the adhesion or interacti on energy during a loading-unloading cycle. There is a high degree of correlation in the way that normal forces and friction forces are affe cted by changes in applied load or pressure, sliding velocity, loading -unloading rates and temperature. These systematic correlations can be conveniently represented by non-equilibrium 'adhesion' and 'friction' phase diagrams.