DOES HERTZIAN CONTACT AREA ACT AS AN EFFECTIVE ZONE GENERATING THE FRICTION RESISTANCE

Coulomb's law of friction, in which the coefficient of friction is con stant independently of apparent area of contact and applied load, is d educed from the modern adhesion theory. That is, the friction resistan ce is caused by shearing of solid/solid junctions which are formed thr ough plastic deformation of surface asperities of mating solids. In so -called point contact, on the other hand, different experimental resul ts from Coulomb's lan have been sometimes reported. In these cases, co efficient of friction is not constant, but reduces with increasing nor mal load. A weighty interpretation for these facts developed formerly is that Hertzian contact area acts as an effective zone to generate th e fricton resistance. This interpretation has. however, an important d oubt, as the Hertzian contact area is not formed through plastic but t hrough elastic deformation of solids. If the friction resistance is ge nerated in an elastic contact area, the adhesion theory of friction wo uld be shaken at its standing basis. To give an explanation of this in consistency between the experimental facts reported previously and the adhesion theory of friction, the authors propose a new idea in this p aper. The plastic deformation occurs at surface asperities even in Her tzian contact. If the rubbing condition is kept dry, the fricton resis tance would be generated only at those plastically deformed zone dotte d in the elastic contact area, so that Coulomb's law is realized. If t he rubbing condition is kept wet, the clearance between mating surface s in the elastic contact zone is filled with any lubricant or contamin ant molecules, the friction resistance would be generated through shea ring of them within the Hertzian area. In this case, the coefficient o f friction would be proportional to (load)(-1/3), which is close to ob servational facts reported previously. An experimental verification ma de in this study can describe the author's proposal.