Single Contacts and Multiple Encounters

The analysis of wear experiments suggests that most of the events which occur in rubbing are contacts between protuberances which are deformed elastically and which separate without damage; an asperity encounter with damage is a relatively rare event. Apparatus for the study of isolated single contacts is described. A single contact which is deformed elastically does not obey Amontons's law, but an assembly of such contacts (multiple contact conditions) should do so. It is shown that under multiple contact conditions the load which can be borne by elastic deformation of the protuberances may be as much as a million times larger than that which can be borne by each individual asperity contact. Reflection electron microscopy shows that many irregularities on worn surfaces must bear their share of the load without plastic flow. Recent experiments suggest that, although a worn particle is produced very infrequently, it is nevertheless the direct consequence of the many preceding encounters which occurred without apparent damage.