STABILITY OF LUBRICATING PROPERTIES OF GRAPHITE BY ORIENTATION OF THECRYSTALLITES IN THE PRESENCE OF WATER-VAPOR

The friction of graphite is characterized by different stable regimes which are quantified by a friction coefficient mu ranging in the inter val 0.1-0.6 and by a wear rate varying similarly. It changes from one stable regime to another according to experimental conditions. The pas sivation of the dangling bonds of superficial carbon atoms created dur ing the wear process by chemisorption of adsorbable gases (atomic hydr ogen, atomic or molecular oxygen, dissociation of water vapour) reduce s surface energy, contact adhesion and thus friction (its coefficient falls from mu = 0.6 to 0.3); it reduces the wear rate by a factor of a bout 100. A high orientation of superficial crystallites obtained in t he presence of water vapour during sliding, causes the second transiti on from mu = 0.3 to mu = 0.1. The wear rate is again reduced by a fact or of 10 approximately. An X-ray analysis of crystallites of a surface with a friction coefficient reduced to about 0.1, has shown a high or ientation of the (001) planes within a 5-degrees angular dispersion. T he film composed of these highly oriented crystallites is very stable. It is well resistant to friction under high vacuum and at high temper ature. It is only altered when arching through the contact.