STABILITY OF LUBRICATING PROPERTIES OF GRAPHITE BY ORIENTATION OF THE CRYSTALLITES 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 interval 0.1-0.6 and by a wear rate varying similarly. It changes from one stable regime to another according to experimental conditions. The passivation of the dangling bonds of superficial carbon atoms created during the wear process by chemisorption of adsorbable gases (atomic hydrogen, atomic or molecular oxygen, dissociation of water vapour) reduces 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 about 100. A high orientation of superficial crystallites obtained in the presence of water vapour during sliding, causes the second transition from mu = 0.3 to mu = 0.1. The wear rate is again reduced by a factor 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 orientation of the (001) planes within a 5-degrees angular dispersion. The film composed of these highly oriented crystallites is very stable. It is well resistant to friction under high vacuum and at high temperature. It is only altered when arching through the contact.