Characterization of graphite superficial thin films achieved during friction

Abstract

Lubricating properties of graphite are strongly dependent on the possibility to form thin transfer films from powder to metallic surfaces during friction. These films are hardly achieved when pure graphite is used. When graphite is activated with sodium thiosulfate, the formation of films is possible and the tribological properties are highly improved: longer endurance, lower value of the friction coefficient, higher load-carrying capacity. In order to understand mechanisms achieved during formation of the superficial films, tribological tests have been coupled with physico-chemical analyses of the surfaces using Raman microspectrometry and grazing-incidence X-ray diffraction. Owing to these techniques, several parameters have been studied such as the physical state of graphite (order/disorder, oxidation,…) and the chemical state of the mixture graphite/additive. Various chemical reactions have been observed during formation of the films. Hypotheses are proposed for the mechanism of film formation and to explain the role of additives.