Abstract
Thin transfer films were formed on steel surfaces during friction against graphite-based pins. Sodium thiosulfate Na 2S 2O 3 or sodium molybdate Na 2MoO 4 were added to graphite to improve the film formation. In order to understand the solid lubricant properties of the graphite + additive mixtures, physico-chemical characterization was performed using classical θ−2θ and grazing-incidence X-ray diffraction, thermogravimetry, Raman and micro-Raman spectroscopy and transmission electron microscopy. The graphite + additive pins before and after friction tests and the transfer filsm were analysed. Graphite was shown to become more and more defective during the transfer process. A turbostratic structure was detected. Crystallite size became as small as a few nanometers. Chemical reactions of the additives were observed as soon as the mixtures were prepared. Decomposition of thiosulfate led to the formation of sulfate Na 2SO 4 and free sulfur. No such reaction was seen with molybdate. The results are discussed in terms of disorder in the two- and three-dimensional ordering of the graphitic structure and are related to the role of interstitials between graphite planes.
Published Version
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