The lubricating properties of graphite at elevated temperatures

Levens, MB 1973, The lubricating properties of graphite at elevated temperatures , PhD thesis, University of Salford.

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The wear rate and the magnitude of friction of commercially available graphites in air have been shown to be dependent on the corrosion resistance of the graphite. In air the wear rate of graphite was observed to increase with increasing temperature up to 6000 C. A very rapid type of wear was found to occur when graphite rubbed against austenitic stainless steel over a certain temperature range. The occurrence of this rapid wear was shown to be dependent on the surface finish of the steel, the composition of the steel, the state of oxidation of the metal, the corrosion resistance of the graphite and on the speed of sliding. It is concluded that this rapid wear is a result of oxidation of the graphite at the rubbing surface which leads to a rapid deformation and work-hardening of the normally soft austenite. Both the wear rate and friction of electrographites have been shown to decrease with increasing temperature in the inert environments of argon, helium and vacuum, though an exception to this general trend was found in one case. The general downward trend with temperature is ascribed to the increased activity of incident reactive species, particularly oxygen, towards the rubbing graphite. The importance of metal oxides in determining the magnitude of graphite wear rate in inert environments was investigated and it was shown that pre-grown oxide films could, in certain Circumstances, reduce wear rate by several orders of magnitude. It was also noticed that when low wear was induced by the presence of a metal oxide no transfer of a graphite film to the metal journal took place. The efficacy of a run-in film in allowing low wear to obtain under normally adverse conditions was investigated and it was shown that the substrate against which the graphite was rubbing was important. Only against copper could a low wear rate be sustained in vacuum after prior running-in in air had taken place. The topography of several rubbed graphite surfaces was investigated using a scanning electron microscope but no firm conclusions could be drawn with regard to particular mechanisms of wear, though agglomerates of very fine particles of graphite were often observed and it appeared, therefore, that a gradual attrition of the ends of firmly bonded crystals had taken place to produce these. X-ray diffraction confirmed that wear debris was amorphous in circumstances of both low and high graphite wear

Item Type: Thesis (PhD)
Depositing User: USIR Admin
Date Deposited: 14 Jul 2017 11:09
Last Modified: 27 Aug 2021 20:41

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