Instabilities in the frictional behaviour of carbons and graphites

Abstract

Both graphitic and non-graphitic carbons are widely used in a variety of tribological applications. A recent development is the use of carbons for aircraft brake discs and this has prompted an experimental investigation to determine the most important factors influencing the magnitude and stability of the coefficient of friction and rate of wear. Graphitic carbons sliding against themselves, or against metals, sometimes exhibit large and sudden transitions in friction and wear at critical conditions of sliding. These conditions are defined and the mechanisms involved are described. Carbons of low or negligible graphiticity are less prone to transitions than graphitic materials, but their friction and wear properties depend markedly on the way in which the conditions of sliding influence the structure and topography of the surface layers developed during sliding. Examples are given of typical variations of friction and wear with distance of sliding, speed and temperature. Discontinuities in friction and wear can also arise from localised surface misalignments induced either mechanically or thermally. The stability of the coefficient of friction and the rate of wear for both graphitic and non-graphitic carbons can be greatly improved by the incorporation of additives or by the presence of organic vapours in the environment. Examples are described.