The wear of graphite sliding on a steel surface and the influence of an electric current on the rate of wear

A detailed study of the rate of wear of one grade of natural graphite, in the form of two brushes sliding on steel rings, has been made under various conditions of load, apparent contact area, and velocity of sliding Equilibrium conditions are established fairly rapidly at the graphite-metal interface, and the rate of wear is then proportional to the load and independent of the apparent area of contact within the experimental range considered. The passage of a direct electric current increases the rate of wear of graphite brushes, the effect on the positive brush being the more marked. This asymmetrical rate of wear increases with current, is more pronounced at the higher velocities of sliding, and does not depend, in a simple manner, on the load. As the load is increased, the rate of wear for both the positive and the negative brushes decreases initially, though at the higher loads the positive rate of wear increases rapidly. There is evidence that this is due to a changed mechanism of wear. Experimental results on a second grade of brush have shown similar asymmetrical wear at low sliding velocities, which is removed by increased loads, and is also absent at a higher sliding velocity. The experimental results are discussed in terms of an asperity model of the mechanism of wear, analogous to that generally used for friction and metallic wear, with the addition of a rubbed graphitic layer between the steel asperities. It is concluded that asymmetric wear is due to an increased removal of graphite from the positive track, thereby producing increased wear of the brush by the metal asperities. Measurements with a Talysurf profilometer, and experiments with multiple brushes running on a single track, support this view.