Found this on another forum.
"An investigation of the effects of polarity and current density on the rate of wear of electrical brushes is reported in this paper, following a brief review of previous experimental work in this field. For carbon and copper impregnated carbon brushes the rate of wear of the positive brush is considerably less than that of the negative brush, while for metallic brushes the situation is reversed. The rate of wear of metallic brushes was found to be much greater than that of carbon brushes, under conditions of these tests. While no complete explanation of brush wear can be derived from the results, the rate of wear seems to be related intimately to the conduction of current across the contact."
https://ieeexplore.ieee.org/document/6540712