The material for commutator brushes is generally

Copper is a good conductor of electricity, but it has a high coefficient of friction. This means that it can wear down the commutator more quickly than carbon.

Cast iron is a strong material that is sometimes used in commutator brushes to withstand the wear and tear of the commutator. However, it is not a good conductor of electricity, so it is not used as the primary material for commutator brushes.