Lead-free carbon brushes for automotive starters

Historically, carbon brushes for automotive starters contain lead as an additive. The desired effect is an increase in starter performance without raising the copper to current-density ratio, which is directly correlated to the brush wear rate. However, lead dust is toxic or may cause genetic changes if incorporated, even in small quantities. In addition, lead oxidizes easily in air, which leads to dimensional and resistivity changes of the brushes over time. The role of lead in the contact has been attributed to its ability to control the film, and lowering the temperature by dissipating heat through phase transition. Only zinc and tin fulfill the requirements of non-toxicity, low melting point and low hardness of the oxide. Tests were performed, and it turned out to be necessary to separate the additive from the copper, to prevent complete migration into the copper and loss of its necessary properties as a metal or oxide. Several different methods were tested to make zinc a usable replacement for lead and the final version yields a lower wear rate with increased performance.