Analysis of Tribofilm Formed by Electric Brush Sliding for Long Life Starter Motor

Global exhaust emission regulations are becoming stricter, and vehicles equipped with the idle stop system (ISS) are increasing. Recently, starters for vehicles equipped with ISS are required to improve operation feel when speedily restarted. To satisfy this demand, starters must rotate at higher rotational speeds, and heavier wear in their brushes may cause problems. Tribofilm formed on commutators surface by the brush sliding is an important factor in the brush commutator wear, because tribofilm is said to have a property to increase lubricity and decrease mechanical wear in brushes and commutator, as well as to improve commutation and decrease arc wear. Therefore, for reducing brush commutator wear, it is considered effective to promote film formation by improving materials. However, few researches have been conducted to evaluate the relationship between brush materials and tribofilm formation. In this research, we investigated the influence of the brush materials on tribofilm formation and its mechanism. We conducted brush durability tests changing the amount of WS₂ and the kind of graphite, and analyzed the hardness of the tribofilm formed by the tests using Nanoindentation technique. As a result of changing these materials, brush commutator wear was reduced and an increase in film thickness was confirmed. At this time, it was also found that the film hardness significantly increased. It is considered that wear of the tribofilm is reduced due to an increase in film hardness, and sufficient film formation is achieved, leading to a reduction in brush commutator wear.