Analysis of tribofilm formed by electric brush sliding for long life starter motor 2019-01-0181

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 between the brush and commutator is an important factor in brush wear, because tribofilm is said to have a property to increase lubricity and decrease mechanical wear in brushes, as well as to improve commutation and decrease arc wear in brushes. Our investigation of tribofilm on ISS starters with high-rotational speeds revealed that the starters have less tribofilm than those formed on conventional ISS starters, suggesting that improving materials will promote the formation of tribofilm and decrease brush wear. 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 material on tribofilm formation and its mechanism. We conducted brush endurance tests changing the kind of graphite and the amount of WS2 and analyzed the hardness of the tribofilm formed during the tests using nanoindentation technique. The test results showed that using small-diameter graphite and increasing the amount of WS2 decreased brush wear, formed a larger amount of tribofilm, and achieved tremendously higher hardness in the tribofilm. Transmission Electron Microscope (TEM) analysis of the tribofilm revealed that it includes tungsten carbide as the reaction product. We conclude that the hard tungsten carbide contributes to the higher hardness and larger amount of tribofilm, leading to the decreased brush wear. By using this knowledge to optimize the composition of brush materials, we successfully reduced brush wear to one-third or more of that of conventional brushes.