The structure of tribolayers at the commutator and brush interface: A case study of failed and non-failed DC motors

The aim of this study is to understand the tribological mechanisms that lead to the failure of the commutating system of a DC motor. Brush and commutator are the most stressed components of a DC motor because of their role in transferring the current from the power unit to the rotor. Interrupting one of these parts leads to an unusual current transition and mostly to a complete damage of the motor. Two DC motors were analyzed after continuously running in a laboratory environment under the same conditions. Failure of one of the motors was observed after 1200 h, while the other one completed the test (1500 h) without any interruption. The analysis included the characterization of the structure and the chemical composition of the surface of the commutator and the brush using different complementary surface analytical methods. It was found that the failure of the motor is related to a very rough and thick layer that was formed both on the commutator and the brush surface. On the brush surface this layer mostly consists of oxidized bulk material (oxidized graphite) with small changes in the mechanical properties. On the commutator however, a combination of oxide material from the bulk (Cu<inf>2</inf>O and CuO) and a significant amount of oxidized carbon coming from the brush surface was found to form a layer. Furthermore, spherical wear particles were found within the microstructure of this layer. These layers were not observed on the surface of the motor that did not fail.