Introduction:
A comparison was made between copper motor brushes with the same product number but from different manufacturers.  The motor brushes from one manufacturer were causing excessive wear on the slip ring elements.

Procedure:  

• Photo documentation
• Hardness and chemical measurements
• Microstructural analysis: grain size, shape, distribution
• Photo documentation of microstructure
  

Conclusion:
It was determined that graphite size and distribution were key in affecting the wear performance of the brushes. The graphite phase was twice as dense in one sample as compared to the other.  There were also significant differences in the size and shape of the graphite particles.  The graphite boundary along the metallic interfaces of the brushes allowed for increased metal-to-metal interface and, consequently, accelerated wear.

Figure 1 – Chemical Composition

Figure 2 – Graphite Count