High-Current Brushes, Part III: Performance Evaluation for Sintered Silver-Graphite Grades

The results of varying the material composition, specifically the metal mass fraction, on the performance of silver-graphite brushes at high-current densities (0.8 MA/m²) in a carbon dioxide atmosphere, have been evaluated. Material properties which were measured included the brush density, electrical conductivity, and hardness. These properties were used in the evaluation of the brush performance when sliding on copper slip rings at a speed of 12.7 mis. Measured parameters included the contact resistance, friction coefficient, and wear rate. The results of this analysis indicate that the resistive behavior was dominated by constriction effects, with the number of contact spots apparently ranging from 2-15. The film contribution to the total resistance was small, the resistivity being less than 6 X 10-¹³ ·m². Friction measurements indicated a specific shear strength of 10-24 MN/m²which, if caused by an adsorbed water film, would correspond to a film thickness in the range from about 0.2-0.4 nm. Wear rates well into the microwear regime have been observed, even at the high-current densities used. However, a satisfactory explanation for the strong dependence of wear on metal content (from 0.7-60 mm³/Mm) as the metal mass fraction varied from 0.4-0.9 has not yet been evolved.