NASA considers several qualities of motor-brush materials when determining suitability for a given mission. Among the most important are the wear rate and the cohesiveness of the debris. Engineers note if there's a tendency for brush debris to pack into commutator slots.
They also look at the electrical properties of the debris and whether there's a tendency to short between commutator segments.
In picking brushes for the rover project six different brush materials in 18 identical motors operated under various load conditions for four weeks in a low-pressure CO2 environment. The atmosphere on Mars is chiefly carbon dioxide (95.3%), nitrogen (2.7%), and argon (1.6%). The atmospheric pressure on Mars is less than 1/100th of Earth's average. All motors performed without failure accumulating between 98 and 144 million revolutions.
The same motors were then cleaned and run in a vacuum for two weeks. Five failed catastrophically and three brush materials survived. The failures in a vacuum all followed the same sequence. Brush debris in the commutator slots forms a partial short, leading to increased power draw and heating. The heat forces up the rotor temperature until the winding insulation fails. In a vacuum, where heat dissipation from the rotor is poor, this problem is especially serious.