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Friction materials with three different formulations containing different solid lubricants were investigated to study the role of lubricants on the friction performance. The three friction materials contained 10 vol.% graphite, 7 vol.% graphite + 3 vol.% Sb2S3, and 7 vol.% graphite + 3 vol.% MoS2, respectively, with the same amount of other ingredients. Results of this work showed that each formulation had advantages and disadvantages. The friction materials containing two lubricants (graphite + MoS2 or Sb2S3) showed better resistance to fading and improved friction stability than the friction material containing only graphite. However, the friction materials with two lubricants showed disadvantages on anti-fading, wear resistance, and DTV generation.

An NAO friction material (low-steel type) containing 15 ingredients was investigated to study the role of transfer film on the coefficient of friction, friction oscillation, and fade resistance. The friction material specimens containing extra 100% of each ingredient were tested using a pad-on-disk type tribotester. A non-destructive method of measuring the transfer film was developed by considering the electrical resistance of the transfer film. Results showed that solid lubricants and iron powder assisted transfer film formation on the rotor surface and abrasive ingredients tended to remove the film. No apparent relationship between transfer film thickness and the average friction coefficient was found in this experiment. Fade resistance was also found to be independent of the transfer film thickness when the stable film was formed on the rotor surface at elevated temperatures. On the other hand, the transfer film on the rotor surface reduced the amplitude of friction oscillation.