Date of Award
Doctor of Philosophy
The purpose of this research was to generate the knowledge for formulating low-Cu and Cu-free brake friction materials without using and releasing hazardous materials that are listed on Washington State and California State Senate Bills. Model brake material samples were manufactured and tested in the Friction Assessment and Screening Test (FAST) and the full scale automotive brake dynamometer (Dyno) using a SAE J2430 test procedure. The SAE recommended a J2430 test procedure which provided the necessary data for the Brake Effectiveness Evaluation Procedure (BEEP) by the Brake Manufacturers' Council. The newly developed low-Cu and Cu-free brake friction materials were formulated by modifying a typical Non Asbestos Organic (NAO) (T-Baseline) formulation and a typical Semi-Metallic (M-Baseline) formulation. The NAO Cu-free brake friction materials contain geopolymer and natural hemp fibers as a partial replacement of phenolic resin and synthetic Kevlar fibers, respectively. Friction performance and wear data from a series of FAST tests were used to train an artificial neural network, which was used to optimize the NAO Cu-free formulations. Then, the optimized low-Cu and Cu-free brake friction materials were tested on the Dyno. Dyno test results showed that all NAO Cu-free brake friction materials have passed the Brake Effectiveness Evaluation Procedure (BEEP), did not exhibit thermal fade when temperature was increased and were slightly sensitive to speed. The NAO Cu-free brake friction materials exhibits slightly lower average friction level when compared to the baseline materials (T-Baseline). The Cu-free brake friction materials, as well as the rotors, exhibit higher wear than the detected wear on the T-Baseline material. The semi-metallic low-Cu and Cu-free brake friction materials have also passed the BEEP. Dyno test results indicated that the semi-metallic low-Cu and Cu-free friction materials did not exhibit thermal fade and were slightly sensitive to speed. The semi-metallic low-Cu and Cu-free materials exhibited lower friction level and higher wear on the pads when compared to the M-Baseline material. The semi-metallic Cu-free material outperformed the M-Baseline material in term of rotor wear. Analyses using scanning electron microscopy (SEM) and energy dispersive x-ray (EDX) microanalysis on the tested materials show that the friction surface of the T-baseline material was covered with fully developed and stable friction layer (third body) consisting mostly of Fe-oxides, different forms of carbon and compounds of materials originally present in the bulk material. The NAO Cu-free materials (T30-15 and T30-16) did not develop a sufficient friction layer. The friction layer seemed to be responsible for the detected lower wear in the T-Baseline compared to the NAO Cu-free materials (T30-15 and T30-16), and it acted as a solid lubricant on the interface between the rubbing pad and the cast iron rotor lowering the adhesive forces. The friction and wear of the T-Baseline material was controlled by adhesive mechanisms. The NAO Cu-free materials (T30-15 and T30-16) with content of geopolymer replacing phenolic resin matrix exhibited extensive abrasive wear in addition to adhesive mechanisms. The capacity to form a friction layer on the surface plays a considerable role when lowering the wear of NAO brake friction materials. The friction layer was formed by compaction and interaction of brake wear particles, and its stability and character depend on the chemistry of the bulk materials in contact as well as the temperature, pressure and sliding speed during a friction process. SEM and EDX analyses also showed that the semi-metallic pads have developed friction layer on the friction surfaces. The difference was that the M-Baseline material had well developed friction layer, but the Low-Cu (M4) and Cu-free (M5) materials, had many smaller patches of friction layer disturbed on the surfaces.
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