Impact of Acrylic Fiber on the Performance of Newly Developed Friction Materials for Vehicles with Regenerative Braking
Regenerative braking in conjunction with friction braking represents one of the most common braking strategies employed in electric and hybrid vehicles. The considerably different role of the friction brake in these ?blended scenarios?, when compared with braking solely by friction brakes, creates potential for development of new advanced and sustainable brake materials and brake designs. Fiber reinforcements used in friction brake material play an important role in improving strength, stability, and frictional properties [1, 2, 3]. This study concentrated on the development of a new generation of lightweight, ?quiet? and environmentally sustainable brake pad materials to be used with coated cast iron rotors in vehicles with regenerative braking. Acrylic fibrillated fibers (CFF V110-1, by Sterling Fibers Inc.) combined with biodegradable mineral fibers (RB215ELS, by Lapinus / Rockwool B.V), phenolic resin and selected fillers and friction modifiers were used in the multifactorial design of experiment approach to develop a formula passing the standardized SAE performance test. Small button pad samples with a diameter of 25.4 mm and height of 8 mm were prepared by using the Ninja 1000W professional mixer, Buehler Simplimet 2 hot press, and post cured in the programmable Fischer Scientific Isotemp muffle furnace. Density, porosity (distilled water, analytical balance AWS ALX ? 310 with accuracy of 0.001g), and shore D hardness (Durometer CV Instruments - ASTM 2240) were reported and compared with typical commercially available NAO pads currently used in friction brakes. The small-size pad samples with diameter of 13.12 mm and height of 8 mm were cut-off from the post-cured buttons by use of diamond core drill (Bosch Mini Diamond Hole Saw 5/8" HDG58). Pad materials were tested in a scaled-down ISO SAE J2522 test using the ?bench-top? Universal Mechanical Tester (UMT-Tribolab by Bruker). They were tested against commercially available gray cast iron (C30, ASTM A48) rotors with diameter of 95.5 mm, coated with ceramic material . Scaling philosophy was based on the characteristic dimensions of length, area, and volume. Surfaces of tested pads and rotors were analyzed using scanning electron microscopy (FEI, Model Quanta FEG450) equipped with energy dispersive x-ray microanalysis (EDX, Oxford Instruments). The newly developed lightweight and ?quiet? optimized pad materials containing up to 3 wt. % of fibrillated acrylic exhibited excellent performance and were ?gentle to the rotor?. Particular attention was paid to the friction level and stability at elevated temperatures and in the fade sections. Importantly, wear of optimized pads and corresponding wear of rotors were extremely low, and no vibration or noise were detected during testing. This could be ascribed to the controlled mechanical properties of pad material and the specific and stable friction layer developed on the surfaces of both pads and rotors. The fibrillated acrylic as well as the biodegradable mineral fiber not only reinforced the bulk of the pad composites and provided damping capacity, but also were detected in the developed friction layers, increasing their strength and stability. It could be concluded that when mixed with an appropriate amount of additional reinforcement(s), the Sterling fiber could offer very attractive properties for pads used in friction brakes of vehicles with the regenerative braking.