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Applied Material Science: Automobile Brake Pads and Friction Composites; Three important aspects of a conventional automotive brake system are design and fabrication of frictional materials, frictional material's wear behavior, and how the wear, which results in the transfer film formation on the surfaces, affects the performance of the brake. Most automotive friction brake pad technology is shrouded in secrecy with each manufacturer claiming his proprietary material, whose constituents are furtive, is the best. The effective performance of the advanced frictional material demands the optimization of the constituents' composition and a thorough understanding of their role in determining the effectiveness of the brake. To better comprehend the role of various constituents and their interplay, if any, we are currently fabricating 2.5 inch diameter frictional material disks in which the following parameters are in particular being studied: a) cross-linking behavior of resin and fibers, b) pre-wetting and its mechanical consequences for the material, c) interaction with salts and water, d) thermal characteristics, e) shape of the constituent particles and their effects on frictional coefficient, f) wear resistance and durability, and g) wear films and their role. The answers to the aforementionedquestions are being sought by undertaking novel in-situ diffuse reflectance-Fourier transform infrared (ISDR-FTIR) measurements at 30^oC < T < 800^oC on fabricated brake pads. In addition, differential scanning calorimetry (DSC), differential thermal analysis (DTA), dynamic mechanical analysis (DMA), thermal mechanical analysis (TMA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and specific heat capacity data are being collected to help us better understand the role of various brake constituents. We are also exploring how the surface structure of the brake shoe pads, which have undergone wear tests using a dynamometer, controls the wear and frictional behaviors of the material. This will be achieved by undertaking surface sensitive spectroscopic experiments, e.g., grazing angle-Fourier transform infrared, diffuse reflectance-FTIR, and photoacoustic-FTIR measurements.; Figure 1. SEM microphotograph of the surface of the composite subjected to friction test. Note the tracks formed by the metallic particles.

Figure 2. This figure depicts how the post-curing affects the frictional behavior of a composite formed from phenolic polymer.

Polymers and Structural Composites
An alternative to the carbon-carbon composite as structural materials is ceramic matrix composites. The high fracture toughness and strength of fiber-reinforced ceramic matrix composites make them attractive alternative materials, especially for the aerospace industry. Among the ceramic matrix composites, silicon nitride (Si₃N₄) and silicon carbide (SiC) can withstand high temperatures and corrosive environments. Therefore, it is not surprising that these ceramic materials are beginning to be considered necessary in advanced aircraft engine structures and heat management applications. The wider application and adaptation of silicon-based ceramic matrix composites by the industry, however, hinge on their economical production and their improved mechanical properties. The current technologies of fabricating ceramic composites, especially the oxides, SiC, and Si₃N₄, rely on powder processing at high temperatures and pressures. Another approach of fabricating fiber-reinforced ceramic composites, which has recently attracted attention, is the use of pre-ceramic polymers. At present we have focused our attention on two silicon based polymers, i.e., polymethyloctadecylsiloxane and polycarbosilane. These two polymers were chosen because they were commercially available.
In addition to silicon-based polymers, we are pursing research on thermosetting polymers, like phenolic or polyimide resins. We are especially interested in these polymers because of their use in friction composite materials. Typical phenol resins used in these composites are novolaks, resols, and cresol resins. Phenolic resin are frequently modified by novolak/resol blends, rubber, or epoxy, etc., to improve the friction materials’ performance. Since the main role of a phenolic resin in a friction composite is to keep the composite’s structural integrity under various mechanical and thermal stresses, it is important to understand the polymer’s thermosetting behavior. Therefore, we have initiated systematic vibrational, thermal, and thermomechanical studies on phenolic resins, used in the fabrication of frictional materials, to understand not only their curing behavior but also how such properties are altered by various friction composite ingredients.

Publications
- A. Radisic, P. S. Valimbe, and V. M. Malhotra, "Effects of Additives on Thermomechanical Properties of Composites formed from Thermoset Polymers" Materials Research Society Symposium, 542, 13-18 (1999).
- P. S. Valimbe and V. M. Malhotra, " Surface oxidation of Phenolic Disks and its Effects on Frictional Behaviors", ACS Polymeric Materials Div. 83, 15 – 16, (2000).
- V. M. Malhotra, P. S. Valimbe, and M. A. Wright, "Fabrication of Automotive Brake Composites from Unburned Carbon", ACS. Fuel Div. 45(3), 504 – 508 (2000).
- V. M. Malhotra and P. S. Valimbe "Is There a Role for Coal Combustion By-Products in Frictional Materials” Proc. 14^th International Symp. On Management and Use of Coal Combustion Products, Vol. 2, EPRI, 62.1 – 62.10 (2001).
- V. M. Malhotra, P. S. Valimbe, M. L. Gray, and Y. Soong, “The Role Unburned Carbon Concentrates on the Frictional Properties of Composites”, ACS. Fuel Div. 46, 593 – 594 (2001).
- V. M. Malhotra, P. S. Valimbe, and M. A. Wright, "Effects of Fly Ash and Bottom Ash on Frictional Behavior of Composites", Fuel 81, 235 - 244 (2002).