The friction material in the automotive brake system has been considered as one of the key components for overall performance of a vehicle. This is because it plays crucial roles in various aspects of the brake performance such as a stopping distance, pedal feel, counter disk wear and brake induced vibrations. A great deal of effort has been given to improve the performance of the friction material and multiphase composites have been used as a brake friction material from the early stage of vehicle development since a monolithic material has never been successful for commercial brake friction materials. More than 10 ingredients have been used to produce commercial brake friction materials, expecting that each ingredient provides beneficial roles for brake performance under different braking conditions. Brake pads typically comprise the following sub-components a) Frictional additives, which determine the frictional properties of the brake pads and comprise a mixture of abrasives and lubricants b) Fillers, which reduce the cost and improve the manufacturability of the brake pads; c) a binder, which holds components of a brake pad together d) reinforcing fibers, which provide mechanical strength.
Before the ban on usage of asbestos in brake linings was imposed in 1989, Asbestos was the most preferred filler material as it is thermally stable upto 5000C, helps regenerate friction surface during use, insulates thermally, strong and flexible and mostly, it is available cheap. Since the ban on asbestos, researchers have struggled to come up with an equally efficient alternative. Barites, mica and cashew dust are amongst some of the materials that have been considered for use as fillers.
FLY ASH AS FUNCTIONAL FILLER IN BRAKE LINING:
Coal combustion byproducts are the inorganic residue left behind during the coal combustion process. Fly ash is one such byproduct. Disposal of Fly ash is an environmental and economic liability for power plants across India. Other than disposal in surface impoundments, about 40% of the total amount of Fly ash generated in India is used in applications such as concrete, embankments, etc. Fly ash is composed of fine size particles (mean size 10 – 30 µm), with uniform physical and engineering characteristics. It possesses low specific gravity in the range of 2-3, as compare to ingredients used in brake linings. They are typically generated at very high temperatures i.e 1000oC upwards. Hence, they should provide a thermally stable bulk for high-temperature environments that a friction composite experiences. The specific heat of fly ash particles is also high (~ 800kJ/kgK). This attribute of the fly ash help store the excess heat generated at the braking interface of lining and rotor. The poor conductivity of fly ash may help in preventing the heat generated to travel towards the backing plate side of the brake, which may vaporize the braking fluid. The above mentioned characteristics present fly ash as a potential ingredient in a brake lining composite.
Fly ash Molecular Structure
On the downside, fly ashes all over the world differ mineralogically from each other due to variability in quality of coal used and over all combustion processes adopted in the respective power plants. Fly ash particles used in this study were obtained form Neyveli Lignite Corporation limited, Tamil Nadu. Some efforts had already made to incorporate fly ash in brake pad composites and several materials were optimized with fly ash content up to 25 wt %. In one paper Malhotra metal have found that the incorporation of fly ash and bottom ash particles in friction composites increases the over all coefficient of friction. They also concluded that the behaviour of ash particles were similar to abrasive like Al2O3, Cr2O3 and SiC. In another paper Filip and...