Abestos has not been used for a long time in brake pads, because of political bullsh*t. While abestos was shown to be carcinogenic when used for building insulation, it was not carcinogenic when used for brake pads, because the shape of the particles are different - but alas, this is a different discussion altogether.
Abestos was replaced by carbon...and most pads nowadays are carbon-metallic compounds. Carbotech Panther Plus and Panther XP pads are ceramic based. We have replaced approximately 90% of the carbon typically found in a pad with ceramic. The main benefits of doing so is that high carbon content pads will forms carbides on the pad surface, and the carbides are very rotor aggressive. Ceramic, on the other hand, does not form carbides, as it is very stable and inert. The high ceramic content of our pads allows them to withstand a high level of heat without changing chemical composition, both on the interface surface and within the pad material itself. We have found in testing that when using our ceramic based compounds, rotors run COOLER by 10-15%. The heat is being retained in the pad compound, and then dissipated through the caliper and surrounding atmosphere. Thus, if there is MORE pad material present, the pad is more able to absorb heat and dissipate it out the edges of the pad. As the pad wears, the surface area of the pad around the edges is decreased.
Further, brake pads produce braking torque via two main methods: (1) chemical interaction and (2) abrasive qualities. Without getting into a lengthy discussion on this, I'll just point out one example. When you overexceed the upper temperature limit of a specific pad, it will fade. And in order to get the car to stop, you have to press harder on the pedal, and thus mash the pad into the rotor with more force. In this way, you will obtain stopping power, but with the abrasive qualities of the pads, and not by any chemical interaction. Does this make any sense to anyone here?
Heat moves from the rotor/pad interface TO the pad, TO the backing plate, TO the pistons and caliper, TO the brake fluid, and then TO the atmosphere. As an aside, this is why you should discharge air from cooling ducts to the CALIPER and NOT the rotor. The rotor will cool itself as long as it is spinning - most rotors are internally vaned. The caliper is a heat-sink for the brake pad, and you MUST cool the caliper if you want to efficiently dissipate heat that is in the pad, and keep your fluid from boiling.
Another aside. Heat _IS_ stored in the pad material...this is a fact. The rotor cools much more efficiently because it is much more mass and surface area, and is always moving. The pad, on the other hand, is much smaller and has far less surface area. For those who dispute the level of heat a pad endures, let me just say that I have personally pulled brake pads out of IT-class race cars at various tracks to measure thickness (wear rate) after ever test session run, and these pads are 800degF+ when measured with a pyrometer, and wear rate IS accelerated on a geometric scale as the pad wears down. The backing plate is cold-rolled steel, and after a few minutes of hard driving it will heat up to the same temperature as the pad itself, with only a slight time lag. The key to everything here is SURFACE AREA. Pistons are made hollow like cups so that the amount of surface area contacting the backing plate of the pad is kept to a minimum. The additional surface area on the "inside of the cup/piston" is there to provide additional cooling. You will find calipers and drums that have fins on them to increase the surface area for cooling - similar to the heat sink on a computer CPU. Rotors have a higher effective thermal emissivity than the pad and caliper, as it is always moving. The pad is stationary, as mentioned before (along with the caliper), so the effective thermal emissivity is far lower for the pad, and it will retain/store heat after it is retracted from the rotor (i.e. you release the brake pedal)...after which it begins to cool. Upon a subsequent brake pedal application, it will heat up again. So long as you keep the maximum temperature reached within the design limits of the pad compound, it will continue to perform well. However, as the pad becomes thinner, there is less edge surface area, so it cools at a rate more similar to the backing plate, which has very low effective thermal emissivity...and it stays hot...and you can more readily exceed the design temperature limit of the pad, and wear accelerates.
Small changes can make a big difference. For example, if you slot a pad, it will last longer, because even the added surface area along the edges from the slotting allows the pad material to dissipate heat faster.
Andie
[This message has been edited by HomeDepotNSX (edited 28 November 2001).]
