The Evolution Of Brake Pad Friction Materials(1)
When I started with Brake & Front End, there were two prominent friction material families – semi-metallics and non-asbestos organics. Semi-metallics were known for being more aggressive or suited for applications with heavier loads and braking forces. Non-asbestos organics were for lighter passenger vehicles where noise and longevity were the top concerns.
About 20 years ago, a new family of friction materials was being offered on passenger vehicles. The new category of ceramic friction materials was part science and part marketing. Technically, a ceramic could be classified as a non-asbestos organic friction material when it was first introduced. The promises of low noise and brake dust had a lot of people listening to the marketing messages.
There are three advantages of ceramic friction material in specific applications. First, ceramic materials offer stable performance under a wide range of temperatures. This friction stability provides both performance and noise advantages.
Ceramic materials that go into a brake pad are tiny strands engineered to be a certain length and width. Some of the early articles on ceramic say the fibers were a replacement for asbestos fibers. But, these synthetic ceramics fibers also increased the performance of the friction materials.By having a consistent coefficient of friction over a broad range of conditions, the engineers can match the caliper, rotor and attached components. Second, ceramic brake pads manage heat in the caliper better on some vehicles than some non-ceramic applications.This is because the brake dust from a ceramic material has a different electrical charge that makes it less likely to stick to wheels.
When the first ceramic replacement brake pad lines came out 25 years ago, the pads were marketed as a noise solution for all vehicles. But, that is where some lines ran into trouble by selling ceramic materials for all vehicles. On some applications, the ceramic pads were having issues with pedal pulsation and judder. Some blamed the high-temperature ceramics for warping rotors. But, most engineers knew the problem was runout and something called a “transfer layer.”
Ceramic friction material uses adhesive or adherent friction properties. This type of friction works best if the friction material creates friction against itself. These types of pad material transfer a very thin layer of pad material onto the surface of the rotor. The transfer layer is bonded to the rotor’s surface and cannot be washed away by water or wheel cleaners.
When the brakes are applied, the transfer layer is broken down and replenished on the rotor. Adherent friction is easier on rotors, but the pads become the primary wear component. This is also one of the disadvantages of a ceramic friction material. If the rotor has excessive lateral runout or poor surface finish, the transfer layer could be laid down unevenly. An inconsistent transfer layer can lead to brake pedal pulsation and brake judder.
Twenty years ago, some ceramic brake pad lines got a bad reputation for pedal pulsation. The issue was not the friction material, but the reconditioning of the rotor. Often, the rotor was not refinished properly, and the transfer layer was not consistent across the entire surface of the rotor. Also, just replacing the pads often lead to the contamination of the new ceramic brake pads due to incompatibility of the transfer layer material.
These large discs simulate the weight of a vehicle and braking forces. These dynos allow engineers to evaluate friction materials under controlled conditions.