The Evolution Of Brake Pad Friction Materials(2)
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.
Semi-Metallics
In the first part of the 20th century, friction materials were woven with asbestos fibers. The materials were sold in rolls and trimmed to fit drum brake shoes. As material engineers improved resins, they could ditch the woven materials and make a better brake material called a semi-metallic. These materials still had asbestos content, but the ability to mold and bond a friction material to a shoe or backing plate had a definite advantage.
Semi-metallic pads first use metal fibers to give structure and provide friction. The metals used are typically high-quality steel, copper and other exotic metals. The other part of the semi-metallic mix consists of a variety of materials like glues, lubricants and structural fibers like Kevlar. The manufacturer will blend the components to provide the best performance for that application.
Semi-metallic friction materials are still used by OEM and aftermarket manufacturers because they can produce predictable levels of hot and cold torque.
NAO
Non-asbestos organic (NAO) friction materials typically wear more than harder semi-metallic compounds. Generalizing the wear characteristics of NAO and ceramic-based compounds is difficult because there are so many variations depending on the manufacturer. Wear varies depending on the formula the friction supplier chooses for a particular application.
APPLICATION-SPECIFIC
When a new vehicle or platform is introduced, the friction material and brake pad design will likely be specific to the application. It could be a ceramic, semi-met, or something in between. The OEM and supplier work together to achieve the best pedal feel, performance and NVH.
Aftermarket brake pad suppliers are taking the same approach to their premium friction lines. Instead of classifying the line as a ceramic or semi-met, they sell it as a match to the original friction material.
THE UNSEEN ASPECTS
When you are buying a brake pad, you are also buying the engineering and research behind it. When an aftermarket brake pad manufacturer is developing or reverse-engineering an application, they rarely test on an actual vehicle. This type of testing is expensive and time-consuming. A brake dynamometer can test brake systems in a controlled environment that mirrors the real world. A brake dynamometer can run 24 hours a day and measure the performance of a braking system over its entire lifetime. Plus, would you want to be a brake pad guinea pig for 25,000 miles?