In the automotive industry, new lightweight and high-strength steel and aluminum are getting attention for reducing vehicle weight and thereby improving fuel efficiency. This is one good example of how the performance of an automotive component depends on its composition. Another important example is the brakes.
Current automobile braking systems are of two types: drum/shoe and disk/pad. Both are “friction” type brakes designed to decelerate a car by transforming the kinetic energy of the vehicle into heat by friction and dissipating the heat into the surrounding environment. Brakes are composed of many different materials depending on the type of vehicle in which they are used (automobile, truck, railroad, aircraft) and factor heavily in the overall performance of the brake. Currently there are four recognized types of friction brake compositions: asbestos, semi-metallic, non-asbestos organic (NAO), and ceramic.
Semi-Metallic Friction Formulations
A semi-metallic brake pad consists of a lining that uses steel wool as a reinforcing fiber; most contain at least 60% steel by weight. The steel fibers act as the framework to lock the friction ingredients together. The most common metal brake pad materials are metal oxides, typically aluminum oxide and iron oxide, brass filings or chips, metal sulfides, including copper sulfide, antimony sulfide, and lead sulfide, steel wool, and copper. One car manufacturer posted a table of common brake pad materials, their functions, and the percentages found in brake pads on its website.
In the 1990s, copper was discovered in urban run-off flowing into San Francisco Bay. Brake pads were found to be the source; small bits of the metal flake off every time drivers apply their brakes. According to the Copper Development Association, the Brake Pad Partnership, a cooperative effort among representatives of the auto industry, brake pad manufacturers, environmental groups, storm water agencies, and coastal cities, determined that brakes account for as much as 35 to 60% of copper in California’s urban watershed run-off. As a result, California and Washington passed laws in 2010 mandating a reduction in the amount of copper used in automotive brake pads. Auto manufacturers will seek out a mix of elements to provide the smooth breaking, efficient heat transfer, and cold weather tolerance currently provided by copper.
In both new and existing formulations, controlling the distribution of the metals and other components used to manufacture automotive brake pads/shoes is a major challenge. It’s also important to be able to reproduce the formulation to ensure good quality control of the final pad or shoe. Read Statistical Approach to EDS Analysis of an Automotive Disc Brake Pad describing a study in which a a semi-metallic automotive disc brake is examined using a scanning electron microscope (SEM) and an energy dispersive spectroscopy (EDS) system to characterize the composition of the brake pad materials. The addition of a statistical approach to the analyses was used to determine the distribution and phases of the components of brake pads/shoes.
The approach of using spectral imaging to obtain element maps followed by principal component analysis (PCA) to derive principal components and then using these components to determine phases is less time consuming than using X-ray maps alone. Further, there is very little human bias introduced into the analysis using this approach. A SEM/EDS application protocol for determining the components and their distributions when verifying a particular formulation for brake pads or linings may be established using this statistical approach and will save time for the analyses.