The Global Brake Safety Council sees an increase in disc brake pads that are prematurely replaced before the end of the friction lining life cycle, due to:
- 1
Rust related issues such as separation of friction lining from the disc brake shoe
- 2
Fluctuation in critical dimensions.
A leading cause for both issues is the use of mill scale steel, or ‘black steel’ (non-pickled and oiled). In the North American aftermarket, as there are little or no steel specifications for disc brake shoes, black steel is increasingly used.
GBSC conducted research of discarded disc brake pads from job-shops and engaged in discussions with metallurgists, major pad manufacturers and OE brake foundation engineers to identify root causes of premature pad replacement and the effects of black steel used for disc brake shoe manufacturing.
Mill scale is embedded in and around the bond line of the friction lining and the disc brake shoe, causing a weaker bond, susceptible to rust jacking. These oxides are also painted over after the pad is assembled, compromising paint adhesion.
Manufacturers using black steel shot blast the disc brake shoes after stamping, attempting to remove mill scale. Shot blasting can deform/compromise critical shoe dimensions causing fit, function, and safety issues in the caliper assembly.
GBSC studied randomly selected leading aftermarket brands to further analyze the above mentioned disc brake pad failures and the effects of black steel.
Parts were put through a 96 hour salt spray test in which the brake shoe must meet 5% red rust maximum. All painted brake shoes failed before the 20 hour interval. The only pad sets to meet the rust requirements had zinc coated shoes. Upon inspection, almost all pad sets showed out of tolerance measurements in critical areas.
Sections of some brake pads, black steel samples, and suspected black steel pads, were prepared and examined using ASTM metallographic procedures. Scale was embedded in the black steel samples and in the bond layer of suspected black steel pads. No iron oxides found in pads with shoes made from pickled/oiled steel.
GBSC recommends all disc brake shoes be manufactured using pickled and oiled steel.