Establishing a Correlation Between Friction Performance and Tribolayer Formation Using Engineered Mineral Fibers

Abstract

This article relates the friction performance to the tribolayer formation when using engineered mineral fibers in the friction formulation. A nonasbestos organic (NAO) nonsteel copper-free passenger car disc pad (PC-DP) nonsteel Cu-free formulation is used to investigate the impact of the fibers on the third body formation and its influence on performance and wear. Full-scale dynamometer performance (AK-Master) test results show the pronounced effect of the fiber length on the friction performance. While a similar overall coefficient of friction (CoF) of around 0.33 is observed in all three cases, more stable friction behavior is observed for the longer fibers, with less variation. This behavior is even more observed in the fade and high-temperature sections. Especially the use of fiber spheres shows quite some unique features over the other tested mineral fibers, especially with respect to pad/disc wear. To understand these differences in performance, the transfer layer and tribolayer on the disc and brake pad are investigated, respectively, by means of scanning electron microscopy (SEM). Formulations showing excellent performance results so far show the formation of a smooth transfer film on the disc. No grooves are visible, and a continuous iron oxide layer is formed at the surface. The tribolayer of the corresponding brake pads nicely shows the formation of a compacted thin layer at the surface. In addition, the importance of mineral fibers in the formulation is highlighted as they are at the core of the tribolayer formation. This study is a step forward toward understanding the role of mineral fibers in a friction formulation and shows its importance toward reducing nonexhaust emissions.