Enhancing Brake Pad Performance: Microstructural Analysis of Composite Iron Sulfides

The research explores the tribological characteristics of brake friction materials, specifically examining synthetic iron-based sulfides with distinct microstructures, pure iron sulfide, and composite iron sulfide. Notably, the composite iron sulfide consisted of an intimate blend of iron sulfide and magnesium oxide, with the latter manifesting as darkened regions in the accompanying image.

Fig.1. SEM micrographs of the two different syntectic iron-based sulfides used in this study: (a) pure iron sulfide and (b) composite iron sulfide FeS+MgO.

The outcomes of tribological testing, conducted across various temperatures according to the SAE J2707 standard, indicated the superior performance of brake pads with composite iron sulfide. These pads exhibited higher friction levels and lower wear compared to those utilizing pure iron sulfide, signifying a noteworthy enhancement in the overall tribological properties of brake pads containing the composite variant.

These differences found corroboration through two analytical methods, Thermogravimetric Analysis (TGA) and X-ray Diffraction (XRD). TGA results depicted a distinctive weight increase in the composite iron sulfide beyond 400°C, suggestive of an alternative oxidation mechanism when contrasted with pure iron sulfides. XRD findings further validated the distinctions observed in TGA, revealing the emergence of magnesium sulfate within the composite iron sulfide.

To examine chemical alterations within the synthetic iron-based sulfides, a comprehensive cross-sectional analysis of the friction materials was conducted utilizing Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM/EDS). The investigation revealed that pure iron sulfide experienced extensive oxidation, with oxidation depths reaching up to 200 µm beneath the friction surface. Conversely, composite iron sulfide primarily demonstrated oxidation near the friction surface, with a maximum penetration depth of 50 µm.

The important role of microstructure in influencing the kinetics of thermal oxidation emerged as a salient theme in the research. An alternative oxidation mechanism was postulated for composite iron sulfides, shedding light on discrepancies in oxidation processes between pure iron sulfides and their composite counterparts.

Fig. 2. Schematic representation of the thermal oxidation mechanism for pure iron sulfide and composite iron sulfide.

For the pure iron sulfides, the oxidation process initiated at the surface and progressed inward. On the contrary, the composite iron sulfide tells a different story. The presence of magnesium oxide in this material effectively functioned as a protective shield, enveloping the iron sulfide and acting as a formidable barrier against oxidation reactions. This protective layer substantially impeded the oxidation of iron sulfide. This hypothesis is substantiated by elemental mapping of partially oxidized particles of pure and composite iron sulfide.

In conclusion, these findings pointed out the potential for optimizing the microstructure of metal sulfides, such as iron sulfide, to yield superior brake pad materials, ultimately contributing to the development of safer and more durable braking systems.

by Diego Chavez Jara Senior R+D+I Engineer, Friction Division