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EuroBrake 2022: Advances in friction material formulation session overview

The Advance in friction material formulation session will take place on Tuesday May 17th and will be chaired by Anne-Lise Cristol of University of Lille and Kai Bode of Audi AG.

Topics and speakers for the session include:

Effect of space fillers in the brake friction material on airborne particle emission: A case study with BaSO4, Ca(OH)2, and CaCO3

Jongsung Park, Korea University

Particulate matter (PM) emissions from the brake friction material were investigated, focusing on the effect of space fillers on the concentration and size distribution of airborne particles. Brake emission tests were carried out using a 1/5 scale brake dynamometer enclosed in a chamber, and the particle concentration was measured using an electrical low-pressure impactor (HR-ELPI+).

Results show that brake emission, disc wear, and pad surface characteristic were considerably affected by the choice of the filler among BaSO4, Ca(OH)2, and CaCO3. The brake emissions ranked in descending order were BaSO4, Ca(OH)2, and CaCO3. The friction material with CaCO3 showed thicker friction films on the steel fibers than Ca(OH)2, whereas bare steel fibers were found with BaSO4, indicating considerable effects by the filler-dependent coverup layers on the steel fibers, which was confirmed by microhardness tests. The filler effect on PM suggests that a proper selection of space fillers can reduce brake emissions from grey iron discs by preventing direct adhesion with steel fibers.

Effect of addition of aluminium anodizing waste on the wear and emission behaviour of a low metallic friction material for automotive braking applications

Priyadarshini Jayashree, University of Trento

Keeping sustainability in mind, the need to recycle/reuse industrial waste is of paramount concern. Numerous industrial wastes like red mud and fly ash have already been extensively employed in automotive brake pads, yielding desirable tribological characteristics. These unusual, yet effective additions, have paved a path to explore and utilize alternative kinds of wastes in braking applications.

One such example is the waste generated during the aluminium anodizing process, termed aluminium anodizing waste (AAW). Currently, the AAW is being utilized in the field of construction (bricks). However, a large chunk of the waste remains underutilized, generating environmental and economic problems. Furthermore, the AAW remains unexploited, as its minor constituents keep varying, depending on the agents utilized in the anodizing process. Additionally, a streamlined procedure of storage and treatment of this waste is still in its nascent stage.

Taking this into consideration, the present study focuses on the storage, characterization, and utilization of AAW in a low metallic friction material composition, with only the essential constituents. The friction material composition was selected to highlight the role of the AAW. The AAW was subjected to initial treatment and an appropriate procedure was established for safe storage.

Next, the AAW was characterized using TGA, FTIR, SEM/EDS, and XRD to obtain its composition and particle morphology. Prioritizing economic feasibility, the powder properties of the waste (heat treatment and particle size) were determined and then subjected to preliminary dry sliding wear tests and emission analysis on a pin on disc tribometer at a constant contact pressure of 1 MPa, sliding velocity of 1.51 m/s, and at room temperature. The testing conditions were selected to replicate mild braking conditions in automotive applications. The tests were compared to a reference composition containing alumina instead of AAW. The worn surfaces of the pairings were evaluated through SEM/EDS analysis to understand the nature and the characteristics of the friction layer. It was observed that the AAW behaved as an abrasive, similar to the functioning of alumina.

Moreover, the AAW containing composition had a similar friction coefficient, wear, and average particle concentration of emissions, when compared to the reference alumina composition. The properties of the friction layer of both the compositions were fairly similar to each other. Through this initial investigation, the prospect of AAW utilization as an alternative for abrasives in brake pads could be further explored through additional specific dynamometric bench tests to obtain relevant data for automotive braking applications.

Effects of titanate on brake wear particle emission

Emiko Daimon, Otsuka Chemical Co., Ltd

Titanate is used as a friction modifier, for noise reduction, fade resistance, stability of friction co-efficient, and wear resistance.

In our previous study, we focused on the chemical reaction on the friction surface. We reported that the reaction between titanate and phenolic resin affects the friction property. In this study, we focused on the effect of titanate on the brake wear emission.

Our friction tester is a scale inertia dynamometer with a blower using clean air through a HEPA filter. We use the 1/7 area size of the full-size pad. In the case of testing by the same slide speed condition, absorbed energy per unit area is almost same. We can reproduce the phenomenon of the frictional surface of the real size in a small specimen. By analysis of the frictional surface, we can also clarify the mechanisms of titanate.

In addition, by using both a CPC (Condensed Particle Counter) and a MCI (Multi-Nozzle Cascade Impactor) sampler, it has become possible to count the number of particles and measure the mass of the particles. In our test results, titanate reduced both particle number and particle mass (both PM2.5 and PM10).

This study suggests that frictional behaviour and wear particle amounts differ depending on the presence or absence of titanate, and reports on the mechanisms of those differences. We assume that these mechanisms are related in our previous studies of the chemical reaction between phenolic resin and titanate.

Performance of environmentally sustainable NAO Cu-free brake pads containing nitrile rubbers and recycled friction material

Vishal Reddy Singireddy, Southern Illinois University

As the automotive industry tends towards development of sustainable and environmentally friendly friction material, studies on potentially recycling and re-using friction material have become increasingly important. In this research, two brake pads, made of sustainable recycled friction material (ACI Industries, Ltd.), identical in formulation except the type of rubber (Zeon Chemicals L.P.), were developed in the laboratory.

Rubbers are a key component in brake friction material and impact dampening the friction level and stability, wear, vibration, and noise, by contributing to formation of friction layers and influencing mechanical and thermal and corrosion properties of brake pads. These aspects become even more relevant when electric vehicles are considered since they are almost noise-free.

The laboratory-developed samples were tested by adopting the scaled-down SAE J2522 brake effectiveness procedure [1, 2, 3] against surface treated commercially available pearlitic gray cast iron rotors (Waupaca Foundry Inc.), [4, 5]. Universal Mechanical Tester (UMT, Tribolab by Bruker) was used to perform the test. Vibrational response was characterized by using a triaxial ICP accelerometer (PCB Electronics, Model = 356A45), sound pressure levels were monitored by a ¼” free-field prepolarized microphone (PCB Electronics, Model = 377C01) and data from them were collected using a high-performance oscilloscope (Agilent Technologies, Model = MSOX2024A) and DAQ (NI USB - 6218). Wear debris and the friction surfaces of tested samples were analyzed using Scanning Electron Microscopy (FEI, Model: Quanta FEG450) and Energy Dispersive X-ray spectroscopy (EDX, Oxford Instruments). Mechanical properties, density and porosity were measured using CV Shore D durometer (ASTM D2240) and AWS ALX – 310 precision balance. The developed lightweight samples exhibited extremely low open porosity (< 3.5 %) and optimal (with respect to compressibility) hardness (~ 55). The newly developed pads when tested against coated rotors developed optimal friction layer responsible for very stable and relatively high friction levels, very low wear of pads and rotors, and a extremely "quiet" operating conditions. This performance was ascribed to a combined effect of the i) appropriate friction layer, ii) hardness and compressibility, and iii) the low porosity.

[1] Rohith Redda Boyna, “Impact of Friction Test Scale on Brake Friction Performance”, Master’s thesis, Southern Illinois University Carbondale, December 2016

[2] Vishal Reddy, et al., "Impact of Acrylic Fiber on the Performance of Newly Developed Friction Materials for Vehicles with Regenerative Braking," 38th Annual SAE Brake Colloquium (Online and on-demand), Oct. 2020 [oral presentation only]

[3] Vishal Reddy, et al., “On Scaled-down Bench Testing to Accelerate Development of Novel Friction Brake Materials” (to be published)

[4] Filip, Peter, and Nathan K. Meckel. "Wear resistant braking systems." U.S. Patent 10,895,295, issued January 19, 2021.

[5] Filip, Peter, and Nathan K. Meckel. "Wear resistant braking systems." U.S. Patent 10,197,121, issued February 5, 2019.

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