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EuroBrake 2022: Recent developments in brake friction materials session overview

The Recent developments in brake friction materials session will take place on Thursday May 19th and will be chaired by Raffaele Gilardi of Imerys Graphite & Carbon and co-chaired by Georg-Peter Ostermeyer of TU Brunschweig.

Topics and speakers for the session include:

Carbon based friction modifiers for high compressibility brake pads

Henrique De Lima Secco, Superior Graphite

Compressibility is a key parameter for brake pad producers, its increase is linked to brake comfort for the drivers and with lower noise levels during braking. The tolerance to noise tends to be even lower for electric and plug-in hybrid vehicles since those operate at a considerably low noise level, compared to the internal combustion engine vehicles. The resilient graphitic carbon (RGC) from Superior Graphite has been the preferred choice for both OEM and Aftermarket brake pad producers as a friction modifier given its proven capability to increase the compressibility of the pads, even with low added amounts in the formulation.

To overcome the current and future challenges in compressibility increase and noise reduction for brake pads, Superior Graphite has developed a new high resiliency synthetic graphite, and other grades in earlier stage of development. These products can be integrated into a formulation as a drop-in solution for traditional carbonaceous materials or as a performance additive, which provides flexibility to the brake pad producers to adapt their current formulations to versions with higher compressibility. The possibility of combining different resilient grades in the same formulation ensures even more flexibility for the formulators to achieve application-wise tailored compressibility and noise performance targets.

The scope of this study was to compare the compressibility of aftermarket brake pads using new Superior Graphite grades as a partial replacement of the carbonaceous materials in the same formulation. The brake pads were prepared and tested for compressibility, shear strength and wear by development partners which have provided the data for this study as a courtesy. The results show significant increases in compressibility of the brake pads using all Superior Graphite resilient products versus baselines, notably the best new product exhibits a 20-25% of increase in compressibility compared to the current industry standard using RGC. If to take into the fact that RGC’s already delivering 30-50% improvement vs baseline, this results in around 50% improvement with the new resilient product.

An investigation of moisture sorption and its influence on brake pad modulus and compressibility: tangent modulus of compression vs. dynamic modulus: review of compressibility vs. hardness

Seong Kwan Rhee, SKR Consulting LLC

The current compressibility measurement method generates compression stress-strain curves. The strain during the 3rd compression is called “compressibility”.

In physics, compressibility is defined as a reciprocal of compression modulus. Using this definition, the tangent and secant compression moduli are obtained from the compression stress-strain curves and corresponding compressibility numbers are obtained. The tangent modulus increases while the secant modulus decreases with increasing moisture content of brake pads. When disc pads are exposed to humidity, the pads gain weight following a linear relationship between the weight and the square root of exposure time in the initial stage of moisture sorption. As disc pads contain 2 – 5 v.% of elastomers and 12-22 v.% porosity, the possibility of viscoelastoplastic and poroelastoplastic behaviour of pads during compression is discussed. A possible relationship between compressibility and hardness is reviewed.

In situ determination of the temperature profile and oxygen concentration in the brake pads using metal sulphides as a marker

Gabriela Macías Benalcázar, Rimsa Metal Technology S.A.

Friction systems reach high temperatures during braking because they transform kinetic energy into heat. High temperatures jointly with oxygen induce the reactivity of susceptible materials towards oxidation such as the phenolic resin and metal sulphides. The decomposition of the phenolic resin contributes to the decrease and instability of the coefficient of friction and increases the wear of the pad. Thus, the determination of the oxidation reactions is a key point in knowing the depth at which reactivity can take place and understanding the friction behaviour.

This study is focused on determining the reactivity through the brake pads pad (NAO Cu-Free formulation) using the oxidation conditions of metal sulphides as a marker. The reactivity characterization through the brake was done using EPMA/WDS and raman techniques, after SAE J2522 and J2707 tests. The results illustrate that there is oxidation underneath the brake pad surface during braking due to the oxygen presence and the high temperatures reached. Depending on the testing schedule the temperature profiles and concentration of oxygen on the pad are different and consequently also is the tribochemistry. In pads tested according to SAE J2707, the tribochemistry is observed in deeper sections than in pads tested according to SAE J2522.

The results of this study will help to determine how far from the surface tribochemistry takes place and to understand the behaviour of friction materials.

Comparative study of tribological behaviour of brake lining materials in road test and laboratory-scale tribometer

Norton Wille, Fras-le SA

This work compares the friction and wear mechanisms data of heavy vehicle brake lining materials from laboratory and field tests. Friction material operating conditions, such as braking pressure, temperature, and speed, were obtained from braking applications of the vehicle. The laboratory tribological evaluation was carried out using pin-on-disc dry sliding tests. The pins were fabricated from the brake lining material and the metallic discs from the gray cast iron drum material. The response surface methodology (RSM) approach using a central composite design was proposed to predict the effects of the contact pressure (P) and sliding velocity (V) on the friction and wear of the brake lining materials.

The morphology and chemical composition of the worn surfaces from laboratory and field tests were analyzed with scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and Raman spectroscopy. The laboratory-scale test results were used to adjust an empirical P.V (pressure × sliding velocity) RSM model that predicts the effect of contact conditions on friction coefficient and volume loss by wear of the brake lining materials. Abrasion grooves were observed on the worn surfaces of the brake lining and drum pair. The hard primary plateaus observed on the brake lining worn surface promoted the drum material transfer and the formation of secondary plateaus from the agglomerated iron oxide. For higher pressure and sliding distance, the transfer of particles from the drum surface to form a compact layer stabilized the friction coefficient. The brake lining material worn surface from the vehicle road tests also exhibits abrasive wear and plateaus formation.

In addition, cracks due to contact tensile stress were observed. Laboratory tribometer alloyed identifying the predominant wear mechanisms of brake lining materials. These laboratory-scale tests can be used to predict the tribology behaviour and develop new brake friction materials formulations before evaluating the results from road tests.

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