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EXETER, U.K. — Phenolic composite brake pad backing plates are a disruptive idea that has yet to find acceptance, which is surprising considering they are 75-percent lighter, inert and corrosion-free, thermally insulating and have superior damping properties – just the characteristics required for electric vehicle braking duty cycles, as outlined in the following article by Dr. Luke Savage, Programme Manager, University of Exeter. Dr. Savage is a senior Materials Scientist with 22 years’ experience of researching answers to real-world industrial problems over a broad range of scientific areas.
Electric cars are heavy, usually much heavier than their ICE counterparts. This is due mainly to the additional weight of the battery packs, adding up to 800kg to the weight of the vehicle, directly affecting the braking system design and sizing. The rise in weight is not insubstantial, with models such as the Mercedes-Benz EQV Luxury passenger van approaching three tons, whilst the Audi e-tron 55 SUV and several other luxury models weigh in at over 2,600 kg. Even the lighter mid-range C-segment electric models average at almost 1,940 kg, – more than 600kg heavier than ICE versions.
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Where Lightweighting and Brakes Intersect
Electric vehicle regenerative braking systems are transforming the duty cycle required of foundation brakes, where regenerative systems account for the majority of vehicle braking during day-to-day driving. However, conventional “foundation” braking systems cannot yet be dispensed with, and now provide the back-up braking system on E-vehicles. Foundation brakes are an absolute necessity in emergencies, but also a more basic reason is that regen. systems are not effective if the battery charge is greater than 80 percent and require foundation braking to provide a means of bringing the vehicle to a controlled and complete stop, plus providing parking braking.
The increased weight of electric vehicles results in the prospect that larger, and hence heavier, foundation brakes need to be carried around on E-vehicles for years, even though the systems are largely under-utilised for their design. On top of this, the extra weight from a heavy braking system has a disproportionately large impact on efficiency.
This is because braking systems are part of the vehicle’s unsprung mass, where heavier unsprung weight is never welcomed by vehicle designers, requiring additional material and hence weight, elsewhere, e.g. in the suspension system to control vehicle handling.
On the other hand, whilst larger and consequently heavier braking systems will be required, duty cycles promise to be drastically reduced, leading to much lower pad/lining wear, directly affecting after-market pad sales. Whilst claims of brake pads never needing to be changed (Musk 2018) might be overblown, it is commonly believed that corrosion will become the primary reason people will replace brake pads, – as is often the case for current cast-iron brake discs.
Another impinging factor is the Euro 7 emissions standards, proposals due to be presented to the European parliament at the end of 2021 likely be adopted in the fourth quarter of 2021 and put into force around 2025. This initiative, which is part of the European Green Deal, will develop stricter emissions standards (Euro 7) for all petrol and diesel cars, vans, trucks and buses.
NEE (Non-Exhaust Emissions) from road traffic refers to particles released into the air from brake wear, tire wear, and road surface wear. These emissions arise regardless of the type of vehicle and its mode of power and contribute to the total ambient particulate matter burden associated with human ill-heath.
Whilst legislation has been effective at driving down emissions of particles from the exhausts of internal-combustion-engine vehicles, the NEE proportion of road traffic emissions has increased. This is certainly set to continue with the upward expansion of the EV market.
Brake wear is estimated to contribute up to 55 percent by mass to non-exhaust traffic-related emissions1. However, lighter duty cycles for foundation brakes would automatically lead to lower brake pad wear, and other emission sources such as corrosion products will form a higher proportion of emissions, where Iron in the form of rust is the most abundant metal (up to 60 percent) found in brake wear debris2.
Particle-induced X-ray emission analysis of wear debris revealed that Fe dominated metallic content in both fine and coarse fractions, while TEM analysis showed the presence of maghemite (γ-Fe2O3), magnetite (FeO-Fe3O4), amorphous carbon in the nanoparticle fraction, γ-Fe2O3, FeO-Fe2O3, and hematite (α-Fe2O3) in the fine fraction3. Suffice to say, controlling emissions in the future is likely to involve controlling corrosion in braking systems.
The main sources of iron-rich emissions are from the pad friction lining, the iron disc and corrosion products from the 4mm -10mm thick, mild-steel backing plates conventionally used as part of all pads.
One answer is to galvanising the steel –where NRS brakes have introduced this technology where galvanized pads targeting the EV market are now available for several models including the Audi E-tron SUV. Given the need to reduce vehicle weight, especially unsprung weight, but also to meet Euro 7 standards and drive down further on NEE emissions, manufacturers are starting to consider more radical innovations in the traditionally very conservative area of foundation disc brakes.
One of these is the full-composite brake pad – where the steel backing plate is replaced with a polymer composite alternative, offering the prospect of a corrosion-free solution and 75-percent reduction in weight compared to steel, plus other advantages such as reduced thermal conductivity and superior NVH damping characteristics.
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The idea has been around for some time, with Phenolic resin maker Sumitomo Bakelite Corporation announcing a polymeric backing plate technology in an SAE technical paper in 20124.
Sumitomo have also teamed up with NRS brakes to produce a lighter hybrid metal/phenolic resin backing plate5.The main challenges for this technology surround composite stiffness and how well backing plate stiffness is retained at elevated temperatures of 150oC+.
The plate must remain rigid and intimately bonded to the friction lining even after multiple high pressure and high-temperature duty cycles. The plates must also experience no adverse effects from other environmental conditions (e.g. moisture, freezing etc.).
The University of Exeter in collaboration with a consortium of materials suppliers, brake manufacturers EBC Brakes Ltd and 3 OEM’s have sought a workable solution robust enough to be used within the electric vehicle sector. The innovation was one aspect of a four-year study into lightweight foundation braking systems for electric vehicles, funded by the U.K. government.
The Exeter solution involves the use of a multi-layered ply structure, where layered phenolic prepreg (woven and uni-directional fiber) sandwiching a specially formulated phenolic SMC (Sheet molding compound) Developed by partner FTI Group (Fiba Tech Industries) – a specialist provider of phenolic composite materials. The resulting plates are optimized for high stiffness at elevated temperatures and can be either bonded like conventional backing plates or co-molded and cured, along with the pad lining, in one operation making for very efficient fabrication, with no need for further treatments.
Prototype plates have been subjected to a series of industry-standard tests where results look promising. ECE Regulation 90 dynamometer evaluation and AK Master test protocol have been applied where no discernible differences were encountered in full-composite pads compared to conventional.
Shear testing is the key strength test when it comes to bond integrity, where the ISO 6312:2010 standard brake linings shear test procedure was applied. Room temperature shear strength performance was over three times higher than required by R90, and still double the requirement after a the very tough 400oC, 90-minute heat soak test.
Additionally, composite pads have been tested under compression both at room and elevated temperatures, where pedal feel was simulated to optimize the structure and achieve a similar response to brake pads using a steel backing plate. The pads were also subjected to a series of environmental tests, tested in the part-worn condition, and put on several different vehicles to gauge performance under normal driving and under extreme race conditions using a Mazda RX-8 track test car. Longer term on-vehicle tests are still on-going but look promising.
An important additional benefit is the thermally non-conductive nature of the composite plates as opposed to steel. Lower thermal conductivity plates directly reduce heat transfer from the disc/lining interface, into the caliper and onwards into the brake fluid, where the fluid wet boiling point can be as low as 120oC, leading to catastrophic brake failure.
Manufacturers are particularly concerned with this eventuality and conduct exhaustive tests involving repeated Vmax to zero decelerations or rapid alpine descents followed by a heat soak, to mimic the very worst conditions imaginable.
Disc temperatures are known to rise to over 700oC during such tests which is then allowed to percolate through the braking system during the stationary “soak”. Brake fluid temperature is monitored, and the brakes must remain effective at all times. A low thermal conductivity composite plate would provide another line of defence in such circumstances.
Composite plates may well also help with tackling long-standing Noise Vibration and Harshness issues (NVH) that have dogged the industry for years. The natural damping properties of composite plates possess different resonant harmonics to conventional pads offering an interesting aspect yet to be fully explored.
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[1] Riediker et al., 2008; Bukowiecki et al., 2009a; Gasser et al., 2009; Harrison et al., 2012; Lawrence et al., 2013.
[2] Gadd and Kennedy, 2000; Chan and Stachowiak, 2004; Kukutschová et al., 2011,
[3] Kukutschová J, Moravec P, Tomášek V, Matějka V, Smolík J, Schwarz J, Seidlerová J, Šafářová K, Filip P (2011) On airborne nano/micro-sized wear particles released from low-metallic automotive brakes. Environ Pollut 159:998–1006.
[4] Inokuchi, H., “Integral Molded Brake Pad with Long Fiber Thermosetting Molding Compound for Automotive Brake System,” SAE Technical Paper 2012-01-1834, 2012, https://doi.org/10.4271/2012-01-1834.
.[5] https://thebrakereport.com/sbcl-and-nucap-develop-composite-material-to-handle-extreme-vehicle-heat
