Source: The following is a two part TBR Technical Corner article written by Dr. Raj Shah, a Director at Koehler Instrument Company, Kora Farokhzadeh, Ph.D., an application scientist at Bruker Nano Surfaces, Tribology, Stylus and Optical Metrology Group, and Blerim Gashi, a student in the Department of Material Science and Chemical engineering at State University of New York (SUNY), Stony Brook. The authors open by examining the environmental impact of brake wear, then follow by analyzing emerging technological trends aimed at mitigating these issues. The second part of the article will appear in tomorrow’s edition of The BRAKE Report.
HOLTSVILLE, N.Y. – A majority of attention, both in research and regulatory legislation, is primarily placed on exhaust-related pollution. During operation of a vehicle, particulate matter can also be emitted through mechanical processes, including tire and brake wear, and resuspension of road dust particles. These emissions are classified as non-exhaust emissions, and comprise a considerable proportion of total vehicular emissions, specifically airborne particulate matter (PM). PM pollution has been identified as a possible cause of cardiovascular and pulmonary diseases and reduction of life expectancy. Despite this, research in the non-exhaust emissions sector is minimal and vital information in preventing further pollution is lacking. As society pushes forth towards a greener future for the environment, though, innovation in ‘zero-emission’ electric vehicles (EVs) is accompanied by several challenges such as driving range and non-exhaust PM emissions due to weight differences. One of the strategies developed and adopted to further decrease non-exhaust emissions is regenerative braking.
The regenerative braking system (RBS) harvests the kinetic energy of the vehicle for storage and re-use, saving up to 60 percent of the kinetic energy otherwise wasted in the form of brake dust and frictional heat dissipated in the surrounding environment. Although insufficient braking torque from the RBS requires conventional friction braking to remain present, the regenerative efficiency associated with the RBS, coupled with reduction of brake wear/ pollutant byproducts, and increased driving range cements the development of the RBS and EVs in the automotive industry.
Impact of Brake Wear on the Environment
Friction brake wear emissions, both in the gaseous form and particulate matter (PM), considerably contribute to air pollution, leading to dire implications on the environment and human health. In urban areas with frequent braking and stop-and-go driving patterns, brake wear emissions account for 55 percent of non-exhaustive emissions (NEEs), and contain particulates small enough, PM10 or PM2.5, to become airborne and inhalable. In addition to their threatening mobility in human respiratory system due to their fine size, they often contain toxic elements (heavy metals, polycyclic aromatic hydrocarbons and phenols, etc.) associated with different types of cancer as well as high oxidative potential causing cardiovascular and inflammatory complications.
Brake Dust: The Inconvenient Truth
Despite its importance, reliable estimation and in-depth characterization of NEE particulate matter is quite challenging due to many contributing factors including but not limited to, geographical/climate variations, road surface characteristics, vehicle types, and traffic conditions. In a global effort, researchers around the world, have focused on the emission sources and characterization of airborne non-exhaust particles collected from roadside environments. Thorough reviews of recent worldwide investigations have been published by Amato et al. and Pant and Harrison.
The chemical composition and size distribution of non-exhaust particles covers a wide spectrum, mostly because their processes of formation are stochastic and involve mechanical abrasion, grinding, crushing, oxidation and corrosion. Hence, uncertainties overshadow identifying their sources and toxic potency. Targeted regulations initiated with banning asbestos in brake pad formulations, followed by recent restrictions to monitor the concentration of Cu, Cd, Ni, Zn, CrVI, Pb, Sb, Hg and their compounds in friction brake materials. Brake pads are broadly classified as metallic, semi-metallic, and non-asbestos organic (NAO), and contain a variety of ingredients such as organic resins, metal sulfides, titanates, barium sulphate, antimony sulfide, metal oxides and silicates, graphite, mica, carbon fibers, glass, steel, silica, and brass. Notably, the detection of one or several of these elements (Sb, Ba, Cu, Fe, Zr, Zn, Cd, Cr, Mo, Sn, Pb, Ti, Ni) has been used as key tracers of brake wear emissions.
Systematic analysis of the source(s) of increased PM from higher traffic volumes is the vital first step in the goal towards reducing atmospheric pollutants. Investigations focused on wear emissions from different classes of brake pads – namely, NAO (high Cu), low-metallic (low Cu) and semi-metallic (no Cu) – with respect to their endotoxin content reported that in terms of pulmonary inflammation, NAO particulateemissions had the highest toxic potential. This was attributed to the chemical composition of the released particles, containing Cu, Ti, Ba, and TiO2 (anatase). It is equally important to keep in mind the PM emission rate (wear resistance) of the brake pads. For instance, higher wear rate of Cu-free pads, even if less toxic, increases exposure concentrations and may result in higher biological adverse effects. Thus, factors affecting brake wear directly contribute to emitted brake wear particles during various braking behaviors such as vehicle weight, speed, and deceleration which in turn induce rotor temperature, sliding speed, and contact pressure.
Al-Thani et al. studied the chemical composition and shape and size distribution of PM from NEEs in Qatar, and further consolidated the prominence of brake wear as a source of traffic-related air pollution. They reported a wide size distribution of sub-micron to 600 μm with high S, Ca, Si, Ni, and Cr contents and noted the significant effect of heavy-duty diesel vehicles on traffic dust composition and distribution. Importantly, the appearance of Fe as well as Ba, Sb, Cu, Si, Ti, Ni, Zn, Cr and Pb in PM chemical compositions was considered characteristic indicators of brake wear pollution. As such, PM10 pollution sourced by brake wear made up 19 percent of the total NEEs pollution recorded (Figure 1).
A similar investigation was carried out by Amato et al., which analyzed PM10 emissions in three European urban environments: the Swiss city of Zurich, and the Spanish cities of Barcelona and Girona. The detection of Fe, Cu. Zn, Cr, Sn and Sb in compositional analysis of particles convincingly indicated brake dust as a key contributor in addition to tire wear and road dust resuspension. The presence of Ba (all friction brakes contain barium sulfate), as well as Fe, Cu and Sb was also utilized as tracer indicators for brake-induced airborne particles sourced in London urban and suburban areas and roadside environments of Birmingham and New Castle, U.K. Source apportionment was done quantitatively based on a Positive Matrix Factorization (PMF) outlined in Tapper et al., yet also focused on tracer species like the previous study, specifically Sn and Cu. The loadings (mg m-2) of the samples were measured for various streets in the cities, while also taking into account vehicle day-1 and average vehicle speed. In grouping prominent components of samples found in varying traffic volumes, as depicted in Figure 2, a relationship between absolute loading, or PM10, and traffic volume can be observed. Clearly, as the volume of traffic increases, the loading of PM less than also increases. More specifically, from <15 kveh to 15-40 kveh day-1, different by a factor of 2, the pollutant loads ( m-2) also increased by a factor of 1.2-2.2. At road sites with high traffic volume >40 kveh day-1, road dust loadings also increased by a factor of 2.6-10.1. It is important to mention that many of the high traffic volume samples were taken from Barcelona. Nonetheless, crustal species such as Si or Na, which are not generated from road traffic, varied independently from traffic volume. Therefore, a general proportional relationship between traffic volume and PM10 pollution is most probable.
As mentioned, Cu and Sb are key indicators in sourcing PM10 to brake wear. As such, Figure 2 represents a clear increase in the loading of both Cu and Sb as the traffic volume increased. The higher traffic volume leads to further brake wear, and thus a greater presence of brake dust in the atmosphere. Amato et al. also focused on the relationship between the loadings of Cu/Sb in the road and the concentration of Cu/Sb in the air as determined by previous literature. The emission of PM10 in the air is primarily initiated by traffic resuspension, which is a process dictated by the quality of the roads. However, a distinguishable relationship is observed between air concentration and road loadings in Figure 3. Barcelona, being a city with heavy traffic, had a Cu loading of ~13 μg m-2 and air concentration of ~80 ng m-2, as compared to the light traffic in Zurich with a Cu loading of ~2 μg m-2 and air concentration of ~19 ng m-2. Barcelona experienced a greater air concentration of Cu pollution by a factor of ~4, due to the greater Cu loading produced from brake wear during heavy traffic.
In a recent study, Beddows and Harrison confirmed earlier findings of Timmers et. al. in that electric vehicles generate higher non-exhaust emissions compared with internal combustion engine vehicles due to their increased weight. They estimated the change in total emissions due to the electrification across different road types and concluded that in order to make any reduction in PM emissions from the electrification of vehicles, regenerative braking must be used. Realistic usage of RBS is expected to result in more than 20 percent reduction in overall emissions (PM2.5 and PM10), with higher reduction gain in urban environments (Figure 4).
The data from studies presented herein and similar investigations prove the negative effects of the much neglected problem that is non-exhaustive emissions, namely tire wear, brake wear, and road resuspension. Among the three, improvements in brake technology and mechanical performance provides the simplest and most efficient path towards reducing overall PM10 concentration and pollution due to NEEs in the atmosphere. The introduction of regenerative braking associated with the boom of EVs is a promising and innovative approach, which may hold the solution to brake wear emissions.
Further information about the authors:
Dr. Raj Shah is a Director at Koehler Instrument Company in New York, where he has worked for the last 25 years. He is an elected Fellow by his peers at IChemE, STLE, AIC, NLGI, INSTMC, CMI, The Energy Institute and The Royal Society of Chemistry. A Ph.D in Chemical Engineering from the Pennsylvania State University and a Fellow from the Chartered Management Institute, London, he is also a Chartered Scientist with the Science Council, UK a Chartered Petroleum Engineer with the Energy Institute and a Chartered Engineer with the Engineering council, UK. An ASTM Eagle award recipient, Dr. Shah recently coedited the bestseller, “Fuels and Lubricants handbook,” details of which are available at: https://www.astm.org/DIGITAL_LIBRARY/MNL/SOURCE_PAGES/MNL37-2ND_foreword.pdf
A 2020 recipient of the illustrious Tau Beta Pi eminent engineer title, he is an active volunteer and on Advisory board of directors at several US universities. A recent recipient of the prestigious P.M. Ku medal from STLE, he has more than 300 publications in numerous journals. More information on Dr. Shah can be found at: https://www.che.psu.edu/news-archive/2018/Alumni-Spotlight-Raj-Shah.aspx
Kora Farokhzadeh, Ph.D., is an application scientist at Bruker Nano Surfaces, Tribology, Stylus and Optical Metrology Group. Her role lies at the forefront of new surface characterization techniques, in particular for mechanical and tribology behavior such as wear resistance assessment, evaluation of mechanical properties using indentation hardness and scratch test methods, analysis of friction properties and lubricants performance for a variety of automotive, biomedical and microelectronics applications. She has been collaborating with Southern Illinois University on introducing a new benchtop technique to evaluate the behavior of friction brake materials.
Blerim Gashi is a student in the Department of Material Science and Chemical engineering at State University of New York (SUNY), Stony Brook, where Dr. Shah is an Adjunct Professor and current chair of the External Advisory Board of directors, and Gashi is also an intern at Koehler Instrument Company.