Airborne Particle Emissions Research Articles

Tribology and airborne particle emissions from grey cast iron and WC reinforced laser cladded brake discs

Laser cladding (LC) is a promising technique to overlay a protective coating on grey cast iron (GCI) brake discs to enhance the wear and corrosion resistance. This study utilized a pin-on-disc tribometer in an aerosol chamber to investigate the tribology and airborne particle emissions from tungsten carbides (WC) reinforced coating overlayed onto GCI substrate through laser cladding. Uncoated GCI brake discs served as reference material, while low-metallic (LM) and non-asbestos organic (NAO) brake pads were used as counterparts. The results indicate that LC coating exhibited slightly higher coefficient of friction and significantly lower wear than uncoated GCI discs. Abrasive wear is the dominant wear mechanism for both uncoated GCI brake discs and LC coatings. LC coatings substantially decreased the particle mass concentrations. All three friction pairs displayed a mass weighted size distribution with a major peak around 2–3 μm. The number size distribution was dominated by a mode below 1 μm. Emissions by number were generally low. Meanwhile, all three friction pairs emitted sheared off and agglomerated particles, with iron being the dominant element. Tungsten was identified in the particles emitted from LC coatings, indicating that the hard coating has a potential to wear off and become airborne particles.

Emission and properties of airborne wear particles from train brake friction materials based on novolac phenolic resins and butadiene rubbers

The emission of airborne particulate matter from a train brake depends on the formulation of its friction material. This study investigates the emission and properties of wear particles from train brake friction materials based on straight or resorcinol-modified novolac phenolic resin and nitrile or styrene butadiene rubber used as binding ingredients. The wear particles are generated by a pin-on-disc tribomachine inside an aerodynamic chamber, counted and collected using aerosol measurement techniques, subjected to microscopic and elemental analysis. It has been found that the modification of novolac phenolic resin with resorcinol has no considerable effects on the emission of wear particles and their properties. By contrast, replacing styrene butadiene rubber with nitrile butadiene rubber leads to a significant decrease in the emission of 0.1–10 μm wear particles.

On the impact of pad material ingredients on particulate wear emissions from disc brakes

Besides friction and wear, airborne particle emission has become a central parameter when evaluating disc brake performance due to its potential adverse health effects as component of ambient air pollution. The pad friction material of brake system is composed of a mixture of ingredients grouped into four material categories: abrasives, reinforcing fibres, lubricants, and fillers. Some other ingredients which do not typically belong to aforementioned categories can be attributed to “fixed material”, such as aramid fibre, which is usually added into brake pad to harmonize the overall tribological properties. There is a gap of knowledge about how one ingredient from one category contributes to the emissions of particle number (PN) and mass (PM2.5, PM10). To investigate this, one ingredient from each category was chosen and produced as pins. As a reference, pins made of a commercial European brake friction material were also produced. The pins were tested using a pin-on-disc tribometer designed for airborne emission studies. Coefficient of friction, particle mass and number concentrations were measured during the tests. The results indicate that the abrasive and metal fibre have PN, PM2.5, and PM10 emission factors that are orders of magnitude higher than the lubricant and aramid fibre.

Size effect of CrFe particles on tribological behavior and airborne particle emissions of copper metal matrix composites

Brake wear particles emitted from vehicles during friction have become an important source of airborne particulate matter (PM) that causes adverse health effects. Particle emissions might be reduced by optimizing the composition of friction materials. Effects of CrFe particle size on tribological behavior and airborne particle emissions of copper metal matrix composites (Cu-MMCs) were investigated using a pin-on-disc tribometer at sliding speeds ranging from 2.5 to 5.0 m/s. The results indicate that the frictional properties, particle emission behavior and worn surface characteristics of Cu-MMCs were strongly influenced by the size of CrFe particles. With decreasing the CrFe particle size, the friction coefficient was relatively stable at 3.8 m/s, decreased gradually at 2.5 m/s, and decreased first then increased at 5.0 m/s. Basically, Cu-MMCs with medium-sized CrFe particles (100–150 mesh) produced the highest levels of particle emissions, followed by Cu-MMCs with large-sized CrFe particles (60–100 mesh), and then Cu-MMCs with small-sized CrFe particles (150–250 mesh) in succession. The level of particle emissions from Cu-MMCs increased substantially with increasing sliding speed. The obtained results were explained considering the evolution of the worn surface characteristics during the friction process.

Airborne Brake Wear Emissions from a Battery Electric Vehicle

Although traffic exhaust emissions in Europe have been drastically reduced, airborne particle emissions caused by brakes and tires are still increasing with the number of vehicles. The measurement of non-exhaust emissions is an emerging technological challenge. We present a custom measurement setup to investigate the brake- and tire-wear emissions of an in-use battery electric vehicle. A separate brake housing and HEPA ventilation enabled airborne brake wear emissions to be measured under realistic conditions without external influences. The emission tests on a chassis dynamometer included particle number concentrations and particle size distribution for diameters of 4 nm to 10 μm. Emission indices were determined for three driving cycles: WLTC Class 3b, WLTC Brake Part 10, and a real driving cycle. Further investigations focused on emission control through regenerative braking and brake coating. Driving with regenerative braking reduced emissions by up to 89.9%, which related to the concentration of particles in the ultrafine/fine size range. Hard-metal brake coating led to a further significant reduction in emissions of up to 78.9%. The results point the way to future RDE measurement of non-exhaust emissions and show the potential of regenerative braking and brake coating to reduce airborne brake wear emissions.

Friction, wear, and airborne particle emissions from rail-wheel contact with laser cladded overlays - A pin-disc tribometer simulation

The present study uses a pin-on-disc tribometer to evaluate friction, wear, and airborne particle emissions for a rail-wheel contact. Test pins from UIC60 900A rail carbon steels were in contact with three types of test discs surfaces: R7 wheel carbon steel, laser cladding overlayed martensitic stainless steel, and laser cladding overlayed Ni-based-8% MnS self-lubricating alloy. Test results show about halving of the coefficient of friction, 0.42 to 0.22, and one ten-power lower specific pin and disc wear of discs with self-lubricating overlay compared to standard railway carbon steel contacts. Using stainless-steel overlayed discs also resulted in one ten-power lower specific disc wear, but pin wear is unchanged. Particle emission for the tests with discs with self-lubricating overlay is constant at almost 200 particles/cm3 while running in the distance is needed for the other tests. Almost all generated airborne wear particles were in the sub-100 nm range. The use of laser-cladded (LC) overlay reduced the number of airborne wear particles in the sub-100 nm range by more than a factor of 10.

Effects of abrasive particles on the particulate matter emission of brake friction composites

The correlation between the type of abrasive in the brake friction composite and the braking induced emission was studied. Brake pads with four different abrasives; silicon carbide, alumina, zircon, and magnesia, were evaluated using a reduced scale dynamometer with an airborne particle counter. The results indicated that the particle abrasivity, comprising aspects such as hardness, fracture toughness, and angularity, strongly affected the disc wear and airborne particle emission, whereas the pad wear was more influenced by the strength of the friction film that was changed by the iron moved from the grey iron disc. The fracture toughness of the abrasives had a significant effect on the disc wear rate and mass concentration of airborne wear particles emitted during brake application. In contrast, the number concentration was considerably influenced by the wear mode of the abrasives against the grey iron disc.

Airborne wear particle emission from train brake friction materials with different contents of steel and copper fibres

This study investigated the influence of the amount of steel and copper fibres in a train brake friction material on the tribological performance, emission intensity and characteristics of airborne wear particles. The particles were generated on a pin-on-disc tribometer under controlled friction and environmental conditions. It was found that the steel fibre results in a more intensive emission of 0.3–10 μm particles compared to the copper fibre. The abrasive wear of the steel disc sample is a predominating source of iron in 1–10 μm particles. The content of iron in these particles is proportional to the relative wear of the disc sample, whilst the content of copper increases with that in the friction material.

Impact of vehicle type, tyre feature and driving behaviour on tyre wear under real-world driving conditions

Tyre wear generates not only large pieces of microplastics but also airborne particle emissions, which have attracted considerable attention due to their adverse impacts on the environment, human health, and the water system. However, the study on tyre wear is scarce in real-world driving conditions. In the present study, the left-front and left-rear tyre wear in terms of volume lost in mm3 of 76 taxi cars was measured about every three months. This study covered 22 months from September 2019 to June 2021 and included more than 500 measurements in total. Some of the data was used to evaluate the effects of vehicle type and tyre type on tyre wear. In addition, a machine learning method (i.e., Extreme gradient boosting (XGBoost)) was used to probe the effect of driving behaviour on tyre wear by monitoring real-time driving behaviour. The current statistical results showed that, on average, the tyre wear was 72 mg veh−1 km−1 for a hybrid car and 53 mg veh−1 km−1 for a conventional internal combustion engine car. The average tyre wear measured for a taxi vehicle configuration featuring winter tyres was 160 mg veh−1 km−1, which was 1.4 and 3.0 times as much as those with all-season tyres and summer tyres, respectively. The wear rate of left-front tyres was 1.7 times higher than that of left-rear tyres. The XGBoost results indicated that compared to driving behaviour, tyre type and tyre position had more important effects on tyre wear. Among driving behaviours, braking and accelerating events presented the most considerable impact on tyre wear, followed by cornering manoeuvres and driving speed. Thus, it seems that limiting harsh braking and acceleration has the potential to reduce tyre wear significantly.

New insights into reducing airborne particle emissions from brake materials: Grooved textures on brake disc surface

Airborne particles from brake systems have been widely recognised as a great contributor of particulate matter (PM) in railway environments, such as micro- and nano-size airborne particles. Increasing attention is being paid to airborne particles due to their potential for adverse health effects. In the brake system, particulate matter and wear debris mostly come from the wear of brake pair materials. Reducing-friction and anti-wear effect can be achieved by means of a reasonable grooved texture. In this paper, evaluation of friction, wear and particle emission behaviour of commercial brake pads for high-speed trains were studied using a pin-on-disc tribometer. Effect of grooved textures on the characteristics of airborne particles and deposited debris particles, as well as the wear behaviour of brake materials, was investigated on the basis of the brake discs with different numbers and angles of grooved textures. Results show that the grooved texture on the brake disc can effectively restrain airborne brake particulate emissions, particularly particles in the size range of 1.0 µm ≤ d ≤ 5.0 µm. Compared with the brake discs with the grooved texture, the size of the abrasive particles generated in the non-textured test group become smaller, irregular agglomerates of wear debris are more easily formed, and the debris particles are mostly lamellar with smooth surfaces and sharp edges. The wear rate of the pins against the brake disc with the grooved texture slightly increases, but the surface wear of the brake discs is effectively mitigated. In comparison with the number of grooves, the angle of the grooved texture has a more significant effect on the particle emission and wear behaviour of the brake disc. Additionally, the brake disc with 6 grooves, as well as the angle of the grooves face the friction direction, exhibits the best comprehensive performance.

Measurement report: Interpretation of wide-range particulate matter size distributions in Delhi

Abstract. Delhi is one of the world's most polluted cities, with very high concentrations of airborne particulate matter. However, little is known about the factors controlling the characteristics of wide-range particle number size distributions. Here, new measurements are reported from three field campaigns conducted in winter and pre-monsoon and post-monsoon seasons at the Indian Institute of Technology campus in the south of the city. Particle number size distributions were measured simultaneously, using a scanning mobility particle sizer and a GRIMM optical particle monitor, covering 15 nm to >10 µm diameter. The merged, wide-range size distributions were categorized into the following five size ranges: nucleation (15–20 nm), Aitken (20–100 nm), accumulation (100 nm–1 µm), large fine (1–2.5 µm), and coarse (2.5–10 µm) particles. The ultrafine fraction (15–100 nm) accounts for about 52 % of all particles by number (PN10 is the total particle number from 15 nm to 10 µm) but just 1 % by PM10 volume (PV10 is the total particle volume from 15 nm to 10 µm). The measured size distributions are markedly coarser than most from other parts of the world but are consistent with earlier cascade impactor data from Delhi. Our results suggest substantial aerosol processing by coagulation, condensation, and water uptake in the heavily polluted atmosphere, which takes place mostly at nighttime and in the morning hours. Total number concentrations are highest in winter, but the mode of the distribution is largest in the post-monsoon (autumn) season. The accumulation mode particles dominate the particle volume in autumn and winter, while the coarse mode dominates in summer. Polar plots show a huge variation between both size fractions in the same season and between seasons for the same size fraction. The diurnal pattern of particle numbers is strongly reflective of a road traffic influence upon concentrations, especially in autumn and winter, although other sources, such as cooking and domestic heating, may influence the evening peak. There is a clear influence of diesel traffic at nighttime, when it is permitted to enter the city, and also indications in the size distribution data of a mode < 15 nm, which is probably attributable to CNG/LPG vehicles. New particle formation appears to be infrequent and is, in this dataset, limited to 1 d in the summer campaign. Our results reveal that the very high emissions of airborne particles in Delhi, particularly from traffic, determine the variation in particle number size distributions.

A Qualitative and Quantitative Occupational Exposure Risk Assessment to Hazardous Substances during Powder-Bed Fusion Processes in Metal-Additive Manufacturing

Metal-additive manufacturing (AM), particularly the powder-bed fusion (PBF) technique, is undergoing a transition from the short-run production of components to higher-volume manufacturing. The industry’s increased production efficiency is paired with a growing awareness of the risks related to the inhalation of very fine metal powders during PBF and AM processes, and there is a pressing need for a ready-to-use approach to assess the risks and the occupational exposure to these very final metal powders. This article presents a study conducted in an AM facility, which was conducted with the aim to propose a solution to monitor incidental airborne particle emissions during metal AM by setting up an analytical network for a tailored approach to risk assessment. Quantitative data about the respirable and inhalable particle and metal content were obtained by gravimetric and ICP-MS analyses. In addition, the concentrations of airborne particles (10–300 nm) were investigated using a direct reading instrument. A qualitative approach for risk assessment was fulfilled using control banding Nanotool v2.0. The results show that the operations in the AM facility are in line with exposure limit levels for both micron-sized and nano-sized particles. The particulate observed in the working area contains metals, such as chromium, cobalt, and nickel; thus, biological monitoring is recommended. To manage the risk level observed for all of the tasks during the AM process, containment and the supervision of an occupational safety expert are recommended to manage the risk. This study represents a useful tool that can be used to carry out a static evaluation of the risk and exposure to potentially harmful very fine metal powders in AM; however, due to the continuous innovations in this field, a dynamic approach could represent an interesting future perspective for occupational safety.

Thermal performance and emissions analysis of a new cooling tower prototype

The design of a wet cooling tower involves a number of trade-offs including the efficiency of the device to remove waste heat (thermal performance) and environmental aspects (emissions level). This paper deals with the experimental characterisation of a new cooling tower prototype. The novelty of the work relies on the study of a cooling tower designed to avoid the emission of airborne particles to the atmosphere. The experiments were conducted in a pilot plant built ad hoc for this purpose. The sensitive paper method was used in the environmental impact assessment (emissions level). The comparison between the obtained results and those found in the literature for similar cooling towers indicates that performance of the inverted cooling tower in terms of emissions is remarkable: D = 1 . 47 ⋅ 1 0 − 6 (0.00015% of the circulating water). This value is up to 300 times lower than the limits imposed by several international standards and involves a reduction in terms of emissions ranging from 40.21% to 82.54% compared to commercial towers. The maximum size of the measured escaping droplets is d d = 50 μ m, and the Sauter Mean Diameter of the ensemble of droplets is 31 . 42 μ m. Concerning thermal performance, the observed results are more modest. Operating at nominal conditions, the inverted cooling tower cools the water up to 33.5 °C. The efficiency of the tower is similar to that of a commercial tower equipped with a film-flow distribution system (up to 6.98% better). The difference is larger (41.16% lower) when compared to the same tower equipped with a splash distribution system. • This paper addresses the experimental evaluation of a new cooling tower prototype. • The measured rate of drift emissions is 0.00015% of the circulated water. • The emissions are reduced 40.21–82.54% compared to commercial towers. • The cooling tower cools the water up to 33.5 °C operating at design conditions. • The thermal performance is similar to a commercial tower with a film-flow system.

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

This study investigated particulate matter (PM) produced during brake application, focusing on the effect of space fillers in low-steel friction composites. The composites ranked in terms of brake emissions in a descending order were BaSO4, Ca(OH)2, and CaCO3. CaCO3 produced thicker friction films on the steel fibres than Ca(OH)2, whereas bare steel fibres were found with BaSO4, indicating strong effects of the filler-dependent coverup layers, which was confirmed by microhardness tests. The filler effect on PM suggests that proper selection of space fillers can reduce brake emissions from grey iron discs by preventing direct adhesion with steel fibres.

Tribology and Airborne Particle Emission of Laser-Cladded Fe-Based Coatings versus Non-Asbestos Organic and Low-Metallic Brake Materials

Laser cladding is a promising surface treatment for refurbishing worn-out cast-iron brake rotors. Previous studies on laser-cladded brake rotors have demonstrated their extensively higher wear and greater airborne particle emissions, compared with traditional cast iron rotors. In order to overcome this, a commercial non-asbestos organic (NAO) brake material is tested against Fe-based laser-cladded and traditional cast-iron brake rotors. Two low-metallic brake pad materials are also tested as references. The materials’ coefficients of friction, specific wear rates and particle number concentrations are evaluated. The results indicate that the NAO brake material showed lower wear and had fewer particle emissions than the low-metallic brake materials when deployed against both cast iron and laser-cladded brake rotors. The NAO/laser-cladding friction pairing showed wear, particle concentration and fraction of fine particles (sub 1 μm) equivalent to those of the low-metallic/cast-iron friction pairing, creating significant potential for application in refurbishing worn-out cast-iron brake rotors.

A study of friction, wear and particulate emissions during the bedding stage of a Cu-free friction material

In this study, the tribological behaviour and airborne particulate emissions during a bedding procedure for a Cu-free friction material for brake applications were investigated. Different tribological stages were identified. During the first stage, called run-in, friction coefficient, system wear, and emissions increased. At the end of the first stage, the friction coefficient reached a stable value. On the other hand, wear continued to increase, albeit at an increasingly lower rate, and the emissions of airborne particles started decreasing. Subsequently, also system wear and the emission of airborne particles reached a steady-state stage. The obtained results were explained considering the evolution of the characteristics of the friction layer that forms on the friction material during braking.

Effects of reinforcing fibers on airborne particle emissions from brake pads

The effect of reinforcing fibers in the brake friction material on brake emission was investigated. Fibers made of steel, mineral, cellulose, and aramid were used to reinforce the friction materials, and friction tests were performed using a Krauss-type tribometer in an environmental chamber with an optical particle counter. The results showed that the use of steel fiber and aramid fiber resulted in the highest and lowest brake emissions, respectively. The fibrillation of cellulose and aramid fibers strengthened the friction films on the pad surface and reduced the brake emissions by improving wear resistance, which was promoted by re-embedding of wear debris in the pad surface during sliding. This suggests that airborne particle emission from a brake system could be reduced by reinforcing the friction material with fibrillating fibers. However, the analysis of the settled wear particles in the chamber indicated that the total brake emission could be increased since the large and settled particles from the friction materials reinforced with fibrillating fibers can be resuspended after a collision since they were agglomerated ultrafine particles.

PM10 in underground stations from different types of trains

• A positive correlation between hourly train frequency and PM10 level is verified. • The type of train passing an underground metro platform affects the PM10 level. • In Stockholm metro, the PM10 could be reduced by up to 30% by train type selection. In underground metro systems, airborne wear particles generated during the operation of metro trains are the predominant airborne contaminants on platforms. Previous researchers have found that several factors influence the airborne particle emission rate during metro train operations. However, the effect of train type has rarely been studied. In 2016 and 2020, field measurements were taken on six underground metro platforms in Stockholm, Sweden. Old and new types of trains (CX and C20) were in traffic at the same time. In the present study, based on the field measurements, the correlations between the hourly average PM10 level and the hourly frequency of the two types of trains were studied. A linear mixed model that includes the effect of hourly train frequency and train type was set up. Based on this model, CX trains have a larger particle emission rate than C20 trains.

Tribological and Emission Behavior of Novel Friction Materials

The tribological behavior and the related airborne particles emission of three copper-free automotive friction materials are investigated. The tests were conducted using a pin-on-disc tribometer equipped with a specifically designed clean-enclosure chamber for the emission measurement. Particle number concentration from particle size 0.3 µm up to 10 µm and the mass of emitted particles between 1 µm to 10 µm were measured. Particular emphasis was given to the chemical composition of the bulk materials, the friction layers and the emissions, in order to explain the acting wear mechanisms, and the recorded emission of airborne particles. The results indicate that the recorded emissions do not correlate with the friction coefficient and the wear rates, since the wear mechanisms exert a different influence on the tribological and emission behavior of the materials under study.

Airborne particle and microbiological human emission rate investigation for cleanroom clothing combinations

The control of airborne particles and bio contaminants is a key factor in several industries in order to avoid product defects, to assure process quality and to protect personnel and outdoor environment. Cleanrooms and controlled environments are spaces with strict control of airborne contaminants and thermo-hygrometric parameters. Humans are one of the main sources of contamination in clean environments. A correct technical clothing system reduces the contamination released by humans. Despite substantial work done in this field, it is difficult to compare previous results due to differences in test rig, procedures, gowning systems and sampling locations. This study implements a dispersal chamber test methodology and an experimental campaign on 7 combinations of technical clothing and undergarments. It presents comparative experimental results of total particle and microbiological contaminants released by humans in terms of emission rate (ER). It is found that ageing factor, sterilization, physical movements, donning and warping type, and materials influence the human ER in clean environments. Sterile garment systems entirely composed of synthetic materials decrease the particle release compared to the non-sterile mixed ones. Sterile garment systems show a better performance, achieving 10 to 30 times fewer particle emission rates in wider movements and almost zero microbiological release. A low human contamination ER benefits the cleanroom design and operation, either reducing the energy consumption or increasing the number of personnel in clean environments.