The wheel–rail contact is a safety critical interface. Wear, particle emission and adhesion are all wheel–rail contact phenomena and are discussed here. All three phenomena are material and system parameters and are linked together. Different countermeasures to one phenomenon such as adhesion enhancement with a friction modifier can increase the wear in the contacting bodies. The wear of railway wheels and rail is linked to the number of airborne particles generated, but the exact number and size distribution of the aerosols is unknown. The main objective of this study is to review the recent work in this field and to discuss future trends.

Airborne wear particle emission has been investigated in a pin-on-disc tribometer equipped with particle analysis equipment. The pins are cut out from commercial powder metallurgy automotive brake pads as with and without copper content. The discs are cut out from a commercial grey cast iron automotive brake disc as cut out and as in addition to a laser cladded with a powder mix of Ni-self fluxing alloy + 60% spheroidized fused tungsten carbide and then fine-ground. Dry sliding wear testing runs under a contact pressure of 0.6 MPa, sliding velocity of 2 m/s and a total sliding distance of 14,400 m. The test results show both wear and particle emission improvement by using laser cladded discs. The laser cladded discs in comparison to the reference grey cast iron discs do not alter pin wear substantially but achieves halved mass loss and quartered specific wear. Comparing in the same way, the friction coefficient increases from 0.5 to 0.6, and the particle number concentration decreases from over 100 to some 70 (1/cm³) and the partition of particles below 7 µm is approximately halved.

Brake-related airborne particulate matter contributes to urban emissions in the transport sector. Recent research demonstrated a clear dependence of the number of ultra-fine particles on the disc brake temperature. Above the so-called transition temperature, the number of ultra-fine particles increases dramatically (several magnitudes). As for exhaust emissions, part of the emissions released during braking can be in the volatile fraction. For this reason, a disc brake test stand specifically designed for aerosol research was equipped with three different aerosol sampling instruments: (i) a standard cascade impactor, (ii) a cascade impactor operating at high temperature with a heated sampling line, and (iii) a standard cascade impactor with a thermodenuder. Tests with a brake assembly representative of European passenger vehicles were executed, and the concentration of released airborne particles was determined. The results showed a decrease by several magnitudes in the concentration (in the size range of below 200 nm) using the cascade impactor operating at 180 °C with the sampling line heated to 200 °C. A further decrease in the concentration of airborne particles with size fractions below 200 nm was measured using a standard cascade impactor with a thermodenuder heated to 300 °C.

Non-exhaust emissions from brakes and tyres are becoming the major transport-related contributor of particulate matter (PM) pollution in cities. Furthermore, tyre microplastics are the major contributor of unintentionally released microplastics in all environmental compartments. The European Union introduced for the first time worldwide limits for brakes (PM₁₀) and tyres (total abrasion mass) with the Euro 7 regulatory step. Thus, the interest in brake and tyre particles regarding health and environmental impacts has significantly increased in recent years. In this review, we summarise studies that assessed the impact of brake and tyre particles on human, mammalian, aquatic, and terrestrial cells and organisms. Furthermore, we summarise the studies that compared the impact of brake and tyre particles to other sources. We also critically examine the sampling methodologies of brake and tyre particles for health and environmental impact studies.

In trains with tread brakes, the coefficient of friction between the brake block and the railway wheel determines the stopping distance. The blocks have traditionally been manufactured from cast iron. Although these blocks have good braking capacity, their use is often restricted due to the squealing noise they emit. Tests of alternative composite block materials have been successful under summer conditions; however, in regions with snowy winters the use of such materials has been limited due to problems with braking capacity under snowy conditions. This research aims to develop a laboratory-scale test methodology for evaluating the braking capacity of tread brake materials under winter and snowy conditions. A pin-on-disc machine placed in a climate chamber was used for testing, and a block of standard cast iron was compared with blocks of standard composite materials. The results indicated that the blocks of standard composite materials generate a much smoother surface on the counter wheel and a significantly lower friction coefficient under snowy conditions. A second test series evaluated blocks of alternative composite materials, and a candidate material with low noise and a sufficiently high sliding friction coefficient was selected for further study. A third test series examining geometrical changes in the contact surface in terms of milled parallel tracks was performed; it revealed that the braking capacity under winter conditions can be increased by milling actions if the parallel tracks are properly oriented – in this case, at an angle of 45° to the sliding direction.

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.