Formation characteristics [8, 9, 18] and the effect of train velocity on the formation characteristics [8, 18] confirm prior results under dry conditions in both particle categories. At all train velocities, the total NC dependence of ultrafine and fine particles on the slip rate showed an increasing–decreasing trend, with a transition occurring at an approximately 1% slip rate (Figs. 2(a) and (b)). The maximum NC of ultrafine and fine particles and the NC at the diameter of the predominant peak increased and the particle diameter at the predominant peak decreased with train velocity (Fig. 6(a)). The total NCs in both particle categories increased with train velocity (Fig. 3). Fine particles were dominantly formed at all train velocities, although the average proportion of fine particles in the total measured nanoparticles decreased from 63.2–52.1% with train velocity.
The contact temperature increases with the slip rate [5, 6] and train velocity [5]. Nanoparticles are mainly produced by thermal processes [7], and the amount generated increases with the contact temperature [22, 23]. Thus, increasing slip rate and train velocity are generally associated with an increase in nanoparticle generation due to the increased contact temperature. Here, the total NC of ultrafine and fine particles increased with slip rate up to 1% and increased faster at higher train velocities. However, the formation of both particles categories began to decrease at all train velocities at a slip rate of approximately 1% (Figs. 2(a) and (b)). According to Sundh et al. [24], a sudden change in the NC of generated nanoparticles indicates a transition in the wear mechanism. The transition to pure sliding from rolling/sliding contact is generally observed at a slip rate between 1% and 2% under dry conditions [25]. This contact transition may cause changes in the wear mechanism and lead to the decrease in particle production seen in this study.
Nanoparticles generation tests have never been performed previously above the train velocity of 103 km/h. The results under dry conditions presented here demonstrate an interesting trend at this velocity up to a slip rate of 1% (Fig. 2(a)): the maximum NC of ultrafine particles at 113 km/h was much greater than that at 90 km/h. Meanwhile, the NC of ultrafine particles increased sharply at 113 km/h, at a slip rate of 0.5–1%. Additionally, the maximum NC of ultrafine particles was higher than that of fine particles, whereas prior studies conducted by Lee [8, 9, 18] reported the opposite trend at train velocities ≤ 102 km/h (i.e., the maximum NC of fine particles was greater than that of ultrafine particles). The predominant peak diameter at 113 km/h was categorized as ultrafine particles, i.e., approximately 80 nm, whereas the peak diameters at 73 and 90 km/h were categorized as fine particles, i.e., approximately 170 nm (Fig. 6(a)). The proportion of ultrafine particles at 113 km/h was much higher than those at 73 and 90 km/h at the entire slip rate (approximately 36.8%, 36.2%, and 47.9% at 73, 90, and 113 km/h, respectively) and at low slip zone (approximately 45%, 50%, and 63% at 73, 90, and 113 km/h, respectively).
In addition to the increased generation of nanoparticles owing to the increasing contact temperature, the development of thin oxide layers may have amplified the generation of ultrafine particles at 113 km/h, particularly in a slip rate of 0.5–1%. According to Liu et al. [17], thin oxide layers that generate mostly ultrafine particles can be generated when the contact temperature increases rapidly. Therefore, it is likely that the contact temperature increased rapidly from a slip rate of 0.5–1% at 113 km/h, thus causing the generation of a significant amount of ultrafine particles via thermal processes and the wear of thin oxide layers. Measurement of the contact temperature and particle composition analysis are needed to verify this hypothesis in future studies.
Applying water changed the generation characteristics in both particle categories. At all train velocities, the total NC of ultrafine and fine particles increased up to a slip rate of 0.5%; a near-constant NC was then maintained (Figs. 2(c) and (d)). The predominant peak diameters under water-lubricated conditions (approximately 120, 80, and 30 nm for 73, 90, and 113 km/h, respectively) were considerably smaller than those generated under dry conditions (approximately 170, 170, and 80 nm for 73, 90, and 113 km/h, respectively), and they shifted from the category of fine to ultrafine particles as the train velocity increased (Fig. 6(b)). In addition, the NCs at approximately 10 nm were much higher than those produced under dry conditions, and they increased with train velocity. The measured total NC of ultrafine particles across the entire slip range studied at each train velocity was higher than that of fine particles (Fig. 3); therefore, the proportion of ultrafine particles was higher than 60% at all velocities studied: 66%, 74%, 76% at 73, 90, 113 km/h, respectively.
The train velocity also influenced the generation characteristics of ultrafine and fine particles under water-lubricated conditions, as the total NC of ultrafine and fine particles generated across the entire slip range from wheel–rail contacts increased with train velocity (Fig. 3). The NC at the peak diameters increased with train velocity, and the diameter at the predominant peak was shifted from approximately 120 to 30 nm (Fig. 6(b)). Ultrafine particles were dominantly generated at all train velocities, and the proportion of ultrafine particles increased from 66–76% with train velocity.
Tap water generally contains various minerals such as Na, Mg, Si, and Ca [26]. Small water droplets (≤ 5 µm in diameter) easily evaporate owing to the Kelvin effect [27]. Using a lab-scale nebulizer, Krames et al. [27] demonstrated that vaporizing sprayed tap water leaves residuals (i.e., mineral crystals) in the air with a PSD peak at approximately 30 nm. When a tap water stream is applied to the contact point, most of the water stream bounce off of the disks’ surfaces as small water droplets. Only a small amount of the water stream applied forms a water film at the contact interface. Further, the high contact temperature causes a portion of the water film to evaporate; evaporated droplets can form water vapor via rapid condensation upon meeting the ambient air. However, the formed residuals remain in the air. Therefore, the observed PNC of particles ≤ 35 nm under water-lubricated conditions may comprise mostly mineral crystals and some water vapor.
The reduction rate of particles approximately 10 nm in size was negative for all train velocities (Fig. 4) and all slip rate ranges (Fig. 5). Further, high NCs of particles of this size were present for all train velocities (Fig. 6(b)) and all slip rate ranges (Figs. 7(d)–(f)). See and Balasubramanian [28] reported that the highest NC peak occurred at a diameter of approximately 10 nm when boiling water. During typical train operations under dry conditions, contact temperatures can reach hundreds of degrees Celsius [29], and a flash temperature of up to 900°C at the asperity level can occur [5]. Although applying water reduces the contact temperature, it likely remains above the boiling point of water. Thus, a portion of the water film evaporates, leading to the formation of water vapor via condensation. The amount of water vapor may increase with increasing train velocity and slip rate due to the higher contact temperature. As the train velocity and slip rate increase, the contact temperature increases. Thus, more water evaporates, leading to the formation of more water vapor. This conclusion was supported by the much higher NC of particles approximately 10 nm in size under water-lubricated conditions than under dry conditions for all train velocities and by the increasing NCs at this diameter with increasing train velocity (Fig. 6(b)) and slip rate ranges (Fig. 7(d)–(f)). Therefore, water vapor is likely the main cause of the peak at approximately 10 nm for all train velocities under water-lubricated conditions.
The increase (i.e., negative reduction rate, Figs. 4 and 5) in particles between approximately 10 and 35 nm in diameter under water-lubricated conditions and with increasing train velocity (Fig. 6(b)) and slip rate (Figs. 7(d)–(f)) was likely owing to the formation of various mineral crystals. An increased train velocity causes water film to exit the wheel–rail contact interface faster, whether as water droplets or via evaporation due to the high contact temperature. Evaporated water film and water droplets can cause mineral crystals to linger in the air. The evaporated water film can also form ultrafine particles by condensing on preexisting nanoparticles in the air. Thus, particles approximately 10–35 nm in diameter could be mostly mineral crystals and ultrafine particles formed via condensation. This phenomenon may increase with train velocity and slip rate, as shown in Figs. 4, 5, 6(b), and 7(d)–(f).
The reduction rate of ultrafine particles decreased considerably with increasing train velocity from 69% at 73 km/h to 54% at 113 km/h, whereas that of fine particles was consistently high, i.e., above 87%, as summarized in Table 2. As detailed above, more ultrafine particles ≤ 35 nm in diameter were generated with increasing train velocity owing to generation of water vapor and mineral crystals. Adding a water lubricant increased the NC of particles of this size, thereby drastically reducing the overall reduction rate of ultrafine particles with train velocity.
The increased NCs of ultrafine and fine particles with train velocity can be explained by three main interconnected phenomena. First, the sliding velocity is higher at a higher train velocity at the same slip rate. Thus, a higher contact temperature occurs at a higher train velocity. Second, water film exits the wheel–rail interface faster at higher train velocity, decreasing the thickness of the water film. This may lead to more asperity–asperity contact and/or more contact between oxide layers at the interface, thus increasing the contact temperature. Third, water film in any local area where a high flash temperature occurs evaporates instantaneously, causing locally dry conditions [30], which then can also lead to more asperity–asperity contact and/or more contact between oxide layers at the interface, thereby leading to a higher contact temperature. For each of these phenomena, increasing the contact temperature causes more water film to evaporate, resulting in more ultrafine and fine particles forming by condensing onto preexisting nanoparticles. Additionally, increasing the contact between asperities and/or oxide layers can lead to the generation of more metal nanoparticles; in particular, contact between oxide layers generates mostly ultrafine particles [17]. Overall, the combination of these three phenomena may result in increased NCs of ultrafine and fine particles with train velocity under water-lubricated conditions.
Multimodal or trimodal PSDs were seen in Fig. 6, with the highest peak occurring at diameters > 10 nm for all tested velocities, owing to the presence of mineral crystal and metal AWPs formed at the wheel–rail contact. Mineral crystals could be measured when water film and water droplets evaporate, and their sizes are mostly below approximately 40 nm. Additionally, metal vapors and metal AWPs are formed at the wheel–rail contact interface with water vapor even under water-lubricated conditions owing to the contact between asperities and oxide layers. These mineral crystals and metal vapors may also contribute to the PSD peak at approximately 10 nm; further, these particles may have been enlarged by coagulation and condensation, as the same processes occurring onto preexisting mineral crystals and metal particles or water vapors. Thus, these enlarged particles may contribute to the peaks at particle diameters > 10 nm and NCs at particle diameters greater than approximately 35 nm at each velocity under water-lubricated conditions.
Overall, applying water effectively reduced the NCs of both ultrafine and fine particles at all train velocities. In particular, the NCs of fine particles were reduced by 91%, 89%, and 87% at 73, 90, and 113 km/h, respectively. Water addition also reduced the NCs of ultrafine particles by 69%, 47%, and 54% at 73, 90, and 113 km/h, respectively. This reduction in both particle sizes under water-lubricated conditions is likely due to the reduction of contact temperature, which is the main factor causing nanoparticle generation. Applying water can cause boundary lubrication by forming a water film at the contact interface [19] and causing the development of heavy oxide layers on the wheel and rail surfaces [31]. These two factors can reduce asperity–asperity contacts, resulting in a reduction of the contact temperature. The applied water can also reduce the contact temperature by absorbing the frictional heat and carrying the heat away from the contact interface. Additionally, the water film may trap some of the generated nanoparticles, causing them to be swept away when the water film leaves, rather than becoming airborne [19].