Assessing the Performance of TOR Lubricants in Humid Environments and Under Dew Conditions

Vol.:(0123456789) Tribology Letters (2024) 72:90 https://doi.org/10.1007/s11249-024-01889-7 ORIGINAL PAPER Assessing thePerformance ofTOR Lubricants inHumid Environments andUnder Dew Conditions SimonSkurka1· RadovanGalas1· MilanOmasta1· HaohaoDing2· Wen‑JianWang2· IvanKrupka1· MartinHartl1 Received: 6 May 2024 / Accepted: 3 July 2024 / Published online: 16 July 2024 © The Author(s) 2024 Abstract Top-of-rail (TOR) lubricants are commonly used for friction control in railway operations. They aim to lower friction and reduce noise and wear while ensuring sufficient transmission of traction/braking forces. However, the wheel–rail interface is an open system, so the conditions may suddenly change due to the weather, and different contaminants may enter the contact and influence the performance of these lubricants. Thus, this study examined the effect of humidity and dew on two commercial products, as these conditions often occur on the track. A methodology based on a creep curves measurement approach was used to assess product performance under various scenarios. All measurements were conducted on a universal tribometer in the ball-on-disc configuration covered with a climate chamber. The results show a strong influence of dew on the tested products, as dew lowered their performance parameters and caused low adhesion problems. Possible mechanisms of water–oil interaction and formation of oxidic third body layers were discussed. The main findings indicate that TOR lubricants may cause traction/braking problems if used in dew conditions. The present study may be helpful in optimising friction management methods in the future. Graphical Abstract Keywords Wheel–rail tribology· Friction modification· Humidity· TOR lubricants· Dew· Contamination Extended author information available on the last page of the article Tribology Letters (2024) 72:9090 Page 2 of 19 1 Introduction Railway transportation is an ecologically friendly way of transporting passengers and goods, and its energy efficiency can be further improved by reducing the adverse effects of friction in the wheel–rail (W/R) contact. Too high friction is the cause of excessive fuel consumption [1]. Furthermore, it causes wear of contact surfaces, leading to more frequent wheel changes and rail grinding [2, 3], increasing maintenance costs [4]. Lastly, friction is a source of noise pollution [5, 6]. At the same time, keeping a certain minimum level of friction is necessary. Otherwise, the effectivity of tangential forces transmission from the wheel to the rail will drop, which can result in the vehicle being unable to accelerate and break effectively [7]. A normal and tangential force ratio can quantify frictional conditions in the rolling–sliding contacts. In the literature, this parameter is called the traction (or sometimes adhesion) coefficient (CoT) and depends on several parameters, e.g. load and creepage. Values higher than 0.09 and 0.2 are sufficient for braking and traction, respectively [8]. Top-of-rail (TOR) products were developed to maintain friction at the desired levels, where noise and wear are reduced, but sufficient CoT for traction and braking is maintained. Initially, these products were solid sticks containing lubricants like graphite or molybdenum disulphide. They were used with great success back in the 90s in the Vancouver mass transit system to overcome problems with corrugation [9]. Since then, TOR products have undergone intensive development, and new liquid-form types have been established. Following [10], liquid TOR products can be divided into two groups depending on their base medium type: Friction modifiers (FMs), which are water-based and TOR lubricants (grease/oil-based products). The difference between these two groups lies in the friction modification mechanism [11]: Water in FMs acts as a transport medium and helps to distribute solid lubricants on the rail head. After that, water evaporates, and the friction is reduced by the shear–displacement mechanism of solid lubricants. In the case of TOR lubricants, grease/oil forms a thin lubricating film on the rail head, contact operates in a boundary/mixed lubricating regime, and solid particles in the product composition help maintain the desired CoT levels. Please note that in general tribology, the term "friction modifier" is used for various additives added to the lubricant to modify its frictional properties. However, in W/R tribology, the term is commonly used for one specific group of TOR products to distinguish between different base-medium types [10]. So, in this paper, "friction modifier" (or "FM") always refers to the group of water-based TOR products. Intensive research over the past decade has already proven the benefits of TOR products. Field studies showed that lower friction was measured on rails where TOR products are applied [12–15], resulting in wear [16–18] and noise reduction [5, 6] and even the number of air-born particles generated from the contact decreased [18]. However, the W/R contact is an open system, which means that besides the intentionally applied TOR products, it is also influenced by various contaminants naturally present on the rail [19]. One of the most common is water. Several studies have shown that water is a good lubricant that can reduce CoT significantly and cause low adhesion problems [20–23]. Furthermore, low adhesion problems are typically linked to the so-called "wet–rail phenomenon" [24, 25]. In that case, a small amount of water mixes with iron oxides, which are inevitably present on the rail surface. Together, they form a thick layer with high viscosity, capable of reducing CoT to as low as 0.05 [25]. It was shown that low adhesion problems are most probable in the morning when dew is present on the track [8], concluding that a small amount of water rather than large amounts causes low adhesion problems. Furthermore, several tests in a climate chamber were conducted to examine the effect of humidity on friction [22, 26]. The results show that with the rise in relative humidity (RH) %, the CoT decreases. Moreover, the study [27] found that it is even worse when the dew point is reached and water condenses on the specimen surfaces. CoT drop is significantly more substantial in that case than in the humid conditions without dew. In addition, it was shown that temperature influences this behaviour, as it determines how much water can be contained in the air before dew conditions happen. It was found that the effect of humidity is most substantial for temperatures below 24°C. On the contrary, it is almost negligible for temperatures over 30°C [27]. Even though contamination significantly influences CoT, little is known about the interaction between TOR products and contaminants. Although TOR products are designed to maintain the CoT required for traction/braking, their effect has been tested primarily in laboratoryclean conditions, or the effect of contaminants has not been evaluated. There are only a few studies addressing this problem. In [28], two FMs were tested under water contamination. Water influenced both products and caused a drop of CoT below 0.1 when applied. However, these products contained friction enhancers like stainless steel particles or sand with high particle diameters, which are not typically found in modern TOR products [29, 30]. It can be assumed that the influence of water on products that do not contain such particles can be even more significant. This conclusion is supported by [31], where different amounts of water were applied to three commercial TOR products (one FM and two TOR lubricants). It was shown that water mixes with the oil in TOR Tribology Letters (2024) 72:90 Page 3 of 19 90 lubricants and causes a long-lasting period of very low CoT. This observation is also supported by a study [32] that showed that a mixture of rail oil and water leads to even lower CoT than the oil itself. Finally, in [26], an FM mixed with iron oxide particles was tested in a climate chamber under high-humidity conditions. It was shown that with an increase in % of RH, the performance of FM was influenced, and CoT decreased. It was also noted that for a higher % of RH, FM stayed wet for a more extended period, suggesting that the base medium evaporates slowly in a humid environment. Although the increase in % of RH caused a decrease in CoT, it stayed in the interval of desired friction levels as it did not drop below 0.2. Although testing of FMs in a humid environment did not result in low adhesion in the study [26], it must be noted that as the RH ranged between 40 and 90%, severe water condensation probably did not occur. Previously, in [27], it was shown that there is a significant drop in CoT when dew is present due to the water acting as a lubricant. Furthermore, it was shown in [31] that water influences the performance of TOR products significantly, and oilbased products even cause low adhesion problems in these conditions. So, based on [27, 31], the question arises about what would happen if condensation occurs and dew is mixed with the product. Although this scenario may happen in the field, the interaction between dew and TOR lubricants has not yet been thoroughly examined. So, this study aims to explore the effect of humidity and dew on two commercial TOR lubricants. Experiments follow an approach based on the creep curve measurement described in [33]. A laboratory tribometer in a ball-on disc configuration equipped with a climate chamber was used for all experiments. 2 Material andMethods 2.1 Test Setup andSpecimens A laboratory tribometer MTM (Mini-Traction Machine, PCS Instruments) in a ball-on-disc configuration was used for all tests and wear-ins/run-ins, see Fig.1. The specimens were disc with 46mm in diameter and a ball with 19.05mm in diameter. According to the Hertz theory, the contact area was circular at the beginning with a diameter of 0.2mm (the Poisson's ratio of 0.33 and Elastic modulus of 205 GPa was considered for this material). Both specimens were loaded against each other and driven by independent servo motors so the required slide–roll ratio (SRR) could be achieved according to the given equation: where wball and wdisc are angular speeds of specimens and rball and rdisc stands for its radii. The embedded sensor measures the normal force (N) with ± 0.3 N accuracy with a 1Hz sampling frequency. However, this signal results from averaging a non-specified higher frequency input signal. Traction force (T) is calculated from the torque measured by the transducer attached to the disc shaft, so the traction coefficient (CoT) could be determined as follows: The specimens were made of AISI 52100 bearing steel with a Vickers macro-hardness of 800–920 and 720–780 HV for ball and disc, respectively. Although bearing steel is not a typical rail material, its higher hardness ensures stable conditions during experiments and better repeatability (1) SRR = w ball ⋅ r ball −w disc ⋅ r disc w ball ⋅r ball +w disc ⋅r disc ⋅200 % (2) CoT = T N Fig. 1 Test setup: a climate chamber and b detail of the specimens inside. Part of b redrawn from [31] with the Publisher's permission (Elsevier) Tribology Letters (2024) 72:9090 Page 4 of 19 due to reduced wear compared to standard wheel/rail steel. The choice of specimen material and geometry follows the benchmarking methodology [33] and enables results to be compared between different laboratories. The impact of the selection of bearing steal over authentic rail steal on measured results and all other study limitations derived from the simplified ball-on-disc configuration will be thoroughly discussed in Sect. “Study limitations”. Before tests (after running-in), the surface roughness of specimens was checked using an optical profiler ContourGT (Bruker), and it varied around Ra 0.15µm for the ball and Ra 0.3µm for the disc. The same profiler was used to study topography changes and observe oxide layer formation. The stability of the environment was ensured by a climate chamber mounted on the top of the pot of the tribometer, see Fig.1a. The chamber had two sensors for monitoring RH and air temperature. The first sensor was mounted in the upper part of the chamber and provided feedback to the humidification unit. Based on the data from this sensor, the humidification unit runs the mixture of wet and dry air into the chamber in a specific ratio, achieving the desired value of RH. The second sensor (HTU21D) was placed near the contact area to enable online monitoring of RH and air temperature near the contact inlet. This sensor measures RH in the 5–95% range with an accuracy of ± 2%. It is also possible to measure outside these borders but with slightly lower accuracy. However, this should not affect the tests conducted since the 100% RH was achieved by running only wet air in the climate chamber. The climate chamber did not enable air temperature control, so all tests were conducted at room temperature of 22 ± 1°C. However, due to the rise of air temperature due to heat generated in the contact during tests, the HTU21D sensor was also used for air temperature monitoring with ± 0.3°C accuracy (declared in the range from − 40 to 125°C). 2.2 Tested TOR Lubricants The TOR lubricant used in most tests is referred to as "TORL-A". It is a commercially available product used for friction modification in railway operations. Additionally, several tests were conducted using a second TOR lubricant labelled "TORL-B". These tests investigated whether similar phenomena can occur for products with different compositions, indicating that they are not specific to TORL-A alone. Essential information about the composition of both products obtained from datasheets can be seen in Table1. 2.3 Methodology Studies on friction modification usually use time tests (CoT on time dependency) [17, 30, 31] or creep curve measurements (CoT on SRR dependency) [14, 28, 34] for testing FMs and TOR lubricants. A study [33] shows that results obtained during time tests can be influenced by the everchanging topography of the contact surfaces. On the contrary, results based on creep curve measurements are less influenced by topography changes and enable the study of TOR lubricants in a wide range of conditions typical for railway operations. Also, a higher % of slip prevents the contact path surface smoothening effect (the major drawback of time tests), enabling the testing of TOR lubricants in higher quantities. So, in this study, the authors chose to follow a creep curve measurement approach proposed in [33] to examine the effect of humidity and dew on TOR lubricants, which is the main objective of this paper. As seen in Fig.2, this Sect.“Methodology” is divided into four consecutive parts describing the fundamental principles of the methodology. Readers are encouraged to see the study [33], where the detailed reasoning behind each step was explained. 2.3.1 Cleaning andRunning‑ing Before each test, specimens were cleaned manually with a paper towel and ultrasonically in an acetone bath for 10min. After that, specimens were runned-in for 30min to stabilise surface roughness and remove residual contaminants. The run-in parameters were 2% SRR, a speed of 1000mm/s and a normal force of 18 N (corresponding to 0.8 GPa Hertz pressure for unworn specimens). For a given set of humidity/dew conditions, every test was repeated five times on the same pair of specimens to obtain results from a wide range of conditions, as the contact path width and depth changes during tests due to wear. Before each of these five tests, the run-in was conducted the same way except for the first one, which was extended to 60min to overcome the initial rapid topography changes of unworn surfaces and form a stable contact path. Table 1 Composition and characteristics of tested TOR lubricants TOR lubricant Structure NLGI grade Base oil Thickener Particles Base oil viscosity at 40°C (mm2/s) TORL-A Paste 0 Biodegradable ester Organic Soft metal 41–53 TORL-B Gel paste 00 Synthetic ester Inorganic (silicate) Soft metal 46 Tribology Letters (2024) 72:90 Page 5 of 19 90 2.3.2 Humidification Phase Next, TOR lubricant was applied to the contact path of the disc. Then, the tribometer pot was covered with a climate chamber, and the mixture of wet and dry air in the specific ratio was run inside to achieve the desired % of RH (this step is further referred to as the "Humidification phase"). It can be assumed that water condensation only occurred during this step since after the start of the test, the surface temperature quickly rose due to the heat generated by friction, and water could no longer condense on the specimens. The humidification phase was skipped in Sets 1 and 7 (see Table2), where TOR lubricants were tested under laboratory ambient conditions. 2.3.3 Testing The test starts when the Humidification phase is ended (see Table2). One test consists of repetitive creep curve measurements with the same specimen pair, see Fig.3a. TOR lubricant is applied only once before the first creep curve measurement. So, as the starvation progresses, every consecutive curve reaches higher values of CoT until the threshold criterion is met. This approach enables testing the product under a wide range of topography conditions and evaluating its lasting effect. Every creep curve consists of six points, each evaluated from a 30-s time test as an average CoT at a given SRR value, namely 0, 2, 5, 10, 15, and 20%. Please note that time tests with 0% SRR were only to redistribute Fig. 2 The individual steps of the methodology Table 2 The testing parameters and conditions a Laboratory ambient conditions: RH = 34 ± 2%, temperature = 22 ± 1°C b Tests with RH = 70 ± 5% are labelled as "humid conditions" in the text c Tests with RH = 100% are labelled as "dew conditions" in the text d According to the Hertz theory, 18 N normal force results in 0.8 GPa contact pressure in the given configuration and unworn specimens Set Product Amount (μl) SRR (%) Normal forced (N) Speed (mm/s) RH (%) Humidification phase (min) Specimen temp. (°C) 1 TORL-A 1 0–20 18 1000 Ambienta– Ambienta 2 TORL-A 1 0–20 18 1000 70b20 Ambienta 3 TORL-A 1 0–20 18 1000 70b20 12 ± 2 4 TORL-A 1 0–20 18 1000 100c20 Ambienta 5 TORL-A 1 0–20 18 1000 100c60 Ambienta 6 TORL-A 1 0–20 18 1000 100c20 12 ± 2 7 TORL-B 1 0–20 18 1000 Ambienta– Ambienta 8 TORL-B 1 0–20 18 1000 100c20 Ambienta Tribology Letters (2024) 72:9090 Page 6 of 19 TOR lubricant on the surfaces evenly, and these tests were not used in the evaluation. The threshold criterion was as follows: If two consecutive points exceeded the threshold curve (black dashed line in Fig.3a), the test was ended. If not, another creep curve was measured. "Consecutive points" are two neighbouring SRRs from the sequence 2, 5, 10, 15 and 20% and corresponding values of CoT. Three traction zones located under the threshold curve were defined: Critical traction zone (CTZ), Intermediate traction zone (ITZ) and Retentivity traction zone (RTZ). These zones represent different frictional levels: "CTZ" for levels insufficient for reliable traction/braking, "ITZ" represents levels desired for operating the rail vehicles, and "RTZ" which shows the distance for which the TOR lubricant has a noticeable effect on friction. If a point is placed in "CTZ" (red area in Fig.3a), the CoT decreases to very low values for a given SRR, indicating that over-lubrication occurs. On the other hand, in "ITZ" (green area in Fig.3a), the CoT reaches intermediate values, meaning the product performed well (that is where, on actual track, adverse effects of friction are reduced but sufficient transmission of traction/braking forces is ensured). Finally, "RTZ" is the entire area under the threshold curve and contains all points measured during the test (except those that exceeded the threshold criterion). Lastly, there is a blank area between "CTZ" and "ITZ", in which CoT acquires values that are not considered critical nor optimal. If a point is placed in this area, this does not directly benefit or penalise the product in the evaluation but influences the retentivity and prolongs test time. Exact values for the threshold Fig. 3 a Model example of one test. Each test is repeated five times, so five triplets of "r", "i", and "c" parameters are obtained. b Average parameters "R", "I", and "C" are calculated and used for the "OLF" evaluation. c Product performance is assessed into four categories Q1–Q4 based on "OLF" and "I" for each set of humidity/dew conditions Tribology Letters (2024) 72:90 Page 7 of 19 90 curve and "CTZ", "ITZ", and "RTZ" boundary points were calculated as a defined fraction [33] of dry and clean contact CoT for a given % of SRR and are listed in Appendix A. 2.3.4 Evaluation From each test, three performance parameters were calculated, namely: "c" (critical traction parameter), "i" (intermediate traction parameter), and "r" (retentivity parameter). These parameters represent the cumulative sliding distances for which CoT stayed in "CTZ", "ITZ", and "RTZ" during the test (see Fig.3a) Eqs.3–5) and their unit is one meter. In other words, each parameter is calculated as a sum of weighted distances for which the contact operated in the corresponding zone, where the weight is the value of the corresponding SRR %. The ideal TOR lubricant would aim to maintain the intermediate friction level for as long as possible (represented by the high value of the "i" parameter) and simultaneously reduce the probability of low adhesion problems (zero or low value of the "c" parameter). The values of these parameters are limited by the product retentivity (expressed by the "r" parameter), which shows how long the effect of the product is noticeable (in this methodology, this parameter represents the whole test duration). Five tests were performed for each set of humidity/dew conditions, resulting in the five triplets of these parameters. From those, average parameters "C", "I", and "R" were calculated, and the over-lubrication factor ("OLF") was evaluated, see Fig.3b) Eqs.6–9. The "OLF" describes the probability of over-lubrication (ranges from 0 to 1; the lower the number, the higher the probability of over-lubrication). The ideal product would score "OLF" equal to 1, meaning only desirable CoT values were measured during the test duration. Finally, a 2D Performance map was constructed in which the x-axis contains the value of "I", and the y-axis has the value of "OLF" for each set of humidity/dew conditions (see Fig.3c). In this map, four performance categories Q1–Q4 were defined (these categories divide the performance map into quartiles). To be placed into Q1, both "I" and "OLF" values have to lay in the upper 25% on each axis, for Q2 in the upper 50%, for Q3 in the upper 75% and for Q4 at least one value is in the bottom 25%. If the TOR lubricant scores in Q1, that means that the risk of low adhesion problems due to over-lubrication (or condensed water, in this study) is not significant, and the product maintains the desired frictional conditions for a very long time. A change in the performance category caused by humidity/dew indicates how the product performs under different scenarios that can occur on the rail. 2.4 Test Conditions TOR lubricants were tested in various conditions to evaluate which factors (% RH value, duration of the Humidification phase, etc.) affect their performance the most. TORL-A was chosen for the majority of experiments. However, some experiments were performed with the second lubricant, TORL-B, to ensure that observed phenomenons are not limited to the composition of TORLA alone but can be generalised to other TOR lubricants with different compositions. All the information about humidity/dew conditions and performed tests is listed in Table2. In this table, each set of conditions is labelled as "Set 1–8". The purpose of testing these sets is as follows: (1) Set 1 was conducted under laboratory ambient conditions, specifically RH = 34 ± 2% and temperature of 22 ± 1°C. This set shows how the TOR lubricant performs in "normal conditions" and was used as the base reference. (2) In Set 2, TORL-A was exposed to "humid conditions" (specifically RH = 70 ± 5%) for 20min. (3) Set 3 was also conducted under "humid conditions" (RH = 70 ± 5%) for 20min. However, specimens were first put into the refrigerator and cooled to 12 ± 2°C (measured on the surface). A comparison between Sets 2 and 3 can highlight the importance of the temperature and the dew point. (4) In Set 4, the TOR lubricant was exposed to RH = 100% ("dew conditions") for 20min during the Humidification phase. A comparison between Sets 1, 2 and 4 shows how the change in RH % value influences the performance parameters. (5) In Set 5, TORL-A was exposed to RH = 100% ("dew conditions") for a prolonged duration of 60min. A comparison between Sets 4 and 5 reveals how the duration of exposure to the humidity will affect the performance parameters. (6) In Set 6, TORL-A was exposed to RH = 100% ("dew conditions") for 20min, but first, specimens were put into a refrigerator and cooled down to 12 ± 2°C (same procedure as in Set 3). The difference in temperature between the surface and surrounding air lowers the dew point and speeds up the water condensation. Thus, more water could condense during the Humidification phase in Set 6 compared to Set 4, where the temperature of the specimens was the same as that of the surrounding air. (7) Set 7 was conducted with TORL-B under laboratory ambient conditions, specifically RH = 34 ± 2% and temperature of 22 ± 1°C to obtain the reference for this TOR lubricant under "normal conditions". (8) In Set 8, the TORL-B was exposed to RH 100% ("dew conditions") for 20min during the Humidification phase. Sets 7 and 8 were designed to validate results for TOR lubricants with different compositions. Tribology Letters (2024) 72:9090 Page 8 of 19 2.5 Additional Experiments Before the tests mentioned above, several experiments were performed to quantify how much water condenses on the disc surface during the Humidification phase for conditions defined in Table2, Sets 2–6 (and 8). Only the Humidification phase was performed in these experiments (without any running-in or testing; the goal was to quantify the amount of water). Then, the discs were weighed, and the amount of water was calculated based on the disc weight difference before and after the Humidification phase. So, the change in TOR lubricant performance could be linked to the explicit water quantity. 3 Results andDiscussion First, experiments defined in Sect.“Additional Experiments” were performed to quantify water amounts that condense on the disc surface during the Humidification phase. The results of these experiments can be seen in Table3 (an average value of 5 measurements). Please note that these are the quantities condensated on the entire disc surface. It is expected that most of the water did not enter the contact because it did not lay in the contact path, as can be seen in pictures from the optical microscope, see Fig.4. So, there are two more columns in Table3: corresponding amounts of water in the contact path and amounts of water in W/R contact. These values were calculated based on the ratio between the entire disc surface area, contact path area, and actual W/R contact area. Note that the "contact path area" was measured using an optical profiler, and an approximated value of 1 cm2 was used as the "actual W/R contact area" based on the literature [11]. 3.1 The Effect onTOR Lubricant Creep Curves The results for Sets 1–8 are summarised in Fig.5. From each Set, only one out of five tests was randomly selected to be displayed for the sake of clarity. However, average Table 3 Weighed amounts of water during the Humidification phase and corresponding amounts for the contact path and actual W/R contact a Calculated based on the disc surface and contact path area ratio b Calculated based on the ratio between the MTM and the actual W/R contact areas Amount of water Set 1 (7) Set 2 Set 3 Set 4 (8) Set 5 Set 6 On disc (μl) – 0.2 0.72 7 30 180 In contact path (μl)a– 0.007 0.03 0.26 1.2 7.2 In W/R contact (ml)b– 0.01 0.04 0.37 1.7 10.3 Fig. 4 Photo of the contact path after the Humidification phase in Sets 1–8. A white dashed line highlights water droplets lying in the contact path Tribology Letters (2024) 72:90 Page 9 of 19 90 performance parameters were calculated based on all five tests and will be discussed later in the text. These charts show creep curves constructed from SRR value on the x-axis and CoT on the y-axis. Each creep curve has six points (the last creep curve in each chart may be an exception). The threshold limit is displayed in these charts by a dashed black line. Creep curves are shown by solid lines in shades of blue. If some point of the creep curve is placed outside the black threshold curve, this point is excluded from the evaluation and connected with other points by a dashed line of the Fig. 5 One selected test for each of Sets 1–8. A black dashed line shows the threshold criterion. Creep curves are depicted in shades of blue. "CTZ" and "ITZ" are highlighted by the red and green areas, respectively Tribology Letters (2024) 72:9090 Page 16 of 19 a similar correlation may exist for testing under humidity/ dew conditions, and these tests will be the next step in our future research. 4 Conclusion This study investigated the influence of air humidity and dew on the performance of TOR lubricants. The universal methodology based on creep curve measurement was adapted to enable comparability of results across different workplaces. This methodology uses the coefficient of traction (CoT) and several performance parameters to evaluate the risk of low adhesion problems and the retentivity of TOR lubricants. Based on these parameters, TOR lubricants are assessed into four performance classes: Q1–Q4. The tribometer MTM in a ball-on-disc configuration was used for all measurements. The pot of the tribometer was covered with a climate chamber to control RH %. Tested TOR lubricants were exposed to three RH levels: laboratory ambient (34%), 70%, and 100%, simulating different situations on the track, e.g., morning dew, tunnels, etc. In some tests, specimens were precooled to create a temperature gradient and lower the dew point. The findings can be summarised as follows. In tests with RH 70%, the effect of humidity on TOR lubricant was insignificant, as there was almost no water condensation. Although humidity influences friction in nonlubricated contacts, the effect is negligible compared to the effect of TOR lubricant. The cooling of specimens enhanced water condensation, resulting in worse product performance in terms of CoT. However, the product still performed sufficiently well, showing that TOR lubricants can be safely used in humid environments without significant risk of low adhesion problems. A substantial effect was observed in tests under dew conditions. There is a clear relationship between the amount of water and its impact on performance. With the increase in water amount, the severity of low adhesion problems also increases, and lower values of CoT are measured. However, this behaviour is not linear, as a slight change in water amount causes a significant change for small amounts of water, but a substantial change in water amount causes only a slight difference for larger water amounts. This may be related to the fact that only a limited amount of water/lubricant can enter the contact area. The second TOR lubricant with a different composition was tested to confirm that this behaviour is not exclusive to the first product. The effect of dew was even more significant in this case, meaning that the resistance to contamination, besides others, depended on the product composition. Results presented in this study imply that without significant water condensation, humidity has a less substantial effect on TOR lubricants. If the dew point is not reached, the low adhesion problems are more likely to occur due to the other impacts, e.g. contact over-dosing. However, when the dew point is reached and water condenses, it can significantly affect the performance of the TOR lubricant. This study showed that dew severely worsens the performance of tested TOR lubricants and, in some cases, can even cause low adhesion problems. These findings can be used to optimise friction management methods. Appendix A See Table5. Table 5 The limits of traction zones and the threshold curve a Values for SRR 0% were not used in the evaluation b ITZ Upper limit was also used as a threshold curve SRR (%) 0a2 5 10 15 20 Dry contact CoT 0.008 0.361 0.605 0.724 0.769 0.773 CTZ upper limit CoT 0.001 0.060 0.101 0.121 0.128 0.128 ITZ lower limit CoT 0.002 0.120 0.202 0.241 0.256 0.256 ITZ upper limitb CoT 0.003 0.240 0.403 0.483 0.513 0.513 Tribology Letters (2024) 72:90 Page 17 of 19 90 Author contributions Simon Skurka: Conceptualization, Methodology, Investigation, Data curation, Writing—original draft. Radovan Galas: Conceptualization, Investigation, Methodology, Writing—review & editing. Milan Omasta: Conceptualization, Investigation, Methodology, Writing—review & editing, Project administration. Haohao Ding: Writing—review & editing. Wen-Jian Wang: Project administration. Ivan Krupka: Data curation, Writing—review & editing. Martin Hartl: Supervision, Funding acquisition. Funding Open access publishing supported by the National Technical Library in Prague. This work was carried out in the framework of the project “Bozek Vehicle Engineering National Center of Competence" (TN02000054), which was co-financed from the state budget by the Technology Agency of the Czech Republic within the programme "National Centres of Competence”. The work was also supported by the National Natural Science Foundation of China (No. 52320105007) and the 2024 Excellent Youth Team Training Program of Southwest Jiaotong University. Declarations Conflict of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons. org/licenses/by/4.0/. References 1. Szablewski, D., LoPresti, J., Sultana, T.: Testing of latest top-ofrail friction modification materials at FAST. Railway Track and Struct. 111, 13–16 (2015) 2. Wang, R.X., Zhou, K., Yang, J.Y., Ding, H.H., Wang, W.J., Guo, J., etal.: Effects of abrasive material and hardness of grinding wheel on rail grinding behaviors. Wear. pp 454–455 (2020). https:// doi. org/ 10. 1016/j. wear. 2020. 203332. 3. Niu, L., Yang, F., Deng, X., Zhang, P., Jing, G., Qiang, W., etal.: An assessment method of rail corrugation based on wheel–rail vertical force and its application for rail grinding. J. Civ. Struct. Health Monit. 13, 1131–1150 (2023). https:// doi. org/ 10. 1007/ s1334902300700-w 4. Krishna, V. V., Hossein-Nia, S., Casanueva, C., Stichel, S.: Long term rail surface damage considering maintenance interventions. Wear, pp 460–461 (2020). https:// doi. org/ 10. 1016/j. wear. 2020. 203462. 5. Han, J., He, Y., Xiao, X., Sheng, X., Zhao, G., Jin, X.: Effect of control measures on wheel/rail noise when the vehicle curves. Appl. Sci. 7, 1144 (2017). https:// doi. org/ 10. 3390/ app71 11144 6. Meehan, P.A., Liu, X.: Wheel squeal noise control under waterbased friction modifiers based on instantaneous rolling contact mechanics. Wear 440–441, 203052 (2019). https:// doi. org/ 10. 1016/j. wear. 2019. 203052 7. Galas, R., Omasta, M., Klapka, M., Kaewunruen, S., Krupka, I., Hartl, M.: Case Study: the Influence of Oil-based Friction Modifier Quantity on Tram Braking Distance and Noise. Tribol. Ind. 39, 198–206 (2017). https:// doi. org/ 10. 24874/ ti. 2017. 39. 02. 06. 8. Ishizaka, K., Lewis, S.R., Lewis, R.: The low adhesion problem due to leaf contamination in the wheel/rail contact: bonding and low adhesion mechanisms. Wear 378–379, 183–197 (2017). https:// doi. org/ 10. 1016/j. wear. 2017. 02. 044 9. Kalousek, J., Johnson, K.L.: An Investigation of short pitch wheel and rail corrugations on the Vancouver mass transit system. Proc. Inst. Mech. Eng. F. J. Rail Rapid Transit. 206, 127–135 (1992). https:// doi. org/ 10. 1243/ PIME_ PROC_ 1992_ 206_ 226_ 02 10. Stock, R., Stanlake, L., Hardwick, C., Yu, M., Eadie, D., Lewis, R.: Material concepts for top of rail friction management—classification, characterisation and application. Wear 366–367, 225–232 (2016). https:// doi. org/ 10. 1016/j. wear. 2016. 05. 028 11. Harmon, M., Lewis, R.: Review of top of rail friction modifier tribology. Tribol. Mater. Surf. Interfaces 10, 150–162 (2016). https:// doi. org/ 10. 1080/ 17515 831. 2016. 12162 65 12. Harmon, M., Santa, J.F., Jaramillo, J.A., Toro, A., Beagles, A., Lewis, R.: Evaluation of the coefficient of friction of rail in the field and laboratory using several devices. Tribol. Mater. Surf. Interfaces 14, 119–129 (2020). https:// doi. org/ 10. 1080/ 17515 831. 2020. 17121 11 13. Moreno-Ríos, M., Gallardo-Hernández, E.A., Vite-Torres, M., Peña-Bautista, A.: Field and laboratory assessments of the friction coefficient at a railhead. Proc. Inst. Mech. Eng. F J. Rail Rapid Transit. 230, 313–320 (2016). https:// doi. org/ 10. 1177/ 09544 09714 536383 14. Areiza, Y.A., Garcés, S.I., Santa, J.F., Vargas, G., Toro, A.: Field measurement of coefficient of friction in rails using a hand-pushed tribometer. Tribol. Int. 82, 274–279 (2015). https:// doi. org/ 10. 1016/j. tribo int. 2014. 08. 009 15. Lundberg, J., Rantatalo, M., Wanhainen, C., Casselgren, J.: Measurements of friction coefficients between rails lubricated with a friction modifier and the wheels of an IORE locomotive during real working conditions. Wear 324–325, 109–117 (2015). https:// doi. org/ 10. 1016/j. wear. 2014. 12. 002 16. Eadie, D.T., Elvidge, D., Oldknow, K., Stock, R., Pointner, P., Kalousek, J., etal.: The effects of top of rail friction modifier on wear and rolling contact fatigue: full-scale rail–wheel test rig evaluation, analysis and modelling. Wear 265, 1222–1230 (2008). https:// doi. org/ 10. 1016/j. wear. 2008. 02. 029 17. Galas, R., Omasta, M., Krupka, I., Hartl, M.: Laboratory investigation of ability of oil-based friction modifiers to control adhesion at wheel-rail interface. Wear 368–369, 230–238 (2016). https:// doi. org/ 10. 1016/j. wear. 2016. 09. 015 18. Abbasi, S., Olofsson, U., Zhu, Y., Sellgren, U.: Pin-on-disc study of the effects of railway friction modifiers on airborne wear particles from wheel–rail contacts. Tribol. Int. 60, 136–139 (2013). https:// doi. org/ 10. 1016/j. tribo int. 2012. 11. 013 19. Olofsson, U., Lyu, Y.: Open system tribology in the wheel–rail contact—a literature review. Appl. Mech. Rev., 69 (2017). https:// doi. org/ 10. 1115/1. 40382 29. 20. Wang, W.J., Shen, P., Song, J.H., Guo, J., Liu, Q.Y., Jin, X.S.: Experimental study on adhesion behavior of wheel/rail under dry and water conditions. Wear 271, 2699–2705 (2011). https:// doi. org/ 10. 1016/j. wear. 2011. 01. 070 21. Gallardo-Hernandez, E.A., Lewis, R.: Twin disc assessment of wheel/rail adhesion. Wear 265, 1309–1316 (2008). https:// doi. org/ 10. 1016/j. wear. 2008. 03. 020 22. Zhu, Y., Lyu, Y., Olofsson, U.: Mapping the friction between railway wheels and rails focusing on environmental conditions. Wear Tribology Letters (2024) 72:9090 Page 18 of 19 324–325, 122–128 (2015). https:// doi. org/ 10. 1016/j. w ear. 2014. 12. 028 23. Chen, H., Ban, T., Ishida, M., Nakahara, T.: Experimental investigation of influential factors on adhesion between wheel and rail under wet conditions. Wear 265, 1504–1511 (2008). https:// doi. org/ 10. 1016/j. wear. 2008. 02. 034 24. Kempka, R., Falconer, R., Gutsulyak, D., Lewis, R.: Effects of oxide and water on friction of rail steel–new test method and friction mapping. Tribol. Mater. Surf. Interfaces 15, 80–91 (2021). https:// doi. org/ 10. 1080/ 17515 831. 2020. 17656 11 25. White, B., Lewis, R.: Simulation and understanding the wet-rail phenomenon using twin disc testing. Tribol. Int. 136, 475–486 (2019). https:// doi. org/ 10. 1016/j. tribo int. 2019. 03. 067 26. Lewis, S.R., Lewis, R., Olofsson, U., Eadie, D.T., Cotter, J., Lu, X.: Effect of humidity, temperature and railhead contamination on the performance of friction modifiers: Pin-on-disk study. Proc. Inst. Mech. Eng. F J. Rail Rapid Transit. 227, 115–127 (2013). https:// doi. org/ 10. 1177/ 09544 09712 452239 27. Galas, R., Omasta, M., Shi, L., Ding, H., Wang, W., Krupka, I, etal.: The low adhesion problem: the effect of environmental conditions on adhesion in rolling-sliding contact. Tribol. Int. 151, 106521 (2020). https:// doi. org/ 10. 1016/j. tribo int. 2020. 106521. 28. Arias-Cuevas, O., Li, Z., Lewis, R., Gallardo-Hernández, E.A.: Rolling–sliding laboratory tests of friction modifiers in dry and wet wheel–rail contacts. Wear 268, 543–551 (2010). https:// doi. org/ 10. 1016/j. wear. 2009. 09. 015 29. Kvarda, D., Skurka, S., Galas, R., Omasta, M., Shi, L., Ding, H., etal.: The effect of top of rail lubricant composition on adhesion and rheological behaviour. Eng. Sci. Technol. Int. J. 35, 101100 (2022). https:// doi. org/ 10. 1016/j. jestch. 2022. 101100 30. Galas, R., Kvarda, D., Omasta, M., Krupka, I., Hartl, M.: The role of constituents contained in water–based friction modifiers for top–of–rail application. Tribol. Int. 117, 87–97 (2018). https:// doi. org/ 10. 1016/j. tribo int. 2017. 08. 019 31. Skurka, S., Galas, R., Omasta, M., Wu, B., Ding, H., Wang, W.J., etal.: The performance of top-of-rail products under water contamination. Tribol. Int. 188 (2023). https:// doi. org/ 10. 1016/j. tribo int. 2023. 108872. 32. Wang, W.J., Zhang, H.F., Wang, H.Y., Liu, Q.Y., Zhu, M.H.: Study on the adhesion behavior of wheel/rail under oil, water and sanding conditions. Wear 271, 2693–2698 (2011). https:// doi. org/ 10. 1016/j. wear. 2010. 12. 019 33. Galas, R., Skurka, S., Valena, M., Kvarda, D., Omasta, M., Ding, H., etal.: A benchmarking methodology for top-of-rail products. Tribol. Int. 189, 108910 (2023). https:// doi. org/ 10. 1016/j. tribo int. 2023. 108910 34. Valena, M., Omasta, M., Kvarda, D., Galas, R., Krupka, I., Hartl, M.: An approach for the creep-curve assessment using a new rail tribometer. Tribol. Int. 191 (2024). https:// doi. org/ 10. 1016/j. tribo int. 2023. 109153. 35. Kimura, Y., Okada, K.: Lubricating properties of oil-in-water emulsions. Tribol. Trans. 32, 524–532 (1989). https:// doi. org/ 10. 1080/ 10402 00890 89819 21 36. Fujita, N., Kimura, Y., Kobayashi, K., Amanuma, Y., Sodani, Y.: Estimation model of plate-out oil film in high-speed tandem cold rolling. J. Mater. Process. Technol. 219, 295–302 (2015). https:// doi. org/ 10. 1016/j. jmatp rotec. 2015. 01. 002 37. Nakahara, T., Makino, T., Kyogoku, K.: Observations of Liquid Droplet Behavior and Oil Film Formation in O/W Type Emulsion Lubrication. J. Tribol. 110, 348–353 (1988). https:// doi. org/ 10. 1115/1. 32616 30 38. Wilson, W.R.D., Sakaguchi, Y., Schmid, S.R.: A dynamic concentration model for lubrication with oil-in-water emulsions. Wear 161, 207–212 (1993). https:// doi. org/ 10. 1016/ 00431648(93) 90471-W 39. Cyriac, F., Lugt, P.M., Bosman, R.: Impact of water on the rheology of lubricating greases. Tribol. Trans. 59, 679–689 (2016). https:// doi. org/ 10. 1080/ 10402 004. 2015. 11079 29 40. Cann, P.M.E., Damiens, B., Lubrecht, A.A.: The transition between fully flooded and starved regimes in EHL. Tribol. Int. 37, 859–864 (2004). https:// doi. org/ 10. 1016/j. tr ibo int. 2004. 05. 005 41. Jacod, B., Pubilier, F., Cann, P.M., Lubrecht, A.A.: An analysis of track replenishment mechanisms in the starved regime. In: Dowson, D. etal (eds) Lubrication at the frontier, tribology series, 36,. Elsevier, Amsterdam, pp 483–492 (1999). https:// doi. org/ 10. 1016/ S01678922(99) 80069-8. 42. Baek, K.S., Kyogoku, K., Nakahara, T.: An experimental investigation of transient traction characteristics in rolling-sliding wheel/ rail contacts under dry-wet conditions. Wear 263, 169–179 (2007). https:// doi. org/ 10. 1016/j. wear. 2007. 01. 067 43. Shi, L.B., Ma, L., Guo, J., Liu, Q.Y., Zhou, Z.R., Wang, W.J.: Influence of low temperature environment on the adhesion characteristics of wheel-rail contact. Tribol. Int. 127, 59–68 (2018). https:// doi. org/ 10. 1016/j. tribo int. 2018. 05. 037 44. Shen, M., Li, J., Li, L., Li, S., Ma, C.: Adhesion and damage behaviour of wheel-rail rolling–sliding contact suffering intermittent airflow with different humidities and ambient temperatures. Tribol. Lett. 72, 18 (2024). https:// doi. org/ 10. 1007/ s1124902301817-1 45. Lyu, Y., Zhu, Y., Olofsson, U.: Wear between wheel and rail: a pin-on-disc study of environmental conditions and iron oxides. Wear 328–329, 277–285 (2015). https:// doi. org/ 10. 1016/j. wear. 2015. 02. 057 46. Olofsson, U., Sundvall, K.: Influence of leaf, humidity and applied lubrication on friction in the wheel-rail contact: pin-on-disc experiments. Proc. Inst. Mech. Eng. F J. Rail Rapid Transit. 218, 235–242 (2004). https:// doi. org/ 10. 1243/ 09544 09042 389364 47. Lewis, R., Christoforou, P., Wang, W.J., Beagles, A., Burstow, M., Lewis, S.R.: Investigation of the influence of rail hardness on the wear of rail and wheel materials under dry conditions (ICRI wear mapping project). Wear 430–431, 383–392 (2019). https:// doi. org/ 10. 1016/j. wear. 2019. 05. 030 48. Tyfour, W.R., Beynon, J.H., Kapoor, A.: The steady state wear behaviour of pearlitic rail steel under dry rolling-sliding contact conditions. Wear. 180, 79–89 (1995). 49. Galas, R., Valena, M., Jordan, T., Kvarda, D., Omasta, M., Skurka, S., etal.: A benchmarking methodology for top-of-rail products: Carry distance and retentivity. Tribol. Int. 197, 109810 (2024). https:// doi. org/ 10. 1016/j. tribo int. 2024. 109810 Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Tribology Letters (2024) 72:90 Page 19 of 19 90 Authors and Affiliations SimonSkurka1· RadovanGalas1· MilanOmasta1· HaohaoDing2· Wen‑JianWang2· IvanKrupka1· MartinHartl1 * Simon Skurka [email protected] 1 Faculty ofMechanical Engineering, Brno University ofTechnology, Technicka 2896/2, 61669Brno, CzechRepublic 2 State Key Laboratory ofRail Transit Vehicle System, Tribology Research Institute, Southwest Jiaotong University, Chengdu610031, China