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Article

Tribological Performance of Glycerol-Based Hydraulic Fluid Under Low-Temperature Conditions

by
Paul Okhiria
,
Marcus Björling
,
Pontus Johansson
,
Mushfiq Hasan
,
Roland Larsson
and
Yijun Shi
*
Division of Machine Elements, Luleå University of Technology, 97187 Luleå, Sweden
*
Author to whom correspondence should be addressed.
Lubricants 2024, 12(12), 430; https://doi.org/10.3390/lubricants12120430
Submission received: 23 October 2024 / Revised: 27 November 2024 / Accepted: 29 November 2024 / Published: 4 December 2024
(This article belongs to the Special Issue Recent Advances in Lubricated Tribological Contacts)
Browse Figures
Versions Notes

Abstract

:
This study evaluated the tribological performance of a glycerol-based hydraulic fluid as a green alternative to conventional mineral-based hydraulic lubricants under low-temperature conditions, down to −20 °C. The performance of the glycerol hydraulic fluid (GHF) was compared against that of a mineral hydraulic fluid (MHF) using an SRV tribometer for steel-to-steel sliding contact under boundary lubrication conditions. Comparisons were also made at a moderate temperature to assess the fluids’ performance across different thermal conditions. The results show that the GHF demonstrated up to 55% lower friction coefficients under various test conditions than the MHF. With wear volumes up to 90% lower, the GHF produced thinner and less intense wear scars on the test discs compared to the deeper and more pronounced scars observed with the MHF. We conducted rheological tests which also revealed the green fluid’s stable viscosity transition with temperature changes and its Newtonian behaviour under the measured shear conditions, which may indicate its ability to maintain consistent lubrication.

1. Introduction

Hydraulic systems are vital across various industries, from construction, mining, and automotive engineering to industries that involve mechanical systems. However, traditional mineral oil-based hydraulic fluids present environmental and regulatory challenges due to their poor biodegradability, high toxicity, and negative impacts on occupational health and safety, alongside the significant emissions that they release during their lifecycle. This issue is particularly critical in hydraulic systems, which may experience leaks and spills, leading to environmental contamination including soil and water pollution [1,2]. Consequently, there has been an increased interest in more sustainable lubrication solutions with smaller ecological footprints while maintaining or surpassing the performance of traditional options [3,4,5].
Glycerol-based lubricants have emerged as promising candidates among the sustainable options. Historically, glycerol has been used in various industries, including in food, pharmaceuticals, cosmetics, and more. Glycerol can be produced as a by-product of saponification and hydrolysis reactions in oleochemical plants, as well as transesterification reactions in biodiesel plants [6]. The recent surge in biodiesel production, which produces glycerol as a by-product, has led to an oversupply of glycerol. This surplus of glycerol presents a valuable opportunity before its repurposing as a lubricant for different mechanical applications, such as gears, harvesters, and chainsaws. Hydraulic systems are no exception to this benefit. The use of glycerol as a lubricant brings a double benefit: it makes practical use of a biodiesel byproduct, avoiding waste, and it offers a greener lubrication alternative [6,7,8,9].
Glycerol is generally used as a lubricant in aqueous glycerol solutions, which are water plus pure glycerol, typically in a concentration range of 5–50% water by weight [9]. The use of aqueous solutions is preferred because pure glycerol has a high viscosity of 890 mPa·s at room temperature and a relatively high freezing point of 18 °C, making it unsuitable for low-temperature conditions [6,9,10]. Glycerol-based lubricants are categorized as water-based lubricants (WBLs) [11]. This classification is advantageous because water is considered an environmentally adaptable lubricant (EAL). By combining glycerol with water or other additives, its viscosity and freezing point can be effectively modified to suit low-temperature hydraulic applications, which creates a synergistic approach of improving its lubricating properties while maintaining its environmental compatibility [8].
In a 2022 study [7], we investigated the use of glycerol–water and glycerol–water–glycol mixtures as potential lubricants for the gear systems of battery electric vehicles (BEVs). It was found that these lubricants could reduce the wear and micropitting damage in steel–steel contact under rolling-sliding conditions. The results also indicated that these glycerol-based lubricants provided lower friction compared to traditional gear oils. The friction coefficients were reduced by approximately 47%, and the wear volume was decreased by up to 63% when 30 wt.% of glycol was added to the glycerol–water mixture at 15% and 30% slide-to-roll ratios (SRRs). Furthermore, one of our earlier studies [9] found that aqueous glycerol solutions outperformed traditional rapeseed oil, particularly in reducing friction and wear, especially when the water content was below 20 wt.%. Aside from these studies, research has also shown glycerol’s potential as a lubricant in diverse industrial applications. This includes its effectiveness in sliding contacts, various lubrication regimes, achieving superlubricity, and its use with coatings, which is further enhanced by its ability to accept additives to enhance its performance, and its compatibility with different material contact surfaces [2,7,8,12,13,14,15,16]. However, its potential for hydraulic applications, particularly in low temperature conditions, remains a research gap. This study advances the research in this direction by evaluating the viability of a glycerol-based hydraulic fluid for steel-to-steel sliding contact applications in low-temperature conditions. Its performance is compared with that of a mineral hydraulic fluid (ISO VG 46) within the boundary lubrication regime, where direct asperity contact dominates. The performance comparison is tribological, focusing on friction and wear using an SRV (Schwingungs Reibung und Verschleiß) tribometer supplied by Optimol Instruments GmbH, Munich, Germany. Also, rheological properties such as their viscosity against temperature and their shear rate were examined using an Anton Paar rheometer from Anton Paar GmbH, Graz, Austriato provide a comprehensive analysis of these fluids’ behaviours under cold conditions.

2. Experimentation

This section details the process carried out for the rheological and tribo-testing of an environmentally friendly glycerol-based hydraulic fluid to evaluate its potential in low-temperature hydraulic applications as an alternative to conventional mineral-based hydraulic lubricants.

2.1. Test Lubricants

The glycerol hydraulic fluid (GHF) sample used in this study was provided by Sustainalube AB, Sweden. However, the supplier has not disclosed the specific composition and additives of the lubricant. The mineral hydraulic fluid (MHF), designated ISO VG 46, was purchased from Biltema, Sweden.

2.2. Rheology Tests

The rheological properties of both fluids were evaluated using an Anton Paar MCR 92 rheometer manufactured by Anton Paar GmbH, Graz, Austria. A concentric cylinder geometry was used, with a 25 mm diameter inner cylinder and a 27 mm diameter outer cylinder. This rheometer features a chiller for measurements down to −50 °C and a heating system for temperatures up to 100 °C. Measurements of viscosity with the shear rate were conducted at low temperatures (0 °C and −20 °C) and with a logarithmic increase in the shear rate from 1 to 100 s − 1 . Fifty data points were selected, with a measuring duration of 17 min. Temperature ramp tests were also performed, recording the lubricants’ viscosities from −40 °C to 0 °C at a constant shear rate of 10 s − 1 . A total of 21 data points were selected, with a measuring duration of 7 min. All rheology tests were repeated three times.

2.3. Tribological Tests

Tribological assessments were carried out using an Optimol SRV model 5 tribometer, provided by Optimol Instruments GmbH, Munich, Germany, as shown in Figure 1, which features an electromagnetic linear drive. This system facilitates an oscillating sliding movement between a moving body and a stationary one, with frequencies ranging from 0.001 to 500 Hz and strokes between 0.01 and 5 mm. The advanced OCA (Optimol Control and Analysis) software version 3.0.0.90 accompanying the tribometer allows for the real-time monitoring and recording of critical test parameters such as the load, temperature, frequency, stroke length, linear wear, and coefficient of friction (COF). These features enable precise control and the replication of test parameters when evaluating lubricants in sliding contact conditions, ensuring the attainment of reliable data on the lubricants’ friction-reducing and anti-wear properties [17].
In the test chamber, an upper steel ball slides under oscillating motion in a ball-on-disc setup, as shown in Figure 2. The ball, made of polished 100Cr6 steel with a diameter of 10 mm and a surface roughness (RMS) of 14 ± 4 nm, moves against a stationary steel disc. The disc is made of lapped 100CR6 steel, measuring 24 mm in diameter with a 7.9 mm thickness, with a surface roughness (RMS) of 250 ± 50 nm. Both the ball and disc were also supplied by Optimol Instruments GmbH, 81369 Munich, Germany. Before each test, the specimens were cleaned with heptane and ethanol. Steel-to-steel contact was selected for this study as it represents the dominant interaction in critical hydraulic components, such as motors, actuators, and valves, where metal-on-metal contacts under boundary lubrication are prevalent [19,20]. While other material pairings, such as steel-to-rubber or steel-to-plastic pairings [2], are also present in hydraulic systems, these interactions were outside the scope of the current study.
All tests were conducted at a sliding frequency of 50 Hz and an amplitude of 2 mm, corresponding to an average sliding speed of 0.2 m/s. These parameters were chosen based on the general test conditions of using an SRV machine according to the ASTM D7421 standard. The tests were performed for 120 min to achieve steady-state friction and wear. Given the aim of low-temperature conditions for the tests, 0 °C and −20 °C were chosen for the tests, with contact pressures of 1.35 GPa and 1.7 GPa being chosen to simulate harsh conditions and speed up the evaluation process. These pressures at point of contact of the ball on the disc in the SRV tribometer corresponded to 25 N and 50 N loads. The tested fluids were applied between the test balls and discs in amounts of approximately 0.5 millilitres, as seen in Figure 2. These test parameters are summarized in Table 1.
To properly assess the friction and wear behaviour of both test fluids and to compare them effectively, it was essential to estimate the lubrication regime under which the tests would be conducted. Tribology categorizes lubrication into three predominant regimes: hydrodynamic, mixed, and boundary. These regimes depend on the properties of the lubricant and the system in which they are to be used, or, in this case, the working principle of the SRV tribometer, the test conditions, and the surfaces in contact. In the hydrodynamic regime, a thick lubricant film allows for the separation of interacting surfaces. This setting is characterized by low friction and a more reduced wear of contact surfaces. The mixed regime is an intermediate stage where surface asperities (roughness) and a lubricant film jointly bear the load between the contacting surfaces. The boundary regime is a more direct interaction, where the lubricant forms a very thin film, which leads to more direct contact between surfaces, resulting in higher friction and wear [21,22]. The specific regime can be approximated by calculating the minimum film thickness and then the lambda ratio (λ) using Equations (1) and (2). The minimum film thickness, calculated using Equation (1), for both lubricants is calculated using the Hamrock and Dowson equations for point contacts [7,23,24,25]. If λ is less than 1, boundary lubrication is expected. When λ is between 1 and 3, mixed lubrication is achieved, and, for λ greater than 3, the result is a hydrodynamic/elastohydrodynamic lubrication regime. We have chosen to use this traditional classification despite recent research [26] showing that the lambda ratio is a poor measure of lubrication quality. Here, we believe the lambda ratio gives us reasonably good estimates of the regimes.
h m i n = 3.63   G 0.49 U 0.68 W − 0.73 1 − e − 0.68 k
λ = h m i n R q 1 2 + R q 2 2  
where h m i n is the minimum film thickness and λ is the Tallian film parameter, or lambda ratio. G = α E ′ is the dimensionless materials parameter, U = η o u e / E ′ R x is the dimensionless speed parameter, k = 1.039 is the point contact parameter, and W = w / E ′ R x 2 is the dimensionless load parameter. R q 1 2   and   R q 2 2 are the root mean square (RMS) roughness for the test balls and discs, respectively. Also, u e is the entrainment speed, η o is the lubricant dynamic viscosity, E ′ is the effective modulus of elasticity of the test specimens in contact, R x is the reduced radius of curvature in the x-direction, α is the pressure-viscosity coefficient, and w is the applied load.
This study follows the experimental approach used by Patzer and Woydt [17] under the ASTM D7421 standard for oils [27] and the ASTM D5706 standard for greases [28] to investigate boundary or mixed lubrication regimes using the SRV tribometer. While their typical test temperatures were 50 °C, 80 °C, and 120 °C with load steps, this research uniquely focuses on significantly lower temperatures (0 °C and −20 °C) with constant loading to assess lubricant performance under cold conditions. All the tests were repeated three times to ensure reliability.
The comparison between the glycerol-based and mineral hydraulic fluids is made within the same lubrication regime, boundary lubrication in this case. Due to technical limitations in measuring the pressure-viscosity coefficients of the test lubricants at 0 °C and −20 °C, approximating the lambda (λ) ratio at these temperatures was not feasible. Consequently, λ values are not reported in the experimental test conditions shown in Table 2, but are included in Table 3 which shows measurements obtained at 40 °C. Despite this limitation, the experimental setup and ASTM standards, as applied in [17], allow for the assessment of boundary or mixed lubrication regimes over a range of loads and temperatures. Specifically for oils, it is common that the SRV oscillating tribometer is used as boundary lubrication equipment across different oscillating sliding frequencies, stroke lengths, loads, and temperatures between 20 °C and 100 °C [9,29]. To confirm that the lubrication regime in this study remains within boundary at 0 °C and −20 °C, the coefficient of friction (COF) results presented in Section 3.3 were analysed. The COF values consistently fell within the typical range of about 0.1 or higher, which is characteristic of boundary lubrication regimes [29,30,31,32].

2.4. Comparing the Tribological Properties of the Test Lubricants at Low- (0 °C and −20 °C) and Moderate- (40 °C) Temperature Conditions

In addition to evaluating the tribological performance of the test lubricants at low temperatures (0 °C and −20 °C), tests were also conducted at a moderate temperature of 40 °C to provide a comprehensive analysis across a broader range of operating conditions. Testing at 40 °C with the same parameters shown in Table 1 was intended to establish a reference point that reflects more typical ambient conditions, providing a baseline against which the low-temperature results can be compared. The wear and friction abilities of the test lubricants can also be assessed for consistency across different temperatures, alongside their thermal stability. By including this moderate temperature test, it is possible to determine the suitability of the glycerol-based fluid for applications where hydraulic systems are subjected to varying temperature conditions in order to validate if it can effectively replace mineral oils in both low- and moderate-temperature conditions. It should be noted that the average sliding speed of 0.2 m/s was used as the entrainment speed for the lubrication regime calculations, assuming that the minimum film thickness was stable mainly in the central zones. In the reversal zones of the ball-on-disc setup, the speed drops to near-zero, affecting the film thickness and lubrication conditions. Table 3 shows the experimental test conditions; all the tests were repeated three times to ensure reliability.

2.5. Surface Analysis Techniques

Following the tests, the wear scars and wear volumes of the surfaces of the test discs and balls were captured using a white light interferometer (Zygo Newview 9000), manufactured by Zygo Corporation, Middlefield, CT, USA. The wear scars on the test discs were analysed using a 2.75× lens, which provided an appropriate field of view for capturing the overall scar dimensions and calculating the subsequent wear volume. The Zygo MX software (Mx 8.0.0.26) was used to define regions of interest (ROI) by selecting the wear scar and setting a reference plane outside the scar. The wear volume was then calculated by the software, which integrated the depth data within the wear scar relative to the reference plane. For the steel test balls, a 10× lens was used to obtain higher-resolution images of the wear patterns. The images captured from the interferometer were post-processed using the Mountains® software Version 10, courtesy of Digital Surf, France.

3. Results and Discussion

This section presents the results of the fundamental properties, as well as rheological and tribological evaluations, of a glycerol hydraulic fluid used as a green alternative to a mineral hydraulic fluid across the various test conditions detailed in Section 2.

3.1. Properties of Test Lubricants

At room temperature (25 °C), the density and viscosity of this glycerol-based fluid were measured to be 1.25 g/cm3 and 0.18 Pa · s , respectively. With the aid of a falling body high-pressure viscometer, a pressure viscosity coefficient of 6.1 GPa − 1 was measured at room temperature (27 °C) and a value of 4.7 GPa − 1 was measured at 40 °C. In comparison, the mineral hydraulic fluid (ISO VG 46) was measured as having a viscosity of 0.08 Pa · s at 25 °C. According to the literature, the density of the mineral hydraulic fluid at 15 °C is 0.85 g / cm ³ [33]. The falling ball measurement measured the pressure viscosity coefficient for this mineral-based fluid was 24.6 GPa − 1 at room temperature (28 °C) and 23.3 GPa − 1 at 40 °C.

3.2. Rheology Results

In Figure 3, the dynamic viscosities (presented in logarithmic form) of the glycerol and mineral hydraulic fluids over a temperature range from −24 °C to 0 °C are illustrated. The GHF’s viscosity decreases gradually, maintaining higher viscosity levels and more stability compared to the mineral fluid, particularly at temperatures below −15 °C. The MHF’s viscosity drops more rapidly after −10 °C, reflecting its lower resistance to flow and possibly a lower viscosity index at sub-zero temperatures.
The viscosities of the glycerol and mineral hydraulic fluids are further analyzed using an exponential viscosity–temperature equation:
η o = b e − a T
where η o represents the dynamic viscosity, b and a are fitting parameters, and T is the temperature in Celsius [34,35]. The fitting of the experimental data to this equation is shown in Figure 3. This viscosity–temperature relationship, obtained using the exponential equation, allows for precise prediction of the lubricant performance under a range of low thermal conditions. For the GHF, the viscosity follows the formula η o = 9.01 × 10 − 1 · e − 0.13 T , while, for the MHF, the relationship is given by η o = 4.33 × 10 − 1 · e − 0.15 T .
The viscosities of the GHF and MHF as functions of the shear rate at 0 °C are shown in Figure 4. A common trend is seen, as both lubricants show Newtonian behaviour, maintaining their viscosity regardless of the applied shear rate. The viscosity of the MHF at 0.5   Pa · s remains lower than that of the GHF at 1.2 Pa · s across the shear rate range.
Figure 5 presents the viscosity as a function of shear rate at −20 °C. The GHF continues to show a relatively stable viscosity across the shear rate range. The viscosity levels are higher compared to those at 0 °C, as expected due to the lower temperature, but the overall behaviour remains Newtonian-like. In contrast, the MHF exhibits a noticeable shear-thinning behaviour at −20 °C, with its viscosity decreasing as the shear rate increases. This behaviour indicates that the lubricant becomes less viscous and flows more under higher shear rates, which can be advantageous for reducing friction in dynamic contact surfaces but may also lead to an inconsistent lubrication performance.
The GHF’s viscosity stability between −24 °C and 0 °C, along with its Newtonian-like behaviour at both 0 °C and −20 °C, suggests a good performance in maintaining consistent lubrication under low-temperature conditions. In contrast, the MHF exhibits significant viscosity variability with temperature changes between −10 °C and 0 °C and shows shear-thinning behaviour at −20 °C. This shear-thinning can be attributed to the formation of paraffin crystals. However, both shear thinning and shear heating may occur simultaneously, contributing to the reduction in viscosity. In [36,37], it was observed that the presence of paraffin crystals in mineral oils with similar properties to the MHF in this study causes shear-thinning at low temperatures below −10 °C. As the temperature drops, paraffin crystals form, increasing the oil’s viscosity. However, under shear forces, the crystal network breaks down, allowing the lubricant to flow more easily. In addition, shear heating, a phenomenon common in mineral oils due to their low thermal conductivity and lower heat capacity [38], could further contribute to the breakdown of this paraffin structure, strengthening the shear-thinning effect.

3.3. Friction Results

The resulting coefficients of the friction profiles from the full test matrix highlighted in Table 2 are shown in Figure 6 and Figure 7. The test results stabilized within one minute for the point contact oscillating sliding test conditions. It is observed that the glycerol-based hydraulic lubricant produced lower friction coefficients than its counterpart and shows a relatively stable coefficient of friction (COF).
The measured average values from repeated experiments of the COF for both test lubricants, shown in the complete test matrix highlighted in Table 2, are shown in Figure 8.
The range of the coefficient of friction of the glycerol hydraulic fluid is from 0.094 to 0.145, which is lower than that of the mineral hydraulic fluid, which has a range of 0.154 to 0.203. In each test condition, the GHF has a friction coefficient that is 21–40% lower than that of the MHF, indicating better performance in friction reduction.
Effect of temperature: when tested under similar load conditions, varying only the temperature between 0 °C and −20 °C, it was observed that the average coefficient of friction (COF) values increased for the glycerol hydraulic fluid as its viscosity also increased with the decrease in temperature. However, the mineral hydraulic fluid showed a different pattern, as its COF only increased with 25 N load at −20 °C. At 50 N, moving from 0 °C to −20 °C, the average COF value remained almost unchanged, with only a negligible difference of 0.004.
Effect of load: it is observed that an increase in load leads to a decrease in the coefficient of friction (COF) for both tested lubricants under constant temperature conditions. This trend is well-documented in the tribological literature, particularly for sliding wear experiments involving reciprocating or oscillating motions, and is observed until a certain threshold is reached and new damage regimes emerge [39,40,41,42,43,44]. Although the specific compositions of the lubricants are not disclosed, it is possible that they contain additives [33,45,46,47,48,49] that are more effective at the higher load of 50 N compared to at 25 N. Under the increased load, the anti-wear and/or physisorbed friction modifiers of these additives may become more mobile and active due to the higher contact temperature generated by the load [45,46,50]. Simultaneously, the frictional heat generated by the increased load can modify the properties of the lubricant, particularly its viscosity. As the lubricant becomes more fluid due to the heat, it spreads more evenly and effectively across the contact surfaces, improving the surface coverage, reducing the amount of direct metal-to-metal contact, and, thereby, lowering the amount of friction [51].

3.4. Wear Results

The results for the wear volume loss experienced by the lower disc specimens after testing of the glycerol and mineral hydraulic lubricants are presented in Figure 9.
The analysis of the wear volumes for both lubricants shows clear differences in performance under various test conditions. The glycerol hydraulic fluid has wear volumes ranging from 2.9 × 10 − 4   μ m 3 to 11 × 10 − 4   μ m 3 . In contrast, the mineral hydraulic fluid has much higher wear volumes, from 68 × 10 − 4   μ m 3 to 140 × 10 − 4   μ m 3 . On average, the wear volume of the GHF is approximately 90% lower than that of the MHF.

3.5. Discussion on Friction and Wear Results

The findings that the friction coefficients for both test lubricants were approximately 0.1 or higher and that significant wear was observed on the disc specimens under all test conditions shown in Table 1 and Table 2 are indicative of boundary lubrication, where direct asperity contacts dominate [22,30,31]. Patzer and Woydt [17] showed that using the ASTM D7421 standard for oils and ASTM D5706 standard for greases can result in boundary or mixed lubrication with the SRV tribometer at moderate temperatures. The use of an SRV tribometer for oils has also been documented under similar test conditions to those indicated in the ASTM D7421 standard across various studies [9,29], and a resultant boundary lubrication is seen to have been achieved in these. Observations in this study suggest that even under cold temperature conditions with constant loading (instead of the step loading specified in the standard), the lubrication regime remains predominantly a boundary regime. However, it is important to note that the contact temperature at the interaction points between the test specimens is likely higher than the nominal test temperatures (0 °C or −20 °C) due to frictional heating.
Within the same boundary lubrication regime, it is important to explore why the glycerol hydraulic fluid demonstrates better friction coefficient performance. The previous literature [7,8,14] attributed the improved coefficient of friction (COF) performance of glycerol lubricants to the hygroscopic nature of glycerol, which allows for the formation of a hydrogen-bonded layer between glycerol molecules and water molecules. This mechanism is more associated with full-film lubrication, where the layer can effectively bear loads, thereby reducing the amount of friction. However, in boundary lubrication, the reduced friction may be linked to the formation of an iron oxyhydroxide layer in the contact area [52]. This layer interacts with the hydration layer of glycerol molecules, thus reducing the amount of direct metal-to-metal contact under sliding conditions and effectively reducing the amounts of both friction and wear. In terms of the wear performances of the two test lubricants in the boundary lubrication condition, a significant 90% reduction in wear volume highlights the better wear resistance of the glycerol hydraulic fluid. This suggests that it could be more effective in reducing wear and extending the life of steel contacts, especially in hydraulic applications that require boundary lubrication.

3.6. Comparing the Tribological Properties of Test Lubricants at Low- (0 °C and −20 °C) and Moderate- (40 °C) Temperature Conditions

The coefficient of friction (COF) profiles for the two test lubricants at 40 °C are shown in Figure 10. In Figure 11, the mean COF values at low temperatures (0 °C and −20 °C) are compared to those at a moderate, typical temperature of 40 °C.
It is observed that, for the GHF, the friction coefficient decreases as the temperature increases. As mentioned in Section 3.3, there could be the presence of additives within the lubricant that better adsorb onto the metal surface and form a thin boundary layer due to the rising test temperature, promoting smoother sliding and reducing the amount of direct metal-to-metal contact, thereby lowering the amount of friction. Also, thermally activated processes occur, where the lubricant’s viscosity decreases with the rising temperature, allowing for easier flow and reducing internal fluid friction. The MHF exhibits a different performance where its COF values at lower temperatures (0 °C and −20 °C) are generally lower or comparable to those at a higher temperature (40 °C). At 40 °C, the GHF outperforms the MHF significantly, with friction coefficients that are from 50% to 55% lower across both load conditions. In contrast, at lower temperatures of 0 °C and −20 °C, the glycerol lubricant still demonstrates better performance, although the margin is narrower, with friction coefficients that are from 21% to 40% lower than those of the mineral lubricant. While the advantage of the GHF decreases at lower temperatures, it remains a more effective option than the MHF, consistently providing reduced friction under varying conditions.
As mentioned earlier in Section 3.4, at low temperatures of 0 °C and −20 °C, the glycerol hydraulic fluid showed a wear performance improvement to a value that was 90% lower than that of the mineral-based lubricant under all test conditions. However, as shown in Figure 12, at a moderate temperature of 40 °C and loads of 25 N and 50 N, the performance advantage of the glycerol lubricant decreased to an average of approximately 77%. In a related study by [44], similar experiments were conducted using an oscillating sliding contact in a ball-on-disc setup at low temperatures of −15 °C and 0 °C and a moderate temperature of 25 °C. Their study focused on polypropylene grease with MoS2 and ZDDP additives and observed that the wear volume decreased at lower temperatures, as the grease was better able to adsorb onto the wear surface, forming a protective tribofilm and effectively reducing the amount of metal-to-metal contact. The findings of this study suggest that the GHF may benefit from a similar mechanism, where low temperatures improve its lubricating properties and overall effectiveness in minimizing wear. This supports the conclusion that this GHF under boundary a lubrication regime would be a good option for use in low temperatures where a lower rate of wear damage to hydraulic steel components is preferred.
It is also observed that, at 40 °C and 0 °C, as the load increases from 25 N to 50 N, the wear volume of both test lubricants increases, which is expected and aligns with findings for oscillating ball-on-disc lubricant tribological testing in the literature [44,46]. However, at −20 °C, the observed wear trend shows a reversal: the wear volume for both lubricants decreases as the load increases to 50 N. This reversal is atypical and not widely documented in the literature, suggesting the influence of unique mechanisms at extremely low temperatures.
During oscillating motion in a ball-on-disc setup, the lubricant is continually redistributed as the ball slides back and forth, reaching zones of near-zero velocity at the stroke edges before reversing its direction. At −20 °C, the significantly higher viscosity of both lubricants could play a critical role. Under higher loads, the increased pressure at the contact interface might force the lubricant deeper into the surface asperities, reinforcing the interfacial protective layer. Given the high viscosity at −20 °C, the lubricant may linger longer within the contact zone before being displaced, maintaining better separation between the ball and disc. This could reduce the amount of wear by limiting direct asperity contact and protecting the surfaces more effectively. This hypothesis is speculative and requires further investigation. Therefore, this possibility will be explored in further studies through broader testing and advanced surface analyses to better understand lubricant performance under such conditions.

3.7. Surface Analysis

To gain an insight into the surface changes of the steel contacts (test balls and discs) when lubricated with the glycerol and mineral hydraulic fluids, the morphologies of the worn surfaces were examined. For all test conditions shown in Table 2 and Table 3, the wear scars on the test discs are given in Figure 13.
The test discs lubricated with the GHF showed thinner and less intense wear scars compared to the more pronounced wear scars produced with the MHF. This observation is consistent with the wear volume results in Figure 12 and the fact that the GHF was more able to protect the steel contact surfaces under the oscillating sliding test conditions. Table 4 shows the corresponding wear scar widths for the test discs. At 40 °C, similar wear widths are observed for both lubricants. However, at 0 °C, the average wear scar width for the GHF was approximately 26% narrower than that of the mineral lubricant. At −20 °C, it gets better with an average wear scar width percentage difference of 46%. This trend suggests that the GHF’s protective performance improves significantly with decreasing temperatures, which is consistent with the observations on wear volume reduction noted in Section 3.6. On the other hand, the MHF shows only marginal reductions in the wear scar width and intensity as the temperature drops.
Figure 14 presents optical images of the wear scars on the test ball specimens lubricated with GHF and MHF. All images were obtained at 10 × magnification using a Zygo interferometer, Newview 9000 from Zygo Corporation, Middlefield, CT, USA and post processed using the Mountains® software Version 10, courtesy of Digital Surf, France. The intensity images reveal distinct wear patterns that occurred due to frictional contact during the testing.
The wear scars on the steel test ball specimens lubricated with GHF at 40 °C show uniform parallel scratches across the entire contact areas, indicating consistent abrasive interaction during the testing. The presence of a well-defined, circular wear pattern suggests that the glycerol lubricant maintained a stable protective film, resulting in controlled and uniform abrasive wear across the contact surfaces. At 0 °C, the wear scars on the glycerol-lubricated balls differ slightly from those observed at 40 °C. Notably, there appears to be a transfer film of material on the wear scars, suggesting that, in addition to abrasive scratches, there is also a presence of adhesive interactions. Also, the scratch marks are not as pronounced, which suggest less aggressive abrasive action due to the better lubricating performance obtained under colder conditions. At −20 °C, the wear scar is noticeably shinier, indicative of the initial stages of wear where minimal surface damage allows the steel ball to retain its polished look due to better lubrication performance at this lower temperature. The dark spots are possible areas in which the initiation of wear particle accumulation occurred. These observations correlate with the results shown in Figure 12, where the lowest wear volume was recorded at −20 °C.
The wear scars on the test ball specimens lubricated with the MHF at 40 °C, 0 °C, and −20 °C display irregular patterns with dark regions indicating pronounced material removal and uneven wear. The more severe wear is highlighted by deep linear scratches within the wear track. The least severe wear is observed at −20 °C, consistent with the wear volume results from Section 3.6. This is evident from the reduced presence of deep abrasion marks on the ball surfaces at this temperature, confirming that the lower wear volumes correspond to less severe surface damage.
It should be noted that, after each oscillating sliding wear test, the wear volume on the test ball specimens was negligible compared to that on the test discs, showing that most of the material loss occurred on the discs. The primary wear mechanism under all test conditions was abrasive wear, with adhesion also being present, as indicated by the surface analysis. This observation is consistent with the findings of Patzer and Woydt [17], who identified abrasion and adhesion as the two main wear mechanisms in steel ball-on-disc tests using the ASTM 7421 standard with the SRV 5 tribometer from Optimol Instruments GmbH, Munich, Germany.

4. Conclusions

This study evaluated the tribological performance of two formulated fluids under low-temperature conditions (0 °C and −20 °C). The two lubricants were tested for their rheological, friction, and wear capabilities to assess their potential use in steel-to-steel sliding contact applications, focusing on boundary lubrication and varying load conditions. Also, the friction and wear results of both lubricants were compared across low temperatures and a moderate temperature (40 °C). Based on the results, the following conclusions are presented:
  • The GHF showed good viscosity stability with temperature changes and maintained a Newtonian-like behavior under shear at both 0 °C and −20 °C, suggesting consistent lubrication performance under low-temperature conditions. By comparison, the MHF showed a more significant viscosity transition with temperature changes and exhibited shear-thinning behavior at −20 °C;
  • The GHF showed lower coefficients of friction across all test temperatures (0 °C, −20 °C, and 40 °C) and load conditions (25 N and 50 N), with COF values ranging from 0.086 to 0.145, a reduction of 21–55% compared to the MHF, which exhibited COF values between 0.154 and 0.223;
  • The GHF led to significantly less surface wear damage with lower wear volumes on the test discs, showing reductions of 77–90% compared to the MHF across all test conditions. The steel test balls lubricated with the glycerol lubricant exhibited clear circular wear scars, suggesting stable lubrication and controlled wear, while those lubricated with the mineral lubricant displayed uneven wear patterns with deep scratches;
  • The temperature significantly impacted the friction and wear performance of both lubricants. The GHF had its lowest COF values at 40 °C, although it showed higher wear volumes compared to its wear volumes at the lower temperatures. At 0 °C and −20 °C, it provided wear protection, with reductions of up to 90% compared to the MHF, although its COF values were higher than at 40 °C.
The results of this study highlight the potential and performance of glycerol-based lubricants, which is consistent with recent research on the use of glycerol aqueous solutions with steel, polymer, and coated contact surfaces [2,7,8,9,13,14,15]. In particular, the findings show that the glycerol-based hydraulic fluid could offer a good alternative for steel-to-steel sliding contact applications, especially under boundary lubrication and low-temperature conditions.

Author Contributions

Conceptualization, P.O., M.B., P.J., M.H., R.L. and Y.S.; Data curation, P.O.; Formal analysis, P.O.; Investigation, P.O.; Methodology, P.O., M.B., P.J. and Y.S.; Project administration, M.B., R.L. and Y.S.; Resources, M.B., P.J., M.H. and Y.S.; Software, P.O.; Supervision, M.B., R.L. and Y.S.; Validation, M.B., P.J., M.H., R.L. and Y.S.; Visualization, P.O.; Writing—original draft, P.O.; Writing—review & editing, P.O., M.B., P.J., M.H., R.L. and Y.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Lubricants 12 00430 g001
Figure 1. Overall view of the Optimol SRV Model 5 Tribometer: On the left, the data acquisition system is located; at the centre, the main body of the SRV, featuring the test chamber; and on the right, the chiller used for low-temperature tests [17,18].
Figure 1. Overall view of the Optimol SRV Model 5 Tribometer: On the left, the data acquisition system is located; at the centre, the main body of the SRV, featuring the test chamber; and on the right, the chiller used for low-temperature tests [17,18].
Lubricants 12 00430 g001
Lubricants 12 00430 g002
Figure 2. (a) Schematic representation of the ball-on-disc setup showing test specimens and directions of movement. (b) Detailed view of the ball-on-disc setup in the test chamber of the Optimol SRV model 5 Oscillating Tribometer, highlighting the test components and environmental conditions.
Figure 2. (a) Schematic representation of the ball-on-disc setup showing test specimens and directions of movement. (b) Detailed view of the ball-on-disc setup in the test chamber of the Optimol SRV model 5 Oscillating Tribometer, highlighting the test components and environmental conditions.
Lubricants 12 00430 g002
Lubricants 12 00430 g003
Figure 3. Dynamic viscosities of glycerol and mineral hydraulic fluids at low temperatures (from −24 °C to 0 °C) with an exponential viscosity–temperature relationship fit.
Figure 3. Dynamic viscosities of glycerol and mineral hydraulic fluids at low temperatures (from −24 °C to 0 °C) with an exponential viscosity–temperature relationship fit.
Lubricants 12 00430 g003
Lubricants 12 00430 g004
Figure 4. Viscosity vs. shear rate of the glycerol and mineral hydraulic fluids at 0 °C.
Figure 4. Viscosity vs. shear rate of the glycerol and mineral hydraulic fluids at 0 °C.
Lubricants 12 00430 g004
Lubricants 12 00430 g005
Figure 5. Viscosity vs. shear rate of the glycerol and mineral hydraulic fluids at −20 °C.
Figure 5. Viscosity vs. shear rate of the glycerol and mineral hydraulic fluids at −20 °C.
Lubricants 12 00430 g005
Lubricants 12 00430 g006
Figure 6. COF profiles of test lubricants at a temperature of 0 °C and loads of 25 N (left) and 50 N (right).
Figure 6. COF profiles of test lubricants at a temperature of 0 °C and loads of 25 N (left) and 50 N (right).
Lubricants 12 00430 g006
Lubricants 12 00430 g007
Figure 7. COF profiles of test lubricants at a temperature of −20 °C and loads of 25 N (left) and 50 N (right).
Figure 7. COF profiles of test lubricants at a temperature of −20 °C and loads of 25 N (left) and 50 N (right).
Lubricants 12 00430 g007
Lubricants 12 00430 g008
Figure 8. Mean COF values at 0 °C and −20 °C under 25 N and 50 N loads for test lubricants.
Figure 8. Mean COF values at 0 °C and −20 °C under 25 N and 50 N loads for test lubricants.
Lubricants 12 00430 g008
Lubricants 12 00430 g009
Figure 9. The wear volume loss of the discs lubricated by glycerol and mineral hydraulic lubricants under different load and temperature conditions.
Figure 9. The wear volume loss of the discs lubricated by glycerol and mineral hydraulic lubricants under different load and temperature conditions.
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Lubricants 12 00430 g010
Figure 10. COF profiles of test lubricants at a temperature of 40 °C and loads of 25 N (left) and 50 N.
Figure 10. COF profiles of test lubricants at a temperature of 40 °C and loads of 25 N (left) and 50 N.
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Lubricants 12 00430 g011
Figure 11. Mean COF comparison of the test lubricant at 40 °C, 0 °C, and −20 °C.
Figure 11. Mean COF comparison of the test lubricant at 40 °C, 0 °C, and −20 °C.
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Lubricants 12 00430 g012
Figure 12. Comparison of wear volumes on test discs lubricated with glycerol and mineral-based fluids at 40 °C, 0 °C, and −20 °C.
Figure 12. Comparison of wear volumes on test discs lubricated with glycerol and mineral-based fluids at 40 °C, 0 °C, and −20 °C.
Lubricants 12 00430 g012
Lubricants 12 00430 g013aLubricants 12 00430 g013b
Figure 13. Surface topography of wear tracks of test discs lubricated with glycerol hydraulic fluid (left) and mineral hydraulic fluid (right).
Figure 13. Surface topography of wear tracks of test discs lubricated with glycerol hydraulic fluid (left) and mineral hydraulic fluid (right).
Lubricants 12 00430 g013aLubricants 12 00430 g013b
Lubricants 12 00430 g014aLubricants 12 00430 g014b
Figure 14. Optical images of test ball specimens lubricated with glycerol hydraulic fluid (left) and mineral hydraulic fluid (right).
Figure 14. Optical images of test ball specimens lubricated with glycerol hydraulic fluid (left) and mineral hydraulic fluid (right).
Lubricants 12 00430 g014aLubricants 12 00430 g014b
Table 1. Test parameters for ball-on-disc setup.
Table 1. Test parameters for ball-on-disc setup.
Test ParametersValues
Average Speed0.2 m/s
Frequency50 Hz
Time2 h
Stroke Length2 mm
Contact Pressure1.35 GPa, 1.7 GPa
Load25 N, 50 N
Temperature0 °C, − 20 °C
Table 2. Tribological test conditions for the glycerol and mineral hydraulic fluids.
Table 2. Tribological test conditions for the glycerol and mineral hydraulic fluids.
TestLubricantTemperature (°C)Viscosity (Pa·s)Load
(N)		Contact Pressure (GPa)
1GHF01.2251.35
2MHF
(ISO VG 46)		00.5251.35
3GHF01.2501.70
4MHF
(ISO VG 46)		00.5501.70
5GHF−2010.9251.35
6MHF
(ISO VG 46)		−206.6251.35
7GHF−2010.9501.70
8MHF
(ISO VG 46)		−206.6501.70
Table 3. Experimental conditions for 40 °C tests run for the glycerol and mineral hydraulic fluids.
Table 3. Experimental conditions for 40 °C tests run for the glycerol and mineral hydraulic fluids.
TestLubricantTemperature (°C)Viscosity (Pa·s)Ue
(m/s)		Load
(N)		Contact Pressure (GPa)α
(GPa−1)		λ
1GHF400.0790.2251.354.70.113
2MHF
(ISO VG 46)		400.0430.2251.3523.30.165
3GHF400.0790.2501.704.70.108
4MHF
(ISO VG 46)		400.0430.2501.7023.30.157
Table 4. Average wear scar width of disc specimens lubricated by both test lubricants.
Table 4. Average wear scar width of disc specimens lubricated by both test lubricants.
Test Conditions Average Wear Scar Width (μm)
GHFMHF
40 °C, 25 N353.0327.6
40 °C, 50 N369.1382.3
0 °C, 25 N242.9341.6
0 °C, 50 N274.7354.9
−20 °C, 25 N173.7333.2
−20 °C, 50 N201.0353.6
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Okhiria, P.; Björling, M.; Johansson, P.; Hasan, M.; Larsson, R.; Shi, Y. Tribological Performance of Glycerol-Based Hydraulic Fluid Under Low-Temperature Conditions. Lubricants 2024, 12, 430. https://doi.org/10.3390/lubricants12120430

AMA Style

Okhiria P, Björling M, Johansson P, Hasan M, Larsson R, Shi Y. Tribological Performance of Glycerol-Based Hydraulic Fluid Under Low-Temperature Conditions. Lubricants. 2024; 12(12):430. https://doi.org/10.3390/lubricants12120430

Chicago/Turabian Style

Okhiria, Paul, Marcus Björling, Pontus Johansson, Mushfiq Hasan, Roland Larsson, and Yijun Shi. 2024. "Tribological Performance of Glycerol-Based Hydraulic Fluid Under Low-Temperature Conditions" Lubricants 12, no. 12: 430. https://doi.org/10.3390/lubricants12120430

APA Style

Okhiria, P., Björling, M., Johansson, P., Hasan, M., Larsson, R., & Shi, Y. (2024). Tribological Performance of Glycerol-Based Hydraulic Fluid Under Low-Temperature Conditions. Lubricants, 12(12), 430. https://doi.org/10.3390/lubricants12120430

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