A Comparative Analysis of the Lubricating Performance of an Eco-Friendly Lubricant vs Mineral Oil in a Metallic System

Abstract

Eco-friendly lubricant research continues to increase since it has a comparative performance to commercial mineral lubricants to overcome the effects of environmental impact. However, the efficiency of these green lubricants depends on specific applications. In this study, we analyzed the friction and wear performance of a castor/sesame oil mixture as an eco-friendly lubricant and its comparison to a commercial mineral lubricant tested in a metallic system employed in bearing elements. For this purpose, AISI 8620 steel against ISO 100Cr6 was used as tribological pair. The friction and wear tests were carried out through a Tribometer of ball-on-disk configuration under boundary lubricating conditions, whereas the worn surfaces were investigated by using optical and electron microscopy. The physical properties and the rheological properties of the lubricants were also determined. The friction and wear performance between the eco-friendly lubricant and mineral oil were similar so that the CLE were comparable. The CLE values in terms of friction and wear ranged from 86% to 99.4%, respectively.

1. Introduction

In general, lubrication is the most efficient method to reduce and control friction and wear [1]. The mechanical systems work under optimal conditions when the lubricant film thickness is large enough to prevent permanent contact between the metallic components. Consequently, the adequate selection of lubricant implies considering parameters such as environmental conditions, speeds, loads, operation temperature, and lubrication method, to mention some [2]. Regarding the lubrication method, the use of liquid lubricants can enhance lubrication conditions at higher speeds and load ratio, also offering a cooling effect [3]. At this point, temperature operation is an important parameter because can produce a variation in the viscosity of some oils, causing a change in the rheological and operational properties.

In most applications, lubricants are either mineral or synthetic types. In the past, mineral oils were widely available on earth and inexpensive; therefore, they were used in many different applications and mechanical systems. However, the reserves of petroleum are increasingly facing a decline, leading to higher prices and an uncertain future. Furthermore, mineral oils have the lowest biodegradation rate, high capability for bioaccumulation, and high toxicity for microorganisms and all living beings [4].

The final disposal of mineral lubricants also causes significant problems for the environment since a certain fraction is deliberately dumped into the environment and another part is used as fuel. To deal with this problem, some companies recondition the lubricants to give them a “new use”, but they basically separate the wear debris particles and incorporate some additives without any standard parameter about this process. Likewise, recent investigations are focused on studying the effect of degradation on lubricating performance [5] and finding different ways to reuse degraded lubricants [6]. The interesting results of Hirani H. et al. (2023) [5,6] have demonstrated that by the incorporation of nano-additives, the acidification, pH value, and wear responses can be improved.

The effect of handling mineral lubricants goes beyond damage to the environment. Paulina Nowak et al. [7] published an excellent review of the ecological and health effects of lubricant oils emitted into the environment. There, they explain how the mineral substance in the water resources may cause disturbances in the oxygen–gas exchange between the water and the atmosphere, leading to changes in the functioning and metabolic disturbances of aquatic organisms’ disorder [7]. Moreover, it has reduced the photosynthesis process, leading to an increase in the water temperature by absorbing solar radiation. In terms of healthy, they mentioned that “the routes of penetration of toxic substances from oil mist into the human body are mainly respiratory and skin” [7]. They point out that people exposed to long-term contact oil mist can show a higher incidence of cancer [7].

Then again, due to the environmental effects of mineral lubricants, some researchers have focused on lubricating alternatives that present similar tribological performance, but with better biodegradability and toxicity than mineral lubricants. In this research group, we have found that castor and sesame oils have the potential to replace some commercial lubricants. Castor oil has good viscosity and can form thick lubricating films in most systems. Castor oil’s composition provides appropriate lubricity for different engineering applications since the hydroxyl group present in its main fatty acid forms thick and viscous lubricant films due to intermolecular hydrogen bonding [8]. Tribological studies have reported good lubricant operation of castor oil for several mechanical systems, some of them without additives [9] and others with natural additives such as xanthophylls [10] and curcumin [11]. While sesame oil has been reported to have better thermal stability [12], through binary formulations between castor and sesame oil and by thermodynamic analysis of excess properties, it was possible to obtain the molar concentration where the mixture has an ideal behavior [12]. Additionally, the rheological and tribological study found that this mixture exhibits better performance than the pure components.

In an effort to inquire into the viability of green lubricants in engineering applications, the present investigation aims to analyze the tribological behavior at different loads of the ideal castor/sesame oil lubricant by comparison with a commercial mineral lubricant using a metallic system of AISI 8620 steel against ISO 100Cr6, which exhibit good wear resistance and better fatigue resistance, respectively [13]. The tribological evaluation was completed by sliding friction tests according to the ASTM G99-17 [14].

2. Materials and Methods

2.1. Metallic Materials

The metallic system was ISO 100Cr6 steel balls 3 mm in diameter and AISI 8620 steel disks with a diameter and thickness of 25.4 mm and 5 mm, respectively. These materials were selected since both are one of the most employed steels for bearing applications due to their outstanding mechanical and tribological properties [13,15]. The balls were purchased from the Anton Paar supplier (Graz, Austria), while steel disks were manufactured and thermally treated by an external supplier.

2.2. Materials Preparation

The Rockwell-C Hardness in both samples was determined to be 60 HRC for the ball and 58 HRC for the disks according to the ASMT E18-15 standard [16] by an Universal Hardness Tester (TIME Group Inc., Beijing, China). For the tribological tests, the surfaces of the disks were polished on a rotational machine Labopol-1 in wet conditions using SiC sandpaper according to Section 11 of standard ASTM E3-11 (2017) [17]. The last polish was made on Metkon polishing cloth and diamond past 3 μm, whereas the ball materials were used as received. The initial surface roughness of the ball and disk elements was 0.02 and 0.03 μm in Ra (0.03 and 0.04 μm in Rq), respectively, which was measured by a contact profilometer Mitutoyo SJ 400 under standard ISO 4287:1997 [18] and Gaussian filter.

2.3. Lubricants

The green lubricant employed in this study was the castor/sesame oil mixture (CO/SO) at a molar concentration of 0.7484 of castor oil in sesame oil. This concentration was selected based on a previous study [12], where different CO/SO ratios were analyzed, and by the excess thermodynamic properties (excess molar volume and viscosity deviation), it was determined that this concentration was the optimized CO/SO ratio that also exhibited the best friction and wear resistance. The performance of this green lubricant was compared with that of one commercial mineral-based oil Mobilgear 600 XP 220 (Exxon Mobil Corporation, Houston, TX, USA), which is intended for use in gear and non-gear applications such as highly loaded and slow-speed plain and rolling contact bearings.

2.4. Physical Properties of Lubricants

For both lubricants, physical properties such as density (ρ) and kinematic viscosity (υ) were determined at temperatures of 25, 40, 70, and 100 °C. The density was measured by the pycnometer method, while the dynamic viscosity (µ) was evaluated by a Brookfield rotational viscometer type RV. Later, kinematic viscosity was calculated from the relation between density and dynamic viscosity, υ = µ/ρ. Finally, the viscosity index (VI) was calculated according to ASTM D2270-10 (2016) standard [19].

2.5. Lubricating Regime Estimation

The condition of lubrication was described by the lambda parameter (λ), which is determined by the film thickness (h) and the surface roughness of the ball (R_a) and disk (R_b) by a simple equation (λ = h/√(R_a² + R_b²)). If the lambda parameter is less than 1, the lubrication regime that will govern the mechanical contact will be the boundary lubrication; between 1 and 3, it will be the mixed lubrication regime; and greater than 3, it will be the elastohydrodynamic regime. For this study, the estimation of film thickness (h) was assessed following the methodology reported previously [9] according to the theory for elastohydrodynamic lubrication of point contacts developed by Hamrock and Dowson [20]. Appendix A resumes the determination of the lubricating regime.

2.6. Tribological Evaluation

The friction and wear tests were carried out through a CSM Tribometer with a ball-on-disk configuration according to ASTM G99-17 standard [14] under the lubricating condition at 70 °C. Figure 1 shows the ball-on-disk configuration and experimental setup for the tribological evaluation. The duration of the tests was 30,000 cycles; the wear radius was 0.002 m; and the linear velocity was 0.025 m/s. Prior to tests, counterparts and Tribometer accessories were cleaned with methanol. Different normal loads were studied assuring that the contact pressures between ISO 100Cr6 and AISI 8620 were below and close to the supplier’s recommended limit. The calculus of contact pressures was made by Hertz’s theory for sphere and flat surfaces. The mean and maximum contact pressures in relation to normal load are shown in

Table 1. Mean (P_m) and maximum (P₀) contact pressures between ISO 100Cr6 ^a and AISI 8620 ^b in relation to normal load (F).

F (N)	Pm (GPa)	P0 (GPa)
2.5	0.95	1.42
5	1.19	1.79
7.5	1.37	2.05
10	1.50	2.25

^a Mechanical properties established by the supplier: E = 212 GPa, v = 0.269, Sy = 2 GPa; ^b Mechanical properties established by the supplier: E = 210 GPa, v = 0.30, Sy = 0.9 GPa.

.

At the end of sliding friction tests, the integrity of balls was made by analytical balance Ohaus explorer Pro EPC 214 C and optical microscope. In all cases, the wear of balls was insignificant. The mass loss of the disk was not detected by the analytical balance, so the volume loss (V) of disks was calculated by the mean of Equation (1), according to the ASTM G99-17 standard [14]. In this equation R is the wear track radius in mm; r is the radius of the ball in mm; and d is the wear track width in mm.

V = 2πR[r²sin⁻¹(d/2r) − (d/4)(4r² − d²)^1/2]

(1)

The value of d was determined by optical microscopy. For it, different sections of the wear tracks were analyzed through a Stereoscope Carl Zeiss (Oberkochen, Germany). The value reported was the average of measurements. Consecutively, the specific wear rate (K, mm³/Nm) was calculated by Equation (2), where V is volume loss in mm³, F is normal load in N, and S is total the sliding distance in m [21].

K = V/FS

(2)

The wear mechanisms of the metallic system were analyzed by optical and electron microscopy, employing a Leica ICC50W (Leica Camera AG, Wetzlar, Germany) and a Quanta 3D 200i (Thermo Fisher Scientific, Waltham, MA, USA) equipped with an Oxford X-MaxN-50 EDX (Abingdon-on-Thames, Oxfordshire, UK).

2.7. Effectiveness of Lubricants

The term efficiency, in a general sense, can be defined as the ability to do things well, successfully, and without waste. In terms of lubrication, an efficient lubricant can be defined as the one that reduces friction and wear, helping to reduce energy consumption and material waste. Due to the complexity of this term, different authors have proposed different ways to specify the efficiency of lubricants. For instance, Chen-Ching T. and Chien-Chih C. (2011) [22] proposed the determination of the working efficiency of the soybean oil-based bio-lubricants for applications in pumps and compressors. In a recent study by Trzepieciński et al. (2022) [23], the authors proposed the coefficient of lubricant efficiency (CLE) which relates the capacity to reduce friction by comparing with the unlubricated condition. Both are statistical values that give a measure of the ability of a lubricant to fulfill a specific function.

In this study, the lubricant efficiency of the CO/SO mixture and the mineral oil was determined from the comparison of the coefficient of lubricant efficiency (CLE) described by Trzepieciński [23]. In the present work, in addition to evaluating the CLE in terms of the kinetic friction coefficient (Equation (3)) as reported in [23], the analysis in terms of wear rate was included as a contribution (Equation (4)).

CLE_friction = [(µ_{kdry friction} − µ_{klubricated friction})/µ_{kdry friction}] × 100%

(3)

CLE_wear = [(K_{dry friction} − K _{lubricated friction})/K_{dry friction}] × 100%

(4)

For the purpose of determining the coefficients of lubricant efficiency values,

Table 2. Kinetic friction coefficient (µ_k) and wear rate (K) values obtained on 100Cr6/8620 metallic system under dry conditions and different normal loads (F).

F (N)	μk	K (mm3/Nm)
2.5	0.945	1.16 × 10−4
5	0.612	1.88 × 10−4
7.5	0.602	1.12 × 10−4
10	0.718	2.09 × 10−4

condenses the friction coefficient (µ_k) and wear rate (K) values obtained in the 100Cr6/8620 metallic system under dry conditions previously obtained.

3. Results

3.1. Physical Properties of Lubricants

The properties of the binary mixture of vegetable oils (CO/SO) and commercial mineral oil (MO) are listed in

Table 3. Physical properties of lubricants.

Lubricant	Density (kg/m3)		Kinematic Viscosity (cSt)			Viscosity Index (VI)
	25 °C	70 °C	40 °C	70 °C	100 °C	---
CO/SO	946	925	185	86	57	352
MO	880	852	220	94	19	97

. It can be observed that the properties of the CO/SO mixture are near those of the commercial mineral lubricant. The density of the CO/SO mixture is 7.5% higher than the MO, whereas the viscosity of this green lubricant is 15% lower at 40 °C but 200% higher at 100 °C than the MO. Furthermore, the thermal stability of the viscosity of the CO/SO mixture, described by the viscosity index (VI) parameter, has a marked difference being 263% higher than that of the MO.

3.2. Lubricating Regime Estimation

The comparison in the estimated values of film thickness (h_c) and lambda ratio (λ) for the 100Cr6/8620 metallic system lubricated with the CO/SO mixture and the mineral oil is shown in Figure 2. From values of lubricant film thickness (Figure 2a), it is possible to observe that regardless of the lubricant, at a temperature of 70 °C, the thickness of the lubricant film does not exceed 6 nm and decreases with the increase of normal load. The lubricant film obtained with CO/SO mixture has a thickness of around 10% less than that of the mineral oil in all cases. Because of the lubricant film thickness between the steel counterparts, the λ parameter shown in Figure 2b was between 0.13 and 0.16 and can be associated with the boundary lubricating condition. As mentioned above, this lubricating regime implies that there will be permanent contact between the counterparts throughout the duration of the tribological tests.

3.3. Friction Coefficient Evaluation of CO/SO and Mineral Oil

The behavior of the kinetic friction coefficient of the 100Cr6/8620 metallic system lubricated with CO/SO and mineral oil is shown in Figure 3. The graph in Figure 3 shows that both lubricants exhibited similar behavior with respect to the normal load. It can be also observed that at 2.5 N of normal load, the CO/SO binary mixture offered the best protection against friction, being a friction coefficient 19% lower than the obtained with the mineral oil at the same load. Nevertheless, at higher loads, the friction coefficient achieved with the CO/SO mixture was slightly higher than the mineral oil, 3% and 6% at 5 N and 7.5 N, respectively. However, at 10 N, the difference was insignificant (around 1%). This means that the CO/SO mixture offers a better response to friction at 2.5 N than the mineral oil and slightly similar behavior between 5 and 10 N. The normal load of 5N represents an important point for both lubricants. At this load, the two lubricants presented the highest friction values, and from it, the friction coefficient exhibited a tendency to decrease. This could be due to the fact that this load generates the conditions to prevent the creation of protective films.

3.4. Wear Performance Evaluation of CO/SO and Mineral Oil

Figure 4, Figure 5 and Figure 6 show the wear parameters to evaluate the wear response of the 100Cr6 steel disks lubricated with CO/SO and mineral oil. From the results in Figure 4, it can be observed that both lubricants exhibited an increasing trend in wear with increasing load. It was found that there is a marked difference in wear between the systems lubricated with mineral oil and the CO/SO mixture at 2.5 N. At this load, the wear track width (WTW) obtained with the mineral oil was 90% lower than the obtained with the CO/SO mixture, while the wear rate was four orders of magnitude lower. However, by increasing the normal load, the difference was continuously reduced, and the values became close so that at 10 N the wear track width under mineral oil lubrication was only 5% lower and the wear rate was at the same order of magnitude as the CO/SO mixture. As can be seen, the wear response observed with the CO/SO lubricants, with an exception at the 2.5 N load, was similar to that of the MO. The wear rate values were of the order of 10⁻⁶ and 10⁻⁷, which represent systems with good wear behavior [24].

Figure 5 shows representative optical micrographs of the worn surfaces of AISI 8620 steel lubricated with CO/SO to identify the main wear mechanisms that govern the system. From Figure 5a (the lowest load), it can be seen that the wear mechanism basically consisted of a combination of burnishing and micro-spalling. Burnishing creates smoother and shiner surfaces by the plastic deformation of the surface due to sliding contact, while spalling causes fractures due to surface and subsurface fatigue. It is possible to notice that by increasing the normal load, the wear phenomena were more aggressive leading to the formation of larger spalling and even furrows due to the detachment of abrasive particles. Although to a lesser extent, similar wear mechanisms were observed in the samples lubricated with the mineral oil.

As a comparison, Figure 6 exhibits SEM micrographs of samples lubricated with CO/SO mixture and the mineral oil tested at 10 N as representatives.

It can be observed in Figure 6 that the main differences between the samples lubricated with CO/SO mixture and the mineral oil tested at 10 N are the width of the wear track, the size of spalling areas, and furrow marks. Likewise, through the analysis by energy dispersive spectroscopy it was possible to observe that in some areas of the surface, there is an oxygen amount which could be related to oxidative wear [25,26]. The amount of oxygen in the sample lubricated with mineral oil was three times higher than that of the one lubricated with the CO/SO mixture, which could imply that this bio-lubricant has better antioxidant protection. Moreover, it can be seen that, in the same sample, different regions have different concentrations of oxygen. This may be due to the fact that the oxygen detected is basically from oxidative particles, which are not homogeneously distributed in the wear track.

3.5. Effectiveness of CO/SO and Mineral Oil Lubricants

Figure 7 illustrates the coefficient of lubricant efficiency (CLE) in terms of friction and wear. In general, it can be seen how the efficiency of the CO/SO mixture reduced the friction and wear equivalent to that of mineral oil. The lowest friction efficiency of the CO/SO was observed at 5 N (81.9%) while the highest was detected at 2.5 N (92.2%). Likewise, the efficiency to reduce wear of both lubricating oils is quite close to 100%. The average efficiency to reduce friction and wear of the biodegradable lubricant is 85.9% and 99.4% while that of the mineral lubricant is 85.8% and 99.6%, respectively.

4. Discussion

From the physical and rheological properties of the binary mixture and mineral oil, it can be seen that the CO/SO blend has a higher density at room temperature and higher kinematic viscosity at 100 °C. Therefore, its viscosity index is 2.6 higher times. The reason for the above is that vegetable oils are mainly composed of long-chain fatty acids with strong intermolecular bonds. The presence of these fatty acids leads to a high density and viscosity, and the intermolecular forces give them greater resistance to changes in their properties due to temperature variations [27].

The CO/SO mixture had also a similar film thickness capacity to the mineral oil. At the tested conditions, both lubricants produced comparable lubricant film thickness and worked in the boundary lubricating regime. This impacted the friction and wear behavior of the lubricants. The friction coefficient and wear rate values were very close, and the wear mechanisms showed only variation in intensity.

The efficiency of the CO/SO mixture in reducing both friction, and wear was very similar to that of the fully formulated mineral oil despite the lack of lubricant additives. The lubricant efficiency of the CO/SO mixture is also higher than the reported for other types of vegetable oils such as sunflower, rapeseed, moringa, and karanja which exhibited efficiency from 5% to 35% [23]. This difference could be due to the fatty acid composition, for which, based on previous study [12], this composition of the CO/SO mixture was established as ideal.

By their nature, metallic systems such as bearings help to reduce energy consumption by reducing friction between moving parts in all types of machines. Over the years, efforts have been made to improve bearings’ efficiency and reliability. The efficiency of bearings when well lubricated varies from 95% to 98% for ordinary bearings, 98% for roller bearings, and up to 99% for ball bearings [28]. Efforts to improve such performance range from the design of new bearings to the use of innovative lubricants that reduce friction and wear. In this study, it was observed that the efficiencies of the CO/SO mixture were equal to those of the actually employed mineral oil, and both are within the required efficiencies for bearing elements. Efficient bearings save energy and can make a significant contribution to global sustainable development by saving energy, reducing lubricant consumption, and extending the life of components and machines. Additionally, the CO/SO lubricant has other favorable properties such as higher flash point, stable viscosity over temperature, higher biodegradability, and lower toxicity, causing a less negative impact on the environment.

5. Conclusions

In the present work, the analysis of a binary mixture of castor/sesame with an ideal concentration as a lubricant in the 100Cr6/8620 metallic system was extended. The tribological behavior of this mixture was compared with commercial mineral oil. The study allows concluding the following:

• The CO/SO mixture has similar physical and rheological properties to the mineral oil. The density of this oil is 7.5% higher than the MO, and the viscosity is 15% lower at 40 °C but 200% higher at 100 °C.
• The CO/SO mixture has better thermal stability since its viscosity index is 263% higher than that of the MO.
• Due to the similarity in physical and rheological properties, the CO/SO mixture has a comparable film thickness capacity to the mineral oil.
• The vegetable oil mixture offers a better performance to reduce friction than mineral oil, in working conditions up to 950 MPa of contact pressure.
• The lubricant efficiency of the CO/SO mixture to reduce friction and wear was very close to that of the commercial mineral oil which is fully formulated.
• At 2.5 and 10 N of normal load, the CO/SO mixture caused lower friction than the mineral oil (up to 19% lower at 2.5 N). However, at 5 and 7.5 N, this property was slightly higher.
• The wear performance of the CO/SO mixture, despite the lack of additives, had a mean CLE value of 85.9% in friction and 99.4% in wear.

Based on the above, this study confirms that vegetable oils and their mixtures with or without the incorporation of additives have great capacities to replace mineral oils in engineering applications. The efficiency to reduce friction and wear is competitive to commercial mineral oils. Furthermore, they are environmentally friendly options contributing to generating sustainable processes. Future work will be centered on the formulation of different CO/SO mixture-based green lubricants with different natural additives to improve friction and wear resistance without affecting their biodegradable and non-toxic nature.