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. 2023 Oct 12;8(42):38933–38940. doi: 10.1021/acsomega.3c03147

Lithium–Calcium Greases Having Carbon Nanotubes and Aluminum Oxide Base Nanoadditives: Preparation and Characteristics of Nanogrease

Bahaa M Kamel †,‡,*, Magdy Naeem Awad §,∥, Ahmed Mobasher ⊥, W Hoziefa #
PMCID: PMC10600880  PMID: 37901501

Abstract

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This research presents the results of the effect of different concentrations (2, 4, 6, and 8 wt %) of multiwall carbon nanotubes (MWCNTs) and aluminum oxide nanoparticles (Al2O3) on the characteristics of lithium–calcium grease (LCG). The LCG/Al2O3 and LCG/MWCNT were studied and illustrated by measuring the dropping point, consistency, thermal conductivity, and tribological characteristics of nanograins (wear, friction, and welding point) by using a four-ball tribometer. The morphologies of the additives and the nanogrease were examined and evaluated by X-ray diffraction and transmission electron microscopy. The worn surface of the ball surface was assessed by scanning electron microscopy. Based on the obtained results, the nanoadditive can significantly enhance the grease properties. The grease having a 4 wt % content of Al2O3 and MWCNT presented the lowest wear scar diameter and friction coefficient. Consequently, the welding point, dropping point, and thermal conductivity indicated that adding nanoadditives could strikingly enhance the lubricating effect of grease by 26, 32, and 75%, respectively. Finally, this study provides a lubricant with promising results that can be used under extreme pressure.

1. Introduction

Lithium–calcium grease is one of the most important types of lubricants that is used in many applications, such as gearboxes, bearings, and moving parts of machines. The efficiency of the moving parts in mechanical systems decreases due to the conversion of kinetic energy into thermal energy and friction. The main task of grease is to reduce friction and wear between moving parts and increase the machine lifetime. A large amount of energy is dissipated in mechanical systems due to friction and heat at the contact surface.1,2 Therefore, ordinary greases do not provide the required efficiency to overcome the problems of friction, wear, and the dropping point of grease. With respect to long-term growth and development, the most commonly used lubricating greases can be classified into three categories based on their usage principles: calcium grease,3 lithium grease,4 and sodium grease.5 Therefore, to improve the efficiency of the machines and improve the quality of products, the development of lubricant efficiency is required. Most additives are unsatisfactory for use due to the pungent odor, corrosion, and friction.6,7 So, recently, nanoadditives were used to enhance the properties of lubricants due to their distinctive rheological8−10 and tribological characteristics,3,11,12 such as tribochemical reactions and friction. Kamel et al.2,3,8,9,12 studied the effect of multiwall carbon nanotubes (MWCNTs) and graphene nanosheets (GNs) when added to calcium grease. The results proved that the tribological and rheological characteristics of modified grease were enhanced at the optimal percent of 3% in both MWCNTs and GNs. The use of hybrid nanoparticles as grease additives is a recent idea. A recent paper by Kamel et al.2 discussed the effect of hybrid nanoadditives TiO2/CNTs on the characteristics of grease, and it clearly highly improves the wear and coefficient of friction by about 72.3 and 60%, respectively, and the viscosity by 48%. By using hybrid MWCNT/GNs, Kamel et al.10 showed that the modified calcium grease can effectively enhance the friction and surface regeneration of the base grease. Harne et al.13 studied the wear behavior of lithium grease by adding graphite particles (1.5 μm, 5% graphite); the results showed that the weld point increased by 35.48% and the average wear scar diameter was reduced by 14.91% if compared with base grease. Liu et al.14 evaluated the rheological behaviors and tribological properties of the commercial grease used in the market lithium grease with nanosilica grease. The results presented that lithium grease with nanoparticles had better properties. On the other hand, an increased silica content would enhance grease penetration, and under the same conditions of increased silica content, viscosity and grease consistency were enhanced. Kumar et al.15 used three sizes (micrometer, submicrometer, and nano) of hexagonal boron nitride (hBN) particles to enhance the performance of the base lithium complex grease. The outcomes showed that the greases containing hBN nanoparticles (NPs) demonstrated the best antifriction (AF), antiwear (AW), and extreme-pressure (EP) characteristics. According to Razavi et al.,16 the addition of different concentrations (0.5, 1, and 2 wt %) of calcium carbonate and silica nanoparticles improved the rheological and tribological characteristics of lithium-based greases. The enhancement in friction and wear indicated the ideal percentage of 1 wt %, and this improvment was caused by the development of a protective layer of nanoparticles. Rawat et al.17 added nanosilica to paraffin grease to evaluate the physical and tribological properties. The results showed that adding nanosilica particles could enhance the tribological characteristics compared to using base grease. The greatest reductions in the friction coefficient and average wear volume, respectively, are 20 and 42%, especially at 0.03 and 0.05 wt %. Peng et al.18 proved that adding a small quantity of SiO2 to paraffin oil significantly improved bearing capacity, antiwear, and antifriction performance. However, very few studies on the tribological and rheological properties of aluminum oxide nanoparticles have been reported.19,20 So, in this work, the tribological and rheological properties of lithium–calcium grease having different nanoadditives, MWCNTs and Al2O3 at various concentrations, were examined under different operating conditions. The rubbed surface was inspected by using scanning electron microscopy (SEM); also, the action mechanism, dropping point, penetration, and thermal conductivity were studied to evaluate the effect of nanoadditives on grease. Finally, the goal is to identify the optimal concentration of MWCNTs/Al2O3 nanoadditives to maximally reduce friction and wear and enhance the thermal conductivity and rheological properties of grease.

2. Experimental Procedures

2.1. Materials and Experimental Methods

The main characteristics of the base grease are shown in Table 1. Extreme-pressure lithium/calcium grease is primarily used for automotive and industrial applications to lubricate machine parts and also has better rust and water resistance than conventional grease, and it is suitable for the lubrication of heavily loaded bearings running at medium speed. The nanoparticles used in this work were multiwall carbon nanotubes (MWCNTs) due to their unique properties.9−11,21 MWCNTs have high thermal conductivity and can be used in normally insulative materials to increase their ability to transmit heat due to the phenomenon of the carbon structure, which has high thermal conductivity; also, using MWCNTs in grease reduces wear and friction due to the formation of a hydrodynamic lubricant film. The MWCNTs had a purity of 98%, an average diameter of 9–12 nm, an average length between 1 and 18 μm, and a thermal conductivity of approximately 35 W/m K. On the other hand, Al2O3 nanoparticles formed an adsorbed film between the contact surfaces, in which these particles produced a “ball bearing” effect, which reduced the friction and wear.22−24 The size of aluminum oxide (Al2O3) used in this work had an average diameter of 20 nm and a purity of 99.9%. All nanomaterials were purchased from Nano Tech.

Table 1. Physical Properties of Extreme-Pressure Lithium/Calcium Grease25.

soap/thickener lithium/calcium grease
NLGI grade 2
four-ball weld load 260–280 daN
cone penetration 264
dropping point, °C 185
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2.2. Sample Preparation and Characterization

Quite recently, considerable attention has been paid to the use of nanoparticles to enhance lubrication characteristics; so, this article introduced different types of nanoadditives where MWCNTs and Al2O3 were separately added to four samples (2, 4, 6, and 8 wt %) of lithium/calcium grease (LCG). In the beginning, the four specific ratios of nanoadditives were subjected to ball milling for 10 min followed by dispersing each ratio in N,N-dimethylformamide (DMF),12 by using magnetic stirring for 10 min to minimize the agglomeration of the bundle. LCG was dissolved by chloroform after adding the nanoadditives by utilizing a high-speed homogenizer for 20 min based on the pilot studies;3,11 beyond this time, the nanoparticles started breaking.

2.3. Characterization of MWCNTs and Al2O3 and Nanogrease

The characterization of nanoparticles and nanograins always shows many important details such as the shape, size, and homogeneity of nanoparticles in grease. High-resolution transmission electron microscopy (TEM) is a common analytical technique, which was used to observe the distribution and aggregation of nanoparticles in the base grease before and after the dispersion of the nanoparticles, and X-ray diffraction (XRD) was used to characterize structural properties and phase of MWCNTs and Al2O3. The topography of the surfaces of the tested balls was examined with a scanning electron microscope (SEM).

2.4. Antiwear Test (AW)

The purpose of this test was to characterize and evaluate the grease properties, such as wear, extreme pressure (EP), and coefficient of friction (COF) properties. The tribological properties of grease were characterized by using a four-ball tribotester (SETA-SHELL) as shown in Figure 1, according to ASTM D2266-01,26 in which the balls used in this test had a diameter of 12.7 mm with a surface roughness equal to 0.032 mm and hardness of 59–61 HRC.27 AW tests were performed at a speed of 1450 rpm, a time of 1 h, and different loads. The wear scar diameters (WSDs) of the three lower steel balls were evaluated by an optical microscope and average reading.

Figure 1.

Figure 1

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Four-ball tribotester machine.28

On the other hand, the coefficient of friction was measured automatically by using a data acquisition system attached to the tribological machine according to the following equation:

2.4. 1

where μ is the friction coefficient, T (kg mm) is the frictional torque, W is the applied load in kg, and r is the distance from the contact surface of the lower balls to the axis of rotation. The extreme pressure (EP) or welding point of nanogrease evaluated the load-carrying capabilities of nanogrease according to ASTM D2596.29 In this test, the steel ball on top of the three balls rotated as shown in Figure 2 at a speed of 1770 ± 60 rpm for a series of 10 s. If there was no welding at the specified load, the load was increased to the next load step until welding of the steel balls occurred; this means that the weld load was the load step at which the test ball’s local temperature reached the melting point of steel.

Figure 2.

Figure 2

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Welding load of steel load.

2.5. Dropping Point and Grease Consistency

The dropping point of lubricants is defined as the temperature at which it passes from a semisolid to a liquid state. This test is very important to assess the quality of the grease. This test is evaluated by knowing the first drop of grease falling from the cup as shown in Figure 3 according to ASTM D0566-20.30

Figure 3.

Figure 3

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Koehler dropping point apparatus.

This figure is adapted from refs (31) and (32) and drawn by using Paint.

Grease consistency or penetration is defined as the ability of grease to resist deformation by an applied force by measuring the depth, in tenths of millimeters by a cone penetrometer as shown in Figure 4 where higher penetration numbers indicate softer greases and vice versa. This test was done according to ASTM D0217-21.33 The penetration number was evaluated according to the NLGI system (National Lubricating Grease Institute), which divides greases into groups based on their consistency, as shown in Table 2.

Figure 4.

Figure 4

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Cone penetrometer.

Table 2. Relationship between the Grease Classification System (NLGI) and the Penetration Value34.

NLGI class penetration number consistency at room temperature
000 445–475 very liquid
00 400–430 liquid
0 355–385 semiliquid
1 310–340 very soft
2 265–295 soft
3 220–250 semisolid
4 175–205 solid
5 130–160 very solid
6 85–115 extremely solid
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2.6. Thermal Conductivity of Fluids

A KD2 Pro thermal characteristic analyzer (Figure 5) was directly used to evaluate the thermal conductivity of nanogrease. This test depends on the transient hot-wire technology, and the governing equation of it can be discussed as follows:

2.6. 2

where T(t) is the wire temperature in the lubricant at time t, T1 is the initial temperature of the lubricant in the vessel, K is the thermal conductivity, D is the thermal diffusivity of the lubricant, r is the radius of the sensing wire, q is the input power of the sensing wire, and ln c is Euler’s constant. This test was done according to ASTM D5470-17.35

Figure 5.

Figure 5

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KD2 Pro thermal property analyzer.

3. Results and Discussion

3.1. Characterization of Al2O3 Nanoparticles and MWCNTs

The phase composition and atomic structure of MWCNTs and Al2O3 were investigated by using powder X-ray diffraction (XRD). Figure 6a shows the sharp peak of the X-ray pattern of MWCNTs, which indicates 2θ = 25.9° corresponding to the (002) reflection, which confirms the crystalline nature of the carbon in multiwall nanotubes. Similarly, Figure 6b shows the different peaks [32.7, 36.5, 39.4, 47.8, and 67°] confirming the crystalline nature of Al2O3 nanoparticles.

Figure 6.

Figure 6

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X-ray pattern of MWCNTs (a) and X-ray pattern of Al2O3 (b).

The dimensions of the Al2O3 nanoparticles and MWCNTs were measured by using transmission electron microscopy (TEM). As seen in Figure 7a,b, the average diameter of MWCNTs is ∼12 nm, with a length of 1–25 μm, and the dimension average diameter size of Al2O3 is 13 nm with a density of <0.12 g/cm3.

Figure 7.

Figure 7

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TEM images of MWCNTs (a) and Al2O3 (b).

3.2. Tribological Properties of Modified Nanogrease

The tribological properties of different compositions of LCG/MWCNTs (2, 4, 6, and 8 wt %) and LCG/Al2O3 (2, 4, 6, and 8 wt %) were studied and compared with the base grease. Figure 8 presents the wear scar diameters of the steel ball after the test at different percentages of MWCNTs and Al2O3, and it is clear from this figure that the WSD was enhanced by adding nanoparticles in both additives; in the case of MWCNTs and Al2O3, the WSD improved at concentrations of 2 and 4 wt % according to the phenomena of formation of a hydrodynamic lubricant film and the rolling effect of nanoadditives, respectively; after this concentration, the WSD increased caused by the agglomeration of the nanoparticles on the grease. By these results, the WSD of LCG/MWCNTs and LCG/Al2O3 was reduced and enhanced by 15% at 12 wt %, respectively.

Figure 8.

Figure 8

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Wear scar diameters of LCG/Al2O3 and LCG/MWCNT at different concentrations.

Figure 9.

Figure 9

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Coefficient of friction of LCG/Al2O3 and LCG/MWCNTs at different concentrations. As shown in Figure 9, the coefficient of friction (COF) of LCG/Al2O3 was enhanced gradually by adding Al2O3 at 2 and 4 wt %, where the enhancement was approximately fixed by 11% at 4 wt %, and then, the COF increased at 6 and 8 wt %. On the other hand, the COF reduces as the MWCNT is added to LCG as shown in Figure 9 because the primary function of a lubricant is to lower the COF. The COF was decreased by 29% with the addition of 4% Al2O3 and by 48% with the addition of 4% MWCNTs. Increasing the concentrations may lead to more problems in homogenization such as agglomeration and segregation.10,19,36 Finally, the result shows that using MWCNTs and Al2O3 to enhance the WSD and COF of the base grease is more effective; especially, using MWCNTs gives the best result if compared to Al2O3. Moreover, the extreme pressure of the base grease was 270 daN, but after adding Al2O3 nanoparticles and MWCNTs, the extreme pressure enhanced by 18 and 25%, respectively, at 4 wt % if compared with the base grease as shown in Table 3. It is evident that the nanoparticles can improve the extreme pressure.

Table 3. Extreme Pressure of the Base Grease and LCG/Al2O3 and LCG/MWCNTs.

  base LCG LCG/Al2O3 LCG/MWCNTs
PB (daN) 270 320 340
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3.3. Worn Surface Analysis

In this test, the morphology of the wear scar diameter after the test was measured by using a scanning electron microscope (SEM, JEOL JSM5600 LV). Figure 10 shows the SEM images of the worn steel balls with the base lithium–calcium grease and the lithium–calcium grease having Al2O3 and MWCNTs at the optimal percentage of volume fraction of 4 wt % and a load of 400 N. As can be seen from Figure 10a, the WSD of the lubricated steel ball with the base lithium–calcium grease has deep grooves and extensive adhesion wear. On the other hand, smoothness with only slight adhesion wear appeared by using Al2O3 as can be found in Figure 10b. In the case of using a lubricant with MWCNTs, the surface of the WSD has smooth wear with slight scratches if compared with the base lubricant alone, as shown in Figure 10c.

Figure 10.

Figure 10

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SEM morphologies of WSD of steel balls lubricated by (a) LCG, (b) 4 wt % LCG/MWCNTs, and (c) 4 wt % LCG/Al2O3.

However, with the lubricant with Al2O3 (Figure 10c), the presence of abrasive and adhesive wear can be seen, but slight abrasive tracks are observed on MWCNTs (Figure 10b). The slight abrasive marks on the surface lubricated by the MWCNTs indicate the deposition of tube-shaped MWCNTs on the wear tracks, causing the formation of a tribofilm with lower surface roughness37 than the rolling effect by using Al2O3; this is illustrated in Figure 11a,b. Finally, the optimal concentration of nanoparticles (NPs) in lubricants plays an important role between the interacting surfaces; positive results can be obtained due to the tribofilm formed with MWCNTs or the rolling effect with Al2O3 leading to a lower WSD and COF in lithium–calcium grease.

Figure 11.

Figure 11

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Lubrication mechanisms of nanoparticles.

3.4. Dropping Point and Cone Penetration

Figure 12 depicts the influence of different nanoparticles on the dropping point of base grease with different concentrations. Increasing the weight percentages of both NPs leads to increasing the dropping point of nanogrease. This could be an advantage for new grease because increasing the dropping point can lead to separation from the oil at higher temperatures, maintaining the stability of the structures of grease at high temperatures. On the other hand, as can be seen from Figure 12, modified nanogreases having 8 wt % MWCNTs and Al2O3 showed the largest increase in the dropping point, which was enhanced by 27 and 41%, respectively. The rationale for this increase in the dropping point of LCG/Al2O3and LCG/MWCNTs may be attributed to the increased thermal conductivity of Al2O3 and MWCNTs, which may have dispersed heat and preserved the consistency of the grease at high temperatures.

Figure 12.

Figure 12

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Effect of the MWCNTs and Al2O3 additive on the dropping point test.

Figure 13 depicts the effect of different nanoadditives on the consistency of grease with different concentrations. From this figure, it can be seen that the consistency of the nanogrease is still the same after increasing the percentage of the nanoparticles in the case of MWCNTs and Al2O3. This means that the consistency of greases does not change significantly, and shear stability is maintained, meaning that grease consistency depends on the thickener fiber structure, not the additives.11,38

Figure 13.

Figure 13

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Worked penetration results of base grease and grease with different concentrations.

3.5. Thermal Conductivity of Modified Grease

For the resulting plot, see Figure 14, the thermal conductivity of the grease with different additives is enhanced highly compared with base grease. It is worth noting that the thermal conductivity of LCG/MWCNTs and LCG/Al2O3 enhancements of thermal grease increases linearly with increasing the volume fraction.9,39 Moreover, the thermal conductivity of LCG/MWCNTs is higher than the thermal conductivity of LCG/Al2O3; this enhancement is due to the presence of the metallic nanoparticle structure, which has high thermal conductivity.

Figure 14.

Figure 14

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Thermal conductivity of base grease and grease with different concentrations.

4. Conclusions

In this study, the comparison of the tribological, dropping point, consistency, and thermal properties of MWCNTs and nano-Al2O3 as additives in lithium–calcium grease was investigated. For this purpose, the synthesis of LCG/MWCNTs and LCG/Al2O3 lubricants with different concentrations has been investigated. Lithium–calcium grease with different concentrations of MWCNTs and Al2O3 nanoparticles was tested for wear and the COF at different loads. The results showed that the grease specimen with 4 wt % of both additives reduced wear by up to 13% for LCG/MWCNTs and up to 10% for LCG/Al2O3. However, the COF increased by up to 29% for LCG/Al2O3 and up to 47% compared to the base grease specimen. The effect of increasing the weight percentages of the NPs in nanoflakes increases the dropping point. This is an advantage for the new nanoflake because increasing the dropping point can lead to separation from the oil at higher temperatures; this will help to keep the grease structures stable and effective at high temperatures. The consistency of LCG/MWCNTs and LCG/Al2O3 lubricants with different compositions remains the same regardless of the percentage of nanoparticles added in the case of the MWCNTs and Al2O3. Both lubricants remain semisolid, and MWCNTs and Al2O3 additives do not affect the grade of the grease. Thermal conductivities of LCG/MWCNTs and LCG/Al2O3 composites improved by approximately 73 and 62%, respectively, with an increasing nanoparticle concentration.

Acknowledgments

The authors extend their appreciation to the Mechanical Engineering Department at the National Research Centre and the Performance Sector at the Misr Petroleum Research Center, Cairo, for providing adequate facilities to conduct this research.

The authors declare no competing financial interest.

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