1. Introduction
Lubricants, such as oils and greases play a key role in many industries, including the automotive, transportation, chemical and food industries in particular. The factor driving the growth of the market for these products in the world is the dynamic technological development. Lubricant manufacturers have to keep pace with more and more advanced vehicles as well as innovative machines and devices, where precision and smooth operation are crucial for their efficient and profitable operation. During mechanical processes, as a result of strong friction between the surfaces, wear of the mechanical elements of the friction junction occurs and heat is generated, which ultimately reduces the efficiency of work and shortens the life of the device. It is estimated that about 23% (119 EJ) of global energy consumption is due to anti-friction and regeneration of friction elements [
1]. The reduction of friction not only contributes to the reduction of energy consumption, but also fuel savings and the related financial and environmental benefits, in particular the reduction of CO
2 emissions [
2]. In addition, conventional additives with anti-wear (AW) and anti-seizure (extreme pressure EP) properties are mainly based on sulfur, chlorine or phosphorus compounds, which are not environmentally friendly [
3,
4]. Therefore, it is necessary to develop innovative components, modifiers and additives to lubricants with significantly improved tribological properties and reduced negative impact on the natural environment. It has been found that one such additive may be flake graphene, including its derivatives (graphene oxide—GO; reduced graphene oxide—RGO), due to its unique properties, such as thickness of only one or a few carbon layers, layered structure, small flake size. Having in mind flake graphene materials’ thickness, most of them are classified as nanoadditives. Conventional EP and AW additives act by adsorbing active groups to the friction surface or react chemically with the metal surface to form a modified boundary layer. On the other hand, EP and AW nanoadditives can protect frictional mating surfaces mainly through: mending effect, polishing effect, protective film formation mechanism and rolling bearing effect [
5,
6].
Flake graphene, which is a known 2D carbon nanomaterial, is obtained from graphite. At the same time, it should be emphasized that graphite is characterized with greasing properties and it is widely implemented, i.e., as an anti-wear agent. Graphene is another allotropic type of carbon, next to graphite. GO is the oxidized form of graphene, decorated with oxygen functional groups (e.g., hydroxyl, carboxyl or alkoxy groups). GO particles are highly hydrophilic and form stable dispersions in water and some organic solvents. RGO is obtained by reduction of oxygen functional groups of GO. Contrary to GO, rGO is hydrophobic and show high mechanical strength, large specific surface area and high electric and thermal conductivity [
7].
The use of flake graphene is currently the subject of much scientific research in various areas, such as composites [
8], functional coatings [
9], electronics [
10], energy storage [
11], sensors [
12] and even medicine and tissue engineering [
13,
14]. One of the intensively developed and promising areas of application of flake graphene is tribology. It was found that the introduction of flake graphene as an additive to the lubricant has a positive effect on its tribological properties, reducing the value of the friction coefficient and the degree of surface wear. Better lubricity is achieved by nanostructured arrangement of graphene flakes and even by graphitization. In addition, the presence of a protective film on the surface of the friction junction elements, arranged parallel to the direction of movement, was found, which indicates that the graphene flakes slide between each other due to friction [
15].
As part of the research described in the literature, the tribological properties of various lubricants (lubricating oils, plastic greases) with the addition of various forms of graphene flakes (graphene, graphene oxide, reduced graphene oxide, graphene nanoplatelets) were analyzed. The tribological properties of polyalphaolefin-2 (PAO2) base oil with the addition of multilayer graphene containing 0.05 wt.%, 0.10 wt.% and 0.50 wt.% were investigated using a 4-ball tester. The tests were carried out at loads from 120 N to 400 N and rotational speeds in the range of 100–400 rpm [
16]. It was found that, regardless of the tested concentration, the use of graphene in the oil improves the tribological properties (reduction of the friction coefficient) of the oil compared to the oil without the additive, with the best results being obtained for a concentration of 0.05 wt.%. However, in the case of a 10-fold increase in the concentration of graphene in the oil to 0.50 wt.%, the value of the friction coefficient increased, approaching the results obtained for pure oil [
16]. The reason for this may be the tendency of the graphene material to agglomerate in the base oils. To prevent this from happening, dispersants can be added to the oil or the graphene can be chemically modified [
17,
18].
Graphene as an anti-wear additive to plastic greases was also tested. The properties of mineral plastic grease thickened with lithium saponified, enriched with the addition of graphene nanoplatelets with a concentration of 0.50 wt.%, 1.00 wt.%, 5.00 wt.% and 10.00 wt.% were investigated. Various loads were applied: 5 N, 10 N and 15 N [
19]. It was found that with an increase in the concentration of graphene in the lubricant, the value of the friction coefficient decreased, in contrast to the results presented earlier, where the best results were obtained for samples containing the lowest concentrations of additives. The value of the friction coefficient decreased by 8.5% for the concentration of 0.50 wt.%, while the increase in the concentration of graphene to 10.00 wt.% resulted in a reduction of the friction coefficient by 16.3% compared to the value of the coefficient obtained for pure lubricant [
19]. Fu et al. found that the addition of graphene nanoplatelets to the base grease effectively reduces the value of the friction coefficient and improves the thermal conductivity of the lubricant. The concentration of graphene in the lubricant was 1.00–4.00 wt.%. The best results of the lubricant’s tribological properties were obtained for the graphene concentration in the lubricant of 2.00 wt.% [
20], for which the diameter of the friction scar decreased by approx. 14% compared to the base grease.
Flake graphene offers great research opportunities because its surface can be chemically modified and its properties can be controlled in this way. For example, the lubricating properties of monolayer graphene oxide when used as an additive to water-based lubricants were investigated [
21]. Tribological tests were performed using a reciprocating sliding configuration at node load 1.88 N. It was found that the addition of GO to the water improved lubrication and provided a very low coefficient of friction of about 0.05 with no apparent wear to the surface after 60,000 cycles of friction tests. It was concluded that the adsorption of graphene oxide on the lubricated surfaces of the friction elements is responsible for the protection of the surface and the material behaves like a protective coating. The change in the composition of the graphene surface, in particular the share of oxygen functional groups, also influences the change in the tribological properties of the graphene material [
21]. Wang et al. discovered that in the case of the oxidized form of graphene, the friction between the two surfaces increases with the introduction of epoxy and hydroxyl groups into the molecules. The interaction between the layers is dominated by hydrogen bonds [
22]. Liu et al. investigated the effect of two types of graphene, physical graphene produced by liquid-phase exfoliation and graphene produced by chemical oxidation and reduction, on the tribological properties of the oil. It was found that the addition of reduced graphene oxide results in worse anti-friction and anti-wear properties of the oil due to the presence of oxygen functional groups on the surface, compared to physical graphene without the presence of oxygen groups. The inferior lubricating properties of RGO are explained by the presence of oxygen-containing groups that increase the surface activity of the flakes, as well as many defects in the material and the contact surface [
23]. Another study found that the lubricating properties of the base oil also depend on the number of layers and the size of the RGO particles. It was shown that the best tribological properties were obtained for graphene with a smaller flake size and containing a larger number of layers; the reduction of friction was 37% and wear was 47% lower compared to pure base oil [
24].
Based on the literature review, it can be concluded that the addition of graphene to lubricants improves their tribological properties. However, graphene still remains the object of tribological research and the obtained results are not always unequivocal. This may result from the use of various forms of graphene (graphene oxide, reduced graphene oxide, graphene nanoplatelets), different sizes of flakes, different number of carbon layers, different content of oxygen functional groups on the surface and different media in which graphene and its derivatives are dispersed, such as also different test conditions. Therefore, the available information is still incomplete and requires further research. The complexity of the application of graphene in terms of improving the tribological properties of lubricants is also indicated by Larrson et al. [
25]. Moreover, in the source literature there are significant discrepancies in the results of the research concerning the influence of graphene concentration in the base lubricant on its tribological properties. The literature lacks results of comparative studies on the effect of the type of graphene (e.g., graphene oxide, reduced graphene oxide, graphene nanoplatelets) on the properties of the lubricant. A very important issue is also the development of a method of introducing graphene into grease or oil in order to obtain a homogeneous dispersion of the nanofiller in the lubricant and, consequently, to improve the lubricity of the product. The literature discusses the problem of agglomeration of graphene flakes; in particular, this problem concerns lubricating oils [
20].
Bearing in mind the above-mentioned limitations of the available literature data, this work proposes an innovative approach to the implementation of graphene materials as a nanoadditive with anti-wear properties to greases. For the purposes of the work, graphene materials were designed and manufactured, representing two types of flake graphene: graphene oxide and reduced graphene oxide. The paper presents the comparative results obtained for both materials. In order to obtain the best knowledge about the influence of graphene materials on the anti-wear properties of the grease, the tests were carried out for a wide range of concentrations of graphene materials in the grease, amounting to 0.25–5.00 wt.%. The experiment designed in this way allowed to obtain a detailed quantitative description of the issue under study. In addition, the article attempts to answer which of the two tested types of graphene materials will better meet the requirements for carbon nanoadditives for lubricants, taking into account their practical application.
2. Materials and Methods
The research was conducted using graphene oxide powder and reduced graphene oxide powder by G-Flake
® (Łukasiewicz Research Network—Institute of Microelectronics and Photonics, Poland)—
Figure 1.
Graphene oxide was produced using a modified Hummers method. This method is based on the oxidation of flake graphite (Asbury Carbons, Asbury, NJ, USA) in sulfuric acid VI (96%, pure p.a., Chempur, Poland) with the addition of potassium permanganate (pure p.a., Chempur, Piekary Śląskie, Poland) and potassium nitrate (pure p.a., Chempur, Poland). After the oxidation process, the graphite oxide in the form of an aqueous suspension is exfoliated and then purified by filtration. After the purification process, an aqueous suspension of graphene oxide is obtained. In order to produce reduced graphene oxide, graphene oxide undergoes a reduction reaction, which takes place at high a temperature in the presence of a reducing agent—hydrazine (pure p.a., Chempur, Piekary Śląskie, Poland). After the reduction process, the reduced graphene oxide is purified by filtration. To obtain GO or RGO in powder form, the aqueous material dispersion was dried. The obtained graphene materials (GO and RGO) were then characterized using the following methods: scanning electron microscopy (SEM) (Auriga
® CrossBeam
® Workstation, Carl Zeiss, Oberkochen, Germany, at a voltage of 1 kV, energy selective backscattered EsB detector), Raman spectroscopy (Renishaw Invia Raman Microscope, Wotton-under-Edge, UK, a laser with a wavelength of 532 nm was used for excitation), elemental analysis (628 Series and 836 Series, Leco Corporation, St. Joseph, MI, USA). Regarding the SEM experiment, it was implemented in order to visualize GO and RGO materials. The powders were placed on a silicon wafer and then glued with a carbon tape to an SEM table. No coating with conductive agent was conducted prior to visualization. As for the Raman examination, collected raw data were analyzed with Wire 3.54 software provided by the spectrometer producer. The analysis was carried out in order to discriminate the position of D and G peaks using a Gaussian/Lorentzian. Prior to determination of the peaks, position baseline subtraction was implemented. Detailed characterization of the purified GO water suspension, from which GO powder and RGO powder were then obtained, is presented in our earlier publication [
26].
The tests of the lubricating properties of GO and RGO were carried out with the use of plastic grease. The plastic base lubricant was prepared using Finavestan A 360 B paraffin oil (pure, TOTAL, Gonfreville-l’Orcher, France) and lithium stearate (pure, ROTH, Karlsruhe, Germany) thickener. It was prepared in laboratory conditions by adding the thickener lithium stearate to the paraffinic base oil, mixing and then heating with constant stirring until the thickener completely dissolved (to approx. 220 °C). The dissolved mixture was allowed to cool completely at room temperature.
The lubricant compositions were prepared in laboratory conditions by adding the appropriate amount of GO or RGO in the form of a powder to the lubricant base. The samples were then mixed using a planetary mixer (SpeedMixer DAC 150.1 FV-K, Hauschild Engineering, Hamm, Germany) to obtain a homogeneous dispersion (3500 rpm, 15 min). Mixtures of grease and flake graphene were subjected to tribological tests, in which the concentration of graphene (GO, RGO) was from 0.25 wt.% up to 5.00 wt.%. It was observed that as a result of the shear stresses arising when mixing pure base grease and base grease with the addition of graphene, the viscosity of the grease decreased due to the shear-thinning phenomenon (
Figure 2).
The authors of the study [
19] described analogous observations. The authors of this article have experimentally verified that the occurrence of this phenomenon does not affect the properties of the base lubricant and the obtained test results. On the other hand, the high shear stresses arising during the preparation of the mixtures prevent the agglomeration of graphene; therefore, it was possible to obtain a homogeneous dispersion of grease and graphene.
The tests of the lubricating properties of the obtained plastic greases with the addition of flake graphene were determined by measuring the average diameter of the wear scar. The tests were performed with the use of a 4-ball tester (Stanhope-SETA Limited, Chertsey, UK). In this tester, the elements of the tribological couple are in point contact and a sliding movement is performed. The tribological couple in the tester consists of four balls with a diameter of 12.7 mm, made of bearing steel with a hardness of 62.7 HRC. Three balls (stationary) are placed in the lower holder, into which the tested lubricant is also introduced (in the amount of 8 ± 2 cm
3). These balls are pressed down with a ring and fixed with a tightened clamping nut. The fourth ball (movable) is mounted in the upper holder. During the test, it rotates at a speed of 1450 ± 50 rpm, which corresponds to the speed of the rubbing surfaces of 0.55 m·s
−1. The balls in the lower holder are pressed with the force P against the ball mounted in the upper holder by means of a lever. The friction linkage diagram is shown in
Figure 3.
Before starting the tests, the balls were washed in gasoline and, after drying, stored in acetone. At the end of each run, the stationary balls were washed in extractive gasoline. At least three test cycles were carried out for each lubricant. After the tribological tests, the diameter of the friction scars formed on the test elements was measured. The diameter was measured on three balls from a given run, perpendicular (
) and parallel (
) to the friction scars, and then the arithmetic mean (
) was calculated, which was taken as the mean track diameter in a given test run. The principle of calculating the average diameter of the wear scar is shown in
Figure 4 and Equation (1).
The size of the trace diameter was the basic measure of the tribological properties of the tested substances. The arithmetic mean of the results of at least three determinations which did not differ from their arithmetic mean by more than 10% was taken as the result of the determination. During the research work with the use of a 4-ball tester, two types of tests were carried out: at a load of 60 kgf for 60 min; at a load of 100 kgf for 1 min. The adopted conditions result from the internal procedure for testing anti-wear additives.
4. Conclusions
Grease samples were produced with the addition of GO and RGO with a concentration in the range of 0.25–5.00 wt.%. It was found that the addition of flake graphene improves the anti-wear properties of the lubricant, while the concentration and type of graphene have an impact on the tested lubricant properties and its stability. For lower contents of graphene additive (0.25 wt.% and 0.50 wt.% for GO and 0.25 wt.% for RGO), an increase in wear is observed as compared to the base grease. For low RGO concentrations (0.50 wt.%), the grease properties are similar to those of the base grease without additive. The introduction of flake graphene to the base of the plastic grease may disturb the internal network of the grease due to the high activity of the surface of the graphene flakes. This phenomenon may be the cause of increased node wear at low additive concentrations (0.25 wt.% and 0.50 wt.%), as these contents are probably too low to compensate for the breakdown of the lubricant’s inner network. This effect is particularly evident in GO containing numerous oxygen functional groups on the surface of the flakes.
The use of a higher concentration of graphene 1.00–5.00 wt.% results in a reduction in the average diameter of the wear scar, which is greatest for an additive content of 4.00 wt.% (reduction of the wear scar diameter was 69% for GO, as well as for RGO). A further increase in the concentration of the additive to 5.00 wt.% results in a renewed increase in wear, which may result from the agglomeration of the graphene material and a significant increase in viscosity, so that the lubricant is not able to produce a lubricating film that will protect the friction surface well.
Out of the two tested forms of flake graphene, RGO is a recommended additive for lubricants due to its stability under operating conditions, while GO was reduced during friction. The RGO powder tested in operation can therefore be successfully used as a material improving the tribological properties of lubricants.
The use of flake graphene in lubricants is not a thoroughly researched and explained issue. In order to further develop the application of graphene flakes as a lubricant additive, it is necessary to continue research, including in the direction of chemical modification of the flake surface, and to explain the mechanisms of graphene operation during friction, and also to maintain the properties of lubricants unchanged in subsequent work cycles.