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Reviews
Manufacturing processes and systems УДК 665.765 DOI: 10.17277/jamt.2023.02.pp.157-169
Modification of lubricants with graphite nanoplates
© Adel Baitia, Saif S.Y. Aldavuda, Avzh A.M. Algurabia, Hicham Salhib, Vladimir F. Pershin3^
a Tambov State Technical University, Bld. 2, 106/5, Sovetskaya St., Tambov, 392000, Russian Federation, b University of Batna 2, Fesdis, Batna, 05078, Algeria
Abstract: The review is devoted to the current state of research and achievements in the field of modification of lubricants. To improve the tribological characteristics of lubricants, various additives, in particular, nanoparticles, are used. These additives avoid direct contact, reduce friction and wear. Given that graphene significantly improves the tribological characteristics of lubricants, the main methods for its production are considered. It is shown that the most promising technology for obtaining graphene to modify lubricants is liquid-phase shear exfoliation of crystalline graphite, since oil mixtures with nanoplates are evenly distributed in grease and ensure its stable operation in friction pairs. A variant of mixing graphite nanoplates with grease in a rotary disperser is considered. It is shown that, along with an increase in tribological characteristics, the lubricant modified with graphite nanoplates creates an antifriction film on friction surfaces. The compositions of the antifriction film and the results of its use are analyzed. The ways of improving the technology of modifying greases with graphite nanoplates are outlined. First of all, it is necessary to modernize the main equipment: a drum rod mill; node for classifying graphene-containing suspensions according to the nanoplates size; rotary disperser. Considering that the maximum effect is observed when using an antifriction film + grease modified with graphite nanoplates, it is necessary to study in detail the antifriction film formation and determine its optimal content in the lubricant.
Keywords: tribological characteristics; additives; nanomaterials; graphite nanoplates; friction pairs; wear; antifriction films.
For citation: Baiti A, Aldavud SSYu, Algurabi AM, Salhi H, Pershin VF. Modification of lubricants with graphite nanoplates. Journal of Advanced Materials and Technologies. 2023;8(2):157-169. DOI:10.17277/jamt.2023.02.pp.157-169
Модифицирование смазочных материалов графитовыми нанопластинами
© А. Баитиа, С.С.Ю. Альдавуд3, А. Альгураби3, Х. СалхиЬ, В.Ф. Першин3^
a Тамбовский государственный технический университет, ул. Советская, 106/5, пом. 2, Тамбов, 392000, Российская Федерация, Университет Батна 2, Фесдис, Батна, 05078, Алжир
Аннотация: Обзор посвящен современному состоянию исследований и достижений в области модифицирования смазочных материалов. Для улучшения трибологических характеристик смазок используют различные добавки, в частности, наночастицы. Такие добавки позволяют избежать прямого контакта, снижают коэффициент трения и износ. Учитывая, что графен значительно повышает трибологические характеристики смазок, в обзоре рассмотрены основные способы его получения. Показано, что наиболее перспективной технологией получения графена для модифицирования смазочных материалов является жидкофазная сдвиговая эксфолиация кристаллического графита, поскольку масляные смеси с нанопластинами равномерно распределяются в пластичной смазке и обеспечивают ее стабильную работу в парах трения. Рассмотрен вариант смешения нанопластин графита с пластичной смазкой в роторном диспергаторе. Показано, что наряду с повышением трибологических характеристик, смазка, модифицированная нанопластинами графита, создает антифрикционную пленку на поверхностях трения. Проанализированы составы антифрикционной пленки и результаты ее использования. Намечены пути совершенствования технологии модифицирования пластичных смазок
нанопластинами графита. В первую очередь необходимо модернизировать основное оборудование: барабанную стержневую мельницу; узел классификации графеносодержащих суспензий по размерам нанопластин; роторный диспергатор. Учитывая, что максимальный эффект наблюдается при использовании антифрикционной пленки + смазка, модифицированная нанопластинами графита, необходимо детально исследовать процесс формирования антифрикционной пленки и определить ее оптимальное содержание в смазке.
Ключевые слова: трибологические характеристики; присадки; наноматериалы; нанопластины графита; пары трения; износ; антифрикционные пленки.
Для цитирования: Baiti A, Aldavud SSYu, Algurabi AM, Salhi H, Pershin VF. Modification of lubricants with graphite nanoplates. Journal of Advanced Materials and Technologies. 2023;8(2):157-169. D01:10.17277/jamt.2023.02. pp.157-169
1. Introduction
Tribology plays an important role in solving the problem of friction, which occurs when the parts of mechanisms and machines that touch each other move relative to each other. Friction reduction can prevent costs of countries' gross domestic product from 1 to 1.4 % [1]. In addition to financial benefits, tribology can help protect the environment by improving energy efficiency and reducing CO2 emissions. Thus, important social problems can be solved by improving the tribological characteristics of lubricants. Fig. 1 shows examples of friction pairs from contact between parts of mechanisms to human joints [2]. The study of lubricants makes a significant contribution to the field of tribology.
A lubricant is a substance that effectively improves the movement of solid objects relative to each other by reducing wear and friction on interacting surfaces. In mechanical systems, friction results from sliding, rolling, or rotating contact surfaces. Consequently, friction leads to significant
energy losses, wear and mechanical damage [3]. At least 5 % of the total amount of mechanical energy is converted into useless heat.
Lubricants must be chemically and thermally stable [4], non-volatile, non-corrosive, and durable over time. To meet environmental protection criteria, they must also be environmentally friendly and biodegradable [5]. The main goals and benefits of lubricants can be summarized as follows: reduction in load and increase in service life due to the formation of a lubricating layer in the friction zone; improving useful driving characteristics, such as reducing noise or friction; removal of generated heat to the outside to avoid overheating of bearings and deterioration of lubricant quality; reduction of corrosion by limiting the formation of rust and the penetration of foreign materials; reducing the degree of surface wear due to the application of lubricants between surfaces that rub against each other and the elimination of metal/metal contacts; reduced maintenance costs; reducing power losses of the internal combustion engine.
Fig. 1. Examples of friction pairs [2, © Almqvist]
Lubricants include greases, solid lubricants and lubricating oils. A liquid lubricant such as water, natural or synthetic oils can reduce friction by preventing sliding between contact surfaces (metal-to-metal or metal-to-nonmetal contacts). The effectiveness of lubrication depends on the contact stresses in the friction pair, the sliding speed and the viscosity of the lubricant. To improve the tribological characteristics of lubricants, various additives, in particular nanoparticles, are used. These additives make it possible to avoid direct contact, reduce the coefficient of friction and wear [6]. The efficiency of using nanoparticles as tribological additives was experimentally confirmed in [7-9]. The issue of obtaining carbon nanoparticles was considered in [10].
Due to their small size and unique microstructure, nanoparticles can easily form lubricated tribofilms on substrates by contacting contact surfaces [3]. Good dispersion characteristics allow nanoparticles to penetrate into the friction contact zone [11]. Thus, several modification strategies should be considered to increase the dispersion and stability of lubricant additives.
The use of a large amount of additives increases not only the viscosity of the lubricant, but also the cost, in particular, this applies to molybdenum disulfide [12, 13]. In addition, these additives have varying degrees of toxicity, releasing sulfated ash, phosphorus and sulfur, which lead to air pollution, for example, acid rain and fog, and increase chemical corrosion [14]. Some additives, including ionic liquids, have exceptional tribological characteristics and are environmentally friendly, but their high cost hinders their wide use in industry [15, 16]. In this regard, it is extremely important to create environmentally friendly and durable lubricant nanomaterials based on nanocarbon. Graphene and its derivatives are considered the most promising lubricant additives. However, there are still problems in improving the dispersion stability of nanoadditives [17].
Since graphene was discovered in 2004, it has already amazed researchers around the world with its unique properties [18, 19]. In this review, special attention is paid to the advantages of graphene, including graphite nanoplates, as an additive to lubricants, as well as directly to the methods and equipment for producing graphene.
2. Using Nanomaterials to Modify Lubricants
Before the discovery of graphene, graphite was the most common form of carbon for lubrication [20, 21]. Graphite consists of graphene layers, which provides a low coefficient of friction between the layers [21]. However, the use of graphite as a lubricant has many disadvantages.
Graphite is difficult to disperse uniformly in a liquid, which causes a decrease in fluidity and lubricity [22]. Graphite hardly enters the contact zone of parts and does not form a continuous protective film. Thus, graphite is not an ideal lubricant [23].
Graphene is a single layer of carbon atoms arranged in a two-dimensional (2D) hexagonal lattice. Unlike graphite, a single-layer or few-layer graphene structure can be easily stabilized in the liquid phase using surfactants [22]. The effects of few-layer graphene (FLG) help to reduce friction due to the sliding of graphene layers [24]. Since the terminology corresponding to the state standards of the Russian Federation does not have terms "few-layer graphene" and "multilayer graphene", we will use the term "graphite nanoplate".
In [25], the effect of graphene structure was studied by comparing two forms of commercial graphene nanomaterials, such as 1-2-layer and 1-10-layer graphite nanoplates. In further work, they were used as additives in 1-octyl-3-methylimidazolim tetrafluoroborate in epoxy resin. It has been established that 1-2-layer graphite nanoplates form large agglomerates, which leads to abrasive wear. On the contrary, 1-10-layer graphite nanoplates prevent wear by eliminating direct contact between friction surfaces. In addition, compared to graphite, additives based on graphite nanoplates are more resistant to oxidation and corrosion and have excellent thermal [26] and tribological [27, 28] characteristics.
A number of researchers [29, 30] have demonstrated the excellent chemical resistance of graphene. In addition to chemical properties, graphene has been found to have superior tribological performance compared to other materials [31]. Tribological features are explained by the ability of graphene layers to smoothly slide over each other [32]. These results were confirmed in a molecular dynamics simulation work that demonstrates the relationship between the number of graphene layers in a graphite nanoplateand friction [33]. The model provides an explanation for stability over a wide range of temperatures, velocities, and pressures, predicting that the frictional force approaches zero as the number of layers approaches two or three.
Berman et al. [34] reported the effect of adding graphite nanoplates with multiple graphene layers to steel friction surfaces to reduce friction and wear. The added nanoplates act as a two-dimensional nanomaterial, reducing wear between sliding contacts by almost four orders of magnitude and friction coefficients by a factor of 6.
Graphene has excellent thermal properties, which is confirmed by theoretical approaches [35]
and experimental results [36]. Modification of lubricant with graphene improves its thermal conductivity [37, 38]. The thermal and electrical conductivity of nanofluids containing graphite nanoplates in various mass concentrations was studied [39]. In [40], the tribocharacteristics of lithium synthetic lubricants based on poly-alpha-olefins and polytetrafluoroethylene particles (4 wt. %) of various sizes, namely 50 nm, 6 ^m, 9 ^m and 12 ^m, and shapes as functional additive were studied. The results showed that the smaller the size, the higher the efficiency. However, 6 ^m particles competed with 50 nm nanoparticles. The authors believe that the 6 ^m particles performed well due to their spherical shape. It was concluded that the parameters that determine the efficiency are both the size and shape of particles. Li et al. investigated the tribological properties of 220 centipoise viscosity base grease with graphite nanosheets as an additive using a disk-on-disk test. The results showed a decrease in the friction coefficient at a normal force of 3000 N by 24 % [41]. In [42], MoS2was fixed on the surface of graphene sheets (GNS) using a hydrothermal process and chemical vapor deposition to obtain GNS/MoS2 nanocolors (GNS/MoS2-NF) and GNS/MoS2 nanoplates (GNS/MoS2-NP). The GNS/MoS2 composite obtained by various methods was examined using XRD, Raman, SEM, TEM and XPS. The results confirmed the different morphology of GNS/MoS2-NF and GNS/MoS2 NP. For GNS/MoS2-NF, MoS2 nanocolors were dotted on the graphene surface, while MoS2 nanoplates were uniformly attached to the graphene surface in GNS/MoS2. The resulting composites were examined on a four-ball friction machine at 1200 rpm and a load of 392 N for 30 min. The GNS/MoS2-NF additive performed better than the GNS/MoS2 NP additive even at higher concentrations. For example, the coefficient of friction and base oil wear scar diameter were reduced by 42.8 and 16.9 % with the introduction of 0.02 wt. % GNS/MoS2-NF compared to GNS/MoS2-NP.
Although graphite nanoplates are a popular option, the synthesis of graphene-based lubricants is challenging and further research is needed to understand the best synthesis methods. However, graphene has the potential to revolutionize the lubricant industry, and using it as a lubricant additive could greatly improve existing lubricants.
3. Synthesis of Graphite Nanoplates
The most common method for obtaining graphite (graphene) nanoplates is chemical vapor deposition (CVD) [43]. The obtained CVD graphene/graphite films are often used as a protective
layer for microelectromechanical systems [44], gas barriers [45], and sensor production [46].
One of the options for the synthesis of liquidphase graphene additives is the mechanical exfoliation of graphite oxide or other chemically modified graphite compound [47] followed by chemical or thermal reduction or modification [48, 49].
Graphene is obtained by mechanical exfoliation of graphite oxide into graphene oxide - the Hummers method [50]. Liang et al. [51] introduced a nonionic surfactant (Triton-X) into the graphite exfoliation method. Graphene was mechanically peeled off by ultrasonication after mixing with a non-ionic surfactant. The frictional properties of water-based lubricants improved by 80% due to modification with graphene. Alternatively, Patel et al. [48] used off-the-shelf reduced graphene oxide (rGO) as lubricant additives and reduced wear and friction by 52 %.
Due to the toxicity of the Hammers method, various alternatives have been proposed. Among available alternatives, ascorbic acid is known as the most promising reducing agent due to its low toxicity, low cost, and non-carcinogenic properties.
An alternative GO recovery method involves using thermal recovery. Thermal reduction is usually carried out at an elevated temperature in order to reduce GO to graphene [51].
Although liquid phase exfoliation of graphene may be the best way to obtain graphene for lubricant additives, researchers need to be aware of the fundamental limitations and impurities present in final products. It should be noted that during the reduction of GO on graphene planes, defects are formed at the sites of oxygen removal, which negatively affects non-tribological properties. Exfoliation of graphite using ultrasound requires large amounts of electricity, which significantly increases the cost. In our opinion, the most promising technology is the production of graphite nanoplates by liquid-phase shear exfoliation.
3.1. Preparation of graphite nanoplates by liquid-phase shear exfoliation
Most attention was paid to exfoliation using ultrasound. This is due to the fact that there is no need to make a special installation and it is possible to carry out the exfoliation of graphite particles on standard laboratory equipment. During sonication, cycles of high pressure (compression) and low pressure (rarefaction) alternate, the speed of which depends on the frequency of ultrasonic vibrations. At low pressure, a vacuum is created and bubbles or voids form in the liquid. At high pressure, the bubbles collapse and this phenomenon is called cavitation.
The temperature during cavitation reaches 5000 K, and the pressure is 2000 atm. In addition, liquid jets are formed with a speed of up to 280 m-s-1. Graphite nanoplates are obtained by sonication in organic solvents, in water with the addition of small amounts of surfactants, or in ionic liquids.
Thus, strong oxidizing agents are not used, which has a positive effect on the quality of nanoplates. A method for obtaining polystyrene functionalized with graphene is presented in [52]. Powders of flake graphite and styrene were used as starting materials. During sonication, graphite flakes are transformed into single-layer and few-layer graphene plates. Simultaneously, graphene sheets are functionalized with polystyrene chains. A similar functionalization process was carried out with other vinyl monomers and new composites were created.
Technology using ultrasound has the following disadvantages: very high energy consumption; graphite particles are affected not only by shear forces, but also by compressive and shock forces, which negatively affects the quality of graphene.
The most promising is shear exfoliation. One of the first versions of this method was proposed in [53]. The slurry jet is fed onto a rapidly rotating disk. Under the action of centrifugal forces, the suspension is distributed over the disk surface in a thin film and moves towards the periphery of the disk. In a thin film, shear forces arise and particles of graphite or other layered crystal are stratified. After repeated supply of the suspension to the rotating disk, the particles gradually turn into nanoplates. This method is interesting due to the stratification mechanism of layered crystals, but it has not found application in industrial production.
The most promising method is the production of graphite nanoplates using a stator-rotor mixer with high shear [54]. The mixer consists of a cylindrical body with 96 square holes with a cross section of 2 x 2 mm. Inside the housing is a rotor with four blades. The gap between the edges of the blades and the inner surface of the housing should be no more than 0.1 mm. Particles are mainly affected by shear forces. When the suspension passes through the holes in the housing, shear forces partially act on the particles. In addition, cavitation occurs. The most significant are the shear forces, and the rest of the force effects cause defects in the graphene planes.
In [54], an L5M laboratory mixer manufactured by Silverson Machines Ltd., UK, was used. Electric motor power is 250 W, maximum rotor speed is 8000 rpm (6000 rpm at full load). The inner diameter of the cylindrical body is 50 mm. The rotor has four blades and a gap between the rotor and the body of 0.1 mm.
During the rotation of the rotor, the mixer works like a pump. By centrifugal force, the suspension is ejected through the holes in the housing. A vacuum is created between the housing and the rotor, and the suspension is drawn through the upper and lower ends of the housing into the area between the housing and the rotor. Particles that enter the area between the blade and the inner surface of the housing are subject to shear forces that cause particle separation. Experiments were also carried out using cases with an inner diameter of 19 mm and 16 mm. 5 liters of a mixture of water + crystalline graphite powder + + surfactant were pre-prepared. The rotor speed was gradually increased to a predetermined value. Exfoliation was carried out for a certain time. During the experiments, six main parameters were varied: mixing time; rotor speed, N rpm (converted to s-1 in calculations); the volume of the processed suspension V liters (from 0.25 to 5); rotor diameter, D mm (12, 16 and 32 mm); concentration of graphite in the initial suspension.
The following components were used as surfactants: N-methyl-2-pyrrolidone; N-cyclohexyl2-pyrrolidone; N-cyclohexyl-2-pyrrolidone. The classification of nanoplates was carried out on a Thermo Scientific centrifuge, model: Heraeus Megafuge [54]. Centrifugation was carried out sequentially at different speeds. After centrifugation, particles with an average size (length) remained in the centrifuge: 5000 rpm - 160 nm; 3000 rpm -200 nm; 2500 rpm - 216 nm; 2000 rpm - 282 nm; 1000 rpm - 1000 nm. After separation of the suspension into fractions, the size of the nanoplates and the number of graphene layers in these nanoplates were evaluated using transmission electron microscopy (TEM). A very interesting fact is the relationship between the sizes of nanoplates and the number of graphene layers that make up these plates. For example, with an average nanoplate size of 1.1 ^m, the average number of graphene layers was 2.8. In addition, it was found that the aspect ratio of nanoplates (length/width) was constant and equal to 2.6. This is a very important fact, which further allows one to more accurately determine the concentration of nanoplates in a suspension. Thus, during cascade centrifugation, the suspension is separated into fractions according to the lateral particle size and the number of graphene layers. Currently, most researchers use cascade centrifugation to sort nanoplates.
As the results of long-term operation of the stator-rotor mixer showed, due to the wear of the rotor blades, the gap increases and the exfoliation process stops. This disadvantage is eliminated in a rotary apparatus with movable blades (Fig. 2) [55, 56].
A-A
Fig.
Fig. 2. Scheme of a rotary apparatus with movable blades: 1 - stator; 2 - rotor; 3 - rotation drive; 4 - blade; 5 - cover
[55, © Al-Shiblawi]
The graphene-containing suspension was obtained as follows. Powder of crystalline graphite weighing 50-100 g was poured with 1-2 liters of oil, which is the basis of grease. The apparatus was placed in the container and the rotation drive 3 was turned on. When the rotor 2 rotated, the mixture of oil and graphite was ejected by centrifugal forces through the holes in the upper part of the housing. In the zone between the housing and the rotor, a reduced pressure was formed and the mixture entered the specified zone through the lower end. Under the action of centrifugal forces, the blades 4 were pressed against the inner surface of the body. Particles that fell into the zone of sliding contact of the fixed inner surface of the stator and the movable blades (exfoliation zone) stratified due to shear forces, i.e. two particles were formed from one particle, but with smaller thicknesses. Since the particles fell into the exfoliation zone many times, nanoplates gradually formed from graphite particles. After the end of the exfoliation process by centrifugation, the resulting suspension was divided into fractions by particle size.
The results of further studies made it possible to carry out the transition from a periodic regime to a continuous one [57-59].
Fig. 3 shows a diagram of a cross section of a rotary apparatus with composite movable blades. A rotor 2 is located in the cylindrical body 1. The blade consists of a base 3 connected to the tip 4 by a tongue-and-groove connection. The base is made of metal, and the tip is made of an anti-friction material, such as fluoroplastic. By changing the density of the metal, it is possible to change the magnitude of the centrifugal force acting on the blade, and, consequently, the force of pressing the tip to the inner surface of the stator. For example, the mass of a blade with an axial length of 10 mm, made of PTFE, is equal to m = 0.4 g; the normal force FN acting on the
3. Scheme of a rotary apparatus with movable composite blades: 1 - stator; 2 - rotor;
3 - the base of the movable blade;
4 - the tip of the movable blade [57, © Al-Jarah]
blade is 4.3 N. When the blade is made in two parts, with a base 4 mm thick, with a radial dimension of 8 mm, made of steel, and a tip 2 mm thick and with a radial dimension of 6 mm, made of PTFE, the total mass of the blade m = 1.8 g, and the normal force Fn = 20 N.
Comparisons between the results of experimental studies in batch mode (Fig. 2) and continuous mode (Fig. 3) were made, which showed that the productivity in continuous mode was at least 1.5 times greater than in batch mode.
4. Modification of Lubricants with Graphite Nanoplates
For the industrial modification of grease in the article [59], an installation was developed, the scheme of which is shown in Fig. 4. The process is
4
Fig. 4. Scheme of the graphene-containing suspensions and concentrates industrial production: 1 - liquid dispenser; 2 - rod drum mill; 3 - graphite powder dispenser; 4 - tank with a stirrer; 5 - additional dispenser of graphite powder; 6 - pump; 7 - first rotary apparatus; 8 - final rotary apparatus; 9 - coarse filter; 10 - fine filter; 11 - vessel for preliminary mixing of nanoplates with grease; 12 - rotary disperser [57, © Al-Jarah]
implemented as follows. The base oil and powder of crystalline graphite are fed by dispensers 1 and 3 into a drum rod mill 2. Mechanical activation of graphite particles and their partial exfoliation are carried out in the mill [59, 60].
The treated mixture is fed into tank 4, where oil and graphite powder are supplied by dispenser 5. From tank 4, the diluted mixture is fed by pump 6 (Pi) into the first rotary apparatus, where graphite is exfoliated. From the first rotary apparatus, the suspension enters the second apparatus, etc. Next, the suspension is fed by pump P2 to the coarse filter 9. The filter cake is sent to the mill 2 for repeated mechanical activation. The clarified suspension is fed to the fine filter 10. The precipitate from the filter 10 is fed into the pre-mixing tank of nanoplates with grease 11.
For uniform distribution of nanoplates over the volume of the modified lubricant, a disk disperser 12 is used [61-64]. The disperser (Fig. 5) consists of a stepped disk (rotor) 1, a stator 2, and a pipe for supplying the mixture 3. The mixing of a viscous liquid with graphene plates was carried out as follows. Grease with graphene concentrate was premixed using a laboratory paddle mixer. The content of graphite nanoplates did not exceed 1 wt. %. The mixture was pumped into nozzle 3 with
a rotating disk 1. Compared to a flat disk, vertical sections were added where the mixture moves along helical trajectories.
The gap between the rotor and the stator S = RiS - Rm, when using different pairs of stators and rotors, varied from 0.05 to 0.2 mm. At the beginning of exfoliation, a gap of 0.2 mm was used, and as the thickness of the nanoplates decreased, the gap was reduced to 0.05 mm.
The motion of a viscous fluid in a small gap between a fixed body and a rotating disk was studied [63]. Mathematical dependencies are obtained to determine the main parameters of the motion of a viscous fluid in the gap. Based on the mathematical apparatus of random Markov processes, discrete in space and time, a model of the process of mixing grease with graphene plates was developed. The decomposition of the mixing process into radial and circumferential components made it possible to estimate the intensity of each of these components. Using the results of numerical experiments, ways to improve the organization of loading the lubricant and modifier into the homogenizer were outlined in order to increase the uniformity of the distribution of graphene plates over the entire volume of the lubricant and stabilize the tribological characteristics.
Fig. 5. Scheme of a stepped disk homogenizer [63, © Alhilo] Table 1. Tribological characteristics of modified lubricants
Grease name
Wear scar diameter, mm
Scuff index, N Critical load, N Welding load, N
Integrated Lithium threaded
Complex Lithium threaded with talc
Complex Lithium threaded Surgut
Integrated Lithium threaded from fishing
Integrated Lithium threaded from pipe factory
Complex Calcium
Complex Calcium with 0.1 % multilayer graphene
0.52 0.53 0.55
0.50
0.77 0.69 0.34
511.80 509.83 707.95
536.77
296.15 201.10 598.00
765 1216 451
765
294 197 1040
1932 1932 1932
1932
1932 1098 4140
4
The efficiency of using nanoplates obtained by liquid-phase shear exfoliation was experimentally confirmed in [65, 66]. The results of the experimental determination of the tribological characteristics of the modified lubricant are given in Table. 1.
According to the data obtained, when 0.1 % multilayer graphene was added to the complex calcium lubricant, the wear scar diameter decreased by 50%, the scuff index increased by 2.9 times, and the bearing capacity increased by 3.8 times [65]. Multilayer graphene (graphite nanoplates) was obtained by liquid-phase shear exfoliation of crystalline graphite powder in oil, which was the basis of a complex calcium lubricant. The number of graphene layers did not exceed 25, and the lateral dimensions were on the order of 1 pm.
5. Formation of Antifriction Films on Friction Surfaces
Protective anti-friction films that form on friction surfaces significantly extend the service life of machines and mechanisms. The paper [67] presents the results of comparative tests on an MI-1M friction machine, Litol-24 commercial lubricant and Litol-24 lubricant modified with graphite nanoplates. We used graphite nanoplates obtained by liquidphase shear exfoliation with the number of graphene layers no more than 25 and a concentration of 0.1 %. It has been established that the modified lubricant reduces the mass wear of friction pairs by 43 %. Fig. 6 shows the contact diagram of the roller-roller friction pair. The rollers are made of ShKh-15 steel.
i
Fig. 6. The scheme of contacting the friction pair "roller-roller": 1 - upper roller; 2 - lower roller;
3 - bath with lubrication [67, © Nagdaev]
Roller parameters: top roller width - 10 mm; lower -12 mm; outer diameter - 50 mm; surface roughness -Ra = 0.8 |im; hardness - 60 HRC. The rollers underwent preliminary running-in on Litol-24 lubricant for 3 hours. The bottom roller was 1/3 of the diameter immersed in a grease bath. Evaluation of the running-in efficiency was the stabilization of the friction moment. After running in, the rollers were washed, dried, and weighed with an accuracy of 0.1 g. The rollers prepared for testing were installed in a friction machine and testing began at zero load, which was gradually increased to a selected value. The wear of the friction surfaces was determined by weighing on an analytical balance. The temperature change on the friction surfaces was determined with an MS 6530 instrument.
After testing, the surface of the upper roller had the form shown in Fig. 7.
In conclusion, it is noted that the modification of the lubricant with multilayer graphene improves performance and reduces the wear of friction surfaces.
Fig. 7. The roller surface after testing (100 times magnification) [67, © Nagdaev]
In [69], the authors set the task to study the processes of formation of a carbon film on the surface of the roller-roller friction unit in Litol-24 lubricant with an additive in the form of multilayer graphene in an amount of 0.2 wt. %. The condition of the friction surfaces was assessed using a Kromatech digital microscope and electron microscopy. Surface roughness and carbon film thickness were determined using a profilometer. The tests were carried out on an MI-1M friction machine. The friction pair "cylinder-ball" was investigated and the coefficient of friction and the state of the surface were determined. As a result of the research, it was found that, first of all, the film was formed in depressions on the friction surfaces (Fig. 8).
To determine the composition of the film, it was deformed to destroy it. According to the scanning electron microscopy images obtained by the authors, in several places the film came off the steel surface and cavities formed.
Fig. 8. Micrograph of the metal roller surface: a - at the 110 times magnification; b - the roller surface after 6 hours of operation in Litol-24 + graphene lubricant [68, © Nagdaev]
A similar film was obtained on the surface of a cylinder 26 mm in diameter made of BrShchF10-1 bronze. To determine the elemental composition of the film, mapping of individual sections was carried out and it was found that the flakes were composed of carbon, which confirms the composition of the carbon antifriction film formed on the surface of ShKh15 steel.
With a change in the thickness of the carbon film (the operating time of the friction unit), the roughness of the carbon surface changes. For the first hour, the roughness changes from class 5 to 9-10. From 1 hour to 3 hours the roughness decreases and then increases again. The moment of the friction force increases during the initial 90 minutes of operation of the friction unit from 5.4 to 5.8 (Nm), then gradually decreases to 4.9.
The test results showed the following values of friction coefficients: Steel ShKh15 - 0.288; Steel ShKh15 + Litol-24 - 0.197; Anti-friction film + + Litol-24 - 0.141; Steel ShKh15 + Litol-24 + 0.2 % graphene - 0.113; Anti-friction film + Litol-24 + + 0.2% graphene - 0.06.
Thus, the use of antifriction film and graphene-modified lubricant reduces the friction coefficient by about 3 times. It is also noted that there is a decrease in the oil absorption of a rough surface, which reduces the thickness of the lubricating film [68].
6. Conclusion
The modification of lubricants with nanomaterials significantly improves tribological characteristics and improves heat removal from the friction zone. The most promising antifriction additives are graphite nanoplates, the production of which is quite simple and cheap and, what is very important and environmentally friendly. When obtaining graphite nanoplates by liquid-phase shear exfoliation of crystalline graphite in base oil, the operation of uniform distribution of these nanoplates over the volume of grease is simplified. The addition of 0.1 wt.% graphite nanoplates containing no more than 25 graphene layers to the complex calcium lubricant reduces the diameter of the wear scar by 50%, the scuff index increases by 2.9 times, and the bearing capacity increases by 3.8 times. In addition, when using lubricants modified with graphite nanoplates, the formation of antifriction films on friction surfaces is observed, which reduces the friction coefficient by 3 times. It is necessary to develop an industrial technology for applying antifriction films using graphite nanoplates, which will reduce friction and wear of friction pairs in mechanisms and machines.
7. Funding
This research received no external funding.
8. Conflict of interests
The authors declare no conflict of interest.
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Information about the authors / Информация об авторах
Adel Baiti, Postgraduate Student, Tambov State Technical University (TSTU), Tambov, Russian Federation; e-mail: [email protected]
Saif S. Y. Aldavud, Postgraduate Student, TSTU, Tambov, Russian Federation; e-mail: eng. saif. suhail@ gmail.com
Avzh A. M. Algurabi, Postgraduate Student, TSTU, Tambov, Russian Federation; e-mail: razemr@ yandex.ru
Hicham Salhi, Ph.D., Assistant Professor, University of Batna, Algeria; e-mail: [email protected]
Vladimir F. Pershin, D. Sc. (Eng.), Professor, TSTU, Tambov, Russian Federation; ORCID 0000-0002-02139001; e-mail: [email protected]
Баити Адель, аспирант, Тамбовский государственный технический университет (ТГТУ), Тамбов, Российская Федерация; e-mail: adelbaitil @gmail.com
Альдавуд Саиф Сухаил Юсиф, аспирант, ТГТУ, Тамбов, Российская Федерация; e-mail: eng. saif. suhail@gmail. com
Альгураби Авж Ахмед Махмуд, аспирант, ТГТУ, Тамбов, Российская Федерация; e-mail: razemr@ yandex.ru
Салхи Хичам, Ph.D., ассистент профессора, Университет Батна, Алжир; e-mail: [email protected]
Першин Владимир Федорович, доктор технических наук, профессор, ТГТУ, Тамбов, Российская Федерация; ORCID 0000-0002-0213-9001; e-mail: [email protected]
Received 12 May 2023; Accepted 08 June 2023; Published 06 July 2023
Copyright: © Baiti A, Aldavud SSYu, Algurabi AM, Salhi H, Pershin VF, 2023. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).
