Copyright © 2020 A.G. Kravtsov. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Problems of Tribology, V. 25, No 4/98-2020, 6-12 Problems of Tribology Website: http://tribology.khnu.km.ua/index.php/ProbTrib E-mail: tribosenator@gmail.com DOI: https://doi.org/10.31891/2079-1372-2020-98-4-6-12 Evaluation of tribological characteristics of technical oils with fullerene compositions A.G. Kravtsov Kharkiv National Technical University of Agriculture named after Petro Vasylenko, Kharkiv, Ukraine E-mail: kravcov@gmail.com Abstract The paper presents experimental studies of the tribological characteristics of liquid lubricants of various viscosity classes and various groups of operation when using fullerene compositions. Tribological characteristics were evaluated on a four-ball friction machine according to GOST 9490. The use of fullerene compositions in the form of a finely dispersed fullerene powder, pre-dispersed (dis- solved) in vegetable high oleic oils, for example, rapeseed, with the subsequent addition of the resulting compo- sition to technical oils of various viscosity classes and various groups of operation, leads to the following posi- tive effect. The anti-wear properties of oils, which are assessed by the wear indicator, increase by 20,0…30,7 %, and the critical load on 18,8…25,0%. These indicators significantly exceed similar indicators when using fuller- ene fine powders without preliminary dispersion in vegetable oils, where the effect is on the border 11,1…15 %. Fullerene additives do not affect the extreme pressure properties of base oils, which are assessed by the scuffing load. This result makes it possible to state that the way to improve the tribological properties of lubri- cants by introducing a fine powder of fullerenes into base technical oils is ineffective. The experimental results obtained confirm the hypothesis about the possibility of the micelle formation mechanism in the lubricant under the action of the electrostatic field of the friction surface. The presence of a surfactant solvent (vegetable oil) allows you to "start" the micelle formation process at lower fullerene concen- trations and to obtain the effect of increasing anti-wear properties. Key words: fullerenes; vegetable oils; four-ball friction machine; tribological characteristics; critical load; welding load; bully index; technical oils. Introduction Today, the use of fullerenes is of great interest of C60, as additives to liquid lubricants. In recent years, a number of scientific articles have appeared, where the results of studies of the effect of fullerene additives to lub- ricants on the processes of friction and wear of metals have been presented and a conclusion has been made about the prospects of using such additives. An interesting and important feature of fullerene additives is that fullerenes are readily soluble in a wide class of organic and inorganic solvents. At the same time, poor solubility of fullerenes in technical oils was noted (mineral, semi-synthetic and synthetic). To date, the solubility of C60 in a large number of liquids has been de- termined and analyzed. It is shown that the solubility of fullerenes decreases with increasing polarity of the sol- vent. A number of unusual properties of fullerene solutions have been revealed, so for some solvents the effect of an anomalous dependence of the solubility of fullerene on temperature was found. At a temperature of about 280 о K maximum solubility is observed in these systems C60, after which it starts to decrease. Another interesting phenomenon observed in fullerene solutions С60, are the processes of formation and growth of clusters, which indicate the proximity of many solutions С60 to the class of colloidal systems. The de- fining moment of this phenomenon is the fact that the size of the fullerene lies on the border of the definition concept of a colloidal particle (according to colloidal chemistry, colloidal particles range in size from one na- nometer to several micrometers). The polarity of the solvent also has a great influence on this process. http://tribology.khnu.km.ua/index.php/ProbTrib https://doi.org/10.31891/2079-1372-2020-98-4-6-12 mailto:kravcov@gmail.com Problems of Tribology 7 The use of fullerene additives for technical liquid lubricants raises a number of questions about their ef- fectiveness, i.e. influence on anti-wear and extreme pressure properties. Interest in this phenomenon is of both fundamental and applied nature, which will allow the development of concepts for their application. Literature review The authors of the work [1] provide an overview of the literature on lubricants with added nanoparticles. The effect of nanoparticles on the tribotechnical characteristics of oils has been analyzed. The paper notes that the use of nanoadditives to lubricants leads to an increase in the viscosity of the base medium, a high bearing ca- pacity of the interface, a decrease in the friction coefficient, and an increase in wear resistance. Work [2] con- tains conclusions that the characteristics of a lubricant can be improved by using nanoadditives. Adding nanopar- ticles to conventional base oils is a promising direction. The work is devoted to an informative review of the ap- plication of nanoadditives to liquid lubricants and the prospects for its use in the production of oils. Similar con- clusions about the prospects of using nanomaterials in liquid lubricants have been made by the authors of the work. [3]. Works [4-6] are devoted to fullerenes as additives to lubricants. The authors note that the use of fullerenes reduces the coefficient of friction and increases the wear resistance of mates. In work [6] it is noted that the con- centration of the fullerene additive should be within 0,5…2,0% masses. In work [7] the result of using fullerene is given С60. The authors note a positive effect, however, they conclude that the mechanism of the senergism of fullerenes with base oil is unclear and requires further research. At the same time, it was noted in the work that a decrease in the friction coefficient with the addition of fullerenes to oils can reach 90% compared to the base oil. Analysis of works devoted to the use of fullerenes as additives to lubricants allows us to conclude that fullerenes are not dispersed (dissolved) in all technical oils [8-10]. This conclusion has been confirmed by the author of the work [11]. The introduction of fullerene additives in the form of a finely dispersed powder into technical oils of various viscosity classes and various groups of operation improves the anti-wear properties of oils, which are estimated by the wear indicator (increase by 11,1…15 %) and critical load (increase by 11,8…17,4%). Fullerene additives do not affect the extreme pressure properties of base oils, which are assessed by the scuffing load. This result makes it possible to state that the way to improve the tribological properties of lubricants by introducing a fine powder of fullerenes into base technical oils is ineffective. As follows from the above analysis of scientific works, such an insignificant effect is typical due to the intense clustering of fullerene molecules in the environment of industrial oils containing surfactants. Technical oils act as a highly polar sol- vent. The experimental results presented allow us to conclude that it is necessary to develop other, more tech- nological techniques and methods for introducing fullerene additives into technical lubricants, the theoretical jus- tification of which has been developed in the works [12, 13]. In the presented works, a mathematical model of the interaction of electrically active heterogeneous fine-dispersed systems at the interface friction surface - lubri- cant has been developed. From the analysis of the solution of the differential equation, which describes the pro- cess of interaction of electric fields, it has been found that the introduction of fullerenes into the base lubricant does not bring a large effect. It has been theoretically established that the use of fullerene "solvents", which can be high oleic vegetable oils, you can "start" the process of micelle formation, where the nucleus of the micelle is a fullerene molecule surrounded by molecules, for example, oleic or stearic acid. In work [12] theoretical studies that have shown, that the number of micelles is 50 times higher than the number of clusters in the base lubricating medium at the same concentration of fullerenes, and the dipole moment of micelles is an order of magnitude higher than the di- pole moment of clusters. At the same time, micelles are more effective, where a single fullerene molecule acts as a nucleus, rather than a cluster of fullerene molecules, which affects the size of the formed micelles. The role of the friction surface on the formation of clusters and micelles in the lubricant film at the friction surface is estab- lished. It is shown that under the action of the stress-strain state of the surface layers, the friction surface acts as "Generator of electrostatic force field", which affects the formation of an electric field in the volume of the oil film. Expressions are obtained for calculating the value of the total electric field strength of the system "friction surface + lubricant". In works [12, 13] it has been theoretically established that the electrostatic field of the friction surface is the driving force for the formation of an electrostatic field in the volume of the oil film, which is adsorbed on the friction surface. It shows that the process of cluster formation from fullerene molecules and micelles from fuller- ene molecules and fullerene solvent molecules affects the magnitude of the electrostatic field in the volume of liquid. Based on the review of publications and the performed modeling, it has been found that high oleic vege- table oil can act as a "strong solvent" of fullerenes. Purpose The purpose of this work is to carry out experimental studies of the tribological characteristics of liquid lubricants in the presence of fullerene compositions in their composition, which contain a fine powder of fuller- enes, previously dispersed (dissolved) in vegetable high oleic rapeseed oil in various concentrations. 8 Problems of Tribology Methods The tribological characteristics were assessed on a four-ball friction machine according to the method de- scribed in GOST 9490. 1. Fullerene compositions were prepared in the following mass concentrations. Base oils free from fuller- ene compositions. 2. Fullerene supplement 50 g/kg. It contains 0,5 g of fullerenes and 49,5 g of vegetable rapeseed oleic oil. Mass addition 50 g is introduced in 1000 g of base oil. 3. Fullerene supplement 100 g/kg. It contains 0,75 g of fullerenes and 99,25 g of vegetable rapeseed oleic oil. Mass addition 100 g is introduced in 1000 g of base oil. 4. Fullerene supplement 150 g/kg. It contains 1,0 g of fullerenes and 149,0 g of vegetable rapeseed oleic oil. Mass addition 150 g is introduced in 1000 g of base oil. 5. Fullerene supplement 200 g/kg. It contains 1,5 g of fullerenes and 198,5 g of vegetable rapeseed oleic oil. Mass addition 200 g is introduced in 1000 g of base oil. 6. Fullerene supplement 250 g/kg. It contains 2,0 g of fullerenes and 248,0 g of vegetable rapeseed oleic oil. Mass addition 250 g is introduced in 1000 g of base oil. Experimental studies included the determination of tribological characteristics on a four-ball machine of liquid lubricants of the following operation groups:  hydraulic mineral oil, by classification ISO corresponds to NM, viscosity class 10, trade mark MGP-10;  engine oil, according to SAE 30 classification, API CC operation group, trade mark M-10G2k;  gear oil, SAE 75W90 classification, API GL-5 service group, trade mark VALVOLINE. Fullerenes in the form of fine powder of various concentrations F=0,5…2,0 grams ―dissolved‖ in high oleic rapeseed oil and added to the above-listed technical base oils in the form of a fullerene composition (FK). Results In the process of experimental studies, the following tribological characteristics have been determined: - wear rate, Dw, mm; - critical load, Pcr, N; - welding load, Pweld, N; - bully index, Ib, N. Experimental results for hydraulic oil MGP-10 are presented in the table 1, for engine oil M-10G2k in the table 2, for transmission oil VALVOLINE GL-5 in the table 3. The experimental results were checked for reproducibility by Cochran's criterion according to the formu- las (1), (2):    N i i p S S G 1 2 max 2 , (1) where S 2 max- maximum value of variance for Dw, Рcr, Рweld, Ib in accordance; Si 2 - the value of the variance of the i - th experiment for Dw, Рcr, Рweld, Ib in accordance. The hypothesis was tested: pG < tabG , (2) where Gtab – tabular value of the Cochren's criterion, with a given confidence interval q = 0,90. During the statistical processing of the experimental results, the number of repetitions of the same type was determined, which make it possible to ensure the experimental error at the level of confidence equal to q = 0,90. The obtained experimental values allow us to conclude that tests on a four-ball machine of liquid lubri- cants for various purposes with different concentrations of fullerene compositions are reproducible and reliable under the condition of three repeats of the same type. In tables 1 - 3 the arithmetic mean values of three repetitions of tribological characteristics are given. Analysis of the data given in the tables 1 – 3 allows us to draw the following conclusions. Wear indicator Dw increases by 20,0…30,7 %, at the same time, the larger value refers to the MGP-10 hydraulic oil, and the lower value refers to the transmission oil VALVOLINE GL-5. It follows from the tables that improvements in anti-wear properties for all oils are characteristic up to a concentration 150 g/kg in the base lubricant. If we compare with the data presented in the work [11], where a finely dispersed fullerene powder was Problems of Tribology 9 used as an additive, then the wear index Dw increases by 11,1…15 %, at the same time, the improvement of anti- wear properties begins with a concentration of 0.2% masses, fullerenes in the lubricant. Table 1 Tribological characteristics of hydraulic oil MGP-10 with the addition of fullerenes (F) and with fullerene composition (FC) Lubricant Dw, mm Pcr, N Pweld, N Ib, N MGP-10, HM 0,65 784 1568 24 MGP-10 + 0,05% F 0,6 784 1568 24 MGP-10 +50 g/kg FC 0,55 823 1568 26 MGP-10 + 0,1% F 0,6 823 1568 24 MGP-10 +100 g/kg FC 0,5 921 1568 28 MGP-10 + 0,15% F 0,6 872 1568 26 MGP-10 +150 g/kg FC 0,45 980 1568 30 MGP-10 + 0,2% F 0,55 921 1568 27 MGP-10 +200 g/kg FC 0,45 980 1568 30 MGP-10 + 0,3% F 0,55 921 1568 27 MGP-10 +250 g/kg FC 0,45 980 1568 30 Table 2 Tribological characteristics of M-10G2k, API CC engine oil with the addition of fullerenes (F) and with full- erene composition (FC) Lubricant Dw, mm Pcr, N Pweld, N Ib, N M-10G2k, API CC 0,45 1235 2450 28 M-10G2k + 0,05% F 0,45 1235 2450 28 M-10G2k + 50 g/kg FC 0,40 1235 2450 30 M-10G2k + 0,1% F 0,45 1235 2450 29 M-10G2k + 100 g/kg FC 0,35 1303 2450 32 M-10G2k + 0,15% F 0,45 1235 2450 30 M-10G2k + 150 g/kg FC 0,34 1381 2450 34 M-10G2k + 0,2% F 0,4 1303 2450 31 M-10G2k + 200 g/kg FC 0,33 1381 2450 34 M-10G2k + 0,3% F 0,4 1381 2450 31 M-10G2k + 250 g/kg FC 0,33 1381 2450 34 Table 3 Tribological characteristics of VALVOLINE GL-5 gear oil with the addition of fullerenes (F) and with full- erene composition (FC) Lubricant Dw, mm Pcr, N Pweld, N Ib, N GL-5 0,45 1960 4900 76 GL-5 + 0,05% F 0,45 1960 4900 76 GL-5 + 50 g/kg FC 0,40 2067 4900 82 GL-5 + 0,1% F 0,45 1960 4900 78 GL-5 + 100 g/kg FC 0,38 2195 4900 84 GL-5 + 0,15% F 0,45 2067 4900 80 GL-5 + 150 g/kg FC 0,36 2323 4900 88 GL-5 + 0,2% F 0,4 2195 4900 82 GL-5 + 200 g/kg FC 0,36 2323 4900 88 GL-5 + 0,3% F 0,4 2195 4900 82 GL-5 + 250 g/kg FC 0,36 2323 4900 88 10 Problems of Tribology The results obtained allow us to conclude that the concentration limit for the fullerene composition in in- dustrial oils can be 100…150 g/kg. A further increase in concentration does not bring a positive effect. The positive effect is also characteristic of the indicator – critical load Рcr, which characterizes the range of performance of anti-wear additives. The critical load is increased by 18,8…25,0%, the larger value refers to the hydraulic oil and the lower value refers to the transmission oil. At the same time, an increase in the critical load for all oils is characteristic up to a concentration 150 g/kg, in the base lubricant. If we compare with the data presented in the work [11], where a finely dispersed powder of fullerenes was used as an additive, then the criti- cal load increases by 11,8…17,4%. Changes in the value of the welding load Рweld during the experiments were not recorded, this allows us to conclude that the fullerene composition obtained by dissolving a fine powder of fullerenes in vegetable high ole- ic rapeseed oil does not improve the extreme pressure properties of liquid lubricants, but is only an antiwear ad- ditive. Bully index Ib, which characterizes the integral tribological characteristic of the lubricant increases by 15,7…25,0%, the larger value refers to the hydraulic oil and the lower value refers to the transmission oil. In work [11], where a finely dispersed fullerene powder was used as an additive, it was noted that the bully index Ib increases by 7,8…12,5%. The obtained experimental results confirm the [12, 13] hypothesis about the possibility of the mechanism of micelle formation in the lubricant under the action of the electrostatic field of the friction surface. The pres- ence of a surfactant solvent (vegetable oil) allows «start» process of micelle formation at lower concentrations of fullerenes and to obtain the effect of increasing anti-wear properties on 20,0…30,7 %, while the dissolution of fullerenes in the base oil without the use of a solvent has the effect of increasing the anti-wear properties on 11,1…15%. The obtained values of the improvement of antiwear properties coincide with the values obtained by other researchers, whose work is reviewed above. Conclusions The use of fullerene compositions in the form of a finely dispersed powder of fullerenes, previously dis- persed (dissolved) in vegetable high oleic oils, for example, rapeseed, with the subsequent addition of the result- ing composition to technical oils of different viscosity classes and different groups of operation, leads to the fol- lowing positive effect. The anti-wear properties of oils, which are assessed by the wear indicator, increase by 20,0…30,7 %, and the critical load on 18,8…25,0%. These indicators significantly exceed similar indicators when using fullerene fine powders without preliminary dispersion in vegetable oils, where the effect is on the border 11,1…15 %. Fullerene additives do not affect the extreme pressure properties of base oils, which are assessed by the bully load. This result makes it possible to state that the way to improve the tribological properties of lubricants by introducing a finely dispersed powder of fullerenes into base technical oils is ineffective. As follows from the above analysis of scientific works such an insignificant effect is typical due to the intense clustering of fullerene malecules in the environment of industrial oils containing surfactants. Technical oils act as a highly polar sol- vent. The obtained experimental results confirm the [12, 13] hypothesis about the possibility of the mechanism of micelle formation in the lubricant under the action of the electrostatic field of the friction surface. The pres- ence of a surfactant solvent (vegetable oil) allows you to «start» the micelle formation process at lower fullerene concentrations and to obtain the effect of increasing anti-wear properties. References 1. Anurag Singh, Prashant Chauhan, Mamatha T. G. A review on tribological performance of lubricants with nanoparticles additives // Materials today: proceedings Volume 25, Part 4, 2020, Pages 586-591 https://doi.org/10.1016/j.matpr.2019.07.245 [English] 2. Sheida Shahnazar, Samira Bagheri, Sharifah Bee Abd Hamid Enhancing lubricant properties by nanoparticle additives // International Journal of Hydrogen Energy Volume 41, Issue 4, 2015, Pages 3153-3170 https://doi.org/10.1016/j.ijhydene.2015.12.040 [English] 3. Imran Ali, Al Arsh Basheer, Anastasia Kucherova, Nariman Memetov, Tatiana Pasko, Kirill Ovchinnikov, Vladimir Pershin, Denis Kuznetsov, Evgeny Galunin, Vladimir Grachev, Alexey Tkachev Advances in carbon nanomaterials as lubricants modifiers // Journal of Molecular Liquids Volume 279, 2019, Pages 251-266 https://doi.org/10.1016/j.molliq.2019.01.113 [English] 4. Yanli Yao, Xiaomin Wang, Junjie Guo, Xiaowei Yang, Bingshe Xu Tribological property of onion-like fullerenes as lubricant additive // Materials Letters Volume 62, Issue 16, 2007, Pages 2524-2527 https://doi.org/10.1016/j.matlet.2007.12.056 [English] 5. L. Rapoport, Y. Feldman, M. Homyonfer, H. Cohen, J. Sloan, J. L. Hutchison, R. Tenne Inorganic fullerene-like material as additives to lubricants: structure–function relationship // Wear Volumes 225–229, Part 2, 1999, Pages 975-982 https://doi.org/10.1016/S0043-1648(99)00040-X [English] Problems of Tribology 11 6. F. A. Yunusov, A. D. Breki, E. S. Vasilyeva, O. V. Tolochko The influence of nano additives on tribological properties of lubricant oil // Materials today: proceedings Available online 14 February 2020 https://doi.org/10.1016/j.matpr.2020.01.447 [English] 7. Xiaowei Li, Xiaowei Xu, Yong Zhou, Kwang-Ryeol Lee, Aiying Wang Insights into friction dependence of carbon nanoparticles as oil-based lubricant additive at amorphous carbon interface // Carbon Volume 150, 2019, Pages 465-474 https://doi.org/10.1016/j.carbon.2019.05.050 [English] 8. Bezmel'nitsyn V. N., Yeletskiy A. V., Okun' M. V. Fullereny v rastvorakh // Uspekhi fizicheskikh nauk. — 1998, № 11, 1195—1220 [Russian] 9. Gindzburg B. M., Baydakova M. V., Kireyenko O. F. [i dr.]. Vliyaniye fullerenov S60, fullereno-vykh sazh i drugikh uglerodnykh materialov na granichnoye treniye skol'zheniye metallov // Zhurnal tekhnicheskoy fiziki. — 2000, № 12, 87—97 [Russian] 10. Yakh'yayev N. YA., Begov ZH. B., Batyrmurzayev SH. D. Novaya smazochnaya kompozitsiya dlya modifikatsii poverkhnostey tribosopryazheniy sudovogo malorazmernogo dizelya // Vestnik AGTU. Ser.: Morskaya tekhnika i tekhnologiya. — 2009, № 1, 47—52 [Russian] 11. Kravcov A. G. Evaluation of tribological characteristics of liquid lubricants with fullerene additives // Problems of Tribology, -2020, V. 25, No 3/97, 50-54 https://doi.org/10.31891/2079-1372-2020-97-3-50-54 [English] 12. ravcov . G. a rabot a matematiches oy modeli v aimodeystviya ele triches i a tivny h gete- rogenny h mel odispersny h sistem na granitse ra dela pover hnost treniya — sma ochnaya sreda // Problemi tribolog . — 2017, № 1, 89—99 [Russian] 13. ravcov . G. Modelirovaniye formirovaniya maslyanoy plen i na pover hnosti treniya pri nali-chii fullerenovy h dobavo v sma ochnom materiale i yeye vliyaniye na s orost i nashivaniya tribosistem // Problemi tribolog . — 2018, № 1, 69—77 [Russian] 12 Problems of Tribology Кравцов А.Г. Оцінка трибологічних характеристик технічних олив з фулереновими компо- зиціями В роботі представлені експериментальні дослідження трибологічних характеристик рідких масти- льних матеріалів різного класу в язкості і різних груп експлуатації при використанні фуллеренових ком- позицій. Трибологічні характеристики оцінювалися на четирьохкульковій машині тертя згідно ГОСТ 9490. Використання фулеренових композицій у вигляді дрібнодисперсного порошку фулеренів, попере- дньо диспергированого (розчиненого) в рослинних високоолеінових оліях, наприклад, ріпакової, з пода- льшим додаванням отриманої композиції в технічні оливи різних класів в язкості і різних груп експлуа- тації, призводить до наступного позитивного ефекту. Протизносні властивості олив, які оцінюються по- казником зносу, збільшуються на 20,0 ... 30,7%, а критичне навантаження на 18,8 ... 25,0%. Дані показни- ки значно перевищують аналогічні показники при застосуванні фелеренових дрібнодисперсних порошків без попереднього диспергування в рослинних оліях, де ефект знаходиться на межі 11,1 ... 15%. На протизадирні властивості базових технічних олив, які оцінюються навантаженням зварювання, фулеренові композиції не впливають. Такий результат дозволяє констатувати, що шлях поліпшення три- бологічних характеристик мастильних матеріалів шляхом введення фулеренових композицій в базові те- хнічні оливи є малоефективним. Отримані експериментальні результати підтверджують гіпотезу про можливість механізму міце- лоутворення в змащувальному матеріалі під дією електростатичного поля поверхні тертя. Наявність по- верхнево-активного розчинника (рослинна олія) дозволяє «запустити» процес міцелоутворення при більш низьких концентраціях фулеренів і отримати ефект підвищення протизносних властивостей. Ключові слова: фулерени; рослинні олії; чотирьохкулькова машина тертя; трибологічні характе- ристики; критичне навантаження; навантаження зварювання; індекс задира; технічні оливи.