ANTIFRICTIONAL METALLIC MATERIALS MODIFIED WITH HEXAGONAL BORON NITRIDE A.Gachechiladze, O.Tsagareishvili, M.Darchiashvili, B.Margiev, L.Rukhadze, L.Chkhartishvili F.Tavadze Institute of Metallurgy and Materials Science, Tbilisi, Georgia,
[email protected] Introduction As is known, wear is a common problem for different types of surfaces of engines’ components subjected to dynamic loadings. In the industry, a number of lubricants are used in order to reduce friction and wear in machines with moving parts. Typically, lubricants utilized are in the form of liquids or greases. But, materials of such consistency do not meet all the requirements for heavy-loaded friction pairs operating under extreme conditions – extremely high or extremely low temperatures, in ultra-high vacuum, under extreme contact pressures, at high or low sliding speeds, etc. Usually, this problem is solved by the preparation of liquid lubricants containing solid additives of layered crystalline materials. However, widely used additives of such kind, e.g., molybdenum disulfide (MoS 2), graphite, and the like, contain heavy metals, sulfur, carbon, etc. and then are environmental pollutants. They can be successfully replaced by promising boron-based “green” lubricants [1] or boriding the rubbing surfaces [2]. In particular, the addition of even 1wt.% hexagonal boron nitride (h-BN) powder in oil, a grease or fuel is sufficient to provide the excellent tribological properties. For the first time, Kimura et al. had conducted [3] a series of detailed sliding experiments, which reveals the behavior of h-BN when added to the lubricating oil. In the case of sliding of bearing steel vs. itself, h-BN slightly increases the coefficient of friction, but dramatically decreases wear; and boron is found to remain on rubbing surfaces in form of non-stoichiometric oxide. However, when bearing steel is sliding against cast iron, the powdered h-BN decreases the coefficient of friction and the remnant is mostly BN. There is suggested a number of recent results confirming that h-BN is effective in reducing wear if used as a liquid lubricant additive – see citations in [4]. The increase in dispersion of h-BN suppresses the sedimentation processes in a liquid lubricant that improves its performance. This is particularly important in the metalwork, when the cutting or grinding processes are accompanied by the significant heat-releasing. A graphite-like lamellar structure of h-BN predetermines its tribological properties: intra-layer bonds are strongly covalent with a deal of ionicity, whereas weak van der Waals polarization-forces are responsible for the inter-layers interaction. Therefore, layers of h-BN are easily moving relative to each other. In the process of rubbing, the single-crystalline h-BN particles are spontaneously aligned so that their basal planes are parallel to the rubbing metal surfaces. Thus, the friction between metal surfaces is replaced by the internal friction between BN-hexagonal layers. Sometimes in nanoscale junctions containing h-BN, even the effect of super-lubricity occurs, which means the almost frictionless tribological state. From the friction experiments conducted with h-BN in sliding contacts with various metals, this material is known as a good solid lubricant as well [5].
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An alternative way to reduce the wear and friction is the production of self-lubricating materials by the introducing h-BN particulates in the surface layer or in the bulk of rubbing metallic materials. Such approach has been a major topic of interest in the last decade. It has been successfully examined in different Fe-based alloys – low-carbon [6], austenitic [7], stainless [8], high-Cr [9], CrMo- [10], TiC-steels [11], and distaloy [12], as well as in certain Cu-based pseudoalloys [13]. This issue is closely related to the processes of growth of h-BN layers on Fe[14–18] or Cu-surfaces [19–26] and vice versa – formation of Fe- [27–29] and Cu-coatings [30,31] onto the h-BN basal plane. Here we propose a novel approach to fabrication of antifrictional (self-lubricating) metallic materials containing brass and iron matrices and modifier particulates of h-BN in surface layer or bulk. Experimental In this paper, we study tribological properties of composite materials based on brass (Cu 68.5wt.% and Zn 31.3wt.%) and iron (Fe). As an antifriction component used nanosized h-BN obtained by a specially developed technology [32]. As a precursor of the synthesis of h-BN it was used a mixture of boric acid (H3BO3) and urea (CH4ON2) or sodium tetraborate. Processes were conducted in atmosphere of ammonia (NH3) or ammonium chloride in a temperature range of 850–1100°C. The resulting product is a textured nanocrystalline boron nitride with an average density of ~2.1g/cm3. The Fig.1 presents the XRD pattern of the boron nitride powder synthesized at temperature of 870°C in ammonia atmosphere using as precursors sodium tetraborate and ammonium chloride, and washed in distilled water. This diffraction pattern shows that, obtained boron nitride has a hexagonal structure. The presence of diffraction peaks broadened to varying degree indicates the non-spherical form of crystals. As is known from the literature (see, e.g., [33,34] and references therein), they should be disk-shaped.
Fig.1. XRD pattern of boron nitride powder synthesized at 870°C in ammonia atmosphere using as precursors sodium tetraborate and ammonium chloride and washed in distilled water.
Sizes of disk-shaped particles were evaluated by the Selyakov–Scherer method [35]. It means that, on the basis of measurements of the peak broadenings and the formula
(2) K / Dhkl cos there is estimated the size of a substance crystallite. Here (2) is the half-width of the interference peak (in radians); K is the factor of shape of the crystal (in 122
general, its value can vary in the range from 0.98 to 1.39), which for crystals of hexagonal boron nitride can be taken as 1.20; is the wavelength (in our case, i.e., for Cu K radiation it equals to 0.1539nm); Dhkl is the grain size along the normal to the (hkl ) -plane (in nm); and is the Bragg angle. The peak broadening can include both broadenings due to small grain sizes and so-called apparatus broadening, and the broadening caused by the deformations and defects of different types. The sizes of the grains were estimated from the interference maxima (004) and (100), for which the apparatus broadenings were ≈0.5° and ≈0.3°, respectively. If neglect the broadening caused by strain and defects, we find that the average thickness of the diskshaped grains of boron nitride, ≈25nm, and their average diameter, ≈240nm. Investigations of the products synthesized from various reagents showed that, among the nitrogen compounds in the process of synthesis the most effective is ammonia: if compared with the nitrogen gas its use reduces the temperature of the synthesis beginning.
Fig.2. XRD pattern of copper-plated h-BN.
In order to improve adhesion to the matrix alloy, brass or iron, boron nitride was chemically plated by copper or iron, respectively. XRD image of the copper-plated h-BN is shown in Fig.2.
(a)
(b)
Fig.3. Microstructures of (a) mechanically and (b) chemically polished samples of composite brass+1wt.%h-BN (×400).
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Plated nanocrystalline boron nitride introduced into molten matrix metal (brass or low-carbon iron). The microstructures of the obtained composite material based on brass are shown in Fig.3. The mentioned antifrictional additive has been distributed in the matrix without any preferential accumulation at the grain boundaries or other structural defects. Determination the tribological properties of metallic materials modified with boron nitride conducted using installation SMC–2 specially designed for the study of friction and wear processes in metals and alloys. Its principle of operation is based on the wear of pairs of samples pressed together by a force of a pre-specified value. During the tests, it was measured the friction torque using a non-contact inductive coupler. The value of the electromotive force, which was assumed to be proportional to the friction torque, fixed by the multi-meter X–18 connected to a personal computer for a software-elaboration the obtained data. Frequency of measurements in these experiments was 1Hz. The method used was “disk drive”. The test sample served for the first disc, and the second, socalled pin-body, was selected respectively to a task. The geometrical dimensions of disks were D=45–50mm and h=10–12mm. Slip ratio was about 10 %. Normal load on the drive controlled by means of bagged spring and its value was ranged in 200–1000N in accordance to the characteristics of the material. As is known, due to wavy surfaces of machine parts actual contact area (ACA) – area at which the micro-irregularities touch each other – is less than that of nominal contact surface: usually ACA small and amounts to 0.01–0.1% of the nominal contact area. Accordingly, for the evaluation of actual pressure on parts in contact, it is too important to know their ACA. From the modern methods of determining the ACA, we have chosen the method of thin plates, which is widely used in engineering practice. The method consists in following. Between the contacting surfaces, the dye-containing plate is placed and after unloading, the ACA is estimated by area and geometry of the left imprint. These estimates were conducted using the special software.
Fig.4. The imprint in conditions of actual contact load of 400N.
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Photos of typical imprints are shown in Fig.4. ACA values estimated before and after grinding of contacting parts at different loads. Duration of grinding was determined based on the dynamics of changing in the friction torque. Stable (constant over time) value of friction torque of rubbing parts reached after about 30 min. After a stable value of the friction torque is reaching, the ACA estimated and based on it the value of the actual load selected. Estimates of ACA obtained at normal load of 200N lead to the actual pressure value of 3660N/mm2. Results As is known, the stabilization of geometric sizes is considered as a very important factor in the operation of friction pairs working in extreme conditions. In order to determine the extent to which succeeded in achieving of such kind of stabilization, it was measured thermal expansion coefficient (TEC) of the obtained composite. Fig.5 shows that, introducing of a friction modifier has insignificant effect on the TEC. This fact reveals the stabilization of the geometric sizes for mating friction pair made of a composite brass+1wt.%h-BN. Introducing of 5wt.%h-BN in the matrix alloy at room temperature increases the thermal conductivity of the matrix from 25 to 50W/m∙K. The increase in the thermal conductivity indicates a possible intensification of heat transfer processes in heavily loaded friction pairs made of the composite material brass + 5wt.%h-BN. As it has been noted, the study of the tribological properties of the materials was carried out on a “disk drive”. The sliding speed was 0.9m/s. Fig.6 shows dependence of the degree of wear on number of revolutions and friction modifier content – see Fig.7. This figure shows that the introducing of the friction modifier, h-BN, is essential to reduce the mass losses in the composite brass + 1wt.%h-BN because of changing the mechanism of wear if compared to the initial sample.
Fig.5. Temperature dependence of thermal expansion coefficient of brass and composite material brass + 1wt.%h-BN.
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Fig.6. Dependence of degree of wear on friction modifier content and number of revolutions.
Fig.7. Dependence of degree of wear of composite materials from content of boron nitride (340 rpm).
Fig.8 shows the optical microscopic image of wear for brass L67 and composite L67 + 1wt.%h-BN for extreme load conditions. Testing under extreme load conditions carried out at a “catastrophic” wear as well. Dimensions and morphology of wear products indicate that the friction modifier changes the mechanism of wear and greatly reduces the intensity of the wearprocess. From the Fig.8, one can see that changes in the morphology and the linear sizes of wear particles at almost identical loads are too radical. Further increase in the content of boron nitride up to 5wt.% increases the mass losses in the process of friction, what is probably due to the decrease in the shear modulus and the loosening of the composite structure.
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(а)
(b)
Fig.8. Wear products of (a) brass and (b) brass + 1wt.%h-BN at almost identical loads.
Fig.8 shows the wear products, while Fig.9 shows dependence of the degree of wear of the investigated samples on the speed at different normal loads.
Fig.9. Dependence of degree of wear of investigated samples on speed at different normal loads.
In the Tab.1, there are presented the values of the linear wear rate k d / s for the investigated samples at 12500rpm and normal load of 117N. D0 and D are the initial and final diameters of a test disc-shaped sample, respectively; d ( D0 D) / 2 is the thickness of the removed layer; and s is the distance traveled in mm. Tab.1. Linear wear of brass and iron, and their composites. Sample
D0 , mm
D , mm
d , mm
k
Brass Brass + 1wt.%h-BN Fe Fe + 1wt.%h-BN
49.76 50.33 50.12 50.28
49.63 50.24 49.76 50.02
0.065 0.045 0.180 0.130
0.21 · 10–7 0.14 · 10–7 0.92 · 10–7 0.66 · 10–7
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From the results obtained in this work, it implies that the introducing of the friction modifier, the hexagonal boron nitride, in a metal alloy significantly reduces wear of the matrix-metal due to the changing the mechanism of wear and the optimal amount of modifier should be ~1wt.%. Tab.2. Friction coefficient and efficiency resource. “Third body” Gear oil PEG PEG + 5wt.%h-BN
Friction coefficient without “third body” 0.039 0.039
Friction coefficient with “third body” 0.006 0.007
0.039
0.006
Resource, km 3.0 14.7 17.7
For a “third body”, it was selected the environmentally friendly polymer polyethyleneglycol (PEG) C2nH4n+2On+1 modified with boron nitride. We used PEG with density of 1.1–1.2g/cm3, which is a solid phase substance. Thickness of the coating of the “third body” was ~200μm. For “third body”, it was tested gear oil, BN layers obtained by the aerosol spraying, PEG without modifier, PEG + 10wt.%MoS2, and PEG + 1wt.%h-BN. From the results obtained for the friction coefficient, one can see that, the gear oil and PEG with and without a modifier do not affect significantly the value of the friction coefficient, but significantly (by 5–6 times) increases the efficiency resource of the “third body” – the distance passed to restore the original moment of friction – see, Tab.2. Conclusions A method for producing anti-friction composite material based on brass (Cu 68.5wt.% and Zn 31.3wt.%) is proposed. Nanocrystalline hexagonal boron nitride obtained by low-temperature chemical synthesis used as a friction modifier. The morphology and size of nanoparticles of hBN determined from the broadening of X-ray diffraction maxima. It has been established that they have disk-shape with average diameter of 240nm and thickness of 25nm. To obtain a composite alloy brass + 1wt.%h-BN, the copper-plated nanocrystalline h-BN obtained by chemical method directly was introduced into the melted matrix, the brass. Study of the TCE of the composite material brass + 1wt.%h-BN showed that, a friction modifier has an insignificant effect on the TEC of the matrix, what provides the stabilization of the size factor of the mating friction pair made of this material. Along with this, the increase by twice in the coefficient of thermal conductivity as compared to the matrix indicates the possibility of intensification of heat exchange processes on the mating surfaces of friction pairs within the expected temperature range of operation. This will results in the stable operation of friction pairs at high temperatures and loads. Study of tribological properties shows that, the friction modifier h-BN radically changes the wear mechanism as it is evidenced by changes in the morphology and linear sizes of wear particles at “catastrophic” load (225N). We estimate the optimal amount of h-BN as a friction modifier could be around of 1wt.%. Tribo-measurements conducted at lower loads (20 and 51N) confirm the positive role of the effect of h-BN on the tribological properties of this novel antifrictional material. From above results it is clear that, at the use of third bodies of various types it can be observed a significant change in the moment of friction, but also a significantly (by 5–6 times) 128
increase in efficiency resource of “third body” when as friction modifier is used environmentally friendly composite PEG + h-BN. References 1.
A.Erdemir. Advances in boron-based lubricants and lubricant additives. In: Proc. 4th Int. Boron Symp. 2009, Eskişehir: Osmangazi Univ. Press, 3-9. 2. A.Greco, K.Mistry, V.Sista, O.Eryilmaz, A.Erdemir. Friction and wear behavior of boron based surface treatment and nanoparticles lubricant additives for wind turbine gearbox applications. Wear, 2011, 271, 9-10, 1754-1760. 3. Y.Kimura, T.Wakabayashi, K.Okada, T.Wada, H.Nishikawa. Boron nitride as a lubricant additive. Wear, 1999, 232, 2,199-206. 4. L.Chkhartishvili. Correlation between surface specific area and particles average size: Hexagonal boron nitride nano-powders. Nano Studies, 2012, 6, 1, 65-76. 5. D.H.Buckley. Friction and transfer behavior of pyrolytic boron nitride in contact with various metals. ASLE Trans., 1978, 21, 2, 118-124. 6. Sh.Chatterjee, J.D.Majumdar, S.M.Shariff, G.Padmanabham, A.R.Choudhury. Effect of laser posttreatment on Al2O3–TiB2–TiN composite coating with free hBN. Int. J. Adv. Manuf. Technol., 2012, 61, 5-8, 559-567. 7. S.Emura, M.Kawajiri, X.Min, S.Yamamoto, K.Sakuraya, K.Tsuzaki. Machinability improvement and its mechanism in SUS304 austenitic stainless steel by h-BN addition. J. Iron & Steel Inst. Jpn., 2012, 98, 7, 358-367. 8. S.Mahathanabodee, T.Palathai, S.Raadnui, R.Tongsri, N.Sombatsompo. Effects of hexagonal boron nitride and sintering temperature on mechanical and tribological properties of SS316L / h-BN composites. Mater. & Design, 2013, 46, 1, 588-597. 9. K.Sakuraya, H.Okada, F.Abe. BN type inclusions formed in high Cr ferritic heat resistant steel. Energy Mater., 2006, 1, 3, 158-166. 10. Y.-N.Wang, Y.-P.Bao, M.Wang, L.-C.Zhang. Precipitation behavior of BN type inclusions in 42CrMo steel. Int. J. Min. Metall. & Mater., 2013, 20, 1, 28-36. 11. S.Mridha, N.I.Taib, A.N.Idriss. Composite coating on steel surfaces by adding TiC and h-BN particulates under TIG torch melting. Adv. Mater. Res., 2012, 576, 1, 463-466. 12. J.Karwan–Baczewska, M.Rosso. Effect of boron on microstructure and mechanical properties of PM sintered and nitrided steels. Powd. Metall., 2001), 44, 3, 221-227. 13. V.V.Garbuz, L.I.Kostenetskaya, V.Ya.Petrishchev, M.M.Churakov. Interaction between boron nitride and molybdenum in the conditions of formation of pseudoalloys. Powd. Metall. & Met. Ceram., 1991, 30, 1, 73-75. 14. T.Takahashi, H.Itoh, A.Takeuchi. Chemical vapor deposition of hexagonal boron nitride thick film on iron. J. Cryst. Growth, 1979, 47, 2, 245-250. 15. T.E.Warner, D.J.Fray. Nitriding of iron boride to hexagonal boron nitride. J. Mater. Sci., 2000, 35, 21, 5341-5345. 16. D.Babonneau, F.Pailloux, J.-P.Eymery, M.-F.Denanot, Ph.Guérin, E.Fonda, O.Lyon. Spontaneous organization of columnar nanoparticles in Fe–BN nanocomposite films. Phys. Rev. B, 2005, 71, 035430, 1-11. 17. A.Essafti, A.Abouelaoualim, J.L.G.Fierro, E.ech-Chamikh. Structural and optical properties of amorphous oxygenated iron boron nitride thin films produced by reactive co-sputtering. Thin Solid Films, 2009, 517, 15, 4281-4285. 18. N.A.Vinogradov, A.A.Zakharov, M.L.Ng, A.Mikkelsen, E.Lundgren, N.Martensson, A.B.Preobrajenski. One-dimensional corrugation of the h-BN monolayer on Fe(110). Langmuir, 2012, 28, 3, 1775-1781. 19. M.Hubáček, T.Sató. The effect of copper on the crystallization of hexagonal boron nitride. J. Mater. Sci., 1997, 32, 12, 3293-3297.
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20. Sh.Chatterjee, Zh.Luo, M.Acerce, D.M.Yates, A.T.Ch.Johnson, L.G.Sneddon. Chemical vapor deposition of boron nitride nanosheets on metallic substrates via decaborane / ammoniar. Chem. Mater., 2011, 23, 2, 4414-4416. 21. N.Guo, J.Wei, L.Fan, Y.Jia, D.Liang, H.Zhu, K.Wang, D.Wu. Controllable growth of triangular hexagonal boron nitride domains on copper foils by an improved low-pressure chemical deposition method. Nanotechnol., 2012, 23, 415605, 1-4. 22. S.Joshi, D.Ecija, R.Koitz, M.Iannuzzi, A.P.Seitsonen, J.Hutter, H.Sachdev, S.Vijayaraghavan, F.Bischoff, K.Seufert, J.V.Barth, W.Auwärter. Boron nitride on Cu(111): An electronically corrugated monolayer. Nano Lett., 2012, 12, 11, 5821-5828. 23. K.K.Kim, A.Hsu, X.Jia, S.M.Kim, Y.Shi, M.Hofmann, D.Nezich, J.F.Rodriguez–Nieva, M.Dresselhaus, T.Palacios, J.Kong. Synthesis of monolayer hexagonal boron nitride on Cu foil using chemical vapor deposition. Nano Lett., 2012, 12, 1, 161-166. 24. S.Suzuki, H.Hibino. Chemical vapor deposition of hexagonal boron nitride. e-J. Surf. Sci. & Nanotechnol., 2012, 10, 1, 133-138. 25. J.Gómez Díaz, Y.Ding, R.Koitz, A.P.Seitsonen, M.Iannuzzi, J.Hutter. Hexagonal boron nitride on transition metal surfaces. Theo. Chem. Acc., 2013, 132, 4, 1-17. 26. S.Roth, F.Matsui, Th.Greber, J.Osterwalder. Chemical vapor deposition and characterization of aligned and incommensurate graphene / hexagonal boron nitride heterostack on Cu(111). Nano Lett., 2013, 13, 6, 2668-2675. 27. F.D.Natterer, F.Patthey, H.Brune. Ring state for single transition metal atoms on boron nitride on Rh(111). Phys. Rev. Lett., 2012, 109, 066101, 1-4. 28. F.D.Natterer, F.Patthey, H.Brune. Erratum: Ring state for single transition metal atoms on boron nitride on Rh(111) (Physical Review Letters (2012) 109 (066101)). Phys. Rev. Lett., 2012, 109, 089901, 1-1. 29. H.Yan, A.H.Wang, X.L.Zhang, Z.W.Huang, W.Y.Wang, J.P.Xie. Nd : YAG laser cladding Ni base alloy / nano-h-BN self-lubricating composite coatings. Mater. Sci. & Technol., 2010, 26, 4, 461-468. 30. V.M.Perevertajlo, O.B.Loginova, I.A.Petrusha, A.A.Svirid. Wettability of pyrolytical boron nitride by the metallic melts. Superhard Mater., 1996, 2, 1, 12-16. 31. D.Sen, R.Thapa, K.Bhattacharjee, K.K.Chattopadhyay. Site dependent metal adsorption on (3×3) hBN monolayer: Stability, magnetic and optical properties. Comput. Mater. Sci., 2012, 51, 1, 165171. 32. B.G.Margiev, R.V.Chedia, A.A.Gachechiladze, L.S.Chkhartishvili, I.L.Kupreishvili, A.G.Mikeladze, D.L.Gabunia, O.A.Tsagareishvili. Production of nanocrystalline boron nitride by chemical synthesis. In: Abs. 3rd Int. Samsonov Memorial Conf. Mater. Sci. Ref. Comp. 2012, Kiev: IPMS–KPI, 204-204. 33. L.Chkhartishvili. Morphology model for nano-powdered boron nitride lubricants. In: Cont. 2nd Int. Conf. Nanotechnol. 2012, Tbilisi: Nekeri, 86-99. 34. L.Chkhartishvili, T.Matcharashvili, R.Esiava, O.Tsagareishvili, D.Gabunia, B.Margiev, A.Gachechiladze. Powdered hexagonal boron nitride reducing nano-scale wear. In: Phys., Chem. & Appl. Nanostr. 2013, Singapore: World Scientific, 438-440. 35. Ya.S.Usmanskij, Yu.A.Skakov, A.N.Ivanov. L.N.Rastorguev. Crystallography, X-ray and Electron Microscopies. 1982, Moscow: Metallurgy.
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АНТИФРИКЦИОННЫЕ МЕТАЛЛИЧЕСКИЕ МАТЕРИАЛЫ, МОДИФИЦИРОВАННЫЕ ГЕКСАГОНАЛЬНЫМ НИТРИДОМ БОРА А. Гачечиладзе, О. Цагареишвили, М. Дарчиашвили,B. Маргиев, Л. Рухадзе, Л. Чхартишвили
Институт металлургии и материаловедения им. Ф.Н. Тавадзе
[email protected] Реферат В работе предлагается метод получения антифрикционного композиционного материала на основе латуни (Cu 68.5масс.% , Zn 31.3масс.%). В качестве модификатора трения использован гексагональный нитрид бора h-BN. Интенсификация процессов теплообмена на контактных поверхностях пар трения приводит к их стабильной работе при высоких температурах и нагрузках. Размеры и морфология продуктов износа показывают, что модификатор трения меняет механизм износа и значительно уменьшает его интенсивность. Оптимальное количество модификатора трения составляет приблизительно 1масс.%. В качестве “третьего тела“ в работе предлагается экологически чистый композиционный материал полиэтиленгликол + нитрид бора (ПЭГ+ h-BN). Ресурс эффективности применяемого “третьего тела“, определяемый по изменению момента трения, существенно (в 5-6 раз) выше по сравнению с применяемыми трансмиссионными маслами.
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