ISSN 20751133, Inorganic Materials: Applied Research, 2015, Vol. 6, No. 4, pp. 289–292. © Pleiades Publishing, Ltd., 2015. Original Russian Text © Yu.K. Mashkov, O.V. Kropotin, S.V. Shil’ko, V.A. Egorova, O.V. Chemisenko, 2015, published in Materialovedenie, 2015, No. 1, pp. 22–25.
The Formation of Structure and Properties of Antifriction Composites via Modification of Polytetrafluoroethylene with Polydispersive Fillers Yu. K. Mashkova, O. V. Kropotina, S. V. Shil’kob, V. A. Egorovaa, and O. V. Chemisenkoa a
Omsk State Technical University, Omsk, Russia Institute of Mechanics of Metallopolymeric Systems, National Academy of Sciences of Belarus, Gomel, Belarus email:
[email protected]
b
Abstract—The structural modification in polytetrafluoroethylene with introduction of polydispersive fillers is studied. Nanoscale fillers are shown to exhibit a higher level of structural activity and effectively impact the mechanical and tribotechnical properties of polymeric composites, especially with the use of complex mod ifiers. Keywords: polytetrafluoroethylene, composite material, structural modification, wear resistance DOI: 10.1134/S2075113315040176
ical and tribological properties of PCMs on basis of PTFE [9, 14].
INRODUCTION Development of modern engineering puts forward new and higher requirements on mechanical and tri botechnical properties of polymeric composite mate rials (PCMs), which are applied in the friction joints of machines. In this connection, there is wide use of structural modification of the polymeric matrix via introducing fillers of different nature and morphology, such as dispersive and fibrous, and recently ultrafine and nanoscale ones [1–3], since the developed surface of nanoparticles favors significant changes in physical properties of nanodispersive materials compared to those of the relevant macroscopic systems [4, 5]. For example, using micro and nanoscale fillers which possess high surface activity induces structural modifi cations in the polymeric matrix at various scales, which allows one to obtain a material with unique mechanical and tribotechnical characteristics for engineering and medicine [6, 7]. To date, there is a considerable amount of experi mental data on the influence of fibrous and dispersive fillers, including nanoscale ones, on the structure and phase state and on the parameters of supramolecular structure of polytetrafluoroethylene (PTFE) [8–14]. It is established that the changes in morphology of the supramolecular structure and properties of a polymer depend on nature of the filler and shape and dispersion of particles. For example, high structural activity of carbon fillers in the form of milled carbon fibers and polydispersive cryptocrystalline graphite favors signif icant modification of the structure of the polymeric matrix, as well as improvement of the physicomechan
OBJECTS AND METHOD OF STUDY The PCMs on the basis of 4PN GOST 1000780 polytetrafluoroethylene developed at Omsk State Technical University were used. Three types of modi fiers were utilized as fillers. The first modifier is polydispersive cryptocrystal line graphite (CCG) GLS3 powder with a specific geometric surface of 20–25 m2/g and specific adsorp tive surface after treatment with active alloying of 45– 60 m2/g. The sizes of CCG particles are varied over a wide range from ultrafine (0.36 μm) to coarse (153 μm) scale [15]. The second modifier is multiwalled carbon nano tubes (GraphistrengthTM (Arkema)) with diameters of 10–15 nm and lengths of 1–10 μm (CNTs). The third modifier is nanoscale silicon dioxide SiO2 powder (BS120 white soot (GOST 1830778)) with particle sizes of 19–27 nm and specific surface of 100–140 m2/g (BS). The effectiveness of modification was estimated from the results of mechanical and tribotechnical tests of the PCM samples. According to the technology of synthesis of PCM samples, all fillers were subjected to mechanical activation together with PTFE powder for 1–3 min in a highspeed paddletype mixer. The PCM samples with fillers were prepared via cold pressing at a pressure of 70–80 MPa with subsequent free sinter ing in an oven at a temperature of 360 ± 3°C.
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J × 10–4 g/h 1 8
10
1
2 6
8
2 4
6
2
4 0
1
2
3
4 wt %
Fig. 1. Wear rate as function of concentration for PCM with single modifiers: (1) CNT; (2) BS120.
The tribotechnical properties of the above PCMs were studied on a tribometer and a UMT2168 friction machine, in whose working joint a finger–disk side scheme was implemented. Three samples in the form of cylindrical fingers with a diameter of 5 mm were simul taneously maintained in the sample holder and pressed with a predetermined force to the surface of the match ing part, which was a metallic disk made of quenched steel 45 with a working surface roughness Ra ≤ 0.32 μm. The set of samples was tested for 3 h at a sliding rate of 1.2 m/s and a contact pressure of 2.6 MPa. The mechanical properties of the PCMs were investigated on a ZwickRoell automated tensile test ing machine within the tensile tests of three PCM standard samples with determination of tensile strength σB and elastic modulus E in tension and rela tive elongation in rupture. The supramolecular struc ture and morphology of the modified PTFE were stud ied via electron microscopy and Xray diffraction using a JEOL JCM 5700 scanning electron micro scope and ADVANCE diffractometer. RESULTS OF EXPERIMENTAL STUDIES The results of wear resistance of PCMs are shown in Figs. 1 and 2 as the dependences of wear rate J on the concentration of modifying fillers. An average wear rate obtained for the infilled PTFE is 788 × 10–4 g/h. Introducing CCG ensures a consid erable decrease in wear rate. At CCG contents of 8 to 20 wt %, the minimum and constant wear rate obtained is J = 26 × 10–4 g/h, i.e., 30 times lower than in the case of infilled PTFE. The wear rate of PCM as function of concentration of the first and secondtype nanomodifiers over a
0
1
2
3
4 wt %
Fig. 2. Wear rate as function of concentration for PCM with complex modifiers: (1) CCG + CNT; (2) CCG + BS120 (CCG content is 8 wt %).
range of 0.5 to 4.0 wt % (see Fig. 1) exhibits an extreme behavior with a minimum wear rate at a modifier con tent of 2.0–2.5 wt %. For a PCM with the CNT mod ifier, the wear rate is 4.8 × 10–4 g/h, while for a BS 120modified PCM, it is 5.2 × 10–4 g/h, i.e., higher by 8.3%. In order to increase the efficiency of structural modification, we prepared and studied PCM samples with complex modifiers, wherein the nanoscale CNT and BS components over a content range of 0.5–4.0 wt % were injected separately at a constant polydispersive CCG content of 8.0 wt %. The results are plotted in Fig. 2. The concentration dependences of the wear rate of PCM with complex modifiers exhibit also an extreme behavior, and the minimum wear rates are obtained at the following concentrations: 2.0 wt % for CNT and 3.0 wt % for BS. The minimum J value equal to 3.6 × 10–4 g/h is found for a PCM with complex filler containing 8.0 wt % of CCG and 3.0 wt % of a nanoscale BS120 powder, which is 7.2 times inferior to the wear rate of PCM with a CCG polydispersive single modifier and 30% lower than the wear rate of PCM with a complex filler containing 2.0 wt % of CNTs. The results of the study reveal that application of nanoscale modifiers in complex with CCG polydis persive micromodifier is thus the most efficient. In addition to wear resistance, mechanical charac teristics are of great importance for tribological sys tems. For example, antifriction materials of sealing device elements (cuffs and piston rings) are expected to exhibit comparatively low elastic modulus and suf ficient strength respective to the load conditions. The results of studies of mechanical properties of PCM with complex fillers of two considered types (table)
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The mechanical characteristics of PCM Sample no. 1 2 3 1 2 3
Filler composition
σB, MPa
CCG—8 wt % CNT—2.5 wt %
σB.av, MPa
18.1 14.5 14.2 18.2 13.8 14.8
CCG—8 wt % BS—3 wt %
15.6
As is seen from the table, the composites with dif ferent nanomodifiers (CNT and BS) have the same σB tension strength values. In this case, the mean elastic modulus value for PCM with a BS120 nanomodifier is 27% lower. Such a reduction in elastic modulus of the sealing element material is, however, acceptable,
(а)
171 160 144 129 107 108
15.6
testify to the required mechanical characteristics in these PCMs.
1
E, MPa
2
10 µm
Eav, MPa 158
115
ε, % 148 136 128 129 106 94
εav, % 137
110
since it favors increasing degree of impermeability of sealing interfaces. In order to explore the correlation of mechanical and tribological characteristics for PCMs containing nanomodifiers with type and parameters of supramo lecular structure of the modified polymeric matrix, we conducted structural investigations of PCM samples via Xray diffraction and electron microscopy. Xray diffraction revealed no changes in PTFE lattice parameters (a = b ≈ 0.57 nm; c ≈ 1.6 nm) with intro duction of modifiers. The degree of crystallinity is slightly increased, but this increase is comparable with the measuring error. The polymeric matric retains there fore its amorphocrystalline structure, but there is a slight increase (about 10%) in the average crystallite size. Electron microscopy analysis revealed the changed morphology of the supramolecular structure of the matrix in the PTFE modified with the above modifi ers. The initial band (lamellar) structure of PTFE is transformed into a structure with defective spherolytic areas (Fig. 3). These structural modifications occur in the matrix zones with fibrous or dispersive nanomodi fier particles. The considered modifications in supramolecular structure caused by structurally active modifiers lead to substantial changes in mechanical characteristics and increase wear resistance of antifriction PCMs. CONCLUSIONS
(b)
1. Nanofillers (CNTs and BS) exhibited high struc tural activity to PTFE and affected the morphology and ordering of the modified polymeric matrix, and the degree of this effect depended on the particle dimension and filler concentration. 2. It was established that complex fillers with poly dispersive micropowder (e.g., cryptocrystalline graph ite), as well as different nanomodifiers with various particle geometry, were the most efficient. 3. The highest wear resistance of PCMs was attained with use of complex fillers in the form of nano and microparticles of cryptocrystalline graphite and silicon dioxide (BS120). 4. A pronounced increase in wear resistance of polymeric nanocomposite is due to the changed mor
50 µm Fig. 3. Optical micrograph of the cleavage of the PTFE sample with 1.5% CNT: (a) the closepacked areas of the matrix; (1) with nanotubes; (2) loosely packed areas of the matrix without nanotubes; (b) spherolyticlike structural elements with pronounced boundaries in the matrix. INORGANIC MATERIALS: APPLIED RESEARCH
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phology and phase content of PTFE caused by high structural activity of the modifying nanofillers. 5. The developed PCMs meet the modern require ments for selfbinding materials for friction joints of machines and engineering equipment, including flex ible elements of sealing devices. ACKNOWLEDGMENTS This work was supported by the Russian Founda tion for Basic Research, project no. 140890022 Bel_a. REFERENCES 1. Krasnov, A.P., Aderikha, V.N., Afonicheva, O.V., Mit’, V.A., Tikhonov, N.N., Vasil’kov, A.Yu., Said Galiev, E.E., Naumkin, A.V., and Nikolaev, A.Yu., Categorization system of nanofillers to polymer com posites, J. Friction Wear, 2010, vol. 31, pp. 68–80. 2. Coleman, J.N., Khan, U., Blau, W.J., and Gun’ko, Y.K., Small but strong: A review of the mechanical properties of carbon nanotubepolymer composites, Carbon, 2006, vol. 44, pp. 1624–1652. 3. Ginzburg, B.M., Pozdnyakov, A.O., Tochil’nikov, D.G., Tuichiev, Sh., and Shepelevskii, A.A., Tribological characteristics of composites based on poly(tetrafluo roethylene) and fullerene carbon, Polym. Sci. Ser. A, 2008, vol. 50, pp. 865–873. 4. Morokhov, I.D., Trusov, L.I., and Lapovok, V.N., Fizicheskie yavleniya v ul’tradispersnykh metallicheskikh sredakh (Physical Phenomena in Ultradispersed Metallic Media), Moscow: Energoatomizdat, 1984. 5. Petrov, Yu.I., Fizika malykh chastits (Small Particle Physics), Moscow: Nauka, 1982. 6. Pleskachevskii, Yu.M., Shil’ko, S.V., and Panin, S.V., Mezomechanics, computer design and engineering of polymer micro and nanocomposites for engineering and medicine, Probl. sovr. materialoved.: Tr. XVIII sessii Nauchn. soveta po novym mater. (Proc. 18th Sci. Con. Sess. on New Mater. “Problems of Contemporary Material Science”), Kiev, 2013.
7. Pleskachevskii, Yu.M., Shil’ko, S.V., and Panin, S.V., Micro and nanostructure polymer composites of tech nical and medical destination: Computer design, experiment, implantation, in Dvadtsat’ konkursnykh let (BRFFI: 1991–2011 gg.) (Twenty Competitive Years (BRFFI: 1991–2011)), Orlovich, V.A., Minsk: Belarus. Navuka, 2012, pp. 334–356. 8. Okhlopkova, A.A., Andrianova, O.A., and Popov, S.N., Modifikatsiya polimerov ul’tradispersnymi soedineniyami (Polymer Modification by Ultradispersed Compounds) Yakutsk: Sibir. Otd. Ross. Akad. Nauk, 2003. 9. Mashkov, Yu.K., Ovchar, Z.N., Surikov, V.I., and Kali stratova, L.F., Kompozitsionnye materialy na osnove politetraftoretilena. Strukturnaya modifikatsiya (Com position Materials on the Basis of Polytetrafluoroethy lene. Structural Modification), Moscow: Mashinostro enie, 2005. 10. Conte, M. and Igartua, A., Study of PtFe composites tribological behavior, Wear, 2012, no. 296, pp. 568– 574. 11. Gong, D., Xue, Q., and Wang, H., Physical models of adhesive wear of polytetraftuoroetylene and its com posites, Wear, 1991, vol. 140, pp. 9–24. 12. Mashkov, Yu.K. and Kropotin, O.V., Tribofizika i struk turnaya modifikatsiya materialov tribosistem (Tribo physics and Structural Modification of Tribosystem Materials), Omsk: Omsk. Gos. Tekhn. Univ., 2009. 13. Gogoleva, O.V., Okhlopkova, A.A., Petrova, P.N., and Vasil’ev, S.V., Perspective material development on the basis off polytetrafluoroethylene and natural ceolites, Materialovedenie, 2013, no. 7, pp. 15–19. 14. Grakovich, P.N., Shelestova, V.A., Danchenko, S.G., Zhandarov, S.F., Smirnov, A.V., and Shkurskii, I.A., The problems of application of composite materials “fluvis” and “superfluvis” in the compressor engineer ing, Tekhn. Gazy, 2013, no. 3, pp. 68–72. 15. Kropotin, O.V., Mashkov, Yu.K., Egorova, V.A., Treni khin, M.V., and Voitenko, N.N., Study of structure and phase composition of ultradispersed cryptocrystalline graphite, Omsk. Nauchn. Vestn., 2006, no. 9, pp. 19–23.
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Translated by O. Maslova
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