ISSN 10683666, Journal of Friction and Wear, 2013, Vol. 34, No. 2, pp. 107–113. © Allerton Press, Inc., 2013. Original Russian Text © A.Ya. Grigoriev, I.N. Kavaliova, A.V. Kupreev, E.E. Dmitrichenko, 2013, published in Trenie i Iznos, 2013, Vol. 34, No. 2, pp. 138–145.
Friction and Wear of Polyamide6 Powder Coatings GradientFilled by Metal Nanofilms A. Ya. Grigorieva, *, I. N. Kavaliovaa, A. V. Kupreeva, and E. E. Dmitrichenkob a
Belyi MetalPolymer Research Institute, National Academy of Sciences of Belarus, ul. Kirova 32a, Gomel, 246050 Belarus b Sukhoi Gomel State Technical University, pr. Oktyabrya 48, Gomel, 246029 Belarus *email:
[email protected] Received December 29, 2012
Abstract—The tribological properties of PA6 and PA6based coatings with surface layers gradientfilled with tin, lead, and bismuth nanofilms in a concentration of up to 1.2 wt % are compared. It has been found that differences in the tribological behavior of the composites are due to the adhesion of the filler metals to the counterbody material. It has been shown that the hardness of the friction surface of the composites depends on the susceptibility of the filler metals to strain hardening and explains differences in the values of their wear. Keywords: coatings, polyimide composites, nanofilm metal filler, wear, friction DOI: 10.3103/S1068366613020050
INTRODUCTION Thin polymer coatings on metal surfaces vary sub stantially in their tribological behavior. Due to this, they can be used similarly to bulk antifriction polymer composites or as an analog of common solid lubricants [1, 2]. However, specific features of deformation and thermal processes that develop in thin polymer layers on metals makes them preferable for use in many cases [3–5]. For example, compared to bulk polymers, the coatings increase the stiffness of friction units and the rate of heat removal from the friction zone, which favors the tribological properties of the units [6, 7]. Nonetheless, thin layers of highmolecular com pounds surpass solid lubricants in the vibration resis tance and noisiness due to their rheological properties. These factors have promoted the use of the polymer coatings in various friction units from common bear ings and slide guides to spline joints of cardan shafts of vehicles [8]. The required properties of uptodate antifriction polymer materials are achieved by adding various nanosized disperse substances, such as chalcogenides of transition metals, aluminum silicates, and carbon in all possible allotropic modifications. [9]. Filling a polymer matrix by particles of ductile metals is an especially promising method. The use of this method considerably increases the thermal conductivity and strength of a composite, as well as favors its tribological performance. Unfortunately, the most widespread methods for manufacturing metalfilled polymer composites do not ensure their efficient use as thin coatings. Volume
filling by nanodispersed particles weakens the adhe sion of the coatings due to the appearance of extra interphases and a decrease in the area of the contact of a matrix polymer with a substrate. The incorporation of particles based on the thermal decomposition of metal salts and formats in a polymer melt is accompa nied by substantial gas release and pore formation. One of the methods for overcoming these difficulties is the formation of gradient coatings in which only sub surface layers are filled (modified) [10, 11]. When using metal fillers, the basic problem is to incorporate them into the subsurface layers. In this work, we solved it by implementing a laboratory method for producing powder gradientfilled coatings. This method is based on the vacuum deposition of metal nanofilms on poly mer particles that are preliminarily applied to steel substrates; after nanofilm deposition, the particles are melted. Despite the extensive experimental data available, the mechanisms of the friction of metalfilled polymer composites and coatings have not been adequately explored. The enhancement of the tribological char acteristics of these materials is commonly attributed to a decrease in shear stresses and shear forces that act on adhesive junctions for ductile metals or to the imple mentation of the Charpy principle for hard metals. However, the applicability of this interpretation to composites with polymer matrices, the mechanical characteristics of which are close to those of ductile metals, as well as the implementation of the Charpy principle in these composites, remain to be verified. The aim of this work is to carry out comparative analysis of the tribological characteristics of powder
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(a)
20 μm (b)
150 μm (c)
50 μm
Fig. 1. Formation of polymer coating: (a) St3kp steel substrate surface; (b) coating surface that faces substrate; (c) upper layer of coating after deposition of PA6 powder on heated substrate.
polyamide coatings, the subsurface layers of which are filed by films of various metals, and to identify their friction and wear mechanisms.
polymer layer melted and a 150–300μmthick coat ing was formed.
EXPERIMENTAL Materials We studied coatings formed on metal substrates by the melting of polyamide6 particles covered with films of ductile metals that were deposited in vacuum. 20 × 20 × 2mm substrates were made of the St3kp car bon steel (GOST 380–94, 0.14–0.22 wt % С, 0.25– 0.50 wt % Mn, and no more than 0.05 wt % Si). The surfaces of the plates were ground with emery paper with a grain size of P1000, polished by GOI no. 3 paste to Ra = 0.15 ± 0.05 μm, and washed in an ultrasonic bath filled by ethanol for 10 min. Figure 1a shows the morphology of the treated substrate surface. A polyamide6 powder (OST 60609–83) with an average grain size of 125 μm was used as the base mate rial of the coatings. The process of depositing a powder coating was combined with the process of producing a composite material. The plates heated in an electric furnace to 225°C were placed in a glass for 5–10 s; a suspension of the polymer powder was produced in the glass by blowing compressed air from the bottom and inducing mechanical vibration with an amplitude of 2 mm and a frequency of 25 Hz. This yielded a layer of partially melted (Fig. 1b) and electrostatically chained (Fig. 1c) particles on the substrate. These specimens were placed in the chamber of a VUP4 multipurpose vacuum setup (ZEM, Ukraine) to deposit a metal layer about 10 nm thick on their sur faces. Thermal deposition was carried out using a bas ket tungsten evaporator under a pressure of no more than 0.02 Pa. The distance between the evaporator and substrate was 10 cm. The С2 lead (GOST 3778–98, 99.95% Pb), the O2 tin (GOST 860–75, 99.5% Sn), and the Vi2 bismuth (GOST 10928–90, 97% Bi) were deposited. After deposition, the vertically arranged specimens were heated in an oven until the metal
Methods The morphology of the coating surfaces was exam ined using a VEGAII LSH scanning electron micro scope (SEM) (Tescan, Czech Republic). An ADD250 energydispersion Xray spectrometer (Oxford Inc., Great Britain) installed directly on the SEM was used to determine the concentration of the metals in the coat ings and plot maps of their distribution over the surface. The structure of the subsurface layers of the coatings was examined on angular cuts in a BX45 optical micro scope (OM) (Olympus, Japan) in transmitted light; the microscope was equipped with an Optizer electron optical attachment for digital image recording (Metal Polymer Research Institute of the National Academy of Sciences of Belarus, Gomel). The microhardness of the coatings was measured on a PMT3 instrument (LOMO, Russia) under a load of 0.3 N. The dynamic mechanical characteristics were determined using an IPM1K pulse strength meter (Institute of Applied Physics of National Acad emy of Sciences of Belarus, Minsk). Tribotests were carried out on an MTU2K7 reciprocal tribometer (MetalPolymer Research Institute of National Acad emy of Sciences of Belarus, Gomel) under normal dry conditions (GOST 8.395–80). The counterbody was a 4.7 mm in diameter ShKh15 steel ball. The tests were carried out under loads of 100, 250, 500, 750, and 1000 mN at a relative sliding velocity of 2.5 mm/s; the double stroke length was 10 mm, and the sliding dis tance was 7.5 m. An estimate of the stress state of the materials in the contact area using the Hertz theory and the Tresca criterion has shown that, at the values σy = 55–77 MPa typical of PA6, they undergo elastic deformation under loads of up to 200–300 mN; above these loads, up to 1.2 N, they undergo elastoplastic deformation.
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(a)
50 μm (b)
50 μm (c)
50 μm
(d)
50 μm (e)
50 μm (f)
50 μm
Fig. 2. Morphology and structure of coatings: (a, b) SEM images of coatings surface (c, d) optical images of structures of subsur face layers of pure and tinmodified coatings, respectively (angular cut, transmitted light); (e) surface of composite and (f) map of tin distribution in it.
The linear wear of the coatings was measured by a MII4M Linnik interferometer (LOMO, Russia) using the method reported in [12]. A fringe pattern of the friction track was obtained in white light. The value of the wear was the maximum depth of the cross section of the friction track profile in the middle of its length. RESULTS AND DISCUSSION The morphology of the surfaces and the structure of the subsurface layers of the modified coatings are characterized by marked granularity that has not been found on pure polyamide6 coatings (Figs. 2a–2d). It is apparent that original powder particles do not mix during melting since the deposited metal films form stable interphases that prevent the particles from com plete coalescence during heating. This is confirmed by the close dimensions of observed facet structures and polymer powder particles (Figs. 1c, 2c, and 2f), as well as an increased concentration of the metals along their boundaries (Figs. 2e and 2f). A partial divergence of increased concentration domains from grain bound aries can by explained by the breakage and fragmenta tion of the metal films during the melting and spread ing of polymer particles. Thus, the metals in the coat ings are most likely in the form of discontinuous films. JOURNAL OF FRICTION AND WEAR
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The concentrations of Pb, Sn, and Bi in the com posites averaged over an area of 300 × 300 μm2 are 1.2, 1.0, and 0.8 wt %, respectively. The differences in the concentrations are commensurate with the ratio of the densities of these metals, which is indicative of close thicknesses of the deposited films. The table presents the mechanical properties of the coatings. It can be seen that the differences are insig nificant, except the values of microhardness. Contrary to the expectations, the hardnesses of the composites demonstrate an inverse dependence on the hardness of the metals that are added to them (HB Pb < HB Sn < HB Bi). Apparently, in this case, the strength of the surface layers is more dependent on other factors, such as the interfacial interaction between the particles, than on the mechanical proper ties of metals. The data presented above show a similar structure of the subsurface layers of the coatings and close values of the structure parameters, i.e., the morphology, grain size, and depth, as well as sites of the localization of the metal films. In most cases, the differences in the mechanical characteristics of the coatings are statisti cally insignificant. Thus, when analyzing the friction processes, the effects of the mechanical and structure factors can be assumed to be the same and specific fea 2013
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Mechanical and relaxation properties of coatings Characteristic
PA6
PA6 + Pb
PA6 + Sn
PA6 + Bi
Elastic modulus Pa, 109
1.95 ± 0.22
1.88 ± 0.31
2.2 ± 0.28
1.98 ± 0.37
Hardness HV30, MPa
157 ± 21
194 ± 24
163 ± 22
132 ± 22
Mechanical loss tangent (at 20°C)
0.29 ± 0.04
0.32 ± 0.06
0.24 ± 0.05
0.30 ± 0.04
Relaxation time s, 10–5
3.55 ± 0.11
4.27 ± 0.08
3.20 ± 0.12
3.99 ± 0.05
6.01 ± 0.32
5.78 ± 0.46
6.3 ± 0.48
5.92 ± 0.37
1.63 ± 0.17
1.93 ± 0.26
1.77 ± 0.21
1.89 ± 0.23
Elastic deformation energy J, 10–4 –3
Viscous deformation energy J, 10
tures of the tribological behavior of the coatings can only be attributed to the properties of the metal film fillers and the processes that develop over their inter phases. Figure 3 shows the data on the friction and wear of the coatings. The shape of load dependences of the friction coef ficient of polymers is commonly interpreted in accor dance with known concepts of the deformation com ponent of the friction force [4]. In the general case, these dependences are Ushaped, which is attributed to changing the mode of material deformations in the contact area from elastic to plastic. The slope of the left branch of the dependence is proportional to the elastic modulus, and the minimum of the friction coefficient corresponds to transition from elastic to elastoplastic deformation. The growth and flattening out of the right branch correspond to an increase in the contribution of plastic deformation until the yield stress is reached in the friction contact zone. Based on these considerations, the close values of the friction coefficient under loads of up to 250 mN are explained by almost the same values of the elastic characteristics of the composites (table), and the difference in these values under loads of above 250 mN can be indicative
of a considerable discrepancy in their mechanical characteristics, in particular the yield stress or hard ness. However, taking into account for the data in the table, this difference is inconsistent with the expected regularity of the deformation theory of friction, according to which softer metals have a higher friction coefficient. The behavior of the hardest material (PA6 + Pb) is almost the same as that of the softer material (PA6 + Bi); the pure PA6 and the PA6 + Sn composite that have close hardnesses substantially dif fer in the shape of the load dependences of the friction coefficient. These discrepancies are indicative of the effect of the factors that vary the friction of the mated materials with increasing load. The results presented in Fig. 4a show a statistically significant change in the microhardness in the friction zone compared to the microhardnesses of the original materials. For the pure PA6, as well as the Pb and Bi filled composites, the hardness changes only slightly; it decreases for the first two materials and increases for the third one. An increase in the microhardness of the tinfilled composite is especially pronounced (more than 20%). Nevertheless, the results do not also argue for the deformation theory because the characteristics of the materials in the friction track zone are inconsis l, μm
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1 3 2
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1 3
1.5 1.0
2 4
0.5 0
250
500
750 N, mN
0
250
500
750 N, mN
Fig. 3. Load dependences of (a) friction coefficient and (b) wear: (1) PA; (2) PA + Bi; (3) PA + Pb; (4) PA + Sn. JOURNAL OF FRICTION AND WEAR
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90
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50 μm
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130 θ, °
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Fig. 4. Relation between properties of composites and their tribological behavior: (a) microhardness of materials (1) in original state and (2) after friction; (b) dependence of friction coefficient on angle of wetting of counterbody material by filler metal; (c–f) SEM images of friction surface morphology for PA6 + Sn, PA6, PA6 + Pb, and PA6 + Bi, respectively (friction under load of 750 mN).
tent with the expected order of values of their friction coefficients. The specific features of the morphology of the fric tion surfaces (Figs. 4c–4f) show that the coatings dif fer in the character of their interaction with the indenter material. The surface of the tinfilled com posite is covered by transversal cracks; they may result from the local seizure and fracture of the polymer sur face by the friction forces that stretch the material in the contact area. However, no such damage has been found on the surface of the bismuthfilled composite that has the lowest friction coefficient in the studied load range. The leading role of adhesion in the friction processes is also supported by the fact that the metals used as fillers substantially differ in the characteristics of interphase interaction with the counterbody mate rial. For example, the wetting angle of the steel is 70°– 80° for tin, 90°–100° for lead, and 130°–140° for bis muth [13]. The wetting angle is inversely proportional to the free surface energy, thus playing a role of the measure of adhesive or interphase interaction between the systems in contact. Among the presented materi als, Sn has the highest adhesive interaction potential, and the next are PA6, Pb, and Bi. This sequence JOURNAL OF FRICTION AND WEAR
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agrees well with the results of measuring the friction coefficient (Fig. 1a). Wear. It is known that the wear is inversely propor tional to the hardness of materials, test conditions being equal [4]. If the results presented in Fig. 3b are considered on the basis of the estimated microhard ness of the friction surfaces, the materials that have their extreme values, i.e., the pure polyamide and the tinfilled composite entirely follow this regularity. This is confirmed by subsequent analysis, and the wear of the other composites depends on changes in the strength and structure of their subsurface layers due to friction. These changes are fairly easily explained when the pattern of variations in the mechanical characteristics of the composites under cyclic loading is considered. For example, it is known that the friction of polymers leads to the defragmentation of their supramolecular structure, as well as the reorientation and destruction of molecular chains, thus reducing the strength of a polymer [14, 15]. This apparently explains the data for PA6 presented in Fig. 3b. In our opinion, variations in the strength of the metalfilled composites are explained by a difference 2013
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in the susceptibility of the metals to work hardening [16, 17]. Specifically, tin is prone to severe lowcycle hardening even at low deformations. On the contrary, lead that is recrystallized at –35°C does not undergo work hardening during friction and its ductility does not decrease. As for bismuth, it is not malleable, reaches the maximum hardness even after a single act of deformation, and is disintegrated by brittle fracture during repeated loading. Taking into account all these factors, we can explain the data presented in Fig. 4 as follows. During the friction of the tinfilled composite, a slight decrease in the strength of the subsurface layers of the PA6 is compensated for by the severe work hardening of the metal. The introduction of lead does not change the strength, and its variations during fric tion are governed by the properties of the matrix poly mer. The addition of bismuth has also an only slight effect on the properties of the composite since the competing process of strength decrease due to the dis persion of metal particles in the surface layer develops along with strain hardening. The best tribological characteristics of this material result most likely from the formation of a surface structure that obeys the Charpy principle, i.e., a ductile matrix contains parti cles of strainhardened bismuth. CONCLUSIONS The vacuum deposition of lead, tin, and bismuth on the polyamide6 powder preliminarily applied to steel substrates, as well as its subsequent melting, were used to prepare specimens of composite powder coat ings with a concentration of metals in the subsurface layers of 0.8–1.2 wt %. It has been found that polymer particles do not mix in 50–100μmdeep subsurface layers during melting and form a network structure. This structure has been shown to result from the presence of about 10nmthick metal films; these films play a role of interphases, which prevent the polymer particles from merge during melt ing. The measurement results have not revealed statisti cally significant differences of the mechanical charac teristics of the coatings from the properties of the matrix polymer, as well as their dependence on the metal type. It has been found that distinctions between the fric tion and wear mechanisms of the coatings are explained by specific features of the adhesion interac tion and the deformation of the metal films with the counterbody material. When the contact zone is in the elastic state, the load dependence of the friction coef ficient is similar for all the materials and has a descending pattern, which corresponds to the defor mation theory of friction. It has been shown that, in the elastoplastic deformation range, the dominant friction mode is governed by the adhesive interaction of the counterbody material with the filler metal, which is confirmed by the results of estimating interphase
interaction forces that act between the mated materials, as well as by the morphology of the worn surfaces. The relationship between the wear of the composites and the susceptibility of the film filler metals to work hardening has been revealed using the results of measuring the microhardness on friction tracks. Thus, the formation of nanofilm metal structures in the subsurface layer of the powder coatings substan tially varies their tribological characteristics. This approach holds promise because it offers a possibility of producing antifriction coatings with required char acteristics without considerable variations in their mechanical properties and adhesion to surfaces of articles. ACKNOWLEDGMENTS We are grateful to Prof. V.M. Shapovalov and research engineer A.M. Valenkov for consultation and assistance in coating deposition, as well as to junior researcher K.V. Panteleev for experimental work. This study was supported by the Belarusian Foundation for Basic Research, projects nos. T11PLSh004 and T11SRB003. NOTATION f—friction coefficient; F—friction force; N—nor mal load applied to specimen; I—linear wear; h—inden tation depth during friction; a—halfwidth of friction track; e—elastic recovery of friction track. REFERENCES 1. Encyclopedia of Polymer Composites: Properties, Perfor mance and Applications, Leshkov, M. and Prandzheva, S., Eds., New York: Nova Science Pub lisher, 2011. 2. Belyi, V.A., Dovgyalo, V.A., and Yurkevich, O.R., Polimernye pokrytiya (Polymer Coatings), Minsk: Nauka i Tekhnika, 1976. 3. Belyi, V.A., Sviridenok, A.I., Petrokovets, M.I., and Savkin, V.G., Trenie i iznos materialov na osnove polimerov (Friction and Wear of PolymerBased Mate rials), Minsk: Nauka i Tekhnika, 1976. 4. Myshkin, N.K. and Petrokovets, M.I., Tribologiya. Printsipy i prilozheniya (Tribology. Principles and Applications), Gomel’: IMMS NANB, 2002. 5. Sysoev, P.V., Bogdanovich, P.N., and Lizarev, A.D., Deformatsiya i iznos polimerov pri trenii (Deformation and Wear of Polymers at Friction), Minsk: Nauka i Tekhnika, 1985. 6. Shestakov, V.M., Rabotosposobnost’ tonkikh polimernykh pokrytii (Operability of Thin Polymer Coatings), Mos cow: Mashinostroenie, 1973. 7. Kolzunova, L.G. and Kovarskii, N.Ya., Polimernye pokrytiya na metallakh (Polymer Coatings on Metals), Moscow: Nauka, 1976. 8. Starzhinskii, V.E., Farberov, A.M., Pesetskii, S.S., Osipenko, S.A., and Braginskii, V.A., Tochnye plast
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Translated by D. Tkachuk
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