ISSN 10683666, Journal of Friction and Wear, 2010, Vol. 31, No. 2, pp. 121–127. © Allerton Press, Inc., 2010. Original Russian Text © V.E. Panin, S.V. Panin, L.A. Kornienko, S. Vannasri, L.R. Ivanova, S.V. Shil’ko, 2010, published in Trenie i Iznos, 2010, Vol. 31, No. 2, pp. 168–176.
Effect of Mechanical Activation of UltraHighMolecularWeight Polyethylene on Its Mechanical and Triboengineering Properties V. E. Panina, S. V. Panina, *, L. A. Kornienkoa, S. Vannasrib, L. R. Ivanovaa, and S. V. Shil’koc a
Institute of Strength Physics and Materials Science, Siberian Division, Russian Academy of Sciences, Akademicheskii pr. 2/4, Tomsk, 634021, Russia *email:
[email protected] b Tomsk Polytechnic University, pr. Lenina 30, Tomsk, 634050, Russia c V.A. Belyi MetalPolymer Research Institute, National Academy of Sciences of Belarus, ul. Kirova 32a, Gomel, 246050, Belarus Received October 19, 2009
Abstract—The effect of mechanical activation of initial ultrahighmolecularweight polyethylene powders on the pysicomechanical properties of the polymer is studied. Mechanical activation is found to raise the strain and strength properties, as well as the triboengineering characteristics of ultrahighmolecularweight polyethylene. Xray diffraction analysis, IR spectroscopy, and optical and electronic microscopy show that mechanoactivation of the initial powder defines the polymer’s structural organization. Key words: ultrahigh molecular weight polyethylene, mechanical activation, friction coefficient, wear resis tance, physicomechanical properties, supermolecular structure. DOI: 10.3103/S1068366610020054
INTRODUCTION A number of filled polymeric materials show excep tional antifrictional properties, for which reason they are extensively used in friction joints [1, 2]. In particu lar, nanodispersed fillers present special interest for developing novel antifriction materials based on ultra highmolecularweight polyethylene (UHMWPE). A series of polymer binders have been found to greatly reduce (up to 50%) the friction coefficient and increase several times the wear resistance, compared to micro composites of analogous formulations [3–5]. Works [6–9] represent early investigations of the effect of nanofillers with 40–70 nm particle size on the structure and properties of polymers. Intensive interest has been focused on UHMWPE in connection with information on the effect of low additions of ultradispersed cobalt and copper spinels [10] ensuring severalfold reduction of the wear rate and low amounts of multilayered carbon nanotubes that greatly increase the hardness, shear strength, and wear resistance of the material [11]. It is shown in [8, 10] that mechanical activation of the binder and filler powders of polymer composites defines their mechanical and triboengineering proper ties. Mechanical activation in particular can be treated as an independent means of refining the properties of composites, which is equivalent to the introduction of fillers.
The mechanisms of polymer strengthening by nano particles and nanofibers differ in principle from the known methods of dispersed hardening by micronsize powders and fibers [12]. The search for methods of improving a range of physicomechanical properties of voluminous UHMWPE items, including mechanoacti vation of the binder material and nanofiller reinforce ment, is considered to be a major theoretical and prac tical challenge. The aim of the work is to study the effect of mechan ical activation of the polymer binder on the physicome chanical and triboengineering properties of UHMWPE composites. MATERIALS AND METHODS The object of investigations was UHMWPE of Ticona Co (Germany) with molecular mass 2–7 mln. carbon units, and its composite materials. The samples were obtained by hot pressing under 10 MPa pressure and 190°C temperature followed by cooling at 3– 4°/min. Mechanical activation of the polymer binder powder was conducted in a planetary ball mill MR/0.5 × 4 allow ing for simultaneous mechanical treatment and mixing of up to 2 l of the powder mix. The parameters and time of mechanoactivation were varied. The physicomechanical characteristics of the sam ples were estimated using an Instron5582 test machine.
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50 µm
(a)
(b)
50 µm
Fig. 1. Optical image of UHMWPE powder microstructure in the initial state (a) and after mechanical activation for 20 min (b).
The wear resistance of the UHMWPE samples dur ing sliding was determined following the tests with blockonring geometry according to ASTM G99 and DIN 50324. The tests without lubrication were con ducted under 160 N load and 100 rpm sliding velocity (the test regime was not varied during testing). The fric tion surfaces of the samples were studied on a Zygo New View 6200 optical profilometer. The friction path area was determined by the “Rhino Ceros 3.0” software by way of a manual display of the friction surface contour and computation of its area. The microstructure of both the initial and activated UHMWPE powders was investigated on a Carl Zeiss Stemi 2000C optical microscope. The phase composi tion and crystallinity degree of the samples were found on an XRD6000 Xray diffractometer with CuKα radi ation. The phase composition and the regions of the coherent scattering were found from the PCPDFWIN database, as well as by using a fullprofile POWDER CELL 2.4. The IR spectra were recorded by on the NIKOLET 5700 spectrometer. The samples underwent structural investigations by the SEM method using a JEM100CX microscope fit with an ASID4D scanning adapter with accelerated voltage 40 keV. The cleavage surface on the UHMWPE sample with incision was studied after deep cooling in liquid nitrogen. d, g/cm3 0.20 0.15 0.10 0
10
20
30
τ, min
Fig. 2. Dependence of UHMWPE powder loose density on mechanical activation time.
RESULTS AND DISCUSSION Processing of the UHMWPE powder in a planetary mill enables the realization of several effects, including the following: ⎯enlargement of the effective surface of the origi nally spherical UHMWPE powder particles in the course of interactions (shear strains) with the grinding bodies (balls) of the mill; ⎯recombination of the UHMWPE molecular structure as a result of rupturing of intramolecular bonds and altered packing of carbon chains; ⎯mechanical alloying of UHMWPE particles by nanofiller ones; ⎯mixing of UHMWPE and filler particles to make the distribution of the filler in the binder homogeneous before hot pressing. The first two effects are analyzed in the present paper in order to estimate the role of mechanical activation of the binder. Figure 1 shows the microstructure of the initial UHMWPE powder before and after mechanical activa tion for 20 min. The polymer particles undergo multiple deformation during mechanical activation, which leads to flattening of the almost spherical UHMWPE parti cles down to flakes 50–200 μm in size and to increased efficient area of contact interactions between the mole cules. The loose density of the powder reduces substan tially within the first 20 min of mechanoactivation, while further activation exerts only a negligible effect on the powder density (Fig. 2). The IR spectra of the UHMWPE powders in the ini tial state and after mechanoactivation are illustrated in Fig. 3. The intensity of the oscillations of the C–H2 link (bands 2851, 1467, and 720 cm–1) in the activated UHMWPE powder drops. In addition, the peak inten sity of the oscillations of the C–O link in the IR spec trum drops as well (bands 1072, 1027, and 802 cm–1). The results of study of the physicomechanical and triboengineering characteristics of the initial and mechanically activated UHMWPE after hot pressing
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A 0.8
0.6
0.4
1
0.2
2
0 4000
3500
3000
2500
2000
1500
1000 v, cm–1
Fig. 3. IR spectra of UHMWPE powder in the initial state (1) and after mechanical activation for 20 min (2).
are presented in Table 1. As follows from Table 1, mech anoactivation of UHMWPE powders results in increas ing elasticity modulus, density, and Shore hardness, as well as in the reduction of the friction coefficient of the material. The optimal activation time for the above mentioned conditions turned out to be 20 min. Any fur ther activation does not improve the properties, which is attributed to the agglomeration of powders due to the high efficient surface area of the particles. This is expressed in the dependence of the Brinell hardness of the polymer on activation time (Fig. 4). The structural analysis of the initial and mechani cally activated UHMWPE after hot pressing has shown that mechanoactivation of the powder leads to varia tions in the macromolecular packing of the polymer. It is shown in [13] that PE tends to form various polymorphous modifications by turning its macromol ecules to different angles. This does not result in any fundamental changes, although there appear some new lines or intensity variations on the diffraction patterns. The crystallite sizes were determined judging by the physical broadening of different reflexes based on Sherer’s formula. The data of the Xray phase analysis
for the initial and mechanoactivated UHMWPE sam ples are listed in Table 2. All samples contain a crystalline phase of βpolyeth ylene. The crystallites are oriented predominantly along the plane (110), and the intensity of the reflex responsi ble for this plane greatly exceeds the intensity of the remaining reflexes. Certain asymmetry is observed in the main reflexes upon the mechanoactivation of UHMWPE powders, which is evidence of the polymor phous transformations in the crystalline lattice. The crystallinity degree and crystallite dimensions decline as a result of mechanical activation of the UHMWPE samples. Microdeformations in the lattice grow, whereas the structural parameters stabilize when the time of activation reaches 20 min (Table 2). Figure 5 shows the Xray patterns of the UHMWPE samples in the initial state and after 20 min of activation. To analyze the effect of mechanoactivation of the UHMWPE powders on the formation of the supermo lecular structure, we have studied the IR spectra of the initial and mechanically activated UHMWPE after hot pressing. We have observed that the peak intensity of the C–O link oscillations grows (Fig. 6) after hot pressing of the activated UHMWPE powder, although the peak
Table 1. Physicomechanical and triboengineering characteristics of the initial and mechanically activated UHMWPE Activation time, min
Density, g/cm3
Friction coefficient
Elasticity modulus, MPa
Yield point, MPa
Shore hardness
Initial 10 20 30 40
0.909 0.915 0.928 0.912 0.919
0.143 0.143 0.133 0.137 0.137
669.64 689.48 699.33 626.13 624.16
14.07 15.00 14.17 12.83 12.93
97.59 98.07 98.60 98.25 97.66
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10
20
τ, min
30
Fig. 4. Dependence of Brinell hardness of UHMWPE samples on activation time of the polymer powder.
I, rel. units
1 3000
2 1000
25 2θ, degrees
20
15
Fig. 5. Radiograms of UHMWPE samples made of the polymer powder in the initial state (1) and after mechanical activation for 20 min (2).
intensity of the C–H2 link oscillations differs little from that of the initial UHMWPE. The analogous depen dence of the oscillation peak intensity of the links C–H2 and C–O is true for any time of activation (10–40 min), but the maximal growth of the peak intensity is observed when activation lasts for 20 min. The data obtained by IR spectroscopy of the activated UHMWPE powders prove that the C–H2 links undergo breakage during
mechanoactivation, which is followed by crosslinking of hydrocarbon C–O chains in groups during hot pressing (Figs. 3, 6) [14, 15]. As a result, the hydrocar bon chains of the polymer change their location. The results of electronmicroscopic investigations agree well with the data on the effect of UHMWPE mechanoactivation on the further formation of super molecular structures in the polymer. To determine the
Table 2. Xray phase analysis results of UHMWPE samples in the initial state and after mechanical activation Activation time, min
Crystalline phase
Crystallinity degree, %
Size of coherent scattering regions, nm
Microdeformations of lattice, 10–3
Intitial 10 20 30 40
βpolyethylene βpolyethylene βpolyethylene βpolyethylene βpolyethylene
56.5 56.0 54.5 53.1 53.0
38 36 30 27 27
14 13 12 10 10
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A
0.3
0.2
0.1 1 0 2 –0.1 4000
3500
3000
2500
2000
1500
1000 v, cm–1
Fig. 6. IR spectra of UHMWPE samples after hot pressing of the polymer powder in the initial state (1) and after mechanical acti vation for 20 min (2).
(a)
50 µm (b)
50 µm
(c)
50 µm (d)
50 µm
Fig. 7. SEM images of supermolecular structure of UHMWPE samples made of the initial polymer powder (a) and after mechan ical activation for 20 min (b), 30 min (c), and 40 min (d).
effect of mechanoactivation on the structurization pro cesses in UHMWPE and, correspondingly, on its phys icomechanical and triboengineering characteristics, we have carried out structural investigations by the scan JOURNAL OF FRICTION AND WEAR
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ning electron microscopy. The macroconformation of the chains in the initial state (hotpressing of nonacti vated UHMWPE powder) bears a partially fibrillar character (Fig. 7a) [16]. After mechanoactivation, the 2010
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1
35
4
30
5 3 2
25
20
15 0
20
40
60
80
100
120
140
160
180 t, min
Fig. 8. Time dependence of the wornout area of UHMWPE samples made of the polymer powder in the initial state and after mechanical activation: 1⎯initial UHMWPE; 2⎯τ = 10; 3⎯20; 4⎯30; 5⎯40 min.
(a)
1 mm (b)
1 mm
(c)
1 mm (d)
1 mm
Fig. 9. Optical images of the friction path of UHMWPE samples made of the initial (a, b) and activated for 20 min (c, d) poders at sliding duration 10 min (a, c) and 20 min (b, d).
supermolecular structure of the polymer is of lamellar character (Fig. 7 b–d) and the width of the lamellas reduces with increasing time of activation, thus being in correlation with the reduction of crystallites, according to the Xray structural analysis (Table 2). As a consequence of the variations in the supermo lecular structure of mechanoactivated UHMWPE, its wear resistance increases (Fig. 8). The maximal
increase in the wear resistance (four times) is displayed by UHMWPE samples subjected to mechanoactiva tion for 20 min (curves 1 and 3). The microstructure of the friction surface of the initial and mechanoacti vated UHMWPE is shown in Fig. 9. The minimum friction surface roughness is recorded after 20 min of mechanoactivation (Fig. 10). The friction coefficient of UHMWPE, correspondingly, reaches a minimum after 20 min of activation (Table 1).
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3.
0.12 4.
0.08 0
10
20
30
40 τ, min
Fig. 10. Dependence of the friction surface roughness of UHMWPE samples upon activation time of the polymer powder.
5. 6.
CONCLUSIONS The study of the structural, triboengineering, and strainstrength characteristics of UHMWPE in the ini tial state and after mechanoactivation of the powder prior to its pressing into samples has shown the follow ing results: ⎯mechanoactivation results in shape variations and broadening of the effective surface of the binder parti cles, as well as breakage and further crosslinking of C–O hydrocarbon chains in groups; ⎯a tendency is observed towards the reduction of the crystallite dimensions and corresponding changes in the character of hydrocarbon chains packing in the polymer (from partially fibrillar to lamellar); ⎯the reduction of the structural components and growth of their packing density is observed in the poly mer, which elevates its triboengineering, strain, and strength characteristics (wear resistance, density, elas ticity modulus, yield strength); in particular, the wear resistance is increased by as much as four times; ⎯the optimal time of activation of the initial UHMWPE powders has been found to be 20 min (longer activation does not further improve the above mentioned characteristics). The determined optimal activation conditions of UHMWPE have been used in developing polymer nanocomposites with preset properties.
7.
8.
9.
10.
11.
12.
13.
DESIGNATIONS τ⎯activation time; d⎯density; ν⎯wave number; A⎯optical density; HB⎯Brinell hardness; 2θ⎯diffraction angle; I⎯intensity radiograph peaks; S⎯wear area; Ra⎯ friction surface roughness.
14. 15.
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