Journal of Alloys and Compounds 586 (2014) S443–S445
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Structure, mechanical and tribological properties of radiation cross-linked ultrahigh molecular weight polyethylene and composite materials based on it V.V. Tcherdyntsev a,⇑, S.D. Kaloshkin a, A.A. Lunkova a, A.M. Musalitin a, V.D. Danilov b, Yu.V. Borisova a, A.A. Boykov a, V.A. Sudarchikov a a b
National University of Science and Technology «MISiS», Leninsky Prospect, 4, Moscow, 119049, Russia A.A. Blagonravov Institute of Mechanical Engineering RAS, ul. Bardina 4, Moscow, 117334, Russia
a r t i c l e
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Article history: Available online 5 June 2013 Keywords: Ultrahigh molecular weight polyethylene Carbon nanotubes Radiation Cross-linking Mechanical properties
a b s t r a c t The effect of irradiation with electrons, oriented drawing, and reinforcement with multi-walled carbon nanotubes (MWCNT) on the structure, physico-mechanical and tribological properties of ultrahigh molecular weight polyethylene (UHMWPE) is studied. It is shown that the radiation cross-linking leads to the melting temperature of UHMWPE nearly linear increases with the dose of radiation. The optimal irradiation dose with respect to the mechanical characteristics is found to be 20 Mrad. It is shown that the strength characteristics of UHMWPE can be improved most efﬁciently by a combination of irradiation, oriented drawing, and reinforcement with nanotubes, and the second and the third factors have a stronger effect than the ﬁrst one. A combined effect of three factors enabled us to enhance the yield strength of material by almost four times without a detrimental effect on its plasticity. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In the last decades, the polymer composite materials have been considered as a signiﬁcant alternative to the mechanical alloys. The composite materials improve the functional characteristics of an article together with a decrease in its weight [1,2]. Ultrahigh molecular weight polyethylene (UHMWPE), which exhibits high mechanical and tribological properties and a high chemical resistance, is one of the most promising matrix materials for the polymer composites . The goal of this work was to create the oriented composite materials based on UHMWPE ﬁlled reinforced with the multiwall carbon nanotubes (MWCNT). It is known that the mechanical orientation of these composites enables one to enhance signiﬁcantly their mechanical properties [4–6]. To form the longitudinal and cross links (C–C) between the polymer macromolecules and to ﬁx the oriented structure, the method of radiation cross-linking was used. This method enables one to affect the functional properties of polymer materials within a wide range .
⇑ Corresponding author. Tel.: +7 9104002369; fax: +7 4956384595. E-mail address: [email protected]
(V.V. Tcherdyntsev). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.05.150
2. Experimental procedure The UHMWPE powder (‘‘Kazanorgsintez’’) with a molecular weight of 2 106 g/ mol (Speciﬁcations: 2211-153-00203335-2004, the density of not less than 0.935– 0.947 g/cm3) b multi-walled carbon nanotubes (MWCNT) with an average diameter of 33 nm and a length of 2–3 lm were used as the source materials. The MWCNT were introduced into the UHMWPE matrix (the concentration of MWCNT was varied from 0.1 to 5 wt%) by the joint mechanical milling of the source materials in an APF-3 water-cooled planetary ball mill. Consolidation of both pure UHMWPE and composite powders was carried out by the hot compression molding technique using a TESAR APVM-904 hydraulic press with a force up to 40 t equipped with additional heating elements. The radiation cross-linking was performed with an Elektronika ELU-6 electron accelerator comprising a controlled pulsed source of electrons with an energy E = 6 MeV, the irradiation dose was varied from 0 to 55 Mrad. The effect of irradiation dose on the melting temperature of UHMWPE was determined by the differential scanning calorimetry (DSC), which was carried out in argon atmosphere using an NETZSCH DSC 204 F1 calorimeter. The mechanical tensile tests were performed with a Zwick Z020 universal testing machine at a speed of 10 mm/min. The friction and wear tests of the composites were performed on «block-on-ring» apparatus (IMASH) in a dry sliding regime at a complex of parameters that simulate the behavior of sealing and bearing units of a broad machinery class . A steel 45 ring 98 mm in diameter and 5 mm in thickness with a hardness of 55 HRC and the maximum/average roughness of 1–1.2/0.2 lm was used as the counterpart. The load was 19 N, the sliding velocity was 2.5 m/s, the duration of tests was 30 min; all tests were performed at the ambient temperature. The specimens in the form of rectangular plates were subjected to the uniaxial orientation. A tensile loading was applied to the specimen ends at room temperature. First, the low-oriented specimens with a draw ratio k = 3 were obtained. Then, the specimens were cut into strips and placed into a press mold. Here, they were
V.V. Tcherdyntsev et al. / Journal of Alloys and Compounds 586 (2014) S443–S445
Fig. 1. Dependence of melting temperature on the UHMWPE irradiation dose.
Fig. 3. (a) Wear spot area and (b) friction coefﬁcient measured on the irradiated UHMWPE specimens.
Fig. 2. Plots of (a) yield strength, (b) tensile strength and (c) elongation vs. the UHMWPE irradiation dose. hot pressed at a temperature and a pressure that, on the one hand, ﬁxed the loworiented structure and, on the other hand, provided the monolithic structure and healing of defects, which formed during the orientation.
3. Results and discussion Fig. 1 gives the dependence of UHMWPE melting temperature on the irradiation dose based on the calorimetric data. It is seen
that the polymer melting temperature gradually increases throughout the studied range of irradiation doses. This result would be expected, because the formation of additional chemical bond will enhance the thermal stability of crystal lattice. Fig. 2 gives the dependences of tensile properties on the irradiation dose of pure UHMWPE. The yield strength r0.2 (Fig. 2a) increases to an irradiation dose of 20 Mrad and, then, slightly decreases. At the same time, the tensile strength (Fig. 2b) is almost independent of the irradiation dose. It should be noted that the yield strength is the most practically important strength parameter, because it determines the highest load, which the material can withstand with no irreversible deformation. An elongation at break (Fig. 2c) decreases with increasing irradiation dose and becomes almost zero at the doses of 30 Mrad and higher. Fig. 3 gives the tribological characteristics of pure UHMWPE vs. the irradiation dose. It is seen that small irradiation doses (5– 20 Mrad) improve the tribological characteristics of material: both the friction coefﬁcient and the wear decrease. At a further increase of the irradiation dose, the friction coefﬁcient becomes equal to that for unirradiated UHMWPE, and the wear intensity even exceeds the values for the unirradiated UHMWPE. Fig. 4 shows the effect of the ﬁlling with MWCNT, the irradiation, and orientation on the mechanical properties of UHMWPE. The introduction of carbon nanotubes into the polymer matrix (1) slightly enhances the yield strength and insigniﬁcantly decreases the elongation. The irradiation (2) considerably enhances the yield strength both for pure UHMWPE and ﬁlled with the MWCNT, and the elongation is almost halved. This is the evidence for the radiation cross-linking of the molecules, i.e. the formation of longitudinal-cross links between the polymer macromolecules, which conserve the structure orientation. These links, on the one hand, enhance signiﬁcantly the strength characteristics of material and, on the other hand, restrict the slipping of the molecules against each other under a tensile load, which reduces the ductility characteristics of the specimens. The orientation (3) also leads to a signiﬁcant increase in the yield strength; it should be noted that, in this case, the introduc-
V.V. Tcherdyntsev et al. / Journal of Alloys and Compounds 586 (2014) S443–S445
Yield strength, MPa
ecules, but also the nanotubes, which are randomly distributed over the matrix, are oriented. The orientation is accompanied by approximately ﬁvefold decrease in the elongation at break, and the specimen’s plasticity it retained: the elongation is 25–50%. The coaction of orientation an irradiation (4) leads to an additional increase in the yield strength and an insigniﬁcant decrease in the elongation at break.
The effect of irradiation with electrons, oriented drawing, and reinforcement with MWCNT on the structure, physico-mechanical and tribological properties of UHMWPE is studied. It is shown that the radiation cross-linking leads to an increase in the melting temperature of UHMWPE. From the viewpoint of mechanical characteristics, the optimal irradiation dose is found to be 20 Mrad. The radiation treatment enables one to improve slightly the tribological characteristics of material. It is shown that the strength characteristics of UHMWPE can be improved most efﬁciently by a combination of irradiation, oriented drawing, and reinforcement with nanotubes, and the effects of the second and third factors are stronger than that of the ﬁrst one.
20 10 0 0
MWCNT weight fraction, %
This work was supported by the federal target program ‘‘Scientiﬁc and scientiﬁc-pedagogical personnel of innovative Russia’’, agreement No. 14.A18.21.0376, signed on 06.08.2012. Authors also with to thank Mr. A.V. Maximkin for his kindly assist both in experimental study and in the preparation of manuscript.
MWCNT weight fraction, % Fig. 4. Dependences of (a) yield strength and (b) elongation of composite on a degree of ﬁlling with the MWCNT. (1) Initial untreated specimens; (2) the specimens irradiated with a dose of 20 Mrad; (3) the specimens subjected to the oriented drawing; and (4) the specimens subjected to the oriented drawing and irradiated with a dose of 20 Mrad.
tion of MWCNT into UHMWPE also has a pronounced effect. For pure UHMWPE, as well as for the specimens with a small content of MWCNT (0.1 wt%), the yield strength is doubled as a result of orientation, whereas for the specimens containing 1.0 wt% MWCNT, it is trebled. This indicates that not only the polymer mol-
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