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Morphological study of the wear of crystalline polymers I: High density polyethylene
Wear, 75 (1982)
173
173 - 182
MORPHOLOGICAL POLYMERS I: HIGH DENSITY
STUDY OF THE WEAR OF CRYSTALLINE POLYETHYLENE*
TADASHI KoMOTO and KENJI TANAKA Department of Polymer ku, Tokyo (Japan)
Technology,
Tokyo Institute
of Technology,
Ookayama,
Meguro-
SEIICHIRO HIRONAKA Department Meguro-ku,
of Chemical Engineering, Tokyo (Japan)
Tokyo
Institute
of Technology,
Ookayama,
TAKESHI MATSUMOTO Research Centre of Product Development, Kamiizumi, Sodegaura-machi, Kimitsu-gun,
Zdemitsu Petrochemicals Chiba (Japan)
Co. Ltd.,
1660
NOBUYUKI TAKANO Products Research Laboratory, Chiba (Japan)
Zdemitsu Kosan Co. Ltd., 244
Anesakikaigan,
Zchihara-shi,
(Received May 27, 1981; in revised form July 28, 1981)
Summary The morphology of the worn surface and wear debris of high density polyethylene (HDPE) slid against steel in air and water was studied. Electron micrographs and electron diffraction patterns of carbon replicas of the worn surfaces revealed that the molecular chains (crystallographic c axis) of HDPE were markedly oriented in the sliding direction. Annealing of the wear specimen caused lamellar crystals to form perpendicular to the sliding direction, suggesting folded-chain crystals linked by tie molecules and tie fibrils. Morphological consideration is also given to the wear mechanisms of HDPE in air and water.
1. Introduction High density polyethylene (HDPE), a crystalline polymer, is an important articular cartilage material, generally mating with stainless steel. The mechanisms of wear of polymers including HDPE have been widely studied [ 1 - 281. The morphologies of worn polymer surfaces and films transferred *Paper presented at the International Conference on Wear of Materials 1981, San Francisco, CA, U.S.A., March 30 - April 1, 1981. 0043-1648/82/0000-0000/$02.50
0 Elsevier Sequoia/Printed
in The Netherlands
174
to the countersurfaces have also been examined by optical and electron microscopy to verify proposed wear mechanisms and to reveal orientational deformation of the polymers [2,4,9,11, 13,15,24 - 271. However, morphological examinations of the worn surfaces and the resultant wear particles have not been made in detail with reference to the molecular properties and crystallization behaviour characteristics of individual crystalline polymers. Wear does not take place unless the polymer material is deformed by external shear, e.g. in the sliding of solid polymer against steel. With the formation of fibres and drawn films, molten polymer molecules pass or flow through a metal nozzle, changing the molecular arrangement from that of a random coil to one oriented by the shearing force. The difference between the two cases is that the random coil arrangement is the contact of solid polymer and metal and the shear-oriented arrangement is that of liquid polymer and metal. The morphologies of the wear surfaces and wear particles of polymers formed by steel sliding may, therefore, be compared with the structures and morphologies of fibres and drawn films. In this paper, a morphological study was made of the wear of HDPE by steel balls sliding in air and in water.
2. Experimental
details
2.1. Friction machine An AISI 52100 steel ball of diameter 4.76 mm ($ in) was slid on an HDPE disk rotating at a speed of 18.8 cm s-l in a ball-on-disk friction machine [29]. The friction coefficients as functions of time, load and lubricant were recorded. The loads used were 5,7 and 10 N. The lubricants were air and water. The resultant wear particles and wear tracks on the polymer disk were examined morphologically. 2.2. Polymer HDPE with a melt index of about 0.16 was used. 2.3. Transmission electron microscopy A carbon replica method was used. Carbon was deposited in uacuo at an angle of 30” onto the polymer disk surface with a wear track. A concentrated gelatin solution in water was placed onto the carbon and, when this had solidified, the carbon film and gelatin were peeled from the specimen, floated upside down on an aqueous KSCN solution to dissolve the gelatin and then transferred to distilled water to wash the gelatin away. The carbon film was collected from this water surface on a copper mesh. Observations were made with a JEM-7 electron microscope. Selected area electron diffraction was carried out on the very thin layer of polymer attached to the carbon film, which had been peeled from the specimen surface with the carbon film.
175
2.4. Scanning electron microscopy The wear particles, which were gold coated in uacuo, were also observed with an HSM-2 scanning electron microscope.
3. Results and discussion 3.1. Wear in air Figure 1 shows the friction coefficient of HDPE sliding against a steel ball in air. The value of the friction coefficient was constant at about 0.13 0.15; this is consistent with values reported elsewhere [ 11,251. The free surface of the HDPE disk, i.e. the unworn surface, did not give a preferentially oriented morphology. Figure 2(a) shows a transmission electron micrograph of the worn surface (wear track) of HDPE slid against steel for 2 h under a load of 5 N. The sliding direction is parallel to the direction of the striations. Sharp electron diffraction spots (Fig. 2(b)) were obtained from the replica shown in Fig. 2(a), indicating a very marked chain orientation along the sliding direction; this is in good agreement with the micrograph (Fig. 2(a)). Figure 2(c) shows that the crystallographic c axis, i.e. the chain axis of polyethylene, is parallel to the striations of the micrograph and the sliding direction. Consideration of the surface melting [4] of HDPE by friction and a very high rate of crystallization suggests that the morphology of the worn surface (Fig. 2(a)) seems to be formed by crystallization of HDPE from its melt at a high shear rate. It is important to clarify the molecular chain arrangement of the worn surface which may be related to the wear mechanism. Three types of chain arrangements in the oriented crystal are assumed: (1) oriented lamellar crystals, i.e. folded-chain crystals, are linked by tie molecules and tie fibrils as in synthetic fibres [30] (Fig. 3(a)); (2) a row structure [ 311 such as the shish kebab structure [ 321, which is formed by crystallization of HDPE from its dilute solution in xylene with vigorous stirring, consists of extended chain crystals and overgrown lamellar crystals
Sliding
time, min.
Fig. 1. Friction coefficient of HDPE slid against a steel ball in air at 21 “C under various loads (sliding speed, 18.8 cm s-l): 0, 5 N; 0, 7 N; 0, 10 N.
176
(b)
Fig. 2. (a) Transmission electron micrograph of a carbon replica of an HDPE surface worn by a steel ball (sliding speed, 18.8 cm s- 1 ; duration, 2 h; load, 5 N; atmosphere, air), (b) electron diffraction pattern obtained from Fig, 2(a) and (c) indexing of the diffraction spots.
(Fig. 3(b)); (3) uniaxially oriented extended-chain crystals such as aromatic polyamide fibres in which the extended molecular chains are aligned parallel to each other as in the so-called nematic crystal (Fig. 3(c)). A folded-chain crystal of polyethylene melts below its equilibrium melting temperature (T,,,” = 141 “C), i.e. at about 130 - 136 “C!,depending on the lamellar thickness. The molecular chains partly melt and crystallize; this accompanies an increase in the crystal thickness by annealing at temperatures above about 115 “C and below the melting temperature. However, the extended-chain crystal of polyethylene melts at the equilibrium melting temperature T,” and the molecular chains do not rearrange at temperatures below T," . Therefore, if the molecular chains in the wear track constitute the extended-chain crystals, i.e. if the model shown in Fig. 3(c) is correct, they
(4 Fig. 3. Schematic (b) row structure oriented lamellar
(b)
(d)
illustration of HDPE chain arrangements in (a) fibre and drawn film, or shish kebab structure, (c) extended-chain crystal fibre and (d) crystals formed by annealing of the structure in Fig. 3(a).
should not rearrange, via partial melting and crystallization, into folded-chain crystals by annealing. In other words, the morphology of the wear track (Fig. 2(a)) should not change by annealing when the extended-chain crystal is formed by friction. For fibre-like crystals (Fig. 3(a)) and crystals having a row structure (Fig. 3(b)), annealing may cause the growth of the lamellar crystals on the folded-chain type of crystal nuclei present at the wear surface, where the original crystal nuclei, if they exist, may have been too small to be observed in the electron micrograph (Fig. 2(a)). The lamellar crystals would be expected to be prominent after annealing (Fig. 3(d)). The HDPE disk worn in air was annealed at 120 “C for 10 min in uacuo. Figure 4 shows an electron micrograph of the carbon replica of the annealed worn surface of HDPE. The sliding direction is vertical. An electron diffraction pattern similar to that shown in Fig. 2(b) was obtained from this carbon replica film; this indicated no change in the direction of the molecular chains owing to annealing. Lamellar crystals of thickness 100 - 200 W were well grown. They are seen as striations perpendicular to the sliding direction, i.e. horizontal in the micrograph. The extended chain crystals are, therefore, suppressed for polyethylene in the worn surface. The first model (Fig. 3(a)), in which lamellar crystals are linked by tie molecules and tie fibrils, is the most plausible. The second model, i.e. the row structure, cannot be rejected, however, because lamellar crystals may have overgrown onto the extendedchain fibrillar crystals by annealing. Transmission electron microscopy observation was also made of wear particles which had been collected on a carbon film covering a copper mesh and shadowed with Pt-Pd. Most of the wear particles were sufficiently thick (about several thousand angstroms) that the electron beam did not pass through them. A very small fraction of the wear particles gave an electron micrograph as shown in .Fig. 5. The wear debris is evidently a highly oriented film of a substantial size. Thicker oriented films may be the major fraction of the wear debris.
Fig. 4. Transmission electron micrograph of a carbon replica of an HDPE surface worn by a steel ball (sliding speed, 18.8 cm s-l; duration, 2 h; load, 5 N; atmosphere, air) and then annealed in uacuo at 120 “C for 10 min. Grown lamellar crystals are seen as striations perpendicular to the sliding direction. Fig. 5. Transmission electron micrograph of a wear particle of HDPE in the form of film (sliding speed, 18.8 cm s-l ; duration, 2 h; load, 5 N; atmosphere, air).
Differential scanning calorimetry was also carried out on HDPE wear particles using a DSC-2 differential scanning calorimeter at a heating rate of 5 “C min-’ . A single melting peak was obtained at a temperature as low as 128.9 “C. This clearly reveals that the wear debris does not contain the extended-chain crystals but the folded-chain crystals and such strained chains as tie molecules and tie fibrils. Even if the row structure is contained in the wear debris, the core crystals are negligibly small in number. The results may suggest that films, with a thickness of several thousand angstroms, are worn from the polymer disk surface in such a way that a molten polyethylene film is uniaxially drawn in air, producing oriented crystallization. 3.2. Wear in water Wear of HDPE in water is closely related to wear in artificial joint prostheses [9, 33 - 371. The friction coefficient changed with sliding time as shown in Fig. 6. The friction coefficient reached a minimum of 0.04 after 5 - 10 min and then gradually increased depending on the load. Tanaka and Miyata [ 111 and Tanaka [ 251 reported that moisture (vapour) and water (liquid) reduce the friction coefficient for the combination of HDPE and steel. The effect of water molecules on the friction and wear of HDPE is of interest with respect to the surface morphology of worn HDPE. Figure 7 shows a carbon replica film of the wear track of HDPE. The
179
I
1
0.14 z
0.12
.,” .:
0.10
2 H
0.08
2B
0.06
.,” ii:
0.04 0.02 I
0
0
60
1
I
I
I
720
180
240
s
I
t
300
360
Slldlng time, min
Fig. 6. Plots of friction coefficient US.sliding time of HDPE slid against a steel ball under various loads (sliding speed, 18.8 cm s -I ; sliding in water at 21 “C): 0, 5 N; 0, 7 N; 0, 10 N.
Fig. 7. Transmission electron micrograph of a carbon replica of the HDPE surface worn by a steel ball (sliding speed, 18.8 cm s -’ ; duration, 6 h; load, 5 N; sliding in water at 21 “C). Fig. 8. Electron diffraction
pattern obtained
from the carbon repfica shown in Fig. 7,
surface is rougher than that in air, although a highly oriented morphology was obtained. From the carbon replica film of the wear track (Fig. 7), an electron diffraction pattern (Fig. 8) was obtained, indicating that the molecular chain orientation along the sliding direction was not as high (this is evident from the arc scattering of the 110 and 200) as in the case of wear in air (Fig. 2(b)). From Fig. 8, the electron diffraction intensity is stronger for the 200 reflection than for the 110 reflection, This is the reverse of the diffraction pattern for wear in air (Fig. Z(b)). Bunn [38] obtained the observed and
180
calculated intensities for various X-ray reflections from polyethylene fibre, where the intensity for the 110 reflection was stronger by a factor of 4 than that for the 200 reflection. The same relationship was found from the present results of electron diffraction from the wear surface obtained in air (Fig. 2(b)). In the fibre specimen [38], the crystallographic c axis is parallel to the fibre axis, while the a and b axes are isotropically oriented in a direction perpendicular to the fibre axis. The fact that the 200 reflection is stronger than the 110 reflection suggests an anisotropic crystal orientation on the wear surface of the specimen such that the crystallographic c axis is parallel to the sliding direction, that the b axis is, to a great extent, perpendicular to both the sliding direction and the wear surface and that the a axis is, to the same extent, perpendicular to the sliding direction and parallel to the wear surface. (The unit cell of polyethylene is orthorhombic with a = 7.40 A, b = 4.93 A and c = 2.534 A (chain axis) [38] .) This result means that in water wear takes place on the crystallographic u-c plane, i.e. along a direction perpendicular to the b axis. The u-c plane is the boundary where the neighbouring crystals slip over each other and accordingly the wear debris separates from the crystal substrate. This may also mean that water molecules are preferentially adsorbed on the u-c plane where they act as a lubricant. Figure 9 shows thick (dark) oriented masses which were removed from the worn surface of the disk specimen by carbon replication. This morphology resembles that in Fig. 5. The thick oriented masses will be worn away as thick, oriented films in the next sliding process. Figure 10 shows large filmy wear debris; the supermolecular structure of this debris is that of oriented lamellae as for wear in air.
Fig. 9. Transmission electron micrograph of a carbon replica of an HDPE surface worn by a steel ball (sliding speed, 18.8 cm s-l; duration, 6 h; load, 5 N; sliding in water at 21 “C!). Fig. 10. Scanning electron micrograph of HDPE wear particles formed by sliding an HDPE disk against a steel ball (sliding speed, 18.8 cm s-l; duration, 6 h; load, 5 N; sliding in water at 21 “C).
181
Acknowledgment The authors express their sincere gratitude to Professor Tanaka of Kanazawa University for his kind suggestions.
Kyuichiro
References 1 K. G. McLaren and D. Tabor, Friction of polymers: influence of speed and temperature, Nature (London), 197 (1963) 858 - 860. 2 K. Tanaka, Y. Uchiyama and S. Toyooka, The mechanism of wear of poiytetrafluoroethylene, Wear, 23 (1973) 153 - 172. 3 L. C. Towle, Shear strength and friction measurements on polyethylene under high pressure, J. Appl. Phys., 44 (1973) 1611 - 1615. 4 K. Tanaka and Y. Uchiyama, Friction, wear and surface melting of crystalline polymers. In L. H. Lee (ed.), Advances in Polymer Friction and Wear, Vol. 5B, Plenum, New York, 1974, pp. 499 - 530. 5 W. A. Glae’ser, J. W. Kissel and D. K. Snediker, Wear mechanisms of polymers at cryogenic temperatures. In L. H. Lee (ed.), Advances in Polymer Friction and Wear, Vol. 5B, Plenum, New York, 1974, pp. 651 - 662. 6 L. H. Lee, Effect of surface energetics on polymer friction and wear. In L. H. Lee (ea.), Advances in Polymer Friction and Wear, Vol. 5A, Plenum, New York, 1975, pp. 31 - 66. 7 L. C. Towle, Shear strength and polymer friction. In L. H. Lee (ed.), Advances in Polymer Friction and Wear, Vol. 5A, Plenum, New York, 1975, pp. 179 - 187. 8 D. H. Buckley and W. A. Brainard, The atomic nature of polymer-metal interactions in adhesion, friction and wear. In L. H. Lee (ed.), Advances in Polymer Friction and Wear, Vol. 5A, Plenum, New York, 1975, pp. 315 - 328. 9 K. J. Brown, J. R. Atkinson, D. Dowson and V. Wright, The wear of ultrahigh molecular weight polyethylene and a preliminary study of its relation to the in uiuo behaviour of replacement hip joint, Wear, 40 (1976) 255 - 264. 10 W. Rostoker and J. 0. Galante, Some new studies of the wear behavior of ultrahigh molecular weight polyethylene, J. Biomed. Mater. Res., 10 (1976) 303 - 310. 11 K. Tanaka and T. Miyata, Studies on the friction and transfer of semicrystalline polymers, Wear, 41 (1977) 383 - 398. 12 D. Tabor, Wear -a critical synoptic view, J. Lubr. Technol., 99 (1977) 387 - 395. 13 K. Tanaka, Friction and wear of glass- and carbon fiber-filled thermoplastic polymers, J. Lubr. Technol., 99 (1977) 408 - 414. 14 T. A. du Plessis, C. J. Grobbelaar and F. Marais, The improvement of polyethylene prostheses through radiation crosslinking, Radiat. Phys. Chem., 9 (1977) 647 - 652. 15 M. K. Kar and S. Bahadur, Micromechanism of wear at polymer-metal sliding interface, Wear, 46 (1978) 189 - 202. 16 S. K. Rhee and K. C. Ludema, Mechanisms of formation of polymeric transfer films, Wear, 46 (1978) 231 - 240. 17 D. J. Briscoe and D. Tabor, Friction and wear of polymers: the role of mechanical properties, Br. Polym. J., 10 (1978) 74 - 78. 18 J. H. Warren and N. S. Eiss, Jr., Depth of penetration as a predictor of the wear polymers on hard rough surfaces, J. Lubr. Technol., ZOO (1978) 92 - 97. 19 J. K. Lancaster, Wear mechanisms of metals and polymers, Trans. Inst. Met. Finish., 56 (1978) 145 - 153. 20 H. McKellop, I. C. Clarke, K. L. Markolf and H. C. Amstutz, Wear characteristics of UHMW polyethylene: a method for accurately measuring extremely low wear rates, J. Biomed. Mater. Res., 12 (1978) 895 - 927.
182 21 N. H. Sung and N. P. Suh, Effect of fiber orientation on friction and wear of fiberreinforced polymeric composites, Wear, 53 (1979) 129 - 141. 22 B. J. Briscoe and A. C. Smith, The influence of dynamic loading on sliding friction, Nature (London), 278 (1979) 725 - 726. 23 B. J. Briscoe and T. A. Stolarski, Combined rotating and linear motion effects on the wear of polymers, Nature (London), 281 (1979) 206 - 208. 24 N. S. Eiss, Jr., K. C. Wood, J. A. Herold and K. A. Smyth, Model for the transfer of polymer to rough, hard surfaces, J. Lubr. Technol., 101 (1979) 212 - 219. 25 K. Tanaka, Friction and wear of semi-crystalline polymers sliding against steel under water lubrication. In K. C. Ludema, W. A. Glaeser and S. K. Rhee (eds.), Proc. Int. Conf. on Wear of Materials, Dearborn, MI, 1979, American Society of Mechanical Engineers, New York, 1979, pp. 563 - 572. 26 J. Krzeminski and E. Kowalska, Struktur und Eigenschaften von durch Reibung modifizierten Kunststoff-Oberflachen, Kunststoffe, 69 (1979) 482 - 485. 27 N. S. Eiss, Jr. and K. A. Smyth, The wear of polymers sliding on polymeric films deposited on rough surfaces. In Proc. ASME-ASLE Int. Conf., San Francisco, CA, 1980, in J. Lubr. Technol., 103 (1981) 266 - 273. 28 B. J. Briscoe and T. A. Stolarski, The influence of linear and rotating motions on the friction of polymers. In Proc. ASME-ASLE Znt. Conf., San Francisco, CA, 1980, in J. Lubr. Technol., (1981). 29 S. Hironaka, Y. Yahagi and T. Sakurai, Effect of adsorption of some surfactants on antiwear properties, ASLE Trans., 21 (1978) 231 - 235. 30 T. Kawai, Structure of fibers, Sen’i to Kogyo, 2 (1969) 106 - 115. 31 A. Keller and M. J. Machin, Oriented crystallization in polymers, J. Macromol. Sci., Phys., 1 (1967) 41 - 91. 32 A. J. Pennings and A. M. Kiel, Fractionation of polymers by crystallization from solution. III On the morphology of fibrillar polyethylene crystals grown in solution, Kolloid-Z., 205 (1965) 160 - 162. 33 D. A. Sonstegard, L. S. Mattews and H. Kaufer, The surgical replacement of the human knee joint, Sci. Am., 238 (1) (1978) 44 - 51. 34 A. Unsworth, The effects of lubrication in hip joint prostheses, Phys. Med. Biol., 23 (1978) 253 - 268. 35 W. Rostoker, E. Y. S. Chao and J. 0. Galante, The appearance of wear on polyethylene - a comparison of in vivo and in vitro wear surfaces, J. Biomed. Mater. Res., 12 (1978) 317 - 335. 36 J. Berzen, H. W. Birnkraut and G. Braun, Ultrahigh molecular weight polyethylene (UHMWPE): application in artificial joints, Br. Polym. J., 10 (1978) 281 - 287. 37 H. Stallforth and M. Ungethiim, Die tribologische Testung von Knieendoprosthesen, Biomed. Tech., 23 (1978) 295 - 304. 38 C. W. Bunn, The crystal structure of long-chain normal paraffin hydrocarbons. The “shape” of the =CHz group, Trans. Faraday Sot., 35 (1939) 482 - 491.
173
173 - 182
MORPHOLOGICAL POLYMERS I: HIGH DENSITY
STUDY OF THE WEAR OF CRYSTALLINE POLYETHYLENE*
TADASHI KoMOTO and KENJI TANAKA Department of Polymer ku, Tokyo (Japan)
Technology,
Tokyo Institute
of Technology,
Ookayama,
Meguro-
SEIICHIRO HIRONAKA Department Meguro-ku,
of Chemical Engineering, Tokyo (Japan)
Tokyo
Institute
of Technology,
Ookayama,
TAKESHI MATSUMOTO Research Centre of Product Development, Kamiizumi, Sodegaura-machi, Kimitsu-gun,
Zdemitsu Petrochemicals Chiba (Japan)
Co. Ltd.,
1660
NOBUYUKI TAKANO Products Research Laboratory, Chiba (Japan)
Zdemitsu Kosan Co. Ltd., 244
Anesakikaigan,
Zchihara-shi,
(Received May 27, 1981; in revised form July 28, 1981)
Summary The morphology of the worn surface and wear debris of high density polyethylene (HDPE) slid against steel in air and water was studied. Electron micrographs and electron diffraction patterns of carbon replicas of the worn surfaces revealed that the molecular chains (crystallographic c axis) of HDPE were markedly oriented in the sliding direction. Annealing of the wear specimen caused lamellar crystals to form perpendicular to the sliding direction, suggesting folded-chain crystals linked by tie molecules and tie fibrils. Morphological consideration is also given to the wear mechanisms of HDPE in air and water.
1. Introduction High density polyethylene (HDPE), a crystalline polymer, is an important articular cartilage material, generally mating with stainless steel. The mechanisms of wear of polymers including HDPE have been widely studied [ 1 - 281. The morphologies of worn polymer surfaces and films transferred *Paper presented at the International Conference on Wear of Materials 1981, San Francisco, CA, U.S.A., March 30 - April 1, 1981. 0043-1648/82/0000-0000/$02.50
0 Elsevier Sequoia/Printed
in The Netherlands
174
to the countersurfaces have also been examined by optical and electron microscopy to verify proposed wear mechanisms and to reveal orientational deformation of the polymers [2,4,9,11, 13,15,24 - 271. However, morphological examinations of the worn surfaces and the resultant wear particles have not been made in detail with reference to the molecular properties and crystallization behaviour characteristics of individual crystalline polymers. Wear does not take place unless the polymer material is deformed by external shear, e.g. in the sliding of solid polymer against steel. With the formation of fibres and drawn films, molten polymer molecules pass or flow through a metal nozzle, changing the molecular arrangement from that of a random coil to one oriented by the shearing force. The difference between the two cases is that the random coil arrangement is the contact of solid polymer and metal and the shear-oriented arrangement is that of liquid polymer and metal. The morphologies of the wear surfaces and wear particles of polymers formed by steel sliding may, therefore, be compared with the structures and morphologies of fibres and drawn films. In this paper, a morphological study was made of the wear of HDPE by steel balls sliding in air and in water.
2. Experimental
details
2.1. Friction machine An AISI 52100 steel ball of diameter 4.76 mm ($ in) was slid on an HDPE disk rotating at a speed of 18.8 cm s-l in a ball-on-disk friction machine [29]. The friction coefficients as functions of time, load and lubricant were recorded. The loads used were 5,7 and 10 N. The lubricants were air and water. The resultant wear particles and wear tracks on the polymer disk were examined morphologically. 2.2. Polymer HDPE with a melt index of about 0.16 was used. 2.3. Transmission electron microscopy A carbon replica method was used. Carbon was deposited in uacuo at an angle of 30” onto the polymer disk surface with a wear track. A concentrated gelatin solution in water was placed onto the carbon and, when this had solidified, the carbon film and gelatin were peeled from the specimen, floated upside down on an aqueous KSCN solution to dissolve the gelatin and then transferred to distilled water to wash the gelatin away. The carbon film was collected from this water surface on a copper mesh. Observations were made with a JEM-7 electron microscope. Selected area electron diffraction was carried out on the very thin layer of polymer attached to the carbon film, which had been peeled from the specimen surface with the carbon film.
175
2.4. Scanning electron microscopy The wear particles, which were gold coated in uacuo, were also observed with an HSM-2 scanning electron microscope.
3. Results and discussion 3.1. Wear in air Figure 1 shows the friction coefficient of HDPE sliding against a steel ball in air. The value of the friction coefficient was constant at about 0.13 0.15; this is consistent with values reported elsewhere [ 11,251. The free surface of the HDPE disk, i.e. the unworn surface, did not give a preferentially oriented morphology. Figure 2(a) shows a transmission electron micrograph of the worn surface (wear track) of HDPE slid against steel for 2 h under a load of 5 N. The sliding direction is parallel to the direction of the striations. Sharp electron diffraction spots (Fig. 2(b)) were obtained from the replica shown in Fig. 2(a), indicating a very marked chain orientation along the sliding direction; this is in good agreement with the micrograph (Fig. 2(a)). Figure 2(c) shows that the crystallographic c axis, i.e. the chain axis of polyethylene, is parallel to the striations of the micrograph and the sliding direction. Consideration of the surface melting [4] of HDPE by friction and a very high rate of crystallization suggests that the morphology of the worn surface (Fig. 2(a)) seems to be formed by crystallization of HDPE from its melt at a high shear rate. It is important to clarify the molecular chain arrangement of the worn surface which may be related to the wear mechanism. Three types of chain arrangements in the oriented crystal are assumed: (1) oriented lamellar crystals, i.e. folded-chain crystals, are linked by tie molecules and tie fibrils as in synthetic fibres [30] (Fig. 3(a)); (2) a row structure [ 311 such as the shish kebab structure [ 321, which is formed by crystallization of HDPE from its dilute solution in xylene with vigorous stirring, consists of extended chain crystals and overgrown lamellar crystals
Sliding
time, min.
Fig. 1. Friction coefficient of HDPE slid against a steel ball in air at 21 “C under various loads (sliding speed, 18.8 cm s-l): 0, 5 N; 0, 7 N; 0, 10 N.
176
(b)
Fig. 2. (a) Transmission electron micrograph of a carbon replica of an HDPE surface worn by a steel ball (sliding speed, 18.8 cm s- 1 ; duration, 2 h; load, 5 N; atmosphere, air), (b) electron diffraction pattern obtained from Fig, 2(a) and (c) indexing of the diffraction spots.
(Fig. 3(b)); (3) uniaxially oriented extended-chain crystals such as aromatic polyamide fibres in which the extended molecular chains are aligned parallel to each other as in the so-called nematic crystal (Fig. 3(c)). A folded-chain crystal of polyethylene melts below its equilibrium melting temperature (T,,,” = 141 “C), i.e. at about 130 - 136 “C!,depending on the lamellar thickness. The molecular chains partly melt and crystallize; this accompanies an increase in the crystal thickness by annealing at temperatures above about 115 “C and below the melting temperature. However, the extended-chain crystal of polyethylene melts at the equilibrium melting temperature T,” and the molecular chains do not rearrange at temperatures below T," . Therefore, if the molecular chains in the wear track constitute the extended-chain crystals, i.e. if the model shown in Fig. 3(c) is correct, they
(4 Fig. 3. Schematic (b) row structure oriented lamellar
(b)
(d)
illustration of HDPE chain arrangements in (a) fibre and drawn film, or shish kebab structure, (c) extended-chain crystal fibre and (d) crystals formed by annealing of the structure in Fig. 3(a).
should not rearrange, via partial melting and crystallization, into folded-chain crystals by annealing. In other words, the morphology of the wear track (Fig. 2(a)) should not change by annealing when the extended-chain crystal is formed by friction. For fibre-like crystals (Fig. 3(a)) and crystals having a row structure (Fig. 3(b)), annealing may cause the growth of the lamellar crystals on the folded-chain type of crystal nuclei present at the wear surface, where the original crystal nuclei, if they exist, may have been too small to be observed in the electron micrograph (Fig. 2(a)). The lamellar crystals would be expected to be prominent after annealing (Fig. 3(d)). The HDPE disk worn in air was annealed at 120 “C for 10 min in uacuo. Figure 4 shows an electron micrograph of the carbon replica of the annealed worn surface of HDPE. The sliding direction is vertical. An electron diffraction pattern similar to that shown in Fig. 2(b) was obtained from this carbon replica film; this indicated no change in the direction of the molecular chains owing to annealing. Lamellar crystals of thickness 100 - 200 W were well grown. They are seen as striations perpendicular to the sliding direction, i.e. horizontal in the micrograph. The extended chain crystals are, therefore, suppressed for polyethylene in the worn surface. The first model (Fig. 3(a)), in which lamellar crystals are linked by tie molecules and tie fibrils, is the most plausible. The second model, i.e. the row structure, cannot be rejected, however, because lamellar crystals may have overgrown onto the extendedchain fibrillar crystals by annealing. Transmission electron microscopy observation was also made of wear particles which had been collected on a carbon film covering a copper mesh and shadowed with Pt-Pd. Most of the wear particles were sufficiently thick (about several thousand angstroms) that the electron beam did not pass through them. A very small fraction of the wear particles gave an electron micrograph as shown in .Fig. 5. The wear debris is evidently a highly oriented film of a substantial size. Thicker oriented films may be the major fraction of the wear debris.
Fig. 4. Transmission electron micrograph of a carbon replica of an HDPE surface worn by a steel ball (sliding speed, 18.8 cm s-l; duration, 2 h; load, 5 N; atmosphere, air) and then annealed in uacuo at 120 “C for 10 min. Grown lamellar crystals are seen as striations perpendicular to the sliding direction. Fig. 5. Transmission electron micrograph of a wear particle of HDPE in the form of film (sliding speed, 18.8 cm s-l ; duration, 2 h; load, 5 N; atmosphere, air).
Differential scanning calorimetry was also carried out on HDPE wear particles using a DSC-2 differential scanning calorimeter at a heating rate of 5 “C min-’ . A single melting peak was obtained at a temperature as low as 128.9 “C. This clearly reveals that the wear debris does not contain the extended-chain crystals but the folded-chain crystals and such strained chains as tie molecules and tie fibrils. Even if the row structure is contained in the wear debris, the core crystals are negligibly small in number. The results may suggest that films, with a thickness of several thousand angstroms, are worn from the polymer disk surface in such a way that a molten polyethylene film is uniaxially drawn in air, producing oriented crystallization. 3.2. Wear in water Wear of HDPE in water is closely related to wear in artificial joint prostheses [9, 33 - 371. The friction coefficient changed with sliding time as shown in Fig. 6. The friction coefficient reached a minimum of 0.04 after 5 - 10 min and then gradually increased depending on the load. Tanaka and Miyata [ 111 and Tanaka [ 251 reported that moisture (vapour) and water (liquid) reduce the friction coefficient for the combination of HDPE and steel. The effect of water molecules on the friction and wear of HDPE is of interest with respect to the surface morphology of worn HDPE. Figure 7 shows a carbon replica film of the wear track of HDPE. The
179
I
1
0.14 z
0.12
.,” .:
0.10
2 H
0.08
2B
0.06
.,” ii:
0.04 0.02 I
0
0
60
1
I
I
I
720
180
240
s
I
t
300
360
Slldlng time, min
Fig. 6. Plots of friction coefficient US.sliding time of HDPE slid against a steel ball under various loads (sliding speed, 18.8 cm s -I ; sliding in water at 21 “C): 0, 5 N; 0, 7 N; 0, 10 N.
Fig. 7. Transmission electron micrograph of a carbon replica of the HDPE surface worn by a steel ball (sliding speed, 18.8 cm s -’ ; duration, 6 h; load, 5 N; sliding in water at 21 “C). Fig. 8. Electron diffraction
pattern obtained
from the carbon repfica shown in Fig. 7,
surface is rougher than that in air, although a highly oriented morphology was obtained. From the carbon replica film of the wear track (Fig. 7), an electron diffraction pattern (Fig. 8) was obtained, indicating that the molecular chain orientation along the sliding direction was not as high (this is evident from the arc scattering of the 110 and 200) as in the case of wear in air (Fig. 2(b)). From Fig. 8, the electron diffraction intensity is stronger for the 200 reflection than for the 110 reflection, This is the reverse of the diffraction pattern for wear in air (Fig. Z(b)). Bunn [38] obtained the observed and
180
calculated intensities for various X-ray reflections from polyethylene fibre, where the intensity for the 110 reflection was stronger by a factor of 4 than that for the 200 reflection. The same relationship was found from the present results of electron diffraction from the wear surface obtained in air (Fig. 2(b)). In the fibre specimen [38], the crystallographic c axis is parallel to the fibre axis, while the a and b axes are isotropically oriented in a direction perpendicular to the fibre axis. The fact that the 200 reflection is stronger than the 110 reflection suggests an anisotropic crystal orientation on the wear surface of the specimen such that the crystallographic c axis is parallel to the sliding direction, that the b axis is, to a great extent, perpendicular to both the sliding direction and the wear surface and that the a axis is, to the same extent, perpendicular to the sliding direction and parallel to the wear surface. (The unit cell of polyethylene is orthorhombic with a = 7.40 A, b = 4.93 A and c = 2.534 A (chain axis) [38] .) This result means that in water wear takes place on the crystallographic u-c plane, i.e. along a direction perpendicular to the b axis. The u-c plane is the boundary where the neighbouring crystals slip over each other and accordingly the wear debris separates from the crystal substrate. This may also mean that water molecules are preferentially adsorbed on the u-c plane where they act as a lubricant. Figure 9 shows thick (dark) oriented masses which were removed from the worn surface of the disk specimen by carbon replication. This morphology resembles that in Fig. 5. The thick oriented masses will be worn away as thick, oriented films in the next sliding process. Figure 10 shows large filmy wear debris; the supermolecular structure of this debris is that of oriented lamellae as for wear in air.
Fig. 9. Transmission electron micrograph of a carbon replica of an HDPE surface worn by a steel ball (sliding speed, 18.8 cm s-l; duration, 6 h; load, 5 N; sliding in water at 21 “C!). Fig. 10. Scanning electron micrograph of HDPE wear particles formed by sliding an HDPE disk against a steel ball (sliding speed, 18.8 cm s-l; duration, 6 h; load, 5 N; sliding in water at 21 “C).
181
Acknowledgment The authors express their sincere gratitude to Professor Tanaka of Kanazawa University for his kind suggestions.
Kyuichiro
References 1 K. G. McLaren and D. Tabor, Friction of polymers: influence of speed and temperature, Nature (London), 197 (1963) 858 - 860. 2 K. Tanaka, Y. Uchiyama and S. Toyooka, The mechanism of wear of poiytetrafluoroethylene, Wear, 23 (1973) 153 - 172. 3 L. C. Towle, Shear strength and friction measurements on polyethylene under high pressure, J. Appl. Phys., 44 (1973) 1611 - 1615. 4 K. Tanaka and Y. Uchiyama, Friction, wear and surface melting of crystalline polymers. In L. H. Lee (ed.), Advances in Polymer Friction and Wear, Vol. 5B, Plenum, New York, 1974, pp. 499 - 530. 5 W. A. Glae’ser, J. W. Kissel and D. K. Snediker, Wear mechanisms of polymers at cryogenic temperatures. In L. H. Lee (ed.), Advances in Polymer Friction and Wear, Vol. 5B, Plenum, New York, 1974, pp. 651 - 662. 6 L. H. Lee, Effect of surface energetics on polymer friction and wear. In L. H. Lee (ea.), Advances in Polymer Friction and Wear, Vol. 5A, Plenum, New York, 1975, pp. 31 - 66. 7 L. C. Towle, Shear strength and polymer friction. In L. H. Lee (ed.), Advances in Polymer Friction and Wear, Vol. 5A, Plenum, New York, 1975, pp. 179 - 187. 8 D. H. Buckley and W. A. Brainard, The atomic nature of polymer-metal interactions in adhesion, friction and wear. In L. H. Lee (ed.), Advances in Polymer Friction and Wear, Vol. 5A, Plenum, New York, 1975, pp. 315 - 328. 9 K. J. Brown, J. R. Atkinson, D. Dowson and V. Wright, The wear of ultrahigh molecular weight polyethylene and a preliminary study of its relation to the in uiuo behaviour of replacement hip joint, Wear, 40 (1976) 255 - 264. 10 W. Rostoker and J. 0. Galante, Some new studies of the wear behavior of ultrahigh molecular weight polyethylene, J. Biomed. Mater. Res., 10 (1976) 303 - 310. 11 K. Tanaka and T. Miyata, Studies on the friction and transfer of semicrystalline polymers, Wear, 41 (1977) 383 - 398. 12 D. Tabor, Wear -a critical synoptic view, J. Lubr. Technol., 99 (1977) 387 - 395. 13 K. Tanaka, Friction and wear of glass- and carbon fiber-filled thermoplastic polymers, J. Lubr. Technol., 99 (1977) 408 - 414. 14 T. A. du Plessis, C. J. Grobbelaar and F. Marais, The improvement of polyethylene prostheses through radiation crosslinking, Radiat. Phys. Chem., 9 (1977) 647 - 652. 15 M. K. Kar and S. Bahadur, Micromechanism of wear at polymer-metal sliding interface, Wear, 46 (1978) 189 - 202. 16 S. K. Rhee and K. C. Ludema, Mechanisms of formation of polymeric transfer films, Wear, 46 (1978) 231 - 240. 17 D. J. Briscoe and D. Tabor, Friction and wear of polymers: the role of mechanical properties, Br. Polym. J., 10 (1978) 74 - 78. 18 J. H. Warren and N. S. Eiss, Jr., Depth of penetration as a predictor of the wear polymers on hard rough surfaces, J. Lubr. Technol., ZOO (1978) 92 - 97. 19 J. K. Lancaster, Wear mechanisms of metals and polymers, Trans. Inst. Met. Finish., 56 (1978) 145 - 153. 20 H. McKellop, I. C. Clarke, K. L. Markolf and H. C. Amstutz, Wear characteristics of UHMW polyethylene: a method for accurately measuring extremely low wear rates, J. Biomed. Mater. Res., 12 (1978) 895 - 927.
182 21 N. H. Sung and N. P. Suh, Effect of fiber orientation on friction and wear of fiberreinforced polymeric composites, Wear, 53 (1979) 129 - 141. 22 B. J. Briscoe and A. C. Smith, The influence of dynamic loading on sliding friction, Nature (London), 278 (1979) 725 - 726. 23 B. J. Briscoe and T. A. Stolarski, Combined rotating and linear motion effects on the wear of polymers, Nature (London), 281 (1979) 206 - 208. 24 N. S. Eiss, Jr., K. C. Wood, J. A. Herold and K. A. Smyth, Model for the transfer of polymer to rough, hard surfaces, J. Lubr. Technol., 101 (1979) 212 - 219. 25 K. Tanaka, Friction and wear of semi-crystalline polymers sliding against steel under water lubrication. In K. C. Ludema, W. A. Glaeser and S. K. Rhee (eds.), Proc. Int. Conf. on Wear of Materials, Dearborn, MI, 1979, American Society of Mechanical Engineers, New York, 1979, pp. 563 - 572. 26 J. Krzeminski and E. Kowalska, Struktur und Eigenschaften von durch Reibung modifizierten Kunststoff-Oberflachen, Kunststoffe, 69 (1979) 482 - 485. 27 N. S. Eiss, Jr. and K. A. Smyth, The wear of polymers sliding on polymeric films deposited on rough surfaces. In Proc. ASME-ASLE Int. Conf., San Francisco, CA, 1980, in J. Lubr. Technol., 103 (1981) 266 - 273. 28 B. J. Briscoe and T. A. Stolarski, The influence of linear and rotating motions on the friction of polymers. In Proc. ASME-ASLE Znt. Conf., San Francisco, CA, 1980, in J. Lubr. Technol., (1981). 29 S. Hironaka, Y. Yahagi and T. Sakurai, Effect of adsorption of some surfactants on antiwear properties, ASLE Trans., 21 (1978) 231 - 235. 30 T. Kawai, Structure of fibers, Sen’i to Kogyo, 2 (1969) 106 - 115. 31 A. Keller and M. J. Machin, Oriented crystallization in polymers, J. Macromol. Sci., Phys., 1 (1967) 41 - 91. 32 A. J. Pennings and A. M. Kiel, Fractionation of polymers by crystallization from solution. III On the morphology of fibrillar polyethylene crystals grown in solution, Kolloid-Z., 205 (1965) 160 - 162. 33 D. A. Sonstegard, L. S. Mattews and H. Kaufer, The surgical replacement of the human knee joint, Sci. Am., 238 (1) (1978) 44 - 51. 34 A. Unsworth, The effects of lubrication in hip joint prostheses, Phys. Med. Biol., 23 (1978) 253 - 268. 35 W. Rostoker, E. Y. S. Chao and J. 0. Galante, The appearance of wear on polyethylene - a comparison of in vivo and in vitro wear surfaces, J. Biomed. Mater. Res., 12 (1978) 317 - 335. 36 J. Berzen, H. W. Birnkraut and G. Braun, Ultrahigh molecular weight polyethylene (UHMWPE): application in artificial joints, Br. Polym. J., 10 (1978) 281 - 287. 37 H. Stallforth and M. Ungethiim, Die tribologische Testung von Knieendoprosthesen, Biomed. Tech., 23 (1978) 295 - 304. 38 C. W. Bunn, The crystal structure of long-chain normal paraffin hydrocarbons. The “shape” of the =CHz group, Trans. Faraday Sot., 35 (1939) 482 - 491.