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Single wedge sliding tests to investigate the mechanism of UHMWPE particle generation with microfabricated surface textures
Polymer Testing 25 (2006) 424–434 www.elsevier.com/locate/polytest
Material Behavior
Single wedge sliding tests to investigate the mechanism of UHMWPE particle generation with microfabricated surface textures Hsu-Wei Fang a,*, Stephen M. Hsu b, Jan V. Sengers c a
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1, Sec. 3, Chung-Hsiao E. Road, Mailstop 2521, Taipei, Taiwan 106 b National Institute of Standards and Technology, Gaithersburg, MD 20899, USA c Institute for Physical Science and Technology, University of Maryland, College Park, MD 20742, USA Received 11 October 2005; accepted 18 November 2005
Abstract Microfabricated surface textures have been applied to generate narrowly distributed UHMWPE wear particles with different sizes and shapes. The purpose is to study the effects of UHMWPE induced bioactivity that leads to the failure of total joint implants. Our previous study has developed the principles of surface-texture design to control the particle size and shape. The objective of this paper is to investigate the UHMWPE particle-generation mechanism with the surface textures containing wedgeshaped cutting edges. Single tip sliding experiments have been designed to investigate the kinematics and the material response of the UHMWPE particle-generation process. With constant penetration depth setup, strain hardening of UHWMPE under a wedge-tip sliding process has been quantified. With constant normal load conditions, we are able to simulate the wedge feature sliding over UHMWPE material with a scaled-up wedge tip. From in situ observation of the process, the kinematics of the sliding process of the wedge tip has been elucidated. The shear stress-induced molecular orientation and embrittlement of the material further contribute to the fracture of UHMWPE and formation of the wear particles. Overall, these results provide experimental evidence of the UHMWPE particle generation mechanism with microfabricated surface textures. The basic science behind the generation of UHMWPE particles by surface-texture design has been further illustrated. q 2005 Elsevier Ltd. All rights reserved. Keywords: UHMWPE; Wear; Particles; Surface texture; Mechanism; Wedge; Strain hardening; Molecular orientation; Viscoelasticity
1. Introduction 1.1. Generation of UHMWPE particles with microfabricated surface textures Ultra-high molecular weight polyethylene (UHMWPE) wear particles have been recognized as the cause of aseptic loosening in total joint
* Tel. C886 9303 80200; fax: C886 2274 18575 E-mail address: [email protected] (H.-W. Fang).
0142-9418/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2005.11.009
replacement [1–3]. In order to investigate the effects of size and shape of particles on biological response, particles with controlled sizes and shapes are needed. Our previous studies have shown that narrowly distributed UHMWPE particle size and shape can be produced by rubbing UHMWPE pins against textured surfaces [4,5]. Fig. 1 shows the schematics of the microfabricated surface textures used in the process of UHMWPE particle generation. Wedgeshaped features were designed and fabricated on the surface to act as the cutting devices. Critical dimensions of the surface texture include: (1)
H.-W. Fang et al. / Polymer Testing 25 (2006) 424–434
(a)
425
well as the particle lengths and aspect ratios of the generated particles (the ratio of particle length to particle width) are listed in Table 1. The results indicated that the length of the cutting edge on the surface controls the length of the generated particle. The particle length is about 80% of the cutting-edge length. The aspect ratio of the particle can be controlled by the combination of particle length and particle width control. A smaller normal load, smaller cutting-edge height or a higher sliding speed results in a larger aspect ratio of the generated particles [4,6]. 1.2. Basic principles of particle generation
(b)
Fig. 1. Generation of UHMWPE with microfabricated surface textures: (a) schematic of the critical dimensions of microfabricated surface textures; (b) schematic of linear reciprocating of wear tester.
dimensions of cutting edges: cutting-edge length (Lc), cutting-edge height (Hc), cutting-edge width (Wc) and half angle of the cutting-edge angle (q); (2) arrangement of the cutting edges: pitch distance of the surface texture in the sliding distance (Ds), distance between adjacent cutting edges (D i), distribution density of the cutting edges on the surface, and patterns of cutting-edge distributions. By rubbing the textured surface with UHMWPE in an ASTM F-732 linear reciprocating wear process (Fig. 1b), the UHMWPE wear particles were generated and collected. The principles of surface-texture design have been developed to generate specific sizes and shapes of UHMWPE wear particles [6]. Microfabricated surface textures with various dimensions have been prepared for UHMWPE particle generation. Fig. 2 shows the micrographs of various surface features along with the generated UHMWPE particles under the same contact pressure of 3 MPa and average sliding speed of 57.2 mm/s [5]. The dimensions of the surface textures as
The morphology of the worn surface and the wear particles of UHMWPE have been observed after the particle-generation process. Fig. 3 shows an example of SEM photos of the UHMWPE worn surface, the single surface feature and generated particles. Shear deformation of UHMWPE was observed along the sliding direction. The width of the sliding track seen on the worn surface is consistent with the length of the cutting edge. Based on this observation, we propose the particle-generation process as follows: at the beginning of the wear tests, the cutting edges penetrate into the UHMWPE material under applied normal load. During the linear sliding motion, the UHMWPE material is sheared and accumulated in front of the cutting edge. The increased volume of accumulated material leads to increased material resistant force acting on the cutting edges. Finally, the cutting edges are pushed away from the surface and a wear particle is generated. With uniform wedge features on the microfabricated surface, the particle-generation process can be studied by focusing on a single wedge sliding over the UHMWPE material. Experiments of wedge-shaped edges sliding over metals have been performed by Black et al. [7] to investigate the effects of wedge angle and lubrication condition on material deformation. Slip-line field analyses have been applied to model the purely plastic deformation of metals of a wedge sliding process [8–10]. The wedge penetrates into the material while a normal load is applied. During the sliding process, a horizontal displacement is applied to the wedge tip under a constant normal load and the material is ploughed. The introduction of the horizontal sliding process unloads the left-hand face of the cutting edge. Further vertical deformation takes place by the wedge sliding further deeper in the sliding process. The material displaced by the wedge tip was accumulated in front of the edge. The material resistant force from
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Fig. 2. Micrographs of the surface textures and generated UHMWPE particles listed in Table 1.
the displaced volume acts on the wedge tip as an encountered force to the normal load. The net normal load is decreased and it leads to a decrease of the penetration depth. Slip-line field analyses have provided the explanation of the movement of the wedge and the results are consistent with the experimental outcomes [7–9]. Under the purely plastic assumption of the slip-line field theory, the plastic deformation and strain takes place immediately when a stress which is larger than the yield criterion is induced. An elastic region of material response and the strain hardening behavior are neglected. However, for a relatively high-
speed plastic deformation of a viscoelastic UHMWPE in the UHMWPE particle generation with surface textures, the strain hardening and the viscoelastic characteristics should be further taken into consideration. One needs to further understand the material responses of UHMWPE in order to elucidate the wedge sliding process over a viscoelastic polymer such as UHMWPE. The strain-hardening effect of UHMWPE in the plastic deformation regime has been identified and measured in several studies [11–13]. Kurtz et al. [14] have studied the yielding and plastic-flow behavior of
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Table 1 Dimensions of microfabricated surface textures and generated UHMWPE particles Surface texture
(a) (b) (c) (d) (e)
Dimensions of surface textures
Particle dimensions
Lc (mm)
Wc (mm)
Hc (mm)
Ds (mm)
Di (mm)
Length (mm)
Aspect ratio
55.0 15.0 7.0 3.5 3.6
5.0 5.0 5.0 2.0 2.0
4.2 3.4 3.0 1.4 0.6
200 200 200 20 10
40 80 90 4 4
43.1G7.1 10.2G3.9 6.2G2.1 4.2G0.8 3.9G0.7
6.7G1.1 2.2G0.7 2.6G0.6 1.9G0.5 1.6G0.3
Uncertainty of measurement of the dimensions of surface textures come from the microfabrication process. A relative standard uncertainty is estimated to be less than 10%; Mean particle lengthsGstandard deviations are listed for the particle dimensions.
two medical grades of UHMWPE (GUR1120, GUR 4150) by conducting uniaxial tension and compression tests. They indicated that the strain hardening of UHMWPE appears with a strain larger than 0.15. With a strain larger than 0.5, dramatic strain hardening either under tension or compression was observed. Stain hardening of UHMWPE is expected when the wedge surface pushes the confronted UHWMPE. Further measurement of sliding strain hardening of UHMWPE is necessary in order to understand the
built-up of material resistant force during UHMWPE particle generation process with surface textures. 1.3. Objective and approaches The objective of this study is looking for the experimental evidence of the proposed mechanism of UHMWPE particle generation. A direct observation of the wedge-shaped feature sliding over UHMWPE is desirable. However, the small scale of surface textures
Fig. 3. SEM micrographs of surface cutting-edge feature, UHMWPE worn surface and generated UHMWPE particles.
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in the micrometer range makes in situ observation difficult. A scaled-up model experiment was developed to allow the direct observation of the single wedgeshaped tip sliding on UHMWPE. Experiments with constant penetration-depths have been designed to measure the strain hardening of UHMWPE. On the other hand, the experiments with constant normal loads have been implemented to simulate the mechanical conditions in the real particle-generation process. The UHMWPE material response under a wedge sliding process and the kinematics of the micro-cutting process
are expected to be explored. Understanding the particle-generation mechanism in the rubbing process between UHMWPE material and textured surfaces can further elucidate the effects of surface-texture dimensions on sizes and shapes of generated UHMWPE particles. 2. Experimental procedures The detailed facility setup of sliding movement, microscopic video and force measurement systems for
Fig. 4. Schematic representation of scaled-up single cutting-edge sliding test: (a) with constant penetration depth condition; (b) with constant normal-load condition.
H.-W. Fang et al. / Polymer Testing 25 (2006) 424–434
sliding experiments with constant penetration depth and with constant normal load are described below. 2.1. Sliding tests with constant penetration depth The experimental setup of constant depth sliding tests is shown schematically in Fig. 4(a). Wedgeshaped tips fabricated out of tool steel with two tip angles (608 and 908) were used. The wedge length is 2 mm. The wedge-shaped tip is held in a fixed position on a vertical slider that can be adjusted in the vertical direction to control the penetration depth. A GUR-415 UHMWPE disk (2 in. diameter) is mounted in a lower holder on a horizontal slider in the Y direction. The Y direction slider is attached to another horizontal slider in the X direction. The sliders are driven by a computercontrolled micro-stepping motor. The vertical and horizontal sliders move at the precise distance, speed, and acceleration specified by the computer controller. The contact between the tip and the UHMWPE surface can be detected from the on-line monitoring of the forces. The UHMWPE sample was first leveled on the stage before the tests. The indentation of the cutting edge was conducted at a speed of 0.5 mm/s vertically. Various penetration depths (ranging from 80 to 400 mm) were applied in the tests. After 2 s, the cutting edge was slid sideways for 4 mm with a constant speed of 0.5 mm/s. 2.2. Sliding tests with constant normal load The experimental setup of the scaled-up single cutting-edge sliding test with constant normal load is schematically shown in Fig. 4(b). Two wedge tips (908 and 608 with 2 mm edge length) were prepared. The tip was mounted on a lever arm. A UHMWPE (GUR 415 grade) disk (2 in. diameter) was mounted in the lower holder attached to a horizontal slider. The slider was driven by a rotary motor at an average sliding speed of 57.2G3.2 mm/s (the same speed as in the real particlegeneration process as described previously [5]). 2.3. Force measurement and video capturing A piezoelectrical three-dimensional force transducer was mounted above the tip to measure forces. The load cell sent electrical signals to three charge amplifiers. The output of the amplifier could be adjusted from 1 to 1000 N/V and the voltage signal was then stored by the data-acquisition system. Calibration was made prior the tests to obtain the curve of output voltage versus load. To observe the sliding process, a video camera
429
(Digital 5000 by Panasonic) was mounted on the frame of the test rig. A pair of micro lenses made by Volpi Company was used. The magnifications are 165 and 300 times for the two lenses and the focus distance between the lens and the specimen was 50 mm. A fiberglass light source was applied, arranged behind the specimen to give a bright and clear image of the material in front of the tip. The video pictures picked up by the camera were recorded by a DV system (Sony). Later, the recorded video could be converted as a digital file to the computer. Thus, we can show the video frame by frame allowing the dynamic sliding process to be analyzed. 3. Results and discussion 3.1. Sliding of wedge with constant penetration depth Fig. 5(a) shows a horizontal-force curve for a 608 wedge tip with a constant penetration depth of 400 mm. The sliding motion starts at time tZ0. The horizontal force acting on the wedge grows and approaches a steady value. The video of the cutting-edge sliding process was synchronized with the force curve in Fig. 5(b). By comparing the volumes of accumulated material in the video frames, it is observed that a maximum accumulated volume was achieved between tZ0.8 and 1.2 s which corresponds to the point A in the force curve. After tZ1.2 s, the accumulated material volume in front of the cutting edge remains the same. The wedge surface pushing the confronted UHMWPE results in the increase of material resistant force acting on the wedge. When the resistant force rises to a certain level, UHMWPE starts to plastically flow into the nearby region and the material accumulated in front of the wedge surface reaches a maximum volume. By identifying the time for reaching the maximum accumulated volume, the sliding distance toward this point is determined from the known sliding speed. 3.2. Strain hardening of UHMWPE during sliding Sliding tests with constant penetration depth was designed to investigate the strain hardening of UHMWPE under a wedge sliding process. From point O to A in Fig. 5, the sliding process produces shear of UHMWPE and the material pile-up is accumulated in front of the tip. The increased deformation volume in front of the tip leads to an increase of the material resistant force acting on the tip. Thus, an increased horizontal force is observed. The material accumulated in front of the cutting edge reaches a maximum volume
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at point A. From point A to B, the horizontal force keeps growing without a visible increase of the cumulative volume in front of the tip, but an increasing sidewall of material is observed. The material displaced
by the tip no longer goes in front of the tip. It is suggested that the tip keeps pushing the UHMWPE material and the strain-hardening effect of plastic deformation contributes to the increase of the
Fig. 5. Results of wedge-tip sliding process with constant penetration depth (cutting-edge length, 2 mm, penetration depth, 400 mm, sliding speed, 0.5/s). (a) Horizontal-force curve; (b) microscopic video frames.
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(a) 120
The single wedge sliding tests with constant normal load were designed to simulate the wedge-shaped surface feature sliding over UHMWPE during the particle-generation process. With the scaled-up tips, we are able to observe the processes in situ. Based on the observation of the repeated penetrating and lift-up of the wedge tip, we summarize the movement of the wedge tip schematically in Fig. 8. When the sliding process begins, the material resistant force acting on the
Force (N)
60 40
0 0
2
4
6
Sliding distance (mm)
(b) 250 Horizontal force Vertical force
200
2.4 mm
150 100 50 0 0
2
4
6
Sliding distance (mm) Fig. 6. Force curve of the sliding process by wedge tips under a normal load of 66.75 N with (a) 908 angle tip; (b) 608 angle tip.
wedge-tip face increases with increasing deformed UHWMPE accumulated in front of the tip. Thus, the horizontal force acting on the tip increases. The initial vertical force shown in Fig. 6 represents the applied 3.0
Sliding distance (mm)
3.4. Kinematics of wedge sliding process
1.6 mm
80
20
3.3. Sliding of wedge with constant normal load Various normal loads (22.25, 44.50, and 66.75 N) were applied on the wedge-shaped cutting edges with two different included angles (608 and 908) under a average sliding speed of 57.2 mm/s. The video observation shows that the wedge tip penetrates into the material and is then pushed up away from the surface. Then the tip penetrated into the material again. The phenomena were repeated during the entire sliding process. Normal and horizontal forces were recorded during the wedge tip sliding process (Fx, Fz). Fig. 6 shows the force curves of the sliding process by a wedgeshaped cutting edge with included angle of 908 and 608. The initial penetration depth was also measured prior to the horizontal sliding of the tip. Fig. 7 shows the plot of sliding distance between two cycles versus initial penetration depth under different normal loads.
Horizontal force Vertical force
100
Force (N)
horizontal force from A to B. Since the depth of penetration is fixed in the experiment, the increased force is not able to lift up the tip. Finally, the increased contact pressure of the wedge face forces plastic flow of materials sideways. A steady-state plastic flow is established and the horizontal force approaches a constant value, as seen after point B in Fig. 5. The maximum displaced volume of the material during the sliding process is equal to the product of wedge length, penetration depth and the sliding distance. The resistant force due to strain hardening of UHMWPE is nearly proportional to the displaced volume in the sliding process. We obtained the material resistant force per unit volume of UHMWPE to be about 1.2!10K6 N/ (mm)3 from the results with different penetration depths. The quantification of the strain hardening of UHMWPE during a wedge sliding process has been achieved. This value can be further applied to model the particle generation process. To sum up, this experimental result provides the explanation of the force contributing to the lift-up process of the wedge features.
431
2.5 2.0 1.5 1.0 0.5
60° wedge tip 90° wedge tip
0.0 0
20
40
60
80
100
Initial penetration depth (µm)
Fig. 7. The plot of sliding distance between two cycles versus initial penetration depth from the results of wedge sliding tests with different normal load and wedge angle.
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normal load of the test. We define Frxz is the resultant resistant force acting on the tip. Frz and Frx are the resolved component forces in the vertical and horizontal directions, respectively (see Fig. 8(b)). First, Frz increases with increased accumulated volume of UHMWPE, and Frz also contributes to lift up the tip in the sliding process. The contact area between the tip and the material decreases during the lift-up process and thus the Frz starts to drop. At the moment that the tip is pushed to the apex of the material bump, Frz drops to zero. Because the vertical force shown on the plot presents the difference between the normal force and the material resistant force in the vertical direction, the amount of vertical force returns to the original applied normal load when the tip is fully retracted.
Normal load (W)
(a)
Normal load (W) Sliding
θ Frxz
(b) Frz = Frxz sinθ
θ Frx=F rxz cosθ
3.5. Effect of normal load and wedge angle
Normal load (W) Sliding
(c)
Normal load (W) Material bump
Sliding
(d)
Sliding distance
Fig. 8. Schematic representation of the wedge sliding over UHMWPE under a constant normal load condition.
25 A
B
Load (mN)
20
15
10
5 O
0
C2
C
0 1000
2000
3000
4000
5000
6000
C1
7000
Displacement (nm) Fig. 9. Load–displacement curve of nano-indentation test on UHMWPE with constant strain rate of 0.1 sK1 [4].
The frequency of up and down movement of the tip or the distance between the repeated material bumps provides information on the shear deformation of the material during the sliding process. Thus, by understanding the force curve of the sliding process, the influence of the geometry of the tip and mechanical operation conditions on the wear-particle dimensions can be understood. The sliding distances between two displaced bumps under different normal loads were plotted against the initial penetration depth in Fig. 7. A larger normal load leads to a deeper penetration depth at the beginning of the sliding. The plot shows that increasing initial penetration depth results in a longer sliding distance. Under the same initial penetration depth, a sharper wedge (smaller wedge angle) results in a longer sliding distance. Dependence of the wedge tip lift-up process on the wedge angle needs further examination. As illustrated in Fig. 8(b), the quantity of the resolved component forces in the vertical direction is a function of the wedge angle. Assuming the same penetration depth during the sliding process, the resultant material resistant forces (Frxz) from the shear of UHMWPE material are the same. The resolved force in the vertical direction is FrzZFrxz sinq. A larger angle causes more material resistant force to be distributed to the vertical direction, which increases the lift-up force. Thus, the time required for the tip to be pushed away is shorter. With a constant sliding speed in the horizontal direction, a shorter distance between two peaks is expected. Variation of the wedge angle contributes to the change of penetration depth and the cutting edge lift-up speed. Both consequences have an influence on the sliding distance. This result verifies that a larger normal load and smaller wedge angle lead
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to a larger volume of generated UHMWPE particles, as observed in the previous studies [5,6]. Uni-Directional Stress
3.6. Viscoelasticity of UHMWPE The cutting edges penetrated into UHMWPE at the beginning of the process under a constant normal load. The applied normal load, angle and height of the cutting edge determine the initial penetration depth. Unloading due to the consequent sliding process leads to the recovery of the penetration depth for a viscoelastic polymer. Our previous study performed a nanoindentation test on UHMWPE to examine the deformation recovery due to viscoelasticity [4]. Fig. 9 shows the load–displacement curve of the indentation. Fig. 9 shows that the displacement increases in the loading segment (O/A); at the holding period with constant load (A/B), creep of the material under a constant stress was observed; during the unloading stage (B/ C), the displacement curve shows additional deviation due to viscoelastic recovery of the material. The difference of the displacement between point C1 and C2 (812 mm) represents the elastic recovery of the material deformation. The difference of the displacement between point C and C2 (1563 mm) further indicates the delayed recovery of the material deformation by viscoelastic effect. Compared with the maximum penetration depth, 37.3% of the maximum penetration depth was recovered after the load is removed. Also, comparing the ratio of the area ‘BCC1’ (viscoelastic recovery) to the area ‘OABC1’ (equal to the total energy done during the indentation) in Fig. 9 shows that the effect of viscoelasticity on UHWMPE deformation is dramatic. It should be taken into consideration when we want to correlate the dimensions of the surface textures to the size and shape of the generated UHMWPE particles.
C C C C
C--H
Molecular Axis Fig. 10. Schematic representation of UHMWPE molecular orientation under a uni-directional stress.
molecular orientation at UHMWPE surfaces subjected to different shear motions by rubbing a UHMWPE pin against a polished Co–Cr disk. The results show that a higher degree of chain alignment was induced by unidirectional rubbing the UHMWPE surface. It concluded that the molecular chains tend to align and orient
Break of material (release of energy causing the drop of force)
40 35
3.7. Stress-induced molecular orientation of UHMWPE
Fz Fx
30
Force (N)
The kinematics of the wedge-shaped surface features has been illustrated from the single sliding tests with constant normal load. However, the generation of particles was not observed from a single pass of the sliding process. Particles were generated only after several cycles of sliding. UHMWPE is a semi-crystalline material and the folded-chains in the crystalline region have a specific molecular orientation. We are wondering if there is any change of mechanical property of UHMWPE when a mechanical stress is applied on the material surface. Soft X-ray absorptionspectroscopy has been used [15] to measure the
C--H
25 20 15 10 5 0 0
2
4
6
8
10
12
14
Time (sec) Fig. 11. Force curves of the 11th pass of the wedge sliding test (wedge length, 2 mm, wedge angle, 608, penetration depth, 80 mm, sliding distance, 6 mm/pass, sliding speed, 1 mm)
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to a common direction when the surface is subjected to a directional force such as shear stress from sliding (as schematically shown in Fig. 10). To investigate the influence of multiple scratches on the mechanical properties of the material surface, a multiple-pass wedge-sliding test with constant penetration depth has been performed. Fig. 11 shows the force curve of the 11th sliding pass of the linear reciprocating experiments (with 2 mm wedge length, 608 wedge, and 80 mm penetration depth). Drop in the force during the sliding process indicates damage of the UHMWPE. We think that the multiple sliding processes induces the UHMWPE molecules to be oriented in the sliding direction and the compact arrangement of molecules on the surface makes the material become more brittle. Finally, the UHWMPE material fractures at the tip–material interface. Detachment of the material from the surface leads to the generation of the particle. 4. Conclusions 1. By designing the sliding experiments with constant normal load and constant penetration depth, single wedge sliding tests with scaled-up tips has enabled the direct observation of kinematics of the microcutting process and the measurement of strain hardening of UHMWPE. 2. The recovery of the deformation due to viscoelasticity may affect the size of the generated UHMWPE particles. The degree of viscoleastic recovery of the penetration depth of a tip has been discussed from a result of nanoindentation tests. 3. UHMWPE particles are generated from the following steps: (a) deformation of the material due to the penetration and sliding processes of the surface features; (b) strain hardening occurs under a surface-feature sliding process; (c) UHMWPE molecules align to the direction where the shear stress is applied and the surface layer of material becomes more brittle; (d) Detachment of the particle at the tip edge– material interface according to the fracture of UHWMPE.
References [1] W.H. Harris, The problem is osteolysis, Clin. Orthop. Res. 311 (1995) 46–53. [2] H.C. Amstutz, P. Campbell, N. Kossovsky, I.C. Clarke, Mechanism and clinical significance of wear debris-induced osteolysis, Clin. Orthop. Res. 276 (1992) 7–18. [3] W.J. Maloney, R.L. Smith, Periprosthetic osteolysis in total hip arthroplasty: the role of particulate wear debris, J. Bone Joint Surg. 77A (1995) 1448–1461. [4] H.-W. Fang, S.M. Hsu, J.V. Sengers, Ultra-high molecular weight polyethylene wear particle effects on bioactivity, National Institute of Standards and Technology Special Publication 1002, 2003. [5] H.-W. Fang, S.M. Hsu, J.V. Sengers, Generation of narrowly distributed ultra-high molecular weight polyethylene particles by surface texturing techniques, J. Biomed. Mater. Res. B: Appl. Biomater. 67B (2003) 741–749. [6] H.-W. Fang, S.M. Hsu, J.V. Sengers, Surface texture design to generate specific sizes and shapes of UHMWPE wear particles, Materialwissenschaft und Werkstofftech ( Mater. Sci. Eng. Technol.) 34 (2003) 976–988. [7] A.J. Black, E.M. Kopalinsky, P.L.B Oxley, An investigation of the different regimes of deformation which can occur when a hard wedge slides over a soft surface: the influence of wede angle, lubrication and prior plastic working of the surface, Wear 123 (1988) 97–114. [8] J.M. Challen, P.L.B Oxley, An explanation of the different regimes of friction and wear using asperity deformation models, Wear 53 (1979) 229–243. [9] A.J. Black, E.M. Kopalinsky, P.L.B Oxley, The wave model of metallic sliding friction: an investigation of the transition from initial indentation to steady-state wave formation in a model asperity experiment, J. Phys. D: Appl. Phys. 26 (1993) 1892–1899. [10] K.L. Johnson, Contact Mechanics, Cambridge University Press, Cambridge, UK, 1985. [11] B.J. Briscoe, L. Fiori, Nano-indentation of polymeric surfaces, J. Phys. D: Appl. Phys. 31 (1998) 2395–2405. [12] S.M. Kurtz, L. Pruitt, C.W. Jewett, R.P. Crawford, D.J. Crane, A.A. Edidin, The yielding, plastic flow, and fracture behavior of ultra-high molecular weight polyethylene used in total joint replacements, Biomaterials 19 (1998) 1989–2003. [13] D.J. Krzypow, C.M. Rinmac, Cyclic steady state stress–strain behavior of UHMWPE, Biomaterials 21 (2000) 2081–2087. [14] S.M. Kurtz, C.M. Rimnac, D.L. Bartel, Predictive model for tensile true stress-strain behavior of chemically and mechanically degraded UHMWPE, J. Biomed. Mater. Res. B: Appl. Biomater. 43 (1998) 241–248. [15] S. Sambasivan, D.A. Fischer, M.C. Shen, S.M. Hsu, Molecular orientation of ultrahigh molecular weight polyethylene induced by various sliding motions, J. Biomed. Mater. Res. B: Appl. Biomater. 70B (2004) 278–285.
Material Behavior
Single wedge sliding tests to investigate the mechanism of UHMWPE particle generation with microfabricated surface textures Hsu-Wei Fang a,*, Stephen M. Hsu b, Jan V. Sengers c a
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1, Sec. 3, Chung-Hsiao E. Road, Mailstop 2521, Taipei, Taiwan 106 b National Institute of Standards and Technology, Gaithersburg, MD 20899, USA c Institute for Physical Science and Technology, University of Maryland, College Park, MD 20742, USA Received 11 October 2005; accepted 18 November 2005
Abstract Microfabricated surface textures have been applied to generate narrowly distributed UHMWPE wear particles with different sizes and shapes. The purpose is to study the effects of UHMWPE induced bioactivity that leads to the failure of total joint implants. Our previous study has developed the principles of surface-texture design to control the particle size and shape. The objective of this paper is to investigate the UHMWPE particle-generation mechanism with the surface textures containing wedgeshaped cutting edges. Single tip sliding experiments have been designed to investigate the kinematics and the material response of the UHMWPE particle-generation process. With constant penetration depth setup, strain hardening of UHWMPE under a wedge-tip sliding process has been quantified. With constant normal load conditions, we are able to simulate the wedge feature sliding over UHMWPE material with a scaled-up wedge tip. From in situ observation of the process, the kinematics of the sliding process of the wedge tip has been elucidated. The shear stress-induced molecular orientation and embrittlement of the material further contribute to the fracture of UHMWPE and formation of the wear particles. Overall, these results provide experimental evidence of the UHMWPE particle generation mechanism with microfabricated surface textures. The basic science behind the generation of UHMWPE particles by surface-texture design has been further illustrated. q 2005 Elsevier Ltd. All rights reserved. Keywords: UHMWPE; Wear; Particles; Surface texture; Mechanism; Wedge; Strain hardening; Molecular orientation; Viscoelasticity
1. Introduction 1.1. Generation of UHMWPE particles with microfabricated surface textures Ultra-high molecular weight polyethylene (UHMWPE) wear particles have been recognized as the cause of aseptic loosening in total joint
* Tel. C886 9303 80200; fax: C886 2274 18575 E-mail address: [email protected] (H.-W. Fang).
0142-9418/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2005.11.009
replacement [1–3]. In order to investigate the effects of size and shape of particles on biological response, particles with controlled sizes and shapes are needed. Our previous studies have shown that narrowly distributed UHMWPE particle size and shape can be produced by rubbing UHMWPE pins against textured surfaces [4,5]. Fig. 1 shows the schematics of the microfabricated surface textures used in the process of UHMWPE particle generation. Wedgeshaped features were designed and fabricated on the surface to act as the cutting devices. Critical dimensions of the surface texture include: (1)
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(a)
425
well as the particle lengths and aspect ratios of the generated particles (the ratio of particle length to particle width) are listed in Table 1. The results indicated that the length of the cutting edge on the surface controls the length of the generated particle. The particle length is about 80% of the cutting-edge length. The aspect ratio of the particle can be controlled by the combination of particle length and particle width control. A smaller normal load, smaller cutting-edge height or a higher sliding speed results in a larger aspect ratio of the generated particles [4,6]. 1.2. Basic principles of particle generation
(b)
Fig. 1. Generation of UHMWPE with microfabricated surface textures: (a) schematic of the critical dimensions of microfabricated surface textures; (b) schematic of linear reciprocating of wear tester.
dimensions of cutting edges: cutting-edge length (Lc), cutting-edge height (Hc), cutting-edge width (Wc) and half angle of the cutting-edge angle (q); (2) arrangement of the cutting edges: pitch distance of the surface texture in the sliding distance (Ds), distance between adjacent cutting edges (D i), distribution density of the cutting edges on the surface, and patterns of cutting-edge distributions. By rubbing the textured surface with UHMWPE in an ASTM F-732 linear reciprocating wear process (Fig. 1b), the UHMWPE wear particles were generated and collected. The principles of surface-texture design have been developed to generate specific sizes and shapes of UHMWPE wear particles [6]. Microfabricated surface textures with various dimensions have been prepared for UHMWPE particle generation. Fig. 2 shows the micrographs of various surface features along with the generated UHMWPE particles under the same contact pressure of 3 MPa and average sliding speed of 57.2 mm/s [5]. The dimensions of the surface textures as
The morphology of the worn surface and the wear particles of UHMWPE have been observed after the particle-generation process. Fig. 3 shows an example of SEM photos of the UHMWPE worn surface, the single surface feature and generated particles. Shear deformation of UHMWPE was observed along the sliding direction. The width of the sliding track seen on the worn surface is consistent with the length of the cutting edge. Based on this observation, we propose the particle-generation process as follows: at the beginning of the wear tests, the cutting edges penetrate into the UHMWPE material under applied normal load. During the linear sliding motion, the UHMWPE material is sheared and accumulated in front of the cutting edge. The increased volume of accumulated material leads to increased material resistant force acting on the cutting edges. Finally, the cutting edges are pushed away from the surface and a wear particle is generated. With uniform wedge features on the microfabricated surface, the particle-generation process can be studied by focusing on a single wedge sliding over the UHMWPE material. Experiments of wedge-shaped edges sliding over metals have been performed by Black et al. [7] to investigate the effects of wedge angle and lubrication condition on material deformation. Slip-line field analyses have been applied to model the purely plastic deformation of metals of a wedge sliding process [8–10]. The wedge penetrates into the material while a normal load is applied. During the sliding process, a horizontal displacement is applied to the wedge tip under a constant normal load and the material is ploughed. The introduction of the horizontal sliding process unloads the left-hand face of the cutting edge. Further vertical deformation takes place by the wedge sliding further deeper in the sliding process. The material displaced by the wedge tip was accumulated in front of the edge. The material resistant force from
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Fig. 2. Micrographs of the surface textures and generated UHMWPE particles listed in Table 1.
the displaced volume acts on the wedge tip as an encountered force to the normal load. The net normal load is decreased and it leads to a decrease of the penetration depth. Slip-line field analyses have provided the explanation of the movement of the wedge and the results are consistent with the experimental outcomes [7–9]. Under the purely plastic assumption of the slip-line field theory, the plastic deformation and strain takes place immediately when a stress which is larger than the yield criterion is induced. An elastic region of material response and the strain hardening behavior are neglected. However, for a relatively high-
speed plastic deformation of a viscoelastic UHMWPE in the UHMWPE particle generation with surface textures, the strain hardening and the viscoelastic characteristics should be further taken into consideration. One needs to further understand the material responses of UHMWPE in order to elucidate the wedge sliding process over a viscoelastic polymer such as UHMWPE. The strain-hardening effect of UHMWPE in the plastic deformation regime has been identified and measured in several studies [11–13]. Kurtz et al. [14] have studied the yielding and plastic-flow behavior of
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Table 1 Dimensions of microfabricated surface textures and generated UHMWPE particles Surface texture
(a) (b) (c) (d) (e)
Dimensions of surface textures
Particle dimensions
Lc (mm)
Wc (mm)
Hc (mm)
Ds (mm)
Di (mm)
Length (mm)
Aspect ratio
55.0 15.0 7.0 3.5 3.6
5.0 5.0 5.0 2.0 2.0
4.2 3.4 3.0 1.4 0.6
200 200 200 20 10
40 80 90 4 4
43.1G7.1 10.2G3.9 6.2G2.1 4.2G0.8 3.9G0.7
6.7G1.1 2.2G0.7 2.6G0.6 1.9G0.5 1.6G0.3
Uncertainty of measurement of the dimensions of surface textures come from the microfabrication process. A relative standard uncertainty is estimated to be less than 10%; Mean particle lengthsGstandard deviations are listed for the particle dimensions.
two medical grades of UHMWPE (GUR1120, GUR 4150) by conducting uniaxial tension and compression tests. They indicated that the strain hardening of UHMWPE appears with a strain larger than 0.15. With a strain larger than 0.5, dramatic strain hardening either under tension or compression was observed. Stain hardening of UHMWPE is expected when the wedge surface pushes the confronted UHWMPE. Further measurement of sliding strain hardening of UHMWPE is necessary in order to understand the
built-up of material resistant force during UHMWPE particle generation process with surface textures. 1.3. Objective and approaches The objective of this study is looking for the experimental evidence of the proposed mechanism of UHMWPE particle generation. A direct observation of the wedge-shaped feature sliding over UHMWPE is desirable. However, the small scale of surface textures
Fig. 3. SEM micrographs of surface cutting-edge feature, UHMWPE worn surface and generated UHMWPE particles.
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in the micrometer range makes in situ observation difficult. A scaled-up model experiment was developed to allow the direct observation of the single wedgeshaped tip sliding on UHMWPE. Experiments with constant penetration-depths have been designed to measure the strain hardening of UHMWPE. On the other hand, the experiments with constant normal loads have been implemented to simulate the mechanical conditions in the real particle-generation process. The UHMWPE material response under a wedge sliding process and the kinematics of the micro-cutting process
are expected to be explored. Understanding the particle-generation mechanism in the rubbing process between UHMWPE material and textured surfaces can further elucidate the effects of surface-texture dimensions on sizes and shapes of generated UHMWPE particles. 2. Experimental procedures The detailed facility setup of sliding movement, microscopic video and force measurement systems for
Fig. 4. Schematic representation of scaled-up single cutting-edge sliding test: (a) with constant penetration depth condition; (b) with constant normal-load condition.
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sliding experiments with constant penetration depth and with constant normal load are described below. 2.1. Sliding tests with constant penetration depth The experimental setup of constant depth sliding tests is shown schematically in Fig. 4(a). Wedgeshaped tips fabricated out of tool steel with two tip angles (608 and 908) were used. The wedge length is 2 mm. The wedge-shaped tip is held in a fixed position on a vertical slider that can be adjusted in the vertical direction to control the penetration depth. A GUR-415 UHMWPE disk (2 in. diameter) is mounted in a lower holder on a horizontal slider in the Y direction. The Y direction slider is attached to another horizontal slider in the X direction. The sliders are driven by a computercontrolled micro-stepping motor. The vertical and horizontal sliders move at the precise distance, speed, and acceleration specified by the computer controller. The contact between the tip and the UHMWPE surface can be detected from the on-line monitoring of the forces. The UHMWPE sample was first leveled on the stage before the tests. The indentation of the cutting edge was conducted at a speed of 0.5 mm/s vertically. Various penetration depths (ranging from 80 to 400 mm) were applied in the tests. After 2 s, the cutting edge was slid sideways for 4 mm with a constant speed of 0.5 mm/s. 2.2. Sliding tests with constant normal load The experimental setup of the scaled-up single cutting-edge sliding test with constant normal load is schematically shown in Fig. 4(b). Two wedge tips (908 and 608 with 2 mm edge length) were prepared. The tip was mounted on a lever arm. A UHMWPE (GUR 415 grade) disk (2 in. diameter) was mounted in the lower holder attached to a horizontal slider. The slider was driven by a rotary motor at an average sliding speed of 57.2G3.2 mm/s (the same speed as in the real particlegeneration process as described previously [5]). 2.3. Force measurement and video capturing A piezoelectrical three-dimensional force transducer was mounted above the tip to measure forces. The load cell sent electrical signals to three charge amplifiers. The output of the amplifier could be adjusted from 1 to 1000 N/V and the voltage signal was then stored by the data-acquisition system. Calibration was made prior the tests to obtain the curve of output voltage versus load. To observe the sliding process, a video camera
429
(Digital 5000 by Panasonic) was mounted on the frame of the test rig. A pair of micro lenses made by Volpi Company was used. The magnifications are 165 and 300 times for the two lenses and the focus distance between the lens and the specimen was 50 mm. A fiberglass light source was applied, arranged behind the specimen to give a bright and clear image of the material in front of the tip. The video pictures picked up by the camera were recorded by a DV system (Sony). Later, the recorded video could be converted as a digital file to the computer. Thus, we can show the video frame by frame allowing the dynamic sliding process to be analyzed. 3. Results and discussion 3.1. Sliding of wedge with constant penetration depth Fig. 5(a) shows a horizontal-force curve for a 608 wedge tip with a constant penetration depth of 400 mm. The sliding motion starts at time tZ0. The horizontal force acting on the wedge grows and approaches a steady value. The video of the cutting-edge sliding process was synchronized with the force curve in Fig. 5(b). By comparing the volumes of accumulated material in the video frames, it is observed that a maximum accumulated volume was achieved between tZ0.8 and 1.2 s which corresponds to the point A in the force curve. After tZ1.2 s, the accumulated material volume in front of the cutting edge remains the same. The wedge surface pushing the confronted UHMWPE results in the increase of material resistant force acting on the wedge. When the resistant force rises to a certain level, UHMWPE starts to plastically flow into the nearby region and the material accumulated in front of the wedge surface reaches a maximum volume. By identifying the time for reaching the maximum accumulated volume, the sliding distance toward this point is determined from the known sliding speed. 3.2. Strain hardening of UHMWPE during sliding Sliding tests with constant penetration depth was designed to investigate the strain hardening of UHMWPE under a wedge sliding process. From point O to A in Fig. 5, the sliding process produces shear of UHMWPE and the material pile-up is accumulated in front of the tip. The increased deformation volume in front of the tip leads to an increase of the material resistant force acting on the tip. Thus, an increased horizontal force is observed. The material accumulated in front of the cutting edge reaches a maximum volume
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at point A. From point A to B, the horizontal force keeps growing without a visible increase of the cumulative volume in front of the tip, but an increasing sidewall of material is observed. The material displaced
by the tip no longer goes in front of the tip. It is suggested that the tip keeps pushing the UHMWPE material and the strain-hardening effect of plastic deformation contributes to the increase of the
Fig. 5. Results of wedge-tip sliding process with constant penetration depth (cutting-edge length, 2 mm, penetration depth, 400 mm, sliding speed, 0.5/s). (a) Horizontal-force curve; (b) microscopic video frames.
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(a) 120
The single wedge sliding tests with constant normal load were designed to simulate the wedge-shaped surface feature sliding over UHMWPE during the particle-generation process. With the scaled-up tips, we are able to observe the processes in situ. Based on the observation of the repeated penetrating and lift-up of the wedge tip, we summarize the movement of the wedge tip schematically in Fig. 8. When the sliding process begins, the material resistant force acting on the
Force (N)
60 40
0 0
2
4
6
Sliding distance (mm)
(b) 250 Horizontal force Vertical force
200
2.4 mm
150 100 50 0 0
2
4
6
Sliding distance (mm) Fig. 6. Force curve of the sliding process by wedge tips under a normal load of 66.75 N with (a) 908 angle tip; (b) 608 angle tip.
wedge-tip face increases with increasing deformed UHWMPE accumulated in front of the tip. Thus, the horizontal force acting on the tip increases. The initial vertical force shown in Fig. 6 represents the applied 3.0
Sliding distance (mm)
3.4. Kinematics of wedge sliding process
1.6 mm
80
20
3.3. Sliding of wedge with constant normal load Various normal loads (22.25, 44.50, and 66.75 N) were applied on the wedge-shaped cutting edges with two different included angles (608 and 908) under a average sliding speed of 57.2 mm/s. The video observation shows that the wedge tip penetrates into the material and is then pushed up away from the surface. Then the tip penetrated into the material again. The phenomena were repeated during the entire sliding process. Normal and horizontal forces were recorded during the wedge tip sliding process (Fx, Fz). Fig. 6 shows the force curves of the sliding process by a wedgeshaped cutting edge with included angle of 908 and 608. The initial penetration depth was also measured prior to the horizontal sliding of the tip. Fig. 7 shows the plot of sliding distance between two cycles versus initial penetration depth under different normal loads.
Horizontal force Vertical force
100
Force (N)
horizontal force from A to B. Since the depth of penetration is fixed in the experiment, the increased force is not able to lift up the tip. Finally, the increased contact pressure of the wedge face forces plastic flow of materials sideways. A steady-state plastic flow is established and the horizontal force approaches a constant value, as seen after point B in Fig. 5. The maximum displaced volume of the material during the sliding process is equal to the product of wedge length, penetration depth and the sliding distance. The resistant force due to strain hardening of UHMWPE is nearly proportional to the displaced volume in the sliding process. We obtained the material resistant force per unit volume of UHMWPE to be about 1.2!10K6 N/ (mm)3 from the results with different penetration depths. The quantification of the strain hardening of UHMWPE during a wedge sliding process has been achieved. This value can be further applied to model the particle generation process. To sum up, this experimental result provides the explanation of the force contributing to the lift-up process of the wedge features.
431
2.5 2.0 1.5 1.0 0.5
60° wedge tip 90° wedge tip
0.0 0
20
40
60
80
100
Initial penetration depth (µm)
Fig. 7. The plot of sliding distance between two cycles versus initial penetration depth from the results of wedge sliding tests with different normal load and wedge angle.
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normal load of the test. We define Frxz is the resultant resistant force acting on the tip. Frz and Frx are the resolved component forces in the vertical and horizontal directions, respectively (see Fig. 8(b)). First, Frz increases with increased accumulated volume of UHMWPE, and Frz also contributes to lift up the tip in the sliding process. The contact area between the tip and the material decreases during the lift-up process and thus the Frz starts to drop. At the moment that the tip is pushed to the apex of the material bump, Frz drops to zero. Because the vertical force shown on the plot presents the difference between the normal force and the material resistant force in the vertical direction, the amount of vertical force returns to the original applied normal load when the tip is fully retracted.
Normal load (W)
(a)
Normal load (W) Sliding
θ Frxz
(b) Frz = Frxz sinθ
θ Frx=F rxz cosθ
3.5. Effect of normal load and wedge angle
Normal load (W) Sliding
(c)
Normal load (W) Material bump
Sliding
(d)
Sliding distance
Fig. 8. Schematic representation of the wedge sliding over UHMWPE under a constant normal load condition.
25 A
B
Load (mN)
20
15
10
5 O
0
C2
C
0 1000
2000
3000
4000
5000
6000
C1
7000
Displacement (nm) Fig. 9. Load–displacement curve of nano-indentation test on UHMWPE with constant strain rate of 0.1 sK1 [4].
The frequency of up and down movement of the tip or the distance between the repeated material bumps provides information on the shear deformation of the material during the sliding process. Thus, by understanding the force curve of the sliding process, the influence of the geometry of the tip and mechanical operation conditions on the wear-particle dimensions can be understood. The sliding distances between two displaced bumps under different normal loads were plotted against the initial penetration depth in Fig. 7. A larger normal load leads to a deeper penetration depth at the beginning of the sliding. The plot shows that increasing initial penetration depth results in a longer sliding distance. Under the same initial penetration depth, a sharper wedge (smaller wedge angle) results in a longer sliding distance. Dependence of the wedge tip lift-up process on the wedge angle needs further examination. As illustrated in Fig. 8(b), the quantity of the resolved component forces in the vertical direction is a function of the wedge angle. Assuming the same penetration depth during the sliding process, the resultant material resistant forces (Frxz) from the shear of UHMWPE material are the same. The resolved force in the vertical direction is FrzZFrxz sinq. A larger angle causes more material resistant force to be distributed to the vertical direction, which increases the lift-up force. Thus, the time required for the tip to be pushed away is shorter. With a constant sliding speed in the horizontal direction, a shorter distance between two peaks is expected. Variation of the wedge angle contributes to the change of penetration depth and the cutting edge lift-up speed. Both consequences have an influence on the sliding distance. This result verifies that a larger normal load and smaller wedge angle lead
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to a larger volume of generated UHMWPE particles, as observed in the previous studies [5,6]. Uni-Directional Stress
3.6. Viscoelasticity of UHMWPE The cutting edges penetrated into UHMWPE at the beginning of the process under a constant normal load. The applied normal load, angle and height of the cutting edge determine the initial penetration depth. Unloading due to the consequent sliding process leads to the recovery of the penetration depth for a viscoelastic polymer. Our previous study performed a nanoindentation test on UHMWPE to examine the deformation recovery due to viscoelasticity [4]. Fig. 9 shows the load–displacement curve of the indentation. Fig. 9 shows that the displacement increases in the loading segment (O/A); at the holding period with constant load (A/B), creep of the material under a constant stress was observed; during the unloading stage (B/ C), the displacement curve shows additional deviation due to viscoelastic recovery of the material. The difference of the displacement between point C1 and C2 (812 mm) represents the elastic recovery of the material deformation. The difference of the displacement between point C and C2 (1563 mm) further indicates the delayed recovery of the material deformation by viscoelastic effect. Compared with the maximum penetration depth, 37.3% of the maximum penetration depth was recovered after the load is removed. Also, comparing the ratio of the area ‘BCC1’ (viscoelastic recovery) to the area ‘OABC1’ (equal to the total energy done during the indentation) in Fig. 9 shows that the effect of viscoelasticity on UHWMPE deformation is dramatic. It should be taken into consideration when we want to correlate the dimensions of the surface textures to the size and shape of the generated UHMWPE particles.
C C C C
C--H
Molecular Axis Fig. 10. Schematic representation of UHMWPE molecular orientation under a uni-directional stress.
molecular orientation at UHMWPE surfaces subjected to different shear motions by rubbing a UHMWPE pin against a polished Co–Cr disk. The results show that a higher degree of chain alignment was induced by unidirectional rubbing the UHMWPE surface. It concluded that the molecular chains tend to align and orient
Break of material (release of energy causing the drop of force)
40 35
3.7. Stress-induced molecular orientation of UHMWPE
Fz Fx
30
Force (N)
The kinematics of the wedge-shaped surface features has been illustrated from the single sliding tests with constant normal load. However, the generation of particles was not observed from a single pass of the sliding process. Particles were generated only after several cycles of sliding. UHMWPE is a semi-crystalline material and the folded-chains in the crystalline region have a specific molecular orientation. We are wondering if there is any change of mechanical property of UHMWPE when a mechanical stress is applied on the material surface. Soft X-ray absorptionspectroscopy has been used [15] to measure the
C--H
25 20 15 10 5 0 0
2
4
6
8
10
12
14
Time (sec) Fig. 11. Force curves of the 11th pass of the wedge sliding test (wedge length, 2 mm, wedge angle, 608, penetration depth, 80 mm, sliding distance, 6 mm/pass, sliding speed, 1 mm)
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to a common direction when the surface is subjected to a directional force such as shear stress from sliding (as schematically shown in Fig. 10). To investigate the influence of multiple scratches on the mechanical properties of the material surface, a multiple-pass wedge-sliding test with constant penetration depth has been performed. Fig. 11 shows the force curve of the 11th sliding pass of the linear reciprocating experiments (with 2 mm wedge length, 608 wedge, and 80 mm penetration depth). Drop in the force during the sliding process indicates damage of the UHMWPE. We think that the multiple sliding processes induces the UHMWPE molecules to be oriented in the sliding direction and the compact arrangement of molecules on the surface makes the material become more brittle. Finally, the UHWMPE material fractures at the tip–material interface. Detachment of the material from the surface leads to the generation of the particle. 4. Conclusions 1. By designing the sliding experiments with constant normal load and constant penetration depth, single wedge sliding tests with scaled-up tips has enabled the direct observation of kinematics of the microcutting process and the measurement of strain hardening of UHMWPE. 2. The recovery of the deformation due to viscoelasticity may affect the size of the generated UHMWPE particles. The degree of viscoleastic recovery of the penetration depth of a tip has been discussed from a result of nanoindentation tests. 3. UHMWPE particles are generated from the following steps: (a) deformation of the material due to the penetration and sliding processes of the surface features; (b) strain hardening occurs under a surface-feature sliding process; (c) UHMWPE molecules align to the direction where the shear stress is applied and the surface layer of material becomes more brittle; (d) Detachment of the particle at the tip edge– material interface according to the fracture of UHWMPE.
References [1] W.H. Harris, The problem is osteolysis, Clin. Orthop. Res. 311 (1995) 46–53. [2] H.C. Amstutz, P. Campbell, N. Kossovsky, I.C. Clarke, Mechanism and clinical significance of wear debris-induced osteolysis, Clin. Orthop. Res. 276 (1992) 7–18. [3] W.J. Maloney, R.L. Smith, Periprosthetic osteolysis in total hip arthroplasty: the role of particulate wear debris, J. Bone Joint Surg. 77A (1995) 1448–1461. [4] H.-W. Fang, S.M. Hsu, J.V. Sengers, Ultra-high molecular weight polyethylene wear particle effects on bioactivity, National Institute of Standards and Technology Special Publication 1002, 2003. [5] H.-W. Fang, S.M. Hsu, J.V. Sengers, Generation of narrowly distributed ultra-high molecular weight polyethylene particles by surface texturing techniques, J. Biomed. Mater. Res. B: Appl. Biomater. 67B (2003) 741–749. [6] H.-W. Fang, S.M. Hsu, J.V. Sengers, Surface texture design to generate specific sizes and shapes of UHMWPE wear particles, Materialwissenschaft und Werkstofftech ( Mater. Sci. Eng. Technol.) 34 (2003) 976–988. [7] A.J. Black, E.M. Kopalinsky, P.L.B Oxley, An investigation of the different regimes of deformation which can occur when a hard wedge slides over a soft surface: the influence of wede angle, lubrication and prior plastic working of the surface, Wear 123 (1988) 97–114. [8] J.M. Challen, P.L.B Oxley, An explanation of the different regimes of friction and wear using asperity deformation models, Wear 53 (1979) 229–243. [9] A.J. Black, E.M. Kopalinsky, P.L.B Oxley, The wave model of metallic sliding friction: an investigation of the transition from initial indentation to steady-state wave formation in a model asperity experiment, J. Phys. D: Appl. Phys. 26 (1993) 1892–1899. [10] K.L. Johnson, Contact Mechanics, Cambridge University Press, Cambridge, UK, 1985. [11] B.J. Briscoe, L. Fiori, Nano-indentation of polymeric surfaces, J. Phys. D: Appl. Phys. 31 (1998) 2395–2405. [12] S.M. Kurtz, L. Pruitt, C.W. Jewett, R.P. Crawford, D.J. Crane, A.A. Edidin, The yielding, plastic flow, and fracture behavior of ultra-high molecular weight polyethylene used in total joint replacements, Biomaterials 19 (1998) 1989–2003. [13] D.J. Krzypow, C.M. Rinmac, Cyclic steady state stress–strain behavior of UHMWPE, Biomaterials 21 (2000) 2081–2087. [14] S.M. Kurtz, C.M. Rimnac, D.L. Bartel, Predictive model for tensile true stress-strain behavior of chemically and mechanically degraded UHMWPE, J. Biomed. Mater. Res. B: Appl. Biomater. 43 (1998) 241–248. [15] S. Sambasivan, D.A. Fischer, M.C. Shen, S.M. Hsu, Molecular orientation of ultrahigh molecular weight polyethylene induced by various sliding motions, J. Biomed. Mater. Res. B: Appl. Biomater. 70B (2004) 278–285.