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Wilches2008.pdf
Available online at www.sciencedirect.com
Wear 265 (2008) 143–149
Wear of materials used for artificial joints in total hip replacements
L.V. Wilches, J.A. Uribe, A. Toro ∗
Tribology and Surfaces Group, National University of Colombia, Medellı́n, Colombia
Received 19 July 2007; accepted 19 September 2007
Available online 26 November 2007
Abstract
Two tribological pairs used for artificial joints were studied. Wear tests were performed in a pin-on-disc type machine modified to allow lubrication
with bovine serum. The pins were made of either AISI F138 stainless steel or ASTM F136 titanium alloy, while the discs were manufactured with
ultra-high molecular weight polyethylene (UHMWPE). The tested pairs were ASTM F138 stainless steel–UHMWPE and ASTM F136 titanium
alloy–UHMWPE. The sliding velocity was fixed to 0.58 m/s for all the tests and the variation of friction force was registered as a function of time
and normal load. Also, an electrochemical setup allowed monitoring the variation of the corrosion potential between the metallic pins and the
bovine serum. The lower values of friction coefficient were measured when a thin film of polymer was transferred to the metallic surface, which led
to a smooth interface and avoided debris generation. The viscoelastic behavior of UHMWPE was responsible for the increase in friction coefficient
after a testing period, which depended on the normal load and the specific pair tested.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Total hip replacements; Wear mechanisms; Wear debris; Corrosion potential; Friction coefficient
1. Introduction
The use of alloys and polymers for total hip and knee replacements has increased significantly in recent years, mainly as
a consequence of their good combination of mechanical and
surface properties together with recent improvements in biocompatibility and bioactivity [1–4].
Metallic materials are highly appreciated due to their good
wear resistance and high mechanical properties such as hardness
and ductility, while polymers usually show low friction coefficients and excellent chemical stability when in contact with
physiological fluids. Nevertheless, a significant number of total
joint replacements fail long before the end of their expected
life causing severe traumas to the patients [5]. These failures
are related to surface damage of the parts in contact, generally
represented as wear of the polymers and corrosion of the alloys.
Most of the implants used in Colombia for total hip replacements are manufactured either with stainless steel–UHMWPE
or Ti6Al4V–UHMWPE pairs, so the knowledge of the surface
damage mechanisms that act on these surfaces when put in contact to the human body is crucial to develop better materials
∗
Corresponding author. Tel.: +57 4 425 5339; fax: +57 4 425 5339.
E-mail address: [email protected] (A. Toro).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.wear.2007.09.010
as well as to improve the surgical practices and post-operative
rehabilitation. Although a considerable amount of information
is available regarding tests in joint simulators [6,7], standard test
methods for wear testing of materials used in joint prostheses [8]
and materials response to particular wear–corrosion process in
the human body [9–11], the knowledge of the wear mechanisms
acting on the surfaces in contact is still limited, in particular
when factors such as surface finishing, chemical composition,
stress distribution and medical record of the patient have to be
considered from a comprehensive approach.
The purpose of this work is to study the mechanisms of
surface deterioration of materials used for total hip joint replacements when submitted to conditions close to those found in
the human body, as well as to suggest improvements for future
applications based on friction coefficient and corrosion potential
results.
2. Experimental procedure
2.1. Materials
AISI 316 LVM Stainless steel (ASTM F138 standard) bars
with 6 mm in diameter, Ti6Al4V extra low interstitial (ELI)
alloy (ASTM F136 standard) with 6.35 mm in diameter and ultra
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L.V. Wilches et al. / Wear 265 (2008) 143–149
Table 1
Chemical composition, hardness and microstructure of the metallic materials studied
Chemical analysis carried out by optical spectrometry (wt%).
Fig. 1. Modified pin-on-disc wear testing machine. (a) General aspect of the device, (b) modified disc, and (c) detail of the positioning of pins and rings and
configuration of electrochemical cell.
L.V. Wilches et al. / Wear 265 (2008) 143–149
145
Table 2
Initial roughness of the surfaces in contact
Material
Ra (␮m)
Rq (␮m)
ASTM F138 (AISI 316LVM) pin
ASTM F136 (Ti6Al4V) pin
UHMWPE ring
0.04 ± 0.01
0.04 ± 0.01
0.24 ± 0.02
0.07 ± 0.01
0.07 ± 0.01
0.32 ± 0.03
high-molecular weight polyethylene, UHMWPE (TIVAR 1000)
sheets with 6 mm in thickness were used in this investigation.
The ASTM F138 and ASTM F136 bars were manufactured by Sandvik, while Quadrant Engineering Plastics Products
provided the UHMWPE sheets. The measured chemical composition of the metallic materials, as well as their average hardness
and microstructure are indicated in Table 1. The measured average hardness of the UHMWPE was 60–65 SH D.
2.2. Wear tests
The wear tests were performed in a modified pin-on-disc
machine [12], which allowed lubrication of the contact surfaces
with bovine serum with a protein content of 0.030 ± 0.001 g/cm3
(measured by the Biuret method). The bovine serum was used to
provide chemical and biological characteristics similar to those
of synovial fluids found in natural joints [5]. It was obtained from
adult Holstein cows having an average weight of 650 kg, which
were fed with unfertilized Kikuyo grass plus a 6% P mineral
supplement. Blood collection was made by using a Vacutainer
system; the serum was incubated at 37 ◦ C for 30 min and then
centrifuged at 3000 rpm for 5 min. The whole process of blood
collection, centrifugation and protein content measurement was
performed in the Animal Production Laboratory of the National
University of Colombia in Medellı́n.
In each test, the pin was fixed to a rigid arm and put in contact
with a rotating ring under the application of a normal load by
dead weights, as shown in Fig. 1. All the pins were cylinders with
6 mm in diameter and 12 mm long, and the rings had 170 mm of
internal diameter, 190 mm of external diameter and a thickness
of 6 mm. The homogeneous contact between the pin and the ring
was verified by checking the flatness of the surfaces with a dial
indicator having a reading accuracy of 0.002 mm. The relative
slip speed between pins and rings was fixed to 0.58 m/s and the
normal loads were 5, 15, 30, and 50 N, which correspond to mean
contact pressures of 177, 531, 1060 and 1767 kPa, respectively.
Three tests were performed for each condition of normal load
and tribological pair. The total testing time was 7 h for all the
pairs studied and the temperature of the ambient was controlled
to 25 ◦ C. The tribological pairs tested were as follows: (a) ASTM
F138 stainless steel pins against UHMWPE rings, and (b) ASTM
F136 alloy pins against UHMWPE rings.
The surface of metallic pins was polished with emery papers
and abrasive cloths with diamond particles of 0.1 ␮m in diameter, which led to the roughness values shown in Table 2. On
the other hand, the surface finishing of the UHMWPE rings was
obtained through lathe machining (see Table 2). All the samples were washed, dried and isolated after washing in order to
avoid contamination. The microstructure of the stainless steel
Fig. 2. Variation of friction coefficient with testing time for different normal
loads. AISI 316LVM pin sliding against UHMWPE ring.
was composed of austenite grains with average size of 25–30 ␮m
and the titanium alloy showed an ␣-␤ distribution with a volume
fraction of ␤ of circa 20%.
The time-variation of the corrosion potential between the
working electrode (pin) and the bovine serum during the wear
tests was measured with the aid of the arrangement shown in
Fig. 1c.
Before and after each test, the samples were ultrasonically
cleaned, dried in warm air and characterized by optical and
scanning electron microscopy in order to identify the damage
mechanisms at the surface. The friction coefficient and corrosion potential were registered every second with the aid of a data
acquisition card and software Labview 5.1 provided by National
Instruments under an educational contract.
3. Results and discussion
Figs. 2 and 3 show the variation of the friction coefficient
of ASTM F136–UHMWPE and ASTM F138–UHMWPE pairs
as a function of testing time for different normal loads. It can
be observed that the friction coefficient has a trend to increase
suddenly at a testing time that depends on the magnitude of
the normal load. For both systems studied, when the normal
load was 5 N no significant changes in friction coefficient were
observed, but for a normal load of 50 N the effect was noteworthy. This particular response of the surfaces in contact can be
related to adhesive phenomena at the interface, since UHMWPE
Fig. 3. Variation of friction coefficient with testing time for different normal
loads. ASTM F138 pin sliding against UHMWPE ring.
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L.V. Wilches et al. / Wear 265 (2008) 143–149
Fig. 4. Evidences of adhesion of UHMWPE to metallic pins after 7 h of testing. Normal load: (A) 5 N; (B) 15 N; (C) 30 N; (D) 50 N. The polymer layers correspond
to the clearer areas in the image and the black arrows indicate the sliding direction.
layers were transferred to the pin’s surface during the tests, being
greater the amount of material transferred when the normal load
increased (see Fig. 4). For a normal load of 5 N the amount of
polymer transferred to the pin was small, but in the case of normal load of 50 N practically all the pin’s surface was covered by
the UHMWPE layer, especially in the case of UHMWPE–AISI
316LVM pair. In addition, for a given normal load the increase
of friction coefficient at a specific interval during the tests is
also related to the adhesion of polymer to the metallic surface,
as can be seen in Fig. 5 for AISI 316LVM samples tested using
a normal load of 50 N, which were analyzed in SEM before and
after the sudden increase in friction coefficient reported in Fig. 3.
The increase in the amount of transferred polymer to the pin’s
surface with normal load is stimulated by a marginal lubrication regime at the effective area of contact, which is a result of
localized high contact pressures that prevents a hydrodynamic
regime.
The images in Figs. 4 and 5 suggest that for the most part
of the testing time in the metal–polymer pairs the contact was
of polymer–polymer type, so the measured friction force can
be related to cohesive failures at the sub-surface of the ring
instead of being a consequence of the adhesive forces at the stainless steel–UHMWPE interface, which leads to delamination of
polyethylene. A similar observation was made by Saikko et al.
[12] in CoCr alloy–UHMWPE pairs tested in a three-axis knee
wear simulator with ball-on-flat, where polymer wear debris
between 0.1 ␮m and 1 ␮m were reported. Furthermore, the fluctuations observed in Figs. 3 and 4 could be a consequence of
periodic stick–slip cycles caused by adhesion and subsequent
cohesive failure at the sub-surface of the polymer.
Fig. 5. Pin’s surface of ASTM F138 stainless steel tested against UHMWPE rings under a normal load of 50 N, before (A) and after (B) the sudden increase of
friction coefficient. Parallel marks indicate the sliding direction.
L.V. Wilches et al. / Wear 265 (2008) 143–149
147
The effect of wear of UHMWPE–metal pairs on osteolysis
and early loosening in total hip replacements has been reported
in literature [13]; a polymer transfer layer was observed by
Gispert et al. [14] in UHMWPE–ASTM F138 pairs and by Li
et al. [15] in UHMWPE–ASTM F136 pairs after pin-on-disc
tests similar to those described in the present work. Moreover,
Watters [16] also found these transfer layers at the surface of
ASTM F136 femoral heads in contact with UHMWPE liners,
after experiments carried out in a conventional hip simulator.
Regarding the wear mechanisms, Buford et al. [17], Gongde
[18], Liu [19] and Ahlroos [20] independently concluded that
the formation of adhesive joints and subsequent cohesive subsurface failures govern the initial stages of surface degradation
of UHMWPE in contact with metals, while localized fatigue
effects become more significant for longer testing times. The
results found in the present work support these conclusions.
3.1. Effect of testing conditions on corrosion potential
The variation of corrosion potential with testing time for
the metal–polymer systems studied is shown in Fig. 6. In the
ASTM F138–UHMWPE pair two regions can be distinguished:
the first region is characterized by a strong decrease in the values of potential (circa 350 mV during the first 30 min of the test)
and the second one shows a slight decrease to a stability value
of about −650 mV after 7 h of testing. In contrast, the ASTM
F136–UHMWPE system shows a quite different behavior: there
is an initial increase of the corrosion potential during the first
2 h of the test, followed by a period in which no significant variations are observed. Since the corrosion potential established in
titanium alloy–polyethylene pairs was higher than that of stainless steel–polyethylene pairs, the surface of the titanium alloy
can be considered more resistant to uniform corrosive attack
under the specific corrosion–wear conditions employed. Nevertheless, surface examination of both materials after the tests did
not reveal any evidences of localized corrosion, which means
that the passive layer was unaffected in all cases.
The fluctuations in the curves can be associated to changes in
the thickness of the diffusive layer close to the metal surface and
variations of the concentration of the solution. Small temperature
variations and high contact pressures can induce the precipitation of proteins which are temporally adhered to metal, changing
its electrochemical response and therefore affecting the mea-
Fig. 6. Variation of corrosion potential with testing time for different normal
loads. Metallic pins sliding against UHMWPE ring.
sured corrosion potential. Although no significant changes in
colour and texture of the bovine serum were detected after naked
eye observation, the microscopic examination of the surfaces
after the tests revealed the presence of tiny grains adhered to the
metallic pins (Fig. 7), whose chemical composition is related to
that of the bovine serum. Also, the changes in effective contact
area between the pin and the ring due to adhesion of polymeric
layers to metal slow down the electrochemical reaction since
these layers act as an isolating material.
3.2. Surface damage and wear mechanisms
The predominant wear mechanism in metal–polyethylene
pairs was adhesion, which caused thin polymer films to be transferred to the surface of the pins since the beginning of the tests.
This film probably covers the surface of the pins almost com-
Fig. 7. (a) Grain precipitated from the bovine serum, which was observed at the surface of ASTM F138 stainless steel pin and (b) EDS spectra of the grain.
148
L.V. Wilches et al. / Wear 265 (2008) 143–149
Fig. 8. Evidences of plastic deformation (a) and cracking (b) of UHMWPE films transferred to the surface of ASTM F138 pins. Load 50 N. Testing time 7 h.
Fig. 9. Wear marks and signals of plastic deformation at the surface of UHMWPE ring sliding against an AISI 316LVM pin. Load 50 N. Testing time 7 h.
pletely during the first stages of the tests and acts as a lubricant
afterwards. However, when the normal load and testing time
increase the adhesive forces at the interface overcome the cohesive forces inside the polymeric ring, which leads to failure
beneath the interface. The material that is removed from the
ring is deposited onto the pin’s surface and from that point on
the contact is mainly of polymer–polymer type.
For longer testing times another important change is observed
at the surface of the pins, as can be seen in Fig. 8. The polymer
film shows cracks and signals of plastic deformation such as
prows, especially in the areas close to the borders of the contact zone. It is proposed that cracks could be a consequence of
thermal fatigue related to the difference between the thermal
expansion coefficients of the metal surface and the polymeric
film, while plastic deformation is caused by the high contact
pressures achieved at the interface. On the other hand, analysis
of the worn surfaces of the rings revealed plastic deformation
and damage to the texture left by the machining process prior to
wear tests (Fig. 9).
3.3. Viscoelasticity effects
The effect of viscoelastic response of the UHMWPE surface
sliding against ASTM F138 stainless steel can be seen in Fig. 10.
The average value of friction coefficient did not change considerably when the normal load was gradually reduced from 50 N
to 5 N, which means that the relationship Ff = ␮W applies to
Fig. 10. Variation of friction coefficient with normal load during a single test
for contact between AISI 316LVM and UHMWPE. Load 50 N (a) the load is
reduced during the test. (b) The load is increased during the test.
L.V. Wilches et al. / Wear 265 (2008) 143–149
the tribo-system, i.e., the friction force depends mainly on the
number of plastically deformed asperities (the higher the normal
load, the higher the number of deformed asperities) [21]. On the
other hand, when the normal load was gradually increased during a single test, an increase in friction coefficient with time was
observed. The change in friction coefficient could be the result
of a change in the relation between the number of plastically
deformed asperities and the normal load, and/or a substantial
change in mechanical properties of the polymer due to the high
temperature at the interface, which is clearly more feasible when
the normal load is higher [22,23].
4. Conclusions
• The most important wear mechanisms in the metal–polymer
pairs under the specific conditions studied were the adhesion
of the polymer to the metallic surface and subsequent failure
at the sub-surface of the polymer.
• The transference of a thin polyethylene layer to the metallic
surfaces promoted a lubricating effect at the interface for low
normal loads and short testing times. When this condition was
achieved the friction coefficient was always lower than 0.04.
• The amount of polymer adhered to the surface increased with
the normal load and was affected by the viscoelastic response
of the UHMWPE, especially for normal loads over 30 N. For
higher normal loads and longer testing times the lubricating
effect was not predominant and significant plastic deformation of the polyethylene was observed.
• The measured corrosion potential of ASTM F136 titanium
alloy in bovine serum during the wear tests was higher than
that of ASTM F139 stainless steel, which indicates a better resistance of the former to uniform corrosion under the
specific conditions studied.
Acknowledgement
The authors thank to Research Division of National University of Colombia (Medellı́n) for financial support, project no.
20301004575, Code DIME 30805798.
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