J. Biomedical Science and Engineering, 2011, 4, 375-382
doi:10.4236/jbise.2011.45047 Published Online May 2011 (http://www.SciRP.org/journal/jbise/ JBiSE ).
Published Online May 2011 in SciRes. http://www.scirp.org/journal/JBiSE
Influence of different CoCrMo counterfaces on wear in
UHMWPE for artificial joints
Victor A. González-Mora, Michael Hoffmann, Rien Stroosnijder, Eduardo Espinar1, José Maria
Llamas1, Mariano Fernández-Fairén2, Francisco Javier Gil2
1Department of Ortodoncia, Facultad de Odontologia, Universidad de Sevilla, Sevilla, Spain;
2CREB, Department of Ciencia de Materiales e Ingeniería Metalúrgica, ETSEIB, Universidad Politécnica de Cataluña, Barcelona,
Spain.
Received 20 December 2010; revised 20 January 2011; accepted 1 March 2011.
ABSTRACT
Wear tests were carried out to study the effect of
various counterface materials in the wear behaviour
of Ultra High Molecular Weight Polyethylene
(UHMWPE). The materials used as counterfaces
were based on varieties of CoCrMo: 1) forged (hand-
pol-ished) CoCrMo; 2) forged (mass-finished) Co-
CrMo; 3) cast (mass-finished) CoCrMo. Additionally,
two coatings were proposed: 1) a CoCrMo coating
applied to the forged CoCrMo alloy by means of
physical vapour deposition (PVD); 2) a ZrO2 coating
applied to the forged CoCrMo alloy by means of
plasma-assisted chemical vapour deposition (PACVD).
The reciprocating pin-on-flat (RPOF) device for
pin-on-disk wear testing was used for this study. The
worn surfaces were observed using optical, atomic
force and scanning electron microscopes.
Keywords: Wear; Artificial Joints; Co CrMo
1. INTRODUCTION
Decades of basic and clinical experimentation have re-
sulted in a vast array of prosthetic designs and material
combinations. However, there is still any consensus on
which designs are the most appropriate and successful.
All material combinations have certain drawbacks. Ac-
cording to Wang [1], “All prostheses will fail sometime.
It is a race between the life of the patient and the life of
the prosthesis” [1-3]. Therefore, the duty of researchers
is continuing the search for finding better combinations
of materials for artificial joints.
The material combinations used today include a
polymer component for the acetabular cup in the hip
joint, or a tibial plateau in the knee joint, and a metallic
or ceramic counterface for the femoral head in the hip
joint, or a metallic counterface for the femoral condyle
in the knee joint. For the polymer component, ultra-high
molecular weight polyethylene (UHMWPE) has been
universally adopted. Nowadays, the most widely used
components are couples (or sliding pairs) composed of
UHMWPE and a metal counterface (generally a
CoCr-based alloy). This material combination is called a
“metal-on-polyethylene” artificial joint. In hip joints, a
ceramic material (usually alumina or zirconia) can also
be used as a counterface. This is called a “ceramic-on-
polyethylene” joint. In recent years, there has been re-
newed interest in “metal-on-metal” and “ceramic-on-
ceramic” artificial hip joints, in which both the femoral
and acetabular components are made of metal or ce-
ramic.
Many variables contribute to clinical success or fail-
ure in complex reconstructive procedures, such as a total
joint replacement. These variables include patient selec-
tion, surgical techniques and prosthetic components.
There are many reasons for failure during the life of a
joint prosthesis. Early failures are often caused by infec-
tion, joint dislocation and/or the fracture of the replace-
ment components. However, the main cause for most of
the long-term failures is an aseptic loosening. With the
advances in the design of the prosthesis and the fixation
methods, wears of UHMWPE have replaced loosening
as the main cause of failure in long-term implants [1,2].
It is generally recognized that microscopic polyethylene
wear debris can induce adverse biological tissue reac-
tions and subsequent bone resorption or osteolysis [1-5].
Wear particles of all types of biomaterials, especially
UHMWPE, can apparently cause macrophages which
may lead to an osteolytic reaction (either directly, or via
mediator release) [3].
Concerns over UHMWPE wear have led to new de-
signs and new material combinations for the articulating
surfaces of artificial joints. Thus, metal and ceramic
wear particles must also be considered. Osteolysis is

V. A. González-Mora et al. / J. Biomedical Science and Engineering 4 (2011) 375-382 376
related to particle accumulation. The size, concentration
and, to a lesser extent, shape and chemical composition
of the particles are the most important factors in bioen-
vironmental response to wear debris.
2. MATERIALS AND METHODS
The reciprocating pin-on-flat (RPOF) device is a special
pin-on-disk (POD) wear tester that was designed in ac-
cordance with the ASTM F732-82 standard. This stan-
dard is the first specific standard in the field of biotri-
bology. It sets the guidelines for a “laboratory method
for evaluation of the friction and wear properties of
combinations of materials that are being considered for
use as the bearing surfaces of human total joint replace-
ment prostheses” [7,8]. The standard is mainly intended
for the evaluation of polymer material combinations.
The RPOF wear-test device is a tribosystem, in which
an apparatus produces an oscillatory relative motion
between the pins and plates. Normally, the pins are sta-
tionary while the plates have an oscillating motion. The
motion is always in a horizontal p lane and unidirection al
(Figure 1). These “reciprocating” devices are so called
because of the reciprocating oscillating movement of the
plate with respect to the pin.
The tests on the RPOF wear-test method were per-
formed as follows. The disks were mounted on a linear
bearing while the pins were fixed and pressed against the
disks. The motion of the RPOF machine is unidirectional
and reciprocating, with a stroke length of 17 mm. A load
of 225 N (23 kg) was placed over the pins, resulting in a
contact pressure of 3.5 MPa; this is the total standard
knee replacement stress measured [4]. The pin contact
area is considered to be 63.6 mm2. The frequency of the
motion was 1 Hz, or 1 cycle/second. Two stroke lengths-
those are one back-and-forth motion of the disks are con-
sidered one cycle. The wear on the UHMWPE pins was
determined by weight loss measurements every 250,000
cycles, up to a total t est l engt h of one million cycl es.
The test lubricant was replaced with fresh solution af-
ter every weighing stop. Distilled water was added dur-
ing the test to compensate the water evaporatio n. A solu-
tion consisting of bovine serum and distilled water was
used as test lubricant. The solution had a total protein
concentration of 30 mg/ml, which simulated the clinical
situation [9]. The serum was purchased at Sigma-Aldrich
(calf serum, bovine donor; product No. C9676). The
adsorption of the protein constituents onto UHMWPE
pins was determined using an additional control pin,
which was loaded in the RPOF machine as the
UHMWPE pins were loaded, but not applying any mo-
tion. The UHMWPE pins were cleaned and dried in ac-
cordance with the ASTM 1714 standard. A Mettler
Toledo AT261 Delta Range® microbalance with an ac-
curacy of 10 µg was use d to weigh the pins.
The pins were manufactured from a medical-grade
GUR1120 UHMWPE bar, previously sterilised with
standard 25 kGy (2.5 Mrad) gamma radiation. The den-
sity of the UHMWPE was 0.9737 g/cm3. The pins were
13 mm long and 9 mm in diameter. The disks were
manufactured from five different counterface materials,
all of which were CoCrMo alloys. Ta b l e 1 summarises
the test conditions and materials.
The standard material in this study was a hot-forged
L
Lo oa ad d
P PI IN N
D
DI IS SK K
Figure 1. Motion/loading configuration of a RPOF wear-test
machine showing the translating unidirectional movement of
the pin on the plate. The yellow arrow shows the direction of
sliding and the red arrow shows the direction of the load ap-
plied.
Table 1. Conditions of the RPOF wear tests.
Test parameter Value
Type of motion Unidirectional (reciprocating)
Contact geometry Flat-on-flat
Frequency 1 Hz
Sliding distance/cycle 17 mm
Contact area 63.6 mm2
Applied load 23 kg (225 N)
Contact stress 3.54 MPa
Test length 1 million cy cles (at intervals of 250,000)
Lubricant 30 mg/ml initial protein content
Temperature Room
UHMWPE componentGUR1120
Counterface com ponent
Forged (hand-polishe d) CoCrMo
Forged (mass-finished) CoCrMo
Cast CoCrMo
Forged CoCrMo with a CoCrMo coating
Forged CoCrMo with a ZrO2 coating
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JBiSE
CoCrMo alloy. Table 2 shows the chemical composition
of this material.
 For ged (hand-pol i shed) CoCrM o
 Forged (mass-finished) C oCr M o
 Cast (mass-finished) CoCrMo
 In addition to the materials mentioned above, two
coatings were employed. First, a CoCrMo coating ap-
plied to the forged CoCrMo alloy by means of physical
vapour deposition (PVD). The coating had the same
chemical composition as the substrate. The rationale for
testing this kind of coating was related to the use of
femoral components in Total Knee Replacements
(TKRs).
The second coating was a ZrO2 coating applied to the
forged CoCrMo alloy by means of plasma-assisted
chemical vapour deposition (PACVD). The rationale for
testing this kind of coating was the same as for the Co-
CrMo coating applied to forged CoCrMo.
For each counterface material, four disks were tested
and at least three of them were considered for evaluation .
A total of 40 wear tests were performed. Ta bl e 3 shows
the roughness and hardness of each material.
3. EXPERIMENTAL RESULTS AND
DISCUSSION
Figure 2 shows the wear results obtained with the RPOF
tests for the UHMWPE specimens (pins). The volumet-
ric wear (mm3) of the UHMWPE pins is represented as a
function of test duration (in cycles) and counterface typ e.
The volumetric wear results are calculated according to
the average of weight loss of three specimens for each
material.
The standard deviations ranged from 0.01 to 0.05 mg,
except for the CoCrMo coating. These measurement
variations imply that the gravi metric wear determination
is highly influenced by the intrinsic uncertainty of the
measurement, with little weight losses in the UHMWPE.
Thus, for very early stages and especially for counter-
faces producing very little weight loss in the UHMWPE
specimen, the measurements have a high uncertainty.
The CoCrMo coating had significantly higher standard
deviations, ranging from 0.08 to 0.11 mg. The greater
scatter for the CoCrMo coating is due to the higher sen-
sitivity of the coating to scratches and third-body wear,
which highly influence the surface roughness of the
disks and thus weight loss in the UHMWPE.
The results shown in Figure 2 can be used to ex-
trapolate the wear behaviour of UHMWPE to the next
million cycles. However, it is more interesting to com-
pare the wear rates of the sliding couples than to com-
pare the volumetric wear after one million cycles of the
wear test. Figure 3 shows data from Figure 2 for the
first 250,000 cycles without standard deviations. This
data is related to the initial st age of running-in wear and
do not correspond to the stationary w ear state. Wear-rate
values are calculated from the slope of the linear regres-
sion fitting to the volumetric wear data. The R2 values of
the linear fitting (greater than 0.92) indicate that the
wear tests are highly reproduc ible and the wear rates are
highly linear, except for the mass-finished alloys (both
cast and forged), which had higher uncertainty due to
low measurement accuracy for small weight losses. This
is consistent with other studies [10,11] and clinical ob-
servations [12,13], which have shown that the high ini-
tial wear rate is statistically higher by a significant
amount than the wear rate thereafter.
The results show that the CoCrMo coating causes the
highest UHMWPE wear of all the counterfaces tested.
The CoCrMo coating wear rate is an order of magnitude
higher than that caused by the mass-finished (forged)
alloy, which caused the least UHMWPE wear in this
study. The ZrO2 coating and the hand-polished (forged)
CoCrMo alloy caused intermediate UHMWPE wear
rates. The UHMWPE wear value for the ZrO2 co ating is
Table 2. Chemical composition of the forged CoCrMo alloy (%).
Component Cr Mo Mn Ni Si Fe C N
Balance 26 - 30 5 - 7 max 1 max 1 max 1 max 0.7max 0.35 max 0.25
Table 3. Roughness and hardness of each material tested.
Material Roughness Ra (µm) Hardness (HVN)
Hand-polished 0.03 ± 0.01 673 ± 21
Mass-finished 0.05 ± 0.01 840 ± 62
Cast CoCrMo 0.05 ± 0.01 783 ± 52
CoCrMo coating 0.10 ± 0.01 884 ± 28
ZrO2 coating 0.06 ± 0.01 575 ± 43

V. A. González-Mora et al. / J. Biomedical Science and Engineering 4 (2011) 375-382 378
Figure 2. Average volumetric wear of UHMWPE pins sliding against various counterfaces.
Figure 3. Wear rates of UHMWPE pins sliding aga i n st various counterfac es.
about half the wear value for the CoCrMo coating.
Different surface treatments (mass-finishing and hand-
polishing) on the forged CoCrMo alloys lead to signify-
cant differences in UHMWPE wear. The above data
shows that the mass-finishing treatment on CoCrMo
alloys causes less UHMWPE wear.
After the wear tests, the disk surface was observed
with an optical microscope. All disks showed a certain
degree of scratching. On the hand-polished alloys, the
scratches were always parallel to the sliding direction
(Figure 4(a)) and were deeper and wider than those
found on the mass-finished materials (Figure 4(b) and
4(c)). The scratches were most numerous on the Co-
CrMo coating surface (Figure 4(d)) and on the hand-
polished alloys. However, the scratches on the mass-
finished (forged and cast) alloys were shallow and hard
to see, which may be due to the fact that the mass-fin-
ishing treatment on the surface of the CoCrMo alloys
produces some hardening on the surface, rendering it
more scratch-resistant than the hand-polished surface
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(a) (b) (c)
(d) (e)
0.5 mm
Figure 4. Optical micrographs showing the surface after the RPOF wear test. (a) hand-polished; (b) mass-finished; (c) c ast; (d) Co-
CrMo coating; (e) ZrO2 coating.
[14-17].
For the CrCoMo coatin g samples, the worn UHMW PE
surface presents numerous scars caused by the scratches
of the disk counterfaces. The appearance of greater
damage to the UHMWPE must be due to a rougher disk
counterface. Therefore, greater UHMWPE wear is re-
lated to the roughening of the surface, which causes
abrasive wear on the UHMWPE surface. For the other
pin surfaces, the wear damage appears to be due to ad-
hesion and creep; thus, the surfaces are smooth. As a
result, the UHMWPE surface is flattened and has fewer
scars. The differences in the wear surface of metal coun-
terface are shown in Figure 5. The images were ob-
tained using an atomic force microscope (AFM). This is
consistent with in vivo observations, which showed that
joint replacements which did not fail, or which had very
little linear wear, maintained their initial low surface
roughness counterface and had few scratches [17-21].
For the ZrO2 coating, we identified some surface de-
fects in the coating itself (Figure 6(a)), which occurred
during coating deposition. Those defects, together with
the irregular mass-finished substrate surface and the
well-known brittleness of ZrO2 coatings, may be the
cause of the coating’s fracture and subsequent detach-
ment, which is shown in Figure 6(b). This kind of coat-
ing failure is responsible for the high degree of
UHMWPE wear observed in the RPOF tests.
The hardness rankings are consistent with the wear
results and surface observations for the bulk counter-
faces. Thus, the mass-finished (forged) alloy causes less
UHMWPE wear than the mass-finished (cast) alloy, and
the second one causes less UHMWPE wear than the
hand-polished (forged) alloy. Similarly, the mass-fin-
ished (forged) CoCrMo alloy is harder than the mass-
finished (cast) alloy, and the second one is harder than
the hand-polished (forged) alloy. Therefore, the harder a
surface is, the less UHMWPE wear it causes. The effect
of counterface hardness on UHMWPE wear is due to the
fact that hard surfaces are more resistant to scratching
and, therefore, produce less UHMWPE wear, since an
increase in surface roughness leads to an exponential
increase in UHMWPE wear [22-24].
The UHMWPE wear caused by the ZrO2 coating is
consistent with the assumption that softer counterfaces
cause more UHMWPE wear. Additionally, AFM and
SEM observations of the worn surfaces showed that the
coating itself is highly resistant but tends to delaminate
due to deposition defects [25].
The CoCrMo coating has the highest hardness value.
This should make it extremely resistant to scratching (or,
at the very least, more so than the bulk materials exam-
ined). Under the prevailing experimental conditions,
however, the CoCrMo coating produced the highest
UHMWPE wear in this study. Additionally, the AFM
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V. A. González-Mora et al. / J. Biomedical Science and Engineering 4 (2011) 375-382 380
0 2 4 μm
0 2 4 μm
Forged CoCrMo alloy (mass-finished)
(a) (b)
0 2 4 μm
0 2 4 μm
Forged CoCrMo alloy with a CoCrMo
coating
1 m Before After
( a) (b)
Figure 5. AFM images of the hand-polished (forged) CoCrMo alloy and of the CoCrMo coating, before and after the wear test. The
black arrows show the direction of sliding.
10 m
(a) (b)
Figure 6. SEM micrographs of the ZrO2 coating surface. (a) as-received surface showing coating deposition defects; (b) surface after
he wear test showing the failure of the coating by fracture and detachment. t
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V. A. González-Mora et al. / J. Biomedical Science and Engineering 4 (2011) 375-382 381
observation of the worn CoCrMo-coated surface showed
a large number of scratches and therefore greater surface
roughness, which causes high UHMWPE wear. Fur-
thermore, coating fragments may fa vour third-body wear
mechanisms, which also roughen the coating’s surface.
4. CONCLUSIONS
The results obtained with the RPOF wear tests have
demonstrated that the CoCrMo coating caused the high-
est UHMWPE wear of all the counterfaces tested. The
ZrO2 coating and the hand polished (forged) CoCrMo
alloy produced intermediate UHMWPE wear rates and
the mass finished (forged and cast) alloys produced the
lowest UHMWPE wear. The effect of the counterface
hardness in the UHMWPE wear is due to the fact that
hard surfaces are more resistant against scratching and
consequently produces less UHMWPE wear, since an
increase in the surface roughening produces an expon en-
tial increase in the UHMWPE wear.
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