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Mass Loss and Ion Elution of Biomedical Co-Cr-Mo Alloys
during Pin-on-Disk Wear Tests
Kyosuke Ueda1,+, Kaori Nakaie1, Shigenobu Namba2, Takashi Yoneda3,
Keita Ishimizu4 and Takayuki Narushima1
1Department of Materials Processing, Tohoku University, Sendai 980-8579, Japan2Materials Research Laboratory, Kobe Steel, Ltd., Kobe 651-2271, Japan3Yoneda Advanced Casting Co., Ltd., Takaoka 933-0951, Japan4Kyocera Medical Co., Ltd., Osaka 532-0003, Japan
Pin-on-disk wear tests using biomedical Co-Cr-Mo alloy pins and alumina disks were conducted in Kokubo and 1% lactic acid solutions.
The mass loss and elution of metallic ions were measured and the surface of the pin was observed after the wear test. Mass loss of the alloy pins
in 1% lactic acid solution was 10 times higher than the mass loss in Kokubo solution. In Kokubo solution, the as-cast pins exhibited higher mass
loss and higher total amount of eluted ions than solution-treated pins. The Cr and Mn ion content in Kokubo solution was lower than expected,
based on the chemical composition of the alloy. The incorporation of Cr and Mn ions into the calcium phosphate detected on the wear track of
disks is the possible reason for the small amount of these ions in Kokubo solution. [doi:10.2320/matertrans.ME201316]
(Received March 19, 2013; Accepted May 16, 2013; Published June 21, 2013)
Keywords: cobalt-chromium-molybdenum alloy, precipitate, pin-on-disk wear test, Kokubo solution, lactic acid solution, ion elution
1. Introduction
Cobalt (Co)-chromium (Cr)-molybdenum (Mo) alloys
are frequently used as implants in dental and medical elds
because of their excellent mechanical properties and high
corrosion and wear resistance. The American Society for
Testing of Materials (ASTM) F75 standard permits the
addition of carbon and nitrogen to Co-Cr-Mo alloys at 0.35
and 0.25 mass%, respectively.1) Carbon and nitrogen contentis related to suppression of -phase, stabilization of the
metallic face-centered-cubic (fcc) Co-based phase (-phase),
and strengthening of the material by the formation of carbides
and nitrides. This alloy has been used in the stem, ball and
cup components of articial hip joints in both metal-on-
ultrahigh molecular weight polyethylene (UHMWPE) and
metal-on-metal type implants, as well as in the sliding
components of articial knee joints.
The main concern in using these metal-based hip replace-
ments is the formation of wear debris2) and ion elution,3)
which seem to be closely related to the microstructure and
precipitates of the Co-
Cr-
Mo alloy implants.It is well known that the phase, morphology, size and
distribution of the precipitates affect the properties of Co-Cr-
Mo alloys.4-11) In our previous study, we found that carbon,
nitrogen, silicon (Si) and manganese (Mn) content in the
acceptable range given by ASTM F75 standard, along with
heat treatment, could be used to control the phase of
precipitates.12-19)
In metal-on-metal wear tests of Co-Cr-Mo alloys, many
studies have been reported on the effects of precipitates on
wear behavior. Chibaet al.reported that the marked abrasive
wear was caused by precipitates,20) while other studies
showed that precipitates improved wear properties.5,21) The
combination of Co-
Cr-
Mo alloy and alumina (Al2O3), which
is of the metal-on-ceramic type, is a potential candidate for
articial joints because of its low wear rate.22) As in the case
of metal-on-metal type joints, the wear behavior of metal-on-
ceramic type is speculated to be affected by the precipitates
in the metal, Co-Cr-Mo alloys. There are very few reports
on the wear behavior in metal-ceramic combinations.22,23)
Although Yan et al.23) conducted pin-on-plate wear tests
using wrought Co-Cr-Mo alloy pins and Al2O3 plates and
studied the corrosion behavior of the alloy pins during the
tests, they did not discuss the effects of the microstructureand precipitates in the Co-Cr-Mo alloys on the mass loss and
ion elution.
It has been reported that small amounts of Ni ions tend
to elute from Co-Cr-Mo alloys in spite of their low Ni
content.24) Clarifying the elution of nickel (Ni) ions during
wear tests is an important step towards characterizing the
further suitability of Co-Cr-Mo alloys for medical implants,
due to the toxicity of Ni ions in the human body.25)
In the present study, pin-on-disk wear tests using Co-Cr-
Mo alloy pins and Al2O3disks were conducted using Kokubo
and 1% lactic acid solutions as lubricants. The mass losses in
the pins and ion elution into the lubricants were evaluated.The objective is to clarify the effect of microstructure on the
wear properties and ion elution of biomedical Co-Cr-Mo
alloys.
2. Experimental Procedures
2.1 Specimens
Four types of Co-Cr-Mo alloy ingots (7 mm in diameter
and 50 mm in length) with different Si, Mn, nitrogen and Ni
content were fabricated using an induction melting furnace
with a copper (Cu) mold. Table 1 shows the composition
of the alloys. The carbon content remained constant at
0.25 mass%. To investigate the effect of Ni content on Ni
ion elution, an alloy with a higher Ni content than that of
the ASTM F75 standard was also used. In the present study,
alloy notations were as follows: 0Si0Mn0.3N, 1Si1Mn0.3N,+Corresponding author, E-mail: [email protected]
Materials TransactionsSpecial Issue on New Functions and Properties of Engineering Materials Created by Designing and Processing II
2013 The Japan Institute of Metals and Materials
http://dx.doi.org/10.2320/matertrans.ME201316http://dx.doi.org/10.2320/matertrans.ME201316http://dx.doi.org/10.2320/matertrans.ME201316http://dx.doi.org/10.2320/matertrans.ME2013168/12/2019 co cr mo alloy
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1Si1Mn0N and 1Si1Mn3Ni0N depending on the contents of
added elements. Solution treatment (ST) was conducted at
1508 K for 86.4 ks for complete precipitate dissolution.
The precipitates in the alloys were observed using optical
microscopy (OM, BX60M, Olympus, Tokyo, Japan) and
scanning electron microscopy (SEM, XL30FEG, Philips,
Hillsboro, OR) after electrolytic etching in a 10% sulfuric
acid in ethanol solution. The precipitates were electrolytically
extracted from the alloy in a 10% sulfuric acid solution and
precipitate phases were identied using X-ray diffraction
(XRD, D8 ADVANCE, Bruker AXS K.K., Karlsruhe,
Germany).
2.2 Pin-on-disk wear test
Pin-on-disk wear tests were conducted referring to Japan
Industrial Standards (JIS) T0303 and using Co-Cr-Mo alloy
pins and Al2O3disks. A wear machine designed by our group
can conduct the wear test in lubricant with constant
temperature and collect the lubricant after the test is
completed.26) The Co-Cr-Mo alloy pins were 5 mm in
diameter and 40 mm in length and the contact surface of the
pins was worked to a spherical radius (SR) of 100 mm toavoid stress concentration at the edge of the pins. The Al2O3disks (99.9%, CRA3846-0, KYOCERA, Kyoto, Japan) were
50 mm in diameter and 5 mm thick. Both the contact surfaces
of pins and disks were mirror-polished (Ra < 0.05 m) and
ultrasonically cleaned in ethanol, followed by ultrapure
water for 300 s each. 20mL each of Kokubo solution27)
(pH = 7.4) and 1% lactic acid solution (pH= 2.3) were
used as lubricants maintained at 310 K. The disks rotated
25 mms1 (15.91rpm) at a sliding diameter of 30mm.
The rotation or sliding speed was set at the same value of
a hip joint simulator previously reported.21) Pin load was set
to 1.0 kgf, corresponding to 166 MPa of contact stress as
calculated by Hertzian contact theory.28) To investigate the
effects of wear and stress on metallic ion elution in the
lubricant, a static immersion test was carried out. The Co-Cr-
Mo alloy plates (7 mm in diameter and 2 mm in thickness),
polished with emery papers up to # 1500, were immersed in
either 10 mL of Kokubo solution or 1% lactic acid solutionat 310 K. These solutions with specimens were kept in a
shaking bath at a speed of 0.27 s1.
After the wear test, the pins were ultrasonically cleaned in
ethanol and ultrapure water, and then weighed to evaluate
mass loss. The pin surface was observed with SEM. The
chemical composition of the disk surface was measured with
energy dispersive X-ray spectrometry (EDX). After wear and
static immersion tests, the lubricant was ltered with a
membrane lter (polycarbonate, 0.2 m pore size). The metal
ion concentrations in the ltrate were quantitatively deter-
mined using inductively coupled plasma mass spectroscopy
(ICP-MS, Agilent 7500cx, Agilent, Tokyo, Japan).
3. Results
3.1 Precipitates in pins
Figure 1 shows a representative microstructure of the as-
cast and ST 1Si1Mn0.3N alloys. A dendritic matrix with
interdendritic and grain boundary precipitates were observed
in the as-cast alloy. No precipitates were observed in any of
the ST alloys. The spherical features observed in the image
(Fig. 1(b)) are the pores formed during heat treatment.13)
Figure 2 shows the XRD patterns of the precipitates electro-
lytically extracted from the as-cast alloys. -phase precip-
itates (M2T3X-type precipitate with-Mn structure, where Mand T are metallic elements and X is carbon or nitrogen) 13)
were the main precipitate species in all the alloys. M2X-
type18) and M23X6-type12) precipitates were also detected in
the alloys with and without nitrogen addition, respectively.
The area percents of the precipitates were calculated using
at least three OM images (500). The area percent of each
precipitate was calculated using the ratio of the intensity of
the strongest peak to the sum of the intensities of the
strongest peaks of the precipitates in the XRD patterns.
The area percents of the precipitates are shown in Fig. 3.
Morphologies of precipitates are shown in Fig. 4. The
Table 1 Chemical composition of Co-Cr-Mo alloys used in this study
(mass%).
Notation Co Cr Mo Si Mn Ni C N
0Si0Mn0.3N Bal. 27.97 5.70
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morphology of the -phase precipitates was starlike-dense
with typical sizes of 2 to 5 m. M2X-type precipitates
coexisted with -phase in the lamellar cellular colony.16) As
well, M23X6-type precipitates and -phase exhibited starlike
with patterns of stripes.13)
3.2 Mass loss and surface morphology of pinsThe mass loss of the pins after wear tests in Kokubo and
1% lactic acid solutions is shown in Fig. 5. The mass loss of
the as-cast pins in Kokubo solution was higher than that of
the ST pins, while the mass losses of both the as-cast and ST
pins were comparable in a 1% lactic acid solution. The mass
losses in the 1% lactic acid solution were approximately 10
times higher than those in the Kokubo solution.
The surface morphologies of the as-cast and ST
1Si1Mn0.3N alloy pins after wear tests in Kokubo and 1%
lactic acid solutions are shown in Fig. 6. The wear direction is
top to bottom in the gures. Continuous wear grooves were
observed in Figs. 6(b), 6(c) and 6(d). On the other hand, in
Fig. 6(a), discontinuous deep wear grooves were detected in
the as-cast pin after a wear test in Kokubo solution. Figure 7
depicts the microstructure around precipitates in the as-cast
pins after wear tests. In Figs. 7(a), 7(b) and 7(c) of as-cast pins
after wear tests in Kokubo solution, cavities were observed
next to precipitates, and discontinuous grooves originatedfrom these cavities. This observation suggests that the
discontinuous grooves were formed by wear debris detached
from cavities. On the other hand, for wear tests conducted
in 1% lactic acid solution, such cavities were not observed.
3.3 Total elution of metallic ions
Figure 8 shows the total amount of eluted metallic ions
(Co, Cr, Mo, Si, Mn and Ni) from pins after wear and static
immersion tests. The total amounts of eluted ions after wear
tests were 3 to 4 orders of magnitude higher than the amounts
after static immersion tests. These results indicate that wear
accelerated ion elution, possibly because the passivationlm
on the pin was destroyed during wear, and bare metallic
surfaces were exposed to lubricants. The total amount of
eluted ions from as-cast pins was higher than that for ST pins
in Kokubo solution.
35 40 45
Inten
sity,I(a.u.)
50 55
0Si0Mn0.3N
2(CuK)
1Si1Mn0.3N
1Si1Mn0N
1Si1Mn3Ni0N: M2X
: M23X6
: -phase
: -phase
Fig. 2 XRD patterns of precipitates electrolytically extracted from the as-
cast alloys.
Fig. 4 SEM images of (a) -phase, (b) M23X6-type and (c) M2X-type precipitates.
Areapercentofprecipitates,
Ap
(%)
0
2
4
6
8
10
12
14
0Si0M
n0.3N
1Si1M
n0.3N
1Si1M
n0N
1Si1M
n3Ni0N
M2XM2X M23X6
M23X6
Fig. 3 Area percent of precipitates in the as-cast alloys.
Mass Loss and Ion Elution of Biomedical Co-Cr-Mo Alloys during Pin-on-Disk Wear Tests 3
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4. Discussion
4.1 Formation of cavities and discontinuous grooves
The formation of cavities and discontinuous deep grooves
is likely to cause a higher mass loss and a higher total
elution of ions in Kokubo solution for the as-cast pins than
for the ST pins. Wear debris of both precipitates and metallic
matrix were found in Kokubo solution after wear tests of
as-cast pins. A part of precipitate rst detached from the
metallic matrix, forming cavities during wear tests, and then
caused deep discontinuous grooves (Fig. 6(a)) through
the three-body abrasion. The higher hardness,29) i.e., slower
wear rate, and higher Youngs modulus of precipitates as
compared to the metallic matrix led their detachment during
wear tests in Kokubo solution. The formation of the cavity
and discontinuous groove did not depend on the phase of
precipitates, which suggests that the difference of hardness
and Youngs modulus between precipitates and metallic
matrix are dominant in cavity and discontinuous groove
formation.
Masslossofthe
pin,
Mw
/mg
0
0.2
0.4
0.6
0.8
1.0
0Si0M
n0.3N
1Si1M
n0.3N
1Si1M
n0N
1Si1M
n3Ni0N
(a) : as-cast
: ST
Masslossofthe
pin,
Mw
/mg
0
2
4
6
8
10
0Si0M
n0.3N
1Si1M
n0.3N
1Si1M
n0N
1Si1M
n3Ni0N
(b) : as-cast
: ST
Fig. 5 Mass loss of pins after wear tests in (a) Kokubo solution and (b) 1%lactic acid solution.
Fig. 6 SEM images of the surfaces of (a), (b) as-cast and (c), (d) ST pins of 1Si1Mn0.3N alloy after wear tests in (a), (c) Kokubo and (b),
(d) 1%lactic acid solution.
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In the 1% lactic acid solution, the detachment of
precipitates was not signicant because both precipitates
and metallic matrix worn out relatively uniformly due to the
fact that 1% lactic acid is more corrosive than Kokubosolution. The mass loss of the pins in a 1% lactic acid
solution was much higher than the loss when using Kokubo
solution, while the mass of the total amount of eluted ions
from the pins after wear tests in 1% lactic acid solution was
approximately same as the mass loss of the pins.
Chiba et al. reported that Co-Cr-Mo alloys with -phase
obtained from strain-induced martensitic transformation
exhibited excellent wear resistance because of the increased
hardness.20) In the Co-Cr-Mo alloys, nitrogen addition
stabilizes the -phase.30) On the other hand, in the present
study, XRD and SEM analysis revealed that the formation of
the -phase by strain-induced martensitic transformation
during wear tests was not signicant, even in the alloys
without nitrogen addition. Therefore, there was no signicant
difference in mass loss between the as-cast alloys with and
without nitrogen addition.
4.2 Elution of each metallic ion
The amount of eluted ions for each metallic element is
shown in Fig. 9. Small amounts of Ni ion were detected after
wear tests on 0Si0Mn0.3N, 1Si1Mn0.3N and 1Si1Mn0Nalloy pins, caused by Ni impurities in these alloys.
Decreasing Ni content in these alloys will be necessary to
decrease Ni ion elution and improve the safety of Co-Cr-Mo
alloys.31)
The mass ratios of Co/Cr and Co/Mn for eluted ions from
the 1Si1Mn3Ni0N alloy pin into Kokubo solution were 53.1
and 225.0, which were much higher than the same mass
ratios in the alloy content, 2.1 and 49.3, respectively. This
means that the Cr and Mn ion content in Kokubo solution is
lower than expected, based on the chemical composition of
the alloy. EDX spectra of the surface of the disk on and out of
the wear track are shown in Fig. 10. Calcium and phosphorus
signals were detected on the wear track, which indicates
the formation of calcium phosphate. These species would
precipitate because of the decreasing solubility of calcium
phosphates at the contact points between pin and disk, where
CavitiesPrecipitate(b)
5 m
Precipitate
Cavities
(c)
5 m
(d) SE BSE
2 m
Precipitate
(e) SE BSE
Precipitate
5 m
(f) SE BSE
2 m
Precip- itate
Cavities
(a)
5 mPrecipitate
Fig. 7 SEM images of the surface of the as-cast pins with precipitates of (a), (d)-phase, (b), (e) M23X6-type and (c), (f) M2X-type after
wear test in (a), (b), (c) Kokubo solution and (d), (e), (f ) 1% lactic acid solution.
Mass Loss and Ion Elution of Biomedical Co-Cr-Mo Alloys during Pin-on-Disk Wear Tests 5
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the temperature of the solution locally increased.32) The
calcium phosphate on the disk after the wear test using the
1Si1Mn3Ni0N alloy pin was dissolved using a 0.5% nitric
acid solution and the composition of the calcium phosphate
precipitates were analyzed using ICP-MS. The calcium
phosphate included Co, Cr, Si, Mn and Ni with a mass ratio
of 61.2: 36.2 : 0.5 : 1.5 : 0.6. These results indicated that the
Cr and Mn ions were preferentially incorporated into calcium
phosphate, resulting in the low concentration of these ions inthe Kokubo solution.
In previous study, the formation of calcium phosphate on
the wear track after wear tests of Co-Cr-Mo alloys in Hanks
solution was reported.20) It has also been reported that Cr and
Mn ions were incorporated into the Ca site of calcium
phosphates.33,34) Results obtained in this study agreed with
these previous studies.
5. Conclusion
Pin-on-disk wear tests using biomedical Co-Cr-Mo alloy
pins and Al2O3 disks were conducted in Kokubo and 1%
lactic acid solutions and the effects of alloy microstructure
on mass loss and metallic ion elution were investigated.
Following results were obtained.
(1) The mass loss of Co-Cr-Mo pins in a 1% lactic acid
solution was 10 times higher than that in a Kokubo
solution. In Kokubo solution, as-cast pins exhibited a
higher mass loss and a higher total amount of eluted
ions than ST pins.
(2) Discontinuous deep wear grooves were observed on the
as-cast pins after wear tests in the Kokubo solution. The
discontinuous grooves started from cavities next to the
precipitates. On the other hand, for wear tests conducted
in the 1% lactic acid solution, neither cavities nordiscontinuous grooves were observed.
(3) The total amount of metallic ions eluted during wear
tests was higher than the amount eluted during static
immersion tests. In the Kokubo solution, the total
amount of metallic ions eluted from the as-cast pins was
higher than the amount eluted from the ST pins.
(4) The Cr and Mn ion content in Kokubo solution after
wear tests was lower than expected, based on the
chemical composition of the alloy. The incorporation of
Cr and Mn ions into the calcium phosphate detected on
the wear track of disks is the possible reason for the
small amount of these ions in Kokubo solution.
(5) A small amount of Ni ion elution was detected from
wear tests conducted using the alloy pins where Ni was
not intentionally added. This is likely from Ni impu-
rities in metals used for these alloys.
0
50
100
150
200
0Si0M
n0.3N
1Si1M
n0.3N
1Si1M
n0N
1Si1M
n3Ni0N
(a) : as-cast
: ST
0
0.5
1.0
1.5
2.0
0Si0M
n0.3N
1Si1M
n0.3N
1Si1M
n0N
1Si1M
n3Ni0N
(b) : as-cast
: ST
0
2000
4000
6000
8000
10000
0Si0M
n0.3N
1Si1M
n0.3N
1Si1M
n0N
1Si1M
n3Ni0N
(c) : as-cast
: ST
0
0.5
1.0
1.5
2.0
0Si0M
n0.3N
1Si1M
n0.3N
1Si1M
n0N
1Si1M
n3Ni0N
(d) : as-cast
: ST
Fig. 8 Total amounts of eluted metallic ions (Co, Cr, Mo, Si, Mn, Ni) after (a), (c) wear and (b), (d) static immersion tests in (a), (b)
Kokubo solution and (c), (d) 1% lactic acid solution.
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Acknowledgments
The authors would like to thank Dr. K. Kobayashi of
Tohoku University for analyses of precipitates and wear
debris.
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0
50
100
200
150
0
2
1
3
4
0
10
5
15
20
0
2
1
3
4
0
0.4
0.2
0.6
0.8
0
0.2
0.4
0.6
0.8
8
10
12
Co(a) : as-cast
Amountofelutedions,Mi/g
Amountofelutedions,Mi/g
: ST
Cr
Mo
Si
Mn
Ni
0Si0Mn
0.3N
1Si1Mn
0.3N
1Si1M
n0N
1Si1Mn3N
i0N
1000
2000
5000
3000
4000
0
1000
500
1500
0
200
100
300
400
0
40
20
60
80
0
40
20
60
80
012345
100
150
0
Co(b)
Cr
Mo
Si
Mn
Ni
0Si0Mn
0.3N
1Si1Mn
0.3N
1Si1M
n0N
1Si1Mn3N
i0N
: as-cast: ST
Fig. 9 Amount of eluted Co, Cr, Mo, Si, Mn and Ni ions after wear test in
(a) Kokubo solution and (b) 1% lactic acid solution.
O Al
P
Ca
Cr Co
Out of the wear track
On the wear track
0 1 2 3Kinetic energy, E/ keV
Intensity,I(a
.u.)
4 5 6 7 8
On the wear track
Al2O3disk
Out of the wear track
Fig. 10 EDX spectra of the surface of the disk on and out of the wear trackafter wear test in Kokubo solution.
Mass Loss and Ion Elution of Biomedical Co-Cr-Mo Alloys during Pin-on-Disk Wear Tests 7
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