co cr mo alloy

8/12/2019 co cr mo alloy

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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.ME201316


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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.

K. Ueda et al.4


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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.



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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.

K. Ueda et al.6


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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.

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