TA

Ma

b

a

ARRA

KTCNMS

1

of6amm[lr

a

(

0d

Wear 271 (2011) 1210– 1219

Contents lists available at ScienceDirect

Wear

j o ur nal ho me p age: www.elsev ier .com/ locate /wear

ribocorrosion behavior of CoCrMo alloy for hip prosthesis as a function of loads: comparison between two testing systems�

.T. Mathewa,∗, M.J. Runaa,b, M. Laurentb, J.J. Jacobsa, L.A. Rochab, M.A. Wimmera

Section of Tribology, Department of Orthopedic Surgery, Rush University Medical Center, 60612 Chicago, IL, USACenter for Mechanical and Materials Technologies (CT2M), Department of Mechanical Engineering, University of Minho, Azurém, 4800-058 Guimarães, Portugal

r t i c l e i n f o

rticle history:eceived 4 October 2010eceived in revised form 13 January 2011ccepted 13 January 2011

eywords:ribocorrosionoCrMo alloyormal loadetallic implants

ynergism

a b s t r a c t

Metal-on-metal (MOM) hip prosthesis bearings have enjoyed renewed popularity, but concerns remainwith wear debris and metal ion release causing a negative response in the surrounding tissues. Furtherunderstanding into the wear and corrosion mechanisms occurring in MOM hips is therefore essential.

The purpose of this study was to evaluate the tribocorrosion behavior, or interplay between corro-sion and wear, of a low-carbon CoCrMo alloy as a function of loading. The tribocorrosion tests wereperformed using two tribometer configurations. In the first configuration, “System A”, a linearly recip-rocating alumina ball slid against the flat metal immersed in a phosphate buffer solution (PBS). In thesecond configuration, “System B”, the flat end of a cylindrical metal pin was pressed against an aluminaball that oscillated rotationally, using bovine calf serum (BCS) as the lubricant and electrolyte. System Bwas custom-built to emulate in vivo conditions. The tribocorrosion tests were performed under poten-tiostatic conditions at −0.345 V, with a sliding duration of 1800 s and a frequency of 1 Hz. In System Athe applied loads were 0.05, 0.5, and 1 N (138, 296 and 373 MPa, respectively) and in System B were 16,32, and 64 N (474, 597, and 752 MPa, respectively). Electrochemical impedance spectroscopy (EIS) andpolarization resistance were estimated. The total mass loss (Kwc) in the CoCrMo was determined. Themass loss due to wear (Kw) and that due to corrosion (Kc) were determined. The dominant wear regime

for the CoCrMo alloy subjected to sliding changes from wear–corrosion to mechanical wear as the con-tact stress increases. An attempt was made to compare both system, in their tribochemical responsesand formulate some insights in the total degradation processes. Our results also suggest that the proteinsin the serum lubricant assist in the generation of a protective layer against corrosion during sliding. Thestudy highlights the need of adequate methodology/guidelines to compare the results from different testsystems and translating in solving the practical problems.

. Introduction

Metal-on-metal (MOM) bearings currently constitute about 35%f over 200,000 primary total hip replacement procedures per-ormed annually in the US, a number that is expected to approach00,000 by 2030 [1,2]. However, there are increasing reports ofdverse local tissue responses mediated by degradation products –etal ions and wear debris – generated by wear and corrosion ofetal-on-metal total hip replacements and surface replacements

2]. These degradation products can cause hypersensitivity, toxico-ogical risk to systemic and remote sites and periprosthetic boneesorption [3–5].

� Authors would like to state that the paper received “Kent Ludema”, best paperward at WOM 2011, held at Philadelphia, April 2011.∗ Corresponding author. Tel.: +1 312 942 8310; fax: +1 312 942 4491.

E-mail addresses: mathew t mathew@rush.edu, mathewtmathew@gmail.comM.T. Mathew).

043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2011.01.086

© 2011 Elsevier B.V. All rights reserved.

The great majority of MOM bearings are made of CoCrMo alloys.These alloys have been extensively used in biomaterials for jointreplacement due to their wear and corrosion resistance. A protec-tive Cr oxide (Cr2O3 or other oxidation states) film forms on thesurface of the alloy that inhibits corrosion and the release of metalions [6,7]. The degree of protection depends on the composition ofthe oxide film, which in turn depends on the body fluids [8]. Onthe bearing surfaces, there is in addition the synergistic effect ofwear and corrosion, i.e., tribocorrosion that can markedly increasematerial loss [9–11]. Thus, the total material loss (Kwc) can be muchhigher than the material loss due to pure corrosion (Kc), without theinfluence of wear, or the material loss due to wear in the absenceof corrosion (Kw) [12]. According to Stack and Abdulrahman [13],the dominant regime for material loss in the system can be inferred

from the value of the Kc/Kw ratio, where Kc is calculated with Fara-day’s law from the current measured during the test. A value in therange of 0.1–1 corresponds to a wear-enhanced corrosion mecha-nism, whereas lower values point to a mechanism dominated by

M.T. Mathew et al. / Wear 271 (2011) 1210– 1219 1211

Table 1Source, Rockwell hardness, and elemental composition of the low-carbon wrought CoCrMo alloy used in this study.

Samples Source Original roddiameter (mm)

RockwellC hardness

Chemical composition (wt%)

C Co Cr Mo Si Mn Al

mcc

etlmfecRftcrrht[

buosamc

2

2

wesfia(drtIctrav1msnaccn

USA), the Randles EIS equivalent circuit (Fig. 1a) was used to deter-mine the polarization resistance (Rp) for System B and System Abefore sliding tests. For System A, a specially designed equivalentcircuit was used to model impedance data after sliding test (Fig. 1b).

Table 2Chemical composition of PBS solution, used in System A (total amount: 1 L).

NaCl (g/L) KH2PO4 (g/L) KCl (g/L) Na2HPO4 (g/L)

8.18 0.14 0.22 1.42

Table 3

System A SURGIVAL (Spain) 20 38

System B ALLVAC (USA) 29 42

echanical wear. Further, the value in the range of 1–10 indicatesorrosion enhanced wear mechanism and above 10 corresponds toorrosive process [13].

Although the corrosion resistance of CoCrMo alloys has beenxtensively investigated, little work has been performed to dateo evaluate their tribocorrosion behavior. Yan et al. [6] found thatoad and articulations could increase the corrosion rate and the

etal ion release processes. It was noticed that main ions releasedrom the tested materials were Co ions. They also observed thatlectrochemical methods can affect the protein adsorptions pro-ess, resulting in the transition of wear and corrosion mechanisms.ecently, it has also been determined that CoCrMo hip bearing sur-

aces undergo microstructural changes and chemical reactions withhe joint environment during articulation that produce a mechani-ally mixed zone of nanocyrstalline metal and organic constituents,eferred to as a biotribolayer. This layer appears to be critical toeducing wear and corrosion [14]. Triboelectrochemical studiesave been performed using various sliding contact test configura-ions that include pin-on-disk, pin- or ball-on-flat, and ring-on-disk15].

The purpose of this study was to evaluate the tribocorrosionehavior as a function of load for a low-carbon CoCrMo alloy bysing two different test set-ups, one a linearly reciprocating ball-n-flat configuration and the other a custom-made pin-on-balletup that more closely simulates the hip in vivo conditions. Welso sought to contrast two distinct test configurations, and deter-ine to what extent the simpler configuration matched the more

omplex in vivo-like configuration.

. Materials and methods

.1. Overview

The experimental design for this study consisted of using twoear test configurations or tribo-systems to determine the param-

ters related to the tribocorrosion of a low-carbon CoCrMo alloyubjected to sliding against an alumina counterface. In the first con-guration, “System A”, a linearly reciprocating alumina ball slidgainst the metal flat immersed in a phosphate buffer solutionPBS). In the second configuration, “System B”, the flat end of a cylin-rical metal pin was pressed against an alumina ball that oscillatedotationally, using bovine calf serum (BCS) as the lubricant and elec-rolyte. System B was custom-built to emulate in vivo conditions.n both systems, the test chamber doubled as an electrochemicalell, with the CoCrMo component as the working electrode. All theribocorrosion tests were performed in triplicate (n = 3), to checkeproducibility, under potentiostatic conditions at −0.345 V, with

sliding duration of 1800 s and a frequency of 1 Hz. The main inputariable was load. In System A, the applied loads were 0.05, 0.5, and

N (138, 296, and 373 MPa initial Hertzian contact stress), chosenatch the generally used loads in such tribometer in tribocorrosion

tudies. In addition, such low loads might assist in investigating theature of the passive film. In system B, the applied loads were 16, 32

nd 64 N (474, 597 and 752 MPa (these values are initial Hertzianontact pressure)). After running-in period, with more conformingontact, this corresponded to approximately 15, 30, 60 MPa in theormal hip joint during the daily activities [16,17]. The numerical

0.04 64.81 27.82 5.82 0.36 0.78 <0.020.03 64.96 27.56 5.70 0.38 0.60 <0.02

output variables were the polarization resistance, the total mate-rial loss (Kwc), the loss due to mechanical wear (Kw) and the lossdue to corrosion or “chemical wear” (Kc). Details of the materialsand wear test configurations and test conditions follow.

2.2. The CoCrMo specimens

The rods were originated from two sources, despite havingalmost identical elemental compositions and hardness (Table 1).The CoCrMo specimens used in System B were machined from rodsof a low-carbon (LC) CoCrMo wrought alloy from Alloy 1 specifica-tion in ASTM Standard F1537-07. The tested specimens consistedof disks 20 mm in diameter and 3.67 mm thick for the ball-on-platesystem (System A) and 12 mm in diameter and 7 mm in thicknessfor the pin-on-ball system (System B). The tested surfaces weremechanically polished to a mirror finish (Ra = 0.002 ± 0.01 �m),cleaned with propanol in an ultrasonic bath, rinsed with distilledwater, and dried using warm air prior to testing.

2.3. The electrolytes

The electrolyte used for the ball-on-plate configuration (SystemA) was phosphate buffered solution (PBS) whereas the elec-trolyte and lubricant used for the pin-on-ball configuration (SystemB) was bovine calf serum (BCS), supplied by Invitrogen corpo-ration and diluted with a buffered saline solution to have aprotein concentration of 30 g/L. Their compositions are given inTables 2 and 3, respectively. The pH of both electrolytes wasadjusted to 7.65 ± 0.04, to be comparable to the pH of human jointfluid (synovial fluid).

2.4. Tribocorrosion tests

2.4.1. Common electrochemical protocolThe protocol for all the tribocorrosion tests entailed three

phases: initial stabilization before sliding test, sliding test, and finalstabilization after sliding test. Electrochemical impedance spec-troscopy (EIS) measurements were performed before and aftersliding using two potentiostats: model Reference 600, for System A,and G300, for System B (Gamry Instruments, Warminster, PA, USA).Using the ZView software (Scribner Associates, Southern Pines, NC,

Chemical composition of BCS solution, used in System B (total amount: 600 mL).

NaCl (g/L) EDTA (g/L) Tris (g/L) Protein (g/L)

9 0.2 27 30

1212 M.T. Mathew et al. / Wear 271 (2011) 1210– 1219

a

b

Rsol Rp

CPE

Rsol CPEout

Rpout L

R

Fda

sftEif(atc

22ttm(t(wtdc−mt

FP

The RE used was the same as in System A. A graphite rod was usedas the counter electrode. The area of the sample exposed to theelectrolyte was 1.13 cm2. Tests were also performed at Ecorr andunder potentiostatic conditions with the same applied potential.

ig. 1. Schematic diagram for the three-element Randles equivalent circuit used toetermine the polarization resistance (Rp). Rsol represents the solution resistancend Cdl represents the double layer capacitance.

The applied anodic potential of −0.345 V vs. SCE for the potentio-tatic conditions was chosen based on the potentiodynamic curverom the initial corrosion tests (Fig. 2) to represent a passive poten-ial of the CoCrMo alloy in BCS and PBS. The corrosion potential,corr, and the corrosion current density, Icorr, were obtained accord-ng to the Tafel’s slope method and tabulated in Fig. 2. The changesrom cathodic to anodic reactions occur at the corrosion potentialEcorr). Above this potential the cathodic reactions are negligiblend the current is determined by the kinetics of metal oxidation,he anodic reactions. A passive region can also be observed in bothurves.

.4.2. The tribometers

.4.2.1. System A, conventional CETR tribometer. A CETR tribome-er (Model UMT-2, CETR, Campbell, California, USA) was used inhe ball-on-plate configuration, whereby a 10 mm diameter alu-

ina ball slid against a CoCrMo disk in a linearly reciprocating pathFig. 3a). The stroke length was 5 mm. A saturated calomel elec-rode (SCE) was used as the reference electrode (RE), a platinumPt) electrode as counter electrode (CE) and the CoCrMo disc as aorking electrode (WE), as shown in Fig. 3b). The active area of

he working electrode was 2.29 cm2. Tests were performed in twoifferent electrochemically controlled techniques: (i) free potentialondition (E ) (ii) potentiostatic test with an applied potential of

corr

0.345 V vs. SCE. Each test began with a cathodic cleaning treat-ent (potentiostatic condition at −0.8 V vs. SCE) with the purpose

o remove oxides that were air-formed at the surface. EIS mea-

10-11 10-9 10-7 10-5 10-3-1,5

-1,0

-0,5

0,0

0,5

1,0

1,5

2,0

PBS BCS

Pot

entia

l (V

vs.

SC

E)

Current densi ty (A/ cm2)

Appli ed Poten tial-0.345V

Ecorr Icorr

-0.51 44 3.4 339e-9-0.86 61 2.7 504e-6

ig. 2. Polarization curve of low carbon CoCrMo alloy in PBS and BCS solutions.otential range from −0.8 to 1.8 V, scan rate of 1 mV/s.

Fig. 3. Depiction of the conventional CETR tribometer for System A. (a) Tribometerset-up. (b) Test area. (c) Top view of the test area.

surements were carried out in a frequency range from 63 kHz to0.001 Hz with 10 frequency/decades within. Each test was startedwith a fresh alumina ball surface.

2.4.2.2. System B, customized tribosystem. This tribosystem entaileda pin-on-ball configuration in which the flat end of a cylindri-cal CoCrMo pin was loaded against the equator of a rotationallyoscillating 28 mm diameter alumina ball (Fig. 4). The oscillationfrequency was 1 Hz, with a ball rotation of ±15◦ for 1800 cycles.

Fig. 4. Depiction of the pin-on-ball custom built tribometer, System B, used in thisstudy. (a) The tribometer set-up. (b) The test area. (c) Top view of the test area.

M.T. Mathew et al. / Wear 271 (2011) 1210– 1219 1213

Table 4Test conditions and a comparison between Systems A and B.

Similarities Differences

System A System B

Applied potential −0.345 V Pin-on-plate Ball-on-flatSliding time 1800 s Reciprocating motion Oscillatory/rotary motionNo. of cycles 1800Frequency 1 Hz Area exposed to electrolyte (2.28 cm2) Area exposed to electrolyte (1.13 cm2)Protocol Three phases Low loads (0.05 N, 0.5 N, 1 N) High loads (16 N, 32 N, 64 N)Material (sample) Low carbon-CoCrMo PBS solution (30 mL) BCS solution (150 mL)Counterbody Ceramic ball Total distance (18 m) Total distance (26.4 m)Average velocity Sliding distance (10 mm/s) Sliding distance (14.7 mm/s)Velocity profile Sinusoidal Horizontal position of the plate causes the wear debris to

ct zonVertical position of the pin causes the release of the wear

Tita

a

2

t[

off8b8

F

K

wescq

FD

spread in the vicinity of the conta

he cleaning process was performed at −0.9 V vs. SCE. Electrochem-cal impedance was carried out in a frequency range from 63 kHzo 0.001 Hz, with 10 frequency/decades. Each test was started with

fresh alumina ball surface.The test conditions and a comparison of tribometer Systems A

nd B are given in Table 4.

.5. Mass loss due to wear and corrosion

The total mass loss (Kwc) during tribocorrsion is the sum ofhe loss due to wear (Kw) and that due to corrosion (Kc), so that13]:(1)Kwc = Kw + Kc

To obtain the total mass loss, Kwc, topographical measurementsf the wear scar were made using a scanning white light inter-erometry microscope (Zygo Corporation, Middlefield, CT, USA),rom which the wear volume was calculated using the MetroPro

software (Zygo Corporation). The mass loss was then calculatedy multiplying this volume by the density of the CoCrMo alloy,.30 g/cm3 [18].

The mass loss (in grams) due to corrosion was estimated fromaraday’s law

c = q × M

n × F(2)

here q is the charge in coulombs passed through the working

lectrode, M (g/mol) is the atomic weight of the element being dis-olved, n is the dissolution valence (in this study, n = 2 was used foralculations) and F is Faraday’s constant (96.490 C/mol). The charge

was calculated by integrating the current i measured during the

0 50 0 1000 150 0 2000 2500-2,0x10

0,0

2,0x1 0

4,0x1 0

6,0x1 0

8,0x1 0

1,0x1 0

1,2x1 0

Cur

rent

(A)

Time (sec)

a b

Friction Coefficient

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0

Curr ent

Friction Co eff icient

1NStart End

Z Z’

p

ig. 5. Evolution of the current and the friction coefficient during a 30 minute sliding ruotted line (Z–Z′) shows zero current.

e debris to the solution (under gravity force)

test over time t (see Fig. 5). Hence, the weight loss due to wear isdetermined by Eq. (3)

Kw = Kwc − Kc (3)

2.6. Surface characterization

Morphological characterization of the surface was carried outusing different techniques: scanning electron microscopy (SEM)and energy dispersive X-ray spectroscopy (EDS) (Model-Joel JSM-6490 LV, Oxford Instruments, England), white light interferometry(Zygo Corporation, Middlefield, CT, USA).

3. Results

3.1. Evolution of current and friction coefficient during the slidingtest

During the tribocorrosion tests, the evolution of current and fric-tion coefficient were monitored as a function of time are shownin Fig. 5 for the maximum loads (1 N for System A and 64 N forSystem B). When the sliding starts, the current abruptly increasesto a higher value, corresponding to a sudden increase in the cor-rosion rate of the exposed surface as sliding removes the passivefilm and the surface left behind becomes unprotected. When slid-ing stops, the current decreases abruptly to a value similar to theinitial one, as the metal in the mechanically activated area repas-

sivates. The oscillation of the current and friction coefficient arisefrom the depassivation and repassivation of the metal surface, andfollow the cyclic motion (Fig. 5). In addition, the test configurationand electrolyte influenced the evolution of the current and friction

1500 200 0 250 0 30 00 3500 400 0

-8,0 x10

-6,0 x10

-4,0 x10

-2,0 x10

2,0x10

4,0x10

Cur

rent

(A)

Time (sec)

Current

Friction Coefficient

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0

Friction Coefficient

64N

End Start

Z Z’

Q

n at (a) 1 N in PBS solution (System A) and at (b) 64 N in BCS solution (System B).

1214 M.T. Mathew et al. / Wear 271 (2011) 1210– 1219

10 10 10 10 10 1010

10

10

10

10

10

10

Before Sliding Aft er Sliding

phase angle (degrees)

|Z| (Ω

.cm

2 )a b

1N

0

10

20

30

40

50

60

70

80

90

10 10 10 10 1010

10

10

10

Before sliding Af ter Sliding

0

10

20

30

40

50

60

|Z| (Ω

.cm

2 )

phase angle (degrees)64N

Phase angle

Imped ance

Phase angl e

Impedan ce

more stab elbats sselel

F (a) 1 N

cpr(fpora[ittTiid

3

vowiwepsllss

btlcdtciewd

with wear debris in the contact zone.Examination of the wear scars by SEM revealed some pitting

corrosion at the motional ends of the wear scars (Fig. 9a and c).

fre quency (Hz)

ig. 6. Bode plot from electrochemical impedance spectroscopy measurements for

oefficient. In System A, the current was anodic throughout andeaked midpoint in time (Fig. 5a), whereas in System B, the cur-ent gradually increased to more anodic values throughout the testFig. 5b). The current increase during the first half of the sliding runor System A is thought to be associated with further damage of theassive layer, an increase in the wear scar width, and rougheningf the surface. The decrease in current during the second half of theun suggests the formation of a partially protective film, perhapsssociated with oxidized and compacted wear debris tribomaterial19]. Similar decreases in current with time have been observedn the tribocorrosion of titanium oxycarbide films [20]. For Sys-em B, the gradual increase in the current may be connected withhe gradual increase of the wear scar area as the test progresses.he current increase is not smooth probably because the current ismpacted by the uneven accumulation and egress of the wear debrisn the contact zone and by the dynamic nature of film formation andestruction on the metal surface.

.2. Electrochemical impedance data before and after the sliding

The bode plots (impedance |Z| vs. frequency and phase angles. frequency) are presented in Fig. 6. For both solutions, onlyne time constant can be seen in the phase angle (Fig. 6, lineith symbol) at lower frequencies. At the highest frequencies, the

mpedance (|Z|) and phase angle values tend to become constant,hich is characteristic of resistive behavior and arises from the

lectrolyte resistance [16,17]. It confirms the presence of a com-act, homogeneous and protective passive film on the surface. Afterliding, the impedance has decreased in System A (Fig. 6a, greyine, <0.01 Hz), whereas it has increased in System B (Fig. 6b, greyine, <0.01 Hz). The increase denotes a slowing down of the corro-ion kinetics, possibly due to the presence of proteins in the BCSolution.

For all applied loads, the polarization resistance, Rp, was higherefore sliding than after sliding for System A, while the reverse wasrue for System B. This is shown graphically in Fig. 7 for the highestoad in each system. The low Rp after sliding (Fig. 7a) indicates poororrosion resistance of the surface in System A, which might beue to the large area of the worm surface (no passive film) andhe presence of wear debris in the vicinity of the contact zone. Inontrast, the high Rp after sliding (Fig. 7b) in System B indicates

mproved corrosion resistance of the surface, perhaps connected toxposure to the proteins in solution. The constant coverage of theorn area by the alumina counterface and the possibility for wearebris to fall away might be other reasons for this observation [19].

frequen cy (H z)

in PBS solution and for (b) 64 N in BCS solution, Systems A and B, respectively.

3.3. Wear scar profile and surface characterization

The wear scars shapes were consistent with the motion andshape of the alumina counterface. For System A, they were thereforegrooves with an approximately circular arc cross-section (Fig. 8a),whereas for System B, they consisted of depressions ranging fromalmost spherical at low loads to ellipsoidal at high loads (Fig. 8b).The ellipsoidal shape stemmed from a slight lateral movementof the rotating ball against the pin when subjected to a highcontact pressure. For both systems, the wear volumes could bereadily determined from topographical measurements, and thecorresponding weight losses are given in Table 5. The longitudinalscratches seen in the wear scars from System A may be associated

Fig. 7. Polarization resistance before and after sliding at the highest normal load in(a) System A and in (b) System B.

M.T. Mathew et al. / Wear 271 (2011) 1210– 1219 1215

Fig. 8. 3-Dimensional image of the wear scar using Zygo Microscope for (a) System A and (b) System B, with 1 N and 64 N, respectively.

Table 5Wear–corrosion volume loss for the highest normal loads of both systems.

Normal load Kwc [�g] Kw [�g] Kc [�g] Kc/Kw

System A0.05 N 0.08 ± 0.12 0.07 ± 0.10 0.01 ± 0.02 0.130.5 N 5.10 ± 3.03 3.83 ± 2.68 1.28 ± 0.36 0.301 N 7.09 ± 7.65 5.10 ± 8.03 2.00 ± 0.40 0.35

System B16 N 4.13 ± 0.22 3.06 ± 0.16 1.07 ± 0.05 0.3532 N 7.31 ± 0.36 5.68 ± 0.89 1.63 ± 0.08 0.2964 N 19.40 ± 0.99 18.87 ± 1.06 0.52 ± 0.03 0.03

Fig. 9. SEM micrographs of the wear scars on the CoCrMo surface for System A (left side, (a) and (b)) at 1 N and for System B (right side, (c) and (d)) at 64 N. (e) Typical EDSspectrum for pits and passive film on the surface.

1216 M.T. Mathew et al. / Wear 271 (2011) 1210– 1219

02468

0.0 0.4 0.8 1.2

a b

Wei

ght l

oss [

μg]

Kwc Kc Kw

05

10152025

0 15 30 45 60 75

Loa ds (N)

Wei

ght l

oss [

μg]

Kwc Kc Kw

tion f

Twdayoca[

3

Kai

icmcwitnb

4

c

Loads (N)

Fig. 10. Total weight loss distribu

hese localized imperfections on the oxide layer were made byear debris pushed to the edge where the movement stops. Theebris damages the protective film, leading to pitting corrosionnd corrosion products rich in oxygen as verified by EDS anal-sis (Fig. 9e). Inside the wear track, wear debris particles werebserved (Fig. 9b and d), even though those surfaces had undergoneleaning in an ultrasonic bath prior to examination. These stronglydhering particles can induce current variations during the sliding19].

.4. Weight loss distribution as a function of load

The weight loss distribution in terms of Kwc (total weight loss),c (weight loss due to corrosion), and Kw (weight loss due to wear)s a function of load is shown graphically in Fig. 10 and tabulatedn Table 5 for both systems.

The total weight loss (Kwc) and weight loss due to wear (Kw)ncrease with load in both systems. Although the weight loss due toorrosion increases with load for System A, it is highest at the inter-ediate load of 32 N for System B. The contribution of corrosion is

onsistently small compared with the contribution of mechanicalear (Kwc and Kw, in Fig. 10). Because metal loss due to corrosion

s estimated from the current measured during the tribocorrosionest, the influence of current from the unworn area should not beeglected [19]. In this study, it is compensated using the currentefore sliding as the zero point.

. Discussion

In this study we evaluated the tribocorrosion behavior of a low-arbon CoCrMo alloy, two test systems, namely, a conventional

wc

wc wc

0.00

0.20

0.40

0.60

0.80

1.00

0.05N 0. 5N 1N

Kc/

Kw

wear-corros ion region(0.1 ≤ Kc/Kw ≤ 1)

(150 MPa) (320 MPa) (410 MPa)

System A

Gradua l increm ents Gradu

Fig. 11. Synergistic interactions for each load of two systems. Grey b

or (a) System A and (b) System B.

reciprocating sliding system (System A) and a specially designedtribosystem emulating to some extent the hip joint contact condi-tions (System B). System A is relevant from a practical point of viewbecause it has been used by various research labs. We thereforealso sought to contrast the two test configurations and determineto what extent the simpler configuration matched the more com-plex in vivo-like configuration, with an aim to capture the key testparameters for evaluating the tribocorrosion behavior of CoCrMoalloys used for joint bearings.

The much higher currents observed in System A comparedto System B during sliding (Fig. 5) despite the lower normalloads and electrolyte salt concentration suggest that the proteinsmay have a strong protective effect against tribocorrosion. Themarkedly higher friction coefficient in System A (0.50) comparedto System B (0.25) also suggests that the proteins are acting aseffective boundary lubricant. The electrochemical impedance mea-surements indicate the presence of a compact, homogeneous andprotective passive film on the surface for both systems. Evidencefor this film is seen in SEM micrographs (Fig. 9). However, theimpedance decrease after sliding at low voltage excitation frequen-cies observed in System A (Fig. 6a) suggests that the film formedmay offer less protection after sliding than before. The correspond-ing increase in impedance for System B (Fig. 6b) suggests theopposite, i.e., a more protective film exists after sliding than before.These impedance results are consistent with the trends observedfor the polarization resistance, Rp. Its decrease after sliding for Sys-tem A (Fig. 7a) indicates decreased corrosion protection, whereas

its increase for System B (Fig. 7b) indicates increased protection.The latter is consistent with the formation of protective tribolayer.

The extent and shapes of the wear scars are indicative of theconsiderable difference in the motions and lubrication in the two

wc ww

16N 32N 64N

System B

wear region (Kc/Kw ≤ 0.1)

(515 MPa) (65 0 MPa) (81 5 MPa)

Transition

al reducti on

ar: wear–corrosion regime (wc); Black bar: wear regime (w).

ear 271 (2011) 1210– 1219 1217

sCadtemaSaau

doisTtwtatt

4

ats[aotcc

sKtn

Table 6Wear factors for Systems A and B.

Normal load K – wearcoefficient (Kwc)

K – wearcoefficient (Kw)

System A0.05 N 0.0908 0.10420.5 N 0.5671 0.42521 N 0.3940 0.2832

M.T. Mathew et al. / W

ystems. In System A, in which an alumina ball slides against aoCrMo flat (Fig. 3), the horizontal reciprocating motion (back-nd-forward) causes the oxide film on the metal to be constantlyestroyed and re-formed. When the pin goes forward, it removeshe protective film and the clean metal left behind can corrode moreasily. It yields galvanic coupling of two distinct surface states of theetal: the passive metal (unworn area) and the bare metal (worn

rea) exposed to the solution by abrasion of the passive film. Inystem B, in which an alumina ball rotates right-to-left against

CoCrMo flat (Fig. 4), the contact zone is a small and ellipticalrea (Fig. 8b) that restricts access of the electrolyte to corrode thenprotected surface [20,21].

In addition, the mechanical and electrochemical mechanismsuring rubbing lead to the release of metallic wear particles, asbserved in Fig. 8c. Those detached particles could form third bod-es and be ejected from the contact, and/or be spread on the metalurface, resulting in the formation of solid oxides or dissolved ions.hus, in System A there is accumulation of wear particles aroundhe wear scar on the unworn area. Those small localized particlesill remove the protective film in that unworn area, inducing pit-

ing corrosion, increasing the size of the corroded area and the totalmount of corrosion. On the other hand, in System B the wear par-icles have a greater chance to disperse into the electrolyte underhe influence of gravity [19,22].

.1. Synergistic interactions

A point of particular interest in evaluating tribocorrosionre the synergistic interactions between wear and corrosion, ashey will affect the tribological mechanisms and could have aignificant influence on the amount of material loss. Stack et al.9,13] determined that the ratio Kc/Kw of the chemical wear (Kc)nd mechanical wear (Kw) provides a criterion for the magnitudef this synergism and the ensuing dominant regime present in aribocorrosion system. The value of this ratio of corrosion-to-wearontribution is indicative of the synergistic interaction and isonnected to the dominant wear mechanism as follows [13]:

Ratio of corrosion-to-wear contribution Degradation mechanism

Kc/Kw ≤ 0.1 Wear0.1 < Kc/Kw < 1 Wear–corrosion1 < Kc/Kw ≤ 10 Corrosion–wearKc/Kw > 10 Corrosion

The values of Kc/Kw for Systems A and B are given in Table 5 and

hown graphically in Fig. 11 as a function of load. In System A, thec/Kw ratio lies in the range of 0.1 < Kc/Kw < 1 for all loads, indicatinghe mass loss mechanism is a wear dominated corrosion mecha-ism (wear–corrosion). In System B, a wear–corrosion mechanism

y = 0.7874 x + 0.68 64

0

2

4

6

8

0 2 4 6 8 10

Accumulate d Dissipat e d e ne rgy (J )

a

Wei

ght l

oss

(ug)

Fig. 12. The evolution of weight loss as a function of

System B16 N 0.0105 0.008032 N 0.0086 0.006664 N 0.0111 0.0114

also dominates for the first two loads. Then there is a transition towear dominated degradation mechanism at the highest load. It is ofinterest that the Kc/Kw ratio increases gradually with load for Sys-tem A, but decreases with load for System B, suggesting that thereis a contact stress which maximizes synergism and that mechan-ical wear can dominate at both low and high loads. The latter isunderstandable because the corrosion rate is limited by its kineticswhereas the mechanical wear can continue to increase with load.Further, the increase in corrosion with increasing load in System Amay be partly associated with frictional heating effects. The transi-tion from wear–corrosion to wear dominated regime is importantbecause it may lead to the destruction of a bio-tribolayer that isconsider to increase the wear resistance of CoCrMo alloys [14]. Itis therefore important to determine the critical load at which suchtransition is expected and to compare it with the maximum loadsexpected for the system.

The two systems may also be compared with respect to theirwear factors (Table 6). The wear factors for System A are on aver-age 31 times those for System B, suggesting that the dominantwear mechanism is different in the two systems, perhaps due tothe presence of proteins in System B. However, other factors, suchdifferences in motion and contact stress may also be significant.Feasibly a more fundamental comparison of the two systems mustbe made by considering the total wear as a function of the totalor accumulated dissipated energy [23–25], obtained by integratingthe frictional force over the sliding distance. The wear vs. accumu-lated dissipated energy curves downward for System A (Fig. 12a)but is fairly linear for System B (Fig. 12b), suggesting that in SystemA there is a mechanism rendering material removal less efficientat the highest load. A possibility is that wear particles remainingwithin the wear area protect the surface, either by re-adhering toit or acting as mini ball bearings. The average slope for System A is17 times that for System B, which is lower than the correspondingratio for the wear factors (31), suggesting that the dissipated energy

offers a somewhat better basis for comparison of the two systems.

The comparison of results even for tribocorrosion tests followingsimilar protocols can be difficult. Thus, Mischler et al. [22], con-ducted an multicenter study with seven laboratories in Europe that

by = 0.04 74x + 1.53 33

0

5

10

15

20

25

0 10 0 20 0 30 0 40 0 50 0Acc umulated Diss ipated energy (J)

Wei

ght l

oss

(ug)

dissipation energy for System A and System B.

1 ear 2

etafesaat

4

cwwbvtitbcftew3pat

5

fblifipbwf

•

•

•

[

[

[

[

[

[

218 M.T. Mathew et al. / W

ntailed a prescribed protocol to assure similar conditions with theribometers, test systems, materials, environment, operating vari-bles, surface cleaning, and electrochemical measurements. It wasound that no clear correlation existed between any single param-ters and the measured wear rates, but that the current duringliding was closely related to and increased with the wear trackrea. Consistent with this finding, System A has greater wear trackrea and corrosion (current) than System B, but the difference elec-rolytes confound.

.2. Limitations

Because one of the objectives of this study was to compare aonventional test configuration as used by previous researchersith a test configuration geared to hip bearing applications, thereere multiple test conditions that were simultaneously different

etween the two configurations, making it impossible to decon-olute the effect of each variable. Thus, the difference in motions,he lubricants, normal loads, and contact stresses were differentn the two systems, so determining what was the effect of each ofhese variables when comparing the two systems was not possi-le. Also, although both tests were conducted under potentiostaticontrol at −0.345 V vs. SCE, there was a possibility of a slight shiftrom this value [19,22] during the tribocorrosion tests due to theribochemical events at the surfaces. In using Faraday’s law for thestimation of the mass Kc due to corrosion, the value n = 2 was used,hereas the true value lies somewhere between 2 (Co → Co(II)) and

(Cr → Cr(III)). In addition, Ponthiaux et al. [19] highlighted theossible electrochemical interaction between worn and unwornrea during the sliding, leading to the presence of galvanic couplehat may impact corrosion processes.

. Conclusions

The tribocorrosion behavior of a low-carbon CoCrMo alloy usedor hip bearing applications was evaluated using two distinct tri-ometer configurations. In the first configuration, “System A”, a

inearly reciprocating 1 alumina ball slid against the metal flatmmersed in a phosphate buffer solution (PBS). In the second con-guration, “System B”, the flat end of a cylindrical metal pin wasressed against an alumina ball that oscillated rotationally, usingovine calf serum (BCS) as the lubricant and electrolyte. System Bas custom-built to more closely emulate in vivo conditions. The

ollowing conclusions were drawn:

The tribocorrosion behavior of the CoCrMo alloy is influenced bythe test system and required to be considered while interpretingthe result.It was more favorable in System B, which was closer to in vivoconditions. Thus, comparing System B to System A:◦ The electrochemical impedance after sliding increased,

whereas it decreased in System A.◦ The polarization resistance increased, rather than decreased as

in System A, indicating a protective effect.◦ The friction coefficient in System B was lower than that in Sys-

tem A (approximately half in the case of highest load 1 N and64 N).

◦ The wear factor and energy dissipation per unit mass loss wereover an order of magnitude lower in System B than A.

Except at the normal highest load, the dominant mass lossmechanism was wear–corrosion, suggesting marked synergism

between wear and corrosion. At the highest load, 64 N in SystemB, the dominant mechanism was mechanical wear. Thus, there isa transition from wear–corrosion to mechanical wear somewherebetween 32 and 64 N.

[

71 (2011) 1210– 1219

• The more favorable tribocorrosion behavior of the alloy in SystemB despite the higher contact stresses may stem from the proteinsin the electrolyte lubricant providing boundary lubrication andassisting in the formation of a biotribolayer [14]. Other factorsinclude less direct exposure of the worn area to the electrolyte.

• The results for System B suggest that the dominant mass lossmechanism in metal-on metal bearings is wear–corrosion.

• The notable differences between the two systems indicate thatemulating key aspects of the in vivo conditions is important.

Further work is required to identify all the key factors impactingthe tribocorrosion of CoCrMo in the context of clinical applications.The identification of these factors may allow the formulation of asimplified, canonical tribocorrosion test that embodies the neces-sary aspects to evaluate alloys for hip bearings. An extensive studyis planned to address such issues.

Acknowledgements

This study was funded by Luso-American Foundation (FLAD)grant in Portugal and National Institutes of Health (NIH), RC2(1RC2AR058993-01) grant in Chicago. A special thanks to Prof. A.Fischer (University of Duisburg-Essen, Germany) for valuable dis-cussion and Prof. R. Urban and Ms. D. Hall for the assistance in SEMcharacterization. Acknowledgments also go to Dr. Thomas Pandorfof Ceramtech, Plochingen, Germany for providing ceramic heads.

References

[1] Morbidity and Morality Weekly Report, MMWR, Prevalence and Most Com-mon Causes of Disability Among Adults – United States, Department of Healthand Human Services, Centers for Disease Control and Prevention, May 1, 2009,58(16), pp. 421–426.

[2] J.J. Jacobs, A.K. Skipor, L.M. Patterson, N.J. Hallab, W.G. Paprosky, J. Black, J.O.Galante, Metal release in patients who have had a primary total hip arthro-plasty. A prospective, controlled, longitudinal study, Journal of Bone and JointSurgery – American Volume 80 (1998) 1447–1458.

[3] P.F. Doorn, P.A. Campbell, J. Worrall, P.D. Benya, H.A. McKellop, H.C. Amstutz,Metal wear particle characterization from metal on metal total hip replace-ments: transmission electron microscopy study of periprosthetic tissuesand isolated particles, Journal of Biomedical Materials Research 42 (1998)103–111.

[4] E.F. DiCarlo, P.G. Bullough, The biologic responses to orthopedic implants andtheir wear debris, Clinical Materials 9 (1992) 235–260.

[5] N. Hallab, K. Merritt, J.J. Jacobs, Metal sensitivity in patients with orthopaedicimplants, Journal of Bone and Joint Surgery – American Volume 83 (2001) 428-.

[6] Y. Yan, A. Neville, D. Dowson, Biotribocorrosion an appraisal of the time depen-dence of wear and corrosion interactions: I. the role of corrosion, Journal ofPhysics D: Applied Physics 39 (2006) 3200.

[7] A. Neville, Y. Yan, D. Dowson, Tribo-corrosion properties of cobalt-based med-ical implant alloys in simulated biological environments, Wear 263 (2007)1105–1111.

[8] T. Hanawa, S. Hiromoto, K. Asami, Characterization of the surface oxide film ofa Co–Cr–Mo alloy after being located in quasi-biological environments usingXPS, Applied Surface Science 183 (2001) 68–75.

[9] M.M. Stack, N. Pungwiwat, Erosion–corrosion mapping of Fe in aqueousslurries: some views on a new rationale for defining the erosion–corrosioninteraction, Wear 256 (2004) 565–576.

10] R.I. Trezona, D.N. Allsopp, I.M. Hutchings, Transitions between two-body andthree-body abrasive wear: influence of test conditions in the microscale abra-sive wear test, Wear 225–229 (1999) 205–214.

11] K. Adachi, I.M. Hutchings, Wear-mode mapping for the micro-scale abrasiontest, Wear 255 (2003) 23–29.

12] J. Jiang, M.M. Stack, A. Neville, Modelling the tribo-corrosion interaction inaqueous sliding conditions, Tribology International 35 (2002) 669–679.

13] M.M. Stack, G.H. Abdulrahman, Mapping erosion–corrosion of carbon steel inoil exploration conditions: some new approaches to characterizing mecha-nisms and synergies, Tribology International 43 (2010) 1268–1277.

14] M.A. Wimmer, A. Fischer, R. Büscher, R. Pourzal, C. Sprecher, R. Hauert, J.J. Jacobs,Wear mechanisms in metal-on-metal bearings: the importance of tribochem-ical reaction layers, Journal of Orthopaedic Research 28 (2010) 436–443.

15] D. Landolt, S. Mischler, M. Stemp, Electrochemical methods in tribocorrosion:

a critical appraisal, Electrochimica Acta 46 (2001) 3913–3929.

16] R. Buscher, G. Tager, W. Dudzinskil, Subsurface microstructure of metal-on-metal hip joints and its relationship to wear particles generation, Journalof Biomedical Materials Research. Part B: Applied Biomaterials 72B (2005)206–214.

ear 27

[

[

[[

[

[

[

M.T. Mathew et al. / W

17] H. Yoshida, A. Faust, J. Wilckens, M. Kitagawa, J. Fetto, E.Y. Chao, Three-dimensional hip contact area and pressure distribution during activities of dailyliving, Journal of Biomechaincs 39 (2006) 1996–2004.

18] ATIAllvac, http://www.alleghenytechnologies.com/allvac/pages/Nickel/UNSR31537.htm, in, Pittsburgh, PA, USA, Conculted in September 2010.

19] P. Ponthiaux, F. Wenger, D. Drees, J.P. Celis, Wear 256 (2004) 459–468.20] M.T. Mathew, E. Ariza, L.A. Rocha, A.C. Fernandes, F. Vaz, TiCxOy thin films for

decorative applications: tribocorrosion mechanisms and synergism, TribologyInternational 41 (2008) 603–615.

21] A.W.E. Hodgson, S. Kurz, S. Virtanen, V. Fervel, C.O.A. Olsson, S. Mischler, Pas-sive and transpassive behaviour of CoCrMo in simulated biological solutions,Electrochimica Acta 49 (2004) 2167–2178.

[

[

1 (2011) 1210– 1219 1219

22] A.W.E.H.S. Kurz, S. Virtanen, V. Fervel, S. Mischler, Corrosion characterizationof passive films on CoCrMo with electrochemical techniques in saline and sim-ulated biological solutions, European Cells and Materials 3 (Suppl. 1) (2002)26–27.

23] S. Mischler, Triboelectrochemical techniques and interpretation methods intribocorrosion: a comparative evaluation, Tribology International 41 (2008)573–583.

24] S. Fouvry, T. Liskiewicz, P. Kapsa, S. Hannel, E. Sauger, An energy description ofwear mechanisms and its applications to oscillating sliding contacts, Wear 255(1–6) (2003) 287–298.

25] A. Ramalho, J.C. Miranda, The relationship between wear and dissipated energyin sliding systems, Wear 260 (4–5) (2006) 361–367.