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Journal of Biomechanics 42 (2009) 2708–2711

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Journal of Biomechanics

0021-92

doi:10.1

� Corr

School

Technol

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www.JBiomech.com

Microstructure analysis and wear behavior of titanium cermet femoral headwith hard TiC layer

Yong Luo a,b,�, Shirong Ge a, Hongtao Liu a, Zhongmin Jin b

a Institute of Tribology and Reliability Engineering, School of Material Science and Engineering, China University of Mining & Technology, Xuzhou, Jiangsu, 221116, PR Chinab Institute of Medical and Biological Engineering, School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK

a r t i c l e i n f o

Article history:

Accepted 13 August 2009Titanium cermet was successfully synthesized and formed a thin gradient titanium carbide coating on

the surface of Ti6Al4V alloy by using a novel sequential carburization under high temperature, while the

Keywords:

Sequential carburization

Titanium cermet

Hip joints

Femoral head

90/$ - see front matter & 2009 Elsevier Ltd. A

016/j.jbiomech.2009.08.003

esponding author at: Institute of Tribology

of Material Science and Engineering, Chi

ogy, Xuzhou, Jiangsu, 221116, PR China. Tel./f

ail address: sulyflying@yahoo.com (Y. Luo).

a b s t r a c t

titanium cermet femoral head was produced. The titanium cermet phase and surface topography were

characterized with X-ray diffraction (XRD) and backscattered electron imaging (BSE). And then the wear

behavior of titanium cermet femoral head was investigated by using CUMT II artificial joint hip

simulator. The surface characterization indicates that carbon effectively diffused into the titanium alloys

and formed a hard TiC layer on the Ti6Al4V alloys surface with a micro-porous structure. The artificial

hip joint experimental results show that titanium cermet femoral head could not only improve the wear

resistance of artificial femoral head, but also decrease the wear of UHMWPE joint cup. In addition, the

carburized titanium alloy femoral head could effectively control the UHMWPE debris distribution, and

increase the size of UHMWPE debris. All of the results suggest that titanium cermet is a prospective

femoral head material in artificial joint.

& 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Titanium alloys are generally widely applied for dental andnon-cemented orthopaedic implants due to their similar modulusto human bone, superior biocompatibility and corrosion resis-tance (Conforto et al., 2004; Niinomi, 2007; Zheng et al., 2006;Zeng et al., 2005; Zheng et al., 2008). However, a main limit oftitanium alloys is their poor tribological behavior, characterizedby high coefficients of friction, severe adhesive wear with a strongtendency to seizing and low abrasion resistance (Abkowitz et al.,2005; Ceschini et al., 2008). Thus, a number of different surfacemodification techniques, such as ion implantation, physical vapordeposition, chemical vapor deposition, micro arc oxidation andplasma spraying, have been developed to improve the tribologicalproperties of titanium alloy over the past 4 decades (Braceraset al., 2005; Krupa et al., 2007; Szymanowski et al., 2005; Kwonet al., 2007; Yao et al., 2005; Huang et al., 2004; Uzumaki et al.,2008; Beck et al., 2007; Stoch et al., 2005). It is believed that TiCcoating has an ability to form strong bonds with the titaniumsubstrate which could provide a superior hardness and wearresistance, and it can induce carbon atoms into the metal matrixto enhance the biocompatibility (Brama et al., 2002; Brama et al.,

ll rights reserved.

and Reliability Engineering,

na University of Mining &

ax: +86 516 83591916.

2007). Therefore, forming TiC coating on biomaterials especiallytitanium alloys has attracted extraordinary attention due to theirpotential application as artificial hip joints.

In the present study, titanium carbide layer has been synthesizedon titanium alloy surface by using a sequential carburization underan inert gas environment. The surface composition and micro-structure of carburized titanium alloy were characterized. At last,the biotribological behavior of carburized titanium alloy femur headwas investigated by using CUMT II artificial joint hip simulator.

2. Experimental

A medical grade titanium alloy (Ti6Al4V) femoral head withthe diameter of 28 mm was polished to Rao0.05mm, thenultrasonically cleaned in ethanol and acetone, in turn, for 30 minand inserted into a vacuum gas carburizing furnace. Prior toheating, the reaction chamber was first evacuated by a vacuumpump. Then titanium alloys was carburized in the vacuumcarburizing furnace and finally titanium cermet femoral headwas formed after 4-h carburization with deep-black color andporous structure on the surface shown in Fig. 1.

X-ray diffraction (XRD) analysis was performed on D/mrx-3B,X-ray diffractometer system with Cu-Ka radiation to evaluate thecrystal structure of titanium cermet femoral head. The XRDspectra were obtained by scanning in the 2y range 15–951. Andthe surface topography were characterized using LEO-1450,

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Fig. 1. The photos of titanium cermet femoral head.

200

200

400

600

800

1000

(222

)(3

11)

(220

)

(200

)

Inte

nsity

/ C

ount

s pe

r sec

ond

(111

)

2θ / degree30 40 50 60 70 80 90

Fig. 2. The XRD pattern of the titanium cermet hip femoral head.

Y. Luo et al. / Journal of Biomechanics 42 (2009) 2708–2711 2709

backscatter electron imaging. Then, the surface hardness of thetitanium cermets was measured on a Vickers hardness tester withthe norm load of 1.96 N and loading time of 10 s. And then, a 1million cycle in vitro hip simulator study between titaniumcermet femoral head and UHMWPE cup was performed on a fourstation CUMT II artificial joint hip simulator. Paul cycle (ISO14242-1 (2002)) with a peak of 784 N was applied with 1 Hz cyclerate. 25% Bovine serum with 0.1% sodium azide was chosen aslubricant during the 1 million cycles’ simulation and wear wasdetermined gravimetrically after each 0.1 million cycles.

Fig. 3. BSE image of the surface of titanium cermet hip femoral head.

3. Result and discussion

3.1. Composition analysis

Fig. 2 shows the XRD pattern of titanium cermet femoral headwhich reveals the crystal structure and phase purity of theproducts. All the diffraction peaks can be indexed to the cubicstructure of TiC, which is consistent with the standard value forTiC. There is no evidence of hydrogen which is considered as asignificant source leading to brittle fracture. The results indicatethat the carbon effectively diffused into the titanium alloys andformed a hard TiC layer on the Ti6Al4V alloys surface.

Fig. 4. Typical microstructures of titanium cermet.

3.2. Surface topography

Fig. 3 shows the typical BSE image of the as-synthesizedproducts which provides further insight into the micro-porous TiCstructure. It is found that the as-synthesized products formed auniform micro-porous structure. It is believed that such uniquestructure can deposit lubricant and debris during the wear test,thus improve the lubrication, therefore it would help to reducefriction and decrease wear, which makes micro-porous titaniumcermet be considered as a potential ultrahigh wear resistancematerial for biotribological application.

Fig. 4 shows cross-sectional microstructures of titaniumcermet. It is found that titanium carbide layer has been formedon the surface of titanium alloys after carburization. The gradedstructures exhibited good bonding between the coating andsubstrate, which could prevent the coating delamination fromthe substrate.

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Fig. 5. The hardness profile across the coating-substrate of the titanium cermet

(n=9, error bars=95% confidence limit).

00

5

10

15

20

25

30

35

40

45

50

Counterfaced with Ti6Al4V head Counterfaced with Titanium cermet head

Mas

s lo

ss /

mg

Cycles / x 103200 400 600 800 1000

Fig. 6. Stage mass loss of UHMWPE cup counterfaced with Ti6Al4V head and

carburized Ti6Al4V head (n=3, error bars=95% confidence limit).

3210

50

100

150

200

250

300

350

400M

ass

loss

/ m

g

Samples

Ti6Al4V head UHMWPE counterfaced with Ti6Al4V UHMWPE counterfaced with titanium cermets

Fig. 7. Mass losses of three materials after 1 million cycles (n=3, error bars=95%

confidence limit).

0

0

2

4

6

8

10

Inte

rzon

e co

nten

t / %

UHMWPE counterfaced with Ti6Al4V head UHMWPE counterfaced with titanium cermet head

Particle size / μm50 100 150 200 250 300

Fig. 8. The debris distribution of UHMWPE against titanium and titanium cermet

femoral head.

Y. Luo et al. / Journal of Biomechanics 42 (2009) 2708–27112710

3.3. Hardness

The hardness profile across the coating-substrate of thetitanium cermets was shown in Fig. 5. It is observed that thehardness profile revealed a gradient decrease distribution fromthe surface to the substrate of titanium cermets, quite similarresults to the report of graded Co–Cr–Mo coating on titaniumalloys (Vamsi Krishna et al., 2008). The hardness of titanium alloyswas measured as 341 HV, while the hardness of titanium cermetsat the depth of 30mm is 695 HV, with a significant increase of104% comparing to the titanium alloys, which indicates thattitanium carbide has effectively strengthened the surface. It is alsoshown that the hardness of titanium cermets at the depth of1.53 mm is 414 HV, 21% higher that that of titanium alloys, whichindicates that the carburizing layer was quite thick and couldprovide effective support to the substrate of titanium cermets.

3.4. Biotribological properties

Fig. 6 shows the stage mass loss of UHMWPE cup counterfacedwith Ti6Al4V femoral head and titanium cermet femoral head for1 million cycles. It is noticed that the mass loss of UHMWPE cupper 0.1 million cycles has a different tendency for titaniumfemoral head and titanium cermet femoral head in the firstmillion cycles. The mass loss of UHMWPE cup per 0.1 millioncycles keeps a steady increase when against titanium femoralhead, from 24.2 mg for the first 0.1 million cycles to 30.2 mg forthe tenth 0.1 million. However, the mass loss of UHMWPE cup per0.1 million cycles increases in the beginning and then keeps adecrease tendency when against titanium cermet femoral head. Inaddition, titanium cermet femoral head can effectively reduce thewear of UHMWPE, with a decrease of 63.6% for the first 0.1 millioncycles and 71.5% for the tenth 0.1 million cycles. As the sequentialcarburization increased the surface hardness by forming a hardtitanium carbide layer, thus improved the friction and wear ofUHMWPE–Titanium cermet bearing.

Fig. 7 shows the mass losses of titanium femoral head andUHMWPE cup after 1 million cycles. Titanium cermet femoralhead has no mass loss after 1 million cycles and can significantlyreduce the mass loss of UHMWPE cup compared to titaniumfemoral head, decreases from 272.6 to 98.5 mg. Different from thezero wear of titanium cermet femoral head, there is some wearappearing in the titanium femoral head, and the mass loss is

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Y. Luo et al. / Journal of Biomechanics 42 (2009) 2708–2711 2711

8.1 mg after 1 million cycle against UHMWPE cup. As the wear oftitanium will result in a lot of black debris, which will causereverse effect of cell response and lead to the failure of joints.Therefore, the wear of titanium proves that titanium alloy withoutany surface modification is not suitable for hip joint material.

Fig. 8 shows the debris distribution of UHMWPE againsttitanium and titanium cermet femoral heads. It is found that themedian diameters of UHMWPE wear debris when againsttitanium and titanium cermet femoral heads are 6.56 and8.55mm respectively. In addition, titanium cermet femoral headcould effectively control the UHMWPE debris distribution,avoiding very large debris more than 100mm, thus helps to thefriction and wear of the bearing.

4. Conclusions

The titanium cermet femoral head was successfully synthe-sized for the first time and formed a thin gradient coating on thesurface of Ti6Al4V alloy with a micro-porous structure by using anovel sequential carburization under high temperature. The XRDand surface topography analysis results indicate that carboneffectively diffused into the titanium alloys and formed a hard TiClayer on the Ti6Al4V alloys surface with micro-porous structure.The artificial hip joint experimental results show that titaniumcermet femoral head could not only improve the wear resistanceof femoral head, but also decrease the wear of UHMWPE joint cup.In addition, titanium cermet femoral head could effectivelycontrol the UHMWPE debris distribution, and increase thesize of UHMWPE debris. All of the results suggest that titaniumcermet femoral head is a prospective femoral head material inartificial joint.

Conflict of interest statement

All authors declare that no financial and personal relationshipswith other people or organizations could have inappropriatelyinfluenced this work.

Acknowledgements

The authors wish to thank the National Nature ScienceFoundation of China (Grant no. 50535050), Project 2007CB607605

supported by the Vital Foundational 973 Program of China andfoundation of the China Scholarship Council.

References

Abkowitz, S., et al., 2005. The development of wear resistant titanium-ceramiccomposites for orthopaedic implant devices. Medical Device Materials II:Proceedings from the Materials & Processes for Medical Devices Conference2004, 178–183.

Braceras, I., et al., 2005. Bone cell adhesion on ion implanted titanium alloys.Surface & Coatings Technology 196, 321–326.

Beck, U., et al., 2007. Micro-plasma textured Ti-implant surfaces. BiomolecularEngineering 24, 47–51.

Brama, M., et al., 2002. Protein kinase C alpha/estrogen receptor alpha interactionin osteoblasts during differentiation. Journal of Bone and Mineral Research 17,S436.

Brama, M., et al., 2007. Effect of titanium carbide coating on the osseointegrationresponse in vitro and in vivo. Biomaterials 28 (4), 595–608.

Conforto, E., et al., 2004. Rough surfaces of titanium and titanium alloys forimplants and prostheses. Materials Science & Engineering C—Biomimetic andSupramolecular Systems 24, 611–618.

Ceschini, L., Lanoni, E., Martini, C., et al., 2008. Comparison of dry sliding frictionand wear of Ti6Al4V alloy treated by plasma electrolytic oxidation and PVDcoating. Wear 264 (1–2), 86–95.

Huang, P., et al., 2004. Surface modification of titanium implant by microarcoxidation and hydrothermal treatment. Journal of Biomedical MaterialsResearch Part B—Applied Biomaterials 70B, 187–190.

Krupa, D., et al., 2007. Effect of calcium-ion implantation on the corrosionresistance and bioactivity of the Ti6Al4V alloy. Vacuum 81, 1310–1313.

Kwon, S.Y., et al., 2007. Surface characteristics and biocompatibility of micro arcoxidized (MAO) titanium alloy. Bioceramics and Alternative Bearings in JointArthroplasty (pp. 239–248), 239–248.

Niinomi, M., 2007. Recent research and development in metallic materialsfor biomedical, dental and healthcare products applications. Thermec 2006539–543 (Pts 1–5), 193–200.

Szymanowski, H., et al., 2005. Plasma enhanced CVD deposition of titanium oxidefor biomedical applications. Surface & Coatings Technology 200, 1036–1040.

Stoch, A., et al., 2005. Sol–gel derived hydroxyapatite coatings on titanium and itsalloy Ti6Al4V. Journal of Molecular Structure 744, 633–640.

Uzumaki, E.T., et al., 2008. Biocompatibility of titanium based implants withdiamond-like carbon coatings produced by plasma immersion ion implanta-tion and deposition. Bioceramics 20 (Pts 1 and 2), 677–680 361–363.

Vamsi Krishna, B., et al., 2008. Functionally graded Co–Cr–Mo coating on Ti–6Al–4V alloy structures. Acta Biomaterialia 4 (3), 697–706.

Yao, C., et al., 2005. Titanium nanosurface modification by anodization fororthopedic applications. Nanoscale Materials Science in Biology and Medicine845, 215–220.

Zheng, J.H., et al., 2006. Titanium materials for biomedical applications. Rare MetalMaterials and Engineering 35, 139–142.

Zeng, Q., et al., 2005. Surface modification of titanium implant and in vitrobiocompatibility evaluation. Asbm6: Advanced Biomaterials Vi 288–289,315–318.

Zheng, M., et al., 2008. Microstructure and osteoblast response of gradientbioceramic coating on titanium alloy fabricated by laser cladding. AppliedSurface Science 255, 426–428.