Phase stability after aging and its influence on pin-on-diskwear properties of Ce-TZP/Al2O3 nanocomposite andconventional Y-TZP

Kenji Tanaka,1 Jiro Tamura,1 Keiichi Kawanabe,1 Masahiro Nawa,2 Masaki Uchida,3 Tadashi Kokubo,3

Takashi Nakamura1

1Department of Orthopedic Surgery, Faculty of Medicine, Kyoto University, Shogoin, Sakyo-ku, Kyoto 606-8507, Japan2Advanced Technology Research, Matsushita Electric Works, Ltd., Kadoma, Osaka 571-8686, Japan3Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Yoshida,Sakyo-ku, Kyoto 606-8501, Japan

Received 19 April 2002; revised 10 October 2002; accepted 7 November 2002

Abstract: Recently zirconia/alumina composites havebeen examined by many researchers as the new generationof bearing materials in total joint replacements. In this study,the phase stability of a Ce-TZP/Al2O3 nanocomposite andconventional Y-TZP after aging, and its influence on wearresistance, were investigated. Very slight phase transforma-tion was observed in both types of ceramics 18 months afterthe implantation of Ce-TZP/Al2O3 or Y-TZP samples intorabbit tibiae. However, Y-TZP showed marked phase trans-formation (approximately 80%) after aging in an autoclave(121°C) for 190 h or in physiological saline at 62°C for 18months, whereas the new composite remained almost resis-tant to degradation. According to the results of self-pairingpin-on-disk wear tests using ceramic specimens with or

without autoclave aging, the wear factor was almost thesame between Ce-TZP/Al2O3 samples with and withoutaging (6.74 � 0.36 � 10�8 and 6.04 � 0.95 � 10�8 mm3/Nm,respectively). In contrast, although non-aged Y-TZP had thelowest wear factor (4.88 � 0.51 � 10�8 mm3/Nm) of allspecimens tested, aged Y-TZP showed 10-fold greater wearthan nonaged Y-TZP. The present study suggests that Ce-TZP/Al2O3 nanocomposite has much greater phase stabilitythan Y-TZP, and that its wear properties are not influencedby aging. © 2003 Wiley Periodicals, Inc. J Biomed Mater Res67A: 200–207, 2003

Key words: zirconia; composite; aging; phase transforma-tion; wear resistance

INTRODUCTION

Zirconia ceramics are well recognized as havingbetter fracture toughness and flexural strength thanalumina ceramics. The latter have occasionally beenreported to cause component fractures in total jointarthroplasty due to their brittleness.1–3 Furthermore,many researchers have reported that, in laboratorywear tests, the wear rate of polyethylene articulatingagainst zirconia is lower than against alumina.4,5

Therefore, yttria-stabilized tetragonal zirconia poly-crystals (Y-TZP) ceramics have been used as the fem-oral head component in articulation with a polyethyl-ene cup in total hip arthroplasty for more than 10years. Although the excellent wear and mechanical

properties of Y-TZP are considered very promising, afew problems still remain concerning the low-temper-ature aging degradation (LTAD) caused by phasetransformation, which might cause deterioration inthe surface quality of Y-TZP bearing components. Ac-cording to the experimental observations of Yo-shimura et al.,6 the degradation of Y-TZP is caused bya tetragonal-to-monoclinic transformation accompa-nied by micro- and macrocracking, that proceeds rap-idly at temperatures of 200–300°C. Moreover, thisdegradation is time dependent and enhanced by wateror water vapor.

Some investigators have concluded from the goodresults of long-term aging tests in vivo and in vitro thatwell-manufactured Y-TZP components for medicaluse should show almost no degradation associatedwith phase transformation, even after long-term clin-ical use.7,8 On the other hand, several zirconia/alu-mina composites have recently been developed toeliminate possible degradation phenomena, and tooffer enhanced mechanical properties and wear resis-tance.9–11

Correspondence to: T. Nakamura; e-mail: ntaka@kuhp.kyoto-u.ac.jp

Contract grant sponsor: Grant-in-Aid for Scientific Re-search, the Ministry of Education, Science, Sports and Cul-ture, Japan; contract grant number: 13680933

© 2003 Wiley Periodicals, Inc.

We have already reported the potential of Ce-TZP/Al2O3 nanocomposite, developed by Nawa et al.,12 asa bearing material in total joint replacement. Thisnovel ceramic is composed of 70 vol % TZP stabilizedwith 10 mol % CeO2 (ceria), 30 vol % Al2O3, and 0.05mol % TiO2.

The significant characteristic of its microstructure isan intragranular type of nanostructure.13 As reportedpreviously, this composite possesses both high flex-ural strength and extremely high fracture toughness.In addition, it has been shown that this ceramic has asgood biocompatibility as alumina.12

In the present study, we investigated the wear prop-erties, with ceramic-on-ceramic pin-on-disk tests, andthe phase stability of Ce-TZP/Al2O3 nanocompositeafter long-term aging treatments in vitro and in vivo,compared with conventional Y-TZP.

MATERIALS AND METHODS

Sample preparation

Ce-TZP/Al2O3 nanocomposites were prepared by sinter-ing mixed powders of Ce-TZP and Al2O3 at 1440°C for 4 h.The details of the experimental procedures have been de-scribed previously.12,13 Sintered samples of Ce-TZP/Al2O3

nanocomposite had an average grain size of 0.59 �m and adensity of 5.56 g/cm3, which was almost equal to the theo-retical density.

Alumina and Y-TZP ceramics were supplied by KobeSteel Co. (Kobe, Japan) as reference ceramics. TZP ceramicsstabilized with 3 mol % Y2O3 were sintered in air at 1475°Cfor 2 h. Alumina ceramics were sintered in air at 1500°C for2 h and then subjected to hot isostatic pressing. The averagegrain size and density of Y-TZP ceramics were 0.29 �m and6.02 g/cm3, respectively. Those of alumina ceramics were2.14 �m and 3.91 g/cm3, respectively.

The mechanical properties of the Ce-TZP/Al2O3 nano-composite, Y-TZP, and alumina are listed in Table I.

Aging experiments

Plate-shaped samples (3 � 10 � 1.5mm) of the Ce-TZP/Al2O3 nanocomposite and Y-TZP were prepared by cuttingthe sintered pellets. One side of each specimen was polishedto a mirror-like surface finish, after which three aging ex-periments were performed.

For the in vitro tests, test specimens of both the Ce-TZP/Al2O3 nanocomposite and Y-TZP were treated in an auto-clave at 121°C and 0.15 MPa for 6, 12, 18, 36, 72, 108, or 190 h,or kept in physiological saline solution at 62°C for 3, 6, 9, 12,or 18 months (n � 4 for each set of conditions and eachperiod).

For the in vivo tests, 15 white Japanese rabbits of about3-kg body weight were operated on under sterile conditions.Two pieces of each ceramic were placed in the proximalmetaphysis of rabbit tibiae on each side, after making a drillhole. The animals were reared and the experiments werecarried out at the Institute of Laboratory Animals, KyotoUniversity. The Kyoto University guidelines for animal ex-periments were observed and the operations were per-formed under general anesthesia. Rabbits were sacrificed 3,6, 9, 12, or 18 months after implantation and the ceramicspecimens were removed (n � 6 for each type of ceramic andeach period).

The surfaces of the specimens were examined after theaging treatments by thin-film X-ray diffraction (TF-XRD;RINT-2500, Rigaku, Tokyo, Japan) with CuK �-radiation (50kV, 300 mA) over an angular range of 25–37° in 2�. Thevolumetric fraction of the monoclinic phase was determinedusing Toraya’s equation.14 The flexural strength of the sam-ples was tested by the three-point bending method afterautoclave aging. The span length and the crosshead speedwere 8 mm and 0.5 mm/min, respectively.

Wear experiments

Ceramic-on-ceramic wear experiments were performed atroom temperature using a pin-on-disk machine, in which astationary pin is loaded by a dead weight on a horizontal,reciprocating plate [Fig. 1(a)]. The tangential friction forcebetween the pin and the disk was measured by the straingauges fixed on leaf springs attached to the transverse barholding the wear pin. The strain-gauge output voltage waspassed through an amplifier and plotted on a chart recorder.Subsequently, by converting this voltage to friction force, thedata on friction coefficient were collected. The ceramic wearpin was cylindrical, 5 mm in diameter and 15 mm long, witha truncated conical end giving a flat surface of 1.5 mm indiameter [Fig. 1(b)]. The disk was 50 mm in diameter and7 mm thick.

A constant force of 88 N was applied, resulting in anapparent contact pressure of about 50 MPa. The surfaceroughness of the disk and pin was �0.01 �m. The averagesliding speed was 26 mm/s (1.3 Hz), and the sliding distancewas about 9 km (corresponding to the test duration of 96 h).Bovine serum, diluted to 30% with distilled water, was usedas a lubricant in the wear test; 0.2% (wt/vol) sodium azidewas added to the lubricant to retard bacterial degradation.

To investigate the effect of aging on wear performance forY-TZP and Ce-TZP/Al2O3, we tested samples with and

TABLE IMechanical Properties of Tested Ceramics

Ce-TZP/Al2O3 Alumina Y-TZP

Density (g/cm3) 5.56 3.91 6.02Grain size (�m) 0.59 2.14 0.29Elastic modulus (GPa) 247 376 203Flexural strength (MPa)a 941 441 945Fracture toughness

(MPam1/2)b 20.05 4.35 5.54Hardness (GPa) 11.71 16.21 11.66

aThree-point bending test (span length: 30 mm, crossheadspeed: 0.5 mm/min).

bindentation fracture method (equation of Marshall andEvans).

PHASE STABILITY AFTER AGING 201

without aging in an autoclave at 121°C and 0.15 MPa for 36 hfor each ceramic. Alumina ceramics were used as controls.Five groups of self-mated sliding combinations were exam-ined: nonaged Y-TZP pin versus nonaged Y-TZP disk (Y/Y);aged Y-TZP versus aged Y-TZP (aY/aY); nonaged Ce-TZP/Al2O3 versus nonaged Ce-TZP/Al2O3 (Ce/Ce); aged Ce-TZP/Al2O3 versus aged Ce-TZP/Al2O3 (aCe/aCe); andnonaged alumina versus nonaged alumina (A/A) (n � 3 forA/A and n � 4 for the other groups).

Before and after the wear tests, the ceramic specimenswere cleaned ultrasonically, and then dried in a desiccator.The wear volume of the pin specimen was calculated fromthe weight loss measured with an electronic balance (reso-lution of 0.01 mg; AT201, Mettler Toledo, OH) and thematerial density. The wear-track profiling of the disk spec-imen was measured across the track at three different pointsusing a profilometer (Dektak II, ULVAC, Kanagawa, Japan),and the volumetric wear on the disk was calculated fromthese profile data. The wear factor, Wf (mm3/Nm), wasdetermined using the following formula:

Wf � V/DF

where V (mm3) is the measured wear volume, D (m) is thesliding distance, and F (N) is the applied load. The surfaceroughness of the worn and unworn areas of all the testeddisks was measured perpendicular to the sliding direction,using the same profilometer. After each test, the pin and the

disk were examined with a scanning electron microscope(SEM; S4700, Hitachi, Tokyo, Japan).

All data on the total wear factor were statistically ana-lyzed using a one-way ANOVA, with Fisher’s PLSD methodas a post hoc test. Differences with p � 0.05 were consideredto be statistically significant.

RESULTS

Aging experiments

Figure 2 shows the XRD patterns for the Ce-TZP/Al2O3 nanocomposite and Y-TZP before and after var-ious aging treatments. A remarkable increase in themonoclinic peak was observed in Y-TZP after aging,whereas the Ce-TZP/Al2O3 nanocomposite showedalmost no change in the monoclinic phase. The volu-metric fraction of the monoclinic phase as a function oftime for the three aging conditions is shown in Figure3. In vivo aging for up to 18 months induced onlyslight phase transformation for both ceramics [Fig.3(a)]. However, Y-TZP showed a remarkable increasein the monoclinic phase, from 0.4% before aging to

Figure 1. (a) Schematic of pin-on-disk machine; (b) geometry of pin specimen; (c) optical micrograph of the wear track ofnonaged Ce-TZP/Al2O3 nanocomposite disk after test (original magnification �10); (d) SEM of aged Ce-TZP/Al2O3nanocomposite pin after test (original magnification �25). [Color figure can be viewed in the online issue, which is availableat www.interscience.wiley.com.]

202 TANAKA ET AL.

78.7% after 18 months in physiological saline at 62°C[Fig. 3(b)], whereas Ce-TZP/Al2O3 nanocompositeshowed only a very slight increase (from 5.7% beforeaging to 7.1% at 18 months). Similarly, a dramaticincrease in monoclinic content was observed withtime for Y-TZP subjected to the autoclave aging test(81.0% at 190 h), whereas Ce-TZP/Al2O3 nanocom-

posite showed only a slight change in monoclinic con-tent, verifying its very high resistance to aging degra-dation [Fig. 3(c)].

As shown in Table II, the flexural strength of theY-TZP samples was significantly lower after autoclave

Figure 2. XRD patterns of the surfaces of (a) Ce-TZP/Al2O3 nanocomposite and (b)Y-TZP. (T: Tetragonal zirconia,M: Monoclinic zirconia.)

Figure 3. Monoclinic content of tested ceramics as a func-tion of aging time in various environments. (a) Rabbit tibiae;(b) physiological saline at 62°C; (c) autoclave at 121°C and0.15 MPa.

PHASE STABILITY AFTER AGING 203

aging for 108 h than its initial strength (unpaired t-test,p � 0.0004), but no significant difference in flexuralstrength was observed for the Ce-TZP/Al2O3 samplesaged for 0 h and for 108 h (p � 0.15).

Wear experiments

The present test configuration and conditions al-lowed us to perform stable wear experiments thatresulted in the formation of even, well-margined, andlinear wear tracks on the disk specimens, which wereclearly visible to the naked eye [Fig. 1(c)]. Similarly inthe pin specimens, no pieces broken as the result of alocal concentration of stress were apparent. Therefore,the contact surfaces of the pins appeared macroscop-ically unchanged [Fig. 1(d)].

The wear data are summarized in Table III. Unex-pectedly, the total wear factor of A/A, used as acontrol, showed significantly higher values than thatof Y/Y (p � 0.04). In particular, the pin wear factor ofA/A tended to be high despite the low value of thedisk wear factor. This may be attributable to the testconfiguration, including the pin geometry, and to testconditions such as contact pressure and the materialproperties of alumina, including grain size and me-chanical strength.

It should be noted that aY/aY showed a remarkableincrease in the wear factor (50.21 � 3.15 � 10�8 mm3/Nm), despite the fact that Y/Y showed the lowestvalue of all (4.88 � 0.51 � 10�8 mm3/Nm). The totalwear factor for aY/aY was significantly higher than

those of all other combinations (p � 0.001). In contrast,there were no statistically significant differences in thetotal wear factors for the Ce-TZP/Al2O3 nanocompos-ite between Ce/Ce and aCe/aCe (6.74 � 0.36 � 10�8

and 6.04 � 0.95 � 10�8 mm3/Nm, respectively), andthe values were not significantly different from thoseof Y-TZP (Y/Y) or alumina (A/A).

Figure 4 shows the frictional behavior of the repre-sentative set of each sliding combination. The frictioncoefficient at the beginning of the test for aY/aY wasrelatively high (0.28), whereas those for the other com-binations ranged from 0.12 to 0.14. Finally, however,the steady-state coefficients of friction were 0.11–0.15for all the combinations. These results strongly sug-gest that the increased wear of aY/aY is caused by therugged transformation layer near the surface of theaged Y-TZP specimens. We infer that the decrease inthe steady-state friction coefficient of aY/aY is due tothe removal of the transformation layer during theperiod of the wear experiments.

Figure 5 shows scanning electron micrographs ofthe worn surfaces of tested ceramic specimens. Manypits representing grain-pullout are evident in the alu-mina specimens, which suggests that the current weartest conditions were severe enough for alumina [Fig.5(a)]. The SEM findings for the Ce-TZP/Al2O3 nano-composite were quite similar for both Ce/Ce andaCe/aCe. Although an obvious tendency for inter-granular fracture was observed in both groups, thefrequency of grain-pullout was low, and crack net-work, representing severe wear damage, was notformed [Fig. 5(b)]. Interestingly, the SEM micrographsof aY/aY were similar to those for Y/Y. Some inter-granular fractures and a few small pits were generallyobserved in almost all worn areas for both groups [Fig.5(c)], but the remains of large-scale fractures of thetransformation layer were observed only at the edgesof the aY/aY disk wear track [Fig. 5(d)].

The data on the wear-track profiles and the surface

TABLE IIFlexural Strength after Autoclave Aging

Aging Time(h)

Ce-TZP/Al2O3 Y-TZP

0 1196 � 133

p � 0.15

1360 � 60

p � 0.000418 1150 � 42 1168 � 5736 1116 � 65 1088 � 15072 1123 � 40 1112 � 43

108 1069 � 80 1054 � 91

n � 3–5, unit: MPa, mean � SD.

TABLE IIIWear Data

nPin Wf(�10�8)

Disk Wf(�10�8)

Total Wf(�10�8)

Y/Y 4 2.30 � 0.14 2.58 � 0.44 4.88 � 0.51aY/aY 4 5.51 � 0.61 44.71 � 2.57 50.21 � 3.15*Ce/Ce 4 3.18 � 0.34 3.57 � 0.49 6.74 � 0.36aCe/aCe 4 2.74 � 0.43 3.31 � 0.56 6.04 � 0.95A/A 3 5.13 � 2.04 2.69 � 0.16 7.81 � 1.88**

Mean � SD mm3/Nm.*Significantly higher than all other combinations (p �

0.0001).**Significantly higher than Y/Y (p � 0.04).

Figure 4. Frictional behavior of different material combi-nations.

204 TANAKA ET AL.

roughness of the worn and unworn areas of the disksare summarized in Table IV. The surface roughness ofthe unworn areas of aged Y-TZP specimens (12.9 � 2.8nm) was more marked than that of the nonaged Y-TZP(4.2 � 1.7 nm). Such deterioration of the surface qual-ity seemed to be caused by phase transformation. Incontrast, there were virtually no differences in surfaceroughness of the worn areas between Y/Y and aY/aYsamples (15.8 � 10.8 nm and 13.7 � 5.4 nm, respec-tively), which is consistent with the SEM findingsdescribed above.

DISCUSSION

Until now, Y-TZP ceramics have been widely usedas femoral heads in total hip arthroplasty, and recentlythey have also been applied clinically as femoral com-ponents in total knee arthroplasty.15 As shown byKumar et al.4,5 and other researchers in laboratorywear tests, the Y-TZP-on-polyethylene combinationhas better wear resistance than metal- or alumina-on-polyethylene combinations. Therefore, bearing com-ponents made of Y-TZP are expected to reduce thepolyethylene wear and the resultant osteolysis.

On the other hand, the only drawback of Y-TZP isLTAD, and several studies have focused on the phasestability of zirconia ceramics after in vitro and in vivoaging. Drummond reported a significant decrease instrength (from 13 to 22%) for yttria-stabilized zirconiaaged in a physiological medium, with aging times ofup to 453 days.16 Thompson and Rawlings17 foundthat aging in Ringer’s solution promoted phase trans-formation and reduced the strength of zirconia.17 Incontrast, Shimizu et al.8 found no serious reduction in

Figure 5. SEM of worn ceramic specimens. (a) alumina pin , original magnification �4800; (b) aged Ce-TZP/Al2O3 pin,original magnification �4800; (c) aged Y-TZP pin, original magnification �4800; (d) aged Y-TZP disk, original magnification�18.

TABLE IVSurface Profile of Disk Specimens

Maximum Depth(�m)

Roughness(nmRa, unworn)

Roughness(nmRa, worn)

Y/Y 1.61 � 0.39 4.2 � 1.7 13.7 � 5.4aY/aY 17.62 � 0.55 12.9 � 2.8 15.8 � 10.8Ce/Ce 2.25 � 0.32 3.5 � 1.0 23.7 � 9.6aCe/aCe 2.03 � 0.45 7.1 � 3.5 20.4 � 14.0A/A 1.71 � 0.20 6.1 � 2.5 13.2 � 4.1

Mean � SD

PHASE STABILITY AFTER AGING 205

bending strength for Y-TZP over a 3-year period un-der various aging conditions. They estimated fromtheir various aging test data that zirconia should showa bending strength of over 800 MPa after 80 years invivo. Similarly, Cales et al. investigated the phase sta-bility of commercially available Y-TZP ceramics andfound that no mechanical degradation was induced inresponse to long-term aging, either in vitro or in vivo.They concluded that zirconia ceramics are quite sen-sitive in their properties to composition, grain size,flaw population, etc.7 The influence of phase transfor-mation on the wear properties of Y-TZP was alsoinvestigated by Villermaux et al.18 In hip simulatorstudies using zirconia femoral heads presenting 40%monoclinic phase on their surfaces, phase transforma-tion due to aging did not change the excellent wearproperties of zirconia heads articulated with polyeth-ylene or alumina cups.

In contrast to these encouraging experimental re-sults, clinical reports on total hip arthroplasty usingzirconia femoral heads have been less satisfactory.19,20

Haraguchi et al.21 have recently reported that twozirconia femoral heads retrieved at 3 and 6 years aftercementless total hip arthroplasty showed surface de-terioration associated with phase transformation. Themonoclinic content of the surfaces was about 30 and20%, and the surface roughness increased from aninitial value of 0.006 to 0.12 �m in one case.

Thus, the results for laboratory and clinical studiesare contradictory for the phase stability of Y-TZP. Toovercome the problem of LTAD, several improve-ments have been made to Y-TZP, such as grain-sizecontrol, hot isostatic pressing, and slight alumina dop-ing. As another solution, several zirconia/aluminacomposites have been developed and tested as thenext generation of bearing materials.9–11 For instance,Affatato et al.11 investigated in a hip joint simulatorthe wear behavior (ceramic-on-ceramic) of experimen-tal mixed-oxide ceramics consisting of alumina andY-TZP, compared with commercial alumina. Therewere no significant differences in wear between theexperimental ceramic and commercial alumina.

Ce-TZP/Al2O3 nanocomposite is a very attractiveceramic material, as mentioned above. In the presentstudy, this new composite showed excellent phasestability, even after long-term in vivo and in vitro agingtreatments, although remarkable phase transforma-tions were observed for conventional Y-TZP underconditions of severe aging. Conventional Y-TZPshowed approximately 80% monoclinic content byvolume, whereas it maintained a high bendingstrength of more than 1000 MPa. However, the surfaceroughness of Y-TZP clearly deteriorated, as shown inTable IV. These results suggest that the aging-inducedphase transformation occurred mainly in the superfi-cial layer but not in the bulk of aged Y-TZP specimens.It is still unclear whether surface deterioration leads toincreased polyethylene wear clinically. In contrast,

only very slight changes were seen in the monocliniccontent, bending strength, and surface roughness ofthe Ce-TZP/Al2O3 nanocomposite, even after auto-clave aging. Furthermore, according to the experimen-tal results reported by Uchida et al.,22 Ce-TZP/Al2O3nanocomposite samples showed no apparent changesin XRD patterns after immersion in strong alkali oracid solutions at 95°C for 4 days. Therefore, we believethat Ce-TZP/Al2O3 nanocomposite is very unlikely tobe degraded in the human body, regardless of theduration of implantation.

In the present wear tests, we examined five kinds ofceramic-on-ceramic sliding combinations to clarify therelationship between phase stability and the wearproperties of zirconia ceramics. We chose to examinethe ceramic-on-ceramic combination, but not the ce-ramic-on-polyethylene one, because the former hasrecently drawn the attention of many researchers as analternative bearing couple. In our previous study, weperformed similar wear tests using distilled water as alubricant under relatively low loads (11.3 MPa).12 Di-luted serum and higher loads (50 MPa) were used inthis study, because Clarke et al.23 recently reportedgood results for the zirconia-on-zirconia combinationin hip joint simulator wear tests using serum as alubricant. The data of Clarke et al.23 are in contrast tothe disastrous results for the same combination inlaboratory wear tests conducted by Willmann et al.and Oonishi et al.,24,25 using water or saline solution.Thus, it appears that the zirconia-on-zirconia combi-nation survives well in serum, although it deterioratesin water environment. Interestingly, in the presentstudy, the total wear factor for Y/Y was the lowest ofall the combinations, which is partly consistent withthe results of Clarke et al. However, the aY/aY com-bination produced much more wear. From these fric-tion and wear data, we infer that the massive wear onaged Y-TZP samples is due to the brittle transforma-tion layer. In contrast, no significant difference in wearfactor was observed between nonaged and aged Ce-TZP/Al2O3 nanocomposite specimens, which was re-flected in the SEM findings.

Riche et al.26 have described, in detail, the wearpatterns of Y-TZP visualized with SEM. They ob-served four types of results, corresponding to mild tosevere damage: (1) grooving, (2) intergranular fractureand grain-pullout, (3) network of cracks on a largerscale, and (4) much larger fracture, presenting flakes.On the basis of their observations, intergranular frac-ture and grain-pullout, which were most commonlyobserved for both Ce-TZP/Al2O3 and Y-TZP in thisstudy, are not considered to represent severe damage.However, large-scale fractures observed only at theedges of aged Y-TZP disk specimens indicate severedamage in aged Y-TZP.

Therefore, the present data from aging and wearexperiments strongly suggest that Ce-TZP/Al2O3nanocomposite should be highly resistant to degrada-

206 TANAKA ET AL.

tion in any severe environment. We consider that thepoor clinical results for total hip arthroplasty usingY-TZP bearing components may be related to LTAD.Although there is almost no concern for LTAD withtoday’s medical-grade Y-TZP, it is appropriate thatorthopedic surgeons should select the most reliableand promising biomaterials for patients. We believethat Ce-TZP/Al2O3 nanocomposite is a good candi-date in the new generation of ceramic bearing mate-rials for total joint replacement, and we are conductingfurther studies of this ceramic.

CONCLUSION

The phase stability and wear resistance of a newlydeveloped Ce-TZP/Al2O3 nanocomposite were inves-tigated and compared with those of Y-TZP and alu-mina. The Ce-TZP/Al2O3 nanocomposite showed nophase transformation, no decrease in strength, and nodeterioration of surface quality after various agingtreatments, in contrast to the dramatic changes foundin Y-TZP. Moreover, the present wear tests have dem-onstrated that the excellent wear properties of thisnew ceramic should never be influenced by aging.Therefore, we conclude that this new composite is amore reliable bearing material than conventional Y-TZP because of its extreme resistance to LTAD.

We thank Dr. Masanori Oka for his technical advice in thewear experiments. We also thank Kobe Steel Co. for thesupply of materials.

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