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 DOI: 10.1177/1350650114541106

2014 published online 19 JuneProceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology

Dipankar Choudhury, Taposh Roy, Ivan Krupka, Martin Hartl and Rajshree Mootanahsurfaces in total hip joint replacement

Tribological investigation of ultra-high molecular weight polyethylene against advanced ceramic  

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Special Issue Article

Tribological investigation of ultra-highmolecular weight polyethylene againstadvanced ceramic surfaces in total hipjoint replacement

Dipankar Choudhury1,2, Taposh Roy3, Ivan Krupka1,2,Martin Hartl1 and Rajshree Mootanah4

Abstract

The aim of the study was to investigate whether a modified ceramic head surface could reduce the friction and wear rate

of simulated ceramic-on-polyethylene hip joints. To address this aim, ultra-high molecular weight polyethylene

(UHMWPE) was made to slide on aluminium oxide (Al2O3), dimpled Al2O3, diamond-like carbon (DLC) coated and

DLC-coated dimpled substrates. The experiment condition was replicated to simulate artificial hip joints in terms of

contact pressure, speed and temperature. UHMWPE on non-dimpled Al2O3 showed lower friction coefficient and wear

rate compared to other advanced surfaces. Lower wettability, and higher hardness and surface adhesion of DLC resulted

in increased friction and wear. The high difference in modulus of elasticity and hardness between UHMWPE and both,

Al2O3 and DLC, reduced the effectiveness of textured surface techniques in friction and wear reduction. Therefore, no

tribological benefit was found by fabricating either DLC coating or surface texturing on hard surface when rubbed against

softer UHMWPE.

Keywords

Ceramic on polyethylene, diamond-like carbon, micro-dimpled surface, friction coefficient, wear, tribology, textured

surface, prosthesis design

Date received: 8 November 2013; accepted: 12 May 2014

Introduction

Total hip replacement is one of the most successfulachievements in orthopaedic surgery. Since successfulintroduction of hip replacement operation in the1960s, the procedure has been increasing at a rate of10% every year.1,2 Successful hip replacements restorepatients’ mobility, enabling them to have a comfort-able and independent life. However, there are stillmajor challenges that need to be overcome, includingimplant durability and the biological response fromjoint-induced wear debris,3,4 both associated withwear rate, as well as other material- and patient-related factors. A reduction of wear rate, not onlyincreases the durability of the reconstructed joint,but also reduces the risk of biological response,which is dependent on the dose level (concentration)and sizes of wear debris.5

Polyethylene cups, combined with hard metallic orceramic spheres, are widely used in artificial bearingsurfaces because of their excellent tribological charac-teristics and elastic property, which helps reduce stress

shielding. Following its initial success as cup materialin total hip replacement for 30 years, ultra-highmolecular weight polyethylene (UHMWPE) hasbeen the dominant orthopaedic material in totaljoint replacements (TJRs).6 However, their wear rateis still high (4–10mm per million motion cycles7) andthe revision rate of a metal or ceramic on UHMWPEis as high as 10% after 10 to 15 years of joint

1Faculty of Mechanical Engineering, Brno University of Technology,

Technicka, Brno, Czech Republic2CEITEC-Central European Institute of Technology, Brno University of

Technology, Technicka, Brno, Czech Republic3Department of Biomedical Engineering, Faculty of Engineering,

University of Malaya, Kuala Lumpur, Malaysia4Medical Engineering Research Group, Department of Engineering and

the Built Environment, Faculty of Science and Technology, Anglia Ruskin

University, Chelmsford, UK

Corresponding author:

Dipankar Choudhury, Faculty of Mechanical Engineering, Brno

University of Technology, Technicka 2896/2, 616 69 Brno,

Czech Republic.

Email: Dipankar.choudhury78@gmail.com

Proc IMechE Part J:

J Engineering Tribology

0(0) 1–10

! IMechE 2014

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replacement.8 High wear rate results in high dose ofwear debris, a major influencing factor on biologicalresponse.5

It is therefore important to reduce wear rate forincreased durability and effective performance of hipimplants. Polyethylene, being relatively softer thantheir counterface materials, contributes to most ofthe wear. Attempts to reduce the friction and wearrate of polyethylene include improving its materialproperties (cross-link UHMWPE)9,10 and changingits surface properties (surface energy, wettability).11

However, failure still occurs in highly cross-linkedUHMWPE, in particular, an increase in rim fractureis caused by a decrease in fatigue resistance.12

Several studies have been conducted to reduce fric-tion and polyethylene wear rate in artificial hip jointsby applying surface texturing. For example, Itoet al.13 and Swano et al.14 fabricated micro-dimplesurfaces on metallic femoral head and confirmedreduced friction and wear in their simulated joints.More recently, Sagbas et al.15 found that a dimpledUHMWPE cup resulted in lower friction temperaturecompared to the non-dimpled UHMWPE. They alsoshowed that, when blended with vitamin E, dimpledUHMWPE cups could further lower the rise in fric-tion temperature. A rise in friction temperature influ-ences the rate of wear, fatigue, creep and oxidativedegradation of bearing materials. This can contributeto cup loosening by causing bone necrosis and sur-rounding tissue damage.16–18

Aluminium oxide (Al2O3)-based femoral head hasa number of advantages compared to CrCoMo-basedmetallic head, including better mechanical properties,such as higher hardness, smother surface profile and,therefore, lower friction coefficient and wear. Urbanet al.19 monitored 64 ceramic-on-polyethylene (CoP)hip joints and reported lower and more consistentwear rates than previously reported metal-on-poly-ethylene (MoP) wear rates. Furthermore, the wettabil-ity of Al2O3 is higher than the CrCoMo; water contactangles on Al2O3 and CrCoMo are around 59� and75�, respectively.20,21 Therefore, Al2O3 attracts morewater, thus ousts the protein towards the courterfacesubstrate, which helps to reduce wear on the poly-ethylene surface.

Liu et al.22 reported that hard-coated surface, suchas diamond-like carbon (DLC) or nitrogen ion-implanted CoCrMo, can increase the wear rate ofUHMWPE, which has a much lower hardness.However a meta-analysis, conducted by Roy andLee,23 showed that DLC-coated femoral head againstUHMWPE cup had impressive performance. In theirreview article, Love et al.24 reported that wear ratesproduced between UHMWPE acetabular cupsagainst DLC-coated metallic femoral heads werereduced by up to 14 times when the DLC coatingmaterials were based on tetrahedral carbon (ta-C).

To our knowledge, no study has investigated theinfluence of ceramic femoral ball surface texture and

coating, combined, on the tribological outcomes ofCoP hip joints. A recent study25 showed that a com-bination of dimple and DLC coating can significantlyreduce friction and wear. However, the study was notperformed in a biotribological environment.Therefore, the aim of the study is to compare thetribological performance of Al2O3, dimpled Al2O3,DLC coating and dimpled DLC against UHMWPE,using simulated hip joint conditions.

Material and methods

Specimen preparation

A 99% Al2O3 plate (AdValue Technology Tucson,USA) was trimmed into 15� 15� 6mm3 blocks,using a diamond cutter (IsoMet� 5000 PrecisionsSaws Buehler, China) to simulate the femoral headsurface. The cup surface was represented by cylindricalpin UHMWPE (Goodfellow Cambridge Ltd, UK) of6mm diameter and 6mm length (n¼ 12). The bulksurface roughness of Al2O3 and UHMWPE were rela-tively high (�1 mm). The Al2O3 blocks were processedin a number of polishing steps, including diamondgrinding, using a 30 mm disc for initial polishing, fol-lowed by 9 mm, 6 mm, 1 mm and 0.05mm diamond poly-crystalline suspensions. Following the polishing steps,the Al2O3 surface profiles had a mirror finish (mean0.1mm). Surface polishing on the UHMWPE cylinderswere not carried out to avoid any risk of misaligning ofthe line contact.

A CNC micro-machine (Mikrotools DT110,Singapore) was used to produce the micro-dimpleson the Al2O3 samples (n¼ 4) (Figure 1). A set of dia-mond drill bit (M.A. Ford, USA) with diameter of300mm was used to create the dimples. In order toachieve effective machining, the spindle speed andfeed rate were maintained at 55,000 r/min and334mm/min, respectively, as per manufacturer’s rec-ommendation. A new drill bit was used in eachsample, as there was a visible sign of wear on the tipof the drill bit.

Following the surface finish and dimple fabricationprocedures, a set of dimple (n¼ 3) and non-dimple(n¼ 3) block was sent to a specialised physicalvapour deposit coating (PVD) company (NoconTechnology Limited, Malaysia) to produce a tetrahe-dral amorphous carbon (TA-C) coating. TA-C is oneform of DLC which is considered to be one of thehardest form of DLC.24 The experimental samplesdescribed are shown in Figure 1.

Mechanical and surface characterisation

The material and surface properties of the specimenswere determined from the following tests:

Surface roughness. The sample surface roughness, Ra,was measured, using a surface profilometer

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(Mitutoyosj210, USA), at random locations acrossthe surface (5 times in parallel, and 5 times trans-versely along 4.5mm spans).

Surface profiles. A number of mechanical testing wasperformed to determine the dimpled and coatingproperties. The dimpled properties26 included diam-eter, depth and distance measurements. The surfaceprofiles (Figure 1) were inspected before and afterthe tests, using a field emission scanning electronmicroscopy (FESEM, AURIGA, Zeiss, Singapore).

Hardness, elasticity and wettability. Nano-indentation(DUH-211/DUH-211S Dynamic Ultra MicroHardness Tester, Shimadzu, Japan) technique wasused to measure the hardness and elasticity of eachsample. A 150 mN load was applied to each of thesample for 15 s. The hardness and elasticity testswere conducted at three random locations on everysample. UHMWPE pin hardness was taken from the

supplied material specification. The contact angle wasmeasured, using a contact angle analyser (OCA15EC,Dataphysics Instruments, Germany). To increaseaccuracy, five resultant-water-contact-angle readingswere taken for each specimen and the mean valueswere used.

Details of experiment

A tribometer (TR 283 Series, DUCOM, Bangalore,India), suitable for low frequency, was employed tosimulated hip joints in terms of contact pressure,speed and temperature. Figure 2 shows the tribometerexperimental set-up and the schematic diagram of thespecimen. To simulate a human medium walkingspeed of 10mm/s, the stroke length was set at 2mmand the frequency at 5Hz. Extracted bovine serum(50% with distilled water) was used as lubricant andtemperature was maintained at 37 �C to simulate bodytemperature. Three sets of static loads of 10, 15 and

Figure 1. Experimental specimen: disc (15� 15� 6 mm3) and pin (Ø 6� 6 mm2).

UHMWPE: ultra-high molecular weight polyethylene; DLC: diamond-like carbon.

Figure 2. (a) Tribometer experimental set-up and (b) schematic diagram of the specimen.

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20N were applied, which were equivalent to 17.86, 21.419, 24.733MPa Hertz Pressure to UHMWPE/ Al2O3

interfaces and 15.486, 18.967, 21.901MPa HertzPressure to UHMWPE/DLC interfaces. The threecontact pressures refer to normal weight, overweightand obese patients. The experimental conditions aresummarised in Table 1.

Friction coefficient was computed from the appliedload and friction force data, which were measured6000 times per minute on the tribology machine.The average friction coefficient was calculated forevery 6000 data. Since the experiment was run for30min, a total of 30 average friction coefficient data

were obtained for each sample and used in Figure 3.Wear rate was calculated by measuring the weight ofUHMWPE pin before and after the test. Before mea-suring weight of the pin after the test, an ultrasoniccleaning was performed to wash out the generatedwear debris. Following the cleaning process, wedried the sample by flowing hot air of about 25 �Cfor 15min to eliminate any weight gain from the lubri-cant contamination. A digital weighing scale of0.01-mg accuracy was used to record the change inweight. The disc wear rates of the Al2O3 and DLCdiscs were not measured as they were much harderthan UHMWPE and were expected to produce negli-gible wear.

Statistical analysis. The IBM SPSS statistics 21 softwarewas used to perform statistical analysis to determinewhether there was any significant difference in frictioncoefficient value between the applied loads. Singlefactor analyses were performed to check there wasno significant difference between the testing resultsof same sample and under same loads. In that case,p-value was greater than 0.05 in 95% confidence levelinterval (CI), suggesting no significant differencebetween the testing results of same sample andunder same load. A Tukey post hoc analysis wasthen conducted for multiple comparisons betweenthe different samples under the different loads.

Figure 3. Friction coefficient profiles under: (a) 10 N, (b) 15 N and (c) 20 N loads.

DLC: diamond-like carbon.

Table 1. Experimental parameters of wear tests.

Items Description

Specification of pin 6� 6.35 mm2 (Ø)

Specification of block 15� 15� 6 mm3

Lubricant Bovine synovial fluid

Speed 10 mm/s

Hertz pressure UHMWPE/Al2O3 17.86, 21.419, 24.733 MPa

UHMWPE/DLC 15.486, 18.967, 21.901 MPa

Temperature 37�C

UHMWPE: ultra-high molecular weight polyethylene; DLC: diamond-

like carbon.

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In this case, the p-value was less than 0.05 in 95% CI,suggesting a significant difference between the differ-ent samples under different loads.

Results and analysis

Mechanical properties and experimental conditions

Results of the mechanical tests are presented inTable 2. Al2O3 is considered to be one of the hardestmaterial, but the hardness of DLC (TA-C) is 53%higher. Moreover, the modulus of elasticity of DLCis 30% lower than that of Al2O3, indicating that DLChas a higher flexibility. However, the wettability ofAl2O3 is higher than that of DLC. The presence ofdimples increases the wettability of both of the sub-strates. The dimple parameters were precisely definedand were similar for both the Al2O3 and DLC sub-strates. The selection of dimple parameter was basedon our previous study.26

Friction coefficient

The comparison of friction coefficient for each sampleand loading conditions is presented in Figure 3 andTable 3. For every loading condition (10, 15 and20N), the non-dimpled Al2O3 surface generatedlower friction coefficients (0.055, 0.050 and 0.049,respectively) compared to the other surface profiles.Friction coefficients were much higher for the

DLC-coated surfaces, ranging from 0.170 to 0.184.A general decrease in friction coefficient was observedwith increasing load for the dimpled and non-dimplednon-coated Al2O3 surfaces. The reverse was true forthe dimpled and non-dimpled DLC-coated surfaces.

For the lower 10 N loading condition, both theAl2O3 and DLC non-dimpled surfaces producedlower friction coefficients compared to the corres-ponding dimpled surfaces. In both cases, the differ-ences between dimpled and non-dimpled weresimilar (0.0044) (Figure 3(a)). For the 15 N loadingcondition, the non-dimpled Al2O3 surface produced alower friction coefficient than the dimpled Al2O3 sur-face and the difference between the two increased to0.008. However the friction coefficient profile for theDLC-coated non-dimpled and dimpled surfacesalmost overlapped each other, the difference in fric-tion coefficients being only 0.002 (Figure 3(b)). Underthe 20N loading condition, the non-dimpled Al2O3

surface still produced a lower friction coefficientthan the dimpled Al2O3 surface and the differencebetween the two had further increased to 0.012. TheDLC-coated non-dimpled surface exhibited a higherfriction coefficient than the dimpled DLC-coated sur-face (Figure 3(c)).

Therefore, it can be deduced from the results that,with increasing loading, the dimpled surface becameincreasingly effective for the DLC-coated surfaces(decreasing friction coefficient compared to corres-ponding non-dimpled surface), but not for the

Table 2. Mechanical and surface properties of test specimens.

Samples

Mechanical properties Surface properties

Hardness (GPa) Elasticity (GPa) Roughness (mm) Water contact angle

Dimple profile (mm)

Diameter Depth Distance

UHMWPE (pin) 0.02 0.69 1� 0.5 – – – –

Al2O3 5.2� 0.5 370� 3 0.12� 0.01 77 – – –

Dimpled Al2O3 5.2� 0.5 370� 3 0.12� 0.01 61 300 20 950

Non-dimpled DLC 8.� 0.5 260� 5 0.29� 0.1 88 – – –

Dimpled DLC 8.1� 0.5 260� 5 0.29� 0.05 83.5 300 20 950

UHMWPE: ultra-high molecular weight polyethylene; DLC: diamond-like carbon.

Table 3. Mean friction coefficient of different material combinations at different loading conditions.

Loads

Mean coefficient of friction

UHMWPE/

Non-dimpled Al2O3

UHMWPE/

dimpled Al2O3

UHMWPE/

Non-dimpled DLC

UHMWPE/

dimpled DLC

10 N 0.055� 0.07% 0.059� 0.12% 0.170� 0.22% 0.174� 0.19%

15 N 0.051� 0.16% 0.058� 0.12% 0.179� 0.39% 0.181� 0.27%

20 N 0.049� 0.11% 0.059� 0.07% 0.188� 0.12% 0.183� 0.06%

UHMWPE: ultra-high molecular weight polyethylene; DLC: diamond-like carbon.

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Al2O3 non-coated surfaces (increasing friction coeffi-cient compared to corresponding non-dimpledsurface).

Effect of contact pressure on various interfaces. Withincreasing applied loads, the friction coefficients ofboth dimpled and non-dimpled Al2O3 surfacesdecreased and the rate of decrease in the friction coef-ficient for the non-dimpled Al2O3 surface was higher(10.1%) than that for the dimpled (3.4%) Al2O3 sur-face. The trends were different for the DLC-coateddimpled and non-dimpled surfaces. With increasingapplied loads, the friction coefficients of both dimpledand non-dimpled DLC-coated surfaces increased andthe rate of increase in friction coefficient for the non-dimpled DLC-coated surface was higher (10.5%) thanthat for the dimpled (5.7%) Al2O3 surface. Figure 4shows the scanning electron microscope (SEM)images of non-dimpled Al2O3 and non-dimpledDLC surfaces (Figure 4). The DLC surface is visiblymore flecked with a surface roughness of 0.29mm,which was higher than that of Al2O3 (0.12 mm).

Wear and surface conditions

The wear rates are presented in Figure 5. The non-dimpled Al2O3/UHMWPE pair exhibits the lowestweight loss as a result of wear (0.04mg), followedby dimpled Al2O3/UHMWPE (0.06mg), non-dimpledDLC/UHMWPE (0.1mg) and dimpled DLC/UHMWPE (0.13mg). These values were obtainedafter all the loading conditions, combined. For exam-ple, the weight of the non-dimpled UHMWPE/Al2O3

specimen was measured after completing all the testsunder 10, 15 and 20N loads. Since each test for thedifferent specimens and loading conditions lasted foronly 30min, weight loss was measured for each spe-cimen before and after each test series, including allthree loading conditions, not for every set of loads.

Wear rates exhibit similar trends as the frictioncoefficient profiles. Wear rate for non-dimpledAl2O3/UHMWPE<wear rate for dimpled Al2O3/UHMWPE<wear rate for non-dimpled DLC/UHMWPE<Wear rate for dimpled DLC/UHMWPE. The SEM images before and after

Figure 5. Comparison of UHMWPE mass loss against different surfaces (mean� 95% confidence limits).

UHMWPE: ultra-high molecular weight polyethylene; DLC: diamond-like carbon.

Figure 4. Morphology of surface profiles prior to wear testing for: (a) non-dimpled Al2O3 and (b) non-dimpled DLC.

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loading shows some evidence of wear sign in theUHMWPE specimen (Figures 6 and 7).

Figure 6 shows the textured Al2O3 and UHMWPEsurfaces prior to testing. The entire SEM images werefocused on the polar zone.10 The dimples on the Al2O3

are clearly visible and precise. Some valleys are visibleon the UHMWPE surface, indicating its high rough-ness (1.5 mm) compared to commercially availablepolyethylene cup.

SEM images of the UHMWPE pin after the tests(Figure 7) indicated some wear signs, which wasexpected due to the weight lost measurement. Forexample, Figure 7(c) to (d) shows the polyethylene sur-faces against non-dimpled and dimpled DLC surfaces,

respectively, following the wear tests. These surfacescan be easily distinguished from UHMWPE surfacesagainst non-dimpled and dimpledAl2O3. There are vis-ible wear signs: plastic deformation along with thepresence of cracks and flakes in both images, however,these deformations weremore visible in theUHMWPE/dimpled DLC image (Figure 7(d)). The weight lostmeasurement confirmed higher wear rate associatedwith the dimpled DLC surfaces. Figure 7(a) and (b)shows more wear on UHMWPE surface againstdimpled Al2O3, compared to non-dimpled Al2O3. Thewear rate was found to be higher for the UHMWPE/dimpled Al2O3 pair. However, no visible changes onany Al2O3 or DLC disc surface was observed.

Figure 7. Morphology of UHMWPE surface against: (a) non-dimpled Al2O3, (b) dimpled Al2O3 and (c) non-dimpled DLC,

(d) textured DLC, after testing.

Figure 6. Morphology of surface profiles prior to wear testing for: (a) textured Al2O3 and (b) UHMWPE pin.

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Discussion and conclusions

Tribology involves many factors, including interfaceparameters, such as contact area and surface topog-raphy, material properties, such as hardness, elasti-city, and wettability, loading conditions, lubricationmechanism and temperature, amongst others. Whenreporting tribological outcomes, it is important todefine these experimental and material parameters.In this experiment, almost all of these parameterswere precisely measured.

The coating was performed from a specialised coat-ing company and mechanical tests were carried outunder controlled conditions in a university-basedlaboratory. The study was conducted in an environ-ment which replicated implanted CoP hip joints. Oneof the hardest ceramic, 99% Al2O3, and one of thehardest DLC, TA-C were used in this study. Theexperimental conditions, including contact pressure,temperatures and speed, matched those of othertribo-tester experiments.7,27,28 The bovine serumused in this study is a commonly used lubricant in abiotribological tests.26,29

Although modern hip and knee joint simulatorscan simulate a dynamic loading, based on gait data,and provide wear rate, they cannot measure real-timefriction coefficient. In this study, the CoP hip jointwas simulated by a UHMWPE pin rolling on Al2O3

flat surfaces with and without dimples and DLC coat-ing to obtain friction coefficient. Although the pin-on-disc tribo-tester cannot accurately replicate anactual knee or hip joint prosthesis, it can replicateHertzian contact pressure and gait speed at the artifi-cial joint. For example, in this study, three types ofcontact pressure were considered to simulate normalweight, overweight and obese patients (Table 1). Thespeed of 10mm/s in Table 1 replicates medium walk-ing speed.

The micro indentation test showed that Al2O3

and DLC hardness were 5.1 and 8.3GPa, respectively.These values were likely to have been underestimatedas they were carried out, using with a micro indenter,as opposed to a general indenter.26,27 The othermechanical properties, presented in Table 1, includingmodulus of elasticity, surface roughness andwettability were similar to those reported in otherstudies.23,24,30

This study shows that, for the different loadingconditions, the non-dimpled ceramic surface gener-ated a lower friction coefficient (0.049–0.055)compared to the dimpled non-coated Al2O3 (0.057–0.059), the dimpled DLC-coated (0.174–0.184) andthe non-dimpled DLC-coated (0.170–0.188) surfaces.Our previous study26 reported a significant reductionin friction coefficient in ceramic on ceramic interfacewhen a dimpled surface is used. However, the samedimple parameters increased the friction coefficient inthe simulated CoP joint. The friction coefficients wereeven higher for the DLC surface (0.17–0.188), which

is considered to be a very potential substrate for jointinterface.

In many studies, dimpled surfaces were found to beeffective in reducing friction coefficient of MoP,13,14

metal-on-metal31,32 and ceramic-on-ceramic26 sur-faces. Similarly, at least two review papers reportedthat the DLC surface is promising in orthopaedicimplants.23,24 However, none of these studies actuallyreported friction coefficient profiles of polyethylene-on-DLC compared to polyethylene-on-Al2O3 sur-faces. The material properties, including modulus ofelasticity, hardness and wettability are likely to influ-ence these outcomes.

The modulus of elasticity (E) of ceramic is veryhigh (E¼ 370GPa) compared to that of UHMWPE(E¼ 0.69GPa) material. Therefore, ceramic does notdeform as much as the softer UHMWPE.Consequently, part of the deformed UHMWPEcould have come into contact with the edge of thedimples, resulting in increased friction force. Similarsituation could have been the case with the DLC sur-faces. However, DLC has a lower modulus of elasti-city (260GPa) and, hence, experiences a higherdeformation under high pressure (21.901MPa), com-pared to Ceramic. Therefore, dimpled DLC surfacehas better wear performance compared to non-dimpled DLC at the same pressure.

DLC is considered a very promising surface inimplant interfaces. It is approximately 1.5 timesharder, but much more flexible than Al2O3. Heeet al.33 reported that the ratio of hardness (H) tomodulus of elasticity (E) plays an important role infriction and wear reduction – a high H:E ratio lowersfriction and wear. Despite its very high H:E ratio(0.3115) compared to Al2O3 (0.0141), DLC showedhigher friction coefficient profiles for all loading con-ditions. This counter-intuitive result could be because:(i) the DLC surface is more hydrophobic compared toAl2O3, (ii) the surface roughness of DLC was higherthan that of Al2O3, and (iii) the DLC/UHMWPE pairmight have higher adhesion force compared to theAl2O3/UHMWPE pair.

Dong et al.34 pointed out the possibility of forma-tion of a number of pits on the transferring layer ofDLC surfaces during coating fabrication process.These pits could have embedded into UHMWPElumps, thereby producing a strong adhesive forceand, as a result, a higher friction coefficient. Thehigher DLC surface roughness (DLC: 0.29 mm;Al2O3: 0.12mm) is also an evidence of pits formationin our experimental DLC surfaces. Wettability is akey surface property that plays an important role intribological performances of DLC and other inter-faces.30 Our experimental DLC surface had a lowerwettability than Al2O3 surface. This also contributedto generating a higher friction coefficient.

The UHMWPE surface used in this study had ahigher surface roughness (1.5 mm) than those of com-mercially available implants. Although the absolute

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wear-rate values might have been affected, the com-parative values were not likely to have been since theexperimental conditions were kept consistent acrossall samples. Moreover, the use of rough polyethylenesurfaces is commonly used in wear studies.27–29

Wear is a complex, non-linear and multi-factoredprocess. It is therefore difficult to identify which fac-tors have higher influence on wear. Recent wear stu-dies on UHMWPE against DLC surfaces reportedincreased wear rate27–29 due to very high hardness ofDLC against soft polyethylene. In addition to com-paring wear between UHMWPE/Al2O3 andUHMWPE/DLC surfaces, we also investigated theeffects of dimpled DLC and Al2O3 surfaces on wearrates. Our results were counter-intuitive and did notshow improved wear performance for UHMWPE/dimpled DLC interface. This could be explained by(i) the fact that DLC has a very high modulus of elas-ticity compared to UHMWPE, which could result indeformed UHMWPE coming in contact with thedimple edges, resulting in increased wear and (ii) theexplanations given by Dong et al.34 about the pit for-mation during DLC fabrication.

Textured surfaces are believed to reduce the realcontact area and provide better lubrication distribu-tion.13,31,32 However, in this experiment, these fea-tures might not have been adequate to overcome theinfluences of material and surface properties, includ-ing hardness, surface roughness, adhesion, modulusof elasticity and wettability. Segbas et al.15 recentlyshowed a dramatic reduction of friction temperaturewhen surface-textured UHMWPE was rubbed on ametal counterpart. Similarly, Lui et al.22 and Sheejaet al.29 recommended the modification of UHMWPEsurface by DLC coating for better tribologicaloutcomes.

The main limitations of this study are the numberof cycles and unidirectional sliding motion, which donot fully replicate the motion in implanted joints.However, although this might have affected the abso-lute friction coefficient and wear rate values, the com-parative values across specimens are not likely to havebeen affected since the experimental conditions werekept consistent for all the tests.

To conclude, neither DLC nor surface texture onceramic (Al2O3) alone can improve tribological out-comes of ceramic on UHMWPE sliding pair. This isthe first study to have investigated the influence ofceramic femoral ball surface texture and coating,combined, on the tribological outcomes of CoP hipjoints.

Mechanical properties such as hardness, elasticity,surface roughness and wettability could potentiallyplay dominating roles compared to other factors,such as lubrication and contact area. In future studiesto investigate whether the durability of CoP hip jointscan be further improved, either coating or texture orboth can be produced on UHMWPE surfaces andinvestigated under a higher number of test cycles.

Funding

The research was gratefully funded by: the University of

Malaya, UMRG Grant (Project no. RG147-12AET);Brno University of Technology Excellent YoungResearcher Fund (Project no. CZ.1.07/2.3.00/30.0039); the

European Regional Development Fund (Project noCZ.1.05/1.1.00/02.0068).

Conflict of interest

None declared.

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