Fabrication and characterization of DLC coated microdimples on hip prosthesis heads

Fabrication and characterization of DLC coated microdimples on hipprosthesis heads

Dipankar Choudhury,1,2,3 Hee Ay Ching,4 Azuddin Bin Mamat,5 Jan Cizek,6

Noor Azuan Abu Osman,4 Martin Vrbka,1,2,3 Martin Hartl,1,3 Ivan Krupka1,2,3

1Faculty of Mechanical Engineering, Brno University of Technology, Technicka 2896/2, 616 69 Brno, Czech Republic2Central European Institute of Technology, Brno University of Technology, Technicka 3058/10, 616 00 Brno, Czech Republic3Institute of Machine and Industrial Design, NETME Centre, Brno University of Technology, Technicka 2896/2, 616 69 Brno,

Czech Republic4Department of Biomedical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia5Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia6Institute of Materials Science and Engineering, NETME Centre, Brno University of Technology, Technicka 2896/2, 616 69

Brno, Czech Republic

Received 10 March 2014; revised 26 July 2014; accepted 8 August 2014

Published online 00 Month 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.b.33274

Abstract: Diamond like carbon (DLC) is applied as a thin film

onto substrates to obtain desired surface properties such as

increased hardness and corrosion resistance, and decreased

friction and wear rate. Microdimple is an advanced surface

modification technique enhancing the tribological perform-

ance. In this study, DLC coated microdimples were fabricated

on hip prosthesis heads and their mechanical, material and

surface properties were characterized. An Electro discharge

machining (EDM) oriented microdrilling was utilized to fabri-

cate a defined microdimple array (diameter of 300 mm, depth

of 70 mm, and pitch of 900 mm) on stainless steel (SS) hip

prosthesis heads. The dimpled surfaces were then coated by

hydrogenated amorphous carbon (a-C:H) and tetrahedral

amorphous carbon (Ta-C) layers by using a magnetron sput-

tering technology. A preliminary tribology test was conducted

on these fabricated surfaces against a ceramic ball in simu-

lated hip joint conditions. It was found that the fabricated dim-

ples were perpendicular to the spherical surfaces and no

cutting-tools wear debris was detected inside the individual

dimples. The a-C:H and Ta-C coatings increased the hardness

at both the dimple edges and the nondimpled region. The tri-

bology test showed a significant reduction in friction coeffi-

cient for coated surfaces regardless of microdimple arrays: the

lowest friction coefficient was found for the a-C:H samples

(m 5 0.084), followed by Ta-C (m 5 0.119), as compared to the

SS surface (m 5 0.248). VC 2014 Wiley Periodicals, Inc. J Biomed

Mater Res Part B: Appl Biomater 00B: 000–000, 2014.

Key Words: surface modification, micro dimple, diamond like

carbon, hip prosthesis, implant interface

How to cite this article: Choudhury D, Ching HA, Mamat AB, Cizek J, Abu Osman NA, Vrbka M, Hartl M, Krupka I. 2014.Fabrication and characterization of DLC coated microdimples on hip prosthesis heads. J Biomed Mater Res Part B 2014: 00B:000–000.

INTRODUCTION

A total hip replacement is one of the most successfulachievements in orthopedic surgery. Well-designed artificialhip joints may perform well for up to 25 years with a goodadjustment and under normal walking conditions. However,young patients (45–64 y),1 with more active lifestyles havetheir implants only last for 10–15 years.2 Further to that, arevision hip replacement is always a more complex andcostly surgical procedure when compared with the primaryreplacement, partially because of the need to counteract thebone loss that requires sophisticated anchoring devices fit-

ted to support the prosthesis.3 Therefore, the implant dura-bility is still a major challenge that needs to be overcomealong with the possible biological response from joint-induced wear debris—both associated with wear rate aswell as other material–and patient-related factors.4,5

Microdimple is an advanced surface modificationtechnique, proven to be effective in many functional andengineering surfaces including the surface of a golf ball,6

engine cylinders,7,8 sliding bearings and mechanicalseals,9,10 and artificial hip joints.11–14 These studies haveshowed that a well-defined microdimple surface has the

Correspondence to: D. Choudhury; e-mail: [email protected] grant sponsor: NETME CENTRE PLUS (LO1202) with financial support from the Ministry of Education, Youth and Sports under the

National Sustainability Programme I

Contract grant sponsor: Excellent Young Researcher Project, Brno University of Technology; contract grant number: CZ.1.07/2.3.00/30.0039

Contract grant sponsor: Internal Grant Agency of the Ministry of Health, Czech Republic; contract grant number: NT/14267-3/2013

Contract grant sponsor: High Impact Research Grant UM.C/HIR/MOHE/ENG/10, University of Malaya

VC 2014 WILEY PERIODICALS, INC. 1

potential to reduce friction by increasing the hydrodynamicpressure and acting as a lubricant reservoir. Few studiesreported a reduced wear rate by utilizing microdimplesurfaces. In that case, an increased hardness due to themachining process was reported.15,16 Diamond like carbon(DLC) coating such as hydrogenated amorphous carbon (a-C:H) and tetrahedral amorphous carbon (Ta-C) are theexamples of advanced surface modifications allowing anachievement of the required surfaces properties such ashardness,14,17,18 wettability,19 corrosion resistance,20 andcontrolled friction and wear.21–23 However, Haque et al.24

reported that a severe damage occurred to DLC coated sur-face by a third body abrasive wear of medium sized par-ticles—thus a removal of these debris from the contactregion is certainly important for protecting the DLC coating.For example, Amanov and Sasaki16 reported that Cr-dopedDLC-coated dimpled specimen exhibited a lower frictioncoefficient (>50%) and wear rate (>50%) when comparedwith those of their polished counterparts under oil-lubricated sliding conditions; this was achieved by thecombination of the storage of wear debris in the dimplesand having high hardness of the DLC layer. Tarabolsi et al.5

also reported that microdimpled CoCrMo and Al2O3 surfa-ces significantly reduced the amount of wear debris in arti-ficial joint arthroplasty. However, the combination ofmicrodimple and hard coating such as DLC has not beeninvestigated to the artificial hip joints yet. Therefore, wehypothesize a combination of well-defined microdimplesand coated surfaces would enhance the tribological per-formance of the hip joint prosthesis thus increasing itsdurability.

The fabrication technique of precise microdimple array isa key concern, especially on complex shaped substrates.Laser surface texturing (LST), microtooling, electro dischargemachining (EDM), and chemical etching are examples ofavailable techniques. Among them, laser texturing is widelyused25,26; however, it is mostly applied in flat substrates.Matsumura and Takahashi27 fabricated microgroves on cylin-drical surfaces by using a two-flute ball end mill, a mechani-cal machining. Fabricating dimples perpendicular to the hardspherical surfaces is a major challenge of the mechanicalmachining. Excessive tool wear is also a concern in mechani-cal machining, especially microdrilling on hard surface. Fur-thermore, there is a possibility of changing in mechanicalproperties to the operated surfaces. Roy et al.28 reported anexcessive wear of microtool induced cutting tools operatedon a hard substrate such as Al2O3, and also a 13% reducedhardness around the dimple edge compared with non-dimpled region. The decreased hardness was because ofthe geometry of larger sized dimple (Ø400 mm and 30 mmdepth). Although, The LST oriented dimpled surfaces werefound to have an increased hardness,16 this could bebecause their fabricated dents were smaller in geometry(diameter: 200 mm, depth 0.5 mm), thus geometrical effectwas negligible.25 A larger and deeper dimple is preferableto hip joint prosthesis because the contact area of hip jointprosthesis is larger (8–10 mm2),29 and the larger sizedwear debris30 are expected to remove into dimples from

the contacting region, and subsequently reducing the thirdbody abrasive and their possible biological effect surround-ing tissue.13

In this study, a microdimple array was fabricated onstainless steel hip prosthesis heads by using EDM orientedmicrodrilling, and was coated with a-C:H and Ta-C layersusing a physical vapor technique (magnetron sputteringtechnology). These two types of DLC coatings wereselected with regard to their different mechanical and tri-bological properties.31 The dimple parameters were meas-ured, and their mechanical and material properties wereanalyzed. A friction test was then conducted to determinethe influence of the coated dimpled surfaces as comparedto the bulk SS specimens.

MATERIALS AND METHODS

Sample preparationHip prosthesis heads (25 mm in diameter) were machinedfrom a stainless steel (316L) bar using a CNC lathe machine(Miyano BNC-42CH), and a surface polishing up to a 0.5 mmdiamond paste was then adopted to reduce the surface rough-ness. The dimple arrays were then fabricated into the pros-thesis heads by using EDM technique (Sodic A30R) equippedwith 300 mm diameter tungsten carbide (WC) rod as the dril-ling bit. The dimple parameters (diameter of 300 mm, depthof 70 mm, and pitch of 900 mm) were selected in accordancewith the study,12 which reported a significant reduction infriction (30%) and wear rate (68%) when compared withnondimpled prostheses after 106 cycles. The operating param-eters of EDM are provided in Table I. Three circles of dimplesat various distances from the central dimple were produced:a rotation by 45�, 30�, and 15� from the initial position wasselected for the fabrication, yielding 8, 12, and 24 dimples inthe individual circles, respectively. An inclination of 1�, 2�,and 3� was used for the circles to maintain dimples-surfaceperpendicularity. The designed and fabricated dimpled surfa-ces could be seen from Figure 1(a and c), respectively. Theoperating EDM cutting tools is shown in Figure 1(b).

Following the surface finishing and dimple fabricationprocedures, two set of specimens (2 nondimpled, 2dimpled) were coated with a-C:H and Ta-C using a hybridmagnetron sputtering method whereby the coatingmachine is equipped with integrated arc and multitargetmagnetron sputtering technology. The coating process wasconducted using high purity carbon graphite and chro-mium targets of about 100 mm in diameter and 3 mm inthickness coupled with radio frequency power source. Thedeposition parameters of a:C-H are summarized in Table II.Cr was used as an interlayer,32,33 and Ag were used as adoping material in order to increase bonding strength anddecrease residual stress, respectively.22 The coating fabri-cation procedure was performed by the Nocon Technology,Selangor, Malaysia.

Mechanical and surface propertiesNanoindentation testing was carried out on the coated andnoncoated surfaces (DUH-211/DUH-211S Dynamic UltraHardness Tester, Shimadzu, Japan) in order to measure the

2 CHOUDHURY ET AL. DLC COATED MICRODIMPLE CHARACTERISTICS ON HIP PROSTHESIS HEADS

hardness and modulus of elasticity. The instrument is capa-ble of measuring dynamic indentation depth to very thinfilms and post-treated surface layers that are impossible tomeasure with conventional methods.34 This kind of mea-surement is dependent on the geometry of the indenter. The100 mN load was applied at a speed 0.66 mN/sec with aholding period of 10 sec. An 115� triangular pyramidintender was used as a tip. The parameters of the testingare provided in Table III. Five repeated readings were takenfor each of the samples.

To measure the water contact angle, a contact angle ana-lyzer (OCA15EC, Dataphysics Instruments, Germany) was used.In total, five readings were carried out for each specimen. Thesurface roughness and dimple profiles were inspected by usinga 3D optical microscopy (Contour GT-I, Brucker, Italy).

For investigating the effect of geometry on hardness amicroindenter (LECO LM247AT) was utilized and 10 read-ings were taken for each sample. In the fabricated prosthe-

sis heads, two contradictory effects on the mechanicalproperties (hardness) of the surfaces could be expected inthe vicinity of the dimple edges: an increase in hardnessdue to the machining process (i.e., work hardening effect)and a reduction because of the geometrical effect.25,28 Toidentify the relative incidence of these two influences,microhardness was measured at two different locations: ata free surface of the specimens relatively far from the fabri-cated dimples (> 50 mm), and �10 mm from the dimplesedges. As the indentations resulted in �10 mm indentationmarks, the closest corner of each indentation was �5 mmfrom the dimple edge.

To identify the chemical composition of coated surfacecross sectional analyses were performed under Gemini FieldEmission Scanning Electron Microscope (FE-SEM), Zeiss.Before that a milling operation was conducted on the coatedsurface layer by using a focused beam of ion. Because theEDM induced fabrication process involved tungsten carbide

FIGURE 1. Fabricating procedure of dimple on hip joints head (a) drawn dimple, (b) drilling process, (c) dimpled head. [Color figure can be

viewed in the online issue, which is available at wileyonlinelibrary.com.]

TABLE I. EDM Operating Parameters for Fabrication of

Defined Dimple Arrays

Parameter Value

Pulse ON time 10 secPulse OFF time 28 secPeak Current 1.0 XServo Voltage 5 VPolarity Z1

TABLE II. Deposition Parameters of a:C-H

StageTime(min.)

Current(mA)

Voltage(V)

Gas Pressure(mBar)

Pumping 30 – – –Argon Cleaning 30 – 75 2.5 (Argon)Cr Deposition 30 60 40 –DLC Deposition 90 20 160 0.5 (Acetylene)Cooling 30 – – –

ORIGINAL RESEARCH REPORT

JOURNAL OF BIOMEDICAL MATERIALS RESEARCH B: APPLIED BIOMATERIALS | MONTH 2014 VOL 00B, ISSUE 00 3

(WC) drill bit, an EDX analysis was carried out to determinethe chemical composition inside the dimples of SS and DLCsubstrates and to detect any WC-based particles in thedimpled area.

Tribology testThe tribology tests were performed using a newly designedpendulum hip joint simulator, the details of measurementprinciple of the simulator was described somewhere else.35

In total, five hip prosthesis heads (dimpled SS, nondimpledDLC, dimpled a-C:H, nondimpled Ta-C, and dimpled Ta-C)were tested against a ceramic acetabulum cup (28 mmdiameter, Biolox Delta, Zimmer). The experimental parame-ters are described in Table IV and the pendulum hip simula-tor is depicted in Figure 2. The prosthesis head is loaded bytwo handed weight bar that can be loaded and unloadedmanually. The pendulum is pulled up to 16�, and thenallowed it to be free for swinging. The frequency and theinitial peak amplitude were 0.5 Hz and 0.9 radians, respec-tively, and the peak amplitude trends to zero over timebecause of the friction coefficient. The measured angularvelocity profile is transferred to the PC and friction coeffi-cient is calculated from the recorded peak angular ampli-

tudes. The applied load was 1760 KN, considered three-time body weight during a standard walking gait for metalon ceramic hip joint.36 A bovine serum (25%) was used asa lubricant, and temperature was maintained at 37�C.

RESULTS AND DISCUSSION

Dimple profilesThe average surface roughness before and after the coatingand dimples fabrication procedures could be seen fromTable V. The surface roughness profile of the noncoated SSis shown in the Figure 3(a). The average surface roughnessof the SS samples (50 6 5 nm) is considered a mirror pol-ished surface that is equivalent to the surface roughness ofcommercially available metallic hip joints head.37 Figure3(b) shows a spectrum of DLC coated substrate—Ta-Ccontains more Sp3 bonding and fewer hydrogen ions thana-C:H, thus makes Ta-C exhibit higher hardness and modu-lus of elasticity. The surface roughness of a-C:H coatedheads either dimpled or nondimpled also was adequatelypolished (50–60 nm); however, Ta-C coated heads werefound to be considerably rougher (90–110 nm). There werea number of pits observed in the Ta-C surface, which aremainly liable of the increased roughness. Dong et al.38 alsopointed out the possibility of formation of a number of pits

TABLE III. Operating Parameters of Nanoindenter

Parameter Value

Test mode Load-unloadTest force 100 mNMinimum force 0.2 mNLoading speed 0.66 mN/secHold time at load 10 secHold time at unload 30 sec

TABLE IV. Experiment Parameters for Tribology Test

Parameters Magnitude/Description

Applied Load 1750 KNFrequency 0.5 HzStarting inclined angle 16�

Lubricant Bovine serum (25%)Temperature 37�C

FIGURE 2. Pendulum hip simulator. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]


on the transferring layer of Ta-C surfaces during coating fab-rication process. However, the obtained roughness of thesamples is still acceptable considering the surface roughnessreported in other studies investigated on artificial hipjoints.21,22 Figure 3(c,d) shows the images of a-C:H (lightdark) and Ta-C (dark)—the SEM images shows the mor-phologies of both form of DLC that are similar to the DLCimages published in other studies.39,40 However, the ratio ofSp3 and Sp2 bonding could not be measured in this study.

Table V shows the surface and geometrical parametersof the fabricated dimples. All of the fabricated dimples are

consistent and well acceptable (a standard deviation of4–7%).25,28 The 3D profile of the dimple is shown in theFigure 4(a)—a clearly visible dimple with depth of 70 6 5mm. The circular shaped of the dimple [Figure 4(a)] refersthe perpendicular position of dimple on the sphericalshaped prosthesis head. These dimples are supposed to actas reservoirs of lubricant thus promote a better lubricationdistribution.41 Moreover, they should hold wear debris, as aresult, the participation of third body abrasive wear ratereduce.13 A well-defined dimple is preferable in the contactinterface so that they can be repeatable and perform theirroles in sliding or rolling condition. The diameter and spac-ing of dimple on dimpled a:C-H and Ta-C surfaces could beseen from Figure 4(b,c). The location of the central dimplewas found to be deviated slightly from the theoretical cen-tral point due to a slip-movement of the drill bit during thedrilling process. The dimples within the three respectivecircles were fabricated accurately. The dimple parameterswere chosen because the similar profile was found to beeffective in hip joints prosthesis although these were testedin metal on polyethylene hip joint prosthesis.12

Coatings analysisThe respective thicknesses and morphological features ofthe deposited layers were determined on the cross-sectionsurfaces. It could be seen from Figure 5(a) that the a-C:Hcoated surface consists of three distinct layers—a diamond

FIGURE 3. (a) Surface roughness profiles of SS, (b) DLC family diagram, and SEM images of (c) a-C:H and (d) Ta-C. [Color figure can be viewed

in the online issue, which is available at wileyonlinelibrary.com.]

TABLE V. Surface Roughness and Dimple Parameters of the

Fabricated Samples

Materials Ra (nm) Ø (mm) Dimple parameter (mm)

DimpledSS (Head)

50 6 5 25 Ø 300 6 10; depth 70 6 5;Spacing 900 6 50

NondimpledDLC (Head)

55 6 5 25 –

DimpledDLC (Head)

60 6 5 25 Ø 300 6 10; depth 70 6 5;Spacing 900 6 50

NondimpledTaC (Head)

100 6 5 25 –

DimpledTaC (Head)

110 6 5 25 Ø 300 6 10; depth 70 6 5;Spacing 900 6 50

CeramicAcetabulumCup

256 5 28 –



structured carbon uppermost layer (�1 mm), followed bythin Cr layer (�0.15 mm) and the original SS substrate. Aslightly different structure was observed in the Ta-C coatedsamples: four layers consisting of diamond structured car-bon (0.9 mm), followed by Cr layer (0.066 mm), and a sec-

ond carbon layers in contact with the SS substrate. Thedetected chromium layer is commonly used as an interme-diate layer allowing a stronger bonding between the carbonlayers and the steel.22 According to Hauert et al.,42 very thinDCL surface can have pinholes during sliding and body fluid

FIGURE 4. Images of dimpled (a) stainless steel (under 3d profiler), (b) a:C-H under SEM, and (c) Ta-C under SEM. [Color figure can be viewed

in the online issue, which is available at wileyonlinelibrary.com.]

FIGURE 5. Cross section images of deposited (a) a-C:H and (b) Ta-C on stainless steel. [Color figure can be viewed in the online issue, which is

available at wileyonlinelibrary.com.]


can penetrate through these holes and increase the corro-sion rate. In this case they42 suggest increasing the coatingthickness up to 1 mm or more and adding an interlayer. Theinterlayer does not only increase the adhesion strength butalso acts as a protector from the corrosion.32 In this study,the both a:C-H and Ta-C surfaces had both Cr interlayer andalso coating thickness is more than 1 mm, thus suitable forartificial hip joints. Although there are arguments regardingthe biocompatibility of the Cr layers, the possibility of con-tamination of the surrounding tissue is probably unlikely inthis case, as Cr was only used as interlayer.

Mechanical and surface properties of specimenMechanical properties such as hardness, elasticity, and sur-face property such as wettability are important factors tothe tribological performance of an interface. Usually hard-ness is a very desirable parameter to reduce wear rate;however, a modulus of elasticity usually increases with theincrease of hardness, and the substrate becomes stiffer orbrittle. Moreover, a higher modulus of elasticity refers alower elastic deformation, consequently results in highercontact pressure and increased linear wear rate. Hee et al.21

mentioned in their systemic review review that the ratio ofhardness to elasticity (H/E) plays an important role inreducing friction and wear. Table VI summarizes theobtained results of the hardness and elasticity measure-ments for SS, a-C:H and Ta-C samples. The measured valuesare in accordance to similar published studies.21,22,43

Figure 6 (a,b) shows the mechanical behavior of SS, a-C:H and Ta-C samples over time and applied load, respec-tively. In accordance with the hardness and modulus of elas-ticity measurements, the residual depth was created intoTa-C sample (0.74 mm), followed by a-C:H (0.80 mm) and SS(0.93 mm). The values of the residual depth vs. the elasticrecovery (0.12 mm, 0.14 mm, and 0.20 mm for SS, a-C:H andTa-C samples, respectively, after the external load removal,confirm the elastic-plastic properties of the deposited a-C:Hand Ta-C coatings.

The superior hardness of a-C:H and Ta-C coated surfacesare in contrast with the calculated H/E ratio (Table VI); thea-C:H surfaces seem to have similar H/E value (2.9) to theSS (2.93), even, the ratio for Ta-C is smaller (2.68) than SSmaterial. The lower H/E ratio refer a material to moreceramic type (hard but brittle), which may not be preferablefor artificial hip joints considering their effect on stress

shielding into bone-implant interface.44 From this point, a-C:H could be superior among the three surfaces—harderthan SS, and more ductile than Ta-C. Hard on hard such asceramic and ceramic (CoC) hip joints is consideredadvanced hip joint prosthesis and getting popularity,45 butDCL (specially a-C:H) can even offer higher hardness andmore flexibility compare to conventional Al2O3, or ZrO2.

In this study, the water contact angles were found to be 79�

(SS), 76� (a-C:H), and 72� (Ta-C). Usually, a Hydrophilic DLC(8.3�) was found to be very effective in generating low frictioncoefficient, even in dry conditions.19 Wettability is considered amajor surface property influencing the tribological perform-ance. Moreover, it can have influence in protein deposition.However, there is a contradiction in literature whether hydro-philic or hydrophobic is better in protein deposition specially inloading bearing interfaces.22,46 In this study, the differences inwettability between the coated and noncoated surfaces are sig-nificant (a maximum difference of 9%); the obtained values aresimilar to DLC coatings published elsewhere.47,48

Geometrical effect of dimple edge on hardnessHardness was measured in two sample regions: free surfaceand near the dimple edge (Figure 7). The correspondinghardness of the SS samples was found to be HV 454 6 52and HV 391 6 30, respectively. Likewise, the hardness ofthe coated surfaces were HV 535622.7 and HV 461 6 24(a:C-H), and HV 7436 66.6 and HV 513 6 65 (Ta-C). It is tobe noted that the hardness (HV) values are different fromhardness values (HT) of Table VI as these wear measured in

TABLE VI. Mechanical and Surface Properties of the

Experimental Hip Prosthesis Heads

Property SS a:C-H Ta-CChange

(DLC/TaC)

Hardness (HT) 565 6 21 729 6 10 819 6 9 129/145%Elasticity [GPa] 193 6 17 251 6 22 306 6 27 130/158%Hardness/

Elasticity2.93 2.90 2.68 21/28%

Poisson’s ratio 0.189 0.189 0.189 –Water contact

angle (�)79 6 2 76 6 2 72 6 2 24/29%

FIGURE 6. Material behavior under nanointender (a) over time and (b)

over load. [Color figure can be viewed in the online issue, which is




different apparatus (microintenders and nanointenders). Thegeometrical effect of the dimple edges was therefore signifi-cant and prevailed over the effect of the microdrill machin-ing. This result is in accordance with the previous studywhere a reduced hardness due to dimple microtooling inAl2O3 surfaces was reported.28 Larger dimple can downgradehardness, especially at the edge of the dimple. Thus, thebreakdown could start from the edge area and eventuallypropagated to the other area. However, due to a:C-H or Ta-Ccoatings, the lowered hardness of the dimple edge recovered,as a result the durability of dimple will last longer. Similarincidences was reported by Amanov and Sasaki.16

Dimple chemical compositionsThe EDX analysis provided a composition of the dimplesurfaces for both coated and noncoated samples. The results

could be seen from Table VII. No traces of tungsten (an indi-cator of a potential presence of the drill bit debris) weredetected inside the dimples, confirming the suitability of theselected dimple manufacturing method. Furthermore, 1.5%(wt) of Silver (Ag) was detected in the DLC surfaces. The Agwas added as doping element in the DLC because it reducesresidual stress in the coating.22

Friction testThe results of the friction coefficient (COF, m) measurementsfor the investigated surfaces are shown in Figure 8. Theaverage friction coefficient trends were found to be as fol-lows: m-a:C-H < m-dimpled a:C-H < m-Ta-C < m-dimpled TaC< m-dimpled SS at 1760 N load. The relative differences offriction coefficient between the dimpled and nondimpledsurfaces are negligible and the presence of the dimplestherefore does not negatively influence the friction perform-ance of the prosthesis heads. This result is in direct contrastwith the previous studies: a significant reduction of thefriction coefficient by using dimpled ceramic on ceramic(15%)28 and metal on metal (50%, after 106 cycles)11 hipjoints was recorded. Simultaneously, a significant effect ofthe a:C-H and Ta-C layers deposition was detected: the fric-tion coefficients dropped by 61% (a:C-H) and 52% (Ta-C)when compared with SS samples, respectively. The post-testsurfaces analysis showed a minor surface roughnessincrease of the coated specimens, whereas the SS sampleshad some visible scratches.

A deep dimple can be utilized as a reservoir of micro-sized wear debris. The previous studies indicate that nano-sized wear debris is more biologically active especiallywhen under ionized conditions.5,13 Therefore, we predictthat trapped wear debris into the dimples will be less bio-logically reactive to the surrounding tissue thus improvingthe acceptability. The wear debris does not come only fromrubbing of the femoral head and the cup but also from thedrilling process during fixation, corrosion, and fretting wear.Subsequently, the debris could transfer to the surroundingtissue or the joint interface. If any hard particles migrate tothe interface, they will likely damage its surface severely.Usually, fine debris generated from rubbing area has lowerinfluence in friction and wear—third body abrasive wear

TABLE VII. EDX Analysis of the Dimpled SS and Coated a:C-

H Prosthesis Heads

Element Composition [wt%] SS DLC

Si 0.3 0.3Cr 18.0 86.1Fe 71.9 –Ni 9.6 –Ag – 1.5W – –Other 0.2 12.1

FIGURE 8. Friction test for different prosthesis heads at 1760 N load.

[Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

FIGURE 7. Geometrical effect on hardness on various experimental

surfaces. [Color figure can be viewed in the online issue, which is



rate of fine is 10 times slower than second body abrasivewear.49 However, the slow progression of the third bodyabrasive can decrease the surface fatigue strength, andmakes a propagating to development of additional wear dueto fatigue wear mechanism after certain period. The keyconcern to resolve is whether the trapped debris will beretained in the dimple for a long-term basis or releasedafter a while?

A number of studies mentioned the possibility of trap-ping the wear debris inside the dimple.13,28 In this regard,there could be two phenomena—(a) dry or starving lubrica-tion conditions, and (b) full or mix lubrication conditions.Varenberg and Etsion (2002)50 described behavior of debrisin dry condition (fretting wear)—the escaped oxide weardebris propagated from the contact region into the dimples.As a result, up to 84% reduction of debris presence fromthe contact region was observed. The debris trapped to thedimples were larger in dimensions: diameter 100–120 mm,depths 47–50 mm, density 42–47%. Again, the interface waswithout present of lubricant; thus, the debris stayed insidethe dimples until these were fully filled up. As a result, thirdbody fretting wear rate was largely reduced.50 Although theparticle mechanism in the dimple at dry condition is under-standable without any in situ observation, the behaviorunder lubrication condition is more complex. A post vitroexperiment (lubricated interfaces) surface morphologyshowed that the dimples were filled with the wear debris toa high extent—a indication of capability of entrappingdebris by the dimple in mix or full lubrication regime.5

However, a microscale flow vortex can exist inside the dim-ple during the sliding and rolling conditions.51 Althoughthese vortex flows are mainly credited for enhancing hydro-dynamic pressure, these could also influence in releasing ofthese entrapped particles from the dimples. However, in ourknowledge, there is no study available in the literature thatdescribes a complete understanding of debris behaviorinside the dimple with a presence of the microvortex flows.Kai et al.52 tried to monitor the velocity profiles of particlesinside a lubricated dimple by using fluorescent particles.Their result demonstrated that the depth of the dimplesinfluenced the particle speeds. For shallow dimple (less orequal to 10 mm depth)—the velocity of each fluorescentparticles was higher in the contact region (non-dimplearea), and decreased to the inlet and outlet of the dimples,which gradually decreased further, as these particlesapproached to the center of the dimples. Again, for deeperdimples (30–40 mm), the particles velocity approachednearly zero at the center of the dimple because the effect ofthe vortex becomes neutral in the deep zone of the dimple.However, a number of limitations can be addressed in theKai et al.52 study including the dissimilarity of materialproperty of fluorescent particles from the wear debrisincluding their affix to the contacted substrate. Furthermore,it could be imagined that the nanosized particles could becontaminated by the proteins or other biological compo-nents of the synovial fluid, and as a result, their behaviorcould be completely changed. Similarly, the clearance of thefemoral head and acetabular cup could have an impact (act

like a membrane/filter) to the entrapped debris. In sum-mary, a tight clearance and deep microdimple should havebetter capacities of trapping wear debris; however, verydeep dimple could decrease the other tribological outcomes,for example, a slight higher friction coefficient was found inthis study. Therefore, a complex study need to be conductedfor optimizing the dimple size for trapping the debris andfilm formation with different material combinations andgaits data.

The tribology experiment was studied for short durationand it is a limitation of this study. Despite of the limitationother experimental conditions were closely matched to thein vivo hip joints conditions. For example, it was conductedby using a pendulum hip simulator.35 The contact pressurewas maintained at the similar Hertzian contact pressure ofmetal on ceramic hip joints,53 temperature was maintainedat 37�C11,37 and bovine serum (25%)54 was used as a lubri-cant. The surface finish of the modified surfaces was accept-able37 (exception, a slightly rougher surface of the Ta-C).The Zimmer ceramic cup (Biolox Delta, 82% Al2O3 and17% of ZrO2) was used as an acetabulum, which indicatedhighly replication of simulated hip joints.

The studies21,22,43 pointed out fabulous performances ofTa-C in lowering friction coefficient. In our study, the a:C-Hcoated surfaces exhibited better performance compare to Ta-C.Such difference could be caused by the slightly higher surfaceroughness of Ta-C as compared to that of a:C-H. Further tothat, it has to be noted that only one set of dimple parameterswere tested and these need to be further optimized. A longrun experiment should to be investigated to justify the benefitof DLC coated dimple surfaces to the artificial hip joints.

CONCLUSIONS

This study consists of three major parts: (a) fabrication ofwell-defined dimples on spherical hip prosthesis heads, (b)deposition of a:C-H and TaC coatings over the dimpled surfa-ces, and (c) conducting a tribology test replicated of artificialhip joints. From these results, it could be concluded that:

� Controlled electro discharge machining (EDM) orientedmicrodrilling is a suitable method for a precise fabricationof dimples on curved (spherical) surfaces, yielding dimplesperpendicular to the surface of prosthesis head with nodetectable wear debris or cutting chip found in the dimples.

� A multilayer a:C-H /Ta-C film structure was detected dueto the presence of a relatively thin Cr layer, included forincrease in mutual layers cohesion.

� The a:C-H /Ta-C films significantly improve the surfacehardness of the samples; the Ta-C surface exhibitedhigher values of wettability and hardness, whereas supe-rior surface finish (roughness) and modulus of elasticitywere obtained for a:C-H coated samples; both a:C-H andTa-C coated samples showed similar dynamic materialbehavior, different from that of SS samples.

� The geometrical effect of dimple reduces hardness of thematerial in a stronger manner than the material workhardening from the EDM drilling procedure



� The friction coefficient of the SS samples is significantlyreduced after the deposition of a:C-H and Ta-C layers; theinfluence of dimples on the friction coefficients is negligible;the changed roughness was negligible to a:C-H /Ta-C surfaceswhereas it was remarkable to stainless steel prosthesis head.

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Fabrication and characterization of DLC coated microdimples on hip prosthesis heads

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