CERAMICSINTERNATIONAL

Available online at www.sciencedirect.com

http://dx.doi.org/0272-8842/& 20

nCorrespondinnnCorrespond

versity of Malayfax: þ60 3 7967

E-mail addreah_sarhan@um.e

(2015) 13055–13063

Ceramics International 41 www.elsevier.com/locate/ceramint

Enhancing the adhesion strength of tantalum oxide ceramic thin film coatingon biomedical Ti–6Al–4V alloy by thermal surface treatment

B. Rahmatia,n, E. Zalnezhadb, Ahmed A.D. Sarhana,d,nn, Z. Kamiaba, B. Nasiri Tabrizic, W.A.B.W. Abasc

aDepartment of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, MalaysiabDepartment of Mechanical Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 133-791, Korea

cDepartment of Biomedical Engineering, University of Malaya, 50603 Kuala Lumpur, MalaysiadDepartment of Mechanical Engineering, Faculty of Engineering, Assiut University, Assiut 71516, Egypt

Received 25 June 2015; received in revised form 13 July 2015; accepted 14 July 2015Available online 22 July 2015

Abstract

In this study, Titanium alloy (Ti–6Al–4V) substrate was coated with 4 μm tantalum oxide (Ta–O) ceramic layers using physical vapordeposition magnetron sputtering (PVDMS). The coated sample surfaces were heat treated at 300, 400 and 500 1C in a box furnace to enhance theadhesion strength. Field Emission Scanning Electron Microscopy (FESEM) together with energy dispersive X-ray spectroscopy (EDX), X-raydiffraction analysis (XRD) and Vickers microhardness testing was used to distinguish the Ta–O coatings in terms of morphology, surfaceelements, structure and surface hardness, and to compare untreated and hardened sample conditions. The effect of heat treatment on adhesion wastested with scratch test equipment. The coating thin film adhesion strength improved from 713 to 1907 mN throughout the thermal surfacetreatment duration. The effect of thermal treatment on surface hardness was examined, and evidently, the surface hardness increased from 446.24to 535.5 HV on account of the surface treatment at 500 1C.& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: PVD magnetron sputtering; Coating; Ti–6Al–4V; Ceramic tantalum oxide; Biomaterial

1. Introduction

Metallic implants fabricated with materials such as titanium,tantalum, chrome, cobalt and stainless steel have been inroutine clinical use for several years. Medical grade Titaniumalloys (Ti–6Al–4V) are widely used as implant materials dueto their high strength-to-weight ratio, corrosion resistance,biocompatibility and osseointegration properties [1]. Thecombination of favorable mechanical properties and highbiocompatibility is the reason why titanium and its alloys arestandard in implant applications, as these come in contact withhard and/or soft tissue [2,3]. These alloys are a preferred

10.1016/j.ceramint.2015.07.09015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author.ing author at: Department of Mechanical Engineering, Uni-a, 50603 Kuala Lumpur, Malaysia. Tel.: þ60 3 7967 4593;5330.sses: r.bijan@yahoo.com (B. Rahmati),du.my (A.o.s.A.D. Sarhan).

choice, particularly for the replacement of hard tissue, e.g., asanchoring parts in total hip and knee arthroplasty, due to thebulk material's low elastic modulus combined with highfatigue strength [4]. On account of the high biocompatibilityof titanium, it is not recognized as a foreign material by thecellular environment [5,6]. Seldom, a short while afterimplanting, aseptic loosening of titanium-based prosthesesmay occur. The relative movement between hard tissue/bonecement and the implant may provoke debris abrasion from theprosthesis surface. These particles form on the native oxidelayer that is only a few nanometers thick, has low mechanicalstability and does not tolerate such movement [7]. Smallabrasive particles generated cause an inflammatory reaction ofthe surrounding tissue, which leads to inevitable bone loss[8,9]. Researchers have studied the improvement of titaniumsurface tribological properties in several investigations. Dif-ferent coating technologies, such as thermal and electrochemi-cal oxidation techniques [10] as well as physical vapor

Table 1Composition of Ti–6Al–4V.

Component Wt%

Al 6V 4O Max 0.2Fe Max 0.25Ti Balance

Table 2Deposition parameters.

PVD variable factors Quantity

Base pressure 2� 10�5

Power DC(watt) 200Argon flow rate (%) 50Oxygen flow rate (%) 6DC bias (volt) 0Temperature (1C) 140Time (min) 120

Time (min)

T(°C)

TargetTemperature

AmbientTemperature

Steady (60min)

Heatin

g (5°Cmin

¹ )Cooling (Naturally)

-

Fig. 1. Temperature vs time schematic graph of thermal processing.

B. Rahmati et al. / Ceramics International 41 (2015) 13055–1306313056

deposition (PVD) [11,12] and chemical vapor deposition(CVD) [13,14] have particularly been evaluated. Metal oxidesand nitrides as hard material layers can enhance the abrasionresistance of bulk material. The unexpected transition from ahard/brittle surface coating to a softer/brittle substrate surfaceis a significant disadvantage of metal oxide and nitridematerials. Transition from a hard to soft coating layer wouldbe more favorable to avoid coating delamination whenmechanical load is applied. In case of bulk titanium and Ti-alloys, surface hardness enhancement can be achieved byoxygen diffusion hardening [15,16].

Good biocompatibility, ductility and excellent corrosion resis-tance render tantalum metal an excellent biomaterial [17,18].Hence, tantalum metal is considered for clinical applications as agood titanium biomaterial. However, tantalum is more expensivethan titanium and has much greater density, which prohibits itsapplication in weight-bearing implants. For these reasons, sometechniques have been developed to facilitate depositing tantalumand its oxide films onto other substrates, such as silicon as well astitanium and its alloys [19,20]. The refractory metal Ta is chosen,for it is a transition metal (atomic number 73; atomic weight180.05 amu), it remains relatively inert in vivo and its reactionwith oxygen (O2) occurs much faster compared to Ti–6Al–4V[21]. Due to the volume increase accompanying this reaction,self-induced crack healing will occur on the damaged surface[22]. Researchers have reported that TaOx are porous [23].Moreover, has also been claimed that Ta2O5 film, when lessthan 8 nm thick, has superior electron contributing properties[24]. This amplifies the need to comprehend TaOx species thatare present not only on the surface but also throughout thematerial depth. Although the most stable form of tantalum metalis the pentavalent state, it was demonstrated equally capable ofhaving multiple oxidation states in unsaturated TaOx [25,26].

The aim of the present study is to further develop the surfacetreatment in order to generate a Ta–O hard coating onto a Ti–6Al–4V substrate surface. For this purpose, the substrates arecoated with a thin Ta/Ta–O layer of approximately 4 μm usingphysical vapor deposition magnetron sputtering (PVDMS).Thermal surface treatment is applied to the Ta–O coatings toachieve both adhesion strength and a hardened surface withhigh abrasion resistance. Scratch testing is done to identify thecoating-substrate adhesion strength. The microstructure ofuntreated and treated Ti–6Al–4V coating is analyzed usingField Emission Scanning Electron Microscopy (FESEM),energy dispersive X-ray (EDX) and X-ray energy diffraction(XRD). Scratch and microhardness testing serve to distinguish

the coatings' adhesion strength and surface hardness. Thesurface roughness of the coated Ti–6Al–4V substrate surfaceis measured using Atomic Force Microscopy (AFM).

2. Materials and experimental design

2.1. Sample preparation and coating process

Ti–6Al–4V substrates were cut into 25 mm� 10 mm� 2 mmpieces. The Ti–6Al-4V composition is specified in Table 1. Beforethe experiment, the specimens were ground with SiC paper (800–2500 grit), after which the surfaces were polished to a mirror finishwith diamond liquid [27]. To remove contamination, the sampleswere washed with distilled water and blow dried using a hair dryer,then ultrasonically cleaned in acetone for 10 min. The substrateswere ground and polished to 0.05 μm roughness.An SG Control Engineering Pte Ltd series magnetron

sputtering system was utilized to deposit thin Ta/Ta–O film.The samples were placed in the deposition chamber of thePVD magnetron sputtering system. The chamber was thenevacuated to a base pressure of about 2� 10�5 Torr. In orderto reduce water vapor content in the vacuum chamber [28], Tialloy was sputtered under argon during substrate ion etchingprior to deposition. Pure Ta (99.99%) film was deposited ontothe substrate surface for 20 min, and the process was continuedby depositing Ta and oxygen (O2) onto Ti–6Al–4V substratesurface for 120 min. Ta as target and oxygen as additional gaswere used to deposit tantalum oxide (Ta–O) thin film ontoTi–6Al–4V substrate. The sputter parameters were adjusted asshown in Table 2 to achieve maximum coating stability atacceptable deposition rates and homogeneity based on pre-vious pilot experiments. Zero bias voltage was applied toTi–6Al–4V, while argon flow rate and oxygen flow rate wereadjusted to 50% and 6% respectively, during deposition at140 1C substrate temperature with 200 W DC power voltage.

Ta-O

Ti-6Al-4V 5

Ta-O

Ti-6Al-4V 5μm

Ta-O

Ti-6Al-4V 5μm

μm

Ta-O

Ti-6Al-4V 5μm

Untreated thin film coating

Thin film coating treated at 300°C

Thin film coating treated at 400°C

Thin film coating treated at 500°C

Fig. 2. FESEM cross-sectional view and intensity micrographs of coated samples: (a) untreated surface, (b) thermally treated surface at 300 1C, (c) thermally treatedsurface at 400 1C, and (d) thermally treated surface at 500 1C.

B. Rahmati et al. / Ceramics International 41 (2015) 13055–13063 13057

2.2. Thermal treatment of Ta–O coating

The thermal treatment in this research was carried out in abox furnace to improve the adhesion of Ta–O coating ontoTi–6Al–4V substrate. This required careful monitoring of theheat-up rate until the target temperature was reached. The three

annealing temperatures for heat treatment were set at 300, 400and 500 1C [21,29–31]. The heating rate was adjusted to5 1C min�1 until the target temperature was reached, at whichthe specimens were kept for 60 min [32,33]. In the final step,the samples were left in the furnace to cool naturally. Fig. 1shows a schematic graph of the thermal treatment process.

Fig. 3. XRD patterns of the substrate (Ti–6Al–4V) and as-prepared coatingsbefore and after thermal treatment at 300, 400, and 500 1C for 1 h; S1 forsubstrate; S2 for coated substrate; S3, S4 and S5 for coated samples treated at300, 400 and 500 1C, respectively.

B. Rahmati et al. / Ceramics International 41 (2015) 13055–1306313058

2.3. Adhesion and morphological analysis

The samples were tested with a scratch test device to determinethe thin film coating's adhesion strength. Scratch testing is aconvenient means of studying the adhesion strength of thin film

coatings [34]. Scratch testing is suitable for thicknesses rangingfrom 0.1 to 20 μm [35]. This test is commonly carried out in bothacademic and industrial fields. A scratch tester quantitativelymeasures film-to-substrate adhesion strength. In this study, aninitial load of zero was applied onto a sample with a Rockwell-type diamond indenter (25 μm radius). The load was graduallyincreased by 9.2 mN/s at a sliding velocity of 2 μm/s. During thepresent test, the scratch length was 700 μm. In addition, thecritical load (Lc) in the scratch test was used to calculate theadhesion strength. In order to obtain the Lc value, an acousticsignal, friction curve and microscope observation were utilized.Acoustic signal produced by film delamination may serve incharacterizing Lc. The thin film coating's surface and crosssections' microstructure were analyzed using FESEM and energydispersive X-ray (EDX). Specimens for cross-sectional analysiswere cut with a high-precision diamond cutting saw in thepresence of coolant. The transverse sections of the coatedspecimens were mounted and polished metallographically to beexamined under FESEM. Phase structure and purity wereexamined by grazing incidence X-ray diffraction (GIXRD)analysis with a PANalytical Empyrean X-ray diffractometer(Cu–Kα radiation) over a 2θ range from 101 to 801.

3. Results and discussion

3.1. Thin coating layer deposition (cross sectional graphanalysis)

FESEM images of the cross sections before and after thermalsurface treatment, and an intensity graph of the related samplesare shown in Fig. 2(a)–(d). Fig. 2(a) shows the cross section andintensity graph of an untreated specimen. Fig. 2(b)–(d) illustratethe cross section and intensity graph of samples treated at 300,400 and 500 1C, respectively. The FESEM images indicate thecoatings' lamellar structure. Also, Fig. 2 presents the diffusion rateof Tantalum and oxygen and chemical composition of Ti–6Al–4V, Ta–O and Ti. The influence of film morphology on theelectrochemical properties of tantalum oxide is evident.

3.2. Coating phase XRD analysis

The XRD patterns were compared to standard cardJCPDS005-0682 for Ti, JCPDS004-0788 for Ta, JCPDS015-0206 for Ta6O and JCPDS008-0255 for Ta2O5. Fig. 3 displaysthe XRD patterns of the substrates (Ti–6Al–4V) and as-prepared coatings before and after thermal treatment at 300,400, and 500 1C for 1 h. In accordance with this figure, theXRD profile of the substrate (S1) contains several peakslocated at approximately 35.41, 38.51, 40.41, 53.31, 63.61,711 and 76.91, which were attributed, respectively, to the(1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 1 2) and (2 0 1)orientations of the Ti–6Al–4V substrate (JCPDS005-0682).According to the S2 XRD profile following the PVD process,the main characteristic peaks of the as-prepared coating belongto high crystalline Ta (JCPDS004-0788) and Ta6O(JCPDS015-0206), which overlap due to significant phaseevolution after the PVD process. In this case, no Ta2O5

Fig. 4. FESEM surface topography of coated Ti–6Al–4V samples: (a) untreated, (b) treated at 300 1C, (c) treated at 400 1C, and (d) treated at 500 1C.

B. Rahmati et al. / Ceramics International 41 (2015) 13055–13063 13059

characteristic peaks were detected in the XRD patterns,suggesting that no crystallization occurred during the coatingprocess. During surface thermal treatment at 300 (S3), 400(S4) and 500 1C (S5), tantalum oxide layer crystallizationoccurred, and consequently, Ta2O5 characteristic peaks withorthorhombic structure (JCPDS008-0255 for Ta2O5) wereidentified.

3.3. FESEM and EDX coating surface characterization

Fig. 4(a)–(d) represents the FESEM of Ta–O coatings onspecimen surfaces before and after thermal treatment. Fig. 4(a)shows the surface coating before thermal treatment and Fig. 4(b)–(d) shows the surface coatings after thermal treatment at300, 400 and 500 1C, respectively. According to these figures,the surface coating became denser as the temperature increasedfrom 300 to 500 1C. Above 300 1C, the Ta–O density growthrate increased strongly due to Ta self-oxidation from theoxygen in the furnace along with thermal treatment at highertemperature. The FESEM characterization reveals that different

thermal treatment temperatures produced different surfacemorphologies during surface treatment. The dense and smoothcoating in the current study is possibly a result of the presenceof additional Ta–O, high oxygen ion concentration, hightemperature and thermal treatment time, all of which favoredthe formation of a uniform coating.The elemental composition of Ta–O coating surface was

identified before and after thermal treatment by energydispersive X-Ray analysis (EDX). Fig. 5(a)–(d) shows theEDX before and after thermal treatment. From these graphs, itis found that the oxygen element density on the coated surfaceincreased during surface thermal treatment from 300 to 500 1C.Fig. 5(a) shows the wt% of Ta and oxygen before thermaltreatment and Fig. 5(b)–(d) presents the densities of Ta andoxygen elements after thermal treatment at 300, 400, and500 1C, respectively. Fig. 5(d) indicates the highest percentageof oxygen on the treated surface at higher thermal treatmenttemperature (500 1C). In contrast, Fig. 5(a) shows the lowestpercentage of oxygen before thermal treatment. Besides, Fig. 5(b) and (c) indicates that the percentage of oxygen was higher

2 4 6 8 10keV

0

2

4

6

8

10

12

14

16

18

cps/eV

O

Ta

Ta

Ta

Tantalum

Oxygen

2 4 6 8 10keV

0

2

4

6

8

10

12

14

16

18

cps/eV

O Ta

Ta

Ta

2 4 6 8 10

keV

0

2

4

6

8

10

12

14

16

18

cps/eV

O Ta

Ta

Ta

2 4 6 8 10keV

0

2

4

6

8

10

12

14

16

18

cps/eV

O Ta

Ta

Ta

Fig. 5. EDX of coated Ti–6Al–4V substrate surface: (a) untreated Ta–O layer, (b) Ta–O layer treated at 300 1C, (c) Ta–O layer treated at 400 1C, and (d) Ta–Olayer treated at 500 1C.

B. Rahmati et al. / Ceramics International 41 (2015) 13055–1306313060

after treatment compared to the untreated thin film coating.Moreover, these graphs (Fig. 5(b) and (c)) illustrates that theformation of tantalum oxide was higher at 400 1C compared with300 1C thermal treatment. Essentially, the tantalum oxide layerhad formed before surface treatment but concluded after thermaltreatment completed. In particular, oxidation increased graduallyduring thermal surface treatment from 300 to 500 1C. Theseresults are in complete agreement with the XRD results in Fig. 3.

3.4. Scratch test analysis of thin layer coating

In this study, scratch adhesion testing was performed onuntreated and treated samples to measure the Lc. The criticalloads are specified in Fig. 6(a)–(d), which show the failurecharacter of Ta–O-coated Ti–6Al–4V. Fig. 1(a) represents thescratch failure load before thermal treatment. Fig. 6(b)–(d)illustrates the Lc after thermal treatment at 300, 400 and

Fig. 6. Load and depth vs distance graphs for: (a) untreated Ta–O layer with 713 mN Lc, (b) Ta–O layer treated at 300 1C with 911 mN Lc, (c) Ta–O layer treatedat 400 1C with 1617 mN Lc, and (d) Ta–O layer treated at 500 1C with 1907 mN Lc.

B. Rahmati et al. / Ceramics International 41 (2015) 13055–13063 13061

500 1C, respectively. In Fig. 6(a) it is observed that the criticalload to remove the coating layer is 713 mN. However, inFig. 6(b)–(d), the adhesion strengths observed are 911, 1617

and 1907 mN, respectively. According to the scratch testresults, thermal treatment affects thin film coating adhesion,whereby thin film layer adhesion to the substrate is stronger at

Fig. 7. Critical load vs temperature graph, before and after thermal surfacetreatment at different temperatures.

B. Rahmati et al. / Ceramics International 41 (2015) 13055–1306313062

500 1C (Fig. 6(d)) than at lower temperatures. This may be dueto layer atoms penetrating into the substrate surface, creating amore powerful bond at temperatures increasing from 300 to500 1C.

As per Fig. 2(a), a Ta–O thin film coating formed on the Ti–6Al–4V substrate surface during the PVDMS process. XRDanalysis (Fig.3 (S2)) was conducted for Ta6O on Ti–6Al–4Vsubstrate surface. Moreover, in the EDX analysis of non-thermally treated specimens illustrated in Fig. 5(a), thepresence of Ta and O in the thin film coating compositionwas proven. The untreated sample scratch test demonstrated713 mN adhesion, which is not sufficient to deter layerdelamination from the Ti–6Al–4V substrate. Due to theshallow diffusion of Ta–O into the substrate surface, surfacethermal treatment was carried out on the coated substrates atdifferent temperatures (300, 400 and 500 1C). According toFig. 2(b)–(d), the layers became denser and the percentage ofoxygen element rose throughout thermal treatment from 300 to500 1C, respectively. Moreover, the diffusion of Ta andoxygen into the substrate surface is evident, as the surfacegradually grew. This indicates that coating adhesion increasedfrom 713 to 911, 1617 and 1907, as represented in Fig. 6(a)–(d), respectively. The rise in oxygen element in the Ti–6Al–4Vsubstrate surface coating layer may have happened owing tothe free oxygen in the furnace and oxygen captured in thecoating layer. The oxygen reacted with free Ta in the coatinglayer. Sufficient reaction time and proper conditions wereavailable for tantalum oxide to form during thermal treatment.As seen in the XRD analysis (Fig. 3 (S3, S4, S5) oxidationcompleted at 300, 400 and 500 1C, respectively. Theseconditions facilitated strong adhesion between the Ta–O thinfilm layer and Ti–6Al–4V substrate surface. Essentially, Ta–Odiffused into the substrate surface throughout thermal treat-ment. Besides, transition metal oxides are theoretically morestable due to a large decrease in the standard Gibbs energy.Thus, TaO2 and Ta2O5 as transition metal oxides are stable[36]. The reaction of TaOx is:

2TaO2þO�22Ta2O5þ2e ð1Þ

This formula indicates that both TaO2 and Ta2O5 are stable [37].

Fig. 7 displays the increasing trend of the thin film-substratesurface critical load. The Lc improved after thermal treatmentat 300, 400 and 500 1C, respectively (Fig. 6), meaning the Ta–O thin film coating elements diffused properly into the Ti–6Al–4V substrate surface, which did not easily delaminate.

4. Conclusions

This study demonstrated that the adhesion strength of Ta–Othin film coating is more sensitive to thermal surface treatmentat increasing temperature from 300 to 500 1C. The diffusion ofTa–O layer elements and Ti–6Al–4V surface elements intoeach other as a result of temperature plays a major role in theadhesion strength between the coating layer and Ti–6Al–4Vsubstrate surface. Processing temperature seems to consider-ably affect adhesion strength. Thus, higher temperature favorssuperior adhesion strength. The higher adhesion strength(1907 mN) observed at higher thermal treatment temperature(500 1C) can be explained by the better penetration of Ta–Oelements into the substrate surface as well as the formation of adense, thin layer coating. Moreover, thermal treatment is agood reason why Ta2O5 forms, which is a type of Ta–O withstable oxidation and whose adhesion strength is affected bythermal treatment temperature.

Acknowledgments

This research was supported by the High Impact Research(HIR) Grant UM.C/HIR/MOHE/ENG/44 and Institute ofResearch Management & Monitoring (IPPP) Grant PG100/2013B from University of Malaya.

References

[1] J. Parthasarathy, et al., Mechanical evaluation of porous titanium (Ti6–Al–4V) structures with electron beam melting (EBM), J. Mech. Behav.Biomed. Mater. 3 (3) (2010) 249–259.

[2] D.M. Brunette, et al., Titanium in medicine: material science, surfacescience, engineering, biological responses, and medical applications,Springer-Verlag, Berlin, Heidelberg, 2001.

[3] H. Farnoush, et al., Modification of electrophoretically deposited nano-hydroxyapatite coatings by wire brushing on Ti–6Al–4V substrates,Ceram. Int. 38 (6) (2012) 4885–4893.

[4] P. Dearnley, A review of metallic, ceramic and surface-treated metalsused for bearing surfaces in human joint replacements, Proc. Inst. Mech.Eng., H J. Eng. Med. 213 (2) (1999) 107–135.

[5] F. Guillemot, Recent advances in the design of titanium alloys fororthopedic applications, Expert Rev. Med. Dev. 2 (6) (2005) 741–748.

[6] C.-K. Lee, Fabrication, characterization and wear corrosion testing ofbioactive hydroxyapatite/nano-TiO2 composite coatings on anodic Ti–6Al–4V substrate for biomedical applications, Mater. Sci. Eng.: B 177(11) (2012) 810–818.

[7] N.J. Hallab, J.J. Jacobs, Orthopedic implant fretting corrosion, Corr. Rev.21 (2–3) (2003) 183–214.

[8] E.A. Salvati, F. Betts, S.B. Doty, Particulate metallic debris in cementedtotal hip arthroplasty, Clin. Orthop. Relat. Res. (1993)160–173293 (1993) 160–173.

[9] H.-G. Willert, et al., Crevice corrosion of cemented titanium alloy stemsin total hip replacements, Clin. Orthop. Relat. Res. (1996)51–75333 (1996) 51–75.

B. Rahmati et al. / Ceramics International 41 (2015) 13055–13063 13063

[10] D. Velten, et al., Preparation of TiO2 layers on Cp–Ti and Ti6–Al–4V bythermal and anodic oxidation and by sol–gel coating techniques and theircharacterization, J. Biomed. Mater. Res. 59 (1) (2002) 18–28.

[11] E. Zalnezhad, A.A. Sarhan, M. Hamdi, Optimizing the PVD TiN thinfilm coating's parameters on aerospace AL7075-T6 alloy for highercoating hardness and adhesion with better tribological properties of thecoating surface, Int. J. Adv. Manuf. Technol. 64 (1–4) (2013) 281–290.

[12] E. Zalnezhad, A.A.D.M. Sarhan, M. Hamdi, Prediction of TiN coatingadhesion strength on aerospace AL7075-T6 alloy using fuzzy rule basedsystem, Int. J. Precis. Eng. Manuf. 13 (8) (2012) 1453–1459.

[13] J.-H. Ouyang, et al., Mechanical and unlubricated tribological propertiesof titanium-containing diamond-like carbon coatings, Wear 266 (1–2)(2009) 96–102.

[14] C. Moseke, et al., Hard implant coatings with antimicrobial properties, J.Mater. Sci.: Mater. Med. 22 (12) (2011) 2711–2720.

[15] Z.X. Zhang, et al., The effect of treatment condition on boost diffusion ofthermally oxidised titanium alloy, J. Alloy. Compd. 431 (1–2) (2007)93–99.

[16] C. Hertl, et al., Oxygen diffusion hardening of cp-titanium for biomedicalapplications, Biomed. Mater. 5 (5) (2010) 054104.

[17] Y.X. Leng, et al., The biocompatibility of the tantalum and tantalumoxide films synthesized by pulse metal vacuum arc source deposition,Nucl. Instrum. Methods Phys. Res. Sec. B: Beam Interact. Mater. Atoms242 (1–2) (2006) 30–32.

[18] C. Balagna, M. Faga, S. Spriano, Tantalum-based multilayer coating oncobalt alloys in total hip and knee replacement, Mater. Sci. Eng.: C 32 (4)(2012) 887–895.

[19] L. Chongyan, et al., Tribological performances of Ti6Al4V alloymodified by Ta ion implantation, Rare Metal Mater. Eng. 37 (3) (2008)556–560.

[20] W. Yang, et al., Biomedical response of tantalum oxide films depositedby DC reactive unbalanced magnetron sputtering, Surf. Coat. Technol.201 (19) (2007) 8062–8065.

[21] B.R. Levine, et al., Experimental and clinical performance of poroustantalum in orthopedic surgery, Biomaterials 27 (27) (2006) 4671–4681.

[22] F. Macionczyk, B. Gerold, R. Thull, Repassivating tantalum/tantalumoxide surface modification on stainless steel implants, Surf. Coat.Technol. 142–144 (0) (2001) 1084–1087.

[23] T.M. Suzuki, et al., Visible light-sensitive mesoporous N-doped Ta2O5

spheres: synthesis and photocatalytic activity for hydrogen evolution andCO2 reduction, J. Mater. Chem. 22 (47) (2012) 24584–24590.

[24] V. Macagno, J. Schultze, The growth and properties of thin oxide layerson tantalum electrodes, J. Electroanal. Chem. Interfacial Electrochem.180 (1) (1984) 157–170.

[25] K. Wang, et al., In situ spectroscopic observation of activation andtransformation of tantalum suboxides, J. Phys. Chem. A 114 (7) (2010)2489–2497.

[26] M. Khanuja, et al., XPS depth-profile of the suboxide distribution at thenative oxide/Ta interface, J. Electron. Spectr. Relat. Phenom. 169 (1)(2009) 41–45.

[27] Zalnezhad, E., A.A. Sarhan, and M. Hamdi. Adhesion strength predictingof Cr/CrN coated Al7075 using fuzzy logic system for fretting fatigue lifeenhancement, in: Proceedings of the World Congress on Engineering andComputer Science, 2013.

[28] S. Gangopadhyay, et al., Composition and structure-property relationshipof low friction, wear resistant TiN–MoSx composite coating deposited bypulsed closed-field unbalanced magnetron sputtering, Surf. Coat. Tech-nol. 203 (12) (2009) 1565–1572.

[29] H. Grüger, et al., High quality r.f. sputtered metal oxides (Ta2O5, HfO2)and their properties after annealing, Thin Solid Films 447–448 (0) (2004)509–515.

[30] K.-M. Yin, et al., Oxidation of Ta diffusion barrier layer for Cumetallization in thermal annealing, Thin Solid Films 388 (1–2) (2001)27–33.

[31] D. Dorranian, et al., Effects of low temperature on the characteristics oftantalum thin films, Vacuum 86 (1) (2011) 51–55.

[32] C. Hertl, et al., Structural characterisation of oxygen diffusion hardenedalpha-tantalum PVD-coatings on titanium, Mater. Sci. Eng.: C 41 (0)(2014) 28–35.

[33] Y. Leng, et al., The biocompatibility of the tantalum and tantalum oxidefilms synthesized by pulse metal vacuum arc source deposition, Nucl.Instrum. Methods Phys. Res. Sec. B: Beam Interact. Mater. Atoms 242(1) (2006) 30–32.

[34] A.-T. Akono, F.-J. Ulm, Scratch test model for the determination offracture toughness, Eng. Fract. Mech. 78 (2) (2011) 334–342.

[35] J. Li, W. Beres, Three-dimensional finite element modelling of the scratchtest for a TiN coated titanium alloy substrate, Wear 260 (11–12) (2006)1232–1242.

[36] T. Ushikubo, Recent topics of research and development of catalysis byniobium and tantalum oxides, Catal. Today 57 (3–4) (2000) 331–338.

[37] Wei, Z., et al. Highly reliable TaOx ReRAM and direct evidence of redoxreaction mechanism. in: Proceedings of the Electron Devices MeetingIEDM 2008, IEEE International, 2008.