Improved tribological, electrochemical and biocompatibility properties of Ti6Al4V alloy by gas-nitriding and Ti–C:H coating

Improved tribological, electrochemical and biocompatibility properties of Ti6Al4V alloy by gas-nitriding and Ti–C:H coating

    Improved tribological, electrochemical and biocompatibility properties of Ti6Al4V alloy by gas-nitriding and Ti–C:H coating W.H. Kao,...

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    Improved tribological, electrochemical and biocompatibility properties of Ti6Al4V alloy by gas-nitriding and Ti–C:H coating W.H. Kao, Y.L. Su, J.H. Horng, H.C. Huang, S.E. Yang PII: DOI: Reference:

S0257-8972(15)30332-7 doi: 10.1016/j.surfcoat.2015.10.035 SCT 20649

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

24 June 2015 26 September 2015 18 October 2015

Please cite this article as: W.H. Kao, Y.L. Su, J.H. Horng, H.C. Huang, S.E. Yang, Improved tribological, electrochemical and biocompatibility properties of Ti6Al4V alloy by gas-nitriding and Ti–C:H coating, Surface & Coatings Technology (2015), doi: 10.1016/j.surfcoat.2015.10.035

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ACCEPTED MANUSCRIPT Improved tribological, electrochemical and biocompatibility properties of Ti6Al4V

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alloy by gas-nitriding and Ti-C:H coating

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W. H. Kao a*, Y. L Su b, J.H. Horng c, H.C. Huanga, S. E. Yangd Institute of Mechatronoptic Systems, Chienkuo Technology, Changhua, Taiwan.

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Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan

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Department of Power Mechanical Engineering, National Formosa University, Yunlin, Taiwan.

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Department of Beauty Science and Graduate Institute of Beauty Science Technology, Chienkuo

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Corresponding author

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Technology University, Changhua, Taiwan.

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a

Wen-Hsien Kao, Ph. D.

Tel: +886-4-7111111 ext. 3921

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Fax: +886-4-7111164

E-mail: [email protected]

ACCEPTED MANUSCRIPT ABSTRACT

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Ti-C:H coatings are deposited on untreated Ti6Al4V alloy samples and high-temperature

nitrided

Ti6Al4V

(N-Ti6Al4V),

T-C:H-coated

Ti6Al4V

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Ti6Al4V,

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gas-nitrided Ti6Al4V alloy samples by means of CFUBMS. Therefore, four types of specimen, i.e., (Ti-C:H/Ti6Al4V)

and

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T-C:H-coated nitrided Ti6Al4V (Ti-C:H/N-Ti6Al4V) are considered. The microstructure, adhesion and hardness of the Ti-C:H coatings are examined using a Raman spectrometer, scratch tester and

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nanoindenter, respectively. The tribological properties of the coated and uncoated samples are

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investigated using a reciprocating sliding wear tester in NaCl solution. The corrosion performance of

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the four specimens is evaluated by means of potentiodynamic polarization tests. Finally, the

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biocompatibility of the various samples is investigated using purified mouse leukaemic monocyte macrophage cells (Raw 264.7). Overall, the results show that the gas-nitriding treatment increases both

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the hardness and the surface roughness of the samples. Moreover, of the various samples, the Ti-C:H/Ti6Al4V sample exhibits excellent tribological properties and the best corrosion resistance and biocompatibility performance. Keywords: Gas-nitriding; Ti-C:H coating; Tribology; Corrosion; Biocompatibility.

1.

Introduction Titanium alloys such as Ti6Al4V are widely used in the biomedical engineering field due to their

high strength-to-weight ratio, low density, high corrosion resistance, and good biocompatibility [1].

ACCEPTED MANUSCRIPT However, although Ti6Al4V is often used for orthopaedic implants [2, 3], it has a low surface hardness

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and a poor wear resistance [4-6]. The particles or metal ions subsequently produced at the joint surface

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enhance osteolysis, and therefore increase the risk of healthy osseous tissue deterioration [7, 8]. In

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order to overcome this problem, and prolong the implant life, various surface treatments have been proposed for improving the wear resistance properties of Ti6Al4V, including thermal oxidation [9],

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thin film coatings [10-12], and nitrogen diffusion hardening [13].

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Nitrogen diffusion hardening is one of the most commonly applied surface treatments and results in an effective improvement in both the hardness and the wear resistance. However, nitriding also

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leads to a high surface roughness and a poor friction coefficient. Consequently, the biocompatibility

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and tribological properties are severely degraded. Diamond-like carbon (DLC) coatings have many advantageous properties, including a low coefficient of friction, good wear resistance, high hardness

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[14], excellent biocompatibility [15], and high corrosion resistance [16]. These properties render DLC coatings an ideal choice for the surface treatment of load-bearing metal implants. Previous studies by the present group have investigated the incorporation of various metals into DLC coatings, including Ti [17], W [18], Zr [19] and Cr [20]. Among these various coatings, Ti-doped DLC (Ti-DLC) coatings have attracted particular interest in the literature due to their superior biocompatibility. For example, it was shown in [21, 22] that the addition of Ti improves the surface absorption of proteins, and therefore enhances the initial hemocompatibility of the coated surface. When choosing potential materials for orthopaedic prostheses, it is important to consider three properties, namely the corrosion resistance, the

ACCEPTED MANUSCRIPT wear behavior and the biocompatibility. However, previous studies on Ti-DLC coatings have typically

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considered only one property, i.e., the wear resistance [17], biocompatibility [21, 22] or corrosion

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resistance [16]. Accordingly, the present study performs a systematic investigation into the mechanical,

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tribological, electrochemical and biocompatibility properties of nitrided and non-nitrided Ti6Al4V samples with and without Ti-C:H coatings, respectively. Bharathy et al. [23] showed that a low Ti

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addition (less than 4.1 at%) prompts an effective improvement in the biocompatibility of DLC films.

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Moreover, in [17], the present group showed that Ti-DLC coatings with a low Ti content (3.0 at.%) have superior tribological properties. Thus, in the present study, the Ti-C:H coatings were also

2.

Experimental

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prepared using a low Ti concentration (3.3 at.% ).

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2.1.Nitriding treatment

Ti6Al4V specimens with a diameter and thickness of 24 mm × 7.9 mm (wear tests) and 15 mm × 1.25 mm (corrosion and biocompatibility tests) were cut from cylindrical bars. The specimens were ground using P180, P320, P600, P1200 and P2000 grade paper and then polished mechanically with 1, 0.3 and 0.05 µm Al2O3 powder particles. The final roughness of the untreated Ti6Al4V alloy specimens was around Ra=0.021 μm. The specimens were heated to 900℃and then maintained at this temperature for 120 minutes under ambient pressure conditions in a high purity N2 atmosphere. To enhance the adhesion of the Ti-C:H coatings, the nitrided samples were polished with 0.05 µm Al2O3

ACCEPTED MANUSCRIPT powder for around 20 seconds in order to remove any surface impurities. The final roughness of the

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nitrided samples was approximately Ra=0.127 μm.

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2.2.Ti-C:H coating deposition

Ti-C:H coatings were deposited on untreated and nitrided Ti6Al4V samples using a Closed Field

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Unbalanced Magnetron Sputtering (CFUBMS) system with a single Ti target, three C targets and CH4

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reaction gas. Prior to the deposition process, the specimens were cleaned for 20 minutes in Ar plasma using a pulsed-DC voltage with a magnitude of -340 V and a pulse frequency of 150 kHz. In order to

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enhance the adhesion of the Ti-C:H coating, a Ti interlayer was grown on the sample surface for 20

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minutes using the Ti target with a current of 1 A and a chamber pressure of 0.4 Pa. The Ti-C:H coating was then deposited on the interlayer using a Ti target current of 0.4 A, C target currents of 2.3 A, a

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chamber pressure of 0.4 A, a CH4 flow rate of 3 sccm, and a deposition time of 100 minutes. For both the Ti interlayer and the Ti-C:H coating, the deposition process was performed using a pulsed-DC power supply with a frequency of 50 kHz, a substrate bias voltage of -42 V, a sputtering source of Ar plasma (30 sccm flow rate), and a table rotation speed of 3 rpm.

2.3. Characterization of Ti-C:H coating, Ti6Al4V and N-Ti6Al4V samples The structural features of the Ti-C:H coatings were examined using a Raman spectrometer (RM1000, Renishaw, England) fitted with an argon ion laser system with a central bandwidth of 633

ACCEPTED MANUSCRIPT nm. The average thickness of the Ti-C:H coatings was determined via a Field-Emission Scanning

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Electron Microscope (FE-SEM, XL-40-FEG, Philips). In addition, the wear surfaces of the sample and

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316L counterbody were examined using an FE-SEM equipped with an energy dispersive spectrometer

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(EDS, operated at 10 kV and 60 seconds). The hardness of the Ti-C:H coatings was evaluated by means of nano-indentation tests (UNAT-M BMT, Germany, 10 tests per sample) performed using a

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Berkovich diamond tip under maximum applied loads of 35 mN and 5 mN, respectively. Meanwhile,

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the hardness of the Ti6Al4V and N-Ti6Al4V samples was measured by means of Knoop hardness tests (Matsuzawa MXT-70, Japan, 10 tests per sample) using a load of 10 g for 15 s. The surface roughness

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of the samples was measured using an optical profilometer (WLI BMT, Germany). Finally, the

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adhesion performance of the Ti-C:H coatings was investigated by performing scratch tests (FM-POD-200NT, Taiwan) using a Rockwell-C diamond indenter with a tip radius of 300 μm, a

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loading rate of 1 N/sec, a maximum load of 100 N, and a scratch length of 10 mm. For each coating, the critical load was defined as the load at which the Ti6Al4V substrate first became visible at the base of the scratch track.

2.4. Tribological properties The tribological properties of the uncoated and coated samples were evaluated using a reciprocating sliding wear tester (SRV, Optimol, Germany). To simulate the human body environment, the wear tests were performed in a NaCl solution with a concentration of 8.9 g/l (0.89 wt.%). In each

ACCEPTED MANUSCRIPT test, a 316L stainless steel ball (diameter 10 nm) was used as the counterbody and the sliding load was

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set as 10 N. Moreover, the frequency, stroke length and sliding time were set as 50 Hz, 1 mm and 24

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minutes, respectively. Each specimen was tested twice. Following each test, the average volume of the

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wear scars on the specimen was determined using an optical profilometer.

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2.5. Anti-corrosion properties

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The anti-corrosion performance of the various specimens was investigated by performing polarization tests in a NaCl solution (0.89 wt.%) using an ECW-5000 potentiostat (Jiehan, Taiwan).

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Prior to the tests, the NaCl solution was thoroughly deaerated of oxygen by bubbling high-purity

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nitrogen gas through the solution for 30 minutes. The tests were performed using a saturated calomel electrode and a platinum slice as the reference electrode and counter electrode, respectively. The

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specimens served as the working electrode, with the controlled potential measured using the ECW-5000 potentiostat. The exposed area was equal to 1 cm2 for every specimen. The specimens were immersed in the NaCl solution for 60 minutes in order to establish the open circuit potential (OCP). During the immersion process, the electrode potential was increased from -200 mV to 3000 mV at a scanning rate of 1 mV/s.

2.6. Biocompatibility properties Ti6Al4V, N-Ti6Al4V, Ti-C:H/Ti6Al4V and Ti-C:H/N-Ti6Al4V samples were seeded with

ACCEPTED MANUSCRIPT purified mouse leukaemic monocyte macrophage cells (Raw 264.7) expanded in American type

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culture collection. The cells were cultured in Dulbecco-Modified Eagle Medium supplemented with

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10% fetal bovine serum, 500 UI/ml penicillin and 0.1 mg/ml streptomycin at a temperature of 37°C in

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a humidified incubator with a 5% CO2 atmosphere. The specimens were placed in a 24-well polystyrene plate and exposed to ultraviolet radiation overnight prior to culturing in order to ensure the

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complete sterilization of the sample surface. The Raw 264.7 cells were seeded with a density of 1×104

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cells/well in 2 ml of culture medium, and cultured for 1, 3 and 5 days, respectively. For control purposes, Raw 264.7 cells were also cultured on tissue culture-treated polystyrene 24-well plates

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(Nunc, Roskilde, Denmark) under identical conditions. The cell viability was assessed by means of a

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3-(4, 5-dimethylthiazol -2-yl)–2, 5-diphenyltetra sodium bromide (MTT) assay performed following incubation with 0.5 mg/ml of MTT for the final 4 hours of the designated culture period (i.e., 1, 3 or 5

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days). For each specimen, the cell viability was quantified in terms of the cell density (i.e., the number of cells per square centimeter), as determined by an Enzyme-Linked Immuno-Sorbent Assay (ELISA) reader with a wavelength of λ=600 nm. Finally, the morphologies of the Raw 264.7 cells grown on the various samples were observed using an FE-SEM.

3.

Results and discussion

3.1.Structure, hardness, surface roughness and adhesion properties Figure 1(a) presents a cross-sectional optical microscope (OM) image of the N-Ti6Al4V sample.

ACCEPTED MANUSCRIPT It is observed that the nitridation depth is around 50 μm. Figures 1(b) and 1(c) present cross-sectional

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SEM images of the Ti-C:H/Ti6Al4V and Ti-C:H/N-Ti6Al4V samples, respectively. The interface

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between the Ti-C:H coating and nitrided Ti6Al4V substrate has a wavy characteristic (Fig. 1(c)) due to

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the high surface roughness of the nitrided Ti6Al4V substrate. From inspection, the Ti interlayer and Ti-C:H coating have thicknesses of approximately 0.1 μm and 0.9 μm, respectively. The elemental

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compositions of the Ti–C:H coatings were evaluated using a glow discharge spectrometer (LECO

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GDS-750 QDP) operated with a DC source (600~1500 V). The results showed that the Ti–C:H coatings contained 96.7 at.% carbon and 3.3 at.% titanium.

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Figure 2 shows the Raman spectrum of the Ti-C:H coating. As shown, the spectrum is deconvoluted into two bands (both Gaussian fitted), namely a D (disordered) band and a G (graphitic)

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band. Moreover, the ID/IG ratios near the centers of the two bands (i.e., 1356 cm−1 and 1541 cm−1,

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respectively) are equal to approximately 1.12 in both cases. In accordance with [14], the Ti-C:H coating structure is thus classified as sp2 a-C nano-crystalline graphite. Table 1 presents the hardness and surface roughness values of the four samples considered in the present study. It is seen that for both the coated and the uncoated samples, the nitriding treatment increases the surface hardness of the Ti6Al4V sample. Moreover, it is observed that the hardness of the coated samples (both untreated and nitrided) is significantly higher than that of the uncoated samples. According to [24], the coating hardness is unaffected by the underlying substrate if the maximum indentation depth is less than one tenth of the coating thickness. In the nanoindentation tests performed

ACCEPTED MANUSCRIPT in the present study, the maximum applied load was set initially as 35 mN. The maximum indentation

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depths of the Ti6Al4V and N-Ti6Al4V samples with a Ti-C:H top coating (0.9-μm thickness) were

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found to be 310 nm and 283 nm, respectively. Thus, the indentation depth was greater than one tenth of

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the coating thickness in both cases. Consequently, a notable difference in the hardness of the two coatings was observed (i.e., 14.91 GPa and 16.65 GPa, respectively). Accordingly, the hardness of the

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Ti-C:H coatings was re-measured under a maximum applied load of 5 mN. The maximum indentation

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depths were equal to 73 nm (with an error of 8%) and 71 nm (with an error of 6%), respectively. In other words, the indentation depth was less than one tenth of the Ti-C:H coating thickness (900 nm).

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The Ti-C:H/Ti6Al4V and Ti-C:H/N-Ti6Al4V coatings were found to have hardness values of 16.71

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GPa and 16.90 GPa, respectively. In other words, in the absence of the substrate effect, the difference between the hardness of the two coatings is less than 5%. Table 1 also shows that for all samples, the

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surface roughness increases following the nitriding and coating processes. Figure 3 presents SEM images of typical scratch marks on the surfaces of the Ti-C:H/Ti6Al4V and Ti-C:H/N-Ti6Al4V specimens. Note that the dark region of the scratch track corresponds to residual Ti-C:H coating, while the bright region corresponds to the exposed Ti6Al4V substrate. The critical loads of the Ti-C:H/Ti6Al4V and Ti-C:H/N-Ti6Al4V coatings were found to be 33 N and 38 N, respectively. It is thought that the difference in the critical loads of the two coatings is due, at least in part, to a difference in the effects of the two substrates on the Ti-C:H coating. The N-Ti6Al4V substrate has both a higher hardness (9.05 GPa) and a higher elastic modulus (159.7 GPa) than the

ACCEPTED MANUSCRIPT Ti6Al4V substrate (hardness: 3.21 GPa; elastic modulus; 114.2 GPa). The hardness and elastic

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modulus properties of the N-Ti6Al4V substrate are thus closer to those of the Ti-C:H coating

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(hardness: 16.90 GPa; elastic modulus:164.2 GPa). In general, the resistance of a thin-film coating to

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deformation increases as the elastic modulus increases. Moreover, the coating adhesion improves as the elastic modulus of the coating approaches that of the substrate since the deformation discontinuity

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at the coating/substrate interface reduces. In other words, the coating deforms in sympathy with the

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substrate, and hence the adhesion between the coating and the substrate is improved. According to previous studies [25-27], the mechanical properties and structure of Me-doped a-C:H

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films vary greatly depending on the coating deposition conditions and final composition. However,

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through an appropriate control of the deposition process, an excellent quality of a-C:H coatings can be achieved. Nonetheless, DLC films readily spall from most metal substrates due to their high intrinsic

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compressive stress. For example, in the studies of [26, 28] on TiC/a-C:H nanocomposite coatings deposited on Si wafers using CFUBMS and a-C:H films deposited on nitrided and non-nitrided AIS 420 using PACVD (Plasma Assisted Chemical Vapor Deposition), respectively, both films were found to be under compressive stress. It is thus speculated that the Ti-C:H coatings deposited in the present study are also under compressive stress. A future study will therefore investigate the stress state of the upper coating layer and explore the relationship among the structure, residual stress, hardness, adhesion strength and wear resistance properties of the film.

3.2.Tribological properties

ACCEPTED MANUSCRIPT Figure 4 shows the friction coefficient profiles of the Ti6Al4V, N-Ti6Al4V, Ti-C:H/Ti6Al4V and

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Ti-C:H/N-Ti6Al4V specimens during sliding against the 316L stainless steel ball. The friction

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coefficient of the Ti6Al4V sample has an average value of 0.73 and exhibits large fluctuations over the

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text period. Thus, it is inferred that significant and persistent plastic deformation and adhesion occur at the contact surface between the sample and the steel ball. For the nitrided sample (N-Ti6Al4V), the

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average value of the friction coefficient is slightly lower (i.e., 0.57) and the fluctuations are notably

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reduced. This suggests that the nitriding treatment increases the surface hardness and changes the dominant wear mechanism from one of plastic deformation and adhesive wear to one of abrasion. The

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average friction coefficients of the Ti-C:H/Ti6Al4V and Ti-C:H/N-Ti6Al4V samples (0.09 and 0.10,

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respectively) are significantly lower than those of their uncoated counterparts. Moreover, both traces remain stable over the entire sliding test. These findings suggest that the Ti-C:H coating provides a

operation.

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solid lubricant effect, which prevents plastic deformation and adhesive wear during the sliding

Figure 5 shows the wear rates of the various specimens. The results show that the nitriding treatment leads to a significant reduction in the wear rate. Furthermore, the wear rate is further reduced following the deposition of the Ti-C:H coating. From inspection, the Ti-C:H/Ti6Al4V and Ti-C:H/N-Ti6Al4V samples have wear rates of just 0.005 and 0.019 µm3/N·m, respectively. In other words, the wear rate of the Ti-C:H/Ti6Al4V sample is approximately 1575 times lower than that of the original Ti6Al4V sample (7.875 µm3/N·m).

ACCEPTED MANUSCRIPT Figure 6 presents SEM images of the coated and uncoated Ti6Al4V specimens and 316L

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counterbodies. The EDS analysis results for the Fe, Ti, C and O contents of the two bodies are also

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shown in every case. Figures 6(a) and 6(b) show that the untreated Ti6Al4V specimen and 316L ball

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both undergo severe plastic deformation and adhesive wear during sliding. Furthermore, the EDS analysis results reveal that significant material transfer occurs between the two bodies. For example,

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the Ti6Al4V surface contains 10.0 at.% Fe transferred from the 316L steel ball, while the surface of

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the 316L ball contains 10.7 at.% Ti transferred from the Ti6Al4V disk. As discussed above, the nitriding treatment increases the surface hardness and wear resistance of

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the Ti6Al4V sample surface, and therefore reduces the plastic deformation and adhesive wear. Figure

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6(c) shows the wear scar on the surface of the N-Ti6Al4V sample. It is observed that the surface is smoother and has fewer abrasive scratches than the non-nitrided surface (Fig. 6(a)). An EDS analysis

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of the black regions on the sample surface reveals the presence of 11.0 at.% Fe and 48.1 at.% O, which suggests the transfer and subsequent oxidation of material from the 316L sliding ball to the sample surface. The abrasive scratches seen in Fig. 6(c) are thought to be caused by the sliding of hardened debris fragments broken off from the nitrogenized layer on the sample surface and iron oxide particles originating from the sliding ball surface. As shown in Fig. 6(d), the hardened debris fragments also result in severe abrasive scratches on the relatively softer surface of the 316L ball (hardness: 2.81 GPa). In addition, the surface contains a large number of lumps and pits; indicating significant adhesive wear. However, the EDS analysis results show that the surface contains very little Ti and O (i.e., 1.4 at.%

ACCEPTED MANUSCRIPT and 14.2 at.%, respectively). Thus, it is inferred that only limited material transfer from the N-Ti6Al4V

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specimen occurred during sliding.

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The SEM images presented in Figs. 6(e) and 6(g) show that the Ti-C:H/Ti6Al4V and

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Ti-C:H/N-Ti6Al4V samples exhibit no plastic deformation, adhesive wear or residual debris particles. However, slight abrasive scratches are observed on both surfaces as a result of oxidized debris

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fragments originating from the 316L steel ball. It is seen in Fig. 6(f) that the surface of the 316L ball

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shows signs of slight plastic deformation following sliding against the Ti-C:H/Ti6Al4V sample. Moreover, the EDS analysis results show the presence of a small amount of C (16.6±2.2 at.%)

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transferred from the sample. By contrast, the surface of the 316L ball sliding against the

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Ti-C:H/N-Ti6Al4V sample shows more prominent scratch marks and a higher C content (36.2±4.3 at.%) scattered in the form of black spots and bands (see Fig. 6(h)). Thus, it appears that the high

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surface roughness of the Ti-C:H/N-Ti6Al4V sample (i.e., 0.175 µm, see Table 1) results in a greater fracturing and C transfer of the Ti-C:H coating to the steel ball than in the case of the Ti-C:H/Ti6Al4V sample with a lower surface roughness (i.e., 0.030 µm). In other words, the wear rate of the Ti-C:H/N-Ti6Al4V sample is higher than that of the Ti-C:H/Ti6Al4V sample, as shown in Fig. 5. It is noted that this finding is consistent with the results presented by Jiang and Arnell [29] for the effects of the substrate roughness on the wear rate of DLC coatings. Specifically, the results showed that as the surface roughness increases, both the contact pressure and the lateral impact of the ball on the tips of the coating asperities increase. Consequently, the wear rate also increases.

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3.3. Electrochemical properties

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Figure 7 shows the polarization curves of the various samples. Table 2 presents the results obtained

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in the potentiodynamic polarization tests for the corrosion potential (Ecorr) and corrosion current density (Icorr) of the four samples. It is seen that the Ti-C:H/Ti6Al4V specimen yields the highest

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corrosion potential (-0.114V) and the lowest corrosion current density (2.38E-05 Acm-2). In other

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words, of the four specimens, the Ti-C:H/Ti6Al4V sample has the greatest corrosion resistance. The good anticorrosion performance of the Ti-C:H coating suggests that the coating has good chemical

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inertness in NaCl solution (concentration 0.89 wt.%). Moreover, a lower corrosion rate implies a more

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limited release of metallic ions from the coated substrate. Thus, the smaller corrosion current (Icorr) of the Ti-C:H coating indicates that the coating has better biocompatibility than the uncoated Ti6Al4V

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substrate.

3.4.Biocompatible properties Figure 8 shows the cell densities on the control substrate and Ti6Al4V substrates, respectively, following culturing periods of 1 to 5 days. After 1 day, all of the substrates have a similar cell density of around 10000~13000 cell/cm2. After 3 days, the cell density increases by around five times. Moreover, after 5 days, the cell density increases further and approaches a saturated value. As shown, the four Ti6Al4V substrates can be ranked in terms of a decreasing cell density as follows:

ACCEPTED MANUSCRIPT Ti-C:H/Ti6Al4V (115746 cell/cm2), Ti-C:H/N-Ti6Al4V (99439 cell/cm2), Ti6Al4V (99004 cell/cm2)

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and N-Ti6Al4V (94282 cell/cm2). Wei et al. [30] reported that for human endothelial cells (ECV304)

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cultured on DLC films, the cell number reduces with an increasing surface roughness. In other words,

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a rough surface induces hydrophobicity, low surface free energy, low hydrogen content and high residual stress, and therefore adversely affects cell viability. In addition, several researchers have

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shown that hydrophobic surfaces tend to induce a less favorable cell response than hydrophilic

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surfaces due to a lower protein absorption [31-33]. These findings are consistent with the present results, which show that the N-Ti6Al4V sample (Ra=0.127 μm) possesses fewer cells than the

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Ti6Al4V sample (Ra=0.021 μm), while the Ti-C:H/N-Ti6Al4V sample (Ra=0.175 μm) possesses

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fewer cells than the Ti-C:H/Ti6Al4V sample (Ra=0.03 μm). However, it is noted that the cell densities on the Ti-C:H/Ti6Al4V and Ti-C:H/N-Ti6Al4V samples are greater than those on the uncoated

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specimens despite their greater surface roughness. In other words, the results suggest that the Ti-C:H coating also has an effect on the cell viability. Figure 9 presents SEM micrographs of the Raw 264.7 cells on the four Ti6Al4V samples following a culture period of three days. It is seen that for all of the samples, the cells are well attached to the substrate and have a regular morphology. No significant morphological differences are found between the cells grown on the different samples (Fig. 9(b), (d), (f) and (h)). However, observing the SEM images in Figs. 9(a), (c), (e) and (g), it is seen that the density of the cells on the coated samples is greater than that on the uncoated samples. In other words, the Ti-C:H coating is beneficial in

ACCEPTED MANUSCRIPT enhancing the biocompatibility of the Ti6Al4V samples (both untreated and nitrided). Furthermore,

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comparing Figs. 9(e) and 9(g), it is seen that the cells on the Ti-C:H/Ti6Al4V sample are more densely

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packed than those on the Ti-C:H/N-Ti6Al4V sample. The surface roughness of the Ti-C:H/N-Ti6Al4V

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sample is greater than that of the Ti-C:H/Ti6Al4V sample. Thus, it appears that the surface roughness contributes to the difference in the cell density in the two cases. More specifically, the smoother

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surface of the Ti-C:H/Ti6Al4V coating yields a superior biocompatibility performance.

Conclusions

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An experimental investigation has been performed into the mechanical, tribological,

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electrochemical and biocompatibility properties of nitrided and non-nitrided Ti6Al4V samples with and without Ti-C:H coatings, respectively. The results have shown that the nitriding treatment and

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Ti-C:H coating both yield an effective improvement in the specimen surface hardness. Moreover, the wear tests have shown that the Ti-C:H coating results in a greatly improved tribological performance (i.e., a higher wear resistance and a lower friction coefficient). Of the various specimens, the Ti-C:H/Ti6Al4V sample possesses the highest wear resistance and the lowest friction coefficient. The electrochemical tests have shown that the Ti-C:H/Ti6Al4V sample also has the greatest resistance to electrochemical corrosion. Finally, the cell viability tests have shown that the cell density reduces with an increasing surface roughness for both the coated and the uncoated samples. However, the Ti-C:H coating yields a significant improvement in the biocompatibility of the Ti6Al4V samples (both original

ACCEPTED MANUSCRIPT and nitrided). Of the four samples, the Ti-C:H/Ti6Al4V specimen exhibits the greatest cell density, i.e.,

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the greatest biocompatibility. Thus, overall, the present results suggest that Ti-C:H/Ti6Al4V is an ideal

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candidate for the fabrication of robust and durable load-bearing artificial implants.

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Acknowledgements The authors gratefully acknowledge the financial support provided to this study by the Ministry of Science and Technology of Taiwan under Contract No. MOST 103-2221-E270-002.

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ACCEPTED MANUSCRIPT Figure Captions Figure 1 (a) Cross-sectional OM image of N-Ti6Al4V nitrided layer, (b) cross-sectional SEM image of

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Ti-C:H/Ti6Al4V coating, (c) cross-sectional SEM image of Ti-C:H/N-Ti6Al4V coating.

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Figure 2 Raman spectrum of Ti-C:H coating.

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Figure 3 SEM images of: (a) Ti-C:H/Ti6Al4V scratch trace, and (b) Ti-C:H/N-Ti6Al4V scratch trace. Figure 4 Friction coefficient traces of various specimens sliding against 316L stainless steel ball. Figure 5 Wear rates of various specimens sliding against 316L stainless steel ball.

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Figure 6 Wear surface morphologies and EDS analysis results for Fe, Ti, C and O contents of various

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Ti6Al4V/316L ball wear pairs: (a) untreated Ti6Al4V, (b) 316L ball sliding against Ti6Al4V, (c) N-Ti6Al4V, (d) 316L ball sliding against N-Ti6Al4V, (e) Ti-C:H/Ti6Al4V, (f) 316L ball

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sliding against Ti-C:H/Ti6Al4V, (g) Ti-C:H/N-Ti6Al4V, and (h) 316L ball sliding against

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Ti-C:H/N-Ti6Al4V.

Figure 7 Potentiodynamic polarization curves of uncoated, nitrided and coated Ti6Al4V specimens in

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NaCl solution. Note that inset shows magnified view of turning point in every case. Figure 8 Number of Raw 264.7 cells on surfaces of Ti6Al4V, N-Ti6Al4V, Ti-C:H/Ti6Al4V and

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Ti-C:H/N-Ti6Al4V specimens after culture periods of 1, 3 and 5 days. Figure 9 SEM micrographs showing proliferation of Raw 264.7 cells on sample surfaces after culture period of 3 days: (a) Ti6Al4V, (b ) Ti6Al4V (×1000 magnification), (c) N-Ti6Al4V, (d) N-Ti6Al4V (×1000 magnification), (e) Ti-C:H/Ti6Al4V, (f) Ti-C:H/Ti6Al4V (×1000 magnification), and (g) Ti-C:H/N-Ti6Al4V, (h) Ti-C:H/N-Ti6Al4V (×1000 magnification).

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Nitrided layer

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Fig. 1. (a) Cross-sectional OM image of N-Ti6Al4V nitrided layer, (b) cross-sectional SEM image of Ti-C:H/Ti6Al4V coating, (c) cross-sectional SEM image of Ti-C:H/N-Ti6Al4V coating.

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Fig. 2. Raman spectrum of Ti-C:H coating.

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Fig. 3. SEM images of: (a) Ti-C:H/Ti6Al4V scratch trace, and (b) Ti-C:H/N-Ti6Al4V scratch trace.

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7.000 6.000

5.000

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Wear rate (μm3/N˙mm)

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Fig. 4. Friction coefficient traces of various specimens sliding against 316L stainless steel ball.

4.000 3.000 2.000 1.000 0.000

316L 10N Standard deviation

Ti6Al4V 7.875 0.557

N-Ti6Al4V 0.316 0.022

Ti-C:H/Ti6Al4V 0.005 0.0004

Ti-C:H/N-Ti6Al4V 0.019 0.0013

Fig. 5. Wear rates of various specimens sliding against 316L stainless steel ball.

Fe

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Fe

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2.6

97.4

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(h) Fig. 6. Wear surface morphologies and EDS analysis results for Fe, Ti, C and O contents of various Ti6Al4V/316L ball wear pairs: (a) untreated Ti6Al4V, (b) 316L ball sliding against Ti6Al4V, (c) N-Ti6Al4V, (d) 316L ball sliding against N-Ti6Al4V, (e) Ti-C:H/Ti6Al4V, (f) 316L ball sliding against Ti-C:H/Ti6Al4V, (g) Ti-C:H/N-Ti6Al4V, and (h) 316L ball sliding against Ti-C:H/N-Ti6Al4V.

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Fig. 7. Potentiodynamic polarization curves of uncoated, nitrided and coated Ti6Al4V specimens in NaCl solution. Note that inset shows magnified view of turning point in every case.

Fig.8. Number of Raw 264.7 cells on surfaces of Ti6Al4V, N-Ti6Al4V, Ti-C:H/Ti6Al4V and Ti-C:H/N-Ti6Al4V specimens after culture periods of 1, 3 and 5 days.

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(g) (h) Fig. 9. SEM micrographs showing proliferation of Raw 264.7 cells on sample surfaces after culture period of 3 days: (a) Ti6Al4V, (b ) Ti6Al4V (×1000 magnification), (c) N-Ti6Al4V, (d) N-Ti6Al4V

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(×1000 magnification), (e) Ti-C:H/Ti6Al4V, (f) Ti-C:H/Ti6Al4V (×1000 magnification), and (g) Ti-C:H/N-Ti6Al4V, (h) Ti-C:H/N-Ti6Al4V (×1000 magnification).

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Table 1 Hardness (H) and surface roughness (Ra) of various specimens. (Note that reported hardness values are obtained under maximum applied load of 5 mN.)

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Table 2 Corrosion potential (Ecorr) and corrosion current density (Icorr) of various specimens

ACCEPTED MANUSCRIPT Table 1 Hardness (H) and surface roughness (Ra) of various specimens. (Note that reported hardness values are obtained under maximum applied load of 5 mN.) 3.16 9.05 16.71 16.90

0.021 0.127 0.030 0.175

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Ra (µm)

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Ti6Al4V N-Ti6Al4V Ti-C:H/Ti6Al4V Ti-C:H/N-Ti6Al4V

H(GPa)

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Ecorr (V)

Ti6Al4V N- Ti6Al4V T-C:H/Ti6Al4V

-0.131 -0.149 -0.114

T-C:H/N-Ti6Al4V

-0.116

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specimen

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Table 2 Corrosion potential (Ecorr) and corrosion current density (Icorr) of various specimens Icorr (Acm-2) 3.84E-05 3.77E-05 2.38E-05 2.45E-05

ACCEPTED MANUSCRIPT Highlights Ti6Al4V alloy was hot gas nitrided and coated with Ti-C:H



Tribology, corrosion and biocompatibility properties were investigated



Ti-C:H coated Ti6Al4V alloy has good potential for durable biomedical implants

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