A comparative study of titanium nitride (TiN), titanium oxy nitride (TiON) and titanium aluminum nitride (TiAlN), as surface coatings for bio implants

Surface & Coatings Technology 205 (2011) 5014–5020

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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t

A comparative study of titanium nitride (TiN), titanium oxy nitride (TiON) and titanium aluminum nitride (TiAlN), as surface coatings for bio implants B. Subramanian a,⁎, C.V. Muraleedharan b, R. Ananthakumar a, M. Jayachandran a a b

Central Electrochemical Research Institute, Karaikudi - 630 006, India Sree Chitra Tirunal Institute for Medical Science and Technology, Thiruvananthapuram - 695 012, India

a r t i c l e

i n f o

Article history: Received 14 December 2010 Accepted in revised form 5 May 2011 Available online 13 May 2011 Keywords: Thin films Magnetron sputtering XRD AFM XPS Corrosion resistance

a b s t r a c t In the present study, the performance of three titanium nitride coatings: TiN, TiON, and TiAlN for biomedical applications were assessed in terms of their surface properties electrochemical corrosion in simulated body fluid and cytotoxicity. Layers of TiN, TiON and TiAlN were deposited onto CP–Ti substrates by DC reactive magnetron sputtering method using a combination of a Ti, Ti–Al targets and an Ar–N2 mixture discharge gas. The presence of different phases was identified by XRD analysis. The morphology was determined through atomic force microscopy (AFM) imaging. The XPS survey spectra on the etched surfaces of TiN film exhibited the characteristic Ti2p, N1s, O1s peaks at the corresponding binding energies 454.5, 397.0, and 530.6 eV respectively. The characteristic Raman peaks were observed from the Laser Raman spectrometer. Platelet adhesion experiments were done to examine the interaction between blood and the materials in vitro. On Control samples (CP Ti), platelets were seen as aggregates, whereas on coated samples, platelets were seen as singles, without any significant spreading. Cytocompatibility studies of coated samples were carried out with bare titanium (CP Ti — ASTM B 348) as controls. L-929 mouse fibroblast cells were used for samples. All materials showed good cytocompatbility with cell lines used. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Recently various surface coating technologies have been performed to enhance the important functional properties such as lubricity, biocompatibility and antimicrobial effect for medical devices and surgical tools. Commercially pure titanium (CP–Ti) and Ti–6Al–4V, remain the two dominant titanium alloys used in implants. The commercially pure Ti has a higher ductility than the biphsic alloy Ti– 6Al–4V. However it is also less strong so cold work only allows a similar strength to be developed. The advantage it does have is phase stability. In most porous coating processes the 6Al–4V alloy develops brittle structures not seen in the pure Ti. This has led to the use of CP Ti in porous coatings (e.g., fiber metal) & TJA components. Ti-based alloys are widely used for orthopedic implants because of the high biocompatibility of titanium and its high corrosion resistance [1]. However, Ti is a soft material with a low shear resistance of the surface, what should be mainly due to the naturally formed surface oxide, and thus, hardening of the titanium surface is performed [2]. Technically, nitriding of titanium is frequently performed for hardening of the surface; a good biocompatibility of this material has been shown in blood and in bone [2,3]. Most transition metals form binary or ternary nitrides, with good mechanical, tribological, anticorrosive and biocompatibility properties

⁎ Corresponding author. Tel.: + 91 4565 227555; fax: + 91 4565 227713. E-mail address: [email protected] (B. Subramanian). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.05.004

[4,5]. Therefore, nitrides coatings are well suited to the protection of the medical implant surfaces. In recent years transition metal nitrides like TiN, ZrN, TiAlN, NbN, TaN and VN were successfully used as protective coatings against wear and corrosion in order to increase the life expectancy of surgical implants and prosthesis [6]. Chung et al. [7] found that the TiN based coatings significantly improve electrochemical and biocompatibility properties of the base material. Mitamura et al. [8] showed that the blood compatibility of TiN films was as good as that of low-temperature isotropic pyrolytic carbon and the durability of titanium heart valve cages coated with TiN increased seven-fold. Dion et al. [9] demonstrated the use of TiN in left ventricular assisted devices. Scarano et al. [10] and Koerner et al., [11] showed reduced bacterial adhesion on TiN surfaces. Coll and Jacquet [12] observed great reduction of wear rate in implants with TiN surface coating. TiAlN coatings have been attracting more attentions since last decades owing to the superiorities such as higher hardness, super wear resistance, lower thermal expansion, lower thermal conductivity and enhanced erosion resistance compared to TiN [13,14]. Particularly, the excellent antioxidation and thermal stability performance at high temperature and chemical stability to rigorous environment make TiAlN coatings have more applications. In fact, the evaporation point of the TiN film was estimated to be 550 °C. The (Ti,Al)N film demonstrates even more stable pyrochemical properties and an evaporation point higher than 800 °C [15,16]. It is argued that a (Ti,Al)N film can be used as an intervening layer to improve the biocompatibility of metal substrate in dental application.

B. Subramanian et al. / Surface & Coatings Technology 205 (2011) 5014–5020 Table 1 Optimized deposition parameters for TiN, TiON and TiAlN reactive sputtering. Objects Specification Target (2 in. Dia) Substrate Target to substrate distance Ultimate vacuum Operating vacuum Sputtering gas (Ar: N2) Sputtering gas (Ar: N2:O) Power Substrate temperature

Ti (99.9%), Al (99.9%) (TiAlN) Ti (99.9%) (TiN, TiON) CP–Ti 60 mm 1 × 10−6 m bar 2 × 10−3 m bar 2: 1 (TiN, TiAlN) 2: 1:1 (TiON) 200 W 400 °C

Titanium oxynitride (TiNxOy) films have recently attracted much attention owing to their remarkable optical and electronic properties, which depend significantly on the N/O ratio. Nitrogen-rich TiNxOy has been widely used in many applications, such as anti-reflective coating [17] and biomaterials [11], while oxygen-rich TiNxOy has been applied in thin film resistors [18], etc. There are various experimental approaches to the synthesis of the TiN, TiAlN and TiON layers like physical vapor deposition (PVD) [19,20], chemical vapor deposition (CVD) [21], ion beam assisted deposition (IBAD) [22] or hollow cathodic ionic plating [23]. In the particular case of d.c. reactive sputtering used in our experiments, optimal conditions for producing layers of stoichiometric TiN, TiAlN and TiON phase, have been achieved, by properly tuning the N2 pressure, deposition rate, as well as the film thickness and substrate temperature. The purpose of this investigation was to develop a TiN, TiAlN and TiON films and to study the surface characteristics of these films on CP Ti substrates. The coatings were investigated as possible candidates to be applied as protective layers on medical implants or prosthesis. 2. Experimental methods 2.1. Substrate pretreatment Titanium substrates were machined using conventional turning and wire cut electric discharge machining (EDM) from commercially pure titanium conforming to ASTM B348 Grade 2. This was followed by hand emerying using silicon carbide emery papers, (240 to 600 mesh sizes) to remove the turning marks and the recast zone generated by the EDM process. Final polishing was carried out using a centrifugal tumbling machine, by wet cut down, dry debarring and ball burnishing. This process generated a surface finish better than 0.1 micron Ra, which is considered suitable for blood contact applications. 2.2. Deposition of TiN, TiON, TiAlN and their characterization The layers of TiN, TiON and TiAlN were deposited on well-cleaned substrates using a DC magnetron sputter deposition unit HIND HIVAC. The base vacuum of the chamber was below 10 − 6 Torr (1.33 × 10 −4 Pa) and the substrate temperature was kept at 400 °C. High purity argon was fed into the vacuum chamber for the plasma

Table 2 Solution composition of simulated bodily fluid. Components

Concentration, g/l

NaCl KCl CaCl2 MgCl2 H2N CO NH2 Egg white

0.40 0.40 0.795 0.780 1.0 0.005

Fig. 1. Survey spectra for (a) TiN (b) TiON and (c) TiAlN thin films.

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imaging Atomic Force Microscope. The Raman spectroscopy measurements used an excitation wavelength of 632.8 nm. The data were collected with a 10 s data point acquisition time in the spectral region of 200–1000 cm−1. 2.3. Electrochemical corrosion studies

Fig. 2. X-ray diffraction pattern for (a) TiN (b) TiON and (c) TiAlN films.

generation. The substrates were etched for 5 min at a dc power of 50 W and an argon pressure of 10 mTorr (1.33 Pa). The deposition parameters for TiN, TiON and TiAlN sputtering are summarized in Table 1. The chemical nature of the outermost part of the films was obtained by X-ray photoelectron spectroscopy (XPS) using Multilab 2000. The XPS measurements were performed at a base pressure of 10 −8 Torr using MgKα (X-ray of 1253.6 eV source). X-ray diffraction (XRD) was used to examine the changes in preferred grain orientation. XRD patterns were recorded using an X'pert pro diffractometer using Cu Kα (1.541 Å) radiation from 40 kV X-ray source running at 30 mA. The surface of the coatings was characterized by a molecular

Conventional three-electrode cell assembly was used for polarization studies as well as for impedance measurements. Electrochemical polarization studies were carried out using Autolab Electrochemical workstation. Experiments were conducted using the standard threeelectrode configuration, with a platinum foil as a counter electrode, saturated calomel electrode as a reference electrode and the sample as a working electrode. Specimen (1.0 cm 2 exposed area) was immersed in the test solution of simulated bodily fluid (SBF) [24]. The composition of SBF is given in Table 2. Experiments were carried out at room temperature (28 °C). The system was allowed to attain a steady potential value for 10 min. The steady state polarization was carried out from −550 mV vs SCE from the OCP and +200 mV vs SCE from the OCP separately using separate electrodes [25] at a scan of 10 mV/s. Electrodes of the same specification employed in polarization studies were used for impedance studies. In order to establish the open circuit potential (OCP), prior to measurements, the sample was immersed in the solution for about 60 min. Impedance measurements were taken after attainment of steady state, an AC signal of 10 mV amplitude was applied and Impedance values were measured for frequencies from 0.01 Hz to 100 kHz. 2.4. Cytotoxic effects Cytotoxicity was studied by assessing the cellular response to L 929 fibroblast cells. Direct contact method based on ISO-10993-Part 5 was carried out on the test samples with CP Titanium as control. Mouse fibroblast cells (L929, ATCC, USA), were cultured in culture flasks with Dulbecco's Minimum Essential Medium supplemented with 10% fetal bovine serum and incubated at 37 °C and 5% CO2 under humidified conditions. Cells were subcultured to 24 well plates

Fig. 3. AFM image of (a) TiN (b) TiON and (c) TiAlN thin films on CP–Ti.

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using Trypsin–EDTA solution and allowed to form a monolayer. Once the cells attained confluency, test samples were kept in contact with cells and cytotoxicity test based on ISO-10993-5 was performed.

Table 3 Polarization and electrochemical impendence data obtained for TiN, TiON and TiAlN and CP–Ti substrate. Sample

Ecorr V

Icorr × 10−7 A cm−2

Corrosion rate mm/y × 10−3

Rct Ω cm2

Cdl × 10−9 F/cm2

CP Ti TiN TiON TiAlN

−0.205 −0.198 −0.174 −0.128

8.13 1.34 0.67 0.28

28.3 5.2 3.1 1.9

1114.0 2722.0 14727.7 20244.1

453.7 44.6 3.2 1.3

2.5. Qualitative analysis of platelet aggregation Materials after exposure to Platelet rich plasma PRP for 30 min were rinsed thoroughly with phosphate buffered saline and fixed with 2% glutaraldehyde overnight. They were then rinsed and dyhydrated with graded concentration of ethyl alcohol. The samples were critical point dried, gold sputter coated and viewed under the scanning electron microscope. 3. Results and discussion 3.1. XPS analysis The XPS survey spectra of 1 minute etched surfaces of the TiN films (Fig. 1a.) on CP–Ti substrate exhibited the characteristic Ti2p, N1s, O1s peaks at the corresponding binding energies 454.5, 397.0, and 530.6 eV respectively [26]. From high resolution XPS measurements of the normal surface of the films, the spin orbit doublet Ti 2p1/2 and Ti 2p3/2 peaks at binding energies 463.1 and 460.3 eV respectively was found in the Ti spectra as shown in Fig. 1a. The Ti 2p3/2 peaks included three components whose peaks centered at 460.3 (I), 456.9 (II) and 454.5 eV (III). In line with the existing literature [27], we can easily associate these components with TiO2, TiOxNy and TiN phases, respectively. The C1s peaks in the spectra (at 284.9 eV) may be the contribution from organic carbon which is unavoidable while using oil diffusion pump for evacuating the deposition chamber and XPS sample holding compartment [28]. Additionally, the Ar 2P peak identified in the spectra of the etched surface may be from the adsorbed argon during etching or Ar species incorporated into the films during growth [29]. N1s spectrum showed peaks centered at 397.0 eV as shown in Fig. 1b. The low nitridation in the films may be due to high surface oxidation. The source of oxidation may be the nitrogen gas. The commercial nitrogen gas was used and it might have considerable percentage of oxygen impurities [30]. Fig. 1b. shows the XPS spectra of TiON film deposited on CP–Ti substrate. These occur both Ti–N and Ti–O2 bonding in the deposited film. Initially Ti atoms

Fig. 4. Laser Raman spectra of (a) TiN, (b) TiON and (c) TiAlN thin films on CP–Ti.

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(ions) will first react with O due to their strong affinity with the increasing power, more Ti atoms are sputtered from Ti target and more Ti ions are created due to charge exchange. There is more opportunity to form the TiN bonding of the Ti–O2 bonding. The XPS survey spectra of normal unetched surface of the Ti–Al–N film on steel exhibited the characteristic Ti2p, Al 2p, N1s, O1s peaks at the corresponding binding energies 454.5, 74.3, 399.6 and 532.1 eV respectively [31]. The Al 2p photo electron spectrum is shown in Fig. 1c. A binding energy of 74.3 eV is observed which is corresponding to an Al–N bonding state within the film [32]. Based on the analytical results, it can be concluded that the TiAlN films are composed of the AlN and TiN phase mainly, on which a thin TiAlON surface layer is formed upon exposure of the films to the air after deposition. 3.2. Structural analysis The XRD pattern exhibiting the structural changes of TiN, TiAlN and TiON are shown in Fig. 2. For a TiN film, the B1–NaCl TiN peak with strong (111) grains is observed. When oxygen atoms were introduced into the TiN film, TiON films remain the NaCl structure, the same as TiN film. The oxygen atoms just replace part of the nitrogen atoms by forming TiO2 in the films. For sample TiAlN, a two phase structure with both B1–NaCl TiN (200) grains and B4-wurtzite AlN (101) and (100) grains are clearly evidenced, indicating that the hexagonal B4 structure is well developed. It is also noticed that the XRD 2θ peak shift toward higher Bragg angles as the Al content increases, which indicates the contraction of lattice by the incorporated aluminum atoms. This could be because of the fact that the covalent radius of Al (0.143 nm) is smaller than that of Ti (0.146 nm). In addition to incorporation into the TiN lattice, aluminum would also react with nitrogen to form a new phase locally in the grain boundary.

Fig. 5. The potentiodynamic polarization curve in simulated bodily fluid solution for CP–Ti substrate, (ii) TiN (iii) TiON and (iv) TiAlN on CP–Ti.

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3.3. Laser Raman studies

Fig. 6. Nyquist plots (i) CP–Ti substrate (ii) TiN (iii) TiON and (iv) TiAlN on CP–Ti.

The typical 2D AFM images of TiN, TiAlN and TiON films are shown in Fig. 3a, b and c, respectively. From the horizontal cross section analysis of AFM, the minimum and maximum grain size was estimated to be in the range of 100 to 150 nm. The value of the mean roughness Ra was calculated as the deviations in height from the profile mean value [33]. AFM results of TiAlN film indicated that an increase in Al content resulted in a decrease of both the grain size and surface roughness due to nucleation matters. The columnar nature of the microstructure in TiN films has been demonstrated using crosssectional SEM [34].

Raman scattering is a powerful and non-destructive technique to study the as-synthesized new materials. It can provide important evidence on the microstructure of films [35]. The Raman spectra in the range of 100–1100 cm −1 for TiN, TiAlN and TiON are shown in the Fig. 4. For the Raman Spectrum of TiN film (Fig. 4a.), it has been evidenced that the scattering in the acoustic range is mainly determined by the vibrations of the heavy Ti ions (typically 150– 350 cm −1) and in the optic range by the vibrations of the lighter N ions (typically 550–650 cm −1), which is consistent with the previous results [36]. The peaks at 246 and 638 cm −1 arise from first –order transverse acoustic (TA) and transverse optical (TO) modes of TiN film, respectively. Due to the oxide layer formation on the surface of TiON, the additional peaks are observed. Peaks at 148 and 518 cm −1 are typical characteristics of rutile TiO2 films [37]. Features at 200, 398 and 641 cm −1 indicate the presence of anatase structure that coexists in the film [37]. A peak at 266 and a broad peak at 650 cm −1 related to transverse acoustic (TA) and transverse optical (TO) modes were observed in the Raman spectra of TiAlN films (Fig. 4c) prepared by reactive sputtering process. This is in good agreement with the reported values for Ti–Al–N films [38]. 3.4. Electrochemical corrosion behavior The results of corrosion testing for the CP–Ti substrate, TiN, TiAlN and TiON in simulated bodily fluid solution are given in Table 3. The corrosion potential of the CP–Ti substrate is about −0. 205 V. The positive shift of Ecorr to −0.128 V for TiAlN indicates better corrosion resistance of the TiAlN coatings as shown in Fig. 5. The corrosion current Icorr of CP–Ti is greater than those of TiN, TiAlN and TiON. For the TiAlN, the corrosion current is reduced to 0.282 × 10 −7A cm −2, as indicated in Table 3. The same three electrode cell assembly, as used

Fig. 7. L-929 fibroblast cells around a) CP–Ti b) TiN c) TiON and d) TiAlN samples.

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Fig. 8. SEM images of platelet aggregation on a) CP–Ti b) TiN samples.

for the potentiodynamic polarization experiments, was employed for the AC impedance investigations. When the sample is immersed in the electrolyte the defects in the coating provide the direct diffusion path for the corrosive media. In this process the galvanic corrosion cells are formed and the localized corrosion dominates the corrosion process. The electrochemical interface can be divided into two subinterfaces: electrolyte / Coating and electrolyte / Substrate. The single semicircle behavior obtained for the samples is believed to be due to the short exposure time (60 min), which is not sufficient to reveal the degradation of the substrate [39]. The Rct increases (Table 3) in the following order: CP–Ti b TiN b TiON b TiAlN, which shows (Fig. 6) that TiAlN coating on Ti substrate has higher corrosion resistance. 3.5. Cytotoxic effects and assessment of platelet adhesion by SEM SEM images of CP–Ti, TiN, TiON and TiAlN samples attached with L 929 fibroblast cells are shown in Fig. 7a, b, c and d. All the three materials did not show any cytotoxic reaction in comparison with Titanium when the materials were kept in contact with Fibroblast cells. Cytotoxicity studies revealed that all these coatings passed the test and hence could be classified as non-cytotoxic material. Fig. 8a, b shows the platelet adhesion of the representative SEM images of TiN coated sample along with the control CP–Ti. Platelets were seen rarely on all samples in the UHMWPE (Historical Control) series and were towards the edge of the materials. On all the other samples, platelets were seen throughout the material; however, more adhesion was seen near the edges. On Control samples (CP Ti), platelets were seen as aggregates, whereas on TiN, TiAlN and TiON samples, platelets were seen as singles, without any significant spreading. On all samples of TiN TiAlN and TiON group, surfaces showed some defects, and platelets seemed to have trapped into the defective area rather than the adhesion with spreading. 4. Conclusions TiN, TiON and TiAlN thin films were successfully prepared by using reactive direct current (DC) magnetron sputtering onto CP–Ti substrates. XPS analyses show that all prepared TiN, TiON and TiAlN films exhibited a mixture of Ti–N, Ti–O–N, Ti–Al–N chemical binding states. For sample TiAlN, a two phase structure with both B1–NaCl TiN (200) grains and B4-wurtzite AlN (101) and (100) grains are clearly evidenced, from XRD. AFM results indicated that an increase in Al content resulted in a decrease of both the grain size and surface roughness. Characteristics peaks of rutile and anatase structure of TiO2 were found to coexist in the TiON film as observed from Laser Raman. The Potentiodynamic polarization and EIS measurements showed that the TiAlN coatings on CP–Ti exhibited superior corrosion resistance compared to the TiON, TiN and the bare CP–Ti substrate. Cytotoxicity

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