Influence of CrN Coating on Electrochemical Behavior of Plasma Nitrided Pure Titanium in Bio-simulated Environment

Influence of CrN Coating on Electrochemical Behavior of Plasma Nitrided Pure Titanium in Bio-simulated Environment

Journal of Bionic Engineering 13 (2016) 150–155 Influence of CrN Coating on Electrochemical Behavior of Plasma Nitrided Pure Titanium in Bio-simulate...

3MB Sizes 0 Downloads 0 Views

Journal of Bionic Engineering 13 (2016) 150–155

Influence of CrN Coating on Electrochemical Behavior of Plasma Nitrided Pure Titanium in Bio-simulated Environment İlhan Çelik Department of Mechanical Engineering, Gümüşhane University, Gümüşhane 29100, Turkey

Abstract Titanium and its alloys are widely used as materials for bio-medical applications, such as implants. However, ions of the alloy can release to the body region and spread into the blood circulation. In this study, plasma nitriding and CrN coating techniques are used in order to overcome the problem of ion release. The objective of this study was to investigate the effects of plasma nitrided pure titanium on the structural properties and corrosion behaviors before and after CrN coating in Ringer’s solution at 37 ˚C. The structural properties were investigated using Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD). A diffusion layer and a compound layer composed of δ-TiN and ε-Ti2N phases were observed on the surface of nitrided pure titanium. Corrosion tests were made for the determination of electrochemical properties with the help of Potentiostat/Galvanostat device. The results show that corrosion behaviors of untreated and treated samples have similar characteristic. Keywords: titanium, corrosion, CrN coating, plasma nitriding, duplex treatment Copyright © 2016, Jilin University. Published by Elsevier Limited and Science Press. All rights reserved. doi: 10.1016/S1672-6529(14)60169-4

1 Introduction Titanium and its alloys have been widely used in chemical, aerospace, food industries and sports equipment manufacturing because of their good fatigue properties, easy fabrication, perfect corrosion resistance, and high strength to weight ratio[1]. With the realization of its biocompatibility[2,3], titanium has also been used in dentistry applications as an implant material and medical applications commonly. The use of titanium in medical field as a biomaterial became possible in 1960s[4]. Titanium and its alloys meet the desired properties compared with other materials used in medical applications such as stainless steel, chrome-cobalt (Cr-Co) alloys, pure niobium (Nb) and pure tantalum (Ta). They are, therefore, the most appropriate materials for medical applications among metallic materials[5]. Another reason why titanium is more advantageous than other metallic biomaterials is its modulus of elasticity value. Titanium which has a lower modulus of elasticity (110 Gpa)[6] is a material that is more similar to bone's modulus of elasticity value (~18 GPa) than other materials such as stainless steel (~200 GPa)[7] and Cr-Co alloy (210 GPa – Corresponding author: İlhan Çelik E-mail: [email protected], [email protected]

253 GPa)[8]. Metallic biomaterials may cause harmful impacts in extended use even if they are biocompatible. The longer implant stays in the body, the more metal ions release from the implant into the body. Thus, when it is considered that the implant will stay in the body for 30 – 40 years after it is implanted on a young and active person, there is still a concern that the release of metal ions in the body may cause cancer. Many researchers have benefited from surface engineering to deal with this problem[9–11]. Scientific studies of the biological behavior of metallic elements have shown that the elemental composition of biomaterials should be meticulously designed to reduce negative body reactions as a cause of local negative tissue reactions or elicit allergic reactions in patients with dental or orthopedic implants[12]. While CoCrMoNi alloys, stainless steels, and Ti and its alloys, and Pt, Nb, Ta, and Zr composed the class of ‘metallic resistance biomaterials’ based on its corrosion rates, many studies showed that these materials release toxic elements such as Co, Ni and V[13]. At the same time, the biomaterials used in biomedical applications should have some properties such as corrosion resistance, high

Çelik: Influence of CrN Coating on Electrochemical Behavior of Plasma Nitrided Pure Titanium in Bio-simulated Environment

surface hardness, and low wear rate[14]. In previous studies, various surface treatments on different metallic biomaterials were applied in order to improve different properties (wear, corrosion, etc.)[15–17]. Thus, positive results were obtained for surface properties of these materials. The aim of this study was to investigate the effects of plasma nitrided pure titanium on the structural properties and corrosion behaviors before and after CrN coating in bio-simulated environment (Ringer’s solution). After the surface treatments, the structural analysis and the surface morphology of the coatings were investigated with X-ray diffraction (XRD) and Scanning Electron Microscope (SEM), respectively. Potentiodynamic polarization measurements were conducted to determine the corrosion resistances of the untreated and treated surfaces.

2 Materials and methods 2.1 Surface treatments In this study, commercially pure titanium (Grade-2) with impurities of 0.006 wt.% N, 0.15 wt.% O, 0.008 wt.% C, 0.041 wt.% Fe and 0.002 wt.% H was used as the initial material. For surface treatments, specimens with the dimensions of 15 mm × 15 mm × 5 mm were machined from pure titanium, polished and then cleaned in a mixture of distilled water and ethanol. The prepared specimens were conducted to duplex surface treatment. The duplex process consisted of two main sections. In the first section, the specimens were treated by plasma nitriding process. Prior to the plasma nitriding treatment, specimens were cleaned with hydrogen sputtering at a pressure of 5×102 Pa for 15 min in order to remove the impurities on the surfaces[18,19], and then the plasma nitriding treatment was performed with a mixture of 75% N2 + 25% H2 under 5×102 Pa pressure at 500 ˚C and 700 ˚C for 4 h. The samples were cooled down inside the vacuum chamber down to room temperature after the plasma nitriding treatment. In the second section, the specimens were coated with CrN by Physical Vapour Deposition (PVD) at the Barlok PVD Coating Company (Istanbul, Turkey). The deposition process was characterized in a condition with the following parameters: substrate bias DC 100 V, working pressure (0.33 ± 0.03) Pa, working temperature 300 ˚C – 350 ˚C, gas rate 70 ps – 75 ps (N), gas flow 40%, deposition time 60 min, target materials Ti and CrN, glow time 40 min,

151

glow stages 20 min 240 V pulse 20 min 280 V, glow gas values 2.0 ps Ar and 1.6 ps H, preheating time 60 min. 2.2 Surface characterizations The structures of commercially pure titanium with and without a coating were studied using SEM (Quanta FEG 250) and XRD (Rigaku diffractometer). The polarization curves were taken by means of a Potentioscan Gamry Series G750TM Potansiyostat /Galvanostat/ ZRA device. One side of the sample with the area of about 0.3848 cm2 was exposed to the solution. The corrosion cell contained 250 ml Ringer’s solution at 37 ˚C. The cell consisted of the sample that served a working electrode, a graphite rod counter electrode, and an Ag/AgCl reference electrode. The rate of scanning of the polarization curve toward the nobler potentials was 1 mV sec after the attainment of a steady state potential. Whole corrosion test processes were repeated at least two times in order to confirm the reproducibility of experimental results.

3 Results and discussion 3.1 Structural Analyses The XRD patterns of untreated and treated pure titanium are given in Fig. 1. The XRD results indicated that α-Ti peaks appeared in the spectra from untreated sample because the commercially pure titanium has an alpha-titanium alloy (ICDD 01-089-3725). ICDD 01076-0198 card number for ε-Ti2N phase and ICDD 01087-0633 card number for δ-TiN phase were used in the analysis of XRD patterns. When the graphic of pure titanium specimen nitrided at 500 ˚C (Nit.500) is examined, it is seen that peaks coming from the base material at 2θ=35˚ and 2θ=77.4˚ disappeared. In the meantime, ε-Ti2N phase

Nit.700+CrN

βα λ β β

Nit.700

βα λ α λα α

Nit.500+CrN Nit.500

20

Untreated Ti α 30

α=α-Ti β=δ-TiN λ=ε-Ti2N θ=δ-TiN/α-Ti λ x x=CrN λβ

θ

x

α

α

β λα β α α

α

α α

40

2θ (°)

50

α 60

α 70

θ

α αα 80

Fig. 1 XRD spectra of untreated and treated pure titanium.

152

Journal of Bionic Engineering (2016) Vol.13 No.1

3.2 Electrochemical Analyses The untreated and treated specimens were examined in Ringer’s solution for comparison of their corrosion behavior. The open circuit potential curves of untreated and treated samples are presented in Fig. 3. It was determined during the preliminary corrosion tests that the specimens reached a stable condition after two hours. The open circuit potential shifts for surface of the treated pure titanium specimens were measured at different coating thicknesses. OCP values of the untreated sample exhibit a stable structure as a function of time. The open circuit potential decreases for the Nit.700 specimen. This situation can be explained by the fact that more corrosive liquid penetrates through the pores in the case of a higher roughness. According to Fig. 3, whole treated samples reach steady state in about 100 min when submerged into electrolyte. In addition, all the treated samples exhibit a noble behavior than the untreated sample. Corrosion rate is very difficult to determine in more complex corrosive solutions such as physiological body fluids. The most common method to determine the corrosion rate under in-vitro conditions is electrochemical

Vref. (V)

and δ-TiN phase appeared at 2θ=39.2˚ and 2θ=42.7˚, respectively. δ-TiN appeared at 2θ=42.7˚ in Nit.500 became much more apparent in pure titanium nitrided at 700 ˚C and turned into a phase with high density. Moreover, it is seen that δ-TiN phase at 2θ=36.7˚ and 2θ=62.2˚ and ε-Ti2N phase at 2θ=61.1˚ appeared in Nit.700. When nitrided specimens were coated with CrN (Nit.500+CrN and Nit.700+CrN), it was detected that the density of the peaks coming from the nitrided surface decreased, and CrN phase appeared at about 2θ=62.5˚[20]. The plasma nitrided surfaces of pure titanium specimens were composed of a structure of δ-TiN and ε-Ti2N phases according to the data obtained from the XRD data analysis. Fig. 2 shows the cross-section SEM micrographs of the plasma nitrided and duplex treated specimens. The cross-section morphology of plasma nitrided pure titanium consisted of diffusion and compound layers (Fig. 2a). While the diffusion layer had a thickness of about 150 μm, the compound layer had a thickness of about 5 μm – 6 μm. It can be seen that CrN film thickness is about 2 μm and its morphology exhibits homogenous (Fig. 2b).

E (V, vs. Ag/AgCl)

Fig. 3 Open circuit voltage curves of pure titanium. (1) Untreated; (2) Nit.500; (3) Nit.500+CrN; (4) Nit.700; (5) Nit.700+CrN.

Fig. 2 Cross-section SEM micrographs of (a) plasma nitrided and (b) duplex treated pure titanium specimens.

Fig. 4 Potentiodynamic polarization curves of the untreated and treated pure titanium in Ringer's solution.

Çelik: Influence of CrN Coating on Electrochemical Behavior of Plasma Nitrided Pure Titanium in Bio-simulated Environment

153

Table 1 Corrosion parameters obtained from polarization curves of the untreated and treated pure titanium specimen in Ringer's solutions (E vs. Ag/AgCl)

(a)

Specimen

Ecorr (mV)

Icorr (A/cm2)

βa (mV/decade)

βc (mV/decade)

Chi Squared

Untreated

−55.6

47.9×10−9

510

263

15.77×10−3

Nit.500

48.2

107×10−9

1396

133.5

11.43×10−3

Nit.500+CrN

129

225×10−9

329.1

113.3

21.58×10−3

Nit.700

−38

209×10−9

259.2

135.5

17.34×10−3

Nit.700+CrN

165

105×10−9

367

103.2

11.35×10−3

(b)

(c)

Fig. 5 SEM images of specimens after corrosion tests at 37 ˚C in Ringer solution. (a) untreated; (b) Nit.500; (c) Nit.500+CrN; (d) Nit.700; (e) Nit.700+CrN.

154

Journal of Bionic Engineering (2016) Vol.13 No.1

measurement like polarization[21]. Electrochemical parameters such as corrosion current density (Icorr), anodic/cathodic Tafel constants (βa and βc), and corrosion potential (Ecorr) were derived from polarization curves using Tafel extrapolation. It was determined that the untreated pure titanium exhibited some characteristic properties in simulated body fluid and showed significant passivation behavior (Fig. 4). Some fluctuations were seen in the some regions (active or passive) of all the curves (Fig. 4). For protection from negative effects of corrosion, as low as possible current density and as much as noble potential are preferred. A comparison of the critical potentials and current density values obtained for all specimens is given in Table 1. As seen in Table 1, untreated specimen and specimens with treated surface have similar Icorr values. However, the lowest Icorr value belongs to the untreated pure titanium. When anodic polarization curves are examined in Fig. 4, it is understood that Nit.500 exhibits a better resistance than the untreated specimen up to average 1.12 V potential value. According to the XRD data in Fig. 1, the density of ε-Ti2N phase in Nit.700 is higher than the one in Nit.500. The active character of ε-Ti2N phase causes a higher corrosion current density[22]. In addition, δ-TiN phase was detected on the surfaces of all specimens with surface treatment (Fig. 1). δ-TiN phase is an inert phase[23] so it has been a factor helping to increase the tendency to noble behavior of the specimens. The CrN coating on the nitrided specimens affected the corrosion resistance negatively. The surface morphologies of untreated and treated pure titanium substrates after corrosion tests have been examined by SEM and are shown in Fig. 5. This figure (Fig. 5) shows the images of the sample surface after cleaned with ethanol. It is observed that pitting corrosion appeared on the surfaces of all specimens. Especially the formation of pitting on CrN coated surfaces can be explicitly observed (Figs. 5c and 5e). It is also understood that local corrosions were appeared on nitrided surfaces and they were less corrosion-damaged (Figs. 5b and 5d).

4 Conclusion In this study, commercially pure titanium specimens were plasma nitrided and CrN coated (duplex

treatment). The electrochemical properties of untreated and treated specimens were experimentally investigated. A compound layer and a diffusion layer were observed on the surface of pure titanium after plasma nitriding. In the modified layer of plasma nitrided pure titanium specimens mainly ε-Ti2N and δ-TiN phases were formed depending on treatment temperature. CrN, ε-Ti2N, and δ-TiN phases formed on the pure titanium as a result of the duplex surface treatments. The corrosion potential values of surface treated specimens have more positive characteristics than the untreated specimen. It is evidence from the results that a pitting-type corrosion mechanism was the dominant corrosion mechanism which was effective on the surfaces of CrN coated specimens. Eventually, taking into account all these factors mentioned above, it can safely be arrived at the conclusion that the surface treated specimen and the untreated specimen have similar corrosion current densities and when considering the tribological benefits, plasma nitrided and CrN coated pure titanium can be preferred in biomedical applications.

References [1]

Yetim A F. Investigation of wear behavior of titanium oxide films, produced by anodic oxidation, on commercially pure titanium in vacuum conditions. Surface & Coatings Technology, 2010, 205, 1757–1763.

[2]

Lemons J E, Niemann K M W, Weiss A B. Biocompatibility studies on surgical-grade titanium-, cobalt-, and iron-base alloys. Journal of Biomedical Materials Research, 1976, 10, 549–553.

[3]

Kasemo B. Biocompatibility of titanium implants: Surface science aspects. Journal of Prosthetic Dentistry, 1983, 49, 832–837.

[4]

Balazic M, Kopac J, Jackson M J, Ahmed W. Review: Titanium and titanium alloy applications in medicine. Internal Journal of Nano and Biomaterials, 2007, 1, 3–34.

[5]

Lütjering G, Williams J C. Titanium, Springer Press, Newyork, 2007.

[6]

Inoue A. Stabilization of metallic supercooled liquid and bulk amorphous alloys. Acta Materialia, 2000, 48, 279–306.

[7]

Nunes J, Piedade A P. Nanoindentation of functionally graded hybrid polymer/metal thin films. Applied Surface Science, 2013, 284, 792–797.

[8]

Pham V H, Yook S W, Li Y, Jeon G, Lee J J, Kim H E, Koh Y H. Improving hardness of biomedical Co-Cr by deposition of dense and uniform TiN films using negative substrate bias

Çelik: Influence of CrN Coating on Electrochemical Behavior of Plasma Nitrided Pure Titanium in Bio-simulated Environment during reactive sputtering. Materials Letters, 2011, 65, [9]

155

oxidation treatments on structural, mechanical and tri-

1707–1709.

bological properties of ultrafine-grained titanium. Surface &

Yetim A F, Alsaran A, Celik A, Efeoglu I. Corrosion be-

Coatings Technology, 2014, 258, 842–848.

haviour of Ti DLC deposition on prenitrided 316L stainless

[17] Daudt N F, Bram M, Barbosa A P C, Alves C. Surface

steel and Ti-6Al-4V alloy. Corrosion Engineering Science

modification of highly porous titanium by plasma treatment.

and Technology, 2011, 46, 439–444. [10] Alemon B, Flores M, Ramirez W, Huegel J C, Broitman E.

Materials Letters, 2015, 141, 194–197. [18] Çelik İ, Karakan M. Investigation of structural and tri-

Tribocorrosion behavior and ions release of CoCrMo alloy

bological properties of duplex surface treated pure Titanium.

coated with a TiAlVCN/CNx multilayer in simulated body

Kovove Materialy-Metallic Materials, 2016, 54.

fluid plus bovine serum albumin. Tribology International, 2015, 81, 159–168. [11] Frutos E, Alvarez D, Fernandez L, Gonzalez-Carrasco J L. Effects of bath composition and processing conditions on the

[19] Karakan M, Denktas O. Effect of post-oxidizing on tribological and corrosion behaviour of plasma nitrocarburized AISI 4140 steel. Kovove Materialy-Metallic Materials, 2009, 47, 367–374.

microstructure and mechanical properties of coatings de-

[20] Chang Z K, Wan X S, Pei Z L, Gong J, Sun C. Microstruc-

veloped on 316 LVM by hot dipping in melted AlSi alloys.

ture and mechanical properties of CrN coating deposited by

Journal of Alloys and Compounds, 2014, 617, 646–653.

arc ion plating on Ti6Al4V substrate. Surface & Coatings

[12] Long M, Rack H J. Titanium alloys in total joint replacement—a materials science perspective. Biomaterials, 1998, 19, 1621–1639.

Technology, 2011, 205, 4690–4696. [21] Esen Z, Dikici B, Duygulu O, Dericioglu A F. Titanium-magnesium based composites: Mechanical properties

[13] Steinemann S G. Metal implants and surface reactions.

and in-vitro corrosion response in Ringer's solution. Mate-

Injury-International Journal of the Care of the Injured, 1996,

rials Science and Engineering a-Structural Materials Prop-

27, 16–22.

erties Microstructure and Processing, 2013, 573, 119–126.

[14] Yetim A F, Yazici M. Wear resistance and non-magnetic

[22] Rossi S, Fedrizzi L, Bacci T, Pradelli G. Corrosion behaviour

layer formation on 316L implant material with plasma ni-

of glow discharge nitrided titanium alloys. Corrosion Sci-

triding. Journal of Bionic Engineering, 2014, 11, 620–629.

ence, 2003, 45, 511–529.

[15] Celik A, Aslan M, Yetim A F, Bayrak O. Wear behavior of

[23] Bes R, Gaillard C, Millard-Pinard N, Gavarini S, Martin P,

plasma oxidized CoCrMo alloy under dry and simulated

Cardinal S, Esnouf C, Malchere A, Perrat-Mabilon A. Xe-

body fluid conditions. Journal of Bionic Engineering, 2014,

non behavior in TiN: A coupled XAS/TEM study. Journal of

11, 303–310.

Nuclear Materials, 2013, 434, 56–64.

[16] Celik I, Alsaran A, Purcek G. Effect of different surface