Journal of Alloys and Compounds 581 (2013) 873–876
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Surface modiﬁcation of NiTi alloys using nitrogen doped diamond-like carbon coating fabricated by plasma immersion ion implantation and deposition Yan-Ling Pei a,⇑, Yang Luan b a b
School of Materials Science and Engineering, Beihang University, Beijing 100191, China Liaoning University of International Business and Economics, Dalian 116052, China
a r t i c l e
i n f o
Article history: Received 7 June 2013 Received in revised form 2 July 2013 Accepted 10 July 2013 Available online 18 July 2013 Keywords: NiTi alloys Nitrogen doped diamond-like carbon coating Plasma immersion ion implantation and deposition Adhesion strength Corrosion resistance
a b s t r a c t Nitrogen doped diamond-like carbon (N-DLC) coating was prepared on NiTi alloys using plasma immersion ion implantation and deposition (PIIID). The Raman spectrum conﬁrms the coating processes the structure of diamond-like carbon. The XPS results reveal the nitrogen is doped into the diamond-like carbon, and CAN bonds are formed in the coating. The scratch test displays that the DLC coating has good adhesion strength with the substrate. Anodic polarization has been carried out to examine the electrochemical corrosion behavior of the coated and uncoated samples. The result indicates that the corrosion resistance of the NiTi alloys is improved by the coating. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction NiTi alloys have attracted much attention due to their unique shape memory effect and superelasticity. For biomedical application, the corrosion resistance of NiTi alloys becomes extremely important, as the amount and toxicity of corrosion products control their biocompatibility. The Ni content in the alloy is approximately 50 at.%, and the release of Ni ions in all metallic implants occurs during the physiological environment [1,2]. The studies on the corrosion resistance of NiTi alloys in the environment of the simulated human body ﬂuids had been reported. The results showed that NiTi alloys exhibited poor resistance to localized corrosion in chloride-containing environments. It is well known that small amount of Ni is an essential element in the body. However, excess of Ni ions may cause allergic reactions and promote carcinogenesis and toxic reactions [3,4]. Therefore, it is very necessary to improve the corrosion resistance of NiTi alloys [5–7]. Surface modiﬁcation is considered an affective method to improve the corrosion resistance of NiTi alloys, which is closely related to the biocompatibility of the materials. Recently, diamondlike carbon has attracted much attention because of its hardness, wear resistance, chemical inertness, low coefﬁcient of friction ⇑ Tel.: +86 1082338173. E-mail address: [email protected]
(Y.-L. Pei). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.07.067
and good biocompatibility. There have many reports on the corrosion behavior of DLC coated TiNi alloys, and the corrosion resistance of these alloys is improved by the DLC coating [8–10]. However, the low adhesion strength caused by high residual stress still limits the practical application of the DLC coating. Fortunately, the nitrogen doped diamond-like carbon (N-DLC) coating exhibits higher adhesion strength than that of pure DLC coating, which is ascribed to the decrease of residual stress caused by the formation of CAN bonds [11–13]. It should be noted that N-DLC coating on the Si substrate also displays excellent mechanical property and biocompatibility [14–16]. However, to our knowledge, no studies on the N-DLC coating on the NiTi alloys have been reported. In this study, N-DLC coating is fabricated by plasma immersion ion implantation and deposition (PIIID) on the NiTi alloys. The surface characteristic, mechanical property and electrochemical corrosion behavior are also investigated. 2. Experimental procedure 2.1. Preparation of DLC coating All samples were cut to 13 13 1 mm3 from a sheet of cold rolled Ni50.8Ti49.2 (at.%) alloys. For all samples, 13 13 mm2 surface was successively polished down to 2000 grit speciﬁcation and mirror polished with 1 lm diamond paste. The sample was then cleaned ultrasonically with distilled water and dried with ﬂowing air. Fabrication of N-DLC coating was carried out by plasma immersion ion implantation and deposition (PIIID). Prior to PIIID, the NiTi samples were ultrasonically
Y.-L. Pei, Y. Luan / Journal of Alloys and Compounds 581 (2013) 873–876 Table 1 Parameters of PIIID fabrication for N-DLC coating.
P = 6.0 10 V = 6 kV s = 60 ls f = 100 Hz T = 30 min
P = 2.0 10 V = 25 kV s = 60 ls f = 100 Hz T=2h v = 15 sccm
PIIID processing steps
N=C N-C or N=C
cleaned with acetone and alcohol for 10 min, respectively. The vacuum chamber was evaluated to a base pressure of 5 10 3 Pa, then Ar+ sputtering was introduced into the chamber to remove undesirable oxide and contamination layers for 30 min prior to coating deposition. Carbon plasma was generated by pulsed cathodic arc plasma source from graphite with a curved magnetic duct (pulse width 1 ms). Similarly, nitrogen gas was supplied via gas lines that controlled by mass ﬂow meters. The detailed experimental parameters were listed in Table 1.
Binding Energy (eV)
2.2. Characterization Chemical compositions were determined by X-ray photoelectron spectroscopy (XPS) (PHI-5700 ESCA System) with a monochromatic Al Ka source. Raman measurements were performed at room temperature using a spectrometer of the type Jobin Yvon T64000, France. In the quasi-back scattering geometry of an Ar laser operating with a power of 200 mW, the 488 nm radiation was focused on a 2 lm spot on the coating. The bonding strength of the coating was evaluated using the scratch test to measure the critical load with one-pass sliding. The test principle was based on driving a hemispherical diamond indenter on the coating surface to produce a scratch. The diamond stylus used in the unit was in the form of a cone with spherical tip of 0.2 mm in radius. The scratch speed was maintained at 10 mm/min. Specimens were scratched while increasing the normal force. The morphology of the scratch track was observed by optical microscope. Electrochemical experiments were carried out with a standard three-electrode system. A saturated calomel electrode (SCE) was used as the reference electrode with a platinum counter electrode. The corrosion resistance was examined in Hank’s solution at 37 ± 0.5 °C (pH 7.4). The composition of Hank’s solution is given as follows: NaCl 8 g/l, Na2HPO4 0.0475 g/l, NaHCO3 0.35 g/l, KCl 0.4 g/l, KH2PO4 0.06 g/l, MgCl2-6H2O 0.10 g/l, MgSO4-7H2O 0.10 g/l, CaCl2 0.18 g/l and Glucose 1 g/l. Potentiodynamic polarization experiments started after the specimen immersed in the experimental solution for an hour under open-circuit conditions and performed at a rate of 20 mV/min.
3. Results and discussion Fig. 1 shows a Raman spectrum of the as-prepared coating. The spectrum of the coating shows a broad peak at approximately 1520 cm 1 and an obvious shoulder at a lower wave number. The broad peak of the ﬁgure can be decomposed into the Gaussian
Raman shift (cm ) Fig. 1. Raman spectrum of the as-prepared coating on NiTi alloys.
Binding Energy (eV) Fig. 2. XPS survey scan spectra for the N-DLC coating on NiTi alloys (a) and the deconvolution of the N1s line (b).
centered at 1551 cm 1 (G peak ascribed to the graphite carbon) and at 1329 cm 1 (D peak ascribed to the disordered graphitic carbon), and the ratio of the integrated areas under the D and G peaks (ID/IG) is 1.93. The spectrum possesses the most dominant characteristic of the typical DLC ﬁlm . This result indicates that the as-prepared coating has a diamond-like carbon structure. Fig. 2a shows the survey scan spectrum of the XPS detected from the surface of N-DLC coating. The C1s, N1s and O1s peaks are clearly identiﬁed, which come from graphite source, nitrogen injection and the reaction of oxygen with the free radicals at the ﬁlm surface [18,19]. In order to further clarify the CAN bonds, the N1s line is decomposed shown in Fig. 2b). The deconvolution of the N1s line gives four peaks at 398.6, 400.2, 401.4 and 402.9 eV, which are assigned to NAC, N„C, [email protected]
, NAN and NAO bonds, respectively. These values agree quite well with the previous publication [20,21]. This indicates the nitrogen is not only doped into diamond-like carbon, but also chemically bonds with carbon. It is beneﬁcial to release residual stress, and consequently improve the adhesion strength. Strong adhesion of the N-DLC coating to the substrate material is very pre-requisite for application as a biomaterial coating for implants. Fig. 3 shows the optical micrograph and corresponding curve of the scratch track in the N-DLC coated NiTi alloys. It can be seen from the scratch track curve, obvious ﬂuctuation, which is come from the acoustic emission signal caused by occurrence of the fracture behavior, is not observed along the scratch curve. It can be concluded that the N-DLC coating possesses excellent adhesion strength. In order to further ﬁnd out the adhesion strength between the coating and the substrate, the local magniﬁed scratch tracks are also shown in Fig. 3. It is clearly seen that no any chipping and ﬂaking is observed along the scratch track when the load is up to 100 N. This indicates that the good adhesion strength between the N-DLC coating and NiTi substrate can be obtained by the PIIID technique and nitrogen doping. Fig. 4 shows the potentiodynamic polarization curves of N-DLC coated and uncoated NiTi alloys in Hank’s solution. Generally, the
Y.-L. Pei, Y. Luan / Journal of Alloys and Compounds 581 (2013) 873–876
cracks, is beneﬁcial to good corrosion resistance. The other factor is the adhesive strength between the coating and the substrate. Poor protection is sometimes not due to the corrosion resistance property of the coating itself, but to the ﬂaking away of the coating due to lack of sufﬁcient adhesion strength. Therefore, these two factors must also be considered during the coating deposition process. In this study, PIIID technique was used to deposit DLC coating on the NiTi alloys. Since the duration of the bias pulse (60 ls) is shorter than that of the arc discharge pulse (1 ms), this technique combines pulsed plasma deposition and pulsed high-energy ion implantation. In addition, the uniform and dense DLC and ZrO2 coating had been prepared by the same technique [22,23]. Therefore, it can be safely expected that the dense coating can be fabricated by the PIIID technique. Prior to the PIIID treatment, the substrate ﬁrst is cleaned by pre-sputtering with an Ar ion to remove the surface contamination and air-formed oxide layer. The very clean substrate then is exposed to the carbon plasma assisted by high bias voltage. In the initial stage of DLC formation, due to the high bias voltage (25 kV), plasma can be implanted into the surface of NiTi alloys, thereby leading to a mixing layer between the coating and substrate. During the formation of N-DLC coating, the alternative implantation and deposition processes relax the residual stress in the ﬁlms. Last but not least, formation of the CAN bonds during the preparation of N-DLC coating is also beneﬁcial to relax residual stress. Therefore, the above factors can well explain the high adhesion strength (shown in Fig. 3) between the substrate and the coating. N-DLC acting as a coating possesses good corrosion resistance due to its chemical inertness. The dense and well adhered N-DLC coating has been fabricated by the PIIID technique. Therefore, the N-DLC coating can improve the corrosion resistance of TiNi alloys.
Friction force (N)
10 8 6 4 2 0
Load (N) Fig. 3. Optical micrograph (a) and corresponding curve (b) of the scratch track on the surface of the N-DLC coated NiTi alloys.
Potential vs SCE/V
N-DLC coated NiTi alloys
4. Conclusions N-DLC coating has been successfully fabricated on NiTi alloys by plasma immersion ion implantation and deposition. Nitrogen is doped into diamond-like carbon, and CAN chemical bonds are formed in the N-DLC coating. High adhesion strength between NDLC coating and NiTi substrate is obtained by the PIIID technique and nitrogen doping. The N-DLC coating markedly improves the corrosion resistance of NiTi alloys.
Uncoated NiTi alloys
Log current density (A/cm 2 ) Fig. 4. Potentiodynamic polarization curves of the N-DLC uncoated and coated NiTi alloys.
samples with lower current density and higher potential indicate better corrosion resistance. It can be clearly seen that a signiﬁcant improvement in corrosion resistance of the NiTi alloys due to the N-DLC coating is evidenced by a shift of the whole polarization curve towards the region of lower current density and higher potential. Therefore, it can be concluded that N-DLC coating improves the corrosion resistance of the NiTi alloys in Hank’s solution. As a coating for improving corrosion resistance, apart from the good corrosion resistance of the coating itself, two important factors affecting corrosion resistance are also taken into account. One is the coating quality. The high-quality coating, which should have few small structural defects such as pinholes, pores and
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