Tribocorrosion and electrochemical behaviour of nanocrystalline diamond coated Ti based alloys for orthopaedic application
Introduction
Load bearing implants, such as total hip and total knee replacements (THR and TKR) have more extensive consideration among clinicians, because of the tremendous increment in individuals who suffers from arthritis and joint related issues. It has been reported that by the end of 2030, the demand for THR and TKR procedures is projected to grow by 174% and 673% respectively, in the USA alone [1]. Further, another report estimated that the percentage of younger patients (<65years) for arthroplasty procedures is expected to exceed 50% between 2010 and 2030 [2]. Owing to these demands, the life expectancy of the implants is of crucial importance and expected to serve for a longer period. However, the commercial metallic materials currently used for the load bearing implants, such as 316 L stainless steel, Co-Cr alloys and titanium based alloys fails within 15–20 years which in turn leads to revision surgery. By 2030, hip and knee revision surgery is expected to increase by 137% and 601% respectively. But the success rate of the revision surgery is low and leaves the patient with undesirable pain [2], [3], [4]. The fundamental reasons for the failure of implants are due to the generation of wear debris and metal ion release from the bearing surface during the wear and corrosion process which in turn induces osteolysis and results in aseptic loosening of implants [5]. It is perceived that the articulating motion of hip and knee joint implicates a sliding contact in a corrosive biological environment, thus by making the artificial joints subjected to corrosion and mechanical wear simultaneously. The synergism between the chemical/electrochemical and wear phenomenon is often referred as tribocorrosion [6], [7]. It has been proved that the synergistic effect, i.e., wear induced corrosion and corrosion induced wear prompts in altering the nature of the implant material loss [8], [9]. It is noteworthy to mention that, despite titanium alloys possess excellent corrosion resistance; but under mechanical loading, they exhibit higher corrosion rates [10]. Further, the work was done by Yan et al., [11] demonstrated that the corrosion and wear induced corrosion mechanism led to 20–30% of material loss in Co-Cr alloys, which usually serves as the femoral head of an implant.
Degradation of metallic implants due to wear and corrosion pose a stringent challenge for researchers. To obviate this issue, it is necessary to design the implant surface by introducing a protective coating, in such a way it enhances the wear and corrosion resistance which ultimately improves the longevity of implants. Several coating techniques such as ceramic coating by thermal spraying, TiN coating and diamond/diamond like carbon (DLC) coatings by physical vapour deposition (PVD) and chemical vapour deposition (CVD) have been currently used to modify the metallic implant surface. Among them, nanocrystalline diamond (NCD) coating on metallic implants by chemical vapour deposition method is one of a kind, due to its inherent properties such as superior hardness, chemical inertness, high fracture toughness and low friction coefficient. These unique properties of NCD coatings make it a potential candidate for wear and corrosion resistant coatings for orthopaedic implant applications [12], [13], [14]. Many authors have investigated the tribological behaviour of chemical vapour deposited (CVD) NCD coatings on different substrates [15], [16], [17], [18]. Amaral et al., [19] observed an extremely low wear rates (0.005 mm3/Mc or 0.08 µm/Mc) in the case of self mated NCD coatings on silicon nitride ceramics in a hip simulator wear test. They also emphasised that the wear rate was lesser than the best reported ceramic on the ceramic hip joint. Besides tribological properties, NCD film possesses required biocompatibility similar to titanium and 316 stainless steel [20]. In addition to biocompatibility, the NCD coatings also elicit osteoblast adhesion [10], [21]. Cytotoxicity evaluation of NCD coatings reveals less toxicity and also provides a suitable surface for cell attachment which induces slight enhancement in the proliferation of cells as assessed by the behaviour of L929 and HG cells [22]. The combination of physio chemical properties and biocompatibility of the NCD coatings makes it a promising material not only in the orthopaedic implant application but also in dental implants, dental burrs and scalpels [23], [24].
The previous study of submicrometric diamond coated on Ti-13Nb-13Zr and Gum metal shows insignificant enhancement in corrosion resistance compare to that of the bare substrate [25]. In the light of above advantageous aspects of NCD coating and taking into the account of the previous study, which prompts us to go for NCD coatings on titanium based alloys. In order to make it even more appropriate and considering the wear corrosion synergistic effect of the orthopaedic implants, it is imperative to study the tribocorrosion behaviour of the NCD coatings. To the best of our knowledge, there are no reports on tribocorrosion behaviour of NCD coatings on titanium based alloys till date. Further, there are only limited papers on corrosion behaviour of NCD coatings on titanium based alloys.
In this work, titanium based alloys such as commercially pure Ti (Cp-Ti), Ti-6Al-4V and Ti-13Nb-13Zr have been chosen. Ti-6Al-4V is a α+β alloy, which possesses excellent corrosion resistance and biocompatibility. However, it is still under scrutiny due to the toxicity of vanadium. Ti-13Nb-13Zr is a near β alloy, which has low modulus and has been approved by FDA. Furthermore, it was reported that the Ti based alloys provide good adhesion to diamond, whereas Co-Cr alloys showed poor adhesion due to the formation of graphitic interlayer [13]. The Hot filament CVD (HFCVD) method is deployed to grow the NCD coatings on titanium based alloys. This technique is one of the most common methods for depositing diamond on various substrate and also useful for growing uniform diamond over a large area in a simple way at low cost [26]. In this study, corrosion and tribocorrosion behaviour of NCD coatings on titanium based alloys have been investigated in detail and reported.
Section snippets
Experimental procedure
As received forged Cp-Ti, forged Ti-6Al-4V and hot rolled Ti-13Nb-13Zr in the dimension of 20 mm diameter and 5 mm thickness were used for the present study. The alloys were mirror polished using SiC papers (400–3000 grits) followed by diamond polishing using one μm diamond paste and finally, the samples were ultrasonically cleaned in acetone for 20 min. The surface roughness (Ra) of the polished Cp-Ti, Ti-6Al-4V and Ti-13Nb-13Zr were 21 nm, 23 nm and 35 nm respectively, as measured using a stylus
X-ray diffraction (XRD) analysis
In order to evaluate the crystallinity of the coatings, XRD analysis was performed. Fig. 1 shows the diffraction pattern of NCD coated Cp-Ti, Ti-6Al-4V and Ti-13Nb-13Zr. In the XRD pattern, the peak at 44.2° and 75.4°corresponds to (111) and (220) planes of diamond respectively. This confirms that the deposited coatings were composed of diamond grains. Besides the diamond peaks, the presence of Ti and TiC crystal planes were confirmed by the appearance of peaks at 36.4°(020), 38.6°(002),
Summary and conclusion
In this study, the corrosion and tribocorrosion behaviour of NCD coatings deposited on Cp-Ti, Ti-6Al-4V and Ti-13Nb-13Zr have been investigated. Concluding statements are as follows.
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Nanocrystalline diamond coatings were deposited on Cp-Ti, Ti-6Al-4V and Ti-13Nb-13Zr using HFCVD method. XRD pattern confirms the crystallinity of the coatings. Raman spectrum revealed the formation of nanocrystalline diamond coatings. Deposited coatings possess compressive stress due to the thermal expansion
Acknowledgments
The authors M.C and M.S.R.R. acknowledge the Department of Science and Technology (DST) of India for the financial support (Grant no. SR/NM/NAT-02/2005).
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