Bio-tribocorrosion behavior of a nanocrystalline TiZrN coating on biomedical titanium alloy
Introduction
Titanium nitride coatings on biomedical metals, which were produced by physical vapor deposition (PVD) processes, have been applied for joint prosthesis, orthopedics instruments and dental implant etc. to protect the implants from wear and corrosion in physiological environment [[1], [2], [3], [4]]. However, some clinical cases reported that the scratching and spalling phenomena happened to the fretting surface of the retrieved TiN coated joint prosthesis that has served for more than 5 years [5,6]. This indicates that the adhesion strength, anti-friction and wear performance of TiN coating on metal substrate cannot meet the requirements of long lifetime for the implants.
Up to date, many approaches have been suggested to increase the mechanical properties of TiN coating, such as metal or non-metal alloying [7], nano-multilayer modulation structure of hetero nitride [8], biphasic composite structure [9] and transitional components distribution at the interface of coating and substrate [10] etc. But the problem is that the improvement of one performance is often at the expense of sacrificing another performance. For example, the increase of hardness results in a brittleness and decrease of adhesion strength. And low hardness is adverse to friction and wear performance. According to Evans' wear theory, the anti-wear performance (W) of the ceramics material needs a good combination of fracture toughness (KIC) and hardness (H) [11]. And increasing the adhesion of ceramic coating to metal substrate requires a good match in chemical bonding, elastic modulus and mechanical properties at interface [12,13]. Based on this, it is indispensable to find a new way to synthetically improve the mechanical properties of TiN coating.
Recently, a nanocrystalline TiN coating on biomedical titanium alloy has been produced by DC-reactive magnetron sputtering deposition [14]. Through the grading increase of nitrogen content along the film growth direction, the coating behaves greatly increased adhesion to titanium substrate, lowered friction coefficient and improved wear resistance in physiological environment compared to the columnar crystal TiN coating. The nanocrystalline TiN coating also has good biocompatibility after in-vitro and in-vivo biological evaluation.
As a bio-coating used for the metal joint prosthesis or dental implant, it is crucial to investigate its bio-tribocorrosion behavior in physiological environment. Although TiN coating exhibits significantly reduced corrosion rate and good wear resistance compared to metal substrate [15,16], the synergic effect of wear and corrosion will accelerate the degradation in the aspects of the chemical and mechanical properties of TiN coating [17]. In addition to this, the friction coefficient of the available TiN coating in Hank' solution is 0.2–0.3, which is far higher than that of human joint (0.01–0.1) [18,19]. Thus, it is necessary to further improve the tribocorrosion and friction performance of TiN coating.
Since ZrN has similar chemical and physical properties and good biocompatibility with TiN, ZrN bio-coating has attracted more and more attention [20,21]. Ripoll et al. found out that Zr-based nitride coating exhibits a better friction and wear behavior when compared to Ti-based coating, which is attributed to the ability to form stable oxide layers [22]. Floroian et al. also obtained the similar results for TiN and ZrN coated titanium implant in simulated body fluid (SBF) [23]. It has been reported that the partial replacement of Ti by Zr atoms will give TiZrN coating, a superior wear resistance and fracture toughness compared to TiN and ZrN coating [24,25]. Nevertheless, the corrosion and bio-tribocorrosion performance of the TiZrN coated titanium alloy in simulated body fluid (Hank's solution) are not clear. Furthermore, calf serum is often used for simulated synovial fluid to reduce the wear of metal artificial joint head [26]. It is of significance to study the effect of calf serum in Hank's solution on the corrosion, friction and wear performance of the coated Ti alloy. The present paper focus on a nanocrystalline TiZrN graded coating. The phase, microstructure and compositions of TiZrN coating were characterized. The bio-tribocorrosion behavior of the TiZrN coated titanium alloy under different anodic potentials was investigated in Hank's solution with and without 25% calf serum. The strengthening and bio-tribocorrosion mechanisms of TiZrN coating were analyzed. In order to clarify the role of Zr in TiN coating, the corrosion and bio-tribocorrosion performances of TiN and TiZrN coating were compared under the same testing conditions.
Section snippets
Materials and methods
TiN and TiZrN coating were deposited on medical grade Ti6Al4V alloy by DC reactive sputtering magnetron deposition with commercial pure Ti and Ti50Zr (mass %) alloy target. High purity argon and nitrogen (99.99%) were introduced into the chamber. Prior to deposition, the chamber was pumped down to a pressure of 4 × 10−3 Pa. The target with the size of ϕ 92.8 mm in diameter and the distance of 100 mm to the substrate was pre-cleaned via Ar+ bombardment for 10 min in order to remove surface
Phase and microstructure characterization
The XRD patterns of the TiN and TiZrN coatings are shown in Fig. 2. TiN coating displays strong (200) texture orientation. The weak diffraction peaks of α-Ti and Ti2N transitional phase from the inner layer of the coating are also seen in the XRD pattern. The diffraction peaks of TiZrN coating are between TiN and ZrN standard diffraction peaks. This indicates that Zr dissolved in TiN lattice produces strong lattice distortion, which causes the shift of diffraction peaks to low diffraction
Conclusions
- 1.
Compared to TiN coating, the nanocrystalline TiZrN coating which is produced by PVD process has higher hardness and more excellent corrosion resistance in physiological environment, which are attributed to the Zr solid solution strengthening and the rapid formation of the stable Ti, Zr oxide or oxynitride passive film.
- 2.
During friction under OCP condition, TiZrN coating behaves smaller corrosion tendency and lower friction coefficient than TiN coating. When applying anodic potentials in the range
Acknowledgment
This work is supported by State Key Laboratory of Powder Metallurgy, Central South University, China (621011823).
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