Bio-tribocorrosion behavior of a nanocrystalline TiZrN coating on biomedical titanium alloy

https://doi.org/10.1016/j.surfcoat.2019.04.036Get rights and content

Highlights

  • Nanograin TiN and TiZrN coating are deposited on Ti alloy by PVD method.

  • TiZrN coating has better anti-tribocorrosion performance than TiN coating.

  • Synergistic effect of corrosion and wear promotes the damage of TiZrN coating.

  • Calf serum improves friction and wear performances of the coated Ti alloy.

Abstract

A nanocrystalline TiZrN graded coating was deposited on biomedical titanium alloy by DC reactive magnetron sputtering process. The microstructure and compositions of the coating were characterized by XRD, TEM and EPMA. The electrochemical corrosion and bio-tribocorrosion behavior of the coated titanium alloy under open circuit potential (OCP) and applied potentials (−0.15 V ~ +0.25 V) were investigated in Hank's solution with and without 25% calf serum. Compared with TiN coating, the hardness of TiZrN coating is greatly increased due to the Zr solid solution strengthening and nanocrystalline strengthening. Under the conditions of OCP and applied anodic potentials, the TiZrN coated Ti alloy exhibits significantly improved anti-tribocorrosion and anti-friction performances, which are attributed to that the stable Ti and Zr oxide or oxynitride passive film on coating near surface and the increased mechanical properties of the coating decrease the synergistic effect of corrosion and wear. 25% calf serum in Hank's solution enhances the chemical stability of the TiZrN coating through adsorption and bio-lubrication mechanism, and therefore further improves the tribocorrosion performance of the coated Ti 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).

References (33)

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