Corrosion and tribological properties and impact fatigue behaviors of TiN- and DLC-coated stainless steels in a simulated body fluid environment
Introduction
Biomaterials used for load-bearing applications including orthopedic implants, pacemakers, surgical instruments, orthodontic appliances and dental instruments, usually have to withstand some aggressive biomedical conditions. Those biomaterials are normally used in a physiological environment with a pH level of 7.4 and a temperature of 37 °C (98.6 °F). The physiological solution is oxygenated and contains organic components in addition to various salts [1]. The biomaterials are also required to have an excellent mechanical strength and wear resistance [1]. Metals are the most commonly used for the load-bearing biomaterials, among which, the most widely used metallic biomaterials for implants devices are 316L stainless steels, cobalt alloys, commercially pure titanium and Ti–6Al–4V alloys [2], [3], [4], [5], [6]. However, these materials suffer from some drawbacks, in case of sustained and long-term use, such as cytotoxicity due to releasing of metallic ions, corrosion and wear. Additionally, in many designs of biomaterial structure, a metal-polyethylene (PE) bearing couple is frequently involved. The wear debris generated during applications of polyethylene may cause substantial negative biological effects such as chronic inflammation, when it is transported to the soft tissue surrounding the implant. Thus, besides corrosion and wear protection properties, the compatibility of biomaterials with the polyethylene material is another important issue to be concerned.
PVD and CVD hard ceramic coatings are expected to be the emerging biomaterials for load-bearing medical devices due to their excellent corrosion and wear protection performance. The ceramic coatings TiN, DLC, TiAlN, TiN/TiAlN, TaN and ZrN [7], [8], [9], [10], [11], [12], [13], [14], [15] prepared by the cathodic arc method, reactive magnetron sputtering, etc. have been studied on their corrosion resistance, biocompatible characteristics and mechanical properties. Until now, the TiN and diamond-like carbon (DLC) coatings are still considered as the most promising coatings used for load-bearing biomaterials and have been used in orthopaedic prostheses, cardiac valves and dental prostheses [16], [17], [18]. Both TiN and a-H:C DLC are found to be biocompatible and have been reported to achieve corrosion protection under most of the biomedical environments [10], [11], [12], [19], [20], [21], [22], [23], [24], [25], [26]. However, the test results in wear and compatibility performances of those coatings against different counter contacting surfaces are rather controversial probably due to different research groups using different experimental setups (pin-on-disk, hip or knee simulator, different surface roughness) and different liquids as lubricants. In addition, few works were conducted to investigate the fatigue behavior of coatings under a load condition simulating some special activities such as climbing, stumbling, and jumping. In those cases, an impact load can be involved, and the impact fatigue can be one of the most likely failure mechanisms of the coated-biomaterials.
Bearing this in mind, in this study, the corrosion performances of TiN and DLC coatings were evaluated in a simulated body fluid by electrochemical methods. Pin-on-disc sliding wear tests (under a fixed static load) and impact fatigue tests were also conducted. The results of those coatings tested under the simulated biomedical environment and load condition were discussed.
Section snippets
Preparation of substrates and coatings
TiN [27] and a-H:DLC (Diamolith®) coatings [27], [28] were prepared using standard commercial processes at Tecvac Ltd. DLC coating was prepared by plasma-assisted chemical vapour deposition (PACVD). Samples were sputter cleaned in an Ar–H2 discharge prior to DLC coating and a thin Si bond layer was first deposited at a thickness of 0.4–0.5 μm. A total pressure of 0.8 Pa was used during deposition and the substrates were R.F. biased to a total power of 500–550 W. The maximum coating temperature did
Electrochemical corrosion properties
Fig. 2 shows the potentiodynamic polarization curves of uncoated and coated stainless steel 316 L samples in the SBF solution at 37 °C. The polarization data are listed in Table 1. The corrosion test results showed that higher Rp and lower Icorr values were measured for the TiN- and DLC-coated samples than the uncoated SS316L (Rp = 7.63 kΩ cm2 and Icorr = 5.36 μA/cm2). The DLC coating exhibited the highest Rp (1543 kΩ cm2) and lowest Icorr (0.055 μA/cm2) among all the three samples. Since the corrosion
Conclusions
The results of potentiodynamic polarization tests and corrosion potential vs. testing time (10 h) indicated that TiN and DLC coatings could achieve a higher corrosion polarization resistance and relatively stable corrosion potential in the SBF environment than the uncoated SS316L. Therefore, the coated samples would have a lower corrosion and metallic ion releasing rate. The superior corrosion protection performance of TiN was due to formation of the Ti–O passive layer on the coating surface,
Acknowledgements
L. Wang would like to acknowledge NSERC for the award of a Canada Graduate Scholarship. The authors also would like to thank the Tecvac Ltd. for the provision of PVD and CVD coatings.
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