One of the major drawbacks for some of these coatings is delamination, which occurs due to poor adhesion on substrates resulting from the high level of internal stress generated during the coating process. While some of these coatings are either in their early or advanced stages of investigation, others such as TiN and TiNbN are already in use in commercially available hip and knee prostheses. Several coating materials such as diamond-like carbon (DLC), chromium nitride (CrN), hydroxyapatite (HA), titanium nitride (TiN) and titanium niobium nitride (TiNbN) have been explored by different researchers for orthopaedic implant applications. These properties can be enhanced by applying a wear and corrosion protective coating material on the implant surface to eliminate or minimize surface damage and degradation while retaining the bulk material properties. The biocompatibility of an implant material is strongly linked to its resistance to tribological and corrosion processes. Cobalt chromium molybdenum alloy (CoCrMo) implant, on the other hand, releases cobalt and chromium ions into the surrounding tissue fluid because of wear at the bearing surface and the corrosion of the implant material. Nevertheless, its low cost and poor corrosion resistance compared with other biomaterials (like cobalt chromium molybdenum and titanium alloy) make it suitable for temporary implants like screws and plates that are normally removed after a few years. Studies conducted on retrieved 316 L stainless steel implant show that more than 90% of failures of the material is due to pitting and crevice corrosion attack, resulting in the release of Fe, Cr and Ni ions, which are toxic to the human body system. However, the above-mentioned implant materials are not immune to wear, corrosion and biocompatibility issues. Materials such as titanium alloys, 316 L stainless steel, cobalt-based alloys, zirconium alloy, ultra-high molecular weight polyethylene and alumina-based ceramics are widely used as implant materials due to their high corrosion resistance, mechanical properties and biocompatibility. The implant failure in vivo is well known to involve corrosion, which leads to the release of wear debris/ions and have been linked with possible adverse health effects such as pains, pseudo-tumour formation and inflammation in patients. One of the major challenges faced by patients with metal-on-metal hip replacements is the potential of implant failure. We expect this finding to be significant for future orthopaedic implants where chromium ion release is still a major challenge. The coatings were further exposed to Ringer's solution for one month and tested for adhesion strength changes, and we found that they retained substantial adhesion to the substrates. We investigated for the presence of chromium ions in Ringer's solution after all of the above electrochemical tests using atomic absorption spectroscopy, without finding a trace of chromium ions at the ppm level for coatings tested under open circuit and at the lower potentials implants are likely to experience in the human body. The coatings were found to be predominantly Cr 2O 3, based on the observation of the dominance of A 1 g and E g symmetric modes in our Raman spectroscopic investigation and the E u vibrational modes in our Fourier transform infrared spectroscopic measurements on the coatings. Chromium oxide coatings prepared by radiofrequency reactive magnetron sputtering on stainless steel substrates were exposed to Ringer's physiological solution and tested for their electrochemical corrosion stability using an open circuit potential measurement, potentiodynamic polarization, electrochemical impedance spectroscopy and Mott–Schottky analysis.
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