The Influence of Plasma-Based Nitriding and Oxidizing Treatments on the Mechanical and Corrosion Properties of CoCrMo Bi
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COCRMO alloy (ISO 5832-12) is known as a metallic biomaterial. Its mechanical properties and corrosion resistance render it one of the dominant alloys for total joint arthroplasty.[1–3] Since hundreds of thousands of patients around the world have used implant materials for years, the quality and lifetime of the implants as well as the biological effects are of great concern. The requirements of the prosthesis materials such as high wear resistance, excellent corrosion properties, and biocompatibility are generally fulfilled using
FOTINI NOLI is with the Department of Chemistry, Aristotle University, 54124, Thessaloniki, Greece, Contact e-mail: [email protected] LUC PICHON is with the Institut Pprime - UPR 3346 CNRS, Universite´ de Poitiers, ISAE-ENSMA, 86962, Poitiers, France. ORHAN O¨ZTU¨RK is with the Department of Physics, Izmir Institute of Technology, Gulbahce-Urla, 35430, Izmir, Turkey. Manuscript submitted May 17, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
conventional biomaterial. However, there is a general recognition that the long-term total joint replacement may be associated with metal-degradation products. Biological reactions induced by corrosion and wear products of the implants are still a major focus in biomedical material research.[4,5] Biomaterials fabricated from CoCr-based alloys are reported to release increased concentrations of Co, Cr, and Ni ions in simulated body fluids.[6–8] Therefore, efforts have mainly focused on the modification of the surfaces of the implants to improve their mechanical properties, specifically lifetime. Nitrogen implantation is one of many surface treatment techniques forming protective layers on Fe- and Co-based alloys and inducing enhanced surface properties, especially improved mechanical properties, without affecting the bulk.[5,9,10] Plasma-assisted nitriding[6,11–19] and plasma-based ion implantation (PBII)[15,19–21] are found to improve considerably the mechanical properties of CoCr alloys (e.g., wear and fatigue resistance) producing thick modified surface layers. Furthermore, both surface treatment techniques can be achieved on
complex shape parts and significantly improve the tribological and corrosion properties of the treated surfaces.[22,23] At moderate nitriding temperatures (typically below 400 C), nitrogen is incorporated in solid solution in the initial FCC c phase. This leads to the so-called metastable expanded FCC cN phase, also called S-phase, which is characterized by high in-plane residual compressive stresses.[11,24] With increasing temperature and treatment duration, the cN phase is partially decomposed into nitride precipitates (CrN, Cr2N) and residual c phase with lower Cr content.[12,13,16] Although nitriding treatment has been observed to have beneficial effect on the mechanical properties of CoCrMo, its influence on the corrosion behavior is still under investigation. Nitriding treatments are usually observed to be detrimental to the corrosion resistance.[23] Chromium atoms, more or less bonded to nitrogen, are no longer mobi
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