Effects of plasma nitriding on mechanical and tribological properties of CoCrMo alloy (original) (raw)
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Metallurgical and Materials Transactions A, 2018
Plasma-based nitriding and/or oxidizing treatments were applied to CoCrMo alloy to improve its surface mechanical properties and corrosion resistance for biomedical applications. Three treatments were performed. A set of CoCrMo samples has been subjected to nitriding at moderate temperatures (~400°C). A second set of CoCrMo samples was oxidized at 395°C in pure O 2. The last set of CoCrMo samples was nitrided and subsequently oxidized under the experimental conditions of previous sets (double treatment). The microstructure and morphology of the layers formed on the CoCrMo alloy were investigated by X-ray diffraction, Atomic Force Microscopy, and Scanning Electron Microscopy. In addition, nitrogen and oxygen profiles were determined by Glow Discharge Optical Emission Spectroscopy, Rutherford Backscattering Spectroscopy, Energy-Dispersive X-ray, and Nuclear Reaction Analysis. Significant improvement of the Vickers hardness of the CoCrMo samples after plasma nitriding was observed due to the supersaturated nitrogen solution and the formation of an expanded FCC c N phase and CrN precipitates. In the case of the oxidized samples, Vickers hardness improvement was minimal. The corrosion behavior of the samples was investigated in simulated body fluid (0.9 pct NaCl solution at 37°C) using electrochemical techniques (potentiodynamic polarization and cyclic voltammetry). The concentration of metal ions released from the CoCrMo surfaces was determined by Instrumental Neutron Activation Analysis. The experimental results clearly indicate that the CoCrMo surface subjected to the double surface treatment consisting in plasma nitriding and plasma oxidizing exhibited lower deterioration and better resistance to corrosion compared to the nitrided, oxidized, and untreated samples. This enhancement is believed to be due to the formation of a thicker and more stable layer.
Fatigue life determination of plasma nitrided medical grade CoCrMo alloy
Fatigue & Fracture of Engineering Materials & Structures, 2010
A B S T R A C T In this paper, the fatigue behaviour of plasma nitrided medical grade forged CoCrMo alloy was studied. Since metallic biomaterials are used for implant applications where high and/or cyclic stresses along with corrosive effects of human body are of concern, enhancing mechanical and surface properties of implant alloys is crucial. Plasma nitriding was implemented at three different temperatures as 600, 700 and 800 • C for time intervals of 1 and 4 h. S-N curves of untreated and nitrided specimens were obtained via axial tension compression fatigue tests. It was found that plasma nitriding treatment reduces the fatigue resistance of forged CoCrMo alloy by the ratios of 7-23% depending on the surface roughness, phase structure and hardness of the modified layer which are determined by the treatment parameters.
High intensity plasma ion nitriding of orthopedic materials
Surface and Coatings Technology, 2004
Orthopedic materials (Ti-6Al-4V and CoCrMo) were nitrided at temperatures up to 850 jC using a newly developed technique-high intensity plasma ion nitriding (HIPIN)-in order to enhance lifetime wear properties. Near surface microstructure and phase formation of both nitrided Ti-6Al-4V and CoCrMo alloys were evaluated using cross-sectional SEM and X-ray diffraction techniques. Nitrogen was found to most readily incorporate into the Ti-6Al-4V structure at around 750 jC (slight expansion of the lattice) with additional formation of TiN and Ti 2 N at higher temperatures. In the case of the CoCrMo alloys (forged and cast), a metastable, high nitrogen phase was also found to form at between 300 and 600 jC along with the formation of CrN/Cr 2 N/j phases above 700 jC. It was concluded that increased wear properties, as a result of HIPIN processing, can be related to both the formation of deep nitrided layers and the formation of specific microstructures at the surface (for both alloy types).
Low pressure plasma nitrided CoCrMo alloy utilising HIPIMS discharge for biomedical applications
Journal of the Mechanical Behavior of Biomedical Materials, 2020
CoCrMo is a biomedical grade alloy which is widely used in the manufacturing of orthopaedic implants such as hip and knee replacement joints because of it has high hardness, high corrosion resistance, and excellent biocompatibility. However, the release of metal ions due to corrosion and wear of the alloy over time may cause allergic or other adverse reactions in some patients. To date, various surface modification techniques including nitriding, have been used to improve the performance of CoCrMo (F75) alloy. 15 m 3 N-1 m-1 (-1100 V), which were one order of magnitude lower than the untreated substrate, Kc = 6 ×10-14 m 3 N-1 m-1. The Knoop microhardness (HK) of nitrided samples significantly increased by a factor of 5 (HK= 2750 at-1100 V) as compared to the untreated substrate, HK=525, demonstrating the high efficiency of the process. The samples nitrided at-700 V and-900 V exhibited enhanced corrosion resistance as compared to untreated alloy by avoiding the formation of CrN based compounds which adversely affect the corrosion performance.
Effect of nitrogen on the microstructure and mechanical properties of a CoCrMo alloy
Materials & Design, 1996
The effect of nitrogen on the microstructure and mechanical properties of a cobalt base alloy was studied. The treatments were carried out for contact times of 0, 15, 30, 45 and 55 min, between the gas and the liquid metal. The treatment temperature was 1723 K at a nitrogen pressure of 0.67 atm. The results showed that as the nitrogen content increased, the tensile and fatigue strengths increased, the hardness increased slightly and the ductility decreased. The metallographic analysis showed that the nitrogen content had little effect on the grain size. When the nitrogen content in the alloy was increased, the formation of finer carbides of the M,C type was promoted ahead of the M2sCs type. The fine carbides tended to accumulate along the grain boundaries.
Modeling of Nitrogen Penetration in Medical Grade CoCrMo Alloy during Plasma Nitriding
Materials Science, 2014
For analysis of plasma nitriding process and nitrogen penetration into CoCrMo alloy the trapping-detrapping model is applied. This model is commonly used for analysis of stainless steel nitriding, however, in this work it is shown that the same nitrogen penetration mechanism takes place in CoCrMo alloys. From the fitting of experimental curves, taken from literature, it is found by the proposed model that diffusion coefficient depends on nitrogen concentration according to Einstein-Smoluchowski relation D ∝ 1/C N. The diffusion coefficients for 400 o C temperature nitriding of in CoCrMo are calculated. The shape of nitrogen depth profile curves are analyzed showing influence of different parameters such as detrapping activation energy, chromium concentration, etc.
Surface & Coatings Technology, 2020
CoCrMo alloy specimens were plasma nitrided using a High Power Impulse Magnetron Sputtering (HIPIMS) discharge. In this work the effect of nitriding voltage (-700 V to-1100 V) on the microstructure, surface hardness, impact load fatigue resistance and fracture toughness (KIc) of the alloy has been investigated. Results revealed that the specimens treated at lower nitriding voltages (-700 V and-900 V) develop a nitrided layer consisting a mixture of Co4N+Co2-3N phases. As the nitriding voltage increased (-1000 V and-1100 V), this transformed into a thick layer consisting mainly of Co2-3N with a minor contribution from CrN/Cr2N phases. Accordingly, surface hardness tests after nitriding showed a significant improvement in hardness value (H= 23 GPa) as compared to the untreated specimen, (H= 7.9 GPa). The impact resistance of the alloy also increased with the nitriding voltage. Impact crater profiling of the specimens subjected to impact load tests showed that the depth of the crater decreased significantly, 2 especially at higher nitriding voltages. At the end of the impact load test (one million impacts), the crater depth for an untreated alloy (12.78 μm) was found to be twice to the crater depth measured for the specimen nitrided at-1100 V (7.1 μm). Impact testing results indicate that the fatigue endurance limit of the CoCrMo alloy increased steadily and considerably with the increase of the nitriding voltage. HIPIMS plasma nitriding resulted in a layer material with improved plain strain fracture toughness (KIc), with higher values (KIc) = 1011 MPamm 1/2 (-700 V specimen) were calculated as compared to KIc = 908 MPamm 1/2 for the untreated specimens. Critical material parameter ratios such as H/E (elastic index or elastic strain to failure) and H 3 /E 2 (plastic index) of the nitrided layers were calculated using surface hardness (H) and elastic modulus (E) values obtained with the help of nanoindentation tests. Systematic improvement in the values of H/E and H 3 /E 2 ratios calculated for all nitrided specimens validated the increase in fracture toughness and impact load fatigue resistance of the nitrided specimens as compared to the corresponding properties of the untreated CoCrMo base alloy.
Plasma nitriding behavior of Ti6Al4V orthopedic alloy
Surface and Coatings Technology, 2008
The influence of plasma nitriding on mechanical, corrosion and tribological properties of Ti6Al4V has been investigated using X-ray diffraction, microhardness tester, scanning electron microscopy, pin-on-disc tribotester, electrochemical polarization and impedance spectroscopy. Plasma nitriding treatment of Ti6Al4V has been performed in 25%Ar-75%N 2 gas mixture, for treatment times of 1-4 h at the temperatures of 650-750°C. The corrosion tests were carried out in Ringer solution at 37°C, and the wear tests were performed in dry sliding conditions. XRD analyses confirm the formation of δ-TiN and tetragonal ɛ-Ti 2 N phases in the modified layer. It was observed that the surface hardness and wear resistance increase as the treatment time and temperature increase. The electrochemical impedance measurements indicate a decrease in double layer capacitance value and increase in charge transfer resistance for the nitrided specimens compared to the untreated ones.