Medical Grade Cobalt Chromium Research Articles

In vitro corrosion and hemocompatibility evaluation of electrical discharge treated cobalt–chromium implant

Abstract

Effect of Mechanical Tumbling Parameters on Surface Roughness and Edge Radius of Medical Grade Cobalt Chromium Alloy

Smoothly polished prosthesis surface is a crucial requirement in medical application and important feature that determine the proper response to corrosion and biocompatibility in the human body. Tumbling is one of the pre-polishing processes that can be conducted in order to improve the surface roughness of the machined prosthesis. However, the using of ceramic media for the tumbling process in medical application is not widely reported. This study was conducted to investigate the effect of mechanical tumbling parameters on the surface roughness of medical grade CoCr alloy by using alumina (Al2O3) based ceramic media. The experiment was performed with a different level of rotational speed (35, 55, and 75 rpm) and soaking time (4, 6 and 8 hours) of the tumbling process. The surface roughness of specimens before and after the process was measured using Mitutoyo Formtracer CS-5000 and the edge radius was measured using Olympus SZX9 microscope integrated with I-solution Lite software. It was figured that 6 hours tumbling time at 55 rpm showed the most significant reduction (32 %) in surface roughness (Ra) of the CoCr alloy specimen, while 8 hours tumbling time at 75 rpm showed the highest effect on the edge radius of the specimen at 63 % increment.

The fundamental mechanisms of wear of cemented carbide in continuous cutting of medical grade cobalt chromium alloy (ASTM F75)

Medical grade cobalt chromium alloys are one of a select number of material types approved for use in the components of orthopaedic implants. However, there has been minimal published research into the fundamental mechanisms in the cutting processes used in their manufacture. These processes affect the form accuracy, surface finish and integrity of these safety critical devices.The present work reports on a scientific investigation into the fundamental mechanisms of tool wear in continuous cutting of the biomedical grade Co-Cr-Mo ASTM F75 alloy using uncoated tungsten carbide tools. Confocal and scanning electron microscopy (SEM) is used to examine and characterise the progression of tool wear at intervals up to a defined “end of life limit” for each combination of speed (vc) and feed rate (f) in experiments where the range of these parameters vary from 20 to 60 m/min and 20–60 μm/rev respectively to reflect the ranges used in industry.An empirical model is then established for progressive tool wear which demonstrates that the wear rates vary by over an order of magnitude. The parameters for the Taylor equation are also determined for this material for the first time. Microscopic examination shows the presence of characteristic tool wear “zones” with consistent size, shape and composition. These zones evolve depending on combination of speeds and feeds as determined in the full factorial experiment. Detailed characterisation of the wear surfaces and cross sections provide a number of insights into the possible mechanisms of wear. It is hypothesised that micron thick layers of Co-Cr-Mo, found on the rake and flank wear surfaces, indicate primary wear mechanism involving adhesion of the work material and subsequent removal of WC hard particles by entrainment in the work material flow.

Effect of Ultrasonic Electro-Chemical Polishing Parameters on Surface Roughness and Material Removal Rate of Medical Grade Cobalt Chromium Alloys

The fabrication of prosthesis from superalloys in bio-medical field is a challenging task as a result of strict tolerance requirement in surface finish. Precision surface finish is desired to avoid biocompatibility issue resulting from poor surface finish. Such surface finish requirement can be achieved by using Electro-Chemical Polishing (ECP) process. This research investigated the effect of applying ultrasonic vibration on ECP with process parameters, namely current (0.5A-2.5A) and time (2min-10min) on the surface roughness (Ra) and material removal rate (MRR) of cobalt chromium molybdenum (CoCrMo). The experiments were conducted using ECP and ultrasonic assisted ECP (UECP) processes whereby phosphoric acid (H3PO4) and SS 304 steel were used as electrolyte and electrode respectively. The result showed an increasing trend of MRR and Ra as the time and current increased in both processes. It was also found that ultrasonic assisted ECP at current (2.5A) and time (30 min) recorded the highest MRR (0.047 g/min) and the lowest Ra (2.139 µm).

Fundamental mechanisms of chip formation in orthogonal cutting of medical grade cobalt chromium alloy (ASTM F75)

This paper reports on research into the fundamental mechanisms of chip formation in cutting of biomedical grade cobalt chromium molybdenum (Co-Cr-Mo) alloy conforming to the ASTM F75 standard. A full factorial orthogonal cutting experiment was undertaken to examine the effect on chip morphology of the basic control parameters: undeformed chip thickness (h) and cutting speed (vc) over ranges from 20 to 140μm and 20 to 60m/min respectively. The morphological characteristics considered include: segmentation spacing, frequency, segmentation and deformation angle where all were found to show a statistically significant dependence on the control parameters. This dependence was evident notwithstanding the “within sample” variation of these characteristics associated here with crystallographic anisotropy due to grain size far exceeding the maximum undeformed chip thickness (h) and giving rise to conditions normally found in micro-cutting. Analysis of the morphological and microstructural characteristics of the chip has led to a hypothesis that the heat treated ASTM F75 Co-Cr-Mo exhibits bifurcation in work hardening rates, possibly due to strain-induced martensitic transformation (SIMT). It is further hypothesised that an increase in cutting energy due to work-hardening depends on temperature and therefore vc and h but also, strain rate, grain size and orientation. Moreover, it is proposed that SIMT dominates over thermal softening and decreases the amount of plastic deformation prior to the onset of crack initiation and propagation, a mechanism identified to result in saw tooth morphology of Co-Cr-Mo chip.

Prediction of tool-wear in turning of medical grade cobalt chromium molybdenum alloy (ASTM F75) using non-parametric Bayesian models

We present a novel approach to estimating the effect of control parameters on tool wear rates and related changes in the three force components in turning of medical grade Co–Cr–Mo (ASTM F75) alloy. Co–Cr–Mo is known to be a difficult to cut material which, due to a combination of mechanical and physical properties, is used for the critical structural components of implantable medical prosthetics. We run a designed experiment which enables us to estimate tool wear from feed rate and cutting speed, and constrain them using a Bayesian hierarchical Gaussian process model which enables prediction of tool wear rates for untried experimental settings. The predicted tool wear rates are non-linear and, using our models, we can identify experimental settings which optimise the life of the tool. This approach has potential in the future for realtime application of data analytics to machining processes.

Fundamental mechanisms in orthogonal cutting of medical grade cobalt chromium alloy (ASTM F75)

Cobalt chromium alloys are sui generis materials for orthopaedic implants due mainly to unique properties of biocompatibility and wear resistance in the demanding in-vivo environment. Notwithstanding the importance of both defined and undefined edge cutting processes on form, finish and surface integrity of orthopaedic components, there has been minimal research reported in the public domain on the fundamental mechanisms in cutting of these alloys. Accordingly, this paper reports on initial research into cutting of the biomedical grade cobalt chrome molybdenum (Co-Cr-Mo) alloy, ASTM F75.Following a brief overview of physical and mechanical properties of this class of Co-Cr-Mo alloys, the results of a full factorial, orthogonal cutting experiment are presented. This involved measurement of force components (Ft and Ff) as a function of the undeformed chip thickness (h) and cutting speed (vc) which were varied over ranges from 20 to 140μm and 20 to 60m/min respectively. The results demonstrated an expected linear increase in force components with h at speeds of 20 and 60m/min. However, at the intermediate speed of 40m/min, there was a transition between about 60 and 80μm indicating a discontinuous rather than continuous effect of speed. The results also enabled determination of the cutting force coefficients Ktc, Kte, Kfc and Kfe, as well as ki1,0.1 and mi0.1 of the Kienzle equations. These relations will enable macro-mechanic modelling of more complex cutting operations, such as milling, in the future.

An investigation of force components in orthogonal cutting of medical grade cobalt-chromium alloy (ASTM F1537).

An ageing population, increased physical activity and obesity are identified as lifestyle changes that are contributing to the ongoing growth in the use of in-vivo prosthetics for total hip and knee arthroplasty. Cobalt-chromium-molybdenum (Co-Cr-Mo) alloys, due to their mechanical properties and excellent biocompatibility, qualify as a class of materials that meet the stringent functional requirements of these devices. To cost effectively assure the required dimensional and geometric tolerances, manufacturers rely on high-precision machining. However, a comprehensive literature review has shown that there has been limited research into the fundamental mechanisms in mechanical cutting of these alloys. This article reports on the determination of the basic cutting-force coefficients in orthogonal cutting of medical grade Co-Cr-Mo alloy ASTM F1537 over an extended range of cutting speeds ([Formula: see text]) and levels of undeformed chip thickness ([Formula: see text]). A detailed characterisation of the segmented chip morphology over this range is also reported, allowing for an estimation of the shear plane angle and, overall, providing a basis for macro-mechanic modelling of more complex cutting processes. The results are compared with a baseline medical grade titanium alloy, Ti-6Al-4V ASTM F136, and it is shown that the tangential and thrust-force components generated were, respectively, ≈35% and ≈84% higher, depending primarily on undeformed chip thickness but with some influence of the cutting speed.

Electrochemical etching of micro-pores in medical grade cobalt-chromium alloy as reservoirs for drug eluting stents.

Drug eluting stents (DES) have shown efficacy in reducing restenosis after angioplasty followed by application of a coronary stent. However, polymer matrices typically used for immobilizing drugs on the stent surface may cause irritation and have limited drug loading capacity. In contrast, drug loading into micro- or nanopores created within the stent material could avoid these problems. We present a technology based on electrochemically induced pitting corrosion to form pores in medical grade steel, followed by loading with rapamycin. This process is applied to pore formation and drug loading in coronary stents consisting of L605 medical steel. Sustained release of the drug over 28days at rates comparable to established DES was demonstrated. This technology is capable of creating pores with well-defined pore size and filling of these pores by a drug employing a crystallization process thus completely avoiding polymer matrices to immobilize drugs. Electrochemically induced pitting corrosion provides a generic means to introduce micro-pores suitable as drug reservoirs into medical grade steel without the need for any further matrix material. Further research will expand these findings to other materials and types of implants that could benefit from the additional function of drug release and/or improved implant/tissue integration.

Finite Element Analysis of Porous Medical Grade Cobalt Chromium Alloy Structures Produced by Selective Laser Melting

The recent introduction of selective laser melting (SLM) for the processing of medical grade cobalt chromium (CoCr) alloy has led to a complex shape fabrication of porous custom CoCr alloy implants with controlled porosity to meet the requirements of the anatomy and functions at the region of implantation. This paper discusses finite element (FE) analysis and mechanical characterization of porous medical grade CoCr alloy in cubical structures with volume based porosity ranging between 60% and 80% produced using SLM rapid manufacturing process. Analysis by FE is considered beneficial to predict the effective mechanical properties of the porous structures manufactured by SLM due to minimization of the need for expensive and time consuming physical testing. Cellular structures modelling for fabrication with Direct Metal Laser Sintering machine were designed to vary between 60% and 80% to study the effect of structural variation on mechanical properties of the cellular porous structure. ANSYS 14.0 FE modelling software was used to predict the effective elastic modulus of the samples and comparisons were made with the experimental data. FE results show that with the material properties in the functions of porosities, minimum mesh size of 0.2 mm for triangular shape mesh and boundary as well as load conditions as applied in this study, agreement in equivalent stress, strain and deformation with the experimental results can be achieved to some extent. The technique for FE in this study can be used to investigate stress distribution in three dimensional model of real bone.

Critical evaluation on structural stiffness of porous cellular structure of cobalt chromium alloy

In order to improve the stiffness characteristics of orthopedic devices implants that mimic the mechanical behavior of bone need to be considered. With the capability of Additive layer manufacturing processes to produce orthopedic implants with tailored mechanical properties are needed. This paper discusses finite element (FE) analysis and mechanical characterization of porous medical grade cobalt chromium (CoCr) alloy in cubical structures with volume based porosity ranging between 60% to 80% produced using direct metal laser sintering (DMLS) process. ANSYS 14.0 FE modelling software was used to predict the effective elastic modulus of the samples and comparisons were made with the experimental data. The effective mechanical properties of porous samples that were determined by uniaxial compression testing show exponential decreasing trend with the increase in porosity. Finite element model shows good agreement with experimentally obtained stress-strain curve in the elastic regions. The models prove that numerical analysis of actual prosthesis implant can be computed particularly in load bearing condition

Influence of roughness on surface instability of medical grade cobalt–chromium alloy (CoCrMo) during contact corrosion–fatigue

Surface roughness and contact load play a major role in contact corrosion–fatigue phenomena that accelerates corrosion pits on the stressed surface area. The local yielding and stress concentration will be produced when the rough surface is brought into cyclic contact. Subsequently, the selective electrochemical attacks on the stressed surface will lead to the roughness evolution during environmental corrosion. The continuous roughness evolution by the stress-assisted dissolution will ultimately nucleate microcracks on the surface. In this article, a new evaluation method was introduced to identify the thermodynamic driving forces responsible for the stress-assisted dissolution. In order for complete understanding of the mechanical and electrochemical response of materials surface during contact corrosion–fatigue, finite element calculations and contact corrosion–fatigue experiments were performed on textured medical grade cobalt–chromium–molybdenum (CoCrMo) specimen surfaces. Consequently, the quantitative model of roughness evolution was developed to predict contact corrosion–fatigue damage of CoCrMo surface.