CoCrMo Alloy Research Articles

Effect of annealing on the mechanical properties and cytocompatibility of CoCrMo alloy with tantalum coating

Cobalt–chromium–molybdenum (CoCrMo) alloy is frequently utilized to replace artificial joints and dental components. Tantalum (Ta) coating is used to improve the osteogenic property of CoCrMo alloy. Annealing treatment can increase the Ta coating adhesion and further improve wear resistance and biocompatibility. In this study, two kinds of thicknesses of Ta coatings were applied on CoCrMo alloy substrates via magnetron sputtering technology. The average thicknesses of the Ta coatings were 4.3 and 9.5 μm. The Ta coatings were annealed for 5 h at 800 °C, 900 °C, and 1000 °C, respectively. Results indicated that heat treatment improved the adhesion of the coatings. The Ta coatings were composed of stable body-centered cubic phase (α-Ta) and metastable tetragonal phase (β-Ta). The Ta coating with a thickness of 4.3 μm exhibited excellent wear resistance after being annealed at 800 °C, with little variability in the friction curve and a low average coefficient of friction. The adhesive force of the two coated samples increased initially and subsequently dropped as the annealing temperature increased. At 900 °C, the Ta-coated samples exhibited the highest adhesive force; after being annealed, the rate of cell proliferation increased. At 900 °C, samples with Ta coatings exhibited the highest rate of cell multiplication and the best cell compatibility compared with the other samples. Ta coatings' adhesive force, wear resistance, corrosion resistance, and cytocompatibility were all significantly enhanced at various annealing temperatures. Thus, the results of this work are a useful benchmark for the surface modification of metal implants used in medical procedures, and Ta coatings are a practical orthopedic material for the replacement of joints and bones.

Metal-Driven Autoantifriction Function of Artificial Hip Joint.

The service life of an artificial hip joint is limited to 10-15years, which is not ideal for young patients. To extend the lifespan of these prostheses, the coefficient of friction and wear resistance of metallic femoral heads must be improved. In this study, a Cu-doped titanium nitride (TiNX -Cu) film with "autoantifriction" properties is deposited on a CoCrMo alloy via magnetron sputtering. When delivered in a protein-containing lubricating medium, the Cu in TiNX -Cu quickly and consistently binds to the protein molecules in the microenvironment, resulting in the formation of a stable protein layer. The proteins adsorbed on the TiNX -Cu surface decompose into hydrocarbon fragments owing to the shear stress between the Al2 O3 /TiNX -Cu tribopair. The synergistic effect of the catalysis of Cu and shear stress between the Al2 O3 /TiNX -Cu tribopair transforms these fragments into graphite-like carbon tribofilms with an antifriction property. These tribofilms can simultaneously reduce the friction coefficient of the Al2 O3 /TiNX -Cu tribopair and enhance the wear resistance of the TiNX -Cu film. Based on these findings, it is believed that the autoantifriction film can drive the generation of antifriction tribofilms for lubricating and increasing the wear resistance of prosthetic devices, thereby prolonging their lifespan.

Influence of Different Sterilization Methods on the Surface Chemistry and Electrochemical Behavior of Biomedical Alloys.

Sterilization is a prerequisite for biomedical devices before contacting the human body. It guarantees the lack of infection by eliminating microorganisms (i.e., bacteria, spores and fungi). It constitutes the last fabrication process of a biomedical device. The aim of this paper is to understand the effect of different sterilization methods (ethanol-EtOH, autoclave-AC, autoclave + ultraviolet radiation-ACUV and gamma irradiation-G) on the surface chemistry and electrochemical reactivity (with special attention on the kinetics of the oxygen reduction reaction) of CoCrMo and titanium biomedical alloys used as prosthetic materials. To do that, electrochemical measurements (open circuit potential, polarization resistance, cathodic potentiodynamic polarization and electrochemical impedance spectroscopy) and surface analyses (Auger Electron Spectroscopy) of the sterilized surfaces were carried out. The obtained results show that the effect of sterilization on the corrosion behavior of biomedical alloys is material-dependent: for CoCrMo alloys, autoclave treatment increases the thickness and the chromium content of the passive film increasing its corrosion resistance compared to simple sterilization in EtOH, while in titanium and its alloys, autoclave and UV-light accelerates its corrosion rate by accelerating the kinetics of oxygen reduction.

Ductility enhancement of additively manufactured CoCrMo alloy via residual stress tailored high stacking fault probability

CoCrMo (CCM) alloys fabricated by laser powder bed fusion (LPBF) normally exhibit low ductility. In the present study, a new plastic deformation mode of residual stress tailored high stacking fault (SF) is proposed for LPBF fabricated CCM alloy, delivering an excellent ductility (∼14.5%) in the as-fabricated state. Elaborate microstructural characterization elucidated that low level of initial residual stress contributes to a high stacking fault probability, generating extensive amounts of SFs to sustain the plastic deformation. Such massive deformation faulting activities mitigated the strain localization between FCC/HCP phase of CCM alloy with high residual stress level. Further low temperature annealing treatment validated the above theory, and increased the ductility to ∼21.4% with a high tensile strength of ∼1181 MPa. The present study demonstrated that the ductility of LPBF fabricated CCM alloy can be significantly enhanced by tuning the internal residual stress and the associated stacking fault probability.

A Response Surface Modeling Study on Effects of Powder Rate and Machining Parameters on Surface Quality of CoCrMo Processed by Powder Mixed Electrical Discharge Machining

Due to the high mechanical strength of the metals used in implant manufacture, which makes them difficult to work with using other machining techniques, electrical discharge machining (EDM) is frequently employed in the production of implants. In this study, the effect of powder ratio and other EDM parameters used in the machining of CoCrMo alloy, which used in implant production widely, with powder-mixed EDM on the surface roughness of the machined part was investigated through the response surface methodology. AISI 316L stainless steel was chosen as the electrode material, and Ti6V4Al was chosen as the additive powder, taking into account their biocompatibility properties. Using a Taguchi L16 array, an experimental design was created by selecting 4 levels for each parameter of additive ratio, discharge current, pulse on time (Ton), and pulse off time (Toff). The response surface method was used, along with the experimental data, to estimate how the parameters affected the arithmetic average roughness (Ra) and mean roughness depth (Rz).

Effect of boriding on high temperature tribological behavior of CoCrMo alloy

In the present work, CoCrMo alloy surfaces were subjected to powder-pack boriding at 950 ℃ for 5-hour and 7-hour treatment periods. The obtained boride layers were characterized with scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, nanoindentation and high temperature wear tests. Characterization results show that boride layers are composed of CoB, Co2B, CrB and Co2MoB2 phases. Up to 4-fold increase is achieved in nanohardness values due to the boride layers in addition to increased elastic moduli. Increased surface hardness coupled with the high temperature stability of boride layers led to nearly 12, 7 and 10-fold increase in the wear resistance of CoCrMo alloys, worn at room temperature, 250 ℃ and 500 ℃, respectively.

Biocompatible as-cast and homogenized TiNbTaZrMoV high entropy alloys: mechanical properties, corrosion resistance and in vitro studies

Commercial 316L SS, CoCrMo, and Ti6Al4V alloys have been widely used as implant materials for biomedical applications. However, the inadequate wear resistance and the released metal ions could result in inflammation, cytotoxicity, and shorten the lifespan of surgical implants. To extend the service life and durability of metallic biomaterials, the TiNbTaZrMoV HEA was developed by integrating biocompatibility characteristics with the unique properties of high-entropy alloys (HEAs). Both as-cast TiNbTaZrMo and TiNbTaZrMoV HEAs consisted of dual BCC phases in the dendritic structure: the dendrite region was enriched with Ta, Mo, and Nb, while Zr, Ti, and V were concentrated in the interdendrite region. The homogenized TiNbTaZrMoV HEA possessed a single BCC phase with a homogeneous component distribution. Moreover, the wear resistance of the homogenized TiNbTaZrMoV HEA was superior to the as-cast TiNbTaZrMo and TiNbTaZrMoV HEAs. In terms of electrochemical behaviors, the homogenized TiNbTaZrMoV HEA exhibited superior corrosion resistance compared to the other commercial metallic biomaterials in phosphate-buffered saline solution at 37 °C. The in vitro biocompatibility evaluation revealed that the homogenized TiNbTaZrMoV HEA could support the attachment and proliferation of the MC3T3-E1 cells, presenting good biocompatibility comparable to the Ti6Al4V alloy. This study demonstrated the feasibility of the homogenized TiNbTaZrMoV HEA for orthopedic bioimplant applications.

Understanding the enhanced corrosion performance of two novel Ti-based biomedical high entropy alloys

The microstructure and corrosion behavior of two novel biomedical high entropy alloys (HEA)s, namely Hf27Nb12Ta10Ti23Zr28 and Hf30Nb14Ta10Ti28Zr18 that were previously designed utilizing machine learning, were investigated in depth. The microstructure of the alloys was determined to be dendritic, with some elemental segregations governed by the solidification kinetics occurring during the arc-melting process. Static immersion experiments were carried out in artificial saliva (AS) and simulated body fluid (SBF) to investigate the ion release behavior of the HEAs and reveal the dissolution kinetics of the passive film forming on the surface. The composition of the corresponding surface oxide layers was examined using X-ray photoelectron spectroscopy, which provided detailed insight into the stability of passive oxide layers and sub-oxide formation. Potentiodynamic polarization experiments performed in AS and SBF at 37 ºC demonstrated that both HEAs exhibit superior corrosion behavior as compared to the CoCrMo alloy, one of the conventional metallic implant materials of choice.

High‐Throughput Alloy Development Using Advanced Characterization Techniques During Directed Energy Deposition Additive Manufacturing

In laser‐based direct energy deposition (DED‐LB) additive manufacturing (AM), wire or powder materials are melted by a high‐power laser beam. Process‐specific characteristics enable robust in situ fabrication of compositionally graded materials, e.g., through an adaption of powder mass flow from independent hoppers. Based on the high flexibility of this approach, pathways toward multimaterial AM have been unlocked. Obviously, such characteristics enable high‐throughput alloy development. However, rapid alloy development demands substantial characterization efforts to assess phase and microstructural evolution. So far, property analysis is considered as the limiting factor for these high‐throughput approaches. Herein, the use of high‐brilliance X‐Ray analysis and subsequent micropillar compression testing are introduced to tackle these challenges. As a proof of concept, their application to a compositionally graded material made from AISI 316L stainless steel and a CoCrMo alloy is presented. The results obtained reveal that X‐Ray analysis can be exploited to evaluate process robustness, chemical characteristics, and phase composition within the gradient regions. Moreover, the use of micropillar compression testing provides spatially resolved insights into the mechanical properties of the gradient regions. The combination of both characterization techniques eventually opens pathways toward a robust and time‐efficient alloy development using powder‐fed DED‐LB (DED‐LB/P).

Investigation of the Influence of High-Pressure Torsion and Solution Treatment on Corrosion and Tribocorrosion Behavior of CoCrMo Alloys for Biomedical Applications

In this study, the influence of the high-pressure torsion (HPT) processing parameters and solution treatment (ST) on the corrosion and tribocorrosion behavior of CoCrMo (CCM) alloys was investigated for possible usage in biomedical applications. The corrosion behavior of the CCM alloys was investigated by using potentiodynamic scanning (PDS) and electrochemical impedance spectroscopy (EIS) tests. Tribocorrosion tests were carried out in a reciprocating ball-on-plate tribometer at 1 Hz, 1 N load, and 3 mm stroke length for 2 h. All electrochemical measurements were performed using a potentiostat in standard phosphate-buffered saline (PBS) solution at body temperature (37 ± 2 °C). The samples were characterized by using a scanning electron microscope (SEM), transmission electron microscope (TEM), optical microscope (OM), and X-ray diffraction (XRD). The deepness and width of wear tracks were examined by using a profilometer. The results showed that HPT and ST processes did not affect significantly the corrosion resistance of samples. However, the ST-treated samples had a higher material loss during sliding in standard phosphate-buffered saline (PBS) at body temperature as compared to HPT-treated samples.

Tailoring the tribology property and corrosion resistance of selective laser melted CoCrMo alloys by varying copper content

In this study, the cobalt-chromium-molybdenum (CoCrMo) alloys containing varying Cu contents (CCM-xCu, x = 0, 2, 3, 4 wt%) were fabricated via the selected laser melting (SLM) method. The influences of Cu content on tribological performance and corrosion resistance were investigated. The 2 and 3 wt% Cu contributed to inhibiting the generation of the HCP phase, whereas the inhibitory effect was quite limited at 4 wt% Cu. Notably, 4 wt% Cu led to the formation of Cr-enrich precipitates in the matrix, which was partially coherent with the Cu nanoparticle. The corrosion resistance of the CCM alloy was enhanced as the 2 and 3 wt% Cu added into the CCM alloy, contrarily, which deteriorated when Cu content reached 4 wt%. For the CCM-2Cu and CCM-3Cu, Cr-precipitates played a major role in enhancing the wear resistance, while the Cu lubrication effect working in coordination with Cr-precipitates determined that for the CCM-4Cu. This study was expected to achieve better tribology and corrosion properties of SLM-produced CCM alloys by tailoring microstructure through the Cu element.

Study on Hardness of Heat-Treated CoCrMo Alloy Recycled by Electron Beam Melting.

The hardness of heat (thermally) treated CoCrMo ingots, recycled by electron beam melting and refining (EBMR) of a technogenic CoCrMo material (waste from the dental technology) under different process conditions (temperature and residence time) is examined. The heat treatment consists of two-step heating up to temperatures of 423 K and 1343 K and retention times of 40 and 60 min, respectively. The influence of various loads (0.98 N, 1.96 N, 2.94 N, 4.9 N, and 9.8 N) on the hardness of the CoCrMo alloy, recycled by EBMR, before and after heat treatment is studied. It has been found that regardless of the EBMR process conditions, the obtained samples after heat treatment have similar hardness values (between 494.2 HV and 505.9 HV) and they are significantly lower than the hardness of the specimens before the heat treatment. The highest hardness (600 HV) is measured in the alloy recycled at 1845 K refining temperature for 20 min. This is due to the smaller crystal structure of the resulting alloy and the higher cobalt content. The results obtained show that the heat treatment leads to considerable changes in the microstructure of the CoCrMo ingots recycled by EBMR. With the increase of the e-beam refining temperature, after the heat treatment, the grains' size increases and the grains' shape indicates an incomplete phase transition from γ-fcc to ε-hcp phase. This leads to a slight increase in the hardness of the alloy.

Amorphous/quasi-amorphous CoCrMo-C coatings for improved electrochemical properties and tribocorrosion resistance of biomedical alloys

Coatings based on CoCrMo cobalt alloy have been designed to increase corrosion resistance of AISI 316L austenitic steel in body fluids. The CoCrMo-C coatings, with different carbon content (from 0 to 65 at.%), were synthesised by reactive magnetron sputtering of the medical CoCrMo (ISO 5832-12) alloy in the argon-acetylene atmosphere. Evolution of their structure, crystalline phase and chemical composition with increasing carbon fraction were investigated by X-ray diffractometry and X-ray photoelectron spectroscopy. Evaluation of corrosion resistance of CoCrMo-C films on 316L steel substrates was performed in Hanks' Balanced Salt Solution (HBSS) by means of electrochemical impedance spectroscopy and potentiodynamic polarisation tests. The studies have revealed high chemical inertness of all coatings. Passive/blocking layers formed on their surfaces exhibit a capacitive character, while their resistivity is nearly two orders of magnitude higher compared to the corresponding values obtained for the CoCrMo and 316L alloys tested under similar conditions. The CoCrMo-C coatings provide pitting corrosion protection for 316L austenitic steel. This is expected to extend the residence time of stainless steel in the tissue environment. The 316L/CoCrMo-C systems were also subjected to tribocorrosion tests in HBSS at three electrochemical potentials, i.e. at open circuit, cathodic, and anodic potential. A model was developed to relate the electrical values recorded during tribocorrosion experiments with the phenomena that characterize the coating wear process. It was found that the friction coefficient systematically decreased from 0.7 to 0.1 with increasing the carbon content under all potentials applied. The wear rate observed for majority of the coatings was lower than that for CoCrMo and 316L alloys. For the samples with the highest carbon content, the wear rate under oxidative conditions became even lower than for the cathodic potential.

Inhibitory effects of Formononetin on CoCrMo particle-induced osteoclast activation and bone loss through downregulating NF-κB and MAPK signaling

Wear particle-induced osteoclast over-activation is a major contributor to periprosthetic osteolysis and aseptic loosening, which can cause pathological bone loss and destruction. Hence, inhibiting excessive osteoclast-resorbing activity is an important strategy for preventing periprosthetic osteolysis. Formononetin (FMN) has been shown to have protective effects against osteoporosis, but no previous study has evaluated the effects of FMN on wear particle-induced osteolysis. In this study, we found that FMN alleviated CoCrMo alloy particles (CoPs)-induced bone loss in vivo and inhibited the formation and bone-resorptive function of osteoclasts in vitro. Moreover, we revealed that FMN exerted inhibitory effects on the expression of osteoclast-specific genes via the classical NF-κB and MAPK signaling pathways in vitro. Collectively, FMN is a potential therapeutic agent for the prevention and treatment of periprosthetic osteolysis and other osteolytic bone diseases.

In vitro corrosion and biocompatibility behavior of CoCrMo alloy manufactured by laser powder bed fusion parallel and perpendicular to the build direction

Biomedical cobalt-chromium-molybdenum alloys (CoCrMo) are frequently used for orthopedic implant and dental materials exposed to mechanical stressors, such as wear and cyclic load. Due to the high demand for customizable implant shapes, these alloys are increasingly manufactured by additive manufacturing methods such as laser powder bed fusion (LPBF). LPBF results in different microstructures and surface roughness as a function of the building direction. This study investigated the corrosion resistance, bioactivity, biocompatibility, and microstructure of LPBF CoCrMo (low carbon content, heat-treated) in the XY (perpendicular) and XZ (parallel) plane of the building direction for as-printed (as-received) and abraded surfaces. A distinct microstructure and different surface roughness were found for the XY and XZ planes. The as-received XY surface showed the lowest corrosion resistance but was still passive in phosphate-buffered saline (PBS, pH 7.4). As-received surfaces were less corrosion-resistant than abraded surfaces. All specimens exhibited lower corrosion resistance in PBS containing citric acid at pH 7.4 than in PBS and citric acid alone. As-received surfaces showed better hydroxyapatite precipitation and cell viability; however, all surfaces had satisfactory biocompatibility and bioactivity. This study showed that the building direction had a minor effect on the corrosion of LPBF CoCrMo.

Differences in In Vitro Bacterial Adherence between Ti6Al4V and CoCrMo Alloys.

Prosthetic joint infection is an uncommon entity, but it supposes high costs, both from the economic side to the health systems and from the emotional side of the patient. The evaluation of the bacterial adherence to different materials frequently involved in joint prostheses allows us to better understand the mechanisms underlying this and provide information for the future development of prevention strategies. This study evaluated the bacterial adherence of four different species (Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli and Pseudomonas aeruginosa) on Ti6Al4V and CoCrMo. The topography, surface contact angles, and linear average roughness were measured in the samples from both alloys. The interaction with the surface of both alloys was significantly different, with the CoCrMo showing an aggregating effect on all the species, with additional anti-adherent activity in the case of Pseudomonas aeruginosa. The viability also changes, with a significant decrease (p < 0.05) in the CoCrMo alloy. In the case of S. epidermidis, the viability in the supernatant from the samples was different, too, with a decrease in the colony-forming units in the Ti6Al4V, which could be related to cation release from the surface. Beyond adhesion is a multifactorial and complex process, and considering that topography and wettability were similar, the chemical composition could play a main role in the different properties observed.

Role of protein in crevice corrosion of CoCrMo alloy: An investigation using wire beam electrodes

In this study, the effects of protein and crevice width on crevice corrosion of CoCrMo alloy were studied using wire beam electrodes. The results show that proteins inside and outside the crevice migrated to the crevice mouth, causing crevice mouth blockage and decreasing the Cl− concentration inside the crevice, which further affected the distribution and rate of corrosion. The accumulation of H+ inside the crevice promoted the occurrence of pitting corrosion, which led to crevice corrosion of CoCrMo alloy. The corrosion rate of the samples with 0.07 mm crevice width was 5 times greater than the samples without crevice.

Investigation of the wear transition in CoCrMo alloys after heat treatment to produce an HCP structure

This study reports that the wear rate of the heat-treated CoCrMo alloy shows a sudden transition of more than 5-fold when the load/contact pressure increases from 45 N (3.6 GPa) to 50 N (3.7 GPa). Heat treatment was used to change the structure of the commercially available CoCrMo from the initial face-centred cubic (γ-fcc) with a small amount of hexagonal close-packed (ε-hcp) to predominantly hcp ε-phase with dispersed nanoscale precipitates of σ-phase. PeakForce QNM was used to quantitatively map the local mechanical properties of the surface at the nanoscale. High-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscope (STEM), energy-dispersive X-ray spectroscopy (EDX), and precession electron diffraction (PED) integrated with the TEM is used to characterize the structure and chemical composition of the worn surface and tribofilm. The possible reason for wear transition was discussed.

Effect of Co-Coated Al2O3 Composite Powders on LPBF-Manufactured CoCrMo Alloy

In this study, we systematically examined the influence mechanisms of introduced cobalt coated alumina (Co-coated Al2O3) particles on the microstructure and properties of cobalt-chromium-molybdenum (CoCrMo) alloy printed by Laser Powder Bed Fusion (LPBF). The Co-coated Al2O3 composite powders with different density of cobalt coating were prepared by varying Al2O3 load from 1 g/150 mL to 2 g/150 mL during the electroless plating process. Then they were mixed with CoCrMo powders in the proportion of 1 wt.% and formed standard samples by LPBF technology. The results showed that the addition of Co-coated Al2O3 particles improved the friction performance of CoCrMo alloys significantly. The wear depth of CCM@2Al2O3 was only 2.18 μm and the wear volume of it was about 10% of pure CoCrMo alloy. The CoCrMo alloy introduced the Co-coated Al2O3 particles with a 1 g/150 mL Al2O3 load formed metal-ceramic bonding interface, which solved the problem of poor wettability between Al2O3 and matrix in LPBF process. Such CoCrMo alloy exhibited excellent tensile properties and the mean microhardness of it reached 379.9 ± 3.5 HV0.5.

Effect of Porosity and Heat Treatment on Mechanical Properties of Additive Manufactured CoCrMo Alloys.

To minimize the stress shielding effect of metallic biomaterials in mimicking bone, the body-centered cubic (bcc) unit cell-based porous CoCrMo alloys with different, designed volume porosities of 20, 40, 60, and 80% were produced via a selective laser melting (SLM) process. A heat treatment process consisting of solution annealing and aging was applied to increase the volume fraction of an ε-hexagonal close-packed (hcp) structure for better mechanical response and stability. In the present study, we investigated the impact of different, designed volume porosities on the compressive mechanical properties in as-built and heat-treated CoCrMo alloys. The elastic modulus and yield strength in both conditions were dramatically decreased with increasing designed volume porosity. The elastic modulus and yield strength of the CoCrMo alloys with a designed volume porosity of 80% exhibited the closest match to those of bone tissue. Different strengthening mechanisms were quantified to determine their contributing roles to the measured yield strength in both conditions. The experimental results of the relative elastic modulus and yield strength were compared to the analytical and simulation modeling analyses. The Gibson-Ashby theoretical model was established to predict the deformation behaviors of the lattice CoCrMo structures.