Uncoated Magnesium Alloy Research Articles

Preparation and characterization of hydroxyapatite coating by magnetron sputtering on Mg–Zn–Ag alloys for orthopaedic trauma implants

The aim of the present paper was to evaluate the effect of hydroxyapatite coatings on the two types of Mg–Zn–Ag alloys as a possible solution to control magnesium alloy degradation. The coatings were prepared by the radio frequency magnetron sputtering method at a deposition temperature of 300 °C. To perform this evaluation, the coated alloys were immersed in a simulated body fluid solution at body temperature (37 ± 0.5 °C) to determine the corrosion resistance through electrochemical and immersion tests. Moreover, the investigation also consisted of the evaluation of microchemical, mechanical, and morphological properties. The deposition temperature of 300 °C was enough to obtain a crystalline hydroxyapatite structure with a Ca/P ratio close to the stochiometric one. The adhesion of coatings was not influenced by the nature of Mg–Zn–Ag alloys, so similar values for both coated alloys were found. The results showed that the coating was homogonous deposited on the Mg–Zn–Ag alloys and the corrosion resistance of uncoated magnesium alloys was improved.

Influence of nanoparticle additions to the electrolyte on the structure, composition and corrosion resistance of oxide layers formed by PEO on cast Mg alloy

The effect of various nanoparticles (NPs) added to the electrolyte on the composition, structure, and properties of oxide layers formed by plasma electrolytic oxidation (PEO) on a cast magnesium alloy AZ81A was studied in this work. The oxide layers were obtained by alternating adding silicon dioxide SiO2, silicon nitride Si3N4, yttrium oxide Y2O3, tungsten carbide WC, and titanium carbide TiC NPs to the electrolyte. The obtained oxide layers were studied by scanning electron microscopy (SEM), energy dispersive X-ray microanalysis (EDXMA), X-ray diffraction analysis (XRD), instrumental indentation and potentiodynamic polarization (PDP). The thickness, roughness, microhardness, adhesion and corrosion resistance of the oxide layers were determined. The greatest increase in the oxide layer thickness, in its hardness and corrosion resistance was observed for TiC and WC NPs added to the electrolyte during PEO. The addition of Si3N4 NPs leads to a decrease in the oxide layer thickness and its hardness, while the corrosion resistance is comparable to the one of the uncoated magnesium alloy.

Mechanical Property of Magnesium Alloy Surface with Dense Oxide

Low Al single-phase magnesium alloy surfaces with dense magnesium oxide films were uniformly formed. The films were deposited with a radio frequency magnetron sputtering process with a planar magnetron sputtering system. The thickness of deposited magnesium oxide thin films was around 240 nm. According to the XRD results, a magnesium oxide phase film was formed on the substrate. The surface was uniform, and no cracks or exfoliation were observed. The deposited magnesium oxide film did not have any cracks or pores, and the surface of the sample was covered by magnesium oxide. The hardness of the magnesium oxide-coated magnesium alloy reached around Hv200, while that of the uncoated Mg-alloy was around Hv80. Moreover, the Vickers indenter under a 10-mN load indented the magnesium alloy substrate coated with the magnesium oxide film to a depth of around 640 nm, while that for the uncoated magnesium alloy substrate was around 620 nm. Meanwhile, the elasticity value for the magnesium alloy substrate coated with magnesium oxide film was around 5.3×1010 Pa, while that of the uncoated magnesium alloy substrate was around 4.2×1010 Pa.

Hemocompatibility of plasma electrolytic oxidation (PEO) coated Mg-RE and Mg-Zn-Ca alloys for vascular scaffold applications

Percutaneous transluminal coronary angioplasty and subsequent vascular scaffold implantation remains the prevalent invasive treatment of coronary heart disease. In-stent restenosis remained a problem with bare metal stents, until drug-eluting stents were introduced. The inhibition of the healing process by the antimitotic drug coating and the permanent metallic remnant can promote sub-acute and delayed stent thrombosis. Thus, the development of biodegradable stents emerged as a subject of research. Magnesium-based bioabsorbable devices can provide sufficient radial force in the acute phase of vessel-treatment and degrade thoroughly in aqueous environment, making them potential new candidates for vascular scaffold applications. Magnesium alloys tend to degrade very quickly due to their high electrochemical corrosion potential. Plasma Electrolytic Oxidation modification of magnesium alloys improves interface and degradadation properties and may therefore enhance the performance and suitability for vascular scaffold applications of these materials. Assuring the hemocompatibility and foremost assessing the thrombogenicity of new biomaterials prior to their use is essential in order to avoid adverse effects. The goal was to assess thrombocyte adhesion on coated Mg-RE and Mg-Zn-Ca alloys. Static experiments with human blood were carried out on the plasma-electrolytically treated or corresponding untreated Mg alloy in order to assess quantity and quality of thrombocyte adhesion via standardized SEM imaging. In a second step, a parallel plate flow chamber was designed in order to examine thrombocyte adhesion under dynamic flow conditions. During flow chamber experiments the test-materials were exposed to human thrombocyte concentrate and the number of adherent thrombocytes was assessed. The flow chamber was additionally perfused with human blood and thrombocyte adhesion was semiquantitatively and qualitatively assessed via SEM imaging and subsequent scoring. In conclusion, a new parallel plate flow chamber design simulating blood-circulation was successfully established, enabling the further assessment of platelet adhesion on bioabsorbable materials under dynamic flow conditions. Static and dynamic experiments showed, that plasma-electrolytically treated specimens showed low thrombocyte adhesion on both alloys, proposing their potential use in vascular scaffolds. The uncoated magnesium alloys showed rapid degradation along with gas formation due to the chemically active surface and therefore give concern regarding their safety and suitability for vascular applications.

Effects of oxidation time on corrosion resistance of plasma electrolytic oxidation coatings on magnesium alloy

Abstract The plasma electrolytic oxidation (PEO) technique was used to coat AZ31 magnesium alloy in a phosphate-based electrolyte. Effects of oxidation time on the morphology, phase structure, and corrosion resistance of the ceramic coatings were systematically investigated by scanning electron microscopy, X-ray diffraction, potentiodynamic polarization and electrochemical impedance spectroscopy testing. The main components of the resulting coatings were MgO, MgF2 and t-ZrO2. It was also found that there was a significant influence of the oxidation time on the corrosion resistance of the coatings. The coating obtained under the oxidation time of 10 min exhibited a corrosion current of 3.89 × 10−6 A cm−2 in 3.5 wt.% NaCl solution, which was a significant decrease compared with the uncoated magnesium alloy. The results of the corrosion tests indicated that the PEO coating formed at 10 min demonstrates a higher corrosion resistance compared to the samples formed at 5, 15 and 20 min.

Wear and Corrosion Properties of 316L-SiC Composite Coating Deposited by Cold Spray on Magnesium Alloy

In order to improve the wear and corrosion resistance of commonly used magnesium alloys, 316L stainless steel coating and 316L-SiC composite coating have been deposited directly on commercial AZ80 magnesium alloy using cold spraying technology (CS). The microstructure, hardness and bonding strength of as-sprayed coatings were studied. Their tribological properties sliding against Si3N4 and GCr15 steel under unlubricated conditions were evaluated by a ball-on-disk tribometer. Corrosion behaviors of coated samples were also evaluated and compared to that of uncoated magnesium alloy substrate in 3.5 wt.% NaCl solution by electrochemical measurements. Scanning electron microscopy was used to characterize the corresponding wear tracks and corroded surfaces to determine wear and corrosion mechanisms. The results showed that the as-sprayed coatings possessed higher microhardness and more excellent wear resistance than magnesium alloy substrate. Meanwhile, 316L and 316L-SiC coating also reduced the corrosion current density of magnesium alloy and the galvanic corrosion of the substrates was not observed after 200-h neutral salt spray exposure, which demonstrated that corrosion resistance of a magnesium alloy substrate could be greatly improved by cold-sprayed stainless steel-based coatings.

In vivo degradation of a new concept of magnesium-based rivet-screws in the minipig mandibular bone

Self-tapping of magnesium screws in hard bone may be a challenge due to the limited torsional strength of magnesium alloys in comparison with titanium. To avoid screw failure upon implantation, the new concept of a rivet-screw was applied to a WE43 magnesium alloy. Hollow cylinders with threads on the outside were expanded inside drill holes of minipig mandibles. During the expansion with a hexagonal mandrel, the threads engaged the surrounding bone and the inside of the screw transformed into a hexagonal screw drive to allow further screwing in or out of the implant. The in vivo degradation of the magnesium implants and the performance of the used coating were studied in a human standard-sized animal model. Four magnesium alloy rivet-screws were implanted in each mandible of 12 minipigs. Six animals received the plasmaelectrolytically coated magnesium alloy implants; another six received the uncoated magnesium alloy rivet-screws. Two further animals received one titanium rivet-screw each as control. In vivo radiologic examination was performed at one, four, and eight weeks. Euthanasia was performed for one group of seven animals (three animals with coated, three with uncoated magnesium alloy implants and one with titanium implant) at 12weeks and for the remaining seven animals at 24weeks. After euthanasia, micro-computed tomography and histological examination with histomorphometry were performed. Significantly less void formation as well as higher bone volume density (BV/TV) and bone-implant contact area (BIC) were measured around the coated implants compared to the uncoated ones. The surface coating was effective in delaying degradation despite plastic deformation. The results showed potential for further development of magnesium hollow coated screws for bone fixation.

Hardness enhancement of PEO-treated Mg alloy for biodegradable implants

In recent times, magnesium alloys have emerged as a possible biodegradable implant material. Plasma electrolytic oxidation (PEO) has been used in the past as a useful surface treatment technique to improve wear resistance and corrosion resistance of magnesium alloys by forming protective coatings. This study focuses on the effect of electrolyte solution on the microstructural and nanomechanical behavior of PEO coatings for possible use in biodegradable implants. Microstructural characterization of the coating was carried out by X-ray diffraction (XRD), scanning electron microscopy followed by image analysis and energy dispersive X-ray spectroscopy. Furthermore, nanoindentation was used to evaluate nanohardness of the PEO coatings. The results show beneficial effects of the PEO coating to improve the hardness of the uncoated AZ31 magnesium alloy. The XRD pattern shows that the crystal structures of the film vary based on electrolyte solution and the effect that has on nanomechanical behavior. In conclusion, this article shows the possible benefit of PEO coating for biodegradable implant use.

Improving anti-wear and anti-corrosion properties of AM60 magnesium alloy by ion implantation and Al/AlN/CrAlN/CrN/MoS2 gradient duplex coating

A novel ion implantation and Al/AlN/CrAlN/CrN/MoS2 gradient duplex coating was used to improve the wear-corrosion synergy performance of AM60 magnesium alloy. The wear and corrosion behaviors of the as-deposited gradient duplex coating were subsequently investigated. The results showed that the gradient duplex coating has excellent wear resistance with the wear rate of 2.31 × 10−6 mm3/Nm, which is far below compared to that of the uncoated magnesium alloy of about 3.15 × 10−4 mm3/Nm. The introduction of intermediate layers between the top MoS2 layer and magnesium alloy substrate remarkably improved the magnesium alloy's load bearing capacity. The hardness and modulus of the Al/AlN/CrAlN/CrN intermediate layer was 20 and 7 times larger than that of the uncoated magnesium alloy, respectively. In addition, the corrosion resistance of the magnesium alloy with the gradient duplex coating was also enhanced with the corrosion current density, which decreased from 1.25 × 10−5 A/cm2 to 6.19 × 10−7 A/cm2. The present work provided a feasible strategy to fabricate wear-corrosion resistant coating via component and structure gradient design.

In Vivo Corrosion Resistance of Ca-P Coating on AZ60 Magnesium Alloy

Magnesium-based alloys are frequently reported as potential biodegradable orthopedic implant materials. Controlling the degradation rate and mechanical integrity of magnesium alloys in the physiological environment is the key to their applications. In this study, calcium phosphate (Ca-P) coating was prepared on AZ60 magnesium alloy using phosphating technology. AZ60 samples were immersed in a phosphating solution at 37 ± 2 °C for 30 min, and the solution pH was adjusted to 2.6 to 2.8 by adding NaOH solution. Then, the samples were dried in an attemperator at 60 °C. The degradation behavior was studied in vivo using Ca-P coated and uncoated magnesium alloys. Samples of these two different materials were implanted into rabbit femora, and the corrosion resistances were evaluated after 1, 2, and 3 months. The Ca-P coated samples corroded slower than the uncoated samples with prolonged time. Significant differences (p < 0.05) in mass losses and corrosion rates between uncoated samples and Ca-P coated samples were observed by micro-computed tomography. The results indicate that the Ca-P coating could slow down the degradation of magnesium alloy in vivo.

In vitro degradation and biocompatibility of Ca-P coated magnesium alloy

Calcium-phosphate compounds(Ca-P) coating was prepared on an Mg-Al alloy(AZ60). Biodegradation of Ca-P coated magnesium alloy was evaluated in simulated body fluid(SBF) by examining the changes in magnesium ion concentration and pH value, which indicated that the Ca-P coating on magnesium alloy strongly affected the corrosion of magnesium alloy. Osteoblast MC3T3-E1 cells were utilized to investigate the cellular cytocompatibility. The cytocompatibility was measured by carrying out a series of tests, such as cholecystokinin-octapeptide(CCK-8) test, alkaline phosphatase activity(ALP) test, cellular morphology of hematoxylin-eosin(HE) staining and the induction of apoptosis. It was found that the cell function showed better in the Ca-P coated Mg-alloy extract than in the uncoated magnesium alloy extract. In summary, the results indicate that the Ca-P coating can improve the corrosion resistance of magnesium alloy and elevate cellular proliferation and differentiation of osteoblast MC3T3-E1 cells.

Characteristics of TiCN Film on AZ31 Magnesium Alloy Deposited by Radio Frequency Magnetron Sputtering

Titanium carbonitride (TiCN) films were deposited on AZ31 magnesium alloy by radio frequency magnetron sputtering. The surface morphology and microstructure were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD), and the corrosion resistance was studied by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). The results show that TiCN coatings deposited on AZ31 magnesium alloy are amorphous, and reveal good adhesion onto the substrate. The corrosion current density of TiCN-coated magnesium alloy was 1.664×10-6 A/cm2, which decreased by more than one order of magnitude compared to the uncoated magnesium alloy (1.785×10-5 A/cm2).

Comparison of the Corrosion Behavior of Coated and Uncoated Magnesium Alloys in an In Vitro Corrosion Environment

AbstractThe in vitro degradation of magnesium alloys with various alloying elements, the effect of coatings, and the impact of an altered experimental environment are investigated. LANd442 and Nd2 alloys are subjected to a continuously moving environment during an immersion test allowing flowing SBF. Applying an MgF2coating to the alloys increases the corrosion resistance of LANd442 but has no effect on the corrosion rate of Nd2 within the period of investigation. It leads to a more‐even degradation with less pitting corrosion in the early stages of corrosion. A bioglass coating on Nd2 increases the corrosion rate. The mass loss, volume loss, and loss in maximum force all show the same trends as the specimens degrade over time.

Spark-deposited coatings on magnesium alloys

The paper examines the mass transfer kinetics, structure, and properties of spark-deposited coatings on magnesium alloys. They are obtained using composite ceramic electrodes in the AlN–Zr(Ti)B2 and B6Si–CaB6 systems. It is revealed that the microhardness and corrosion resistance of the coatings increase in a 3% NaCl solution and the abrasive wear decreases as compared with uncoated magnesium alloy.

Development and biocompatibility of a novel corrodible fluoride‐coated magnesium‐calcium alloy with improved degradation kinetics and adequate mechanical properties for cardiovascular applications

Recently, corrodible magnesium-based alloys have been introduced for use as cardiovascular stents and orthopedic implants. However, rapid corrosion rates have raised questions about their biocompatibility. Therefore, we developed a binary fluoride-coated magnesium-calcium alloy with improved degradation kinetics. Biocompatibility of the alloys was evaluated with metabolic assays (colorimetric WST-1 test). Furthermore, five different probes of magnesium-calcium alloys (MgCa 0.4, 0.6, 0.8, 1.2, and 2.0 wt %) were cocultivated with human smooth muscle cells and endothelial cells. To investigate the decomposition kinetics in a physiological environment the alloys were used untreated and fluoride coated (MgF(2)). Mg and Ca decreased the metabolic activity in vascular cells dose-dependently, with cytotoxic effects only at unphysiological concentrations. Uncoated magnesium alloys showed signs of decomposition after a short incubation time of 24 h in contrast to MgF(2) coated alloys. After 10 days smooth muscle and endothelial cells around the alloys were still alive, whereas colonization of the surfaces was only observed for smooth muscle cells. The fluoride-coated MgCa alloys exhibited good results concerning mechanical properties, degradation kinetics, and biocompatibility in vitro. We conclude that a binary fluoride magnesium-calcium alloy is a promising candidate for the production of cardiovascular stents.

Tribological behavior of plasma electrolytic oxidation coating on magnesium alloy with oil lubrication at elevated temperatures

The ceramic coating with micro-pores structure on AM60B magnesium alloy was prepared in silicate electrolyte by plasma electrolytic oxidation (PEO) method. The PEO coating is mainly composed of percales MgO and forsterite Mg 2SiO 4 phases. The tribological performance of the PEO coatings and uncoated magnesium alloys under oil-lubricated wear conditions at different loads and elevated temperatures (80–120 °C) was comparatively investigated. Results shown that the PEO coating exhibited significantly higher wear resistance and relatively lower friction coefficient when compared with Mg alloy under oil-lubricated conditions. The excellent tribological performance can be attributed to the high hardness and micro-pores structure feature of the PEO coating. Such high-performance PEO coating on magnesium alloys under oil-lubricated conditions could be applied as attractive advanced material for a wide range of automobile engine applications.

Characterization of microarc oxidation coatings formed on AM60B magnesium alloy in silicate and phosphate electrolytes

Microarc oxidation coatings on AM60B magnesium alloy were prepared in silicate and phosphate electrolytes. Structure, composition, mechanical property, tribological, and corrosion resistant characteristics of the coatings was studied by scanning electron microscope (SEM), X-ray diffraction (XRD) and microhardness analyses, and by ball-on-disc friction and potentiodynamic corrosion testing. It is found that the coating produced from the silicate electrolyte is compact and uniform and is mainly composed of MgO and forsterite Mg2SiO4 phases, while the one formed in phosphate electrolyte is relatively porous and is mainly composed of MgO phase. The thick coating produced from a silicate electrolyte possesses a high hardness and provides a low wear rate (3.55×10−5mm3/Nm) but a high friction coefficient against Si3N4 ball. A relatively low hardness and friction coefficient while a high wear rate (8.65×10−5mm3/Nm) is recorded during the testing of the thick coating produced from a phosphate electrolyte. Both of these types of coatings provide effective protection for the corrosion resistance compared with the uncoated magnesium alloy. The coating prepared from the silicate electrolyte demonstrates better corrosion behavior due to the compacter microstructure.

High-purity Magnesium Coating on Magnesium Alloys by Vapor Deposition Technique for Improving Corrosion Resistance

Microstructures in coated magnesium alloy with high purity magnesium fabricated by applying a vacuum deposition technique were investigated. Moreover, relationships between microstructures in coated and un-coated magnesium alloys and corrosion behaviors were interpreted by in-situ laser microscopic observations during salt immersion tests. Magnesium with 3N-grade and AZ31 magnesium alloy were used for an evaporation source and a substrate for deposition. Temperature of the substrates was changed resulting in change in temperature profile in a furnace in order to optimize deposition coating conditions for obtaining homogeneous microstructures and thickness in deposited layer. The coated specimen revealed superior corrosion resistance to those on 3N–Mg, AZ31 and AZ91E alloys, and comparable to that on 6N– Mg in salt immersion tests using 3% NaCl solution at 300 K for 587 ks. In-situ observations showed that inhomogeneity in microstructures, such as second phases and grain boundary segregations, deteriorate corrosion resistance in magnesium alloys. Therefore, pure magnesium coated layer without inhomogeneity in metallographic and electrochemical meanings can improve the corrosion resistance on magnesium alloys.

AFM Observations of Microstructures of Deposited Magnesium on Magnesium Alloys

Microstructures of deposited magnesium coating on a magnesium alloy was examined by an atomic force microscope. The coated surface was composed of facetted magnesium particles which were formed as isolated particles on the surface in the early stage of deposition and then grew to cover the surface. Deposited magnesium coating showed superior corrosion resistance to uncoated magnesium alloy. Homogeneous corrosion occurs on the coated surface, while heterogeneous corrosion preferentially occurs on the magnesium alloy.

Verschleiß- und Korrosionsschutz durch Chrom und Chromnitrid (PVD-Abscheidung auf Al und Mg)

Untersuchungen an den unbeschichteten Magnesium- und Aluminiumgußlegierungen AZ 91 und AlSi 7 Mg zeigen deutlich das schlechte Verschleißverhalten dieser beiden Leichtbauwerkstoffe. Zur Verbesserung des Verschleißverhaltens wurde eine 9 μm dicke CrN-Schicht auf beide Werkstoffe aufgebracht und anschließend tribologisch untersucht. Dabei zeigt sich, daß das Verschleißverhalten beider Werkstoffe enorm verbessert werden kann. Schichtfehler, die auf der Magnesiumlegierung auftreten, haben keinen erkennbaren Einfluß auf das Ergebnis. Das Korrosionsverhalten von Chrom- und Chromnitridschichten wurde auf der gegen Korrosion besonders anfälligen Magnesiumlegierung untersucht. Dabei zeigt sich, daß nur kurzfristig eine Verbesserung des Korrosionsverhaltens erzielt werden kann. Ursache hierfür sind die Schichtfehler. Weiter zeigt sich aber, daß durch den Einsatz von Mehrlagenschichten die „Dichtheit”︁ der Schichten verbessert werden kann. Wear and corrosion protection using Cr and CrN (PVD coating on Al and Mg) Investigations of the wear behaviour of uncoated Magnesium and Aluminium alloys (AZ 91hp, AlSi 7Mg) are showing very high wear rates of these materials. To improve the wear behaviour both materials were coated with 9 μm CrN using PVD (Physical Vapour Deposition) technology. The tribological behaviour of the coated light metals was tested afterwards by using a plate on cylinder tribometer. Looking at the results, wear is reduced enormously. The great number of defects in the coating of the magnesium alloy is showing almost no influence to the wear behaviour. The corrosion behaviour of chromium and chromium nitride coatings was tested on the magnesium alloy. Because of the defects in the coating, caused by defects like pores in the magnesium, only a short term protection of the alloy can be achieved. The corrosion behaviour of multilayer coatings is better than the behaviour of single layer coatings.