Sudheer Neralla

Enhanced mechanical properties and increased corrosion resistance of a biodegradable magnesium alloy by plasma electrolytic oxidation (PEO)

We report the enhanced mechanical properties of AZ31 magnesium alloys by plasma electrolytic oxidation (PEO) coating in NaOH, Na2SiO3, KF and NaH2PO4·2H2O containing electrolytes. Mechanical properties including wear resistance, surface hardness and elastic modulus were increased for PEO-coated AZ31 Mg alloys (PEO-AZ31). DC polarization in Hanks solution indicating that the corrosion resistance significantly increased for PEO-coating in KF-contained electrolyte. Based on these results, the PEO coating method shows promising potential for use in biodegradable implant applications where tunable corrosion and mechanical properties are needed.

Directional-Tribological Investigation of Magnesium Alloys Under As-Cast and Hot Extrusion Conditions

In recent years, magnesium (Mg) and its alloy are being studied for their potential use in orthopedic implants with the novel ability to biodegrade after the implant serves its therapeutic function. Pure Mg, by itself, would not be suitable for use in a load-bearing implant application, due to its high corrosion rate and poor tribological properties. However, through proper alloying, this degradable metal is capable of achieving good mechanical properties reasonably similar to bone, a retarded rate of corrosion and enhanced biocompatibility. Previous studies have shown that alloying Mg with aluminum, lithium, rare earth (RE), zinc (Zn), and calcium (Ca) result in lower corrosion rates and enhanced mechanical properties. Despite the growing popularity of Mg and it alloys, there is relatively little information in the literature on their wear performance. In this paper, we report on an investigation of the directional tribological properties of Mg and Mg-Zn-Ca-RE alloy fabricated via two different manufacturing processing routes: as-cast and hot-extruded after casting, with extrusion ratios of 10 and 50. Pure Mg was cast 350°C. After casting, Mg-Zn-Ca-RE alloy was heat-treated at 510°C. Another Mg-Zn-Ca-RE alloy was hot-extruded at 400°C. Dry sliding wear tests were performed on as-cast and hot-extruded pure Mg and Mg-Zn-Ca-RE alloys using a reciprocating test configuration. Wear rate, coefficient of friction and wear coefficient were measured under applied loads ranging from 0.5–2.5N at sliding frequency of 0.2 Hz for 120 cycles, using microtribometery. Wear properties of the extruded specimen were measured in cross-section and longitudinal section. In the longitudinal section studies, wear properties were investigated along the extrusion direction and the transverse direction. Hardness properties were evaluated using microindentation. Cross-section and longitudinal section were indented with a Vickers indenter under applied load of 2.94 N. Alloying and extrusion enhanced the mechanical properties significantly, increased hardness by 80% and wear resistance by 50% compared to pure Mg. Despite the low hardness in both Mg and the Mg alloy cross-sections, the cross-sections for both displayed higher wear resistance compared to the longitudinal section. In the longitudinal section, wear resistance was higher along the transverse direction of the longitudinal section for both Mg and the Mg alloy. The wear coefficient was used to evaluate how the wear behavior of the material varied with respect to alloying, fabrication process, and direction of wear. The wear coefficient of pure Mg decreased as the extrusion ratio increased, thus, increasing the specific wear rate. The opposite behavior was found in the Mg alloy: as the wear coefficient increases, the specific wear rate decreases. The active wear mechanisms observed on the worn surface of Mg were fatigue, abrasive, adhesive and delamination wear. The same wear mechanisms were observed in the Mg alloy except for fatigue wear. Surface microstructure and topographical characterization were conducted using optical microscopy, scanning electron microscopy mechanical stylus profilometry, and optical profilometry.Copyright

TRIBOLOGICAL STUDY OF MAGNESIUM ALLOYS FOR IMPLANT APPLICATIONS

Magnesium and its alloys have been found to potential candidates for biodegradable implant applications. However, magnesium and its alloys are broadly known to have poor tribological properties, but detailed specifics on wear performance are scarce. This research investigates the tribological characteristics on Mg-Zn-Ca-RE alloys and pure magnesium under as-cast and extruded conditions. Pure magnesium and Mg-Zn-Ca-RE alloys were hot extruded at 350°C and 400°C. Magnesium and Mg-Zn-Ca-Re alloy were also cast at 350°C and heat treated at 510°C. Directional wear properties were investigated using a CETR-UMT 2 microtribometer under unlubricated conditions in a reciprocating configuration for 120 cycles, with normal loads ranging from 0.5N–2.5N. Wear tests were conducted in directions: cross-sectional, longitudinal (along the extrusion direction) and transverse direction (perpendicular to the extrusion direction). Wear properties and friction properties were analyzed using a microtribometer, a mechanical stylus profiler, and microindentation. Surface morphology and microstructure were characterized using optical microscopy, scanning electron microscopy, and optical profilometry. The results show a lower wear rate in the transverse and cross-sectional direction compared to the longitudinal direction.© 2014 ASME

Structural and Mechanical Properties of Mg/MgO and Mg/Al2O3 Nanolaminate Coating for Implant Applications

Nanostructured magnesium coatings have the potential of enhancing the integration of implant to bone tissues due to their ability to regulate the functions of integrin, which modulates cell proliferation and differentiation. However, they are soft, ductile and have low wear resistance. These limitations prevent the practical use of Mg coatings for this application. However, application of Mg thin films in the form of nanolaminate coatings has been found to improve hardness as well as wear properties. In this study, Mg/MgO and Mg/Al2O3 nanolaminates with bilayer thicknesses (Λ) 10, 20, 40, 100, 200, 1000 nm were deposited on glass substrate using the reactive pulsed DC magnetron sputtering process. The Mg/MgO nanolaminates were developed from an Mg target. Λ was controlled by the duration of oxygen flow during the sputtering process. Values of Λ were obtained from low angle-XRD. We found that the rate of MgO deposition significantly depends on water vapor content in the chamber and that a partial base pressure of water below 10−8 Torr is required to achieve repeatable results. Structure and properties of multilayered coatings were studied by X-ray diffractometry, nanoindentation, SEM and AFM. At Λ and respectively, while at higher Λ, other orientations are present in the XRD patterns. The nanoindentation results showed slightly higher hardness of Mg/MgO and Mg/Al2O3 nanolaminate coatings compared to that of pure Mg. Nanolaminates have high ductility compared to MgO and Al2O3. Nanolaminate coatings at Λ < 100 nm exhibit an improvement in the mechanical properties due to the presence of interfaces which act as barrier to dislocation movement.Copyright

Mechanical Characteristics of an Anodized Magnesium Alloy for Biodegradable Implants

In recent years, magnesium (Mg) alloys have emerged as possible biodegradable implant materials; however the degradation rate of Mg occurs at a higher rate than tolerable for the human body. Plasma electrolytic oxidation (PEO) has been used in the past as a useful surface treatment technique to improve the anti-corrosion properties of Mg alloys by forming protective coatings. This present work focuses on the effect of electrolyte solution on the corrosion, microstructural, and nanomechanical behavior of PEO coatings for possible use in biodegradable implants. The experimental parameters applied during PEO process did influence the structure, thickness, and morphology of the coating. Microstructural characterization of the coating was carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM) followed by image analysis and energy dispersive spectroscopy (EDX). Further, nanoindentation was employed to evaluate nanohardness and Young’s modulus of the PEO coating. The results show beneficial effects of the PEO coating to enhance the corrosion resistance of the uncoated AZ31 magnesium alloy. The XRD pattern shows that the components of the film vary based on electrolyte solution. The film composition does affect the nanomechanical behavior.© 2013 ASME

CROSS-SECTIONAL NANOINDENTATION OF ALUMINA THIN FILMS DEPOSITED BY PULSED LASER DEPOSITION PROCESS

Alumina is a widely used ceramic material due to its high hardness, wear resistance and dielectric properties. The study of phase transformation and its correlation to the mechanical properties of alumina is essential. In this study, interfacial adhesion properties of alumina thin films are studied using cross-sectional nanoindentation (CSN) technique. Alumina thin films are deposited at 200 and 700 °C, on Si (100) substrates with a weak Silica interface, using pulsed laser deposition (PLD) process. Effect of annealing on the surface morphology of the thin films is studied using atomic force microscopy. Xray diffraction studies revealed that alumina thin films are amorphous in nature at 200 °C and polycrystalline with predominant gamma alumina phase at 700 °C.Copyright