Novel biodegradable magnesium-graphene composites for biomedical applications
Creators: name: Mia, Md Shahin
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400302
subject: Biomaterials
classificationCode: 401602
subject: Composite and hybrid materials

description: Magnesium (Mg) and some of its alloys as biodegradable implants such as bone plates, screws, pins, and so on have attracted extensive interests in biomedical applications due to their similar elastic modulus to the human bone, which eliminates stress-shielding, whereas the conventional non-biodegradable implant materials such as titanium (Ti) and its alloys, stainless steels (SS), and cobalt-chromium (Co-Cr) alloys may cause stress-shielding, leading to implant loosening and implant failure. However, the low mechanical properties, poor tribological performance, and low corrosion resistance of the conventional Mg alloys restrict their wide range of biomedical applications. The two-dimensional graphene nanoplatelet (GNP) contains a honeycomb lattice structure and a unique sp2-carbon (C) matrix. It has a strong interface bonding ability with the Mg matrix and is impermeable to gas molecules, so it can act as a promising Mg matrix reinforcement. However, producing a uniform dispersion of GNPs in the Mg matrix without agglomeration is a challenge. Further, the optimum content of GNPs in Mg matrices and the fabrication processes of GNP-reinforced Mg matrix nanocomposites (MMNCs) with enhanced properties are still not fully understood. In addition, the strengthening and corrosion mechanisms, tribological and biological behaviours of GNP-reinforced MMNCs are not thoroughly investigated.

In this study, GNP-reinforced MMNCs were fabricated via powder metallurgy, and their microstructures, mechanical, corrosion, tribological, and biological properties were investigated and assessed. The effects of dispersion, the content of GNPs, and fabrication processes on the strengthening and corrosion mechanisms of GNP-reinforced MMNCs were evaluated.

First, the pure Mg, Mg0.5Zr, and Mg0.5Zr0.1GNP nanocomposites were fabricated using powder metallurgy with a pressureless sintering method (Chapter 4). The theoretical yield strength of Mg0.5Zr0.1GNP nanocomposite was determined by considering various synergetic strengthening mechanisms, including the difference in thermal expansion and elastic modulus between Mg matrix and GNP, grain refinement, and load transfer from the Mg matrix to GNP, and precipitation strengthening. The theoretical and experimental yield strengths are in good agreement, validating the synergetic strengthening mechanisms in GNP-reinforced MMNCs. Compared to the pure Mg, the compressive yield strength of the Mg0.5Zr0.1GNP nanocomposite increases by 91%, and the corrosion resistance increases by 48% and 68% tested by electrochemical polarization and H2 evolution in Hanks' balanced salt solution, respectively. The results demonstrated that the Mg0.5Zr0.1GNP nanocomposite exhibits a lower release of Mg ions during degradation. However, the Mg0.5Zr0.1GNP nanocomposite shows lower ductility than the pure Mg. The Mg0.5Zr0.1GNP nanocomposite also shows 92% higher wear resistance at 200 µN applied load, exhibiting lower worn depth and coefficient of friction compared to the pure Mg in wear and scratch tests, respectively (Chapter 5).

Second, Mg0.5ZrxGNP (x = 0.2-0.5 wt.%) nanocomposites were fabricated via a similar fabrication process, and their microstructures, mechanical properties, and corrosion behaviours were characterized and evaluated (Chapter 6). The increase of GNP content in the Mg matrices leads to increases in agglomerations or stacking of GNP layers, which results in insufficient grain refinement and functioning of the strengthening mechanisms. Mg0.5ZrxGNP (x = 0.2-0.5 wt.%) nanocomposites show a decrease in compressive strength and corrosion resistance compared to the Mg0.5Zr0.1GNP.

In the third stage, Mg0.5ZryZn0.1GNP (y = 1-5 wt.%) and Mg0.5Zr3Zn0.2GNP nanocomposites were manufactured by a similar processing method in order to achieve high ductility, yield strength, and corrosion resistance at the same time. In addition, similar characterization and evaluation tests were performed on the nanocomposites (Chapter 6). The Mg0.5Zr3Zn0.1GNP nanocomposite exhibits the highest ultimate compressive strength (387 MPa), compressive yield strength (219 MPa), and lowest corrosion rate (1.5 mm/y) among all the fabricated MMNCs. Therefore, the cytotoxicity of Mg0.5ZryZn0.1GNP (y = 1-5 wt.%) and Mg0.5Zr3Zn0.2GNP were assessed against human osteoblast-like cells (SaOS2). The Mg0.5Zr3Zn0.2GNP exhibits higher cell viability with better cell adhesion and proliferation on its surface (Chapter 8).

In the fourth stage, a hot pressing method was attempted to fabricate MgxGNP (x = 0.1-0.3 wt.%) and their microstructures and mechanical and corrosion properties were characterized and evaluated (Chapter 7). The addition of GNP to the Mg matrix leads to increases in the ultimate compressive strength, compressive yield strength, and corrosion resistance. The hot-press method provides a higher relative density reducing the pores and voids of MMNCs compared to the pressureless sintering method, which increases the interfacial bonding between the Mg and the GNP.

In conclusion (Chapter 9), uniform distributions of low content of Zr (0.5 wt.%) and GNP (0.1 wt.%) in Mg matrices lead to enhanced grain refinement. The addition of 3 wt.% Zn in Mg0.5Zr3Zn0.1GNP nanocomposite exhibits high mechanical strength and excellent corrosion resistance with low Mg ion release. The MMNC surface reveals hydrophobicity (water contact angle &gt;90°) which reduces the affinity to react with water, leading to a decrease in degradation rate. The addition of GNP to Mg matrices increases wear resistance, showing low worn depth and coefficient of friction. The addition of high GNP content (&gt;0.3 wt.%) to Mg matrices results in stacking of GNP layers at grain boundaries and forms agglomeration, which leads to early failure of MMNCs during load transfer from the Mg matrix to the GNP. The difference in thermal expansion, grain refinement, and load transfer mechanisms show higher contributions to the increase in the yield strength of MMNCs among all the strengthening mechanisms. The MMNCs containing Zn exhibit increased cell adhesion and cell viability towards osteoblast-like SaOS2 cells.

In the future, the post hot working processes such as extrusion or hot rolling of the MMNCs are suggested for MMNCs to further reduce pores and voids inside the MMNCs and enhance the grain refinement effect. The reduction of pores and voids can increase the interfacial bonding between the particles, leading to the enhanced mechanical and corrosion properties of MMNCs. It is also suggested to use GNP with a single layer or a few layers in the manufacturing of MMNCs to reduce agglomeration tendency. The low agglomeration will ensure the uniform distribution of GNP in the Mg matrices and achieve better performance of the various strengthening mechanisms.