Fabrication of metal matrix nanocomposite surface on WE43 using friction stir processing for biomedical implant application / Wu Bo

Wu , Bo (2024) Fabrication of metal matrix nanocomposite surface on WE43 using friction stir processing for biomedical implant application / Wu Bo. PhD thesis, Universiti Malaya.

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Abstract

Magnesium (Mg) alloys are favorable for biomedical implants because of their excellent biocompatibility, biodegradability in physiological environment, similar density and Young’s modulus to natural bone, becoming a new generation biodegradable medical implant material. However, the uncontrollable degradation rate in biological environment and bacterial infection restrains their medical application as implant materials. Hence, surface modification techniques are being used to improve the hardness, bio-corrosion, in-vitro biocompatibility and antibacterial properties of Mg-based alloys. In the present work, as a promising surface modification method due to its cost-effective and pollution-free characteristics, friction stir processing (FSP) was utilized to realize surface modification of Mg alloy WE43 with reinforced particles. Initially, the influence of rotational speed, traverse speed and number of FSP pass on the microstructure, stirring zone morphology and hardness behavior of the WE43 alloy were optimized to develop defect-free surface layer on the WE43 alloy. Further, to improve the degradation rate, WE43 reinforced with single nano-hydroxyapatite (nHA) particles was fabricated. Later, the bioactivity, in-vitro cytocompatibility and in-vitro antibacterial properties of WE43 reinforced with hybrid nanocomposites of nHA doped with nano-silver (nAg) for enhanced biocompatibility and osseointegration in orthopaedic implants were verified. Reinforced nHA and nAg particles were filled in a groove at surface of the WE43 matrix. The experiments were performed for different weight percentages of nHA and nAg particles. Microstructural and phase analysis of FSPed samples were observed by OM, SEM with EDS, EBSD and XRD. Corrosion properties were evaluated by electrochemical corrosion test and immersion test. Wettability test and in-vitro bioactivity behavior were performed on all FSPed WE43/nHA composites. In addition, for FSPed WE43/nHA/nAg composites, in-vitro antibacterial test was investigated against Escherichia coli and Staphylococcus aureus bacteria. In-vitro cytotoxicity test was exposed to mouse calvarial preosteoblasts and evaluated using CCK-8 assay. The results showed that FSP is a feasible method to refine the microstructure and improve the properties of WE43 alloy. The microstructure was refined and defect-free WE43 surface metal matrix composites were developed at 1250 rpm and 30 mm/min. Dynamic recrystallization is responsible for the creation of fine equiaxed grains in the stirring zone. After three passes, the FSPed WE43-3P alloy and WE43/nHA-3P composite have the smallest grain size (2.63 and 1.88 μm), which were reduced by 94.29% and 95.92% compared to base metal, respectively. Additionally, the microhardness of FSPed WE43/nHA-3P composite was 105.59 HV increased by 135%. The reinforced HA and Ag particles underwent fragmentation and redistribution, resulting in improved corrosion resistance and lower degradation rate. The wettability test showed an improvement in contact angle and surface energy for FSPed samples. The atomic ratio of Ca/P after immersion 7d reached 1.61 which indicates superior bioactivity for FSPed WE43/nHA/nAg composite. In-vitro cytotoxicity test and in-vitro antibacterial test proved nAg has an impact on hindering bacterial growth on the surface. These results indicate that the grain refinement by FSP and introduction of nHA and nAg particles play a significant role in degradation rate, in-vitro biocompatibility and antibacterial of WE43 alloy for biomedical applications.

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