Surface Modification of Mg-Zn-Dy Alloy Using Plasma Spray and Friction Stir Processing for Biomedical Applications

Abstract

Magnesium (Mg) and its alloys are currently under consideration for use as temporary implants. However, early degradation and maintaining mechanical integrity are causes of concern for the use of Mg alloys as materials for temporary implants. Also, failure due to bacterial infection limits their applications. To this end, surface modification techniques are being used to improve the mechanical, corrosion, biocompatibility and antibacterial properties of Mg-based alloys. In the present work, friction stir processing (FSP) and plasma spray coating techniques were used to tailor the surface characteristics of Mg-1Zn-2Dy (wt.%) alloy. Initially, as-cast (AC) Mg-1Zn-2Dy alloy was subjected to FSP to improve the surface properties. Further, to enhance biocompatibility and antibacterial properties, AC sample was coated with two different coating combinations: hydroxyapatite (HA)/silver (Ag) (C-HAg) and aluminium oxide (Al2O3)/HA (C-AHa). Later, FSP was carried out on coated plates to fabricate composite surface (F-HAg & F-AHa). The FSPed samples were characterized using EBSD to understand the influence of FSP on crystallographic texture, grain size, grain boundaries and thereby their effect on mechanical properties and corrosion behaviour. Microstructural and phase analysis of all samples (AC, FSP, C-HAg, C-AHa, F-HAg & F-AHa) were carried out using SEM, FESEM & XRD. Cytotoxicity and corrosion studies were performed on all samples. In addition, for coated and composite surface samples, antibacterial properties were investigated using Escherichia coli (E. Coli) and Staphylococcus aureus (S. aureus) bacteria. Results showed that the grain size of stir zone (SZ) in the FSP sample was refined to less than 3 μm due to dynamic recrystallization (DRX) during FSP. Further, the FSP sample exhibited better mechanical properties and corrosion behavior when compared to the AC sample. This improvement in mechanical properties and corrosion behavior of the FSP sample compared to the AC sample can be attributed to grain refinement, uniform distribution of secondary precipitates and strong basal texture. The degradation of the FSP sample resulted in the deposition of calcium phosphate-rich minerals, and thereby helping to improve apatite formation on the surface. Cytotoxicity studies using ii MTT assay showed more than 80 % cell viability for both AC and FSP samples suggesting non-toxic nature. Antibacterial studies reveal that both C-HAg and F-HAg samples inhibit Escherichia coli and Staphylococcus aureus bacteria. In comparison, AC, FSP, C-AHa and F-AHa samples exhibit bacterial adhesion on the surfaces. In-vitro cytotoxicity studies reveal that C-HAg, F-HAg & F-AHa samples are non-toxic in nature, while the C-AHa sample alone exhibited toxicity. Results of in-vitro corrosion studies reveal a significant reduction in the corrosion rate for the composite surface samples when compared to the coated samples. In particular, the F-HAg samples showed simultaneous improvement in corrosion resistance and antibacterial properties with good biocompatibility. Overall, the hardness, corrosion resistance, cytotoxicity, and antibacterial properties of F-HAg samples have improved significantly. Results indicate that the F-HAg sample has the potential to be used as material for temporary implant applications.