A Nanostructured Coating for Improved Corrosion Resistance on Lens-Manufactured and Wrought Biodegradable WE43 Mg Alloy

Over the last few decades, magnesium (Mg) and its alloys, due to their inherent mechanical and biological attributes, have been extensively researched for various biomedical implant applications. Mg is a lightweight and a high specific strength material which makes it suitable as a bone fixing device in orthopaedic implant applications. It degrades naturally in a physiological environment with non-toxic corrosion byproducts that are consumable by the body, demonstrating absolute biocompatibility. The main issue that radically limits its wide clinical use as a biodegradable implant material is its rapid degradation rate in the human electrolytic chloride-rich environment, resulting in dramatic changes in local pH and production of a large volume of hydrogen (H2) gas. Furthermore, the Mg implant may lose its mechanical integrity due to fast degradation, posing the risk of premature implant failure. Thus, there is a pressing need to reduce the degradation rate of Mg-based implants in order for them to function safely to the required extent. The use of additive manufacturing (AM) technology for biomedical implant manufacture has resulted in innumerable benefits. Customized, complex-shaped, patient-specific implants fabricated in a single step by AM open up a wide range of applications in the artificial implant sector. However, the issue of rapid degradation with 3D printed Mg-based implants must still be addressed. One of the well-recognized techniques to suppress degradation kinetics of Mg-based implants and to improve their corrosion resistance is surface modification with coating. Surface coating prevents contact with the physiological environment by forming a physical barrier on top of the Mg-based substrate, allowing for a longer healing time. This project aims to develop the surface coating on 3D-printed, using laser engineered net shaping (LENS), and wrought WE43 Mg alloys, in order to improve their corrosion resistance in the physiological environment. For this purpose, thorough study of the protective coating on the LENS-printed WE43 (LW) and wrought WE43 (WW) was carried out. For coating purposes, ZnO was selected as it is a widely used coating material for Mg-based implants for improving their corrosion resistance. Furthermore, a top layer of Ag was deposited to provide antibacterial benefits which helps in avoiding device associated infections especially in the early stage after implant insertion. Magnetron sputtering (MS) was chosen for deposition of high quality thin dense layers of ZnO and Ag on the WE43 alloy surfaces. To obtain maximum corrosion resistance from the deposited ZnOAg coating, the coating deposition parameters including temperature and time were systematically optimized on both WE43 alloys. Furthermore, the microstructural characteristics, and electrochemical and mechanical properties of the alloys with and without coating were evaluated using SEM, EDX, XRD, FIB-SEM, AFM, PDP, and nano indentation testing. The research clearly demonstrated the viability and advantages of applying ZnOAg coating for scaling down the degradation rate of the LW and WW Mg alloys. It also uncovered the competence of fabricating load-bearing biodegradable Mg-based implants using LENS technology. MS of ZnO and Ag resulted in formation of crystalline, uniform, dense, and highly compact thin films of deposited materials on LW and WW surfaces as depicted by FIB-SEM and AFM. ZnOAg coating processed by depositing ZnO for 30 min and Ag for 15 min at 80 °C was found to be optimal as it was more effective in terms of improving electrochemical performance of coated samples by showing maximum corrosion resistance in the physiological medium. Thus, for LW, the optimized ZnOAg film shifted the corrosion potential (Ecorr) from -1.616 V to -1.386 V and reduced the corrosion current density (Icorr) from 70.065 µA to 3.016 µA, as compared to uncoated LW. Whereas for WW, Ecorr was shifted from -1.530 V to -1.296 V and Icorr was reduced from 12.759 µA to 0.456 µA, as compared to the uncoated alloy. In this way, the corrosion reduction efficacy of optimized ZnOAg thin film was around 95.7 % for LW and 96.4 % for WW. The amount of H2 evolved after immersion in Hanks’ balanced salt solution (HBSS) was also reduced during the initial 6 days with ZnOAg coating from 18 ml to 3.2 ml for LW and from 0.7 ml to 0.1 ml for WW. The microstructural investigation of ZnOAg coated and uncoated LW and WW alloys before and after electrochemical corrosion tests provided constructive information regarding the degradation behaviour of substrate materials. Characterization of LW samples revealed spatially varying heterogeneous microstructures with combination of fine equiaxed and columnar grains due to rapid thermo-kinetics of LENS manufacturing process. Evidence of severe evaporation of Mg and increased volume fractions of the alloying contents, intermetallic phases, and oxygen (O) and carbon (C) content was found by EDX. Moreover, 2.5% closed porosity of total volume was also detected using µ-CT scan. On the other hand, WW showed homogeneous, coarse equiaxed grain structure with α-Mg matrix as major constituent of solid solution uniformly distributed throughout the material with minor quantity of secondary phases. The WW was found to be 99.99% dense by µ-CT scan. The average surface roughness of LW was 0.59 µm, while it was 0.04 µm for WW. The surface porosity in LW gave rise to galvanic coupling between Mg and intermetallic phases after immersion in HBSS during electrochemical corrosion testing, resulting in inter-galvanic and localised corrosion. In addition to porosity, the microstructural heterogeneity, elemental segregation, increased concentrations of secondary phases, and zirconium (Zr) precipitates in LW contributed to its corrosion acceleration as compared to WW. The microstructural conditions of LW posed clear effects on ZnOAg coating electrochemical behaviour. The deposited ZnOAg coating could not cover the micro-pores completely, leaving some sites unprotected and open to physiological medium. Moreover, galvanic coupling between Ag, Zn, and Mg speeded up the corrosion process due to simultaneous exposure to the physiological medium, resulting in quick diminishing of ZnOAg protective film as well as corrosion passive byproduct layers of Mg (OH)¬2 and MgO. The nanohardness (NH) and reduced elastic modulus (EMr) of uncoated LW were 1.10 GPa and 47.73 GPa, respectively, while for uncoated WW, they were 1.66 GPa and 57.8 GPa, respectively. The nano indentation test results concluded indicative effect of deposited ZnOAg coating in terms of improved stiffness of LW and WW surfaces, favourable for the orthopaedic implant applications. The NH and EMr of ZnO film were observed to be 4.45 GPa and 67.26 GPa, respectively. While for ZnOAg coating with Ag film on top, NH and EMr were 2.40 GPa and 79.51 GPa, respectively. Similarly, in nano-wear tests, mean wear volumes for uncoated LW and WW were 62.813 µm3 and 42.076 µm3, respectively. While the mean worn volumes for ZnOAg coated LW and WW were 43.332 µm3 and 30.004 µm3, respectively. In this way, the average material volume removed from uncoated alloy surface at 600 µN was reduced by 31% for ZnOAg coated LW. Whereas for ZnOAg coated WW it was 28.69% lower than the uncoated one, indicating towards improved deformation resistance with coating. Furthermore, wettability of uncoated and coated WE43 alloys, evaluated by measuring water contact angle (WCA) was found to be 52.4°, 57.3°, 65.7° and 69.8° for LW, WW, ZnOAg coated LW, and ZnOAg coated WW, respectively, which indicates towards hydrophilicity of substrate materials and applied ZnOAg film. Thus, from the accomplished study, it is anticipated that the ZnOAg coating can effectively reduce degradation rate of both LENS-printed and wrought WE43, creating new possibilities for clinical applications of these biodegradable Mg alloys.<p></p>