/library/oar/handle/123456789/63161 2025-11-05T16:37:04Z 2025-11-05T16:37:04Z /library/oar/handle/123456789/120484 2024-04-16T04:49:20Z 2020-01-01T00:00:00Z

Title: Biodegradable iron-based scaffolds : developing a replication method using additive manufacturing Abstract: Orthopaedic trauma patients may require a load bearing scaffold to assist their recovery. Ideally such a scaffold would be biodegradable, with its degradation rate matching that of bone growth and with pore diameter in the range of 100 µm to 800 µm. Research being carried out on iron-based scaffolds suggests that this can be achieved. This work is aimed to develop a reliable fabrication process for biodegradable iron scaffolds, based on the replication method combined with stereolithography (SLA) 3D printing. The replication method is a powder metallurgy technique which uses a perishable polymer template that is coated with a slurry containing the desired iron-based final material. Instead of using said slurry, a dry coating technique was developed which made use of the inherent tackiness of the 3D printed polymer templates, to attach the powder. The metallic coated polymer template is then heat treated at a low temperature to partially sinter the powder coating to form an interconnected lattice. This is then followed by a high temperature heat treatment to completely burn away the polymer template and fully sinter the metallic scaffold implant. In this work, the technique was developed further by incorporating SLA 3D printing to produce the polymeric templates thus making it possible to produce patient specific scaffolds at a very low price. Two template types were developed namely, cubic and gyroid type templates. To develop this adapted replication method, the SLA 3D printing polymer was analysed using dynamic mechanical analysis, differential scanning calorimetry and furnace heat treatments, to determine the softening and degradation temperatures. The 3D printed templates were analysed using optical microscopy and scanning electron microscopy to analyse their strut and pore size. Coated templates were subsequently analysed using weighted coating mass uptake and X-ray Microscopy. Scanning electron microscopy with electron dispersive spectroscopy was employed to characterise the powder used and the final heat-treated iron lattices. For both template types, the minimum achievable pore and strut size was 600 µm and 420 µm respectively. The optimal pore and strut size was set to 1000 µm and 700 µm, to minimise pore clogging for gyroid templates and to cater for the shrinkage experienced during heat treatment. The best heat treatment achieved used milled iron powder (particle diameter about 1.5 µm), coated using the dry coating method and heat treated with the first dwell at 175°C for 2 hours and a final dwell at 1120°C for 3 hours. Description: B.Eng. (Hons)(Melit.)

2020-01-01T00:00:00Z /library/oar/handle/123456789/119766 2024-05-29T07:55:54Z 2024-01-01T00:00:00Z

Title: Investigating the corrosion behaviour and pressureless sintering of FeMn-alloys for biodegradable implant applications Abstract: FeMn alloys (Mn wt.% 30-35), have emerged as a particularly attractive option as a solution for the limitations presented by pure Fe as a biodegradable metal for temporary implants. Such alloys are antiferromagnetic and have shown in early publications that they could offer adequate mechanical properties and biocompatibility for orthopaedic applications. Another favoured approach involves alloying FeMn with noble elements like Ag, in order to create micro-galvanic couples, which in turn enhance corrosion. Despite the promise shown by these alloys in several publications, the understanding of their degradation mechanism remains limited, with multiple publications sharing similar materials and methodologies, reaching contradictory conclusions. This generally results either from short-span testing providing an incomplete picture of material behaviour, or limited consideration of the impact of testing parameters on the test outcomes. In this work, the first of two major sections were aimed at the study of powderprocessed Fe35Mn, and in select tests also (Fe35Mn)5Ag, using a variety of techniques to determine their behaviour over the first 24 h of testing. Potentiodynamic testing (PDP), electrochemical impedance spectroscopy (EIS) and in situ pH and dissolved oxygen (DO) micro-probe measurements at the sample surface, were some of the techniques used to gather this information. Tests were carried out in HBSS, HBSS containing Ca2+ (HBSS+Ca) and the latter with added bovine serum albumin protein (HBSS+BSA). Static immersion tests of Fe35Mn in the same electrolytes were carried out to provide information regarding the alloy’s long-term degradation behaviour whereas in vivo testing in GAERS rats for 6 months was aimed at demonstrating how considerable the gap between in vitro and in vivo findings, really is. All results pointed towards increased corrosion for both FeMn and FeMnAg when compared to Fe. Outcomes across all testing phases highlighted the impact of Ca2+ ions in the HBSS on the degradation of Fe-based alloys as the same ions interact with phosphates in the solution and metal ions originating from the sample to create Ca/P-rich precipitates that act as a partially-protective barrier to further degradation. Although the same precipitates were unstable on the microgalvanically corroding FeMnAg surface, the charge transfer resistance for this material after 24 h was similar to that of FeMn. This and other supporting results indicated that noble-phase additions, while effective in accelerating corrosion over the short term, might not hold much promise for long-term effectiveness in the body. The same applies to the impact of MnO-inclusions, typically present in powder-processed FeMn samples, on corrosion. Whereas EIS measurements indicated that MnO inclusions could be behaving as micro-cathodes within the austenitic matrix, the behaviour of samples with and without MnO was indistinguishable after the first 24 h. When investigating the influence of BSA over the degradation of FeMn, results indicated that protein reduces the corrosion resistance in vitro. This was likely due to the tendency of BSA to chelate Ca2+ ions, which prevented or delayed the precipitation of Ca/P-precipitates and encouraged localised corrosion as opposed to the uniform corrosion observed in HBSS+Ca. Despite these findings, analysis of FeMn and FeMnAg pin surfaces tested in rat vertebrae for 6 months showed only signs of very limited uniform corrosion, even though FeMnAg accumulated a slightly thicker layer of corrosion products. Moreover, whereas Ca/P-precipitates and metal hydroxides were present on sample surfaces tested in vivo, the major product detected via XRD was CaCO3; a product absent in all in vitro analyses. This highlights the need for further efforts to bridge the gap between in vitro and in vivo testing, as is the aim of international testing standards in the pipeline. Apart from corrosion studies, FeMn is also the centre of development of multiple novel processing methods that allow this material to be processed into implants with the desired shapes and sizes. With most of these methods including the use of powder metallurgy, issues related to the high vapour pressure and high temperature reactivity of Mn need to be addressed. In fact, in the second part of the thesis, blended elemental (B35; blended Fe and Mn elemental powders), milled (M35; Fe and Mn intricately mixed but still present in their elemental form) and alloyed (A35; fully alloyed Fe35Mn) powders, were prepared using high energy ball-milling. Analysis was carried out through microscopy of powders and pressed-and-sintered coupons, carbon analysis and thermal analysis techniques including thermogravimetry with mass-spectroscopy (TG-MS) and differential thermal analysis (DTA). Results indicated that the alloyed powder exhibited a lower tendency to oxidise compared to M35 and B35 powder. Carbon analysis also showed that an increase in ball milling time resulted in an increase in C content in the pre-processed powders due to enhanced diffusion of the organic process control agent over time. This led to the alloyed powders additionally having better carbothermal reduction capacity, allowing for further reduction of oxides as observed both in thermal analyses of powders as well as cross-sectional analysis of pressed-and-sintered samples. However, the high percentage of carbon content also led to precipitation of a higher amount of interconnected M3C carbides at grain boundaries. The same materials were then used in the preparation of cubic scaffolds using a modified replication method wherein the polyurethane foam used as a template in the traditional replication method, was replaced with a 3D-printed customised acrylate-based template, allowing for more control on the structure of the implant. The process was conducted in either of two configurations; exposed, where the sample was exposed to the sintering N2-5H2 atmosphere, and shielded, wherein the sample was covered by a stainless steel shield creating a micro-atmosphere around the sample surface. Results showed that the microstructures of the resulting scaffolds were less affected by the material used, be it M35 or A35, and more impacted by the processing configuration used. In general, samples contained an austenitic matrix from which in excess of 7 wt.% Mn was lost to sublimation and formation of mixed metal oxides and interconnected carbides. All these phases led to rather brittle structures. Shielded structures had superior microstructural homogeneity as a result of the micro-atmosphere formed whereas exposed structures had cores rich in carbides and edge struts richer in oxides. Carbothermal reduction assisted oxide removal from the surface, but kinetic limitations persisted leading to strut cores littered with oxides. This work highlighted the challenges in achieving the right balance to preserve the antiferromagnetic characteristic of these FeMn alloys, reduce oxidation and have effective carbothermal reduction. Description: Ph.D.(Melit.)

2024-01-01T00:00:00Z /library/oar/handle/123456789/119676 2024-03-12T07:24:10Z 2023-01-01T00:00:00Z

Title: Anodic titanium dioxide nanotubes for greywater reclamation Abstract: Photocatalysis has been seen by many as an invaluable eco-friendly tool for environment remediation. Of special interest has been the use of photocatalysis for wastewater treatment. Titanium dioxide (TiO2) has been the photocatalyst of choice for the last few decades. The combination of chemical stability, low-cost, non-toxicity and excellent optical properties have put it at the forefront of wastewater treatment research. Whilst other oxides, such as tungsten trioxide (WO3) and iron oxide (Fe2O3) possess lower band gaps and can thus be activated by visible light, their resistance towards photocorrosion is lower than that of TiO2. The inherent limitation of TiO2 is its wide band gap which means that it can only be activated by UV light. Interest in the material has been consistently growing with strategies being devised to extend its activity into the visible region of the spectrum. Similarly, the advent of LED lights with low power consumption has helped overcome this limitation. In this work TiO2 nanotubes were produced via anodic oxidation. Anodic oxidation or anodising, is a facile way by which nanotube array can be grown. The process also provides a high level of control on the morphology of the produced nanotubes. The effect of different process variables on the photocatalytic activity of the produced was assessed. The process parameters used were dependent on the type of electrolyte being used. In one instance, a solution of 0.5 wt% sodium fluoride in 1 M sodium sulfate was employed. The anodising was carried out at 20 V for 6 hours. An acidic electrolyte consisting of 0.5% wt% sodium fluoride solution in 1 M phosphoric acid was used with a potential of 20 V applied for 3 hours. A non-aqueous solution containing 0.5 wt% ammonium fluoride, 3 wt% water and ethylene glycol as the balance was used for a process at 70 V for 1 hour. One set of surfaces produced in glycol was decorated with silver nanoparticles. The different electrolytes and associated process parameters produced nanotube layers with different morphologies. All materials when irradiated with UVA light showed a high activity towards the degradation of both chemical and bacterial contaminants. The chemical contaminants were the dye methylene blue, the analgesic paracetamol and the surfactant sodium dodecyl sulfate. The ubiquitous E.Coli was used as the bacterial contaminant. All pollutants can be found in all wastewater streams including greywater. The materials produced in glycol had the highest photocatalytic activity towards all pollutants. This is attributable to the increased thickness, tube diameters and high anatase contents. These properties were the result of the high potential used (70 V) and the electrolyte which could support the high potential. The mechanical and chemical stability of the materials was tested using two different aging regimes. A lab-based reactor was constructed to expose the photocatalysts to flowing synthetic greywater and a light source similar in composition to that of the solar spectrum for 12 weeks. The second aging exercise aimed to better replicate the conditions encountered during field use. A second reactor was built where synthetic greywater was circulated over the nanotube array irradiated with solar light and exposed to the different climatic conditions encountered. The morphology of the surface governed the resistance of the materials against fouling. The arrays produced in the ethylene glycol were devoid of spaces between the tubes and this delayed the accumulation of debris and the resulting occlusion of the tube tops. Despite the surfaces showing extensive fouling in both aging exercises, a significant level of activity towards the degradation of both the dye methylene blue and the bacterium E.Coli were retained. The photocatalysts were also free from chemical dissolution and mechanical damage with the crystalline structure being similarly unaltered. A cleaning regime was implemented to determine the ability of the materials to recover their reactivity. Recovery rates were high for all materials with respect to the degradation of the dye and only marginal for the antibacterial activity. The material synthesised in the glycol without the silver decoration, was selected for upscaling. This material retained the highest activity after aging. The nanotube arrays were synthesised on 15 cm X 15 cm plates. These plates were installed in two different prototype flat plate photocatalytic reactors. The first reactor used UVA LED strips to activate the photocatalytic reaction. The effect of changing pollutant concentration, flowrate and the volume of solution were assessed in order to find the maximum productivity of the unit. A synthetic greywater mixture without E.Coli, paracetamol and methylene blue solutions were used as the water matrices to be treated. Another pilot reactor was installed on a rooftop to study the possibility of disinfecting greywater using sunlight. In this case the synthetic greywater mixture was seeded with E.coli. A noticeable level of degradation of the contaminants used was recorded in the UVA unit. The solar disinfection efficiency was only marginal. The efficiency of the reactors was hampered by an unfavourable ratio of photocatalyst surface area to solution volume. The nanotube arrays installed in flat plate reactor show potential for greywater treatment however further work is required before successful commercialisation of the units. Description: Ph.D.(Melit.)

2023-01-01T00:00:00Z /library/oar/handle/123456789/104268 2022-12-09T13:37:07Z 2022-01-01T00:00:00Z

Title: Duplex surface engineered CoCrMo orthopaedic alloy using commercial treatments : a corrosion-wear investigation Abstract: The increase in global population and life expectancy are leading to a rise in the need for total hip arthroplasty (THA). While metal-on-metal (MoM) implants offer a cost-effective solution, the corrosion-wear prone environment in which these operate causes the release of metallic ions and debris, and ensuing complications. In an effort to improve the corrosion-wear performance of MoM implants, in this work a low-carbon wrought ASTM F1537 CoCrMo alloy and a low-carbon cast StelliteTM 21 alloy (similar to ASTM F75 CoCrMo alloy) were surface engineered using commercially available treatments, namely low temperature carburising, and a high-power impulse magnetron sputtered Cr2N coating. The aim of applying such treatments was to limit oxidation-related losses on CoCrMo alloys due to the cyclic formation and damage of the passive film (or Type I corrosion-wear), and suppress coatingsubstrate interface corrosion leading to coating blistering (or Type II corrosion-wear). The dense Cr2N coating was deposited to inhibit any electrolytic pathway to the underlying substrate, while the low temperature carburised diffusion layer was added to reduce the propensity to coating delamination and failure by enhancing the load support to the above Cr2N layer and avoid micro-cracking while further ensuring that any electrolyte passing through the coating does not cause any localised corrosion. A corrosion-wear evaluation of the untreated, carburised, Cr2N-coated, and duplex Cr2N-coated (Cr2N/Carburised) samples was conducted using a custom-built reciprocating sliding tribometer. A novel testing configuration was employed whereby both the disc and counterface materials (jointly referred to as a “tribopair”) were metallic and surface engineered to better replicate the MoM implant conditions. Self-mated tribopairs were made to slide against each other under open circuit potential (OCP) and anodic potential (AP) conditions in both Ringer’s and diluted bovine serum (DBS) solutions at a temperature of 37 ± 1 °C under elastic contact conditions (maximum contact pressure of 437 MPa). Whilst the disc material was always composed from a wrought alloy, the use of both cast and wrought counterface CoCrMo alloys were considered. The untreated Wrought-Cast (W-C) tribopairs exhibited a superior corrosion-wear performance with lower dynamic anodic currents, coefficient of friction (CoF), and total volume losses, compared to the untreated Wrought-Wrought (W-W) tribopairs. The improved corrosion-wear behaviour is being attributed to the in situ formation of an oxidised layer that suppresses oxidation losses on the W-C tribopairs. Testing in DBS under AP conditions resulted in a reduction of around 35% and 90% in Type I corrosion-wear (𝐶𝑊) for the untreated W-C and W-W tribopairs, respectively, when compared to tests conducted in Ringer’s solution. The reduction in 𝐶𝑊 is ascribed to the spontaneous formation of a protein-rich layer which diminishes anodic dissolution and lubricates the tribological interface. When compared to the untreated tribopairs, low temperature carburising did not offer a marked improvement in either W-C and W-W configurations and test solutions. Compared to the untreated W-C tribopairs, the carburised W-C tribopairs displayed higher dynamic anodic currents under AP conditions (~20 μA compared to ~3 μA in untreated W-C), and higher CoF and material losses under both electrochemical conditions, due to the inability of the latter to form an oxidised layer. Similar dynamic anodic currents, CoF and material losses were obtained between the untreated and carburised W-W tribopairs. In Ringer’s solution, the carburised tribopairs exhibited a higher metal ion release than the untreated tribopairs, while in DBS an opposite trend was noted. The Cr2N and Cr2N/Carburised-coated materials successfully mitigated Type I when compared to the untreated and carburised tribopairs, and were also resistant to Type II corrosion-wear. The inherent chemical inertness of the Cr2N coating resulted in extremely low dynamic anodic currents (~2 μA), low metal ion release and low CoF in both test electrolytes. In the absence of third-body particles originating from coating failure either on the disc or counterface, the corrosion-wear scars were characterised by smooth polishing wear free from wear debris. Nano-scratch results and corrosion-wear testing confirm that the underlying carburised layer offers an enhanced load-bearing support to the Cr2N coating with less Cr2N/Carburised samples exhibiting coating failure than the Cr2N samples. The observed coating failures on the counterfaces were attributed to delamination wear occurring upon severe coating thinning. The hard coating debris generated upon coating failure was found to trigger roughening of the otherwise smooth scars via three-body wear. Corrosion-wear testing of ultra-high molecular weight polyethylene sliding against wrought CoCrMo alloy in a metal-on-polymer (MoP) configuration resulted in low dynamic anodic currents, low CoF and negligible material losses. Such results suggest that surface engineering the metallic alloy in a MoP configuration may not impart any substantial benefits. However, this work has clearly shown that the addition of a carburising diffusion layer beneath a dense Cr2N coating layer is promising in MoM configurations. Description: Ph.D.(Melit.)

2022-01-01T00:00:00Z