Development of durable antibacterial stainless steel surfaces through plasma nitriding and ultrashort pulsed laser texturing

Owing to their excellent corrosion resistance, austenitic stainless steels (ASS) have become an important material within the food and medical industries. In particular, the AISI 316L alloys (which are used in this study) are abundantly found in applications where harsh chemical environments are unavoidable. However, due to their poor wear resistance and susceptibility to bacterial colonisation, there are concerns for their further uptake in the future. Low-temperature plasma nitriding can address the poor durability of the ASS alloys by forming the S phase on the surface of the materials, and therefore, providing combined improvement in hardness, wear resistance and corrosion resistance. However, this does not alleviate the worries for biofouling of the surface. Pulsed laser texturing presents a promising and scalable approach for the introduction of functional antibacterial properties on surfaces. However, due to the thermal nature of laser patterning and the thermodynamic metastability of the S phase, almost no research has been conducted prior to this study on the combination of these technologies. Therefore, in this thesis, detailed characterisation of the response of S phase treated surfaces to ultrashort (nano and femtosecond) laser texturing has been carried out. The results within this study have shown that it is possible to texture S phase treated surfaces using femtosecond (fs) pulsed laser texturing with no discernible decomposition or detrimental consequences to the layer structure, hardness or corrosion resistance of the S phase. Given these new findings, and the need to produce durable antibacterial ASS alloys, the focus of this study was then directed towards the identification of bio-inspired surface textures with strong antibacterial efficacy. Four biomimetic fs laser produced structures inspired by features found on lotus leaves, shark skin and springtails were compared to identify an optimal texture. Measurement of the antimicrobial efficacy against S. aureus demonstrated that the springtail inspired triangular LIPSS (laser-induced periodic surface structures) possessed the strongest resistance (over 90 % reduction of viable bacteria). Finally, to fully examine the antibacterial durability of the surfaces, comparisons of wear resistance, texture integrity and long-term antibacterial efficacy against S. aureus and E. coli were carried out on triangular LIPSS textures formed on untreated and S phase treated AISI 316L alloys. Significant improvements of all properties were found with S phase hardened and triangular LIPSS textured surfaces. Using a novel, yet simple, approach to produce uniformly worn textures, which has been a challenge for researchers thus far, it has been possible, within this study, to examine the antibacterial performance of the textured surfaces as a function of surface damage. Almost instantaneous removal of the fine features of the triangular LIPSS, and associated antibacterial efficacy, was observed on untreated samples. On the other hand, retention of texture features and long-lasting antibacterial performance, with at least two fold increase in antibacterial durability when compared to untreated equivalent surfaces, was found on triangular LIPSS textures produced on S phase treated surfaces. Thus, the findings of this thesis are hoped to pave the way towards the generation of long-lasting antibacterial, and possibly multi-functional, stainless steel surfaces for use in the food and medical industries

Processing conditions and mechanisms for the plasma defect-engineering of bulk oxygen-deficient zirconia

In recent years, the utilisation of oxygen-deficient zirconia (ZrO2-α), commonly referred to as black zirconia, has garnered considerable attention due to its potential applications for solid oxide fuel cells (SOFCs), gas sensors, biomedical implant materials, and photocatalysis. However, current methods employed to manufacture ZrO2-α exhibit noticeable limitations regarding their scalability, environmental sustainability, and cost-effectiveness. Our recent work has successfully demonstrated the feasibility for bulk conversion of conventional white zirconia into oxygen-deficient black zirconia through direct current (DC) plasma treatment (i.e. plasma blackening). This study elucidates the conditions for plasma blackening and provides a unique mechanism for the bulk transformation of zirconia. A systematic investigation of different plasma technologies (DC, active-screen plasma), treatment configurations (contact conditions, cathode material, and cathode potential), and treatment parameters (voltage, temperature, duration) uncover the crucial variables that influence the feasibility and rate of the reduction process. The reduction of zirconia is shown to initiate from localised contacting points at the cathode-facing surface and grow, with a hemispherical shape, towards the anode-facing surface. A series of development stages are proposed for the process, namely: bulk oxygen vacancy conductance, surface activation, oxygen vacancy generation and a moving cathode front. The findings of this study provide insights into the underlying mechanisms involved in the bulk-reduction of zirconia and help to pave the way towards future scalable and cost-effective generation of oxygen-deficient zirconia

Plasma-induced repair of macro-indentation formed cracks on yttria-stabilised zirconia

Low-pressure (300 Pa) plasma treatments (N2 gas at 500 °C for 10 h) offer a novel approach to induce crack-healing phenomena, at both the surface and sub-surface, of yttria-stabilised zirconia (YSZ). The plasma-induced restoration process is visualised by surface and transverse imaging using a combination of optical and SEM imaging with in-situ laser milling. XRD and Raman mapping of restored cracks (created using 5 and 20 kgf Vickers indentations) reveal that plasma treatments lead to significantly greater tetragonal to monoclinic phase transition surrounding indents, which enable cracks to close beyond the inherent passive transformation toughening behaviour of YSZ. The findings of this study can help to pave the way towards future repair of damaged YSZ surfaces without the use of healing agents

Nanomaterial-Enhanced Sizings:Design and Optimisation of a Pilot-Scale Fibre Sizing Line

This study focuses on the development of a pilot-scale sizing line, including its initial design and installation, operational phases, and optimization of key process parameters. The primary objective is the identification of critical parameters for achieving a uniform sizing onto the fibres and the determination of optimal conditions for maximum production efficiency. This investigation focused on adjusting the furnace desizing temperature for the removal of commercial sizing, adjusting the drying temperature, as well as optimizing the corresponding residence time of carbon fibres passing through the furnaces. The highest production rate, reaching 1 m sized carbon fibres per minute, was achieved by employing a desizing temperature of 550 °C, a drying temperature of 250 °C, and a residence time of 1 min. Furthermore, a range of sizing solutions was investigated and formulated, exploring carbon-based nanomaterial types with different surface functionalizations and concentrations, to evaluate their impact on the surface morphology and mechanical properties of carbon fibres. In-depth analyses, including scanning electron microscopy and contact angle goniometry, revealed the achievement of a uniform coating on the carbon fibre surface, leading to an enhanced affinity between fibres and the polymeric epoxy matrix. The incorporation of nanomaterials, specifically N2-plasma-functionalized carbon nanotubes and few-layer graphene, demonstrated notable improvements in the interfacial shear properties (90% increase), verified by mechanical and push-out tests

Effect of µplasma modification on the wettability and the ageing behaviour of glass fibre reinforced polyamide 6 (GFPA6)

Glass fibre reinforced polyamide 6 (GFPA6) thermoplastic composites (TPCs) are promising materials with excellent properties, but due to their low surface free energy they are usually difficult to wet, and therefore, possesses poor adhesion properties. μPlasma modification offers potential solutions to this problem through functionalisation of the GFPA6 surface. In this study, the effect of μPlasma on the wetting behaviour of GFPA6 surfaces was investigated. Following single μPlasma treatment scans of GFPA6 samples, a substantial enhancement in wettability was observed. However, the effect of the μPlasma modification was subject to an ageing (hydrophobic recovery) phenomenon, although the enhancement was still partially maintained after 4 weeks. The ageing process was slower when the GFPA6 material was pre-dried and stored in low humidity conditions, thereby demonstrating the importance of the storage environment to the rate of ageing. Orientation of the fibres to the observed contact angle was found to be crucial for obtaining reproducible measurements with lower deviation. The influence of testing liquid, droplet volume and surface texture on the repeatability of the measured contact angle were also investigated