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

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Dashtbozorg, Behnam (2021). Development of durable antibacterial stainless steel surfaces through plasma nitriding and ultrashort pulsed laser texturing. University of Birmingham. Ph.D.

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Abstract

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.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):