A promising laser nitriding method for the design of next generation orthopaedic implants: Cytotoxicity and antibacterial performance of titanium nitride (TiN) wear nano-particles, and enhanced wear properties of laser-nitrided Ti6Al4V surfaces
AffiliationQueen's University Belfast; University of Lincoln; University of Chester; Technical University of Denmark
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AbstractIn this study, fibre laser nitriding in open air was applied to the Ti6Al4V alloy in order to improve the wear resistance, thus minimising the generation of wear debris from the surfaces for load-bearing applications. The recent technological advancement to perform the laser nitriding process in open air allows the opportunity to surface-harden any curved and/or specific areas in the hip implants. The laser nitriding process was modulated between the pulsed mode and continuous wave (CW) mode by varying the duty cycle between 60% (pulsed) and 100% (CW). Our experimental investigations were divided into two stages in sequential order: Firstly, to create crack-free, homogenous and golden laser-nitrided surfaces by the proper selection of duty cycle. Secondly, it was to analyse the properties (both physical and chemical) of the wear debris as well as to evaluate their cytotoxicity and antibacterial performance. The laser-nitrided surfaces were characterised and tested using a variety of techniques, incl. optical microscopy, SEM-EDX, XRD, surface roughness and Vickers hardness measurements, as well as tribological tests (i.e. ball-on-disk wear tests and DLS). The wear debris from the laser-nitrided surfaces (collected in the wear tests) were analysed using TEM, XPS and SEM-EDX. Their toxicity was evaluated using in-vitro cell culture with macrophages at two time points (24 h and 48 h). The antibacterial performance was tested in vitro against two of the most commonly implicated pathogens in orthopaedic infection, namely Staphylococcus aureus and Escherichia coli for 24 h. Our findings indicated that the wear resistance of the surfaces after laser nitriding was significantly improved and the amount of wear debris generated was also significantly reduced. The wear particles from the laser-nitrided surfaces were in the nano-sized scale range (0.01 µm to 0.04 µm or 10 nm to 40 nm). They were found to be less toxic towards RAW264.7 macrophages, yet display antimicrobial properties against Staphylococcus aureus, when compared with the larger particles (1.5 µm in size) from the untreated surfaces. It is envisioned that successful fabrication of the non-toxic and highly wear-resistant TiN layer in Ti6Al4V using the open-air laser nitriding technique can enable progress towards the development of metal-on-metal (MoM) hip implants fully made of Ti-based alloys
Citation"A promising laser nitriding method for the design of next generation orthopaedic implants: Cytotoxicity and antibacterial performance of titanium nitride (TiN) wear nano-particles, and enhanced wear properties of laser-nitrided Ti6Al4V surfaces", Chi-Wai Chan, James Quinn, Issam Hussain, Louise Carson, Graham C. Smith, Seunghwan Lee, Surface & Coatings Technology 405 (2021) 126714
JournalSurface and Coatings Technology
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A single parameter approach to enhance the microstructural and mechanical properties of beta Ti-Nb alloy via open-air fiber laser nitridingChan, Chi-Wai; Chang, Xianwen; Bozorgzadeh, Mohammad Amin; Smith, Graham C; Lee, Seunghwan; Queen's University Belfast, Technical University of Denmark, University of Chester (Elsevier, 2019-12-13)In this study, the idea of applying open-air laser nitriding to improve the microstructural and mechanical properties of beta Ti-45 at.% Nb alloy was demonstrated. Surface cracking after laser nitriding is one of the main reasons impeding direct translation of the laser nitriding technique from the laboratories to industries as cracks can be the weak points to initiate mechanical and corrosion failures in long-term usage. With proper selection of duty cycle (DC) between 40% (modulated mode) and 100% (continuous wave, CW mode) to control the laser energy input and laser-material-gas interaction time, the cracking problems of laser nitriding can be alleviated and even solved. A crack-free and uniformly gold-coloured nitrided surface was successfully obtained at the DC of 40% in this study. The morphology, microstructure, composition and mechanical properties of the nitrided samples were studied and analysed by optical microscopy (OM), scanning electron microscopy (SEM), SEM-energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and Vickers micro-hardness tests. The OM results indicated that minimum overlapping between the laser tracks would give desirable results to obtain the crack-free surface. The measurements from the SEM micrographs indicated the depth of the laser-nitrided areas ranged between 22 and 43 µm. The XRD findings showed that a clear conversion of the TiNb surface to a nitride as a result of laser nitriding was observed. The maximum hardness, as measured by the Vickers method in cross-sections, lay in the range of 780 to 870 HV after laser nitriding. To summarise, control of DC to obtain a crack free and quality surface via fiber laser nitriding in open air is a simpler and quicker approach in comparison with the conventional substrate preheating and nitrogen (N) dilution approaches. The single-parameter approach is more efficient than parameter optimisation via design of experiments (DOE) employed in conventional methods.
Modifications of surface properties of beta Ti by laser gas diffusion nitridingNg, Chi-Ho; Chan, Chi-Wai; Man, Hau-Chung; Waugh, David G.; Lawrence, Jonathan; University of Chester; Queen's University; The Hong Kong Polytechnic University (AIP Publishing, 2016-03-31)b-type Ti-alloy is a promising biomedical implant material as it has a low Young’s modulus and is also known to have inferior surface hardness. Various surface treatments can be applied to enhance the surface hardness. Physical vapor deposition and chemical vapor deposition are two examples of this but these techniques have limitations such as poor interfacial adhesion and high distortion. Laser surface treatment is a relatively new surface modification method to enhance the surface hardness but its application is still not accepted by the industry. The major problem of this process involves surface melting which results in higher surface roughness after the laser surface treatment.This paper will report the results achieved by a 100W CW fiber laser for laser surface treatment without the surface being melted. Laser processing parameters were carefully selected so that the surface could be treated without surface melting and thus the surface finish of the component could be maintained. The surface and microstructural characteristics of the treated samples were examined using x-ray diffractometry, optical microscopy, three-dimensional surface profile and contact angle measurements, and nanoindentation test.
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