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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, lie 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. • Fiber laser nitriding with pulsed modulation was used to treat beta TiNb alloy. • Duty cycle changed between 40% and 100% to control and optimise surface quality. • The nitrided samples had the melt pool depth between 22 and 43 μm. • Maximum hardness of the nitrided samples lie within the range of 780–870 HV. • Crack-free and homogenous nitrided surface can be created at duty cycle of 40%. [ABSTRACT FROM AUTHOR]

A multifunctional beta TiNb surface, featuring wear-resistant and antibacterial properties, was successfully created by means of open-air fibre laser nitriding. Beta TiNb alloy was selected in this study as it has low Young's modulus, is highly biocompatible, and thus can be a promising prosthetic joint material. It is, however, necessary to overcome intrinsically weak mechanical properties and poor wear resistance of beta TiNb in order to cover the range of applications to load-bearing and/or shearing parts. To this end, open-air laser nitriding technique was employed. A control of single processing parameter, namely duty cycle (between 5% and 100%), led to substantially different structural and functional properties of the processed beta TiNb surfaces as analyzed by an array of analytical tools. The TiNb samples nitrided at the DC condition of 60% showed a most enhanced performance in terms of improving surface hardness, anti-friction, anti-wear and anti-bacterial properties in comparison with other conditions. These findings are expected to be highly important and useful when TiNb alloys are considered as materials for hip/knee articular joint implants., (Copyright © 2020 Elsevier Ltd. All rights reserved.)