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Nanosecond laser processing of a Zr-based bulk metallic glass for potential orthopaedic implant applications

Jiao, Yang 2020. Nanosecond laser processing of a Zr-based bulk metallic glass for potential orthopaedic implant applications. PhD Thesis, Cardiff University.
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With continuously improving living standards, the demand for biomaterials in orthopaedic implant applications has increased substantially in recent years. Bulk metallic glasses (BMGs) have recently emerged as promising candidate biomaterials due to their unique mechanical and chemical properties as well as attractive processing capabilities, which result from their amorphous nature. A number of studies exploring the bio-compatibility of BMGs have been reported in the literature, the outcome of which has generally shown the potential of Zr-based, Mg-based and Ti-based BMGs for bio-medical applications. At the same time, surface treatments have also been successfully used on traditional biomedical alloys, especially titanium alloys, to further enhance their bio-compatibility. In this context, the focus of this Thesis was on applying a number of nanosecond (ns) laser surface processing techniques on a specific type of Zr-based BMG, also known as Vitreloy 105, to investigate the potential of this combination of material and surface treatments for the development of a new candidate biomaterial. In the first part of the thesis, reported in Chapter 4, a 2D theoretical model was developed to simulate the thermal history within a Vitreloy 105 workpiece in the context of multiple and moving ns pulses irradiation. In particular, the model was used to predict the ablated feature dimensions and to reveal possible thermal events occurring during the laser micromachining process that could explain the formation of the resulting topography. The combined experimental and theoretical approach also enabled the identification of a suitable set of laser parameters with respect to the process efficiency over a range of pulse durations, fluence values, scanning speeds and track distances between machined grooves. Abstract iii In the second part of the thesis, which is the focus of Chapter 5 and 6, laser surface texturing was employed to fabricate two types of surface patterns, dimple and groove, to study the resulting wettability of Vitreloy 105, and associated bio compatibility characteristics. Firstly, the effect of different laser parameters, including laser fluence, scanning speed, surface texture type and track distance, on the wettability of this BMG was investigated. Following this, through in-vitro cell culture experiments, the bio-compatibility of Vitreloy 105 samples with different surface textures was assessed. The underlying mechanism associated with laser surface texturing which affected the in-vitro bio-compatibility of the material was discussed based on the laser-induced modifications to surface chemistry and surface roughness. In line with the obtained wettability results, the bio-compatibility findings showed that cells on groove-textured surfaces had a higher viability and better adhesion compared to those on the as-cast and dimple-textured surfaces. Moreover, it was also found that cells aligned along the direction of laser-induced groove patterns. In the last part of the thesis, presented in Chapter 7, it was confirmed that laser surface melting (LSM) could be employed to modify the hardness and the shear banding behaviour of Vitreloy 105. More specifically, by conducting ns LSM operations in ambient atmosphere, it was found that surface hardening could be achieved, in addition to the well-known surface softening effect. In addition, it was found that the presence of compressive residual stress and an increased introduction of crystalline precipitates accompanied LSM-induced surface hardening. On the contrary, tensile residual stress and a reduced fraction of crystalline precipitates were observed for the softened surface post-LSM. Finally, differences in shear-banding mechanisms were detected near the surface of the laser irradiated regions. More specifically, overall reduced serrated flow but important surface shear bands events were observed following the LSM-based introduction of compressive residual stress. In contrast, more pronounced serrated Abstract iv flows and the likely distribution of shear banding activity well beneath the irradiated BMG surface was promoted when LSM resulted in the introduction of tensile residual stress. Overall, the results reported in this thesis suggest that nanosecond laser-based surface micromachining, texturing and melting represent promising methods for promoting the potential application of Vitreloy 105 as orthopaedic implants.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: Zr-based Bulk metallic glass; Laser surface melting; Laser micromachining; Wettability; Bio-compatibility; Mechanical Properties.
Date of First Compliant Deposit: 14 September 2020
Last Modified: 25 May 2021 01:25

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