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Title: Titanium foams via metal injection moulding in combination with a space holder
Author: Shbeh, Mohammed
ISNI:       0000 0004 7428 2581
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2018
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In the last decade, there has been increasing interest in the production and characterisation of titanium foams due to the unique combination of properties that are offered by these advanced materials, such as extra light weight, high compressive strength, good energy absorption and high permeability, particularly for open celled foams. These foams have huge potential in a variety of applications, especially for biomedical applications as they offer the possibility to address the stress shielding problem by tailoring their mechanical properties to match that of the human bone. The latter phenomenon results from the non-uniform partitioning of load between the implant and the surrounding tissue or bone as a result of the large difference between the elastic moduli of the dense implant and the bone. Ti foams are mostly produced by powder metallurgical foaming techniques, due to the high melting temperature of Ti and its extreme susceptibility to contamination during melting. One major challenge in producing Ti foams is the inability to easily shape these foams after production to the desired shape without resulting in the closure and smearing of the pores and hence, altering the pore morphology, while creating residual stresses and breaking some of the cells and deteriorating the mechanical properties. This problem could be addressed by developing a foaming technique that makes it feasible to produce foams with the final shapes desired for the component. One promising technique with great potential for solving this problem, which also has the ability to mass produce net shaped open celled Ti foams, is Metal Injection Moulding in combination with a Space Holder (MIM+SH). This technique combines MIM technology, which is commonly used for producing net shaped complex parts in large quantities, with the space holder technique, thus ensuring flexible design with different pore morphologies. The main aim of the project was to investigate the use of MIM+SH technique for the manufacture of Ti foams with different mesostructures with the potential for biomedical applications. The foams produced were characterized using Scanning Electron Microscopy (SEM), Micro-Computed Tomography (Micro-CT) and X-ray Fluorescence (XRF). The results showed that it is possible to produce net shaped Ti foams with different volume percentage of porosities and pore morphologies and with a structure that replicates the structure of the natural bone in having a compact outer layer for increased strength and inner spongy layer for nutrient exchange. The volume percentage of porosity in the foams produced was in the range of 20-64%. In addition, it was found that the shape of the space holder does not have a significant impact on the percentage of final porosity in the samples. However, it has an influence on the mechanical properties of the foams produced, where foams made by spherical space holder was found to have a higher yield stress than those made with the cubic space holder. Digital Image Correlation (DIC) test was also carried out in order to analyse the failure mechanism and figure out the distribution of strain across the samples. The analysis showed that much of the strain is concentrated at 45° in the samples and thus failure is likely to occur by plastic deformation leading to the growth of cracks in these regions driven by the shear forces. After that two approaches were investigated to increase the bioactivity and to potentially address the bioinertness of Ti, which can be an obstacle towards inducing full integration between the bone and implant. The first approach involved introducing a bioactive material known as Hydroxyapatite (HA) into the structure of the foams, while in the second approach the foams were treated using Plasma Electrolytic Oxidation (PEO) to develop a ceramic coating. It was found that adding HA to the Ti can induce brittleness in the structure and reduces the load bearing ability of the titanium foams by resulting in weak ceramic phases. The extent of this brittleness depends on the amount of HA added to the structure. While PEO treatment of the Ti foams produced develops thick surface layers that penetrate through the inner structure of the samples forming a network of surface and subsurface coatings. The results are of potential benefit in producing surface engineered porous samples for biomedical applications which do not only address the stress shielding problem, but also improve the chemical integration.
Supervisor: Goodall, Russell ; Todd, Iain ; Yerokhin, Aleksey Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available