|Author:||Choy, Man Tik Dickson|
|Title:||Rapid microwave sintering of titanium composites for enhanced bioactivity|
|Subject:||Orthopedic implants -- Materials.|
Hong Kong Polytechnic University -- Dissertations
|Pages:||xxii, 199 pages : color illustrations|
|Abstract:||Materials used in orthopedics should possess good biocompatibility, a good combination of high strength and low modulus, and be without cytotoxicity. To achieve a rigid implant fixation, enhanced osteoconductive and osseointegrative characteristics are also needed. Titanium (Ti)-based composites play important roles in orthopedic implant applications, especially in repairing or replacing damaged load-bearing bone tissues. This research study is aimed at developing a rapid microwave sintering scheme to fabricate porous Ti-based composites with enhanced bioactivity. In this rapid microwave sintering scheme, a new and efficient method combining powder metallurgy, microwave heating and in situ synthesis, has been developed to prepare Ti-based composites. The sintering time can be reduced by 90 % using this method, as compared with other traditional powder metallurgy methods. Using the proposed rapid sintering scheme, two Ti-based composite systems have been fabricated, and demonstrated enhanced bioactivity and compressive properties close to natural bone. Recent developments in powder metallurgy provide an effective way for manufacturing complex shaped components with biocompatible surfaces and adequate mechanical strength for bone implant applications. The use of microwave energy for metal powder processing is a modern heating technique, with many distinctive features, such as high heating rate and high thermal efficiency. However, a major challenge in the microwave sintering of metal powder is the non-uniform distribution of the electromagnetic field inside a microwave cavity. As a result, the localized hot or cold spots developed inside a heating target can cause thermal instability. At ambient conditions, the microwave absorption of Ti powder is low at the usual operating frequency of 2.45 GHz. Therefore, microwave sintering of Ti-based materials and their composites is a difficult task.|
To overcome this challenge, the finite element method (FEM) has been applied to identify and estimate the key processing conditions and parameters required for developing a rapid microwave sintering scheme. The results of FEM analysis show that the size of the metal particles, the size of the compact, the choice of microwave susceptor and the configuration of a microwave furnace are the key factors influencing microwave sintering process. In order to realize rapid sintering of Ti-based composites, the particle sizes of the metal powder and carbon nanotube microwave susceptors are required to be within a specific range to boost the microwave absorption of the Ti green compact. A new configuration of a bidirectional microwave furnace has been designed to achieve a more homogenous heating cavity. Using the proposed scheme and the bidirectional microwave furnace, Ti-based composites can be sintered efficiently within two minutes. The drawback of excessive thermal decomposition of bioactive ingredient can also be overcome. In this study, two composite model systems, namely calcium phosphate modified titanium (Ti/CaP) and hydroxyapatite-titanium carbide modified titanium alloy (Ti6Al4V/TiC/HA), have been in situ synthesized using the proposed microwave sintering scheme. The improvement in the bioactivity of the Ti/CaP composite surfaces has been confirmed by extensive apatite formation after incubation in a simulated body fluid for 14 days. The results obtained from in vitro studies using fibroblast L929 and osteoblast-like MC3T3-E1 cells indicate that the Ti6Al4V/TiC/HA composites are non-cytotoxic and exhibit enhanced osteoconductivity, cell viability and biomineralization activity. The improvement in osseointegration of the Ti6Al4V/TiC/HA composites has been further demonstrated by in vivo experiments. The mechanical test results also show that the compressive strength and modulus of the two composite systems fall within the range of the properties of human cortical bone. This study develops a rapid sintering scheme for fabricating Ti-based composite parts with enhanced bioactivity via an in situ synthesis route. The proposed methods in the scheme are capable of speeding up the sintering process and at the same time overcoming the problem of thermal decomposition of the bioactive ingredients. Moreover, the simulation and experimental results also provide information for better understanding of microwave processing of metal powder. Investigations on the tensile and fatigue properties of the composites are recommended for future studies.
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