Abstract
This work comprises of interdisciplinary research on improving the mechanical and biological properties of medical-grade titanium (Ti-6Al-7Nb) alloy for the first-time by using laser shock peening (LSP). Over the years, LSP has been used to enhance the mechanical properties predominantly in the aerospace sector. However, due to the benefits which it could offer, such as enhanced wear, fatigues, hardness, corrosion resistant, just to name a few; it can also be beneficial in the medical sector. Implants failures due to the lack of osseointegration or wear resistance have brought tremendous pain and costs for patients after the primary and often second implant replacement surgeries. In order to avoid such revision surgeries, it is important to extend the service life of the implant to make sure that the implants will not fail until the patients’ demise. With that said, this research is focused on both laser shock peening a medical-grade titanium Ti-6Al-7Nb utilised for orthopaedic implants by employing a nanosecond pulse Nd: YAG laser to modify the mechanical properties and enhance bio-compatibility. The process parameters applied were 3J, 5J and 7J at overlaps of 33%, 50% and 67% at 3 mm spot size, 5 Hz pulse repetition rate, a 1064nm wavelength and 20ns pulse duration.The microstructure examination after laser shock peening showed the average sub-grain size was reduced by 12.5% to 39% for the range of laser energies that were applied. Due to the non-uniform misorientation distributions, heat treatment was conducted to obtain equilibrium and globular microstructure. After heat treatment, the same LSP parameters were applied to the Ti-6Al-7Nb surfaces. The misorientations were dramatically increased after LSP, as evident from the kernel average map (KAM) map. Twinning deformation can be found after the LSP parameter of 7J, 50% for 10 impacts at a spot size of 3 mm.
A range of compressive residual stresses (-42MPa to -512MPa), after LSP were formed at the near-surface region, as characterized by Incremental Hole Drilling (IHD) method. The data was verified for the first-time, by Artificial Neural Network (ANN) technique using a gradient descent learning algorithm. The accuracy of the prediction data are 96.16% and 95.16%. In the case of microhardness, the laser shock peened surfaces were hardened from 14% to 26.5%. The fretting wear in simulated body fluid was conducted using a tribometer to measure the wear behaviour of Ti-6Al-7Nb, before and after LSP. It was found that optimal LSP parameters can improve the wear resistance. What is more, sliding wear behaviour of Ti-6Al-7Nb, subjected to multiple LSP showed that the wear resistance performance was improved by 44%, after 3 impacts.
The surface morphologies and topographies after LSP were also investigated. The surface roughness was increased after LSP due to the dimpling effect particularly at higher laser energies creating high shock pulse pressure. This influenced the contact angle which was measured with the needle-in method using two different liquids (distilled water, ethylene glycol). Surface-free energy including dispersion and polar components, with subsequent calculation of the work of adhesion were acquired. The results showed that laser energy and overlap were inversely proportional to contact angle but proportional to surface free-energy and work of adhesion. The correlation between laser energy (pressure pulse) and contact angle can be explained by Wenzel’s theory, while the relationship between overlap and contact angle is described by the Cassie-Baxter model.
The osteoblast-like MG63 cells were employed to measure the cell viability using MTT assay. The cells were seeded and cultured for 24hrs and 72hrs on the untreated, laser shock peened surfaces and the culture plate as the positive control. The cell morphologies were characterized by fluorescence microscope. The images indicated that the cells can be attached to the surface post LSP, and LSP is not cytotoxic to MG 63 cells. The MTT results show that optimal LSP parameter 5J, 33% and 5J, 50% at the spot size of 3mm can improve cell viability. With considering the wetting properties, the optimal contact angle after LSP is 71.5°.
This research has shown that both mechanical strength and biocompatibility can be controlled and improved for orthopaedic Ti-6Al-7Nb alloy. However, it is not necessary that highest LSP induced compressive residual stress yields the best biological response by the material. Although, a compromise between both can render not only stronger, durable and wear resistant next generation implants, but also those that are more biocompatible. The findings of this research can now be adopted to develop the technique to both strengthen and enable medical metallic implants more bio-compatible, which in turn, improves the standard of living of the end-users.
| Date of Award | Nov 2020 |
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| Original language | English |
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| Supervisor | Jonathan Lawrence (Supervisor) & Pratik Shukla (Supervisor) |