Laser surface modification of biomedical alloys
Chikarakara, Evans (2012) Laser surface modification of biomedical alloys. PhD thesis, Dublin City University.
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This study investigated the effects of high speed laser surface modification on 316L stainless steel and Ti-6Al-4V for biomedical implants application. Laser processing was carried out in an inert argon environment using a 1.5 kW CO2 laser. Parameters investigated in this work included irradiance, residence time, pulse width and sample pre-treatments. Surface topology, microstructure and melt pool depth were characterised using the scanning electron microscope. White light interferometry and stylus profilometry were used to determine the surface roughness. X-ray diffractometry was used to investigate the crystallinity and phase transformation induced by the laser treatment. Micro-hardness was measured using a Vickers micro-hardness indentation apparatus. Wear behaviour was investigated using a pin on disk apparatus. Corrosion behaviour was evaluated using a potentiostat and an electrochemical cell set-up simulating human body conditions. Biocompatibility of the samples was investigated in vitro by monitoring NIH/3T3 fibroblast and MC3T3-E1 osteoblast cell growth via MTT and Hoechst DNA assays.
A strong correlation between irradiance, residence time, depth of processing and roughness was established in 316L. High depth of altered microstructure and increased roughness were linked to higher levels of both irradiance and residence times. At fixed energy density, increase in residence time resulted in growth of the melt pool. In the melted region, a uniform composition in microstructure with fewer impurities was observed. In Ti-6Al-4V alloy, laser treatment resulted in crack-free layers, twenty to fifty microns thick. With increase in both irradiance and residence time, surface roughness was found to decrease while melt pool depth increased. A martensite structure formed on the laser treated region producing acicular αTi nested within the aged βTi matrix. The βTi phase volume fraction was reduced by up to 19%. Microhardness increased up to 760 HV0.05 which represented a 67% increase compared to the bulk material. A homogenous chemical composition of the alloying elements was achieved in laser modified regions. Much lower levels of wear were noted in laser treated samples compared to untreated samples. Stable passive polarisation behaviour and reduction in corrosion rates was noted in treated samples ranging between 86 and 239 nm yr-1 compared to 108 nm yr-1 for untreated samples and 309 nm yr-1 for grit blast samples. Direct contact assays showed that laser treated samples had improved cytotoxicity properties compared to their untreated counterparts.
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