Regarding the higher demand of the total joint replacement (TJR) and revision surgeries in recent years, an implant material should provide much longer lifetime without failure. Nickel titanium (NiTi) is the most popular shape memory alloy in the industry, especially in medical devices due to its unique mechanical properties such as pseudo-elasticity, damping capacity, shape memory and good biocompatibility. However, concerns of nickel ion release of this alloy still exist if it is implanted for a prolonged period of time. Nickel is well known for the possibility of causing allergic response and degeneration of muscle tissue as well as being carcinogenic for the human body beyond a certain threshold. Therefore, drastically improving the surface properties (e.g. wear resistance) of NiTi is a vital step for its adoption as orthopaedic implants. To overcome the above-mentioned risks, different surface treatment techniques have been proposed and investigated, such as Physical Vapour Deposition (PVD), Chemical Vapour Deposition (CVD), ion implantation, plasma spraying, etc. Yet all of these techniques have similar limitations such as high treatment temperature, poor metallurgical bonding between coated film and substrate, and lower flexibility and efficiency. As a result, laser gas nitriding would be an ideal treatment method as it could overcome these drawbacks. Moreover, the shape memory effect and pseudo-elasticity of NiTi from a reversible phase transformation between the martensitic phase and the austenitic phase are very sensitive to heat. Hence, NiTi implant is subjected to the following provisions of the thermo-mechanical treatment process, and this implant provides desired characteristics. It is important to suggest a surface treatment, which would not disturb the original build-in properties. As a result, the low-temperature methods for substrate have to be employed on the surface of NiTi. This present study aims to investigate the feasibility of applying diffusion laser gas nitriding technique to improve the wettability and wear resistance of NiTi as well as establish the optimization technique. The current report summaries the result of laser nitrided NiTi by continuous-wave (CW) fibre laser in nitrogen environment. The microstructure, surface morphology, wettability, wear resistance of the coating layer has been analysed using scanning electron microscopy (SEM), X-ray diffractometry (XRD), sessile drop technique, 3-D profile measurement and reciprocating wear test. The resulting surface layer is free of cracks, and the wetting behaviour is better than the bare NiTi. The wear resistance of the optimised nitride sample with different hatch patterns is also evaluated using reciprocating wear testing against ultra-high-molecular-weight polyethylene (UHMWPE) in Hanks’ solution. The results indicate that the wear rates of the nitride samples and the UHMWPE counter-part were both significantly reduced. It is concluded that the diffusion laser gas nitriding is a potential low-temperature treatment technique to improve the surface properties of NiTi. This technique can be applied to a femoral head or a bone fixation plates with relatively large surface area and movable components.

The control of cell adhesion to synthetic polymers is a key factor in tissue engineering, resting on the ability to direct specific cell types to adhere and proliferate in order to stimulate tissue reconstruction. But often the surface properties are compromised for the sake of the bulk properties, leading to surfaces that do not support sufficiently the level of bioactivity required and accordingly the polymeric biomaterial will fail clinically. Laser treatment offers a unique means of enhancing the osteoblast cell response of the surface of a polymeric biomaterial, whilst keeping the already sufficient bulk properties intact. To this end, infra-red (IR) and ultraviolet (UV) lasers have been employed to modify the wettability characteristics of nylon 6,6, as wetting is often the primary factor dictating the adhesion and bonding potential of materials, as a route to enhancing the surface in terms of osteoblast cell response. What is more, modifying wettability characteristics in this way is a highly attractive means of estimating the biofunctionality of a polymer. IR (CO2) and UV (F2 and KrF excimer) lasers were employed to carry out two different processes: laser whole area irradiative processing and laser-induced patterning. With both CO2 and the excimer lasers changes in the wettability characteristics could be effected with subsequent enhancement of osteoblast cell response. This was also the case with both laser-induced patterning and laser whole area irradiative processing. Essentially, an approach has been established whereby the osteoblast cell response on the surfaces of laser treated nylon 6,6 can be predicted through the laser-induced wettability characteristics modification, particularly for the laser whole area irradiative processed nylon 6,6. This ultimately allows one to determine the osteoblast cell response of the laser surface treated nylon 6,6 surfaces directly from the laser operating parameters. In concurrence with established wetting theory the laser whole area irradiative processing of the nylon 6,6 surfaces caused increased surface roughness, increased surface oxygen content, increased polar component, γP , and increased total surface energy, γT ; thereby generating surfaces displaying reduced contact angle, θ, making the nylon 6,6 surfaces more hydrophilic. The laser-induced patterned samples differed from current theory insofar as the nylon 6,6 surfaces became less hydrophilic due to an increase in θ despite an increase in surface roughness, an increase in surface oxygen content, an increase in γP and an increase in γT . This phenomena can be explained by the transition in wetting regimes from a Wenzel regime to a mixed-state wetting regime. Nevertheless, collation of the wettability characteristics results revealed that θ was a strong correlative decreasing function of both γP and γT , indicating that surface energy played a large role in determining the wetting nature of the nylon 6,6. It was found that for all laser whole area irradiative processed nylon 6,6 surfaces the osteoblast cell response was an increasing correlative and therefore predictive function of θ and was a decreasing function of γP . To an extent, the surface oxygen content and surface roughness could be used indirectly to foretell the osteoblast cell response of the nylon 6,6 surfaces. This is on account of the CO2 and KrF excimer laser whole area irradiative processing bringing about increased surface toxicity, which above a certain level hindered the osteoblast cell response. For the laser-induced patterned nylon 6,6 samples there did not appear to be any particular correlative trend between the modified surface parameters and osteoblast cell response. This can be accounted for by the transition in wetting regimes. Another important factor is that cell morphologies were modulated over all samples which suggests that varying surface parameters on account of laser surface treatment gave rise to variations in cell signaling. It was determined that θ, γP and γT all had very strong correlative relationships with the cytotoxicity. The cytotoxicity reduced upon an increase in θ until a minimum constant was achieved, whereas the cytotoxicity remained constant at low γP and γT until a point at which the cytotoxicity began to increase. These results are noteworthy as they allow one to deduce that, with constant cytotoxicity levels, the osteoblast cell response appeared to be modulated by the wettability characteristics. But once the cytotoxicity increased, the toxicity began to dominate and so negated the identified positive wettability characteristic correlations with osteoblast cell response. Practically, the surface roughness and surface oxygen content could be implemented indirectly to estimate the cytotoxicity. Increase in cytotoxicity was the result of the laser processing with higher fluences generating excessive melting. As a result of this, it is possible to deduce that there was a maximum threshold fluence, beyond which the toxicity of the nylon 6,6 began to dominate, giving rise to a less enhanced osteoblast cell response. On account of the correlative trends which have been identified between the laser surface treatment, wettability characteristics and osteoblast cell response of nylon 6,6 it is likely for one to have the ability to estimate the osteoblast cell response in vitro. This is significant as it indicates that laser surface modification of polymeric materials could have tremendous potential for application within the field of regenerative medicine.

Orientador: Maria Clara Filippini Ierardi / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-11T14:11:04Z (GMT). No. of bitstreams: 1 Nascimento_AlexMaiado_D.pdf: 11719577 bytes, checksum: b0fc7e160f9271407620f8cdd96e489b (MD5) Previous issue date: 2007 / Resumo: Neste trabalho estudaram-se as relações de orientação entre a austenita e ferrita de três aços inoxidáveis duplex fundidos, ASTM A890 1A, 3A e 6A, na condição solubilizada. Logo após, tratamentos superficiais de refusão a laser simples (RL) e com injeção simultânea de partículas cerâmicas (RLI) foram realizados sobre as ligas. As mudanças microestruturais e os efeitos dos tratamentos sobre as propriedades de dureza, resistência ao desgaste e à corrosão das ligas foram analisadas. As mudanças microestruturais foram caracterizadas por microscopia ótica, microscopia eletrônica de varredura com detectores de EDAX e EBSD acoplados, difração de raios-X e microdureza Vickers. Realizaram-se ensaios de desgaste do tipo pino sobre disco sobre as três ligas, nas condições superficiais: inicial, RL e RLI. A análise do comportamento corrosivo das ligas, antes e após os tratamentos a laser, foi realizada usando-se ensaios de polarização em solução de 0,5M de H2SO4 + 0,5M de NaCl na temperatura ambiente. Os resultados obtidos mostraram que as ligas na condição inicial apresentaram a existência de relações de orientação preferenciais entre a austenita e a ferrita, próxima aos modelos teóricos de Greninger-Troiano, Kurdjumov-Sachs e Bain. As superfícies tratadas por laser, apresentaram mudanças microestruturais significativas, as quais produziram durezas e resistências ao desgaste superiores as da condição inicial, e com a manutenção da elevada resistência à corrosão típica dos aços inoxidáveis duplex / Abstract: In this work was studied the orientation relations between austenite and ferrite in three cast duplex stainless steels, ASTM A890 1A, 3A, 6A on solution treatment. After that, simple laser surface remelting (LR) and laser melt injection (LMI) treatments were carried out on the alloys. The microstructural changes and the effects of laser treatments on hardness, wear resistance and corrosion resistance properties were analyzed. The microstructural changes were investigated by means of optical microscopy, scanning electron microscopy with integrated EDS and EBSD systems, X-ray diffraction and Vickers microhardness. The wear properties were measured by pin-on-disc tests on the three alloys in all different conditions : initial, LR and LMI. In order to analyze the corrosive behavior of the alloys before and after the treatments, polarization tests were carried out in the 0,5M de H2SO4 + 0,5M de NaCl solution at room temperature. The results obtained showed that the austenite and ferrite of alloys in the initial condition have orientation relations close to Greninger-Troiano, Kurdjumov-Sachs and Bain theoritical models. However, the laser treated surfaces showed accentuated microstructural changes and the microhardness and wear resistance were increased in comparing with the wear of the substrate. The three alloys in all conditions showed an excellent corrosion resistance what it is typical for the duplex stainless steels / Doutorado / Materiais e Processos de Fabricação / Doutor em Engenharia Mecânica