This thesis presents an investigation of novel laser assisted micromachining processes that addresses the limitations of micromachining of hard-to-machine materials. Two different laser assisted approaches are used to machine hard metals and high strength ceramics. For hard metals, the basic approach involves localized thermal softening of the workpiece material by focusing a solid-state continuous wave near infra-red laser beam in front of the micro milling tool (end mills of 0.1 to 0.5 mm diameter). By suitably controlling the laser power, spot size and scan speed, it is possible to produce a sufficiently large reduction in the flow strength of the work material and consequently the cutting forces and tool deflections. A force model is developed to predict the cutting forces in Laser Assisted Micro Milling (LAMM) of hard metals. For high strength ceramics, the approach involves use of a two step process. In the first step, thermal cracks are generated in a confined volume by the steep thermal gradients generated by laser irradiation of the workpiece. In the second step, the weakened region is removed by a micro grinding tool. The characterization and modeling of the process serve as bases for users of the two approaches to select optimal process parameters.

La bioimpression assistée par laser (LAB) permet de positionner des gouttesde cellules avec une précision micrométrique. Il est ainsi possible de donner uneorganisation initiale aux cellules au sein d’une structure tissulaire 3D. Notre objectif estd’utiliser le LAB pour reproduire l’histo-architecture du stroma cornéen. Le stroma cornéenest un assemblage transparent de lamelles d’une épaisseur totale de 500 μm. Au sein dechaque lamelle, les fibres de collagène ont une même direction, un même diamètre et sontrégulièrement espacées grâce à la présence de protéoglycanes spécifiques du stromacornéen. Pour reproduire cette organisation, nous avons fait l’hypothèse qu’en alignant desfibroblastes du stroma sur un hydrogel de collagène à l’aide du LAB, il serait possibled’aligner les fibres de collagène dans la même direction. Du fait que les cellules impriméessont vivantes et dynamiques, le motif cellulaire initialement imprimé est soumis à desprocessus d’auto-organisation. Il a donc fallu déterminer les paramètres, à la foisd’impression et de culture, permettant d’obtenir de façon reproductible des alignements decellules stables dans le temps. Grâce à la microscopie à génération de secondeharmonique, le remaniement des fibres de collagène par les fibroblastes cornéens a pu êtreobservé. La direction des fibres de collagène correspond à celle de l’alignement cellulaire.En imprimant les fibroblastes de cornée sur des couches successives de collagène, noussommes parvenus à reproduire les variations de direction des fibres de collagène d’unelamelle à l’autre qui sont observées dans le stroma cornéen natif. / Laser-Assisted Bioprinting allows positioning of cell droplets with amicrometric precision. It is thus possible to give an initial organization to the cells within a3D tissue structure. Our objective is to use LAB to reproduce the corneal stroma histoarchitecture.The corneal stroma is a transparent assembly of lamellae with a totalthickness of 500 μm. Within each lamella, collagen fibers have the same direction, thesame diameter, and a regular spacing thanks to the presence of proteoglycans which arespecific from the corneal stroma. To reproduce this organization, we make the hypothesisthat through corneal fibroblasts alignment, using LAB, on a collagen hydrogel, it would bepossible to align collagen fibers in the same direction. Because printed cells are alive anddynamic, the cell pattern initially printed is subjected to self-organization processes. It isthus necessary to determine the printing and culture parameters that promote reproducibleand stable cell alignments. By using second harmonic generation microscopy, collagenfiber reorganization by corneal fibroblasts has been observed. Collagen fiber direction ismatching with cell alignment. Corneal fibroblasts have been printed on successive collagenlayers; it allows reproducing the variations in collagen fiber direction from one lamella toanother that are observed in the native corneal stroma.

Doctor of Philosophy / Department of Industrial & Manufacturing Systems Engineering / Shuting Lei / Laser-assisted machining (LAM) is a promising non-conventional machining technique for advanced ceramics. However, the fundamental machining mechanism which governs the LAM process is not well understood so far. Hence, the main objective of this study is to explore the machining mechanism and provide guidance for future LAM operations. In this study, laser-assisted milling (LAMill) of silicon nitride ceramics is focused. Experimental experience reveals that workpiece temperature in LAM of silicon nitride ceramics determines the surface quality of the machined workpiece. Thus, in order to know the thermal features of the workpiece in LAM, the laser-silicon nitride interaction mechanism is investigated via heating experiments. The trends of temperature affected by the key parameters (laser power, laser beam diameter, feed rate, and preheat time) are obtained through a parametric study. Experimental results show that high operating temperature leads to low cutting force, good surface finish, small edge chipping, and low residual stress. The temperature range for brittle-to-ductile transition should be avoided due to the rapid increase of fracture toughness. In order to know the temperature distribution at the cutting zone in the workpiece, a transient three-dimensional thermal model is developed using finite element analysis (FEA) and validated through experiments. Heat generation associated with machining is considered and demonstrated to have little impact on LAM. The model indicates that laser power is one critical parameter for successful operation of LAM. Feed and cutting speed can indirectly affect the operating temperatures. Furthermore, a machining model is established with the distinct element method (or discrete element method, DEM) to simulate the dynamic process of LAM. In the microstructural modeling of a β-type silicon nitride ceramic, clusters are used to simulate the rod-like grains of the silicon nitride ceramic and parallel bonds act as the intergranular glass phase between grains. The resulting temperature-dependent synthetic materials for LAM are calibrated through the numerical compression, bending and fracture toughness tests. The machining model is also validated through experiments in terms of cutting forces, chip size and depth of subsurface damage.

<p>Due to the small dimensions, nanoparticles and materials consisting of nano-sized building blocks exhibit unique — mostly superior — properties, well differing from their bulk counterpart. Most of the novel properties of nanoparticles (and nanomaterials) are size-dependent, while the majority of the common gasphase methods used for generation of nanopowders result in different, usually wide, size-dispersions. Further understanding of the fundamental processes leading to particle formation is therefore required, leading to better control of size and distribution of the nanoparticles, thus allowing engineering of the desired properties for both nanoparticles and nanomaterials.</p><p>In this present thesis, nanoparticles were produced by two different gasphase techniques activated by lasers, namely laser chemical vapor deposition (LCVD) and pulsed laser ablation (PLA). Optical emission spectroscopy (OES) was performed on thermal (blackbody-like) radiation originating from laser-excited particles during LCVD and coupled to measured size-distributions. In-situ monitoring of size-distributions by a differential mobility analyzer (DMA) was employed during PLA. In addition, deposited nanoparticles were characterized by a variety of standard techniques.</p><p>Different cooling mechanisms of the laser-excited gasphase particles were identified based on temperature and emitted intensity data extracted from OES measurements. The strong evaporation at elevated temperatures also allowed direct size manipulation of the particles. By monitoring the intensity of the emitted thermal radiation and the scattered laser line, strong indications about the so called coagulation limit, where a broadening of the size-distribution occurred, was obtained. The DMA monitoring, supported by modeling, gave information about different mechanisms (thermal and photochemical) of the ablation process, and particle condensation well below the ablation threshold was also found.</p>

Due to the small dimensions, nanoparticles and materials consisting of nano-sized building blocks exhibit unique — mostly superior — properties, well differing from their bulk counterpart. Most of the novel properties of nanoparticles (and nanomaterials) are size-dependent, while the majority of the common gasphase methods used for generation of nanopowders result in different, usually wide, size-dispersions. Further understanding of the fundamental processes leading to particle formation is therefore required, leading to better control of size and distribution of the nanoparticles, thus allowing engineering of the desired properties for both nanoparticles and nanomaterials. In this present thesis, nanoparticles were produced by two different gasphase techniques activated by lasers, namely laser chemical vapor deposition (LCVD) and pulsed laser ablation (PLA). Optical emission spectroscopy (OES) was performed on thermal (blackbody-like) radiation originating from laser-excited particles during LCVD and coupled to measured size-distributions. In-situ monitoring of size-distributions by a differential mobility analyzer (DMA) was employed during PLA. In addition, deposited nanoparticles were characterized by a variety of standard techniques. Different cooling mechanisms of the laser-excited gasphase particles were identified based on temperature and emitted intensity data extracted from OES measurements. The strong evaporation at elevated temperatures also allowed direct size manipulation of the particles. By monitoring the intensity of the emitted thermal radiation and the scattered laser line, strong indications about the so called coagulation limit, where a broadening of the size-distribution occurred, was obtained. The DMA monitoring, supported by modeling, gave information about different mechanisms (thermal and photochemical) of the ablation process, and particle condensation well below the ablation threshold was also found.

Master of Science / Industrial & Manufacturing Systems Engineering / Shuting Lei / Transparent brittle materials such as glass and silicon dioxide have begun to replace the conventional materials due to the advantageous properties including high strength and hardness, resistance to corrosion, wear, chemicals and heat, high electrical isolation, low optical absorption, large optical transmission range and biocompatibility. However because these materials are extremely hard and brittle, development of an ideal machining process has been a challenge for researchers. Non-traditional machining processes such as abrasive jet and ultrasonic machining have improved machining quality but these processes typically results with issues of poor surface integrity, high tool wear and low productivity. Therefore a machining technique that overcomes the disadvantages of existing methods must be developed. This study focused primarily on improving the machinability and attaining crack-free machined surfaces on transparent brittle materials by inducing micro cracks or seed damages on the subsurface of the materials. The hypothesis was that micro-cracks induced by femtosecond laser would synergistically assist the material removal process by a cutting tool by weakening or softening the material, followed by conventional machining process. Laser induced damages due to varying laser intensities and at different depths in bulk BK7 glass was studied in order to select the optimal laser machining conditions for the experiments. Dimensional and structural profiles of laser cracks are observed using an optical microscope. A comparative study of machined untreated BK7 samples and damage induced BK7 samples was conducted. Due to its simple process kinematics and tool geometry, orthogonal machining is used for the study. Results showed that machining laser-treated samples caused an average 75% force reduction on comparison to machining of untreated samples. Laser treated machined samples were produced without subsurface damages, and reduced tool wear was noted. Overall improved machinability of BK7 glass samples was achieved.

<p dir="ltr">Developing functional metallic nanostructured surfaces has seen significant growth in various applications, including sensors, electronics, and biomedical devices. However, conventional fabrication techniques for these nanostructures face limitations such as complexity, high costs, and unstable coatings. Laser-assisted surface processing has emerged as a promising solution to address these challenges by enabling localized processing and modification without altering bulk properties. This dissertation focuses on the development of multifunctional devices using selective laser processing of metallized surfaces, categorized into three routes. The first part explores the utilization of laser-induced oxides (LIO) for simple processing and formation of functional metal oxide nanostructures as electrochemical sensing elements. Different laser processing conditions were systematically studied for cost-effective metals like copper and nickel, evaluating their potential as non-enzymatic glucose sensors. The second part investigates laser selective processing for removing metal coatings on temperature-sensitive substrates, providing a cost-effective and scalable alternative to conventional photolithography and etching processes. Various laser processing conditions were examined to achieve selective patterning of metalized fabric structures for wearable electronics production. The third part explores localized laser processing to create intermetallic nanotexturing mixtures without altering bulk properties. The study involved silver spray- coating onto titanium implants, followed by a post-laser processing. The aim was to achieve simultaneous texturing and intermixing of silver in titanium alloy structures, enhancing antibacterial properties and bone mineralization while preserving mechanical properties.</p><p dir="ltr">Through the comprehensive examination of these three routes, this dissertation demonstrates the immense potential of commercial laser processing systems in the design, fabrication, and characterization of functional metallic nanostructured surfaces. It emphasizes the often-overlooked aspect of chemical alterations in laser-assisted surface processing, bridging the gap between physical and chemical modifications. The research opens new avenues for the development and optimization of multifunctional devices in electronics and biomedical applications.</p>

Doctor of Philosophy / Department of Industrial & Manufacturing Systems Engineering / Shuting Lei / This study experimentally and numerically investigates laser assisted milling (LAMill) of silicon nitride ceramics. Experiments are conducted to study the machinability of Si3N4 under LAMill. The effects of temperature on cutting forces, tool wear, surface integrity, edge chipping and material removal mechanisms are investigated. It is shown that when temperature increases, cutting force and tool wear are significantly decreased, surface integrity is improved, chip size is increased and material removal demonstrates more plastic characteristics. The mechanisms of edge chipping at elevated temperature are investigated theoretically and experimentally. When temperature is above the softening point and below the brittle/ductile transition temperature, the mechanism is mainly through softening. When temperature is above the brittle/ductile transition temperature, toughening mechanism contributes significantly to the reduced edge chipping. The coupled effect of softening and toughening mechanisms shows that temperature range between 1200 to 1400°C has the most significant effect to reduce edge chipping. Distinct element method (DEM) is applied to simulate the micro-mechanical behavior of Si3N4. First, quantitative relationships between particle level parameters and macro-properties of the bonded particle specimens are obtained, which builds a foundation for simulation of Si3N4. Then, extensive DEM simulations are conducted to model the material removal of machining Si3N4. The simulation results demonstrate that DEM can reproduce the conceptual material removal model summarized from experimental observations, including the initiation and propagation of cracks, chip formation process and material removal mechanisms. It is shown that material removal is mainly realized by propagation of lateral cracks in machining of silicon nitride. At the elevated temperature under laser assisted machining, lateral cracks are easier to propagate to form larger machined chips, there are fewer and smaller median cracks therefore less surface/subsurface damage, and crushing-type material removal is reduced. The material removal at elevated temperature demonstrates more plastic characteristics. The numerical results agree very well with experimental observations. It shows that DEM is a promising method to model the micro-mechanical process of machining Si3N4.

Doctor of Philosophy / Department of Physics / Uwe Thumm / The time-dependent Schrödinger equation is integrated on a numerical lattice for up to three-dimensional problems. The wave packet propagation technique has been applied to ion – atom collisions in a strong laser field, the vibrational nuclear motion in small homonuclear diatomic molecular ions, and for the scattering of an ion in front of a metallic surface. For laser-assisted proton – hydrogen collisions it is shown, that strong circularly polarized radiation significantly alters the capture and ionization probabilities and results in a dichroism with respect to the helicity. In a pump – control – probe scheme, “stroboscopic” exposure of a nuclear wave packet of the deuterium molecular ion by a single or a series of short and intense laser control pulses may be used to produce an almost stationary distribution of a single vibrational level, where the nodal structure can be tested using the Coulomb explosion imaging technique. Using a pump – probe setup with variable probe delays it is proposed to use Fourier analysis of the time dependence of the Coulomb explosion kinetic energy release spectrum to reveal insight into the initial vibrational state distribution for small diatomic molecules. A last application demonstrates, that resonant charge transfer for scattering of a negative hydrogen anion on a metal surface depends crucially on the position of surface and image states relative to the conduction and valence band, thereby implying different reaction mechanisms for different surface cuts of a metal.

Le développement des Interventions Médicales Assistées par ordinateur (CAMI) est le résultat d'évolutions convergeantes dans les domaines de la médecine, physique, biomatériaux, électronique, informatique et robotique. CAMI visent à fournir les outils qui permettent au clinicien d'utiliser des données multi-modales de manière rationnelle et quantitative pour planifier, simuler et exécuter des interventions médicales mini-invasives avec précision et sans risque. Parallèlement, les avancées technologiques dans les domaines de l’automatisation, la miniaturisation, la conception assistée par ordinateur et l'usinage ont aussi mené au développement des technologies telles que la bioimpression assistée par ordinateur permettant une impression couche par couche de biomatériaux avec une géométrie contrôlée dans l’espace. Ces résultats ouvrent la voie pour l’utilisation des technologies de bioimpression pour des Interventions Médicales Assistées par ordinateur plus précises et sans risque. Dans ce travail, nous montrons que des constructions tissulaires 3D peuvent être imprimées in vivo et in situ et adaptées à la morphologie d’un défaut. Les résultats ont montré que l'impression de cellules in situ avec une résolution à l’échelle cellulaire a tendance à orienter la réparation tissulaire. / The development of Computer-Assisted Medical Interventions (CAMI) results from converging evolutions in medicine, physics, materials, electronics, informatics and robotics. CAMI aim at providing tools that allow the clinician to use multi-modal data in a rational and quantitative way in order to plan, simulate and execute mini-invasive medical interventions accurately and safely. In parallel, technological advances in the fields of automation, miniaturization and computer aided design and machining have also led to the development of bioprinting technologies which could be defined as the computer-aided, layer-by-layer deposition, transfer and patterning of biologically relevant materials. These results pave the way of using bioprinting technologies for Computer-Assisted Medical Interventions. More precisely, we show that 3D tissue constructs can be printed in vivo and in situ in relation with defect morphology. Interestingly, we demonstrate that printing cells in situ with a cell-level resolution tends to orientate tissue repair.