Search

Some major application areas of microtextured surfaces are found in the energy, biomedical, transportation and aerospace sectors. In relation to the energy sector, microtextured surfaces provide an energy efficient and cost-effective passive mechanism for increased heat transfer during matter’s phase change in energy recovery systems. This study explores the viability of laser microprocessing as an attractive manufacturing route for generating textured surfaces and compares with the results from a previous study involving microgeometries created by micro-wire electro discharge machining (µWEDM). Two types of stainless steel SS316L insert, produced via casting followed by machining, as well as by selective laser melting (SLM), were used as the workpieces. A number of microtextured geometries were produced on the workpieces’ planar top faces using a nanosecond fibre laser (1064 nm wavelength) operating with 0.066 and 0.25 mJ laser energy, 10 and 80 kHz frequencies, 400, 600 and 700 mm/s beam scanning speeds, and 25 and 60 µm set distances between the unidirectional textured grooves/laser tracks. The textured surfaces were subsequently scanned using a 3D optical scanner for evaluating the depth and width of the geometries. The scanning electron micrographs showed comparable groove geometries produced via both laser and µWEDM. During condensation experiments, the laser textured surfaces typically exhibited higher differential temperature, ΔT (~11.9-28.9%) with respect to the unstructured specimens. Additionally, the textured SLM samples generally showed greater heat transfer quality (~3.7-5.7% higher ΔT) than their cast and machined counterparts.

Single, or isolated small arrays of, spherical silica colloidal particles (with refractive index ncolloid = 1.47 and radius R = 350 nm or 1.5 μm) were placed on a silicon substrate and immersed in carbon tetrachloride (nliquid = 1.48) or toluene (nliquid = 1.52). Areas of the sample were then exposed to a single laser pulse (8 ps duration, wavelength λ = 355 nm), and the spatial intensity modulation of the near field in the vicinity of the particles revealed via the resulting patterning of the substrate surface. In this regime, ncolloid < nliquid and the near-field optical intensification is concentrated at and beyond the edge of the particle. Detailed experimental characterization of the irradiated Si surface using atomic force microscopy reveals contrasting topographies. The same optical behavior is observed with both liquids, i.e., the incident laser light diverges on interaction with the colloidal particle, but the resulting interaction with the substrate is liquid dependent. Topographic analysis indicates localized ablation and patterning of the Si substrate when using toluene, whereas the patterning induced under carbon tetrachloride is on a larger scale and extends well below the original substrate surface-hinting at a laser induced photochemical contribution to the surface patterning.

This paper proposes a new tool-path generation approach for layer-based micro machining (“conventional”, laser or EDM milling). With this approach, the laser spot non-conformity or EDM tool wear is compensated for by varying the layer machining angle and start point while the material removal volumes are represented by a sequence of 2D slices. Tool-path generation is performed taking into account the specific requirements of 3D micro-machining. The capabilities of the developed CAM system have been verified experimentally.

A cost effective methodology for pattering of Nano Imprint Lithography (NIL) templates incorporating different length scale features is proposed. The approach relies on selecting the optimum processing window of different technologies for cost effective micro- and nano-patterning. Very promising results were obtained when first fused silica templates were structured by F2 laser ablation at 157nm. It was demonstrated that nano-scale features and complex 3D micro-scale features could be machined with a Focused Ion Beam (FIB) over the existing topography produced by laser ablation. Thus, a large area (up to several square centimeters) of the NIL templates is easily patterned with micro- and even meso-scale features by laser ablation while nano- and micro-scale features could be introduced locally by FIB machining. Through S-FIL process such templates were validated and the nano- and micro-structures were accurately replicated in one imprinting step.

Laser milling of engineering materials is a viable alternative to conventional methods for machining complex microcomponents. The laser source employed to perform such microstructuring has a direct impact on achievable surface integrity. At the same time, the trade-offs between high removal rates and the resulting surface integrity should be taken into account when selecting the most appropriate ablation regime for performing laser milling. In this paper the effects of pulse duration on surface quality and material microstructure are investigated when ablating a material commonly used for manufacturing microtooling inserts. For both micro- and nanosecond laser regimes, it was estimated that the heat-affected zone on the processed surface is within 50 μm. When performing ultra-short pulsed laser ablation, the effects of heat transfer are not as evident as they are after processing with longer laser pulse durations. Although some heat is dissipated into the bulk when working in pico- and femtosecond regimes it is not sufficient to trigger significant structural changes.

Conventional methods of producing ceramic components are based on sintering technology which requires expensive tooling making it uneconomic for small batch fabrication. Laser milling provides a new method of producing parts in a wide range of materials, including ceramics, directly from CAD data. This paper considers the technical capabilities of laser milling when applied to the machining of microcomponents from alumina and silicon nitride ceramics. The main parameters affecting the material removal characteristics of laser milling are reviewed. A new technique for machining alumina components is proposed emphasising the importance of correct set-up design in achieving a high level of accuracy. Process parameters influencing part quality are analysed and guidelines for machine set-ups are formulated. The paper concludes with an assessment of the accuracy of the laser milling process.

We report sub-diffraction limited patterning of Si substrate surfaces by laser-initiated liquid-assisted colloidal lithography. The technique involves exposing a two-dimensional lattice of transparent colloidal particles spin coated on the substrate of interest (here Si) immersed in a liquid (e.g. methanol, acetone, carbon tetrachloride, toluene) to a single picosecond pulse of ultraviolet laser radiation. Surface patterns formed using colloidal particles with different radii in the range 195 nm ≤ R ≤ 1.5 μm and liquids with differing indices of refraction (n(liquid)) are demonstrated, the detailed topographies of which are sensitively dependent upon whether the index of refraction of the colloidal particle (n(colloid)) is greater or smaller than n liquid (i.e. upon whether the incident light converges or diverges upon interaction with the particle). The spatial intensity modulation formed by diffraction of the single laser pulse by the colloidal particles is imprinted into the Si substrate.

Optical near field enhancements in the vicinity of particles illuminated by laser light are increasingly recognized as a powerful tool for nanopatterning applications, but achieving sub-wavelength details from the near-field distribution remains a challenge. Here we present a quantitative analysis of the spatial modulation of the near optical fields generated using single 8 ps, 355 nm (and 532 nm) laser pulses around individual colloidal particles and small close packed arrays of such particles on silicon substrates. The analysis is presented for particles in air and, for the first time, when immersed in a range of liquid media. Immersion in a liquid allows detailed exploration of the effects on the near field of changing not just the magnitude but also the sign of the refractive index difference between the particle and the host medium. The level of agreement between the results of ray tracing and Mie scattering simulations, and the experimentally observed patterns on solid surfaces, should encourage further modelling, predictions and demonstrations of the rich palette of sub-wavelength surface profiles that can be achieved using colloidal particles immersed in liquids.

Laser ablation of a bulk Pd target in DIW, acetone or ethanol was carried out for the production of nanoparticles colloidal solutions. The size distribution of the nanoparticles follows log-normal function for all three liquids, with a median diameter of 3, 1.1 and 1.5 nm and standard deviation of 0.65, 1 and 1, respectively. Amorphous carbon is found on the nanoparticles synthesized in the hydrocarbons (acetone or ethanol). In DIW pure Pd nanoparticles were generated while ablation in acetone or ethanol lead to the synthesis of palladium hydride (PdHx) nanoparticles. These nanoparticles can be used in hydrogen storage applications.

This paper proposes a novel approach and a process chain for precision assembly of nano imprinted Ni shims on a larger Ni master plate and fixing them using laser welding. The capabilities of micro-manufacturing technologies (milling and assembly) were combined with those of nano-manufacturing to stitch produced Ni shims on a template in order to achieve a relatively large area for further imprinting using Roll-To-Roll (R2R) technology. The results of two setups are shown and analysed with the main sources of errors pointed out. Laser welding of the assembly tests were carried out in order to investigate the best of the welding path geometries commonly used in "macro" world.

In order to broaden the application domain of microsystems-based products, a number of processing chains that are complementary to those used for batch-manufacturing of micro electro mechanical systems (MEMS) have been recently proposed by the research community. Such alternative process chains combine micro and nano structuring technologies for master making with replication techniques for high throughput such as injection moulding (IM). In this research, two new process chains were investigated for replicating structured surfaces that are inspired by nature. In particular, a study was conducted to replicate structures incorporating functional features found on the eye of a household fly and on a shark skin. Such features were initially designed by applying a bio-mimetic modelling approach to generate the 3D models necessary to achieve the targeted surface functionality and thus to investigate the feasibility of “embedding” them in existing and new emerging products. The proposed two process chains employ micro-second (ms) and pico-second (ps) laser ablation and focused ion beam (FIB) milling to perform micro and nano structuring, respectively. The feasibility of applying them for producing masters for replicating bio-inspired surface structures was investigated by performing micro injection moulding trials. The results showed that such micro and nano structured surfaces can be replicated successfully, and the two process chains can be considered as promising manufacturing routes for serial production of parts incorporating bio-inspired surface structures.

Emerging micro-engineered products tend to integrate a multitude of functionalities into single enclosures/packages. This trend for “function integration” has to be underpinned by manufacturing capabilities for “length-scale integration” at component and product levels, i.e. the ability to manufacture parts incorporating features from a few hundreds of micrometres to sub-100 nm. In order to design novel process chains that enable such function and length scale integration into miniaturised devices, it is required to utilise materials that are compatible with the various component manufacturing processes in such chains. At the same time, these materials should be able to satisfy the functional requirements of the produced devices. One family of materials, which can fulfil these criteria, is bulk metallic glasses. In particular, the absence of grain boundaries in bulk metallic glasses makes them mechanically and chemically homogeneous for processing at all length scales down to a few nanometres. A number of recent studies have shown that pico-second laser ablation and focused ion beam milling are promising technologies for structuring this type of amorphous alloys at the micro and sub-micro scales respectively without introducing any changes in their non-crystalline structure. In this context, this research presents an experimental investigation to establish the validity of the process pair “pico-second laser” and “focused ion beam” when it is considered for structuring a particular type of bulk metallic glasses to produce replication masters incorporating micro and nano scale structures.

Laser drilling of engineering materials is well established process. Latest developments in laser sources offer enormous flexibility but also present challenges. Laser material interaction governs the material removal process and correct laser parameters are key to successful processing. New generation fiber lasers offer 25 different pulse shapes/durations ranging from 15ns up to 220 ns. Those different waveforms offer different pulse energies as well. The paper discusses effect on the green ceramic tapes when processed with various pulse lengths/shapes in terms of resulting diameter, accuracy and quality

The use of arrays of hollow micro needles offers the potential of pain free drug delivery through the skin. However, their batch manufacture represents a real challenge. Especially, it is difficult to design and implement reliable and cost effective manufacturing solutions suitable for scale up production of such new drug delivery devices. In this paper the authors report an investigation of possible manufacturing process chains for producing steel mould inserts for serial replication of microneedles’ patches employing injection moulding. The focus is on manufacturing solutions that utilise nano-second and pico-second laser milling for machining the necessary replication inserts. The process optimisation issues associated with their manufacture are discussed, and the processing windows that were determined are described. To validate the proposed tool-making process chains some moulding inserts were successfully machined using both laser milling technologies. Finally the limitations and uncertainties of these tool making process chains are discussed.

Laser milling of flat substrates is quite well established process, where laser structuring of rollers is still in development. Due to the large varieties of laser sources, stages, control systems and CAD/CAM systems, it is difficult to create solution that works for all systems. In order to generate control programs for the machine for rollers structuring usually multi axis machining is employed, enabling simultaneous rotation and translations. Taking into account complexity of the task and accuracy required for such applications it takes much longer time for calculation and significantly enlarges the resulting program code. The paper discusses simplified method for laser micro structuring of rollers.

The paper presents the design and manufacturing steps of micro heaters, built on ceramic suspended membranes for gas sensor applications. The micro heaters are designed and fabricated by combining laser milling techniques, and conductive ceramic technology. Trenches are created in the ceramic substrate in order to define the geometry of the heater using laser processing of the substrate. The heater is completed by filling the trenches with conductive ceramic paste and then baking to remove the solvent from the paste. The final step involves releasing the membrane by laser milling, enabling it to be suspended on four bridges, to minimise the dissipation of the heat in the substrate. The temperature of the heater element was measured with a heat camera from FLIR 40 system comparing the case of the heater positioned on top of a released membrane and that of the non-released membrane. The simulation of the heater build on top of a released membrane was compared with the heater measurements.

Laser milling of industrial materials like ceramics, dielectrics and metals is of significant commercial interest for microfabrication of micro-moulds and other micro-system devices. 2.5D laser machined structures were generated in alumina, tungsten and steel substrates using a nanosecond copper vapour laser (511nm) at 10 kHz. Preliminary results in fused silica, alumina and steel are also presented from a high repetition rate amplified mode-locked picosecond Nd:vanadate laser. It is shown that high quality surface finish can be achieved with both laser types; for example, average surface roughness, Ra ~ 300nm has been demonstrated in steel. Fused silica could only be processed with picosecond laser pulses. Volume removal rates are analysed, which are especially high for difficult materials like tungsten (~0.1mm3/min) and are greater compared to other milling technology like micro-EDM. Surface roughness measurements in these materials using white light interferometry are reported along with SEM analysis.

The paper presents the design and manufacturing steps of micro heaters, built on ceramic suspended membranes for gas sensor applications. The micro heaters are designed and fabricated by combining laser milling techniques, and conductive ceramic technology. Trenches are created in the ceramic substrate in order to define the geometry of the heater using laser processing of the substrate. The heater is completed by filling the trenches with conductive ceramic paste and then baking to remove the solvent from the paste. The final step involves releasing the membrane by laser milling, enabling it to be suspended on four bridges, to minimise the dissipation of the heat in the substrate. The temperature of the heater element was measured with a heat camera from FLIR 40 system comparing the case of the heater positioned on top of a released membrane and that of the non-released membrane. The simulation of the heater build on top of a released membrane was compared with the heater measurements.

Laser micro-milling of industrial materials like ceramics, dielectrics and metals is of significant commercial interest for fabrication of micro-moulds and other micro-system devices. 2.5D laser machined structures were generated in alumina, tungsten and steel substrates using a nanosecond copper vapour laser (511 nm) at 10 kHz. Preliminary results in fused silica, alumina and steel are also presented from a high repetition rate amplified mode-locked picosecond Nd:vanadate laser. It is shown that high quality surface finish can be achieved with both laser types; for example, average surface roughness, Ra ~ 300 nm has been demonstrated in steel. Fused silica could only be processed with picosecond laser pulses. Volume removal rates are analysed, which are especially high for difficult materials like tungsten (~ 0.1 mm 3 /min) and are greater compared to other milling technology like micro-EDM. Surface roughness measurements in these materials using white light interferometry are reported along with SEM analysis.

The paper presents the development of a novel suspended membrane resistive gas sensor on a ceramic substrate. The sensor is designed and simulated to be fabricated by combining laser milling techniques, conductive ceramic technology, thin film technology, and semiconductor metal oxides. Trenches are created in the alumina substrate in order to define the geometry of the heater using laser processing of the substrate. The heater is completed by filling the trenches with conductive ceramic paste and then baking to remove the solvent from the paste. The next step consists of polishing the surface to obtain a surface roughness small enough for thin film technology. A dielectric (SiO/sub 2/ or ceramic) material is then deposited, acting as hot plate and also as electrical isolation between the heater and sensing electrode. The sensing electrode consists of an interdigitated resistor made of Au or Pt with thickness in the range of 2000-3000 /spl Aring/. The gas sensitive layer (SnO/sub 2/) is deposited by screen printing or spinning. When heated it react with gas molecules and changes its resistivity, thereby acting as a sensor. The final step involves releasing the sensor, enabling it to be suspended on four bridges, to minimise the dissipation of the heat in the substrate.

We report a comprehensive study of a laser initiated liquid assisted technique to pattern Si and GaAs substrates. Silica colloidal particles masks, sizes 3 μ m and 700 nm, were formed on the substrates and three laser wavelengths: 355 nm/532 nm at 8 ps laser pulse duration and 800 nm at 70 fs laser pulse duration were used. The laser processing was assisted by liquid precursors, namely methanol and carbon tetrachloride. The regular structures develop from holes to bumps and nanorings, depending on the laser power and the size of the colloidal particle mask. Silica colloidal particles act both as a template mask and also based on the liquid-tuned optical field enhancement that shows near field enhancement effects. The patterns are defined by the liquid chemical composition and no influence of the pulse duration (ps or fs) was observed on their design.

Laser milling involves applying laser energy to remove material through ablation in a layer-by-layer fashion. Computer numerical control (CNC) programs for laser milling are obtained directly from a three-dimensional computer aided design (CAD) model of the workpiece. Thus, apart from being a material removal rather than a material accretion system, a laser milling machine operates like any other layered manufacturing technology equipment. This paper reviews the physical phenomena underlying the interaction between a laser beam and a workpiece and the mechanisms governing the ablation process with a view to identifying the key process parameters. The paper then focuses on the use of laser milling to manufacture small parts rapidly in materials that are difficult to machine by conventional means.

Laser milling is a relatively new manufacturing process that has attracted much attention from engineers and researchers for the last decade. The process allows complex parts and tooling inserts to be fabricated directly from 3D CAD data in a wide range of advanced engineering materials such as ceramics, hardened steel, bulk metallic glasses, titanium and nickel alloys. The paper discusses the process design and implementations issues in applying the laser milling technology for tool making. Especially, the focus is on design and implementation of appropriate material removal strategies and the necessary optimisation of processing parameters to produce replication tools with required surface integrity and resolution. Two application case studies are used to demonstrate the capabilities of the laser milling process, in particular for the fabrication of replication tools for serial manufacture of micro needles arrays for drug delivery and bio inspired surface structures for drag reduction. The paper concludes with a discussion of tool making capabilities and limitations of laser milling technology.Laser milling is a relatively new manufacturing process that has attracted much attention from engineers and researchers for the last decade. The process allows complex parts and tooling inserts to be fabricated directly from 3D CAD data in a wide range of advanced engineering materials such as ceramics, hardened steel, bulk metallic glasses, titanium and nickel alloys. The paper discusses the process design and implementations issues in applying the laser milling technology for tool making. Especially, the focus is on design and implementation of appropriate material removal strategies and the necessary optimisation of processing parameters to produce replication tools with required surface integrity and resolution. Two application case studies are used to demonstrate the capabilities of the laser milling process, in particular for the fabrication of replication tools for serial manufacture of micro needles arrays for drug delivery and bio inspired surface structures for drag reduction. The paper conclud...