In this paper we study the influence of the processing wavelength on process efficiency and quality at picosecond microdrilling in steel. Possible optical setups for utilizing the second harmonic will be presented, and the influence of wavelength on the drilling rate will be discussed. The potential of helical drilling with the second harmonic in 1 mm thick CrNi-steel will be investigated with regard to process efficiency and hole quality. An analysis will be given of the role of particle-ignited atmospheric plasma and the relation between isophote contour and hole morphology. Our study reveals that a substantial enhancement of both precision and productivity can be achieved by using frequency-doubled instead of infrared radiation. It is shown that plasma ablation and melt production can be minimized by drilling with the second harmonic.
Surface wear of corresponding tribological pairings is still a major problem in the application of artificial joint surgery. This study aims at developing wear reduced surfaces to utilize them in total joint arthroplasty. Using a pico-second laser, samples of medical CoCrMo metal alloy and Al2 O3 ceramic were patterned by laser material removal. The subsequent tribological investigations employed a ring-on-disc method. The results showed that those samples with modified surfaces show less mass or volume loss than those with a regular, smooth surface. Using calf serum as lubricating medium, the volume loss of the structured CoCrMo samples was eight times lower than that of regular samples. By structuring Al2 O3 surfaces, the wear volume could be reduced by 4.5 times. The results demonstrate that defined surface channels or pits enable the local sedimentation of wear debris. Thus, the amount of free debris could be reduced. Fewer abrasives in the lubricated so-called three-body-wear between the contact surfaces should result in less surface damage. Apart from direct influences on the wear behavior, less amounts of free debris of artificial joints should also be beneficial for avoiding undesired reactions with the surrounding soft tissues. The results from this study are very promising. Future investigations should involve the use of simulators meeting the natural conditions in the joint and in vivo studies with living organisms.
In this paper, for the first time, excimer laser micromachining is applied to create new electrooptical devices called substrate integrated modulators, consisting of new optical and microwave waveguides on LiNbO 3 substrates. The fact of having both optical and microwave waveguides, offers the possibilities to modulate high-frequency optical signal (wavelength of 1.55 μm) by either millimeter or microwave signal (frequency of 60 GHz). Because LiNbO 3 is transparent between 370 nm to 5000 nm, 248 nm KrF excimer laser is one of the best candidates for performing fine micromachining of this material. Cutting the LiNbO 3 wafer, making holes, creating a ridge optical waveguide and fabricating a specific pattern of holes for microwave applications by 248 nm excimer laser are presented in this paper.
During the last decades, riblets have shown a potential for viscous drag reduction. Several investigations and measurements of skin-friction in the boundary layer over flat plates and on turbomachinery type blades with ideal riblet geometry have been reported in the literature. The purpose of the present study is to investigate whether laser machined and ground riblet-like structures could be successfully employed on conventional 2-D (NACA) compressor blades in order to assess the potential of industrial machining processes for the creation of the riblet effect. Perfectly trapezoid riblets were designed specifically for the flow parameters in the wind tunnel. Parameters describing the geometry and the deviation from ideal riblets are developed. Riblet machining by high precision material ablation has the potential of achieving micro-machining quality. In comparison to ns-laser processing using either Q-switched solid-state lasers or excimer lasers, the results for high precision material ablation show the enormous potential of ps-laser radiation and achieve the required quality, free of thermally induced defects and, consequently, with high reproducibility. For grinding riblets, geometrically defined microprofiles must firstly be generated via a profile dressing process and then ground onto the work piece surface. A precise adjustment of the grinding wheel system (grit, bonding) and the dressing/grinding conditions is necessary, in order to satisfy the opposing requirements at both dressing and grinding. The blade specimens were geometrically measured with a confocal microscope as well as secondary electron microscope using a specially developed riblet-oriented analysis. For verifying the measurement results, an Atomic Force Microscope was used. The specimens, i.e. flat plates and compressor blades, are aerodynamically tested in a cascade wind tunnel and properly scaled model surfaces were tested in an oil channel in order to quantify skin-friction reduction. Wake measurements of a cascade with NACA-profiles which have the resulting riblet-like structured surface show that the laser shaped as well as ground riblets reduce skin-friction almost as well as the ideal ones, which means a skin friction reduction of up to 7%.
Pulsed laser radiation is a powerful tool for micro machining of different materials. The laser beam can be focused to spot diameters in the micron range. Depending on the material properties (e.g. heat conductivity), the thermal influence can be limited by using different pulse duration between nanoseconds (10–9 s) and femtoseconds (10–15 s).
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