| Microholes as common structures, have broad applications in aerospace, biology, chemistry, new energy and other fields. However, the difficulties for fabricating microholes with continuously climbing requirements have restricted the improvement of our country’s core competitiveness, especially in some application of national great strategic demand. For example, in the field of new energy, microhole drilling on the pellet is one of the most critical links in Inertial Confinement Fusion(ICF). The fabrication of the microholes directly determines if the project can be carried out successfully. However, the demands of high-quality(no recast, no crack, no heat affected zone and uniform density), high-aspect-ratio(more than tens) make the microheles difficult to fabricate. What’s more, since a variety of constraints, such as non-pollution et al., the various methods of micro-EDM/ECM/acid and alkali assisted etching are not suitable for the drilling of thus microholes. Moreover, e-beam and ion-beam processing can’t get enough depth. Therefore, manufacturing of the microholes has become one of the bottlenecks restricting project progress.With ultra-high power intensities and ultra-short irradiation durations, femtosecond laser presents a new manufacturing mechanism. It provides a processing method with wide material adaptability, minimized heat-affected zones, reduced recast/microcracks, and so on. Also it has advantages of non-contact, non-pollution. Hence, femtosecond laser microholes drilling, based on the ablation, becomes one of the most promising solution to overcome the current manufacturing problem. However, the femtosecond laser ablation is a complex nonlinear and nonequilibrium process. Some challenges remain in achieving deep microholes, such as the unclear laser propagation mechanism, the unclear bending mechanism, and so on. The quality of microholes is difficult to control and the aspect-ratio of microholes is difficult to over 10:1. The improvement of controllability and aspect-ratio of microholes cannot be achieved just by optimizing laser parameters without a fundamental understanding of the drilling process. But, in general, a numerical simulation of process dynamics during laser beam drilling can only consider some certain simplified conditions. Both spatial and temporal scales of the simulation are limited by large amount of calculation. A theoretical research is hardly applied to actual processing, especially in deep-hole drilling. However, it is an urgent need for high-quality and high-aspect-ratio microholes to be obtained at some national great strategic demand.Therefore, based on the aforementioned background and the current status at home and abroad, a comprehensive study of high aspect ratio, high-quality microholes fabrication was conducted. To find out the impact rules and the mechanism underlying different parameters, this study innovatively applied multi-directional and multi-scaled online/ offline observation in femtosecond laser deep-hole drilling. Based on the understanding of the fundamentals in the drilling process at our experiments, high-aspect-ratio and high-quality microholes with high repeatability and controllability were presented.The main research contents and innovations are shown as follows:1. An integrated femtosecond laser drilling system consisting of processing and online monitoring units was built. By using the drilling system, a comprehensive study of high aspect ratio, high-quality microholes percussion drilling process was conducted in ambient air for the first time. The processing results were characterized and analyzed. Based on a large number of experimental analysis, the main rules between microholes morphology and the effective factors(pulse number, the relative position between laser focus and sample surface, pulse energy, the focusing condition) were preliminary revealed. Deep microholes with a diameter of 10- 40 μm and an aspect ratio of over 10- 50 were obtained. The revealed drilling rules can be applied to fabricate various kinds of required microholes within a certain size range. It will actively develop the methods of microholes fabrication and promote the application of microholes.2. The bending effect has seriously affected the controllability of the drilling process and the quality of the hole. To reveal the bending mechanism, a comprehensive investigation of bending, from the physical mechanism to elimination, was conducted firstly. It is revealed that the polarization is not the major factor which leading to bending. But experimental statistics indicate that the deviation of the microhole tends to be perpendicular to the polarization direction. In contrast, a circularly polarized laser has no apparent tendency. To make clear the mechanism of bending effect, the existing phenomenon in various time-scales of drilling process was observed online. The images show that the aerosol, including the vapor cloud and ablated ejection, can last until the milliseconds domain. It is speculated that the disturbance of the laser beam by the dynamic ablated aerosol which have not sufficiently dispersed in the millisecond domain is the main mechanism of bending in the experiment. Based on the study, the main mechanism of bending is further confirmed experimentally. The straight microholes can be obtained in an air environment when the interval between pulses is extended enough. By contrasting the microhole arrayed with bending microholes and straight microholes, the significance of bending effect elimination is demonstrated.3. By the analysis of online observation and experimental results in microholes drilling, vacuum assisted drilling method was proposed. The method not only effectively avoids the microholes bending effect, but also greatly increases the efficiency of drilling and break through the limitation of max-depth. It can greatly expand the available size-range of microholes. High-aspect-ratio(over 100:1) straight microholes with diameter above 10 μm(also 5~10 μm) can be effectively obtain in a vacuum environment. Followed by a large of researches regarding on the contrast between microholes drilling in air and in vacuum(in the aspects of ionization\ejection, etc.), the unique mechanism of femtosecond laser microholes drilling was revealed. Numerous influence factors such as pulse energy, environment, and pulse width are analyzed. The results indicate that the phenomenon of vacuum assisted aspect-ratio improvement is determined by multiple factors. The related mechanism was discussed. This work can be considered as a useful contribution to other experiments on the same subject, making these results significant to the field. Furthermore, considering the limitations of Gauss-beam distribution, it is difficult to obtain high-aspect-ratio microhole structure with smaller diameter. By converting Gauss-beam to Bessel-beam, microholes with samller diameters(below 5 μm) were obtained. The aspect-ratios of microholes can be higher than 100:1(also can up to 330:1 after optimization). The drilling method with Bessel-beam fills the blank of small diameter, high-aspect-ratio microholes drilling, which Gauss-beam can hardly realize. It provides a convenient and effective method for a variety of possible applications. Lastly, through multi-scale observation system, the mechanism of Bessel-beam drilling processing was further revealed.A part of above results have been applied to the fabrication of microholes structures in Inertial Confinement Fusion(ICF). And the results were also reported as highlight works in the National Basic Research Program of China(973 Program) and National Natural Science Foundation of China(NSFC), etc. The main innovations of the thesis were published in the influential journals/conferences. Totally 8 papers are cited by SCI/EI(3 first author papers). |
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