TY - BOOK

T1 - Implementation of femtosecond laser processing for materials testing and research

AU - Pfeifenberger, Manuel

N1 - no embargo

PY - 2019

Y1 - 2019

N2 - In the last two decades research on material processing using femtosecond lasers gained massive interest, due to its unique combination of properties - high material removal rates in addition to a minimized influence on the material surrounding the processing area. Particularly, for micro-processing this opens up a new world of possible applications. The main focus of this thesis was the development of a novel system, which combines a scanning electron microscope, a focused ion beam and a femtosecond laser. This system is based on the Zeiss Auriga laser system, originally equipped with a nanosecond laser. After the successful adaption, it was used to investigate its potential for the processing of specimens in materials research, with an emphasis on the fabrication of specimens for mechanical experiments on the meso-scale. First, the processing of various types of materials, such as metals, polymers or biological materials, was evaluated. Critical parameters for the fabrication of micro-mechanical specimen geometries, regarding structural quality, geometrical precision and processing speed, were identified and an optimization of the laser processing parameters was conducted. In detail, following studies were performed in which the newly developed system was applied to cutting-edge material research questions. The resulting quality of specimens processed by a femtosecond laser and a nanosecond laser was compared by the fabrication of bending cantilevers in rolled tungsten foils. The specimen processed by the femtosecond laser yielded a higher surface quality and no grain coarsening underneath the processed surfaces as it was found for samples cut with the nanosecond laser. Especially, for the fabrication of specimens for mechanical tests on the scale of multiple hundred micrometer this negligible influence represents a significant advantage. Furthermore, the femtosecond laser processing parameters were optimized in terms of the efficiency. The advantage of the femtosecond laser compared to the focused ion beam technique is the high material removal rate and was demonstrated by the fabrication of a set of hundred cantilevers with a size of 420x60x25 µm3 in about 30 minutes, outperforming a conventional focused ion beam system by orders of magnitude. The preparation of meso-scale tensile specimens from heat-sensitive materials by means of the femtosecond laser as well as the mechanical testing of the samples was successfully shown in two further studies. First, experiments on spruce wood displayed the potential of the technique to fabricate pristine biological specimens and to characterize local mechanical properties. Second, tensile experiments on three different polymer foils allowed the local determination of strength and ductility, as well as the investigation of the influence of the femtosecond laser processing on these properties. In addition, these experiments were used to determine the effect of electron irradiation on the local tensile properties of the polymer foils. Finally, an approach for a depth-resolved measurement of tensile properties, was presented. Furthermore, femtosecond laser processed single leg bending specimens enabled the measurement of the fracture toughness for intergranular crack growth in ultrafine grained tungsten materials. These experiments evaluated the crack growth resistance along the elongated microstructure of cold rolled tungsten foils with a thickness of 100 µm and cold drawn tungsten wires with a diameter of 150 µm. Such measurements have not been able to be performed so far with classical approaches. The fracture toughness found for the wires is 5.3 MPa√m. The foils yielded a significantly lower value of 2.4 MPa√m. This difference was linked to a distinct difference of the fracture surface roughness.

AB - In the last two decades research on material processing using femtosecond lasers gained massive interest, due to its unique combination of properties - high material removal rates in addition to a minimized influence on the material surrounding the processing area. Particularly, for micro-processing this opens up a new world of possible applications. The main focus of this thesis was the development of a novel system, which combines a scanning electron microscope, a focused ion beam and a femtosecond laser. This system is based on the Zeiss Auriga laser system, originally equipped with a nanosecond laser. After the successful adaption, it was used to investigate its potential for the processing of specimens in materials research, with an emphasis on the fabrication of specimens for mechanical experiments on the meso-scale. First, the processing of various types of materials, such as metals, polymers or biological materials, was evaluated. Critical parameters for the fabrication of micro-mechanical specimen geometries, regarding structural quality, geometrical precision and processing speed, were identified and an optimization of the laser processing parameters was conducted. In detail, following studies were performed in which the newly developed system was applied to cutting-edge material research questions. The resulting quality of specimens processed by a femtosecond laser and a nanosecond laser was compared by the fabrication of bending cantilevers in rolled tungsten foils. The specimen processed by the femtosecond laser yielded a higher surface quality and no grain coarsening underneath the processed surfaces as it was found for samples cut with the nanosecond laser. Especially, for the fabrication of specimens for mechanical tests on the scale of multiple hundred micrometer this negligible influence represents a significant advantage. Furthermore, the femtosecond laser processing parameters were optimized in terms of the efficiency. The advantage of the femtosecond laser compared to the focused ion beam technique is the high material removal rate and was demonstrated by the fabrication of a set of hundred cantilevers with a size of 420x60x25 µm3 in about 30 minutes, outperforming a conventional focused ion beam system by orders of magnitude. The preparation of meso-scale tensile specimens from heat-sensitive materials by means of the femtosecond laser as well as the mechanical testing of the samples was successfully shown in two further studies. First, experiments on spruce wood displayed the potential of the technique to fabricate pristine biological specimens and to characterize local mechanical properties. Second, tensile experiments on three different polymer foils allowed the local determination of strength and ductility, as well as the investigation of the influence of the femtosecond laser processing on these properties. In addition, these experiments were used to determine the effect of electron irradiation on the local tensile properties of the polymer foils. Finally, an approach for a depth-resolved measurement of tensile properties, was presented. Furthermore, femtosecond laser processed single leg bending specimens enabled the measurement of the fracture toughness for intergranular crack growth in ultrafine grained tungsten materials. These experiments evaluated the crack growth resistance along the elongated microstructure of cold rolled tungsten foils with a thickness of 100 µm and cold drawn tungsten wires with a diameter of 150 µm. Such measurements have not been able to be performed so far with classical approaches. The fracture toughness found for the wires is 5.3 MPa√m. The foils yielded a significantly lower value of 2.4 MPa√m. This difference was linked to a distinct difference of the fracture surface roughness.

KW - Femtosekundenlaser

KW - Mikrobearbeitung

KW - Mikromechanik

KW - Probenpräparation

KW - Kunststoffe

KW - Biologische Materialien

KW - Wolfram

KW - Femtosecond laser

KW - Micro-processing

KW - Micro-mechanics

KW - Sample Preparation

KW - Polymers

KW - Biological materials

KW - Tungsten

M3 - Doctoral Thesis

ER -