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Laser surface treatment of engineering ceramics offers various advantages in comparison with conventional processing techniques and much research has been conducted to develop applications. Even so, there still remains a considerable gap in knowledge that needs to be filled to establish the process. By employing a fibre laser for the first time to process silicon nitride (Si3N4) and zirconia (ZrO2) engineering ceramics, a comparison with the CO2 and a Nd:YAG lasers was conducted to provide fundamental understanding of various aspects of the laser beam-material interaction. Changes in the morphology, microstructure, surface finish, fracture toughness parameter (K1c) were investigated, followed by thermal finite element modelling (FEM) of the laser surface treatment and the phase transformation of the two ceramics, as well as the effects of the fibre laser beam parameter - brightness (radiance). Fibre and CO2 laser surface treatment of both Si3N4 and ZrO2 engineering ceramics was performed by using various processing gases. Changes in the surface roughness, material removal, surface morphology and microstructure were observed. But the effect was particularly more remarkable when applying the reactive gases with both lasers and less significant when using the inert gases. Microcracking was also observed when the reactive gases were applied. This was due to an exothermic reaction produced during the laser-ceramic interaction which would have resulted to an increased surface temperature leading to thermal shocks. Moreover, the composition of the ceramics was modified with both laser irradiated surfaces as the ZrO2 transformed to zirconia carbides (ZrC) and Si3N4 to silicon dioxide (SiO2) respectively. The most appropriate equation identified for the determination of the fracture toughness parameter K1c of the as-received, CO2 and the fibre laser surface treated Si3N4 and ZrO2 was K1c=0.016 (E/Hv) 1/2 (P/c3/2). Surfaces of both ceramics treated with CO2 and the fibre laser irradiation produced an increased K1c under the measured conditions, but with different effects. The CO2 laser surface treatment produced a thicker and softer layer whereas the fibre laser surface treatment increased the hardness by only 4%. This is inconsiderable but a reduction in the crack lengths increased the K1c value under the applied conditions. This was through a possible transformation hardening which occurred within both engineering ceramics. Experimental findings validated the generated thermal FEM of the CO2 and the fibre laser surface treatment and showed good agreement. However, a temperature difference was found between the CO2 and fibre laser surface treatment due to the difference in absorption of the near infra-red (NIR) wavelength of the fibre laser being higher than the mid infra-red (MIR) wavelength of the CO2 laser. This in turn, generated a larger interaction zone on the surface that was not induced further into the bulk, as was the case with the fibre laser irradiation. The MIR wavelength is therefore suitable for Viability and Characterization of the Laser Surface Treatment of Engineering Ceramics 3 the surface processing of mainly oxide ceramics and surface treatments which do not require deep penetration. Phase transformation of the two ceramics occurred at various stages during the fibre laser surface treatment. The ZrO2 was transformed from the monoclinic (M) state to a mixture of tetragonal + cubic (T+C) during fibre laser irradiation and from T+C to T and then a partially liquid (L) phase followed by a possible reverse transformation to the M state during solidification. The Si3N4 transformed to a mixture of α-phase and β-phase (α→ α+β) followed by α+β and fully transforms from α+β → β-phase. What is more, is a comparison of the fibre laser-beam brightness parameter with that of the Nd:YAG laser. In particular, physical and microstructural changes due to the difference in the laser-beam brightness were observed. This research has identified the broader effects of various laser processing conditions, as well as characterization techniques, assessment and identification of a method to determine the K1c and the thermal FEM of laser surface treated engineering ceramics. Also, the contributions of laser-beam brightness as a parameter of laser processing and the influence thereof on the engineering ceramics have been identified from a fundamental viewpoint. The findings of this research can now be adopted to develop ceramic fuel cell joining techniques and applications where laser beam surface modification and characterization of engineering ceramics are necessary.

Zukünftige Beschleunigerprojekte und Nutzerexperimente erfordern für ihren Betrieb einen hochbrillanten Elektronenstrahl mit hohem mittlerem Strom. Eine Elektronenquelle mit dem Potential die Anforderungen erfüllen, ist ein supraleitender Hochfrequenz (SHF) Photoinjektor im Dauerstrichbetrieb. Die Strahldynamik eines solchen Photoinjektor Systems bestimmt die maximal zu erreichende Strahlbrillanz und wird ihrerseits von den Design und Betriebsparametern des Photoinjektors beeinflusst. Ziel ist immer die entscheidenden Design- und Betriebsparameter der Elektronenquelle hinsichtlich einer maximalen Strahlbrillanz zu wählen. Diese Aufgabe verlangt ein detailliertes Verständnis der Strahldynamik-Prozesse. Ferner ist es notwendig, eine Optimierung des Photoinjektors als Ganzes, mit dem Ziel einer maximalen Strahlqualität bei hohem mittlerem Strom, vorzunehmen. Dieses ermöglicht auch, die physikalischen Grenzen eines gegebenen Designs zu ermitteln und im Betrieb vollständig auszunutzen. Diese Doktorarbeit befasst sich mit der Strahldynamik in einem SHF Photoinjektor, unter Berücksichtigung interner Raumladungseffekte. Die Erkenntnisse zur Strahldynamik werden für die Entwicklung eines Optimierungsprogramms verwendet, um die Leistung des Injektors hinsichtlich der Strahlbrillanz zu verbessern. Die entwickelte Methode basiert auf Pareto-Optimierung mehrerer Zielfunktionen, unter Verwendung eines generischen Algorithmus. Das zentrale Ergebnis dieser Arbeit umfasst ein universelles Optimierungsprogramm, das für Photoinjektoren unabhängig von ihrem Design und Anwendungsgebiet genutzt werden kann. Für den Betrieb mit hoher Strahlbrillanz ist es möglich aus den erhaltenen Pareto-optimalen Lösungen einen stabilen Satz an Einstellwerten für den Photoinjektor zu extrahieren. Durch die allgemeine Optimierungsstrategie lässt sich das entwickelte Programm auch für andere Beschleunigerabschnitte, oder die Optimierung einer ganzen Anlage mit erweiterter Zielsetzung anpassen. / An increasing number of future accelerator projects, light sources and user experiments require high brightness, high average current electron beams for operation. Superconducting radio-frequency (SRF) photoinjectors running in continuous-wave (cw) mode hold the potential to serve as an electron source that generates electron beams of high brightness. Different operation and design parameters of the SRF photoinjector impact the beam dynamics and, thus, the beam brightness. Therefore, an in-depth understanding of the beam dynamics processes in an SRF photoinjector and the dependency of the beam dynamics on the photoinjector set parameters is crucial. A high brightness beam operation requires a global optimization of the SRF photoinjector that allows to find suitable photoinjector settings and to figure out and extend the physical performance limits of the investigated injector design. The dissertation at hand offers a detailed analysis of the beam dynamics in an SRF photoinjector regarding internal space charge effects. Furthermore, the impact of the photoinjector elements on the electron beam is discussed. The lessons learned from this theoretical view are implemented in the development of an optimization tool to achieve a high brightness performance. A universal multi-objective optimization program based on a generic algorithm was developed to extract stable, optimum gun parameter from Pareto-optimum solutions. This universal tool is able to optimize and find the physical performance limit of any (S)RF photoinjector independent from the individual application of the electron source (energy recovery linac, free electron laser, ultra-fast electron diffraction). This thesis thereby verifies and complements existing theoretical considerations regarding photoinjector-beam interactions. The global optimization strategy can be introduced to variable optimization objectives as well as it can be extended to an optimization of further parts of the accelerator facility.