CHAPTER 1  

INTRODUCTION: Why laser micromachining of composite ceramics?

Ceramic and ceramic matrix composite materials present an outstanding combination of low thermal and electric conductivity, low specific gravity, high strength, low deformability, high wear resistance and chemical inertness [1]. These properties make ceramic materials extremely well suited to the manufacturing of mechanical and electromechanical miniaturised devices, and they have numerous technological applications in todays industry, ranging from microelectronics to aerospace. However, ceramic materials are typically brittle and difficult to machine. Due to the high contraction experienced during sintering it is often impossible to shape them accurately using conventional techniques. When complicated forms are to be produced with high accuracy, ion beam milling is the main machining technique currently available, but this technique presents some major drawbacks: it requires vacuum, investment and running costs are high and processing is slow. Laser micromachining is increasingly appearing as a worthy alternative because it is fast, it does not require tooling and it is a non-contact process, which is easily automated. When high power infrared lasers are used (i.e., CO₂ and Nd/YAG lasers, emitting radiation with wavelengths of l=10.6 and 1.064 mm, respectively) with typical pulse duration in the millisecond range, considerable melting occurs and a thick resolidified layer forms with extensive cracking. These thermal effects can be minimised using shorter radiation wavelength in the UV range [2] or pulse duration in the nanosecond range and shorter [3-5]. Excimer lasers (l=157-351 nm) with nanosecond duration pulses are currently used in several industrial applications, such as the production of ink jet printer nozzles, medical probes, and microvia drilling [6, 7]. Recently, femtosecond pulse duration lasers opened new perspectives for the shaping of difficult to machine materials and to improve the accuracy of laser machined parts [8]. Materials that are difficult or impossible to machine with nanosecond duration pulses typically include high bandgap dielectrics  that have low absorption [9, 10], and metals, which melt extensively leading to poor finish and low accuracy [3]. Despite excimer laser micromachining being a well established technology, there is still a number of unresolved fundamental questions regarding the ablation mechanisms and the influence of the processing parameters [2, 5]. While this is true for pure materials, ablation of composite materials expectably becomes even further complex. Unlike the micromachining of pure materials, such as metals [11, 12], ceramics [4, 13, 14], and polymers [15-17], the micromachining of composite materials using excimer lasers has not yet been the subject of in-depth investigations. This work is focused on the excimer laser micromachining of Al₂O₃-TiC composite ceramic, and aims to contribute to the understanding of the ablation behaviour of composite materials. Al₂O₃-TiC ceramics have considerable technological importance because of their high hardness, excellent wear behaviour and relatively good toughness, being frequently used in modern hard disk head sliders [18]. The air bearing surface contour of the sliders is typically machined using ion beam techniques. Excimer laser micromachining could be an interesting alternative to shape the sliders, but an extremely good surface finish is required [19]. On the other hand, surface texturing of the sliders is also necessary to provide emergency landing capabilities to the slider on ultrasmooth disks [18]. Consequently, an assessment of the viability of laser micromachining of this material is of considerable technical interest.

In micromachining applications one is interested in several aspects:

-surface quality;

-dimensional control, related to the lateral dimensions and the depth of machined features;

-Efficiency of the process;

-Structural or chemical affectation of the original material;

The relation between optimising the process for specific material and the “process characteristics”

Three-dimensional laser micromachining is a very interesting field for different applications. Especially for manufacturing prototypes or small batch sizes, processes are used that make the structuring of complex features possible, after a short after a short planning stage. There are a large number of publications showing applications of excimer laser micromachining, but usually they do not reveal the practical details of the process, mainly for commercial reasons.

1.1 SCOPE OF THIS WORK

This dissertation is devoted to the study of excimer laser micromachining of Al₂O₃-TiC ceramics. The investigation of the technical viability of laser micromachining of this material required the study of the effects of various processing parameters. Like any other manufacturing technique each process tends to be unique and invariably there are factors such as melting, damaging to surrounding areas, debris, surface roughness, etc., that require experimentation for optimisation. This parametric study was a challenging task, and allowed to gain insight into the ablation behaviour of Al₂O₃-TiC ceramics.

This thesis is organised as follows. In chapter 2 some introductory concepts concerning the properties of laser radiation and its interaction with matter are presented. An overview of the physical aspects of laser ablation is also given. The fundamental aspects of excimer laser technology and micromachining are introduced in chapter 3. In addition, the characterisation techniques used throughout this study are briefly presented, and a description of the physical properties of the materials studied is given.

Chapters 4 to 7 describe and discuss the experimental work carried out in this thesis. Each of these chapters start with a short specific introduction to the subjects covered, followed by a description of the experimental set-up. Next, the experimental results are presented, and then discussed in a separate section. Finally, the conclusions of each chapter are presented. Chapter 4 focuses on the micromachining of Al₂O₃-TiC ceramics in air using nanosecond duration (t=30 ns) KrF (l=248 nm) excimer lasers. The influence of laser fluence, number of laser pulses, beam spot size, angle of incidence, and static or moving sample on the ablation rate, surface topography and machining quality is studied. Chapter 5 studies the influence of the ambient atmosphere (gas type and pressure) on the nanosecond KrF laser machining process. Particular attention is given to the variation of the ablation rate and to the mechanisms responsible for the ejection and accumulation of material around the machined area. In chapter 6, the micromachining of Al₂O₃-TiC ceramics using nanosecond ArF (l=193 nm) and XeCl (l=308 nm) lasers is investigated. A comparative study on the influence of the radiation wavelength is conducted. In chapter 7 the micromachining of Al₂O₃-TiC ceramics using ultra-short pulses (t=500 fs, l=248 nm) is investigated. A comparative study on the influence of the pulse duration (fs vs. ns) is conducted. The main conclusions of this thesis, as well as perspectives about possible future work are presented in chapter 8. Finally, appendixes A and B describe the numerical models developed to study the ablation process. Appendix C displays the list of symbols used throughout this work.

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18. H. Gatzen: Rigid disk slider micromachining challenges to meet microtribology needs, Trib. Int. 33, 337-342 (2000)

19. H. H. Gatsen et al.: Precision engineering of rigid disk sliders, IEEE Trans. on Magn. 32 (3), 1843-8 (1995)