Advances in laser micromachining have resulted in considerable processing capabilities for the growing MEMS/MOMS applications currently being developed. The two distinct temporal regimes for processing that are employed currently are ultrafast timescales at ~150fs and nanosecond timescales at >5 < 250ns. Reported results from various laser interaction studies reveal that the absence of heat affected zones cannot be guaranteed when using ultrafast interactions. This work presents experimental results from ablation studies of Ni in the ns and fs regimes. An important processing parameter, average scanned intensity, is defined along with experimentally derived values for ablation thresholds and the 2ω0 beam diameter for each of the optical setups. We apply electron back scattering diffraction (EBSD) analysis to target machined Ni surfaces from the fs and ns interactions to identify the creation or absence heat affected zones. Results from the study of EBSD data suggest that low intensity ultrafast interactions are capable of eliminating heat affected zones on condition that surface plasmas are not sustained above the interaction site. There is clear evidence of substantial heat affected zones when using nanosecond pulses at a wavelength of 355nm.
The generation of surface periodic structures (SPS) on laser machined surfaces is known to occur when exciting the surface near the ablation threshold using short pulse laser exposure. These effects were first observed in the late 1960s and have remained a laboratory curiosity. Although well studied at nanosecond timescales there have been limited number of studies at ultrafast timescales. We have investigated the conditions necessary to generate short and long-range periodic structures using ultrafast laser pulses at λ =775nm and 387 nm which may find application in the field of surface engineering. This work examines the formation of SPS on a range of materials including Ni, Ti and SS316 and their dependence on fluence and polarisation.
Optimised ultrafast laser ablation can result in almost complete ionisation of the target material and the formation of a high velocity plasma jet. Collisions with the ambient gas behind the shock front cools the material resulting in the formation of mainly spherical, single crystal nanoscale particles in the condensate. This work characterises the nanoscale structures produced by the ultrafast laser interactions in He atmospheres at STP with Ni and Al. High resolution transmission electron microscopy was employed to study the microstructure of the condensates and to classify the production of particles forms as a function of the illumination conditions.
Alumina ceramic, Al<sub>2</sub>O<sub>3</sub>, presents a challenge to laser micro-structuring due to its neglible linear absorption coefficient in the optical region coupled with its physical properties such as extremely high melting point and high thermal conductivity. In this work, we demonstrate clean micro-structuring of alumina using NIR (λ=775 nm) ultrafast optical pulses with 180 fs duration at 1kHz repetition rate. Sub-picosecond pulses can minimise thermal effects along with collateral damage when processing conditions are optimised, consequently, observed edge quality is excellent in this regime. We present results of changing micro-structure and morphology during ultrafast processing along with measured ablation rates and characteristics of developing surface relief. Initial crystalline phase (alpha Al<sub>2</sub>O<sub>3</sub>) is unaltered by femtosecond processing. Multi-pulse ablation threshold fluence F<sub>th</sub> ~ 1.1 Jcm<sup>-2</sup> and at low fluence ~ 3 Jcm<sup>-2</sup>, independent of machined depth, there appears to remain a ~ 2μm thick rapidly re-melted layer. On the other hand, micro-structuring at high fluence F ~ 21 Jcm<sup>-2</sup> shows no evidence of melting and the machined surface is covered with a fine layer of debris, loosely attached. The nature of debris produced by femtosecond ablation has been investigated and consists mainly of alumina nanoparticles with diameters from 20 nm to 1 micron with average diameter ~ 300 nm. Electron diffraction shows these particles to be essentially single crystal in nature. By developing a holographic technique, we have demonstrated periodic micrometer level structuring on polished samples of this extremely hard material.