Single-shot femtosecond laser ablation of nano/polycrystalline titanium investigated using molecular dynamics and experiments
Creators: name: Parris, George
Publisher: name: London South Bank University
Year: 2024

description: Laser ablation, a crucial technique in various scientific and industrial fields, plays a pivotal role in precision manufacturing. Industries such as aerospace rely on laser technology for tasks like drilling microscale holes in jet turbine components to enhance air-cooling efficiency. Moreover, laser-based material processing is indispensable in addressing healthcare challenges with facilitating the postprocessing of 3D-printed bespoke components like patient-specific implants as an example. Ultrashort pulsed laser ablation enables precise micro and nanofabrication, enhancing material properties like wettability, adhesion and biocompatibility. This is particularly important in medical applications like implant development, as it can help reduce the possibility of post-surgery infections. 
Scientifically, understanding the intricacies of ultrashort pulsed laser ablation contributes to ongoing research and development efforts in ablation technology, fostering the enhancement of new material properties related to surface modifications. Additionally, laser ablation plays a crucial role in additive manufacturing technology like 3D printing of metals by facilitating the post-processing stage. 
This thesis investigates the ultrashort pulsed laser ablation of titanium, utilising a combination of molecular dynamics simulations and experiments. Molecular dynamics simulations are used for their capability to model systems at the atomistic scale and ultrashort timescale (femtoseconds in this work), in contrast to the finite element method, and for their computational efficiency compared to methods employing more detailed calculations like density functional theory. The primary focus of this work is on exploring the size effect by examining variations in beam spot diameter and grain size with profound implications for ultraprecision manufacturing of titanium surfaces in sub-micron length scale, produced by casting and additive manufacturing techniques. It contributes a nuanced understanding of ultrashort pulsed laser ablation by bridging the gap between molecular dynamics simulations and experiments. It extends the boundaries by simulating the largest feasible atomistic models and measuring features at the smallest scale permitted by the available metrology devices in experiments. 
The key observations showed the critical importance of the beam spot diameter in determining the laser fluence necessary to achieve average plasma temperatures of around 9,000 K, as well as a direct correlation between the grain size and the response of the material to laser irradiation. Notably, the simulations indicated that the 10 nm laser beam spot diameter compared to the 25 nm requires 59% more absorbed laser energy for ablation. Furthermore, the investigation revealed that by increasing the grain size in alpha-phase titanium, when the number of grains in the volume of 500,000 nm³ were reduce from 500 grains to 10, 36% more absorbed laser fluence was necessary to achieve average plasma temperatures of approximately 9,000 K, despite the material exhibiting higher heat conductivity. 
Additionally, a comparative analysis of ultrashort pulsed laser ablation between atomistic models of pure titanium with single crystal and polycrystalline structures were carried out using molecular dynamics simulations. The results revealed that the nanocrystalline sample modelled in this work, which exhibited lower heat conduction, produced a relatively deeper crater compared to its single crystal counterpart. The single crystal sample had a greater resistance to ablation, leading to the formation of a recast layer with rougher edges in contrast to the nanocrystalline sample. 
In materials science and engineering "size effect" is attributed to a phenomenon where the mechanical, thermal, optical or electrical properties of a crystalline material changes as a function of its physical size where at least one dimension is in submicron length scale. Experimental examination of the size effect was carried out on commercially pure titanium (consisting of 99.6% titanium and the remaining 0.4% containing carbon, nitrogen, hydrogen, iron and oxygen atoms) and Ti-6Al-4V alloy where craters were formed on both materials using single-shots with identical fluence while varying the diameter of the laser beam. It was observed that reducing the beam spot diameter resulted in relatively shallower craters, suggesting an increased threshold for ablation. 
Experiments comparing single-shot laser ablation outcomes between casted and 3D-printed Ti-6Al-4V alloy revealed that the 3D-printed surface (𝑅𝑎 = 32 𝑛𝑚) produced a slightly cleaner crater and smoother recast layer compared to the casted material (𝑅𝑎 = 45 𝑛𝑚). This observation was made after subjecting both substrates to ultrashort pulsed laser irradiation with identical laser parameters.