Cadot, Guillaume Bastien Jérémy
(2017)
Experimental and numerical investigations of diamond and
related materials controlled-depth machining using pulsed laser ablation.
PhD thesis, University of Nottingham.
Abstract
Pulsed laser ablation is a non-conventional machining technique that is used to machine complex parts in ultra-hard materials and for minute part geometry, which are otherwise not readily accessible with conventional tooling. The constant development of new materials with enhanced properties, as well as the demand for products with improved functionality have led to a renewed interest for alternative machining. Pulsed laser ablation is regarded as a promising technology with potential to machine a wide range of materials and shapes. The use of non-mechanical methods is advantageous due to the reduced tool-wear for ultra-hard materials and minute geometry. However, these advantages pose significant challenges since the removal rate of the material in term of shape and amount is controlled through a set of operating parameters. It is therefore necessary to have a comprehensive understanding of the process and the relation between such parameters and the effect of the laser on the surface. Furthermore, the process itself is hard to monitor online due to the short temporal and small spatial space it occurs within, and this makes it more complex to establish a detailed understanding of the process, and the optimum parameters to control the machining.
The main objective of this thesis is to develop mathematical frameworks that have the capability to predict the removal rate of pulsed laser ablation for the main operating parameters (feed speed, power, position, etc.) and the physical processes occurring during pulsed laser ablation of diamond and related materials for nanosecond laser pulses at 1064 nm and 248 nm. This is addressed using two modelling approaches: a physical model that simulates the mass and heat conservation in the system coupled with a collisional radiative model for the plasma, and a simplified approach based on geometrical aspect built on the idea that trenches represent the simplest element of the machining method enabling quantification of the relation between the control parameters and the removal rate.
In the physical approach, the system is modelled using the conservation of mass and energy with the capability to accurately predict the position of the interfaces (graphitisation front and surface), and the amount of material removed. The model is validated against boron doped diamond and is used to estimate the activation energy and rate of graphitisation for tetrahedral amorphous carbon. The framework developed provides accurate results for two different carbon allotropes with a high content of sp$^3$ bounds for a range of fluence.
A geometrical approach for the prediction of the material removal during large pulsed laser ablation machining task has been developed. Since, the objective of this model is for it to be integrated into CAD/CAM packages, the model needs to be computationally efficient and should require as little empirical data as possible to be accurately calibrated. This framework has been validated against three materials, graphite POCO AF-5, a mechanical polycrystalline diamond CVD Mechanical, and a metal-matrix poly-crystalline diamond CMX850. The model enables the prediction of material removal for large machining tasks and is being used with an optimisation procedure for the machining parameters (power, feed speed, etc.) for CAD/CAM packages.
Finally, the physical model is coupled with a collisional radiative model for the plasma, and it enables the prediction of the pressure over the crater. Experimental investigations have confirmed that melting of the graphite only occurs for a fluence over 30 J.cm$^{-2}$. TEM analysis and Raman spectroscopy also show an increase in the disorder of the graphite lattice with an increase of fluence which is coherent with thermal damage and constraint growth of the graphite crystal at the graphitisation front. The fluence threshold for the melting of the graphite lattice is in agreement with the prediction of the model.
The work developed in this thesis contributes to the understanding of the ablation process and graphitisation process during pulsed laser ablation of diamond and related material, and demonstrates how a simplified modelling approach can be used to improve current capabilities of this technology for large micro-machining tasks.
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