Experimental and numerical investigations of diamond and related materials controlled-depth machining using pulsed laser ablation

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

On the relationship between the base pressure and the velocity in the near-wake of an Ahmed body

We investigate the near-wake flow of an Ahmed body which is characterized by switches between two asymmetric states that are mirrors of each other in the spanwise direction. The work focuses on the relationship between the base pressure distribution and the near-wake velocity field. Using direct numerical simulation obtained at a Reynolds number of 10000 based on incoming velocity and body height as well as Bonnavion and Cadot's experiment (Bonnavion et al. 2018), we perform Proper Orthogonal Decomposition (POD) of the base pressure field. The signature of the switches is given by the amplitude of the most energetic, antisymmetric POD mode. However switches are also characterized by a global base suction decrease, as well as deformations in both vertical and lateral directions, which all correspond to large-scale symmetric modes. Most of the base suction reduction is due to the two most energetic symmetric modes. Using the linear stochastic estimation technique of Podvin et al. 2018, we show that the large scales of the near-wake velocity field can be recovered to some extent from the base pressure modes. Conversely, it is found that the dominant pressure modes and the base suction fluctuation can be well estimated from the POD velocity modes of the near-wake

Relationship between the base pressure and the velocity in the near-wake of an Ahmed body

We investigate the near-wake flow of an Ahmed body which is characterized by switches between two asymmetric states that are mirrors of each other in the spanwise direction. The work focuses on the relationship between the base pressure distribution and the near-wake velocity field. Using direct numerical simulation obtained at a Reynolds number of 10000 based on incoming velocity and body height as well as Bonnavion and Cadot's experiment (Bonnavion et al. 2018), we perform Proper Orthogonal Decomposition (POD) of the base pressure field. The signature of the switches is given by the amplitude of the most energetic, antisymmetric POD mode. However switches are also characterized by a global base suction decrease, as well as deformations in both vertical and lateral directions, which all correspond to large-scale symmetric modes. Most of the base suction reduction is due to the two most energetic symmetric modes. Using the linear stochastic estimation technique of Podvin et al. 2018, we show that the large scales of the near-wake velocity field can be recovered to some extent from the base pressure modes. Conversely, it is found that the dominant pressure modes and the base suction fluctuation can be well estimated from the POD velocity modes of the near-wake.Comment: 34 pages, 12 figure

Low-order modelling of the wake dynamics of an Ahmed body

International audienceWe investigate the large-scale signature of the random switches between two mirrored turbulent wake states of flat-backed bodies. A direct numerical simulation of the flow around an Ahmed body at a Reynolds number of 10000 is considered. Using Proper Orthogonal Decomposition, we identify the most energetic modes of the velocity field and build a low-dimensional model based on the first six fluctuating velocity modes capturing the characteristics of the flow dynamics during and in-between switches. In the absence of noise, the model produces random switches with characteristic time scales in agreement with the simulation and experiments. This chaotic model suggests that random switches are triggered by the increase of the vortex shedding activity. However, the addition of noise results in a better agreement in the temporal spectra of the coefficients between the model and the simulation

Effect of a base cavity on the wake of the squareback Ahmed body at various ground clearances and application to drag reduction

Recently, Evrard et al. (2016, Journal of Fluids and Structures, 61) achieved drag reduction by almost 9 % by means of a base cavity on a three-dimensional bluff body, the squareback Ahmed body. The authors associate drag reduction with the suppression of the static asymmetric modes of the wake identified by Grandemange et al. (2013, Journal of Fluid Mechanics, 722) leading to its symmetrization. The beneficial effect of a base cavity on the drag has been known for decades on axisymmetric bluff bodies in the context of aerospace engineering (Morel, 1979, Aeronautical Quarterly, 30) but the phenomenon has not been fully elucidated yet. The present work aims at showing experimentally that the decrease of the asymmetry of the near wake flow is responsible for drag reduction regardless of the ground clearance. With this aim in mind, we do two parametric studies of the ground clearance of a squareback Ahmed body in an industrial wind-tunnel; one without and one with a base cavity. We want to compare the two bifurcations of the wake operated by the ground clearance (Grandemange et al., 2013, Physics of Fluids, 25) depending on the rear geometry. Far enough from the ground i.e. after the bifurcation, the modes indeed disappear in agreement with the work of Evrard et al. the flow is symmetrized and the base pressure increases by 24 %. In the vicinity of the ground however, two new results are reported in this paper. The modes governing the wake are not the same as at higher ground clearances but are rather vertical modes due to the presence of the ground. Besides, even if the cavity does not fully symmetrize the flow in this case, its asymmetry is reduced. Consequently, an important drag reduction of the same order of magnitude as in the far ground regime is observed (7 % associated with a base pressure increase by 20 %). A six-components aerodynamic balance and twenty-one instantaneous base pressure measurements are used to record the data at a high sampling rate to study the dynamics of the changes

SEM picture of the ablation crater of boron doped diamond

SEM picture for several fluences of the ablation crater of a boron doped diamond using a nanosecond pulsed laser ablation

Experimental and numerical investigations of diamond and related materials controlled-depth machining using pulsed laser ablation

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

Topography of Crater after nanosecond ablation pulses

Topography of the crater for several fluences for boron doped diamond

Simulation Results for tetrahedral amorphous carbon

Results from the simulation using the model presented in the paper for tetrahedral amorphous carbo