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Papers by Armelle Vardelle
International Thermal Spray Conference
The use of liquid precursors in plasma spraying makes it possible to produce coatings with more r... more The use of liquid precursors in plasma spraying makes it possible to produce coatings with more refined microstructures than in conventional plasma spraying. Depending on the injection device, the liquid feedstock is injected into the plasma jet in the form of liquid jet or droplets. The instabilities on the liquid-gas interface cause the mechanical break-up of liquids into drops that are subjected to further break-up until the droplets reach a stable state or evaporate. The process break-up may strongly influence the size, trajectories and, therefore, treatment of the droplets in the plasma medium. This study deals with the experimental observation of liquid break-up under plasma spray conditions when using a conventional DC plasma torch with radial injection by means of a pneumatic injection system that can deliver either liquid stream or blobs.
Coatings, 2021
Plasma spray-physical vapor deposition (PS-PVD) refers to a very low-pressure (~100 Pa) depositio... more Plasma spray-physical vapor deposition (PS-PVD) refers to a very low-pressure (~100 Pa) deposition process in which a powder is injected in a high-enthalpy plasma jet, and mostly vaporized and recondensed onto a substrate to form a coating with a specific microstructure (e.g., columnar). A key issue is the selection of the powder particle size that could be evaporated under specific spray conditions. Powder evaporation takes place, first, in the plasma torch between the injection location and nozzle exit and, then, in the deposition chamber from the nozzle exit to the substrate location. This work aims to calculate the size of the particles that can be evaporated in both stages of the process. It deals with an yttria-stabilized zirconia powder and two commercial plasma torches operated at different arc powers with gas mixtures of argon and helium or argon and hydrogen. First, it used computational fluid dynamics simulations to calculate the velocity and temperature fields of the pla...
Numerical simulations can be useful tools for investigating the effect of the torch operating par... more Numerical simulations can be useful tools for investigating the effect of the torch operating parameters and geometry on the arc characteristics and its interactions with electrodes. Up to now, most of the 3-D time dependent models of DC plasma torch are performed without including the solid walls into the computational domain. As a consequence, boundary conditions for the electromagnetic equations are questionable and may affect the discharge characteristics as the arc root size and arc dynamics. Moreover, the heat load to the electrodes, and so electrode erosion, cannot be properly modelled. This work deals with a 3-D time-dependent numerical simulation of a plasma arc ; it involves the arc column region and solid electrode regions in the same computational domain. The differences in arc characteristics when the solid electrodes were included or not into the computational domain were investigated for various operating parameters.
This study deals with the numerical simulations and experimental observation of a new industrial ... more This study deals with the numerical simulations and experimental observation of a new industrial thermal spray process developed at Inasmet Technicalia. The process, called “Oxi-Fuel Ionization” (OFI) combines a plasma torch with a combustion chamber and a nozzle. The plasma jet assists the combustion process; it makes it possible to stabilize it over a large range of fuel/oxidant conditions and also operate the gun with lower gas flow rates than conventional HVOF guns. The mathematical model involves the formation of the plasma jet, combustion process and supersonic flow issuing from the spray torch.It examines the influence of the HVOF nozzle design on the velocity and temperature fields of the gas flow. The model equations were solved using the CFD Fluent 6.3 code and the predictions were validated against experimental observations.
Plasma Chemistry and Plasma Processing, 1995
Journal of Thermal Spray Technology, 2009
Journal of Physics D: Applied Physics, 2013
ABSTRACT Suspension and solution plasma spraying makes it possible to achieve coatings with fine ... more ABSTRACT Suspension and solution plasma spraying makes it possible to achieve coatings with fine microstructural features and is becoming a common route in laboratories to elaborate coatings a few tenths to a few hundreds of micrometres thick. This paper presents the recent developments in direct current plasma spraying of suspensions or solutions. It begins with a short description of the main plasma torches used for liquid feedstock spraying as well as the techniques used to experimentally observe droplets and particles in the plasma jet and characterize the void network of nanostructured plasma-sprayed coatings. The paper then turns to the momentum and heat transfers between fine particles and the plasma jet and the interactions between the plasma jet and a liquid in the form of a jet or drops. It concludes by linking some characteristic features of coating microstructures with the liquid processing in the plasma jet.
Journal de physique. Colloque, Sep 1, 1990
International Thermal Spray Conference
In plasma spraying, the dynamics of droplet deformation and cooling on the substrate determines t... more In plasma spraying, the dynamics of droplet deformation and cooling on the substrate determines the geometry of the resulting splat, the porosity formation and the quality of contact between the splat and the underlying layer, thereby controlling the properties of coatings. The process of splat formation depends on the droplet velocity, size, molten state, chemistry and angle of impact, and on the substrate material, roughness, temperature and surface chemistry. This paper concentrates on investigating the way the droplets impact onto the substrate. A fast-imaging camera is used to observe the impact of plasma-sprayed alumina droplets on cold and hot substrates. Three modes of impact are observed: splashing, deposition or rebounding. A droplet is said to splash when it disintegrates totally or partially into secondary droplets after impacting onto the surface while it is said to deposit if the liquid material spreads and solidifies on the surface with a minimum splashing. The result...
Thermal spray, May 21, 2012
The residual stress level in coatings is a main issue in controlling in-service deformation, spal... more The residual stress level in coatings is a main issue in controlling in-service deformation, spallation or cracking. Residual stress generation has been widely studied for plasma and HVOF sprayed coatings, but only scare data are available for cold sprayed coatings. This paper describes the measurement and analysis of residual stresses in tantalum cold sprayed coatings. Residual stress measurements were performed by the hole-drilling and curvature methods. The former provided a through-thickness residual stress profile in the coating while the latter was used to investigate the in-situ residual stress evolution during the deposition process. The results from both methods were consistent and showed compressive stress of 350 MPa for a tantalum coating deposited on a 3 mm thick copper substrate at 80°C.
Thermal Spray 1998: Proceedings from the International Thermal Spray Conference, 1998
This paper presents a one-dimensional heat transfer model which predicts the solidification and c... more This paper presents a one-dimensional heat transfer model which predicts the solidification and cooling of a plasma-sprayed alumina splat after the flattening process is completed. A heterogeneous nucleation process taking place on the substrate surface was assumed. The density and average size of the formed nuclei were determined from the integration of the nucleation rate calculated from the classical kinetic theory for nucleation. This rate depends on the activation energy required for nucleation which takes into account the effect of the surface via a wetting angle between the growing nucleus and the catalytic surface. This contact angle was estimated from the comparison of the computed grain density with the density observed on splat surface using an atomic force microscope. When 67% of the splat surface in contact with the substrate are covered by grains, a planar solidification front was assumed to move through the melt. The theoretical model accounted also for the selection ...
International Thermal Spray Conference
The use of liquid precursors in plasma spraying makes it possible to produce coatings with more r... more The use of liquid precursors in plasma spraying makes it possible to produce coatings with more refined microstructures than in conventional plasma spraying. Depending on the injection device, the liquid feedstock is injected into the plasma jet in the form of liquid jet or droplets. The instabilities on the liquid-gas interface cause the mechanical break-up of liquids into drops that are subjected to further break-up until the droplets reach a stable state or evaporate. The process break-up may strongly influence the size, trajectories and, therefore, treatment of the droplets in the plasma medium. This study deals with the experimental observation of liquid break-up under plasma spray conditions when using a conventional DC plasma torch with radial injection by means of a pneumatic injection system that can deliver either liquid stream or blobs.
Coatings, 2021
Plasma spray-physical vapor deposition (PS-PVD) refers to a very low-pressure (~100 Pa) depositio... more Plasma spray-physical vapor deposition (PS-PVD) refers to a very low-pressure (~100 Pa) deposition process in which a powder is injected in a high-enthalpy plasma jet, and mostly vaporized and recondensed onto a substrate to form a coating with a specific microstructure (e.g., columnar). A key issue is the selection of the powder particle size that could be evaporated under specific spray conditions. Powder evaporation takes place, first, in the plasma torch between the injection location and nozzle exit and, then, in the deposition chamber from the nozzle exit to the substrate location. This work aims to calculate the size of the particles that can be evaporated in both stages of the process. It deals with an yttria-stabilized zirconia powder and two commercial plasma torches operated at different arc powers with gas mixtures of argon and helium or argon and hydrogen. First, it used computational fluid dynamics simulations to calculate the velocity and temperature fields of the pla...
Numerical simulations can be useful tools for investigating the effect of the torch operating par... more Numerical simulations can be useful tools for investigating the effect of the torch operating parameters and geometry on the arc characteristics and its interactions with electrodes. Up to now, most of the 3-D time dependent models of DC plasma torch are performed without including the solid walls into the computational domain. As a consequence, boundary conditions for the electromagnetic equations are questionable and may affect the discharge characteristics as the arc root size and arc dynamics. Moreover, the heat load to the electrodes, and so electrode erosion, cannot be properly modelled. This work deals with a 3-D time-dependent numerical simulation of a plasma arc ; it involves the arc column region and solid electrode regions in the same computational domain. The differences in arc characteristics when the solid electrodes were included or not into the computational domain were investigated for various operating parameters.
This study deals with the numerical simulations and experimental observation of a new industrial ... more This study deals with the numerical simulations and experimental observation of a new industrial thermal spray process developed at Inasmet Technicalia. The process, called “Oxi-Fuel Ionization” (OFI) combines a plasma torch with a combustion chamber and a nozzle. The plasma jet assists the combustion process; it makes it possible to stabilize it over a large range of fuel/oxidant conditions and also operate the gun with lower gas flow rates than conventional HVOF guns. The mathematical model involves the formation of the plasma jet, combustion process and supersonic flow issuing from the spray torch.It examines the influence of the HVOF nozzle design on the velocity and temperature fields of the gas flow. The model equations were solved using the CFD Fluent 6.3 code and the predictions were validated against experimental observations.
Plasma Chemistry and Plasma Processing, 1995
Journal of Thermal Spray Technology, 2009
Journal of Physics D: Applied Physics, 2013
ABSTRACT Suspension and solution plasma spraying makes it possible to achieve coatings with fine ... more ABSTRACT Suspension and solution plasma spraying makes it possible to achieve coatings with fine microstructural features and is becoming a common route in laboratories to elaborate coatings a few tenths to a few hundreds of micrometres thick. This paper presents the recent developments in direct current plasma spraying of suspensions or solutions. It begins with a short description of the main plasma torches used for liquid feedstock spraying as well as the techniques used to experimentally observe droplets and particles in the plasma jet and characterize the void network of nanostructured plasma-sprayed coatings. The paper then turns to the momentum and heat transfers between fine particles and the plasma jet and the interactions between the plasma jet and a liquid in the form of a jet or drops. It concludes by linking some characteristic features of coating microstructures with the liquid processing in the plasma jet.
Journal de physique. Colloque, Sep 1, 1990
International Thermal Spray Conference
In plasma spraying, the dynamics of droplet deformation and cooling on the substrate determines t... more In plasma spraying, the dynamics of droplet deformation and cooling on the substrate determines the geometry of the resulting splat, the porosity formation and the quality of contact between the splat and the underlying layer, thereby controlling the properties of coatings. The process of splat formation depends on the droplet velocity, size, molten state, chemistry and angle of impact, and on the substrate material, roughness, temperature and surface chemistry. This paper concentrates on investigating the way the droplets impact onto the substrate. A fast-imaging camera is used to observe the impact of plasma-sprayed alumina droplets on cold and hot substrates. Three modes of impact are observed: splashing, deposition or rebounding. A droplet is said to splash when it disintegrates totally or partially into secondary droplets after impacting onto the surface while it is said to deposit if the liquid material spreads and solidifies on the surface with a minimum splashing. The result...
Thermal spray, May 21, 2012
The residual stress level in coatings is a main issue in controlling in-service deformation, spal... more The residual stress level in coatings is a main issue in controlling in-service deformation, spallation or cracking. Residual stress generation has been widely studied for plasma and HVOF sprayed coatings, but only scare data are available for cold sprayed coatings. This paper describes the measurement and analysis of residual stresses in tantalum cold sprayed coatings. Residual stress measurements were performed by the hole-drilling and curvature methods. The former provided a through-thickness residual stress profile in the coating while the latter was used to investigate the in-situ residual stress evolution during the deposition process. The results from both methods were consistent and showed compressive stress of 350 MPa for a tantalum coating deposited on a 3 mm thick copper substrate at 80°C.
Thermal Spray 1998: Proceedings from the International Thermal Spray Conference, 1998
This paper presents a one-dimensional heat transfer model which predicts the solidification and c... more This paper presents a one-dimensional heat transfer model which predicts the solidification and cooling of a plasma-sprayed alumina splat after the flattening process is completed. A heterogeneous nucleation process taking place on the substrate surface was assumed. The density and average size of the formed nuclei were determined from the integration of the nucleation rate calculated from the classical kinetic theory for nucleation. This rate depends on the activation energy required for nucleation which takes into account the effect of the surface via a wetting angle between the growing nucleus and the catalytic surface. This contact angle was estimated from the comparison of the computed grain density with the density observed on splat surface using an atomic force microscope. When 67% of the splat surface in contact with the substrate are covered by grains, a planar solidification front was assumed to move through the melt. The theoretical model accounted also for the selection ...