Suspension plasma spraying (SPS) is increasingly studied to produce finely structured coatings with dense and columnar microstructures for promising thermal barrier coatings especially in aerospace application. However, this process involves many parameters and complex phenomena with large spans of time and space scales in many physical mechanisms, like droplet break-up, liquid droplet evaporation, and various physical phenomena occurring within the suspension droplet, making it difficult to master. Especially, understanding the interactions of liquid drop submitted to plasma with the submicronic suspended particles is essential for material process optimization and control. For SPS understanding, a meaningful modelling of suspension treatment requires a prior analysis of these physical mechanisms and their characteristic times. This study details the different phenomena, their significance and characteristic timescales as well as the selection of the main governing forces acting between the different continuous and discrete phases (plasma, liquid, submicronic particles). We explore associated mechanisms: droplet breakup, carrier liquid evaporation, convective mixing and submicronic particle diffusion within the droplets. These mechanisms involve mass and heat transfer, that should condition particle agglomeration morphology before melting.

This study investigates the effect of water injection in the high pressure chamber of a cold spray nozzle. A de Laval nozzle geometry with constant back pressure and temperature is modeled numerically using Reynolds Stress Model coupled equations. Water spray with a droplet size of 10 – 100 μm is modeled using both uniform and Rosin-Rammler size distributions. The two-phase flow of gas-liquid is modeled using an unsteady discrete phase mass source with two-way coupling with the main gas flow. Upon injection, the droplets in the water spray evaporate while travelling through the nozzle due to momentum and energy exchange with the gas flow. The evaporation behavior in presence of water content is modeled and a correlation between the initial diameter and the diameter just before the throat is obtained. As a result, the proper droplet size distribution with a fully evaporative spray can be used as a carrier of nano-particles in cold spray nozzles. Having the results, guide us to substitute the un-evaporated part of the droplet with an equal diameter agglomerate of nano particles and find a minimum fraction of nano particles suspended in the liquid which guarantees fully evaporative liquid spray injection.

In this study, a downstream injection cold spray nozzle is modeled numerically under various loadings. Instead of micron-sized particles, liquid feedstock as a carrier of nanoparticle suspension is fed into the nozzle through a port located 6 mm downstream of the nozzle throat at the diverging section. Water is used as the liquid carrier with a droplet size distribution of 5-100 µm and liquid-to-gas ratio ranging from 5 to 15%. The radial injection of droplets is simulated by Lagrangian particle tracking which includes the effects of heating and evaporation. The effect of the feedstock on downstream flow is analyzed and the optimum solid-to-liquid fraction in the suspension is determined.

Impact of liquid injection on atmospheric plasma has been measured by an enthalpy probe and visualized by shadowgraphy. Liquid was injected by two nozzles. A solid stream nozzle with 0.3 mm diameter and an atomizing nozzle with a spray angle of 30°. For constant liquid flow rate and varied plasma current an optimal injection parameter has been obtained. Influence of different flow rates on plasma characteristics has been examined. Full profiles along each axis show differences for different injection techniques in enthalpy, temperature, velocity and composition.

High-velocity suspension flame spraying (HVSFS) is used for direct processing of submicron and nano-scaled particles to achieve dense surface layers in supersonic mode with a refined structure, from which superior properties are expected. The application of solutions as a carrier fluid for nano-particles in thermal spray systems is a new approach that requires some thermo-physical and chemical optimization. Three dimensional modeling and analysis of the combustion and gas dynamic phenomena of the three-phase HVSFS process is performed in this study for an industrial TopGun-G torch, based on a numerical model for a conventional HVOF process. Parameter analysis of the solution mixture (proportion between aqueous and organic solvent) in a suspension is performed as well as analysis of the variation of the combustion chamber depending on the torch design, leading to more homogeneous flow properties for an improved HVSFS torch.

The agglomeration process plays an important role in suspension plasma spraying (SPS), affecting the state of particles prior to impact and consequently the mechanical properties of the coated substrate. This paper presents the results of an investigation on the agglomeration of submicron YSZ particles sprayed from a water suspension. The shape and surface morphology of transient and final agglomerate structures was studied by injecting YSZ suspensions with a nebulizer into an inductively coupled plasma torch. It was found that particle agglomeration occurs in successive stages identified as cup or doughnut shaped agglomerates, polycrystalline particles, and amorphous or partially molten particles.

The aim of this work is to develop an analytical methodology for the analysis and prediction of high-velocity suspension flame spraying (HVSFS) under various operating conditions, to determine the effect of individual parameters on the process, and to aid and promote the design of HVSFS torches. A key aspect of the work is the development of a model that accounts for fuel gas combustion, evaporation of organic solvents, and heat, momentum, and mass transfer between the flame and suspension droplets.

The aim of this work is to obtain a better understanding of high-velocity suspension flame spraying though detailed modeling and analysis. A numerical model was developed and is used to analyze the effects of process parameters including droplet size, injection velocity, secondary droplet breakup, solution evaporation and combustion, mass flow rate, flame temperature, and the location of the injection point. The results show that initial injection parameters play an important role in controlling the process and should be specified to minimize cooling effects and promote droplet evaporation inside the combustion chamber. It was also found that the ideal location for the injection point is inside the chamber, which in practice, can be achieved by slightly modifying gun geometry.

Samaria doped ceria (SDC), used as a solid oxide fuel cell (SOFC) electrolyte, was deposited by Solution Plasma Spray (SolPS) process. An aqueous solution of mixed nitrates was axially injected into the Radio Frequency (RF) inductively coupled plasma through an atomization probe. The atomic ratio of the SDC components in solution was (Ce 0.85 Sm 0.15 )O 1.925 . In order to control the initial spray droplet diameter, Malvern RTsizer atomization tests were performed. In-flight particle temperature and velocity were analyzed by DPV 2000. Coatings thickness and morphology were measured and observed by scanning electron microscopy, the substrate porosity being determined through gas permeability measurements. Results indicate that the coating's porosity and density strongly depend on the plasma spraying parameters. Gap size of the injection probe, atomization gas flow rate and the probe position have no significant effects on the coating’s porosity under the spraying conditions applied in this study

For many years, a new interest in nanomaterials, with grain sizes smaller than 100nm, has emerged. This is due to the enhanced properties of the resulting sintered materials or coatings compared to those with coarser-grained materials. This paper is devoted to the feasibility to produce nanomaterial coatings by a dc plasma spray process. Until now, only thick coatings (> 100µm) have been elaborated using this technique, by injecting, with a carrier gas, micrometric particles in the plasma flow. But, it is not possible to inject too small particles (<5µm) without perturbing drastically the plasma jet by the high carrier gas flow rate necessary to give them a high enough momentum. This work presents a new dc plasma spray process, designed to elaborate alumina nanocoatings. The most important step of the process is the control of the ceramic nanometric particle penetration in the plasma. Because of their small size, a liquid, which density made the momentum transfer more efficient, replaced the carrier gas with an injector creating calibrated droplets with controlled velocity and flow rate. To study the liquid-plasma interaction, the penetration of pure water in an Ar/H 2 plasma jet was investigated by means of emission spectroscopy. The modification of temperature field together with oxygen concentration was determined quantitatively. Emission spectra were treated with a new localization method, avoiding the use of Abel's inversion implying a cylindrical symmetry, destroyed by the liquid injection. Such measurements allowed optimizing the liquid penetration in the plasma jet. Alumina nanopowders were dispersed in a liquid to form a stable suspension, which was injected in the plasma. The layered particle morphology, collected on glass substrates at different distances downstream of the injection point, was then studied.

The high technical level of thermal spraying is based on four segments: available know-how, equipment and installations, spray materials, and industrial gases. This article discusses the application of industrial gases used in thermal spraying. It describes the processes involved in the production, provision, and storage of industrial gases as well as their properties. In addition, the article also discusses the influence of quality on the individual fuel gases used in thermal spraying.

This paper investigates the problem of complete evaporation of zirconium oxide powders that are injected into a thermal RF plasma. Particle trajectories and evaporation are studied using optical emission spectroscopy and laser Doppler anemometry. Model calculations are compared with the results of process diagnostics. It is observed that axial emission profiles confirm the influence of the particle size on the evaporation behavior. Paper includes a German-language abstract.

This paper describes a new spraying process that uses a special torch to inject aluminum oxide suspensions into a dc plasma jet. Based on the results obtained, a general mechanism of action is proposed in which factors such as acceleration, impact behavior, and heat transfer phenomena play a role. Paper includes a German-language abstract.

Suspension Plasma Spraying (SPS) is a thermal spray process based on a suspension of fine (<10 μm) or even ultrafine (<100 nm) powders which is axially fed into the induction plasma through an atomization probe. The atomization of the suspension results in microdroplets (20 μm in size). They are flash dried in the plasma, melted and finally can impact a substrate to build a coating or be cooled down and collected as a spheroidized powder. The large industrial potential of this technology results first from the use of fine powder or even sol-gel which is one of the starting step for many ceramic processes, and second from the various side benefits of the liquid phase in the SPS. Indeed, the liquid phase can be simply a carrier for ultrafine powder, or a protection against oxidation in the case of metals, or a protection for health in the case of whiskers, for instance. It can also take a part in chemical reactions when the liquid phase is a solution of chloride, nitrates... or it can be an organic liquid for the synthesis of carbide, where CO is a strong reducer. Furthermore the liquid phase can also release some energy because of its combustion at the very end of the process. It can also change the local atmosphere surrounding the in flight droplets in the plasma where it is possible to use H 2 O 2 as a carrier in order to increase the oxygen partial pressure around sensitive to oxygen decomposition materials. The applications of SPS are in the powder synthesis (in R&D or production), in the spraying of metals, ceramics or composites directly synthesized, or in production of very reactive with air materials. Applications of SPS will be presented for hydroxyapatite (HA) and NiAlMo. Induction plasma SPS coatings and/or powders properties will be discussed as a function of the SPS process variables.

When spraying ceramic particles with a low thermal conductivity such as zirconia using Ar-H 2 direct-current (d.c.) plasma jets where the heat transfer is important, heat propagation phenomena take place with the propagation of melting, evaporation or even solidification fronts. Most models neglect these heat propagation phenomena assuming the particle as a lumped media. This work is aimed at developing a model coupling the effect of heat propagation with the particle dynamic within plasma jets. It uses an adaptative grid in which the coordinates of the phase change fronts are fixed. It allows minimizing the calculation costs (approximately 10 seconds on PC under windows XP against 1hour with an enthalpy model). Such calculations are illustrated for dense and porous agglomerated zirconia as well as iron particles which evaporation in an Ar- H 2 (25 vol %) plasma is important.

The effect of argon carrier gas flow, ethanol- or water injection on the plasma-composition, -enthalpy and – temperature as well as on the entrainment from the ambient was investigated in comparison to the pure plasma condition of an Ar-H 2 -plasma. Additionally, the plasma gas flow rate and power levels were varied. The enthalpy, and thus the plasma temperature, as well as the composition and speed are determined by means of an enthalpy probe system. Spatial resolved data could be obtained by translating the torch via a robot so that a temperature and speed map shows the evolution of plasma features with increasing stand-off distance.

This study deals with the deposition of coating materials that can be difficult to process by plasma spraying, including lanthanum and gadolinium zirconate, two pyrochlores of interest for thermal barrier applications, and lanthanum strontium cobalt ferrite (LSCF), a perovskite of interest for gas separation membranes. In addition to conventional atmospheric plasma spraying (APS), the feedstock powders were applied by suspension plasma spraying (SPS) and plasma spray-physical vapor deposition (PS-PVD). The spraying processes are described in detail along with the characteristics of the powders and coatings and the effects of various spray parameters on splat behavior and coating composition and structure.

Thermal spraying of pure SiC is difficult due to decomposition issues at elevated temperatures. However, the development of suspension plasma spray opens a new path to investigate the deposition of this material since the liquid carrier can hinder this phenomenon. The present work investigates a new route for producing SiC submicron structured coating by suspension plasma spraying (SPS). Classical SiC manufacturing routes using suspension (i.e: spray drying, tape casting) are studied regarding their feasibility to be used on suspension plasma spraying. Aqueous-based suspensions containing 10 wt.% SiC powder (0.60 µm) along with sintering additives are dispersed and stabilized. Both suspensions are sprayed on martensitic stainless steel substrate (AISI 440C) to achieve finely structured and dense coatings. Digital image analysis, X-ray diffraction and scanning electron microscopy are utilized to characterize the coating microstructures. Their dependency on suspension characteristics and spray operation parameters are discussed with respect to the final coating performance.

Suspension plasma spraying (SPS) is a fairly recent technology that is able to process sub-micrometric-sized feedstock particles and permits the deposition of layers thinner (from 5 to 50 µm) than those resulting from conventional atmospheric plasma spraying (APS). SPS consists in mechanically injecting within the plasma flow a liquid suspension of particles of average diameter varying between 0.02 and 1 µm. Upon penetration within the DC plasma jet, two phenomena occur sequentially: droplet fragmentation and evaporation. Particles are then processed by the plasma flow prior their impact, spreading and solidification upon the surface to be covered. Depending upon the selection of operating parameters, among which plasma power parameters (operating mode, enthalpy, spray distance, etc.), suspension properties (particle size distribution, powder mass percentage, viscosity, etc.), and substrate characteristics (topology, temperature, etc.), different coating architectures can be manufactured, from dense to porous layers. Nevertheless, the coupling between the parameters controlling the coating microstructure and properties are not yet fully identified. The aim of this study is to further understand the influence of parameters controlling the manufacturing mechanisms of SPS alumina coatings, in particular the spray patterns influence.

One of the key aspects of particle behavior in the plasma jet is the stochastic nature of the injection process. Particles are injected into the plasma jet with a certain variation in initial velocity which, along with their size distribution, determines the form of the layer structure and the degree of melting. The aim of this work is to model the influence of the initial spray parameters on the main characteristics of spray particles, such as temperature, velocity, and degree of melting, during impact with the substrate. A one-dimensional heat conduction equation that accounts for phase transitions is solved numerically for spherical particle shapes. The dynamic particle problem is also solved and an integral criterion for the coating model is obtained. Paper includes a German-language abstract.