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The suspension plasma spray process is now emerging at the industrial scale, and open questions remain regarding the formation of coatings through this process, especially regarding the relationship among particle impact, spreading and build-up into columns. Furthermore, concerns have been raised regarding the behavior of the suspension droplets in the plasma jet stagnation zone since it is a zone where plasma jet streamline directions and velocities are highly disrupted when the plasma jet impinges on a solid surface. In this study, the particle-laden jet streamlines were experimentally observed very closely to a flat surface using a particle image velocimetry technique. Submicron particles were collected in flight to determine which particles contributed to coating growth. The particle mean impact velocity against the surface was determined through image velocimetry and splat dimensions. The impact angles of the particles were also determined, and a correlation was identified with the growth angles of the columnar coating.

Thermal barrier coatings manufactured either by atmospheric plasma spraying or electron beam physical vapor deposition are widely used in the hot-temperature sections of gas turbines to provide thermal insulation and corrosion protection. Recently, the suspension plasma spraying process has emerged for the manufacturing of thermal barrier coatings. Specific microstructures including columns can be deposited. A narrow window of optimal parameters remains an outstanding problem to produce the suitable microstructure. Lately, studies demonstrated that the substrate topography plays a significant role in the columnar structure formation. This study shows that the substrate topography obtained via laser surface texturing enables a regular columnar structure. The laser beam generates peaks and cavities regularly arranged on the surface. The plasma spray parameters were selected to produce columnar structures linked to the substrate topography. The suspension composition and coating microstructure were studied. Superalloy samples with bond coats were used as substrates. The results showed that the columnar coating microstructure can be adjusted by optimizing the laser process parameters. The improved coating structure had more than 500 columns per mm2. Substrate surface topography optimization and suspension plasma spraying can improve the performances of thermal barrier coating system.

Considerable progress has been made over the last decades in thermal spray technologies, practices and applications. However, like other technologies, they have to continuously evolve to meet new problems and market requirements. This article aims to identify the current challenges limiting the evolution of these technologies and to propose research directions and priorities to meet these challenges. It was prepared on the basis of a collection of short articles written by experts in thermal spray who were asked to present a snapshot of the current state of their specific field, give their views on current challenges faced by the field and provide some guidance as to the R&D required to meet these challenges. The article is divided in three sections that deal with the emerging thermal spray processes, coating properties and function, and biomedical, electronic, aerospace and energy generation applications. Published version

International audience; The plasma spray process with solid feedstock, mainly ceramics powders, studied since the sixties is now a mature technology. The plasma jet and particle in-flight characterizations are now well established. The use of computer-aided robot trajectory allows spraying on industrial parts with complex geometries. Works about splat formation have shown the importance of: the substrate preheating over the transition temperature to get rid of adsorbates and condensates, substrate chemistry, crystal structure and substrate temperature during the whole coating process. These studies showed that coating properties strongly depend on the splat formation and layering. The first part of this work deals with a summary of conventional plasma spraying key points. The second part presents the current knowledge in plasma spraying with liquid feedstock, technology developed for about two decades with suspensions of particles below micrometers or solutions of precursors that form particles a few micrometers sized through precipitation. Coatings are finely structured and even nanostructured with properties arousing the interest of researchers. However, the technology is by far more complex than the conventional ones. The main conclusions are that models should be developed further, plasma torches and injection setups adapted, and new measuring techniques to reliably characterize these small particles must be designed

International audience; Nano-structured coatings should exhibit better properties than micro-structured coatings because of a high volume fraction of internal interfaces. Since the mid-nineties a large body of works have been devoted to suspension and solution plasma spraying for the deposition of finely and even nanometer-structured coatings. The aim of this paper is to take stock of our present knowledge in the field of suspension plasma spraying that is, at the moment, essentially used for oxide ceramic coatings. It will first tackle the injection of the suspension in the plasma jet and the behavior of nano or sub-micro-meter particles processed in the plasma jet core involving the liquid breakup and vaporization that releases the solid particles from the solvent droplets. It will, then, deal with the plasma torches and liquid feeding systems available to suspension spraying. It will finally discuss the key characteristics of suspensions (solvent, dispersant, and particle morphologies), designing of coating microstructure, and potential industrial applications, with the developments requested to cope with these applications.

International audience; Plasma generated by the SG-100 torch was applied to spray suspension formulated with the use of ZrO2 + 8 wt.% Y2O3 (8YSZ) and ZrO2 + 24 wt.% CeO2 + 2.5 wt.% Y2O3 (24CeYSZ) as solid phases. The suspensions were formulated with the use of 20 wt.% solid phase, 40 wt.% water, and 40 wt.% ethanol. The plasma spray parameters were optimized by keeping constant: (a) the electric power of 40 kW and (b) the working gas compositions of 45 slpm for Ar and 5 slpm for H2. On the other hand, the spray distance was varied from 40 to 60 mm and the torch linear speed was varied from 300 to 500 mm/s. The coatings were sprayed onto stainless steel substrates, and their thicknesses were in the range from 70 to 110 μm. The coating microstructures were analyzed with a scanning electron microscope. Mechanical properties were tested with the different methods including the indentation and scratch tests. The indentation test, carried out with various loads ranging from 100 to 10,000 mN, enabled to determine elastic modulus and Martens microhardness. Young's modulus of the coatings was in the range of 71-107 GPa for 8YSZ and 68-130 GPa for 24CeYSZ coatings. The scratch test enabled the authors to find the scratch macrohardness.

International audience; A new finite element model is used to investigate catastrophic failures of a thermal barrier coatings system due to crack propagation along the interfaces between the ceramic top-coat, thermally grown oxide, and bond-coat layers, as well as between the lamellas structure of the ceramic layer. The thermo-mechanical model is designed to take into account a non-homogenous temperature distribution and the effects of the residual stresses generated during the coating process. Crack propagation is simulated using the contact tool "Debond" present in the ABAQUS finite element code. Simulations are performed with a geometry corresponding to similar or dissimilar amplitudes of asperity, and for different thicknesses of the oxide layer. The numerical results have shown that crack evolution depends crucially on the ratio of the loading rate caused by growth and swelling of the oxide layer and also on the interface roughness obtained during the spraying of coatings

International audience; An experimental setup was developed to produce fully melted, millimeter-sized, ceramic or metallic drops with impact velocities up to 10 m/s. Such impact velocities allow achievement of impact Weber numbers, close to those of the plasma spray process (We= 2300) with droplets in the tens of micrometer size range. A fast camera (4000 image/s) combined with a fast pyrometer (4000 Hz), allowed the flattening of the drop to be followed. To study the flattening of micrometer-sized droplets, a direct-current (dc) plasma torch was used to melt alumina particles (around 45 lm in diameter). The experimental setup was composed of a fast (50 ns) two-color pyrometer and two fast (at best an exposure time of 2 ls) charge-coupled device cameras (one orthogonal and other tangential to the substrate). The flattening behaviors of millimeter- and micrometer-sized particles were compared. First, impacts of alumina drops (millimeter-sized) with velocities up to 10 m/s were studied. Results were then compared with micrometer-sized alumina particles (about 45 lm in diameter) sprayed with the dc plasma torch. A correlation was found between both flattening scales, and, in spite of the lower impact velocity at the millimeter scale, droplet ejections were also found as obtained at the micrometer scale. This work shows that for a sound comparison of phenomena at the two different scales it is mandatory to have Weber numbers as close as possible in both cases.

International audience; The development of coating formation processes involving electric arcs depends on process stability and the capacity to ensure a constant reproducibility of coating properties. This is particularly important when considering suspension plasma spraying or solution precursor plasma spraying. Submicron particles closely follow plasma instabilities and have nonhomogeneous plasma treatment. Recently, it has been shown that arc voltage fluctuations in direct-current (dc) plasma torches, showing dominant fluctuation frequencies between 4 and 6 kHz, are linked to pressure oscillations in the cathode cavity of the plasma torch. In this study, first, a method to isolate the different oscillation modes in arc voltage and pressure signals using signal processing methods is presented. Second, correlations between the different modes of oscillations are analyzed following the plasma torch operating parameters. Lastly, it is shown that the use of an acoustic stub, mounted on the torch body, decreases the amplitude of arc voltage fluctuations and slightly increases the mean voltage.

International audience; Solution Precursor Plasma Spraying (SPPS) is a thermal spray process where a feedstock solution is heated and pyrolized to form fine (i.e., 1000 nm) molten particles that deposit onto a substrate to form a layer. The benefits of implementing the SPPS process include, among others: (i) the possibility to create unique microstructures at nanometer scale without the injection feeding problems usually associated to powder feeders and delivery cables and (ii) rapid exploration of novel precursor compositions. In this study, preparation and characterization of porous anode layers with homogeneous Nickel distribution and nanometer sized microstructure are considered for solid oxide fuel cell (SOFC) application. Once the solution is injected, the droplets go through several chemical and physical changes and impact the substrate in different states, from fully molten one to unpyrolized one. The effects of some spray parameters, such as the spray distance and the plasma flow mass enthalpy, on the layer architecture and composition were investigated. The results show that dense or porous layers can be manufactured depending on the operating parameters.

International audience; This study deals with the numerical investigation of a hybrid thermal spray process that combines HVOF and thermal plasma technologies. In this process, a thermal plasma is used to assist the combustion process that proceeds in a quasi-conventional HVOF system. It is expected that this coupling makes the HVOF system more flexible in terms of working parameters and sprayed materials. Also, a rather low fuel gas consumption and high deposition rate compared to that of most of the conventional HVOF guns are sought. Modeling this process can help to understand the phenomena that control the operation of the spray system and, therefore, help to optimize it. The model involves the plasma formation, combustion process, and expansion of the supersonic jet in the ambient atmosphere. In this study, the system uses argon as plasma-forming gas and methane as fuel gas. Fuel and oxidant are not premixed before entering the combustion chamber. In the model, methane oxidation is represented by a single-step global reaction considering only a few chemical species (fuel, oxidant, and product species); the turbulent nonpremixed combustion is modeled by a fast-chemistry combustion model that assumes that the rate of chemical reaction is controlled by turbulence. The model equations are solved using the CFD software Fluent 6.3. The main gas flow characteristics (velocity, temperature, and pressure) in presence and absence of the plasma source are compared and discussed, and the benefits of the plasma source are discussed in the light of predictions and fuel combustion mechanisms.

International audience; In numerous applications developed at the Commissariat a` lEnergie Atomique, Direction de lEnergie Nucle´ aire (CEA-DEN, French Atomic Agency, Atomic Energy Department), particularly those encountered in the processing of nuclear wastes, metallic components are subjected to extreme environments in service, in terms, for example, of ageing at moderated temperature (several months at about 300 C) coupled to thermal shocks (numerous cycles up to 850 C for a few seconds and a few ones up to 1500 C) under a reactive environment made of a complex mixture of acid vapors in the presence of an electric field of a few hundred volts and a radioactive activity. Alumina plasma-sprayed coatings manufactured with feedstock of different particle size distributions, graded alumina-titania coatings, and phosphate-sealed alumina coatings were investigated to improve the properties of metallic substrates operating in such extreme environments. The effects of particle size distribution, phosphate sealant, and graded titania additions on the dielectric strength of the as-sprayed, thermally cycled and thermally aged coatings were investigated. Thermal ageing test was realized in furnace at 350 C for 400 h and thermal shocks tests resulted from cycling the coating between 850 and 150 C using oxyacetylene flame and compressed air-cooling. Alumina coating structures and phase content were characterized in parallel by scanning electron microscopy (SEM) coupled to image analysis and stereological protocols and X-ray diffractometry (XRD). The dielectric strength was assessed by measuring the breakdown voltage at 50 Hz during and after the thermal tests.

International audience; This commentary aims at presenting, from a practical viewpoint, some key points to assess when implementing image analysis coupled to stereological protocols to quantify statistically the architecture of thermal spray coatings and their relevant features (pores, lamellae, and so forth.). This article is the second of a two-part commentary; the first one, published in Journal of Thermal Spray Technology, Vol 16 (No. 1), 2007, detailed those stereological protocols from a historical perspective.

International audience; This article aims at presenting an expert system to assist the design and the simulation of 2-D shapes of alumina-titania (i.e., Al2O3-13 wt.% TiO2) Atmospherically Plasma Sprayed (APS) coatings. Indeed, the expert system derives from a spray deposition mathematic model resulting from experiments. The varied processing parameters were the geometric and the kinematics parameters, mainly, such as: the relative speed gun-substrate, the spray distance, the spray angle, the relative positioning powder injector-spray gun trajectory, the number of passes and the powder feed rate. The variations of the geometry and some of the structural parameters were analyzed relatively to the aforementioned varied parameters. Thus, a large set of spray pattern parameters was designed. This set considers mostly the spray pattern geometry. All the relationships between the processing parameters and the spray pattern parameters were hence grouped in a spray deposition model. The second step of this work consisted in optimizing the robotic (i.e., spray gun) trajectory using a robotic code, which permits a realistic simulation of the spray gun speed and its inertia. Using this simulation software, a trajectory file was built. In the third step of the work, an expert system was developed by combining the spray deposition model with the trajectory. The tasks of the expert system are: (1) to assist the user in designing the coatings by selecting the processing parameters and (2) to simulate the coating shapes by integrating the gun trajectory.

This article presents, from a historical perspective, some stereological protocols of the first order. Such protocols can be implemented to quantify statistically the architecture of thermal spray coatings and their relevant features (pores, lamellas, etc.). A forthcoming Part II of this article will address some key points to implement, from a practical point of view, such protocols.

Editorial; International audience; Why Choose 2006 As The First 100 Years? The Schoop Patent that concerns the origins of thermal spray (TS) can be traced back to 1912 (for example: “Metal Plating with the Air Brush,” Scientific American, Vol 1, 1913, p 346). A tradition exists where chronological anniversaries are recognized by an appropriate public observance. Such an observance recognizes an important historical event and allows a retrospective account of that event. In many instances no specific date is selected, but rather a time frame that may span several days, a few months, or an entire year, since the magnitude of the event is such that its anniversary needs to be placed into the context of its evolution. The observance of the first 100 years of thermal spray falls under a very broad time frame because the early history of this technology is clouded by lack of referred documentation. The early practitioners and inventors of thermal spray were probably unaware that their efforts would spawn multimillion dollar industries across many technological sectors. It is probably safe to assume that these early workers were more interested in preserving jobs rather than history. As well, a certain amount of secrecy would have given these early technologists a commercial edge

International audience; Nanostructured materials offer significant improvements in engineering properties because their grain sizes are smaller than those of conventionally processed materials by a factor of almost 2 orders of magnitude (Ref 1). Since the mid- 1990s, research has been conducted using thermal spray technology for the deposition of finely structured or nanostructured coatings (Ref 2, 3). To produce finely structured coatings by thermal spray techniques, four routes have been suggested