This paper reviews various powder treatments and particle alteration processes and evaluates their effect on the microstructure and properties of thermal spray coatings. It discusses the benefits and drawbacks of thermal plasma treatments for powders, the use of self-propagating high-temperature synthesis (SHS), and different ways nanopowders are produced and sprayed. It covers several spraying methods and a wide range of materials, including ceramics, metals and alloys, cermets, and composites. It also covers mechanical alloying and powder milling processes and addresses the potential risks of inhaling nanopowders.

In this investigation, particle image velocimetry (PIV) diagnostics were employed to analyze the spray produced by a two-fluid atomizer as used in suspension plasma spraying (SPS). This led to a change in the design of the atomizing nozzle in order to achieve a high-speed spray with narrow distributions in droplet size. The resultant spray was characterized and the diagnostic was adapted accordingly. Various suspensions of YSZ powders were then injected into the plasma under different conditions and particle velocities were determined and correlated with the coating morphologies obtained.

In order to achieve SOFC at reduced costs, atmospheric plasma spraying (APS) could be an attractive technique. However, it is difficult to elaborate plasma sprayed coatings with the appropriate porosity for the electrodes and full density for the electrolyte. The spray process has been adapted by providing a suspension as feedstock material. SPS shows important advantages over APS, since it is now possible to spray finer powders to obtain either a thin (10 µm) dense layer as electrolyte or thick and finely structured porous layers for the electrodes. Nevertheless some questions still remain before considering manufacturing SOFC by SPS. The major one is to understand the influence of the suspension and the injection parameters on the drops formation and transformation in plasma before impinging upon the substrate as well as of the suspension characteristics (formulation, particle size and amount, viscosity, surface tension,...). To answer these questions, suspensions based on nickel oxide (NiO) and YSZ (yttria stabilized particles) have been prepared and functional layers have been produced by SPS. This work is compared with previous studies of YSZ sprayed suspensions.

In order to achieve Solid Oxide Fuel Cells (SOFC) with reduced costs, the atmospheric plasma spray process (APS) could be an attractive technique. However, it is difficult to produce plasma sprayed coatings with the appropriate porosity for the electrodes and full density for the electrolyte. The spray process has been adapted by providing a suspension or a powder as feedstock material combined with optimized spray conditions to manufacture SOFC layers with required compositions, microstructures and properties. Suspensions or powders were injected in a plasma jet originating from a conventional torch (F4) implementing external devices. By using optimized suspensions of nickel oxide (NiO) and yttria stabilized zirconia (YSZ) in ethanol, porous anodes have been produced with thicknesses around 50 µm. By the same way aqueous suspensions of yttria stabilized zirconia (YSZ) powders have been prepared to achieve a fine layer as electrolyte to produce the first half cells and the porous cathode layers based on lanthanum strontium manganite (La 0.8 Sr 0.2 MnO 3 or LSM) have been manufactured by conventional atmospheric plasma spraying of powders.

In this paper a comparative study on the microstructure and photocatalytic performances of titanium dioxide coatings elaborated by various thermal spraying methods (plasma spraying in atmospheric conditions, suspension plasma spraying and high-velocity oxy-fuel spraying) was proposed. Agglomerated spray-dried anatase TiO 2 powder was used as feedstock material for spraying. Morphology and microstructural characteristics of the coatings were mainly studied by scanning electron microscopy and X-ray diffraction. The photocatalytic behavior of the TiO 2 -based surfaces was evaluated from the conversion rate of gaseous nitrogen oxides (NOx). It was found that the crystalline structure strongly depended on the technique of thermal spraying deposition. Moreover, a high amount of anatase was suitable for the photocatalytic degradation of the pollutants. Suspension plasma spraying permitted to retain the original anatase phase and to obtain very reactive TiO 2 surfaces for the nitrogen oxides removal.

The suspension plasma spraying using a liquid as material feedstock is a promising technique to elaborate thin and nanostructured coatings. This work deals with the study of suspensions in order to obtain specific microstructural coatings. The slurries were prepared starting from nanosized titania powders. Distilled water and alcohol were used as solvents and appropriate dispersing agents were employed to stabilize the suspensions. An external injection system that ensures the atomisation and radial injection of the suspension into Ar-H 2 plasma under atmospheric conditions was designed. The coatings were characterized by scanning electron microscopy and differences in the microstructure of the deposits were observed. Efforts were made to understand the relationship between liquid feedstock properties, injection/plasma parameters and coatings microstructural characteristics.

Al 2 O 3 -TiO 2 materials are very attractive materials for the manufacturing of wear resistant ceramics coatings. Moreover, nanostructured coatings show superior properties (higher hardness and enhanced fracture toughness) as compared to their counterparts containing microscale grains. The aim of this study is to produce nanostructured Al 2 O 3 -TiO 2 coatings through the plasma spraying technique that can lead to metastable transformations. The starting powders containing 13wt% and 44wt% of titania, have been mechanically activated by high energy ball milling with a planetary mill, (Model P4 by Fritsch, Germany) using steel vials and balls in such conditions to obtained the high pressure TiO 2 II phase. The milled powders were then plasma sprayed with an Ar:H 2 gas mixture, by varying the plasma parameters (gas flow rates, nozzle diameter, aggregates size). The effect of the milling and the spraying conditions on the crystalline phases and the microhardness of the coatings are evaluated.

In order to achieve SOFC with reduced costs, the atmospheric plasma spray process (APS) could be an attractive technique. To obtain dense layer as electrolyte, the route has been adapted by providing a suspension as feedstock material. Suspensions of yttria stabilized zirconia powders in methanol have been prepared with various solid loadings and states of dispersion. An external injection system was used to ensure the atomisation and radial injection of the suspension into the Ar-H 2 plasma under atmospheric conditions. The coatings were characterized by SEM, Archimedes porosity and differences in the microstructure of the deposits were observed. The aim of this work has thus been to study the influence of various parameters (of the suspensions, plasma, torch, injection) on the layer morphology. For this purpose, the atomization has been investigated and efforts have been made to understand relationships between suspension properties, atomization and microstructure.

This paper deals with the elaboration of titanium dioxide coatings designed for photocatalytic applications, obtained by suspension plasma spraying (SPS), an alternative of the atmospheric plasma spraying (APS) in which the material feedstock is a suspension of the powder to be sprayed. Two different TiO 2 powders were dispersed in distilled water and ethanol and injected in Ar-H 2 or Ar-H 2 -He plasma under atmospheric conditions. SEM and XRD analysis were performed to study the microstructure and surface features of the titania coatings. Photocatalytic efficiency of the elaborated samples was evaluated from the conversion rate of different air pollutants: NOx and SO 2 . The present results showed that the suspension plasma spraying allows to produce reactive surfaces for the removal of air pollutants that in the same working conditions, present a higher photocatalytic activity compared to that of the initial raw powders.

In this paper, the modified method of plasma spraying using liquid suspension feedstock was used to elaborate thin titanium dioxide coatings. TiO 2 Degussa P25 powder were dispersed by continuous stirring in distilled water and alcoholic solutions and injected in an Ar-H 2 or Ar-H 2 -He plasma plume under atmospheric conditions. Scanning electron microscopy and X-ray diffraction were performed to study the microstructure and crystalline phases of titanium dioxide coatings. The anatase content in the elaborated TiO 2 deposits strongly depended on the type of the suspension solvent. The photocatalytic activity of the titanium dioxide coatings was evaluated from the conversion rate of nitrogen oxides and compared with that of the initial powder. The obtained results allowed to affirm that the method of the liquid plasma spraying proved to be a promising technique to elaborate photocatalytic TiO 2 coatings for the removal of nitrogen oxides pollutants.

In the present study, TiO 2 and TiO 2 -Al composite coatings were prepared by HVOF spraying using reconstituted nanosized feedstock powders prepared via the spray drying technique. In the flame, the powders were injected by two methods: internal injection i.e. as in conventional HVOF process and external injection i.e. outside the torch nozzle. The microstructure of the coatings was characterized by scanning electron microscopy and X-ray diffraction. It was found that the amount of anatase in the coatings depends on the nature of the powder and also on the type of the injection method. The coatings were tested for their photocatalytic properties regarding the conversion rate of nitrogen oxides. Coatings elaborated by external injection presented a better photocatalytic activity than those obtained by the conventional HVOF process.

TiO 2 and TiO 2 -Al composites coatings were prepared by plasma spraying using a reconstituted nanosized feedstock via a spray drying method. Effects of various spray conditions on the microstructure, porosity, microhardness and wear resistance related to the mechanical performance of coatings were evaluated. The coatings sprayed at relatively low plasma power were composed of two distinct microstructures of well defined lamellar structure, similar to microstructure of conventional plasma sprayed coatings typically observed from fully melted particles, and embedded nano or sub-micron particles originated from partial/non-molten particles of feedstock materials. The fraction of partial/non-molten particles in coating layers was increased by Al additive. Such a characteristic of blended microstructure of coatings was clearly confirmed from a bimodal distribution of microhardness described by Weibull plots. The optimized addition of Al into TiO 2 improved mechanical properties such as microhardness and wear resistance.

Titanium dioxide (TiO 2 ) is one of the most important photocatalyst that allows the environmental purification of water and air by the decomposition of toxic organic compounds and removal of harmful gases. In the photocatalytic applications, TiO 2 can be used in form of powder or coating. In this paper, two techniques of deposition were used to elaborate thin deposits starting from an agglomerated TiO 2 anatase nanopowder: conventional plasma spraying in atmospheric conditions and suspension plasma spraying. The photocatalytic efficiency of the coatings was performed with respect to nitrogen oxides (NOx) and compared with the photocatalytic activity of the TiO 2 Degussa P25 powder. Differences in the photocatalytic efficiencies of the nanocoatings obtained by the two techniques of plasma spraying were obtained. The coatings elaborated by suspension plasma spraying have poor mechanical properties but better photocatalytic efficiencies. This method is a promising technique to elaborate photocatalytic coatings for the removal of different air pollutants.

Titanium dioxide (TiO 2 ) has emerged as an excellent photocatalyst material for environmental purification about two decades ago but only recently few works have focused on the photocatalytic properties of sprayed titanium oxide coatings. So far, the role of oxygen deficiency which can appear as a result of the spray process or by use of titanium suboxide powders on the photocatalytic activity has not been investigated. Also the possible influence of the shear plane structure of titanium suboxides (Magnéli phases) on the photocatalytic activity was not taken into consideration. In the present work, the photocatalytic properties of three powders and coatings sprayed from these powders by APS and VPS are investigated: (1) a commercial fused and crushed titanium oxide powder, (2) an agglomerated and sintered titanium suboxide powder consisting predominantly of Magnéli phases Ti 6 O 11 and Ti 5 O 9 , (3) an agglomerated and sintered powder consisting of Ti 2 Cr 2 O 7 and Ti 6 Cr 2 O 15 (Magnéli phases in the TiO 2 - Cr 2 O 3 phase diagram). The phase compositions of the powders and the coatings were investigated by X-ray diffraction. Neither for the spray powders nor for the coatings any ability to reduce the NOx concentration by the photocatalysis was found. From this it is concluded that both oxygen deficiency as well as Magnéli phase structure are not responsible for photocatalytic properties of materials in the Ti-O phase diagram.

Titanium dioxide (TiO 2 ) is an attractive material for numerous technological applications such as photocatalytic applications. The photocatalysts allow the environmental purification of air and water by the decomposition of toxic organic compounds and removal of harmful gases. This work was focused on the production and evaluation of the environmental properties of titanium dioxide coatings obtained by the plasma spray technique. To carry out the step of validation of the TiO 2 coatings for their environmental functionalities, a control test of the photocatalytic effectiveness was performed with respect to nitrogen oxides. A custom-designed test chamber has been developed. The photocatalytic properties of different coatings were studied as a function of various parameters (porosity, anatase/rutile ratio, nature of the substrate).

Titanium dioxide (TiO 2 ) has emerged as an excellent photocatalyst material for environmental purification about two decades ago. In order to avoid the use of TiO 2 powder, which requires later separation from the treated liquid, few researchers reported on ways of fixing TiO 2 . In these cases, many difficulties must be overcome such as maintaining the photoactive anatase phase as well as the mechanical stability and avoiding the mass transfer limitations. Thermal spraying seems to be an appropriate process to provide ceramic coatings exhibiting such characteristics. The present work assesses the influence of several spray parameters on the morphology of deposits, dominated by two void systems – interlamellar pores and interlamellar cracks – and on their crystalline structures. TiO 2 was atmospherically plasma sprayed by systematically varying the process conditions including torch power, plasma gas mixture Ar/H 2 and Ar/He, and cooling device. An analytical procedure based on XRD patterns was established for the determination of the relative amounts of anatase and rutile in the coatings. An Ar/He mixture as compared to an Ar/H 2 one seems to enhance the formation of anatase from 38vol% to 44vol%. In both cases, a reduction of the plasma power increases the amount of anatase. Several methods of porosity determination were performed: image analysis, Archimedean porosimetry and calculation from phases, mass and thickness. Generally, the results indicate a total porosity varying between 15 and 25% depending on the spraying conditions.

The life and thermal properties of plasma sprayed TBC – widely used in gas turbine engines – are closely related to the microstructure of the ceramic top coating. Especially, the thermal conductivity of this ceramic coating is induced by the void shapes and networks which are in turn determined by both the spraying conditions and the feedstock material. This study has been performed to optimize the ceramic coating microstructure. First, a hollow zirconia powder was elaborated in the laboratory and compared with a commercial hollow powder. Then, the Taguchi method was performed on thermal spray operating factors to optimize their beneficial effects on the Thermal Barrier Coating (TBC). A new hollow powder with small grains was elaborated using spray drying process. That spray dried powder based on yttria partially stabilized zirconia was optimized using drying simulation tests. It was shown that the formulation and the state of dispersion of the slurry allow to control the powder morphology (from dense to hollow shape). Moreover, for hollow powder, it is possible to vary the thickness of the shell. According to these results, a hollow powder (size ranging from 36 to 130 µm and an 11 µm shell thickness) was elaborated as an original feedstock to produce plasma sprayed TBC. The agglomerated powder was consolidated in an oven and then was compared to a commercial hollow yttria zirconia powder during thermal spray tests. Coating experiments were based on a limited number of operating factors, which have an influence on the deposit microstructure. The seven selected spraying factors concerned the plasma (primary and secondary gas flow rates, arc current), the cooling and the powder deposition (spraying distance, spraying angle, traverse speed). Experiments based on these factors were carried out to elaborate a Taguchi fractional-factorial L16 design. The resulting as sprayed coatings characteristics were quantified with respect to deposition efficiency, roughness (Ra) and porosity (image analysis technique). Through statistical calculation, the parameters that have relevant influence on the coating properties were identified, and their relative importance and some of their interactions were studied. The final aim is to produce an effective thermal barrier coating with a reduced thermal conductivity.

In thermal spray, it is well established that tailoring the powder characteristics is of major importance to achieve reproducible coatings on a microstructural and chemical point of view. Among the techniques developed to produce thermal spray powders, spray drying has proved to be the most versatile process. The spray drying method consists in spraying a slurry containing finely dispersed particles of the materials to be agglomerated. However, in order to prepare specific thermal spray powders, two steps have to be mastered: the slurry stability and the spray drying operating conditions. The present study was focused on the relationships that exist between the slurry rheology, the powder morphology and the coating properties. This work was performed on a model material namely Al 2 O 3 . In a first part, the effects of the surfactant percentage and pH on the stability of the suspensions were determined. The evolution of the viscosity of the slurries versus the amount of binder was measured. In a second part, these slurries were used to prepare spray-dried powders. The effects of some process parameters such as atomizing air pressure and slurry feed rate on the granule characteristics (morphology, density, particle size distribution, and powder flow ability) were investigated. Finally, some coatings were deposited using the APS technique on steel substrates from the non-sintered spray-dried powder previously realized. The coating morphology and the crystallographic structure were evaluated as a function of the spraying conditions.

Today, powder particles diameter used for thermal spraying is generally comprised between 5 and 100µm with a preferred range around 40µm for APS applications. Actually, the future trends in plasma spraying are directed to the use of fine or ultrafine powders and the reduction of the steps between raw materials and coatings. So, the present paper investigates the way to use directly spray dried ceramic powders in suppressing the sintering stage. AI2O3 based powders were obtained by the spray drying process. By optimizing the parameters (slurry composition and injection as well as drying characteristics), a narrow grain size distribution was achieved. Chemical composition and shape of synthesized powders were analyzed by scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS). The crystallographic structure was identified by X-ray diffraction (XRD). Demonstration was made that it is possible to obtain coatings using directly spray dried ceramic powders. The plasma spray process parameters (such as current intensity, gas flow rate, powder feed rate and injection mode, cooling stage,...) have to be managed to achieve cohesive coatings. The structure and chemical composition of these coatings were studied. In this way, the direct use of spray dried powders appears as a promising way to realize ceramic coatings.

In general, thermal spraying involves high temperatures that can be deleterious for the microstructure and deformation of the substrate. As a consequence, the use of a cooling system during spraying is often necessary. Meanwhile, in some cases, a too low surface temperature can induce a loss of properties, in particular concerning adherence and coating density. Therefore, it would be sometimes interesting to combine pre-heating and cooling stages with the plasma spray. A specific process, named HeatCool, was developed and patented to ensure a precise control of the temperature at the spraying location. The present work was focused on the study of the influence of pre-heating and cryogenic cooling conditions on the microstructure and mechanical characteristics of NiCrFeBSi self-fluxing alloy deposited by d.c. plasma spray technique. Firstly, a comparison between air and CO2 cooling was conducted to assess the efficiency corresponding to the specific use of cryogenic CO2. The main characteristics studied were the microhardness, roughness, porosity, mechanical deformations, morphology and crystallographic structures. Optimising the cooling methods and conditions combined with the process parameters improved microhardness of the plasma sprayed metal alloy and induced lower strain deformation of the substrate. Secondly, the pre-heating system was added to the device and the HeatCool process was evaluated. The process was demonstrated to be an efficient mean to enhance the structural and mechanical characteristics of coatings made of self-fluxing alloy.