Search

International audience; This work focuses on the comparison of Ln-Ni-O coatings (Ln = La, Nd, Pr) deposited by reactive magnetron sputtering using Plasma Emission Monitoring (PEM) which allows high deposition rate. Each layer is deposited by different steps. The optimal regulation setpoint for oxide rare-earth deposition is determined and then the current dissipated on the nickel target is adjusted to obtain the convenient Ln/Ni ratio and to achieve the K2NiF4 structure. After an appropriate annealing treatment, all coatings exhibit crystalline structures, which depend on the Ln/Ni ratio. Due to the instability of Pr2NiO4 structure at intermediate temperatures, the crystallization step of praseodymium nickelate is performed at higher temperature than the other materials. This further thermal treatment implies a more porous structure. Each coating exhibits interesting properties. Electrical and electrochemical characterizations performed on these deposits prove better properties of the praseodymium nickelate coating.

International audience; Plasma-spray (PS) is the most common technique used to cover orthopaedic titanium-based implant surfaces with hydroxyapatite (HA - Ca10(PO4)6OH2). The objective of the current work was to explore the influence of the precursor nature and characteristics (mainly powder size, Ca/P ratio, and density) on the morphology and the microstructure of HA coatings, deposited either by conventional atmospheric plasma spray (APS) or by rf-suspension plasma spray (rf-SPS). Two powders with different size distribution, density and shape were sprayed with same operating APS conditions. In parallel two water-based suspensions with distinct particle size and chemistry (Ca/P ratio) were implemented in rf-SPS. The morphology of both APS and rf-SPS coatings observed by SEM exhibits micro and sub-micro sized structures respectively, with similar porosity and thickness. The difference in precursor particle size does not affect the coating structure in rf-SPS, unlike in APS conditions. Superimposition of topography and phases maps performed from profilometry and Raman cartographies showed no direct correlation between the surface profile and composition. But such analysis highlighted the heterogeneity of the phases composing the coating surface, opening the possibility of a better understanding of biological behaviour.

Carbonated Biomimetic Nanocrystalline Apatite (BNAc) coatings are obtained for the first time by Cold Spray. The coatings are characterized by FTIR, Raman, XRD, and SEM and compared to the powders. No significant chemical and structural changes are detected and the nanostructure features of these very reactive BNAc are preserved in the coating. These results and preliminary mechanical assays show that Cold Spray can produce an operational biomimetic coatings offering a high potential for implants functionalization and osseointegration. However, these first results need further studies in order to understand the mechanism of adhesion and the interactions at the coating–substrate interface.

International audience; The manufacture of the last generation metal supported IT-SOFC complete cell by dry surface deposition pro-cesses is really challenging. Atmospheric Plasma Spraying (APS) and Reactive Magnetron Sputtering (RMS) processes are respectively adapted to deposit Ni-YSZ anode and YSZ electrolyte layers. RMS is also used to coat a thin and dense La2NiO4 (LNO) cathode layer. In this work, we have realized a complete cell on metallic support (ITM) produced by PLANSEE SE. The innovative LNO cathode layer was compared with screen-printed LNO layers, with and without RMS bonding layer. Electrochemical and Voltammetry tests were performed on these samples. It reveals lower performances than literature due to the high density of the RMS cathode layer, and too high temperature during sintering step which deteriorate layers manufactured by RMS and metallic substrate. Nevertheless, using LNO bonding layer manufactured by RMS seems to be an interesting way to improve the polarization resistance of the cell.

International audience; This work focuses on the structural, microstructural, electrical and electrochemical characterization of La–Ni–O coatings deposited by reactive magnetron sputtering under different argon partial pressures. Deposition was driven by Plasma Emission Monitoring system (PEM) which allows high deposition rate. For each pressure, the optimal regulation setpoint for lanthanum oxide deposition is determined and then the current dissipated on the nickel target is adjusted to obtain the convenient La/Ni ratio to achieve the K2NiF4 structure. After an appropriate annealing treatment, coatings crystallize in the desired structure with more or less impurities. Increasing the device's pressure from 0.4 to 4.8Pa produces more porous samples with different microstructures. Then, a coating with porosity gradient was built by using these four different total pressures. Its electrical performances were tested and allowed its utilization as cathode layer in intermediate temperature-solid oxide fuel cell (IT-SOFC). This modified architecture cathode allows better performances (71mW·cm−2) than dense cathode produced by the same technique (51mW·cm−2).

International audience; The properties and performance of plasma-sprayed thermal barrier coatings (TBCs) are strongly dependent on the microstructural defects, which are affected by starting powder morphology and processing conditions. Of particular interest is the use of hollow powders which not only allow for efficient melting of zirconia ceramics but also produce lower conductivity and more compliant coatings. Typical industrial hollow spray powders have an assortment of densities resulting in masking potential advantages of the hollow morphology. In this study, we have conducted process mapping strategies using a novel uniform shell thickness hollow powder to control the defect microstructure and properties. Correlations among coating properties, microstructure, and processing reveal feasibility to produce highly compliant and low conductivity TBC through a combination of optimized feedstock and processing conditions. The results are presented through the framework of process maps establishing correlations among process, microstructure, and properties and providing opportunities for optimization of TBCs.

Yttria Stabilized Zirconia (YSZ) suspensions were injected in an atmospheric plasma jet using two designs of a home-made two-fluid atomizing nozzle. The sprays of drops were visualized and the behavior of the suspension in the plasma jet was investigated by implementing the Particle Image Velocimetry (PIV) method. The effects of the suspension formulation (surface tension, liquid viscosity, and relative gas-to-liquid mass ratio, GLR) on the distribution and median value of the drop size as well as on the velocity maximum value were evaluated. The interactions between the sprays and the plasma jet were studied. The differences in the behavior of the particle velocity along and radial to the torch axis were pointed out. The validity of PIV measurements was finally demonstrated by the relation established between the in-flight particle velocity and the coating structure.

Several alumina and alumina–zirconia composite coatings were manufactured by suspension plasma spraying (SPS), implementing different operating conditions in order to achieve dense and cohesive structures. Temperatures and velocities of the in flight particles were measured with a commercial diagnostic system (Accuraspray®) at the spray distance as a function of the plasma operating parameters. Temperatures around 2000 °C and velocities as high as 450 m/s were detected. Hence, coatings with high amount of α-alumina phase were produced. The microstructure evolution according to the spray parameters was studied as well as the final tribological properties showing efficient wear resistance.

Thermally sprayed coatings present effective properties strongly different from those of the primary bulk material. In particular, the actual thermal conductivity of Yttria Partially Stabilized Zirconia (YPSZ) coatings is typically twice lower than the thermal conductivity of dense YPSZ. The architecture of the porous network plays a major role on this decrease: thin inter-lamellar cracks act as thermal resistance and contribute to decrease the effective thermal conductivity more efficiently than globular pores. From this situation, an in-house code has been developed since a few years: this code implements a finite difference method to perform calculations directly on micrographs of coating cross-sections obtained by SEM. Each pixel of the intermediate binary picture is interpreted as a cell of integration of the heat conduction equation. A thermal gradient is applied between the top and bottom edges and a system of linear equations is formed and solved, providing the thermal flux flowing through the structure and the corresponding effective thermal conductivity. In the present study, the case of YPSZ coatings before and after thermal treatment was considered. The numerical results are in rather good agreement with experimental data: the thermal treatment tends to close a part of the thinnest pores, thus providing a decrease of the pore level and an increase of the effective thermal conductivity of the produced coatings.

Suspension plasma spray is a promising technique that uses fine particles dispersed in a liquid as feedstock material instead of dry powder as in conventional plasma spraying and has been implemented here to produce layers with appropriate morphologies and microstructures for SOFC applications. This study uses a pressurized gas delivery system to feed the slurry through a homemade two-fluid atomizing nozzle to a conventional plasma torch. The electrodes consist of porous NiO–YSZ as anode and lanthanum nickelate as cathode. The anode and respectively the cathode were deposited onto dense or porous ferritic steel substrates in order to be characterized and optimized. The cell components were examined by scanning electron microscopy (SEM), X-ray diffraction and leakage test. This paper aims at studying the influence of the suspension characteristics (surface tension and viscosity were selected as main parameters), the conditions of injection (nozzle design, gas to liquid ratio, injection angle have been identified as major parameters), the plasma conditions (plasma gas nature and flow rates, spray distance are of major importance) and finally the kinematics on the crystalline phases, the chemical composition (distribution of NiO particles into the layer), the thickness and roughness, the pore ratio and the gas permeability. Then the optimized electrodes have been deposited onto ferritic substrate to perform Open Circuit Voltage and impedance tests at a temperature around 800 °C. This work demonstrated the feasibility for the fabrication of electrodes with interesting performance using suspension plasma spraying technique.

The growing interest for SOFC leads research towards new materials but also towards processes which could be more effective and less expensive to produce fuel cells. Plasma spraying is one of these interesting processes due to its ability to manufacture the whole cell with the same process. The present study uses suspension plasma spraying (SPS) process which consists in injecting a suspension of sub-micrometer particles dispersed in a liquid into a DC plasma torch. This process allows producing finely structured coatings. In the present study, suspensions containing YSZ particles were atomized into a spray by a bi-fluid atomizer. The droplets were then radially injected into an Ar–H2 plasma jet where they were accelerated and heated. This paper aims at studying the importance of the suspension characteristics on the successive steps leading to the coating formation. Various suspensions based on three different media – methanol, azeotropic mixture of methyl–ethyl–ketone/ethanol and water – were evaluated. The amount of dispersant was adjusted in order to obtain well dispersed and stable suspensions. Plasticizers have been used to tailor the viscosity. Each suspension has thus been characterized versus stability, viscosity and surface tension. The influence of these characteristics on the droplets jet (size distribution and jet shape) issued from the atomizer-injector was assessed. In this purpose, two adimensional numbers (We, Re) were determined for the different atomization conditions and were correlated with the droplets jet. Finally, coatings were realised and characterized in terms of porosity by image analysis of SEM pictures. Powder velocity and temperature were evaluated implementing Accuraspray® and drop size distributions (after passing through the plasma jet) were determined as a function of the injection conditions.

The effect of vacancies on structural properties of the perovskite-type oxide was studied using the full-potential linearized augmented plane wave (FP-LAPW) method, within the density functional theory. In this approach, the generalized gradient approximation was used for the exchange-correlation potential. The ground state properties such as lattice parameter, bulk modulus and inter-atomic distances of cubic Ba 1− x ZrO 3 compounds ( x = 0, 0.125 and 0.25) were calculated. Additionally, using a set of total energy versus volume obtained with the FP-LAPW method, the quasi-harmonic Debye model was applied to determine the thermal properties including temperature dependence of bulk modulus, thermal expansion coefficient, specific heats at constant volume and constant pressure. No experimental data are available and our results are considered as purely predictive.

The ground-state equilibrium properties of cubic Ba 1− x Sr x ZrO 3 compounds ( x = 0, 0.125, 0.25, 0.375 and 0.5) were studied by means of ab initio total energy calculations using the full-potential linearized augmented plane wave (FP-LAPW) method. In this approach, the generalized gradient approximation was used for the exchange-correlation potential. These calculations are the first step in the investigation of thermodynamics of Ba 1− x Sr x ZrO 3 compounds. We found out that compressibility decreases with the Sr concentration according to the ionic radii. The sub-stoichiometry also leads to higher hardness of perovskite compounds. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the isothermal bulk modulus, thermal expansion coefficient, specific heats at constant volume and constant pressure as well as Debye temperature were calculated. No experimental data are available and our results are considered as purely predictive. The major trends show that Ba 1− x Sr x ZrO 3 compounds tend to maintain their hardness with the temperature increases. Furthermore, the substitution of Sr 2+ at Ba 2+ sites in BaZrO 3 has a noticeable influence on the thermal expansion behavior which makes Ba 1− x Sr x ZrO 3 compounds unable to be good candidates for application as thermal barrier coating.

Life and thermal properties of plasma sprayed TBCs – widely used in gas turbine engines – are closely related to the microstructure of the ceramic top coating. Especially, the thermal behaviour of this coating is induced by the void shapes and networks which are in turn determined by both the spraying conditions and the feedstock material. A specific hollow yttria partially stabilised zirconia powder was produced in a one-step process by spray drying and an experimental statistical design study was conducted to investigate the influence of spraying variables (primary and secondary gas flow rates, arc current, spraying distance, spraying angle and traverse speed) on structure and properties of resulting plasma sprayed coatings. The coatings were characterized with respect to deposition efficiency, roughness, porosity and thermal conductivity. A reduction of 25% of the thermal conductivity was achieved by improving the spray and powder parameters. A quantitative characterization of the porous structure using image analysis of polished cross-sections was implemented. The parameters that have relevant influence on the coating porous structure were identified, and their relative importance was determined. An attempt was made to identify morphological criteria of the porous network (coarse/fine porosity ratio, cracks total length, cracks orientation) correlating with the thermal conductivity values.

Among processes evaluated to produce some parts of or the whole solid-oxide fuel cell, Suspension Plasma Spraying (SPS) is of prime interest. Aqueous suspensions of yttria partially stabilized zirconia atomized into a spray by an internal-mixing co-axial twin-fluid atomizer were injected into a DC plasma jet. The dispersion and stability of the suspensions were enhanced by adjusting the amount of dispersant (ammonium salt of polyacrylic acid, PAA). A polyvinyl alcohol (PVA) was further added to the suspension to tailor its viscosity. The PVA also improved the dispersion and stability of the suspensions. The atomization of optimized formulations is described implementing Weber and Ohnesorge dimensionless numbers as well as gas-to-liquid mass ratio (ALR) value. Drop size distributions changed from monomodal distributions at low We to multimodal distributions when We number increases. The viscosity of the suspensions has a clear influence on the drop size distribution and suspension spray pattern. The secondary fragmentation of the drops due to the plasma jet was evidenced and the final size of the sheared drops was shown to depend on the characteristics of the suspension. Rather dense zirconia coatings have been prepared, which is a promising way to produce electrolyte.

The full-potential linearized augmented plane wave (FP-LAPW) method within the density functional theory was used to investigate the structural and electronic properties of Y2Ti2O7 and Cd2Nb2O7. In this approach, the generalized gradient approximation was used for the exchange–correlation potential. We have firstly optimized the internal parameter and used it to calculate the ground state properties such as lattice constant, bulk modulus and its derivative as well as angles and inter-atomic distances. The results were in good agreement with available experimental measurements. We find that Y2Ti2O7 and Cd2Nb2O7 are highly incompressible and thus can be good candidates for hard materials. Calculations of band structure, density of state and charge density were also performed to describe the orbital mixing and the nature of chemical bonding. Both Y2Ti2O7 and Cd2Nb2O7 exhibit indirect band gaps. The mixture of B-d and O-2p states emphasis the covalent nature of the B–O bond that should explain the hardness of pyrochlore compounds. The small distances between oxygen and A/B atoms also lead to higher covalency of the bonds.

DOI: 10.1166/jnn.2015.10329; International audience; The deposition of TiN on stainless steel substrates may improve the stability and compatibility of this material with bone, which may be advantageously exploited for the elaboration of advanced prosthetic devices. In this work, TiN-coated 316LSS (by way of DC magnetron sputtering) was used as a starting material for investigating the electrochemical post-deposition of hydroxyapatite (HAp) which has a composition close to that of bone. Electrodeposition was carried out starting from an aqueous medium containing solubilized Ca(NO3)(2) and NH4H2PO4 in the presence of H2O2. We report the influence of experimental conditions on the morphology of the obtained HAp coating on TiN/316LSS. The effect of applied potential, temperature, H2O2 concentration, pH and duration of reaction were thoroughly discussed on the basis of X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier Transform Infrared (FTIR) spectroscopy and Energy Dispersive X-ray Spectroscopy (EDX) results. This method appears advantageous for producing HAp-coated implant materials.

To achieve solid oxide fuel cells (SOFC) at reduced costs, the atmospheric plasma spray (APS) process could be an attractive technique. However, to make dense and thin layers as needed for electrolytes, a suspension is preferably implemented as a feedstock material instead of a conventional powder. Suspensions of yttria-stabilized zirconia particles in methanol have been prepared with various solid loadings and states of dispersion. An external injection system was used to ensure the atomization and radial injection of the suspension into the Ar-H2 plasma under atmospheric conditions. The coatings morphologies were characterized by scanning electron microscopy, and their porosity was evaluated by the Archimedes method. Differences in the microstructure of the deposits were observed depending on the APS operating conditions. Special attention has been dedicated to assess the influence of the suspension as well as the injection on the layer morphology. For this purpose, the atomization has been investigated and efforts have been made to understand relationships among suspension properties, atomization, and coating microstructure.

This paper proposes 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 oxyfuel spraying). Agglomerated spray dried anatase TiO2 powder was used as feedstock material for spraying. Morphology and microstructural characteristics of the coatings were studied mainly by scanning electron microscopy and x-ray diffraction. The photocatalytic behavior of the TiO2-base surfaces was evaluated from the conversion rate of gaseous nitrogen oxides (NOx). It was found that the crystalline structure depended strongly 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 has allowed retention of the original anatase phase and for very reactive TiO2 surfaces to be obtained for the removal of nitrogen oxides.

This paper describes formation of titanium dioxide coatings designed for photocatalytic applications, obtained by suspension plasma spraying (SPS), an alternative of the atmospheric plasma spraying (APS) technique in which the material feedstock is a suspension of the material to be sprayed. Two different TiO2 powders were dispersed in distilled water and ethanol and injected in Ar-H2 or Ar-H2-He plasma under atmospheric conditions. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) analyses were performed to study the microstructure of the titania coatings. Photocatalytic efficiency of the elaborated samples was evaluated from the conversion ratio of different air pollutants: nitrogen oxides (NOx) and sulfur dioxide (SO2). The morphology and crystalline structure of the deposits depended mainly on the nature of the solvent (water or alcohol) used in the preparation of the slurries. Dense coatings were obtained starting from aqueous suspensions and porous deposits were elaborated by plasma spraying of a PC105 alcoholic suspension. A significant phase transformation from anatase to rutile occurred when ethanol was used as a solvent. Different photocatalytic performances were observed as a function of the nature of the liquid material feed-stock, the spraying parameters, and the nature of the pollutant.

Titanium dioxide is one of the most important photocatalysts that allows the environmental purification of various toxic organic compounds in water and removal of harmful air pollutants. In this paper, two techniques of plasma spraying – plasma spraying from an agglomerated nanopowder and liquid suspension plasma spraying – were used to produce thin deposits. The microstructure of coatings was characterized using scanning electron microscopy and X-ray diffraction. The photocatalytic properties of the coatings were evaluated by the decomposition test of nitrogen oxides and compared with the efficiency of the initial agglomerated nanopowder and that of the Degussa P25 powder. Noticeable different behaviours in the photocatalytic tests were observed for coatings performed either from conventional or liquid plasma spraying. The TiO2 deposits realized by spraying of the liquid suspension exhibited higher photocatalytic efficiency. It is found that the plasma spraying of a slurry is an effective method to retain the original anatase phase and prevent the increase of the crystallite size. This new method of modified spraying is proved to be a promising technique to elaborate photocatalytic coatings for the removal of nitrogen oxide pollutants.

This paper deals with the study of the photocatalytic behaviours of TiO2 and TiO2–10 wt.% Al coatings elaborated by high-velocity oxygen fuel (HVOF) spraying using spray-dried nanosized feedstock powders. In the HVOF 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. Scanning electron microscopy and X-ray diffraction were employed to characterize the microstructure and the crystalline structure of the TiO2-based coatings. The photocatalytic activities were evaluated by the nitrogen oxides removal and compared with that of the initial powders. It was found that the amount of anatase in the coatings depends on the nature of the powder and also on the type of injection method. The TiO2 coatings, as well as, the TiO2–Al coatings elaborated using the external powder injection presented a better photocatalytic activity (the nitrogen oxides conversion rates were about 25–42% for NO and 14–18% for NOx) than those obtained by the conventional HVOF process (

TiO2 and TiO2–Al composite 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 using relatively low plasma power were composed of two distinct microstructures of well defined lamellar structure, similar to microstructure of conventional plasma sprayed coatings generated by fully melted particles, and embedded nano or sub-micron particles generated by 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 TiO2 increased the spraying efficiency, and also, improved mechanical properties such as microhardness and wear resistance.

TiO 2 coatings were prepared by plasma spraying using a spray-dried powder as feedstock material. A systematic study has been performed to determine how the titania slurry formulation (e.g. dispersant level, pH, binder addition) affects the granule characteristics. Aqueous slurries consisting of 50 wt.% of TiO 2 particles, 0–1.2 wt.% ammonium polyacrylate as a dispersant and up to 15 wt.% styrene–ester acrylic copolymer as a binder were investigated using settling experiments. It was shown that when the relative sedimentation height (RSH) value is below 0.45 the fully dispersed slurry leads to the fabrication of a hollow powder whereas when the RSH is over 0.50 the flocculated slurry leads to a dense powder. TiO 2 coatings were prepared from a batch of hollow granules of anatase. The microstructure and crystallographic phases of these deposits were characterised using scanning electron microscopy, three porosimetry methods (image analysis, Archimedean and calculation from thickness and X-ray diffraction, XRD) and quantitative XRD. The results showed that the TiO 2 coatings consist of a mixture of anatase and rutile. The content of anatase was varied from 35 to 50 vol.% depending on the operating conditions (mainly plasma power and cooling rate). These great amounts of anatase phase were obtained along with rather high percentage of porosity (between 13 and 25%).

International audience; This work focuses on the structural and electrical characterization of La-Ni-O coatings deposited by reactive magnetron sputtering using Plasma Emission Monitoring (PEM) which allows high deposition rate. The optimal regulation setpoint for lanthanumdeposition is determined and then the current dissipated on the nickel target is adjusted to obtain the convenient La/Ni ratio to achieve the K2NiF4 structure. After an appropriate annealing treatment, all coatings exhibit crystalline structures that depend on the La/Ni ratio. Some cracks appear on samples deposited on alumina substrates depending to the argon flow rate and influence their electrical behavior.

Nitride based hard coatings are widely used for mechanical applications. Among these coatings, chromium nitrides are especially interesting because they combine good tribological properties with relative chemical inertness. Corrosion-resistant nitrogen-doped chromium thin films were deposited by using an industrial magnetron reactive sputtering device. A precise control of the composition has permitted separate synthesis of the defined compounds that exist in the binary diagram Cr–N: Cr 2 N and CrN. Potentiodynamic methods were used to investigate the corrosion protection provided on AISI 304 stainless steel by various coatings. At the same time, coating microstructure was investigated by XRD and SEM/EDS as well as its tribological properties (hardness and wear resistance). The CrN coatings exhibit the higher wear resistance whereas the Cr 2 N films show the better hardness. Cr 2 N coatings or CrN coatings have an inert intrinsic behaviour either in acidic or in chloride solutions. However, their capability of protecting the substrate from any corrosion is limited due to defects and porosity. However, as a function of deposition parameters, these coatings can enhance the substrate lifetime in aggressive environments.

The structural and electronic properties of HfO 2 polymorphs were investigated using density functional theory (DFT). The Kohn–Sham equations were solved by applying the full-potential linearized augmented plane wave (FP-LAPW) method. We used the generalized gradient approximation in the Perdew–Wang formalism to the exchange and correlation energy functional. The ground state properties such as lattice parameter, bulk modulus and its pressure derivative as well as the structural phase stability were calculated and compared to the available experimental data and previous theoretical works. The FP-LAPW method correctly orders the zero temperature energies of hafnia polymorphs. The effect of distortion from the cubic to the tetragonal structure has also been studied on the basis of charge density calculations.

Hard coatings for wear and oxidation/corrosion protection based on chromium nitride were deposited by means of a commercial rf magnetron sputtering unit with a mixture of nitrogen and argon from a target of the pure metal. The present paper reports on the influence of deposition parameters on coating morphology, crystallographic structure, and hardness. The coating surfaces viewed by atomic force microscopy showed that grain shape depends on the composition of the vapour phase, and especially on the presence or absence of reactive gas. In addition, special attention was paid to the first stages of growth. The crystalline phases Cr, Cr2N, and CrN were identified by X -ray diffraction analysis as a function of the film nitrogen content. Finally, hardness measurements were performed and compared to coating structure and composition.

Hard coatings for wear and corrosion protection based on chromium nitride were deposited by means of a commercial RF magnetron sputtering unit. Their structural, compositional and mechanical properties were compared. All films were reactively sputtered with a mixture of nitrogen and argon from a target of the pure metal. This paper reports the influence of deposition parameters on coating morphology, crystallographical structure and hardness. We found that the surface morphology of the coatings is hardly influenced by the volume fraction of the reactive gas. Also, the hardness (HV 0.05 =2500) is closely linked to the crystalline structure of the deposit. Finally, we have paid special attention to the evolution of the deposition rate and the total pressure as a function of nitrogen content in the vapor phase.

International audience; Since the early stages of thermal spray, it has been recognized that the powder composition, size distribution, shape, mass density, mechanical resistance, components distribution for composite particles play a key role in coating microstructure and thermo mechanical properties. The principal characteristics of particles are strongly linked to the manufacturing process. Coatings also depend on the process used to spray particles and spray parameters. Many papers have been devoted to the relationships existing between coating properties and structures at different scales and manufacturing processes. In many conventional spray conditions resulting in micrometric structures, among the different parameters, good powder flow ability, and dense particles are important features. Thermal plasma treatment, especially by RF plasma, of particles, prepared by different manufacturing processes, allows achieving such properties and it is now developed at an industrial scale. Advantages and drawbacks of this process will be discussed. Another point, which will be approached, is the self-propagating high-temperature synthesis, depending very strongly upon the starting composite particle manufacturing. However, as everybody knows, ‘‘small is beautiful'' and nano- or finely structured coatings are now extensively studied with spraying of: (i) very complex alloys containing multiple elements which exhibit a glass forming capability when cooled-down, their under-cooling temperature being below the glass transition temperature; (ii) conventional micrometer-sized particles (in the 30-90 lm range) made of agglomerated nanometersized particles; (iii) sub-micrometer- or nanometer-sized particles via a suspension in which also, instead of particles, stable sol of nanometer-sized particles can be introduced; and (iv) spray solutions of final material precursor. These different processes using plasma, HVOF or sometimes flame and also cold-gas spray will be discussed together with the production of nanometer-sized particles via the chemical reaction method or by a special type of milling: the cryogenic milling process often referred to as ‘‘cryomilling.''

TiO2 coatings were plasma sprayed using two spray-dried powders as feedstock materials with either a dense and solid shape or a hollow morphology. To achieve interesting photocatalytic properties, coatings must be in the anatase phase. The effects of the powder characteristics, especially shape, density and size distribution, and of the plasma conditions on the anatase ratio and porosity rate of the coatings were investigated. To better qualify the characteristics of these coatings produced from spray-dried powders, microhardness was also studied. A bimodal microstructure of the coatings was identified related to the incorporation of non-molten particles that allow to maintain a high ratio of anatase phase. As a consequence a bimodal distribution of Knoop microhardness was also reported for coatings composed of 40 to 60 vol.% anatase.