Manufacturing of MnCo 2 O 4 spinel coatings by solution precursor plasma spraying (SPPS) was studied in order to produce thin ceramic coating on a ferritic stainless steel interconnect for SOFC’s. The main purpose to use MnCo 2 O 4 coating in SOFC devices is to prevent the migration of harmful CrO 3 and Cr 2 (OH) 2 compounds to the triple phase barriers (TPB) of the cathode. In this study Mn(NO 3 ) 2 •4H 2 O and Co(NO 3 ) 2 •6H 2 O were diluted to deionized water and mixture of deionized water and ethanol at 3 M mixture rate. The solutions were sprayed on 0.5 mm thick Crofer 22 APU substrate by Sulzer Metco F4-MB plasma gun with a modified solution feeder. Microstructural characterizations for the as-sprayed coatings were done by using a field-emission scanning electron microscopy (FESEM) with SE-mode. Elemental analyses were done with energy dispersive spectroscopy (EDS) and an X-ray diffraction (XRD) was used for crystallographic studies. The coating with full equivalence of the crystallographic structure of MnCo 2 O 4 spinel was sprayed using argon-helium plasma and water based solution. Plasma gas with hydrogen as a secondary or ternary gas and ethanol based solutions caused the formation of the mixed phases of CoO and MnCo 2 O 4 . Although the microstructures of sprayed coatings were still quite porous, the influence of relevant gun and solution parameters were found in order to improve coating denseness in further studies.

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.

Titanium suboxides form a class of ceramic materials possessing such technically interesting properties as electrical conductivity and solid lubrication. Consequently, these materials have a high potential for application as thermally sprayed coatings. In this paper the preparation and characterisation of two spray powders of different composition by agglomeration and sintering with a narrow range of the value n in Ti n O 2n-1 is described. Powders were characterised by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), thermogravimetric measurements (TG), helium pycnometry, nitrogen adsorption and mercury intrusion techniques. The sprayability was tested by plasma spraying. The coating structures were studied by optical microscopy, XRD, TG and TEM. Although the powders were only partially oxidized as a result of the spray process, the crystallographic structure was changed significantly, according to XRD and TEM investigations. As an alternative method of preparation of titanium suboxide spray powders, the reduction of a fused and crushed spray powder with hydrogen is described. Powder particle shape and size distribution are not changed in this process.

Cold spraying was applied to deposit Ti2AlC on different substrate materials. The study of single impacts by scanning electron microscopy indicates that bonding of the first layer is mainly attributed to the deformation and shear instabilities of the substrates. Nevertheless, the irregularly shaped particles appear to flatten by the impact. This deformation seems to be attributed to local, internal shear, but also to internal fracture. By applying up to five passes under more sophisticated spray parameters, Ti2AlC - coatings with thicknesses of about 110 to 155 µm can be achieved. XRD analysis of the coating proves that the crystallographic structure of the feedstock can be retained during cold spraying. The coating microstructures show rather low porosity, but several cracks between spray layers. Successful build-up of more than one monolayer can probably be attributed to internal deformation and occurring shear instabilities within the highly anisotropic Ti2AlC - phase.

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.

In the continuing progress of fuel cell technology, CeO 2 double doped electrolytes appears to be promising for lowering the SOFC's working temperatures. Ceria electrolytes have better ionic conductivities than YSZ but, at low oxygen partial pressures, the chemical reduction of ceria leads to increasing electronic conduction. Double doping of the ceria increases the electrolytic conduction range without changing its conductivity. To avoid stress development within the ceria crystallographic structure, the dopants mix must have a mean ionic radius as close as possible to the critical ionic radius. Ceria electrolytes with various compositions and dopant concentrations are synthesized with a combinatorial chemistry approach. To synthesize new electrolytes, solution plasma spraying with nitrate salt precursor is used. The reaction is completed and nanocrystalline thin layers of ceramic are formed in the plasma. Comparative studies of plasma spraying techniques, with YSZ powder plasma spraying as electrolyte reference, were performed. Also, comparative impedance spectroscopy measurements are to be performed to validate the double doping hypothesis and thence to identify the best electrolytes in the suite of over 300 new materials.

Epitaxial grain growth during the rapid solidification of molten TiO 2 in plasma spraying was studied. The crystallographic structure of the TiO 2 splats deposited on rutile and α-Al 2 O 3 substrates at 150, 300 and 500 °C was characterized by high resolution transmission electron microscopy and electron back scattering diffraction. The results reveal that homoepitaxial and hetero-epitaxial TiO 2 splats can be formed at the deposition temperature of 500 °C. Epitaxial growth is significantly influenced by the crystal orientation. It is easier to form an epitaxial TiO 2 splat with a <001> orientation in the direction perpendicular to the substrate surface. In order to explain the formation of epitaxial splat during plasma spraying, a competition mechanism between heterogeneous nucleation and epitaxial growth was proposed. It was indicated that the face (001) of rutile crystal exhibits the largest growth velocity, which is conducive to form an epitaxial splat for the melt with a largest undercooling degree. In addition, the effect of deposition temperature and crystalline orientation on the epitaxy was simulated. The simulation results are in agreement with the experimental observations.

Quasicrystals are materials whose structure cannot be understood within classic crystallographic methodology. Quasiperiodic structures have a long-range orientation order but lack transitional periodicity. This non-periodic structure gives quasicrystals a unique range of physical properties including: poor thermal conductivity; excellent crystallographic stability up to their melting point; can be thermally sprayed to display either wear resistant or abradable characteristics; possess excellent release properties (low coefficient of friction); and, they are non-work hardenable. This effort describes parametric conditions utilized to deposit coatings of identical parent metal chemistry but with vastly different end properties.

The metastable anatase structure of titanium dioxide (TiO 2 ) is photocatalytically active and thus able to purify air and water. This makes anatase TiO 2 a desirable material for instance in environmental technology. The present work provides a comparative study on the photocatalytic activity of TiO 2 coatings, prepared by different spray methods: conventional atmospheric plasma spraying, HVOF spraying, suspension spraying and cold spraying. The spray experiments were performed with anatase powder as feedstock and the spray parameters were optimized in order to obtain coatings with high photocatalytic performance. With respect to spray technique and spray conditions, different amounts of the photocatalytic active anatase phase could be retained. The photocatalytic activity of the coatings was evaluated by degradation of dichloroacetic acid. The results show that the photocatalytic activity is highly dependent on the amount of preserved anatase structure obtained under the different spray techniques.

Extensive research activities were conducted over the last few years on coatings made of titanium oxide, an established material for thermally sprayed coating solutions. Multiple existing and potential applications are closely connected with the existence of different titanium dioxide modifications and the formation of suboxides. This provides a basis for discussions on the Ti-O phase diagram as well as the properties and conditions of formation of relevant phases. Coating microstructures, phase compositions and mechanical properties are discussed as a result of interactions of different spray powders in different spray conditions of atmospheric and vacuum plasma spraying (APS and VPS), as well as of high-velocity oxyfuel (HVOF) spraying. The discussion on applications is focused on electrically conductive coatings, coatings with photocatalytic properties and coatings for wear applications.

Plasma sprayed oxide coatings are widely used in applications where the surface properties of the coatings dictate the performance of the coating. The knowledge of chemical surface properties of plasma sprayed oxide coatings is very limited and to our knowledge surface chemical studies of these coatings have not been published. Surface properties of common plasma sprayed oxide coatings exposed to water based electrolytes were evaluated using several surface sensitive techniques such as X-ray photoelectron spectroscopy (XPS) and electrophoretic mobility studies. The dissolution behaviour of the coating materials were studied using inductively coupled plasma-mass spectroscopy (ICP-MS). Other characterization techniques included X-ray diffraction (XRD) and scanning electron microscopy (SEM) combined with energy dispersive spectroscopy (EDS). The correlation between chemical stability, bulk and surface properties of the coatings were evaluated. Obtained results are compared with published values of traditionally manufactured oxide counterparts.

It is generally known that plasma sprayed coatings exhibit rather a low strength thanks to their characteristic microstructure with porosity and microcracks. To determine the role of varying types of deformation in different parts of the coatings profile, 1.8 mm thick chromia coatings on a steel substrate were made using WSP plasma spray. Stress gradients were then measured "in-situ" on a four-point bending device during bend deformation of the coated beam using neutron diffraction. Only compressive loading in coatings increased the resulting stresses. From the plot of stresses vs. applied strain (linear dependence) the Young’s moduli of the substrate and the coating were determined. Both values agree well with those obtained by mechanical testing. In general it is believed that, in a simplified way, interlamellar voids (cohesion defects) roughly parallel to the substrate decrease the tensile strength in the direction perpendicular to the substrate, while intralamellar cracks, roughly perpendicular to the substrate, significantly affect the Young’s modulus of coating. It is supposed that the tensile deformation of coatings opens the cracks and no stress increase is observed. On the other hand the applied compressive deformation in the coating closes the intralamellar microcracks and the internal stresses increase, as suggested by obtained results.

The aim of this paper is to study the behavior of two different sets of as-sprayed coatings obtained from two different starting HA powders, in a defined simulated body fluid (SBF) for a period of 28 days. Spray-dried hydroxyapatite (SDHA) and spheroidized hydroxyapatite (SHA) were the two selected powders. A Controlled Atmosphere Plasma Spraying system (CAPS) was use for the production of the coatings. Effect of pressure and surrounding atmosphere during the spraying of the coatings, were also evaluated. During the in-vitro test, dissolution of the coatings and precipitation of a poorly crystallized apatite in a preferential crystallographic orientation ([001] direction) was observed for the two sets of coatings. Dissolution of the coatings was measured by: 1) weighing the specimen before and after soaking and 2) by measuring the calcium ion concentration in the SBF solution with the Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES). Scanning Electron Microscopy (SEM) exposed the morphology of the coatings and X-Ray diffraction (XRD) and Fourier Transform Infrared Spectrometer (FTIR) revealed their structure and composition.

New advanced polymeric biomaterials such as implantable poly(etheretherketone) (PEEK) are changing the face of the implantable medical device industry. Due to its bioactive behavior in vivo, hydroxyapatite (HA) coatings are used to improve the bone growth and to repair around metallic implant. The objective of this work is to study the feasibility of plasma sprayed hydroxyapatite coating on PEEK material. Different PEEK (unfilled and composite) specimens were successfully coated with a 150 µm thick coating. Chemical and crystallographic compositions, adhesions and microstructures of HA coatings on PEEK and on Ti-6Al-4V were compared. The results showed that the structure of HA coatings were appreciably equivalent. Mechanical tests showed that the plasma spraying process did not severely degrade the initial properties of the PEEK substrate.

Suspension plasma spraying (SPS) is regarded as a promising way to produce new coating structures with improved properties. In this study, SPS was studied as a possible manufacturing process for producing thin MnCo 2 O 4 spinel coatings for used as protective coatings in metallic interconnector plates of SOFC’s. Suspension of nanosized MnCo 2 O 4 powder and ethanol was thermally sprayed by using an F4-MB plasma gun with radial suspension feeding. The influence of spraying parameters, such as plasma gas composition, total gas flow, current and spraying distance for coating architecture was studied by using field-emission scanning electron microscopy (FESEM) and X-ray diffraction method (XRD). Spraying parameters had a strong influence on the coating structure and composition. Coating with the most homogenous structure were formed when sprayed with the low energy spraying parameters whereas high energy parameters resulted in formation of a columnar microstructure containing larger cobalt rich areas.

To clarify the formation micromechanism of plasma spray deposits, internal microstructures have been examined by transmission electron microscopy on the thin films prepared from different depths of the plasma sprayed Al 2 O 3 deposits. Microstructures in the top layer have consisted of both the coarse grains of 1 µm and the fine ones of 0.2 µm in average diameter. Point defects were contained mostly in γ phase grains. In the mid-layer, columnar structures often developed in parallel or in radial directions which adjoined with γ phase grains. From the SAD analyses, columnar grains were identified as α phase and lay in a twin crystal orientation relationship to each other between the joined grains. The bottom layer was composed of a mixture of the coarse grains of 0.2 µm and the fine ones of 0.02 µmin average grain diameter. It was found that the spray deposit was composed of different microstructures in crystallization, depending on depth to the thickness direction of the spray deposit.

Thermally sprayed Fe-based coatings can be applied in conditions ranging from almost solid to complete molten droplets. While spraying under atmospheric conditions, the oxygen content in the coating varies depending on the spray parameters and the portion of molten phases in the droplets. Using vacuum-plasma technology, Fe-based alloys can be sprayed with a significant amount of molten phase without oxidation. This capability can also be used for alloying Fe-based sprays with nitrogen as is done during reactive vacuum plasma spraying. Such alloying promotes the formation of dispersed vanadium-nitride which greatly improves corrosion and wear resistance.

This work investigates the influence of powder characteristics on thermal spray coatings produced from titanium suboxide powders. The powders used were prepared from commercially fused and ground titanium dioxide reduced in a mixture of hydrogen and argon gas. Two particle sizes were produced and subsequently applied by atmospheric plasma and HVOF spraying. Originally melted and crushed powder fractions served as a reference. All deposits showed high wear resistance, particularly the HVOF-sprayed layers. Paper includes a German-language abstract.

Solution precursor plasma spray has been shown capable of depositing high surface area transition metal oxide coatings of interest as ultra-capacitor electrodes. These materials exhibit mixed double layer and pseudo-capacitive properties, enabling larger charge storage capacity than electrical double layer capacitor electrodes such as carbon. This investigation explored potential of SPPS to deposit molybdenum oxide with microstructures suitable for use as pseudo-capacitive electrodes. It further identified a two-step temperature-programmed heat treatment that resulted in the topotactic phase transformation of the α-MoO 3 deposits into high specific surface area molybdenum nitrides exhibiting a higher electrochemical stability window (i.e. a higher specific area capacitance). The electrochemical behavior of molybdenum oxide and molybdenum nitride deposits formed under different deposition conditions was studied using cyclic voltammetry in order to assess the influence of the resulting microstructure on the charge storage behavior and potential for use in ultra-capacitors.