In this study, twin wire arc spraying is used to deposit dense Inconel skins on 40 PPI nickel foam sheets. A design-of-experiments approach is used to investigate the effects of grit blasting on the surface characteristics of paste-filled foam. In-flight behavior of molten droplets and its effect on coating properties is assessed at three spray distances. The Inconel coatings are evaluated based on SEM and EDS analysis, roughness measurements, and adhesion testing. The results indicate that acceptable adhesion, porosity, and oxide content can be achieved over a small range of grit blasting parameters.

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.

Recently, an advanced technique was developed to fabricate sandwich structures for high temperature applications by depositing alloy 625 skins on Ni alloy foam core by thermal spraying. This study tries to utilize an analytical model to estimate the mechanical performance of these structures based on the mechanical properties of the constituents. The mechanical behavior of the Ni alloy foam is assessed via compression testing, while tensile tests are used in the case of the alloy coating. The flexural rigidity of the sandwich structure is calculated using analytical models and experimentally obtained elastic moduli of the alloy 625 coating and Ni alloy foam. The model is also used to calculate the flexural rigidity of sandwich samples with different skin thicknesses to check the accuracy of the model and to understand the effect of skin thickness on the predicted mechanical performance of sandwich structures. The effect of heat treatment on the mechanical behavior of sandwich structures is investigated as well.

Open pore foams can be used as gas filters, catalyst supports, and heat exchangers due to their high gas permeability and heat conductivity. In this study, Ni-Cr skins were deposited on each side of a foam sheet by HVOF spraying to form a sandwich structure for use as a heat exchanger. The microstructure of the skins and the interface with the nickel foam struts were examined and the hydraulic characteristics and heat transfer properties of the sandwich structure were experimentally determined. Pressure drops across the heat exchanger were measured and found to be proportional to the square of the velocity of the coolant and a least square fit was used to solve for the permeability, K, and form coefficient, C, of the foam.

Nickel-based superalloys can be used at temperatures up to 1050 °C in air. Superalloy open cell foam sheets with skin layers plasma sprayed on both sides can be used as high temperature heat exchangers provided that the two deposited skins are dense and well adhered to the open cell foam. In this study alloy 625 skins were deposited on each side of a sheet of metal foam by APS and HVOF to form a sandwich structure. Two densities of open cell foams, 20 and 10 pores per linear inch (ppi), were used in this study as the core. The initial Ni foam was converted to an alloy composition by plasma spraying aluminum and chromium on the foam’s struts with subsequent diffusion/solutionizing heat treatments before the alloy 625 skins were deposited. The microstructure of the coatings and the interface between the struts and skins was investigated. A layer of Ni-Al alloy was formed near the surface of the struts as a result of the heat treatment. The foam struts were imbedded more deeply into the coatings deposited by HVOF than the coatings deposited by APS.

The solution precursor plasma spray (SPPS) process, in which a solution precursor of the desired resultant material is fed into a plasma jet by atomizing gas or high pressure, was developed in the 1990’s and has been studied extensively since then. Recently, it has been shown that the SPPS process is suitable for deposition of porous electrodes for solid oxide fuel cells (SOFC). High efficiency SOFC requires electrodes with 30%-40% porosity. Because of the complexity of the SPPS process and the large number of processing parameters, it is difficult to investigate the effect of each parameter on the two important properties, i.e. coating porosity and deposition efficiency, separately. Design of experiments can use a small number of experimental runs to analyze the effect of each processing parameter on the properties of the fabricated product, after which the processing parameter combinations for fabricating a target product can be found. In this project, a small central composite design, a second order statistical model, was used to analyze and optimize the SPPS process for Ni-YSZ anode deposition. The processing parameters investigated include: 1) Hydrogen flow rate, which determines arc voltage, 2) Current , 3) Solute flow rate, 4) Solution concentration, 5) Distance between nozzle and gun, and 6) Stand off distance. The effects of the selected processing parameters were analyzed, and the resultant model used to select a combination of processing parameters which produced a coating with the desired characteristics.

The purpose of this study was to determine the creep/sintering characteristics of thermally sprayed zirconia coatings and attempt to understand the influence of microstructure on the creep resistance of deposits. The major modification, compared with more typical practice, was employment of powder feedstock with agglomerated sub-micron size particles (Nanox), which is compared to one of the best commercially available powders (HOSP). Thick plasma sprayed coatings were prepared and their physical and mechanical properties were characterized. Creep/sintering experiments were then conducted to investigate the response of the materials when exposed to high temperatures under load. The results showed that it could be possible to correlate the splat thickness to the creep behaviour of the coatings.

CaZrO 3 coatings were alternatively prepared by air plasma spray and flame spray processes. The microstructural characteristics and crystalline phases of the coatings were comparatively studied as a function of the spraying temperature achieved with each technique and the stand off distance. Image analyses were used to estimate their porosity. Thermal diffusivity was measured on free-standing thick coatings using the laser flash technique. Thermal conductivity was obtained from the experimental thermal diffusivity and density data. The hardness of the coatings was evaluated by Vickers indentation tests. Finally, different thermal treatments were carried out to evaluate the evolution of the crystalline phases and the properties of the coatings.

Solution precursor plasma spraying has been used to produce finely structured ceramic coatings with nano- and sub-micrometric features. This process involves the injection of a solution spray of ceramic salts into a DC plasma jet under atmospheric condition. During the process, the solvent vaporizes as the droplet travel downstream. Solid particles are finally formed due to the precipitation of the solute, and the particle are heated up and accelerated to the substrate to generate the coating. This paper describes a 3D model to simulate the transport phenomena and the trajectory and heating of the solution spray in the process. The jet-spray two-way interactions are considered. A simplified model is employed to simulate the evolution process and the formation of the solid particle from the solution droplet in the plasma jet. The temperature and velocity fields of the jet are obtained and validated. The particle size, velocity, temperature and position distribution on the substrate are predicted.

Finely structured ceramic coatings can be obtained by solution precursor plasma spraying. The final structure of the coating highly depends on the droplet size and velocity distribution at the injection, the evolution of the spray in the jet, and droplet breakup and collision within the spray. This paper describes a 3D model to simulate the transport phenomena and the trajectory and heating of the solution spray in the process. O’Rourke’s droplet collision model is used to take into account of the influence of droplet collision. The influence of droplet breakup is also considered by implementing TAB and Wave droplet breakup models into the plasma jet model. The effects of droplet collisions and breakup on the droplet size, velocity, and temperature distribution of the solution spray are investigated. The results indicate that droplet breakup and collision play an important role in determining the final particle size and velocity distributions on the substrate.

In this work was studied the suitability of Solution/Suspension Precursors Plasma Spray (SPPS) technique to produce titanium dioxide (TiO 2 ) and cerium doped barium zirconium titanate (Ce doped Ba(Zr 0.2 Ti 0.8 )O 3 or BZT) coatings. The precursors were sol-gel solutions typically employed to produce powders of those compositions. We report on the formation of porous TiO 2 coatings on stainless steel substrates. The microstructures found this coatings are characterized by features typically found in solution precursor plasma spray processes; a combination of melted and unmelted deposits and small hollow spheres. Transmission electron microscopy revealed submicron/nanocrystalline features forming those deposits. Anatase was the main crystalline phase present in the TiO 2 coatings. The rutile phase was also found in the coatings. The photocatalytic activity of the coatings was measured and compared with the results obtained for commercial anatase powders dispersed in liquid. Preliminary results obtained for Ce doped Ba(Zr 0.2 Ti 0.8 ) O 3 coatings deposited on mild steel substrates showed microstructures and crystalline phases suitable for dielectric applications.

Although the deposition of Ni-YSZ anodes by plasma conventional spray is more successful than other SOFC components, the large NiO and YSZ particles used for the spray process, about 50-150 microns for high porosity coating deposition, reduce the density of three phase sites for electrode reaction. In this paper, the solution precursor plasma spraying (DCSPPS) process, in which solution precursors of the desired resultant materials are fed into a DC plasma jet by atomizing gas, was used to synthesize and deposit porous Ni-YSZ composite anodes. The deposition results show that several process parameters have significant effects on the microstructure and phase composition of the deposited material. The deposits were composed of tower-like, irregularly shaped agglomerates and splats. The sizes of the agglomerates increase with the decrease of the plasma torch power and most are not completely molten during the impact. The amount of splats is proportional to the power and they are much smaller than the agglomerates in volume. After heat treatment to reduce the NiO present in the as deposited coatings, the coatings were found to contain small spherical YSZ particles about 0.5 micrometers in diameter distributed in a continuous Ni matrix. The coatings have 29%-51% porosity depending on the process parameters.

In this work the suitability of two different liquid precursors of hydroxyapatite (HA) as feedstock for the air plasma spray (APS) coating technique was studied and compared. The precursors were organic (calcium nitrate tetrahydrate and triethyl phosphate) and inorganic (calcium nitrate tetrahydrate and ammonium dihydrogen phosphate) sol-gel solution precursors employed in a previous work to produce thin films by a dipping technique. We report on the formation of porous hydroxyapatite coatings on Ti6Al4V substrates. The microstructures found in of both types of coatings are characterized by features typically found in solution precursor plasma spray processes; a combination of melted and unmelted deposits and small hollow spheres. Transmission electron microscopy revealed submicron/nanocrystalline features forming those deposits. Hydroxyapatite was the main crystalline phase present in the coatings. Small amount of other crystalline phases were also found due to the high temperature of the substrates during the spraying.

In this work two different thermal spray techniques were used to deposit La 0.9 Sr 0.1 CrO 3 interconnect material: high velocity oxy-fuel (HVOF) using a modified nozzle and atmospheric plasma spray (APS). Two different APS torches were employed: A commercial torch that uses Ar/H 2 as plasma forming gases and a new torch design that uses CO 2 gas mixtures. A substitute powder with similar physical properties to La 0.9 Sr 0.1 CrO 3 was employed for the development and optimization of the process parameters to achieve the highest density before the deposition of the La 0.9 Sr 0.1 CrO 3 on zirconium oxide substrates. The microstructures observed by scanning electron microscopy (SEM) and the phase composition of the coatings obtained from X-ray diffraction analysis are correlated to the spraying characteristics of the different techniques employed. The electrical resistivity of the as-sprayed coatings is discussed in terms of microstructure features and the phase composition. Post-deposition heat treatments were studied in order to reduce the electrical resistivity.

The current study has been focused on the final morphology of atmospheric plasma sprayed 8% yttria stabilized zirconia single splats. Single splats of two different sizes (-25 µm and +25/-45 µm) of ZrO 2 8Y 2 O 3 powder have been collected on polished stainless steel substrates at three different temperatures (Room, 300°C, and 600°C). The splat morphology and diameter, satellite particles, and splashing behavior were investigated using both scanning electron microscopy and image analysis software. The splat/substrate interface and splat curl up were studied from cross-sections prepared by focused ion beam milling. Results showed primarily pancake morphology and no evidence of delamination along the splat/substrate interface at 300oC substrate temperature and 100 mm spray distance. Overlapped splats showed evidence of melting (microwelding) at the splat boundaries. Splat thickness was measured to be less than 1 µm for all spray conditions. Roughness profiles of the surface of the deposited splats indicated microcracks had formed within the splats. Image analysis results exhibited a higher volume fraction of the splats relative to satellite particles at longer spray distance and higher substrate temperature. The average splat diameter increased as the substrate temperature increased.

Retaining non-melted nano-particles of zirconia in nanostructured coatings has been a challenge in the past. Recently an air plasma spray process was developed to produce coatings which retain up to 30-35% by volume non-melted particles, resulting in a unique structure. The creep/sintering behavior of such thermal barrier coatings deposited from nanostructured feedstock has been measured and compared with deposits produced from hollow sphere powder (HOSPTM). Both feedstocks contain 6-8wt% Y 2 O 3 as stabilizer. Flexure and compression creep testing were conducted under several different loads and temperatures to obtain creep exponents and parameters.

Magnesia-alumina spinel (MgAl 2 O 4 ) is an oxide ceramic with broad applications as a high temperature and corrosion resistant material. Its moderate coefficient of thermal expansion (7-8 x 10-6 K-1) may allow it to be used as a plasma spray coating on metallic substrates with coefficients of thermal expansion of ~12x10-6 K-1. Two important factors affecting the microstructure and properties of plasma sprayed coatings are the particle temperature (Tp) and particle velocity (Vp) in the plasma jet. The particle temperature and velocity are influenced by a substantial number of operating parameters. Using a statistical design of experiments approach, deposition parameters, microstructure, and physical properties of air plasma sprayed spinel coatings are linked through mapping versus in-flight particle temperature and velocity. The Tp-Vp maps provide a basis for depositing coatings with controlled structures.

The structure of thermally sprayed deposit consists of individual lamellae formed from melted and re-solidified particles, along with unmelted and partially melted particles, pores, microcracks, and splat boundaries. The elastic modulus of a vacuum plasma sprayed Ti-6Al-4V alloy parallel to the splat plane determined by standard uniaxial tensile testing was found to be approximately 30% lower than that of conventionally processed materials with the same level of porosity. The relationship between the elastic modulus and the microstructure was studied using an in-situ tensile testing stage in an optical microscope combined with analytical and finite element models. An idealized microstructure was used for the analytical model, which yielded an estimate of the modulus higher than that measured. The finite-element program OOF was also used to compute the elastic modulus based on micrographs of polished and etched surfaces and predicted a reduction of about 37% in the modulus.

Thermally sprayed ceramic coatings deposited from nanostructured feedstock powder have often shown improved mechanical properties comparing to coatings produced from convention feedstock. Bimodal structured ceramic deposits have been reported to demonstrate better properties such as wear resistance, adhesion strength and toughness. For thermal barrier coatings, high temperature performance is a key point especially creep/sintering. In this study creep/sintering rate of plasma sprayed, bimodal structured, yttria stabilized zirconia coatings were investigated. The creep behaviour was investigated using free standing thick (3 mm) coating layers loaded in the four point bending setup at two temperatures : 800 and 1000 ºC in air. Under the same test conditions the creep results for nanostructured coatings and conventional plasma spray coatings have been compared together and the former one in both temperatures showed a lower creep rate.

Boron carbide has been successfully deposited on Ti alloy by vacuum plasma spraying (VPS). Mechanical properties of the deposited structure were assessed by micro-hardness and nano-hardness indentation. Chemical and phase compositions of the starting powder and the as-sprayed structure were characterized using hot gas extraction (LECO), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and Raman spectroscopy. The microstructure consisted of equiaxed boron carbide grains, microcrystalline boron carbide particles, and amorphous carbon regions at the grain boundaries. The amount of boron oxide and amorphous carbon increased during spraying. Carbon segregation to grain boundaries in the as-deposited B 4 C was observed. The measured micro-hardness was slightly higher than values previously reported (1033 ± 2009 HV). There was significant variation of nano-hardness from point to point in the material due to the existence of multiple phases, splat boundaries, and porosity in the deposited structure.