In the present work, a metal-polymer composite coating containing Al and ethylene-vinyl acetate copolymer (EVA) was prepared on the surface of a polymer matrix composite (PMC) using a detonation spraying process. The microstructure and bond strength of the as-prepared coatings were analyzed. The bonding mechanism of the coatings, especially the deposition behavior of the Al and EVA particles on the PMC surface is discussed. Results had shown that detonation spraying technique enables the deposition of metal-polymer coatings directly onto the PMC surface under precise process control. The preparation of metal-polymer composite coating on PMC via detonation spraying process presents promising application as an interlayer for the surface metallization of PMC.

In this work, a three-dimensional coating build-up model is developed to investigate the effect of torch speed, spray angle, and step size on plasma-sprayed coatings. In-flight particle locations and status prior to impact are measured and the data obtained are fed into the model. The model is used to predict the thickness, roughness, and porosity of plasma-sprayed coatings for different spray angles, torch scanning speeds, and scanning steps. Simulation results are presented and discussed along with several rules that govern splat deformation, splat overlap, and pore formation.

There have been recent efforts to expand the thermal spraying capabilities for novel corrosion resistant coatings for metal bipolar plates were produced by thermal spraying for proton exchange membrane (PEM) fuel cell applications. Recently, substrate heated by plasma gun or by external laser beam has been proposed to enhance the mechanical and thermal properties of the coatings. Studies were found that with sufficient substrate heating, substrate melting may happen. When droplets solidified on a thin liquid layer on the top of the substrate, conditions will be similar to crystal growth and Epitaxy film growth will be possible. It is therefore possible that using substrate melting as tool to promote epi-layer growth using plasma spraying. Difficulty is how to control the substrate temperature to cause substrate melting during droplet solidification. In this study we will propose a new idea for better temperature control on the substrate. The capability of epitaxy growth using thermal spraying will be investigated. Molybdenum droplets impact on an Aluminum substrate will be studied. A splat formation model including undercooling, nucleation, and non-equilibrium solidification will be used to study the possibility of the substrate melting and grain size distribution.

Precursor Plasma Spraying (PPS) using Radio Frequency (RF) induction plasma spray is a new process used to synthesize functional materials. RF plasma spray has the advantages of stability, cleanness, high temperature and high chemical reactivity. In this paper, a two-dimensional numerical model has been developed to investigate the induction electromagnetic (EM) field and the thermo-fluid field in a radio frequency inductively coupled plasma (RF-ICP). In flight particle interaction with the plasma jet will be investigated. The traditional micron-size powder particles, e.g. zirconia (PSZ), are injected with carrier gas such as argon. During their interaction with the RF plasma, the powder particles experience acceleration, heating, melting and evaporation and particle heat transfer is considered coupled with the thermo-fluid flow of the RF plasma. A generalized particle model is developed and applied to the precursor plasma spray process operated in a vacuum chamber. The effects of power input, standoff distance and powder size on the RF plasma and particle in flight characteristics are investigated.

A wide range of manufacturing processes are used to supply yttria stabilized zirconia powders for plasma sprayed TBC applications. From previous studies it is known that the difference in coating properties can potentially result from variations in powder feedstock as a consequence of particle inflight behavior and particle impact. An additional strong contribution to splat variation results from changes in the particle in flight behavior. In order to understand the variation in particle condition as a consequence of different powder morphologies, a detailed diagnostic analysis was carried out for plasma densified (PD), fused and crushed (FC) and agglomerated & sintered (A&S) powders. In this study a “3D multiple sensors” based integrated approach was used to evaluate these differences. Direct feed back sensors were used for optimization and combined with sophisticated diagnostics for in-depth studies. To obtain comparable results, three batches of commercial powders were sized to the same specification. For a given set of spraying parameters the recorded spray stream characteristics such as plume position, particle temperature, size and velocity deviated strongly for the given morphologies. By optimizing injection, the different powders can be made to follow nominally similar trajectories. This study reveals the sensitivity of each powder to process parameters and the variability in particle state that can result from it. Some techniques are suggested to optimally inject the different powders and to achieve similar particle states for these morphologies

This work addresses the fabrication of membrane-type SOFCs, operating at an intermediate temperature using all components fabricated by plasma spray technology, and to evaluate the performance of the SOFC single unit at a temperature range of 500-800 0C. Single cells composed of a LaSrMnO 3 (“LSM”) cathode, LaSrGaMgO 3 (“LSGM”) electrolyte and a Ni/YSZ anode, were fabricated in successive atmospheric plasma spraying processes. Plasma spraying processes have been optimized and tailored to each layer in order to achieve a high porosity cathode or anode layer as well as a high density electrolyte layer. Major effort has been devoted to the production of the LSGM electrolyte film with high density and free-cracking. Electrochemical impedance spectroscopy was used to investigate the conductivity of the electrode layers and particularly the resistance of the electrolyte layer. It was revealed that the heat treatment had a great influence on the specific conductivity of the sprayed electrolyte layer, and that the specific conductivity of the heat-treated one was dramatically increased to the same magnitude as that of a sintered LSGM pellet. The experimental results have demonstrated that the plasma spray process has great potential for the integrated fabrication of the medium temperature SOFC units.

Nanostructured materials have a high potential to improve properties of thermal spray coatings. High-energy ball milling is an effective process to produce dispersion-strengthened powders with a homogenous nanoscale microstructure and superior mechanical properties compared to unstrengthened material. Because of the relatively low temperatures and heat flux during HVOF spraying, the microstructure of the powder particles can be maintained in the coating. In this study Al 2 O 3 - dispersion-strengthened NiCr powders were synthesised by high-energy ball milling, using optimised milling parameters. Thermal spray coatings were produced using the HVOF process. The improvement of the tribological behaviour and the potential for technical applications were evaluated. The study shows the favourable impact of mechanically alloyed MMCs on the tribological behaviour of HVOF coatings on the basis of an Al 2 O 3 -strengthened NiCr powder

The melting behavior of in-flight particle and its impact on splat morphology are studied. A group parameter, “melting index”, has been derived to correlate the melting status of inflight particles with particle size, velocity, and temperature which can be measured experimentally. Numerical simulations have been used to determine the unknown parameters in the melting index. The effect of particle size on its melting behavior has been investigated.

Molybdenum powder has been plasma sprayed on stainless steel, brass and aluminum substrates. The substrate melting phenomenon is observed and investigated by means of scanning electron microscopy (SEM) and scanning white light interferometery (SWLI). It is found that the flower-shape splat morphology is typical for molybdenum on all three substrate materials when the substrate is at room temperature. Notable substrate melting is manifested through the energy dispersion analysis of X-ray (EDAX) map and Robinson back-scattered image of cross-sections of splats. It has been shown that the substrate material plays an important role in substrate melting phenomenon. The lift angle of the petals of splats and the maximum crater depth have been characterized and compared. Both of these increase in the sequence, from stainless steel, brass to aluminum. A ‘volume of fluid’ (VOF) based model coupled with rapid solidification has been used to simulate splat deformation, solidification, substrate melting and resolidification. The numerical & analytical results agree quite well with the experimental data. A substrate melting mechanism is proposed based on the time scales of the droplet solidification and substrate melting to explain the formation of flower like splat morphologies.

This work concerns the production and use of aluminum oxide-NiCr dispersion powders for thermal spraying. It investigates the morphology and hardness of the powders and the microstructure and wear resistance of the resulting coatings. The powders are prepared by high energy ball milling and are used to produce oxide dispersion strengthened NiCr coatings via HVOF spraying. Among the key findings is that the powders are less homogeneous when milled with nanoscale aluminum oxide as are coating properties. Paper includes a German-language abstract.

Molybdenum splats were produced at three plasma conditions on steel substrates preheated to three temperatures. Morphology of splats and corresponding craters formed on substrates were observed; dimensions of splats and craters were measured with an optical non-contact interferometer. It is found that substrate is significantly melted and deformed upon impact of the droplet, which leads to the formation of flower like splats and craters. On average, only about 36 to 53 % of the areas covered by splats were in good metallurgical/mechanical contact with substrate. Normalized crater volume increases with droplet size and the contact is improved for the high particle energy/high substrate temperature condition as compared with low particle energy/medium substrate energy condition. Splat morphology and crater formation is explained based on impinging jet heat transfer model.

A model for oxidation of molybdenum particles during plasma spray deposition is developed. The diffusion of metal an-ions or oxygen cat-ions through a thin oxidized film, chemical reactions on the surface, and diffusion of oxidant in gas phase are considered as possible rate-controlling mechanisms with controlling parameters as the temperature of the particle surface, and local oxygen concentration and flow field surrounding the particle. The deposition of molten particle and its rapid solidification and deformation is treated using a Madejski-type model, in which the mechanical energy conservation equation is solved to determine the splat deformation and one-dimensional heat conduction equation with phase change is solved to predict the solidification and temperature evolution. Calculations are performed for a single molybdenum particle sprayed under the Sulzer Metco-9MB spraying conditions. Results show that the mechanism that controls the oxidation of this droplet is the diffusion of metal/oxygen ions through a very thin oxide film. A higher substrate temperature results in a larger rate of oxidation at the splat surface, and hence, a larger oxygen content in the coating layer. Compared to the oxidation of droplet during m-flight, the oxidation during deposition is not weak and can become dominant at high substrate temperatures.

A numerical model is developed to study the effects of the contact resistance, droplet impacting droplet temperature, and substrate temperature on the droplet solidification rate and temperature of the droplet under the condition when the substrate can melt and re-solidify. Two-dimensional simulations show that the interface velocity is small in the area of poor contact with an irregular solidification interface shape. During the impact of Molybdenum on a steel substrate, Mo solidifies while the steel substrate melts.

A high resolution numerical model has been developed to simulate the simultaneous spreading and solidification of single and multiple-splat on a cold substrate. The model combines the level set formulation with curvilinear adaptive finite volume scheme to predict the deforming shape of the splat's free surface as well as the solidification interface shape and dynamics. An adaptive grid generation captures the solidification front and the level set formulation allows the free surface deformation caused by merging and separation. Numerical results on spreading, merging and solidification of a single splat and two splats are presented to demonstrate the capability of the scheme. It also shows that this model can be extended to predict porosity in thermal spray coatings.