The peculiarities of plasma treatment of agglomerated YSZ powders as well as mechanically pretreated metal and metal alloy powder with the aim of their spheroidization and subsequent thermal spraying of produced HOSP powders are analyzed. Formation of splats from hollow droplets deserves a special attention for thermal spraying. In this case, immediately prior to particle – substrate collision, we have a droplet that consists of a liquid shell enclosing a gas cavity heated to a temperature close to the particle surface temperature. The paper presented includes the results of computer simulation and model experiments carried out under full control of pre-impact key physical parameters (KPPs): temperature T p 0 , velocity u p 0 , external diameter of droplet D p 0 , thickness of its shell Δ p 0 , temperature of substrate T b0 , and its surface state. It was shown that formation of splats from hollow droplets proceeds in a manner more stable compared to the case of “dense” molten particles obtained from fused and crushed compacts, and this provides for a more perfect splat-substrate, splat-splat and coating-substrate interface. Advantages of hollow metal, metal alloy and metal oxide powders over corresponding dense powders for thermal spraying by the example of thermal barrier coatings are discussed. Since the splat formation process involving hollow droplets proceeds, following their high-velocity impact deformation on the base/substrate, in an explosive manner, this type of spraying can be referred as HOSP micro explosive thermal spraying.

In waste-to-energy power generation plants, increasing the combustion temperature improves plant efficiency. However, due to problems caused by molten chloride corrosion, the combustion temperature cannot be increased. Consequently, in order to increase the temperature, it is first necessary to protect components from molten chloride corrosion. In this study, CoNiCrAlY coating is proposed as a protective film against molten chloride corrosion. Three kinds of specimens were prepared. One was standard coating made from conventional CoNiCrAlY. The others included the addition of Mo to the CoNiCrAlY by two different techniques. One technique is mechanical alloying (MA), and the other is a gas-atomizing technique. The mechanic-chemical reaction that occurs during the mechanical alloying process can be expected to create new functionality for the material. The effect of Mo content was evaluated for corrosion resistance. These specimens were coated by low pressure plasma spraying (LPPS). The specimens were exposed to NaCl-KCl for the molten chloride corrosion test. The results of the corrosion tests show that corrosion resistance improved in only MA CoNiCrAlY coatings. These results reveal that mechanically alloyed CoNiCrAlY-Mo coating has excellent corrosion resistance, and its corrosion resistance behavior is different from that of gas-atomized CoNiCrAlY-Mo.

A numerical method and software were developed to predict non-stationary conjugated conductive heat transfer, melting and possible evaporation of materials under high energy fluxes impinging onto solid body surface (plasma jet, arc spot, laser or electron beam), and also subsequent cooling and solidification of the melted substrate layer. In the numerical procedure, the finite-element method was employed. The processes of interest can have different characteristic time and spatial scales, which in addition can suffer drastic changes at heat flux densities q ?[108 ;1014 ] W/m2. An advanced procedure was developed to enable dynamic adaptive triangulation of domains involved in the current numerical solution and characterizing the different phase states (liquid or solid) of the materials. This procedure, belonging to the class of the frontal algorithms, allows one to break a solution domain into triangles based only on the domain boundaries. The model applications of the developed simulation software are illustrated.

Within the equilibrium approach, thermophysical foundations were developed which allow one to predict the dynamics of simultaneous deformation and solidification of a metal – oxide droplet impinging on a substrate, with due allowance for conjugated heat transfer under conditions of an ideal contact between the solidifying layer and the base surface. A representative collection of the alumina and YSZ splats was obtained under full control of key physical parameters (KPPs): temperature – Tp0, velocity – up0, and size – Dp, of particle, and temperature –Tb0, of a polished substrate. Without introducing any empirical coefficient, it was obtained quite a satisfactory agreement between the predicted and measured thickness and diameter of splats. In the present paper a comparison of a new solution with known dependences of different authors is carried out. Multiparameter calculations of the YSZ splats characteristics (diameter of splats, rate of cooling and solidification of melt, density of heat flux into substrate, etc.) in a wide range of the KPPs under molten droplets impacted with substrates were carried out.

Formation of splats from hollow YSZ droplets deserves a special attention for thermal spraying. In this case, immediately prior to particle – substrate collision, we have a drop that consists of a liquid shell enclosing a gas cavity heated to a temperature close to the melt temperature. As the drop impinges onto the substrate, it suffers considerable deformation resulting in a rise of the gas pressure and in a decrease of gas temperature. The paper presented includes the results of the model experiments carried out under full control of the key physical parameters (KPPs) - temperature, velocity and size of droplet, and temperature of polished substrate, in which the starting powder was a specially prepared powder consisting exclusively of the hollow spherical YSZ particles. The main goal of the paper is to discuss and study the possible splat formation scenarios under above-mentioned conditions.

Numerical simulation of a plasma spraying process from ceramic particle injection to a coating formation was conducted by integrating particle-laden plasma flow, ceramic splat formation and coating formation models. Velocity and temperature of both plasma flow and ceramic particles under an applied RF electromagnetic field were clarified by using the first model. Radial distributions of particle impact location, velocity and temperature were obtained based on both an unsteady effect of a plasma flow and distributions of particle size and injection velocity. Nextly, splat thickness and diameter of melted ceramic particles after impact on a substrate were clarified by using the particle impact velocity and temperature. Radial distributions of splat thickness and diameter became more uniform. Finally, coating thickness distributions were evaluated by using the last model. They were strongly influenced by particle size and injection velocity distributions.

The present study is conducted to clarify the magnetic control characteristics of a particle-laden plasma jet impinging on a substrate for the improvement of a low pressure plasma spraying process and its controllable optimization. The plasma jet is described by Eulerian approach and each injected particle is described by Lagrangian approach respectively taking into account the compressible effect, variable transport properties and plasma-particle interactions, coupled with the Maxwell's equations. The effects of the location of the applied radio-frequency electromagnetic field, and of the injected particle size on the particle trajectory, particle velocity and its phase change are clarified by numerical simulation. It is concluded that the particle trajectory is influenced effectively and the injected particle temperature can be controlled strongly by applying the radio-frequency electromagnetic field to the nozzle. The reasonable agreement of particle velocity between calculation and experiment is observed.