E. Dongmo, GSaME Universität Stuttgart, Stuttgart, Germany; R. Gadow, M. Wenzelburger, A. Killinger, Universität Stuttgart, Stuttgart, Germany
The interest in submicron and nano-structured layers applied by thermal spray technologies on different surfaces has been significantly increased during the last decade. Conventional HVOF spraying processes are not suitable to achieve submicron and nano-particles. Therefore, High Velocity Suspension Flame Spraying (HVSFS) has been developed for the processing of nano structured spray material to achieve dense surface layers in supersonic mode with a refined micro- or nano-structure, from which superior mechanical and physical properties are expected. However, the chemical and thermodynamic phenomena occurring in the HVSFS reacting flow field are a challenging, multidisciplinary issue. This study is intended to analyze and understand the HVSFS combustion and flow dynamic system on the basis of a CFD model and numerical calculation. Simulation of these phenomena can yield the mass fractions and densities of combustion reactants and products as well as temperature, pressure and particle size and velocity, depending on the position in the combustion chamber and expansion nozzle. In this way, modeling and simulation is suitable to analyze the relevant set of parameters describing the complete spraying process, i. e. the energy balance being influenced by the different combustion mechanisms and the particle properties in the spray jet and during impact on the surface. The final aim is an optimization of the process parameters by variations during simulation experiments. The model considers the following phenomena: combustion of the fuel gas (premixed oxygen/propane), heat, momentum and mass transfer between the flame and the suspension droplets (organic solvent and particles), as well as the injection, vaporization and combustion of the suspension organic solvent.
Summary: The focus of research on nanostructured materials is now shifting from powder synthesis to processing of nanostructured coatings using supersonic thermal spray processes. However, the conventional HVOF thermal spray system is only suitable for the processing of micro-scale powders and has distinct limitations in powder size due to powder supply and safety reasons. This means that some adaptations of the HVOF process are needed for the processing of nano-scale powders.
High velocity suspension flame spraying, HVSFS, uses a solution as carrier fluid (ethanol or isopropanol) to process nano-scale materials. The suspension is injected directly into the combustion chamber. This injection process shows some difficulties compared to conventional powder feeding in terms of pressure distribution and also the possibility of composition variations and in-situ change of the composition. Therefore, a numerical analysis and optimization of the injection behavior of suspensions is helpful.
Using a solution as carrier fluid for nano materials processing in thermal spray systems features some thermophysical and thermochemical changes of the new HVSFS process compared to the standard HVOF process. These include the existence of a third phase (liquid), its evaporation and combustion including a cooling effect in the spray torch, and also the resulting particle morphologies (agglomeration) are different from conventional HVOF processes.
