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In thermal spray technologies and coating industries, increasing research and development efforts have been made in recent years toward submicron and nanostructured layers on different materials and components, promising large potentials in functional and structural coating properties. These potentials have been encouraging researchers to aim for an improved understanding and optimization of the highvelocity oxide fuel (HVOF) system to be able to improve process control, and thus, control coating properties and enable better applications for submicron and nanostructured coatings. Moreover, on the experimental side, new thermal spray technologies have been developed in order to process nanopowders, i.e., mainly suspension-based technologies like suspension plasma spraying or high-velocity suspension flame spraying (HVSFS). HVSFS is a suitable processing method for submicron and nanoscaled particles to achieve dense surface layers in supersonic mode with a refined final structure, which is the prerequisite for improved or even superior mechanical and physical properties. However, theoretical understanding of the chemical and thermodynamic phenomena occurring in the HVOF and HVSFS reacting flow field, which is necessary for process modeling, is a challenging, multidisciplinary issue. In this study, the combustion processes as well as the heat-, mass-, and momentum interactions between the flame, the suspension droplets (including vaporization), and the solid spray particles are analyzed, taking into account both the HVOF and HVSFS spray processes. The processes are modeled and numerical simulation experiments are described. Thereby, the models are giving a detailed description of the relevant set of parameters describing the complete spraying process in the combustion chamber and expansion nozzle, respectively. Simulation results can be applied for improved process control as well as torch design, e.g., adaptation of combustion chamber design to the trajectories and dwell time of spray particles for heat transfer optimization.

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The aim of this work is to develop an analytical methodology for the analysis and prediction of high-velocity suspension flame spraying (HVSFS) under various operating conditions, to determine the effect of individual parameters on the process, and to aid and promote the design of HVSFS torches. A key aspect of the work is the development of a model that accounts for fuel gas combustion, evaporation of organic solvents, and heat, momentum, and mass transfer between the flame and suspension droplets.

High-velocity suspension flame spraying (HVSFS) is used for direct processing of submicron and nano-scaled particles to achieve dense surface layers in supersonic mode with a refined structure, from which superior properties are expected. The application of solutions as a carrier fluid for nano-particles in thermal spray systems is a new approach that requires some thermo-physical and chemical optimization. Three dimensional modeling and analysis of the combustion and gas dynamic phenomena of the three-phase HVSFS process is performed in this study for an industrial TopGun-G torch, based on a numerical model for a conventional HVOF process. Parameter analysis of the solution mixture (proportion between aqueous and organic solvent) in a suspension is performed as well as analysis of the variation of the combustion chamber depending on the torch design, leading to more homogeneous flow properties for an improved HVSFS torch.