Modeling of Combustion as well as Heat, Mass, and Momentum Transfer During Thermal Spraying by HVOF and HVSFS

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 high-velocity 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.