The relevance of nitrogen-based, high-enthalpy plasmas for effective feedstock treatment in thermal spraying of suspensions

This work aims to contribute to our understanding of the importance of nitrogen in enhancing feedstock treatment in plasma spraying. Three different plasma gas compositions-a ternary argon-based composition, a ternary nitrogen-based composition, and a nitrogen-based composition without argon-were used with the Axial III™ plasma torch for suspension plasma spraying. The thermodynamic and transport properties of the plasma gas mixtures were calculated and compared. Based on these calculations, the Ability of Acceleration Factors (AAF) and the Ability of Heating Factors (AHF) were determined and correlated with the measured in-flight particle velocities and temperatures. Computational and experimental results indicate that transport properties, specifically thermal conductivity and viscosity, are key factors in thermal feedstock treatment. In this respect, a high nitrogen content is advantageous. Therefore, nitrogen can be considered a high-heat-transfer component of the plasma. Nitrogen also introduces a relatively high enthalpy. Enthalpy describes the amount of energy stored in the plasma gas that can be released. If the energy output is not enough to heat the feedstock adequately, especially when dealing with high feed rates of liquid feedstocks, the enthalpy of the plasma gas can become a limiting factor. In such cases, a high nitrogen content in the plasma gas is advantageous because the higher enthalpy provides more energy than an argon-based plasma. However, this requires a thermally resilient torch concept.