Plasma sprayed composite coatings obtained by liquid injection of secondary phases

A secondary suspension injection system was designed, manufactured and tested, with the aim of depositing composite coatings formed by a conventional air plasma sprayed matrix embedding heat-sensitive phases sprayed and protected in a liquid suspension flow. The system is composed of two main sections: a pressurized vessel, equipped with regulation and recirculation sub-systems, and an adjustable nozzle holder. Preliminary experimental activities were performed with an alumina/graphite system and focused on the evaluation of the effect of the main SSIS parameters on the amount of retained lubricating phase in the deposits on deposition efficiency and coating microstructure. Alumina powders were fed in the plasma plume by a conventional injection system (argon carrier gas) whereas graphite powder was injected by SSIS from a water-based slurry. A CFD (Computational Fluid Dynamics) software, originally developed for the simulation of plasma spray processes, was implemented in order to confirm the possibility of predicting the effect of the variation of operating parameters on the amount of surviving heat-sensitive phases, potentially embedded in the ceramic matrix. Deposited composite coatings were characterized in terms of thickness, graphite content, phase distribution and tribological properties. (C) 2008 Elsevier B.V. All rights reserved