Influence of Introducing an Organic Pore-Forming Agent on the Porosity and Microstructure of Alumina Coatings Produced by the Atmospheric Plasma Spray Process

Controlling the amount of porosity that can be added to ceramic coatings produced by means of atmospheric plasma spraying has always been a challenging task, especially, when coatings are produced with high adhesion/cohesion strengths combined with higher deposition efficiency. However, coating porosity can be varied in a limited fashion between 2 and 25% by changing the process parameters. The use of pore-forming agent is, therefore, an adequate alternative to add porosity by using the optimal spraying parameters. In this work, baking flour was vertically inserted into the plasma plume at different positions and with different feed rates while depositing alumina particles, as the basic feedstock powder, using optimized spray parameters. According to the spray conditions used in this work, flour particles have successfully contributed to the obtained coating porosity; however, the effectiveness is higher when flour is inserted at about half of the spray distance and depends on the feed rate of the flour. In comparison with flour-free alumina coatings, the porosity of flour-alumina's coatings increased by approx. 50 area% and 170 area% when the flour's weight fraction was equal 10 and 20 wt.% of the feedstock powder, respectively. Adhesion/cohesion strengths, hardness and thickness of coatings were strongly affected.