Porous mullite and mullite–ZrO2 granules for thermal spraying applications

Feedstock powders employed in thermal spraying applications must fulfill certain conditions in terms of particle size, morphology and homogeneity for a continuous and reliable spraying. In this work, two powder processing routes, Spray Drying (SD) and Freeze Granulation (FG), were implemented for single mullite and the equal by volume mullite/YSZ mixture. The particle size, morphology and microstructure of the agglomerated particles achieved by these two methods were studied. Free-flowing and homogeneous powders were obtained with both techniques although FG powders presented a more spherical shape and higher porosity. To evaluate the influence of the powders characteristics on the flame spraying process, both types of powders were flame sprayed under similar conditions monitoring their in-flight characteristics. The temperature reached by the particles depended mainly on their composition. Coatings were deposited on metallic substrates obtaining thicker and denser coatings with SD powders and bimodal structured coatings with FG powders.