Suspension Spraying Tip: High Molecular Weight Solvent

Abstract

In suspension spraying, two most frequently used solvents are water and ethanol. In this study, we test the potential of using alternative, high molecular weight solvent and demonstrate the associated advantages. For that, two organic solvents are directly compared: ethanol (serving as a benchmark, suspension formulated at 10 wt.% solid load) and di-propylene glycol methyl ether (two suspensions at 10 wt.% and 20 wt.% solid load). As a model material, $\hbox {Al}_{2}\hbox {O}_3$ is selected, a frequently sprayed ceramics employed in many industrial sectors. Sub-micron 100% alpha-alumina powder is used to formulate the suspensions. Identical spray conditions are then used to deposit the coatings using hybrid water-stabilized plasma torch. Shadowgraphy monitoring of the suspension fragmentation as well as in situ measurement of the particle in-flight properties is employed, showing no significant differences between the three series. Further, it is shown that the ethanol- and ether-based-feedstock coatings are fully comparable in terms of their microstructure, porosity content, surface roughness as well as hardness and adhesion to the substrates. Importantly, the ether-based coatings exhibit slightly higher levels of $\alpha$-$\hbox {Al}_{2}\hbox {O}_3$ phase when compared to their ethanol-based counterpart (17 wt.% vs. 6 wt.%). The use of 20 wt.% solid load in the ether solvent leads to twofold increase in the deposition rate while (as opposed to ethanol) successfully retaining a dense microstructure. Lastly, the ether is significantly cheaper and safer to handle than ethanol.