Suspension spraying tip: high molecular weight solvent

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, Al₂O₃ 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 are employed, showing no significant differences between the three series. Further, it is shown that the ethanol- and ether-based 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-Al₂O₃ phase when compared to their ethanol-based counterpart (17 wt.% vs. 6 wt.%). The use of 20 wt.% solid load in the ether coating 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.