Suspension spraying tip: high molecular weight solvent Jan Cizek, Radek Musalek, Jan Medricky, Tomas Tesar, Frantisek Lukac, Tomas Chraska Institute of Plasma Physics of the Czech Academy of Sciences, Prague, Czech Republic cizek@ipp.cas.cz, 00420 266 052 096 Daniel Dukovsky Institute of Materials Science and Engineering, Brno University of Technology, Brno, Czech Republic 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. 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.%). Sub- micron 100% alpha-alumina powder is used as a model material to formulate the suspensions. 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. Importantly, the ether-based coatings exhibit slightly higher levels of α-Al2O3 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. Introduction Suspension plasma spray is a liquid feeding-based technique that allows deposition of sub-micron sized feedstock [1][2][3][4]. The coatings fabricated by this approach exhibit properties substantially different from those of the standard dry powders. A number of materials has been successfully suspension-sprayed to date [4], extending the range of potential applications. Owing to its suitable mechanical properties, Al2O3 is an inexpensive ceramic material conventionally used in wear- resistant applications [5][6]. Understandably, it became one of the most studied materials in the field of thermal spray deposition [7][8][9]. Despite all advantages, the Al2O3 coatings plasma sprayed from dry powders (of typical particle size from 30-150 μm) typically contain a dense network of interfaces, i.e., potentially weak regions that may have a detrimental influence on the overall performance of these protective coatings. In fact, spraying of dense Al2O3 coatings from powders is a challenging task; therefore, suspension spraying was attempted and ended up with a success [10][11][12]. Usually, ethanol was used as the solvent. However, two major problems are associated with using ethanol as a solvent. First, there are inherent safety risks in its handling and storage due to its flammability and relatively low flash point (Table 1). Second, in order to spray dense, non-columnar coatings necessary for many applications, diluted ethanol-based suspensions must be used, as it was shown that mechanically weaker columnar coatings form at higher solid load contents [10]. While the former factor can be partially suppressed by employing careful manipulation procedures, the latter factor may require longer spray times to compensate the significantly lower deposition rates of the diluted suspensions. Unfortunately, both arrangements generally trigger increased operation costs, thereby further limiting the use of such suspensions. In our study, an attempt to replace ethanol with another organic- based solvent [13] was made, with Al2O3 serving as the first case study material. Based on its well-proven use as solvent for ceramic inks for digital printing and the good suspension plasma spray results obtained by Canas et al. [14][15], di- propylene glycol methyl ether (Dowanol) was selected for the task. As opposed to ethanol, the safety risks are significantly reduced when using Dowanol; its higher molecular weight results, e.g., in a higher flash point, or lower vapor pressure (i.e., slower evaporation rate, Table 1), enabling safer and easier handling, storage and transport. Secondly, its price is about 80% lower than that of pure ethanol as Dowanol is free of additional taxation and administration (storage regulations, etc.). Thirdly, its evaporation heat (enthalpy of vaporization) is approximately 30% lower compared to ethanol. The lower vaporization enthalpy of Dowanol could potentially be exploited in spraying of higher solid load content suspensions, thereby effectively increasing the deposition rate (and decreasing the process costs) [14][15]. Considering its attractive properties, the three aims of the presented study are to investigate the possibility of using Dowanol as a solvent for Al2O3 suspensions to be used in plasma spraying of dense coatings, investigate the possibility of using higher solid load content in the suspension, and finally, to inquire into the changes in the associated coatings properties. Experimental To investigate the potential of using high molecular weight solvents for suspension spraying, a well-proven Al2O3 oxide ceramic was selected. The fine powder was obtained from Allied High Tech Products (USA). Its average particle size measured by laser diffraction (Mastersizer 3000, Malvern, UK) was approximately D50 ≈ 400 nm (Fig. 1). Thermal Spray 2021: Proceedings from the International Thermal Spray Conference May 24–28, 2021 F. Azarmi, X. Chen, J. Cizek, C. Cojocaru, B. Jodoin, H. Koivuluoto, Y. Lau, R. Fernandez, O. Ozdemir, H. Salami Jazi, and F. Toma, editors DOI: 10.31399/asm.cp.itsc2021p0548 Copyright © 2021 ASM International® All rights reserved. www.asminternational.org 548 Downloaded from http://dl.asminternational.org/itsc/proceedings-pdf/ITSC 2021/83881/548/487968/itsc2021p0548.pdf by guest on 23 September 2021