Abstract
The demand for suspensions that are used in thermal spray processes is expanding from research labs using the lab-prepared suspensions toward actual coating production in different industrial sectors. Industrial applications dictate the reduced production time and effort, which may in turn justify the development of the market for ready-to-use commercial suspensions. To this end, some of the powder suppliers have already taken steps forward by introducing, to the market, suspensions of some of the most used materials, such as yttria-stabilized zirconia (YSZ), alumina, and titania. However, there is a need to compare the suspension characteristics over time and the resultant coatings when using these suspensions when compared with the freshly prepared homemade suspensions. In this work, such a comparison is done using YSZ suspensions of the sub-micron to a few micron powders. In addition, some changes in the suspensions’ formula were performed as a tool to vary the coatings’ microstructures in a more predictable way, without any variation of the spray parameters. The coatings were generated while using both radial and axial injection of the suspensions into Oerlikon-Metco 3MB and Mettech Axial III plasma spray torches, respectively. A clear effect of suspension viscosity on the coating microstructure was observed using the 3MB torch with a radial injection of suspension (i.e., cross flow atomization). However, the viscosity role was not dominant when using the Axial III torch with an axial feed injection system (i.e., coaxial flow atomization).
Highlights
The suspension plasma spray (SPS) process is considered as a technique that enables variations of thermally sprayed coatings e.g., from highly dense and vertically cracked (DVC) to porous, feathery, and columnar microstructures [1]
To satisfy the needs of the industry, the suppliers have offered to the market the ready-to-use suspensions of yttria stabilized zirconia-yttria-stabilized zirconia (YSZ) powders
While while the the surfactant may provide a better dispersion of particles in suspension and help with better stability, surfactant may provide a better dispersion of particles in suspension and help with better stability, it it can havea acontradictory contradictoryeffect effectand andfavor favorthe the formation formation of of large large agglomerates
Summary
The suspension plasma spray (SPS) process is considered as a technique that enables variations of thermally sprayed coatings e.g., from highly dense and vertically cracked (DVC) to porous, feathery, and columnar microstructures [1]. The size of the suspension droplets that were atomized by the plasma jet has a strong influence on the size of the in-flight particles in SPS process [9,10,11] It is known, based on the Stokes law [10], that the smaller particles suffering from low inertia can be distracted from the normal flow by the stagnating gas layer parallel to the substrate on the deposition point. In addition to the droplet size, a variety of morphologies of the in-flight suspension droplets during drying can be formed, such as solid or hollow spheres, as well as large inflated shapes that depend on their heating and drying history [13] Such intermediate shapes of the particles following the solvent evaporation that occur before deposition on the substrate shall affect the coating microstructures [14]. In this work, apart from particle size studies, the viscosity and surface tension of the suspensions are the main parameters of investigation
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