Abstract
Thermal spraying deploying liquid feedstock offers an exciting opportunity to obtain coatings with characteristics vastly different from those produced using conventional spray-grade powders. The most extensively investigated variant of this technique is Suspension Plasma Spraying (SPS), which utilizes a suspension of fine powders in an appropriate medium. The relatively recent advent of axial feed capable plasma spray systems can enable higher throughputs during SPS, provides the possibility for spraying with longer stand-off distances, and also permit the use of suspensions with higher solid loading. The present work investigates axial plasma sprayed coatings produced using a mixed suspension of fine (submicron or nano-sized) powders of Al2O3 and YSZ as a case study. Deposition of the mixed suspension using axial injection plasma spraying, comprehensive evaluation of characteristics of the resulting coatings, and assessment of their tribological behavior were of particular interest. Evaluation of surface morphology, microstructure, and hardness of the coatings reveals that axial SPS of mixed suspensions provides an exciting pathway to realize finely structured multi-constituent coatings using suspensions with as high as 40 wt.% solid loading. The study of scratch, dry sliding wear, and erosion behavior also specifically shows that the addition of YSZ in the Al2O3 matrix can improve the tribological properties of the coating.
Highlights
Thermal spray processes have been widely used for deposition of coatings for various industrial applications, such as aerospace, pulp, and paper, machinery, petrochemical, biomedical, etc. [1].Ceramic coatings are usually deposited by atmospheric plasma spraying (APS) from powder feedstock, with the powder particle size typically varying from 20 to 50 μm, resulting in coarse-structured coatings with typical splat sizes being several tens of microns
Relatively finer arising from the deposition of sub-micron sub-micron sized suspension areThe evident from thesplats surface micrograph
The micrograph clearly sized suspension are distributed evident from shows shows homogeneously greythe
Summary
Thermal spray processes have been widely used for deposition of coatings for various industrial applications, such as aerospace, pulp, and paper, machinery, petrochemical, biomedical, etc. [1].Ceramic coatings are usually deposited by atmospheric plasma spraying (APS) from powder feedstock, with the powder particle size typically varying from 20 to 50 μm, resulting in coarse-structured coatings with typical splat sizes being several tens of microns. Sci. 2020, 10, 5140 spraying of fine (submicron and nano-sized) particles. Use of such fine-sized feedstock poses significant challenges, with the difficulty in injecting nano- and submicron-sized particles into the thermal spray plume/flame being the most dominant [4]. Another major issue related to the injection of fine powders is their tendency to agglomerate, which can clog the delivery line from the powder feeder or choke the plasma torch nozzle itself, thereby hindering smooth and reliable powder flow and producing non-uniform and inferior quality coatings. Attempts to agglomerate the nano-/submicron-sized particles have been made, inhomogeneous melting characteristics of the agglomerated particles in the plume have often been noted [5,9]
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