###### 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 Al\({}_{2}\)O\({}_{3}\) 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 Al\({}_{2}\)O\({}_{3}\) matrix can improve the tribological properties of the coating. S 2020 10 5140 10.3390/app10155140 22 July 2  
spraying of fine (submicron and nano-sized) particles. However, 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. The fine particles also typically possess poor flowability and, because of their inherently low momentum, they cannot easily penetrate the high-velocity gas streams associated with thermal spray techniques, thereby compromising controlled powder feeding [7]. The use of high carrier gas flow rates are required for enabling such particles to penetrate the plasma plume, and this can detrimentally perturb the plasma jet [10]. Spraying sub-micron or even finer powders can also present a health concern at the workplace. Although 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]. To overcome the above challenges associated with the production of finely structured coatings, the submicron/nano-sized powder can be suspended in an aqueous or organic solvent and such a suspension used as the feedstock for thermal spraying. Thus, the process merely requires a stable suspension of the powder material to be coated, prepared in a suitable aqueous or organic solvent. Plasma spraying has been the spray technique of choice for a vast majority of suspension-based coating efforts, due to the availability of a high-energy flux that is needed for the added step of eliminating the solvent in case of a suspension feedstock. The above approach forms the basis for the increasingly interesting Suspension Plasma Spray (SPS) technique, which has already been shown to be capable of producing coatings with diverse microstructural features ranging from extremely porous to very dense, columnar, vertically cracked, etc. [11; 12; 13; 14; 15]. Such unique and novel microstructures are not usually obtained through conventional thermal spraying using powder feedstock.