###### Abstract
Notwithstanding the above, there have been relatively few studies involving the use of suspensions in tandem with an axial feed capable plasma spray system. Several of such prior reports have originated from the authors' group, but have been primarily focused on TBCs [6; 14; 16]. In the present paper, axial SPS deposition of an Al\({}_{2}\)O\({}_{3}\)-YSZ mixed suspension has been used as a case study. In this context, it is relevant to mention that Zirconia Toughened Alumina (ZTA), an important manufacturing ceramic, has been extensively studied both as a structural material [27; 28; 29] and as a protective coating [23]. The combination of high hardness and Young's modulus of the Al\({}_{2}\)O\({}_{3}\) matrix, with the additional toughening effect provided by the dispersed ZrO\({}_{2}\), has been reported to lead to a significant increase in the flexural strength and fracture toughness [27; 28; 29]. Thus, from a potential application viewpoint too, the Al\({}_{2}\)O\({}_{3}\)-YSZ system was deemed to be an ideal demonstrative model system for the present study on plasma spraying with mixed suspensions. However, it must be emphasized that, while the Al\({}_{2}\)O\({}_{3}\)-YSZ deposition is merely an illustrative example, the versatility of the approach is in being able to easily mix suspensions of any two individual materials A and B that can be thermally sprayed, and pick the \"composite\" coating constituents A and B almost at will. Figure 1: Schematic of axial and radial feeding of feedstock. A previous study on APS Al\({}_{2}\)O\({}_{3}\)-ZrO\({}_{2}\) fine-structured composite coating comprising fine ZrO\({}_{2}\) dispersed in a coarse-structured Al\({}_{2}\)O\({}_{3}\) matrix has been shown to yield better wear resistance and lower friction coefficient compared to a conventional coarse-structured Al\({}_{2}\)O\({}_{3}\) coating [30]. An SPS Al\({}_{2}\)O\({}_{3}\)/ZrO\({}_{2}\) laminated composite coating produced by [PERSON] et al. [23] yielded a significant improvement in mechanical and thermal performance compared to Al\({}_{2}\)O\({}_{3}\) and ZrO\({}_{2}\) alone, which can be useful for a variety of applications demanding enhanced resistance against abrasion, wear, oxygen diffusion, elevated temperature exposure, etc. In the case of Al\({}_{2}\)O\({}_{3}\)-YSZ coatings produced by mixed suspensions, very few studies have evaluated the mechanical properties of the coating systems [18; 23; 31]. Most of these earlier studies have also involved radial injection of feedstock and consequently utilized solid loads of 5-20 wt.%, as compared to the much higher 40 wt.% solid loading employed in the present study to highlight the augmented throughput capability that can be realized with axial feeding of suspensions. This work aimed to showcase axial feed suspension plasma spraying as a versatile coating pathway representing one of the emerging and industrially relevant applications of plasma techniques, given its potential to realize diverse functional coatings with refined microstructures. The focus was on studying the processing of mixed suspensions using axial plasma spraying and comprehensively analyzing the resulting coatings. Individual suspensions of Al\({}_{2}\)O\({}_{3}\) and YSZ powders, each with 40 wt.% solid loading, were mixed in appropriate portions and sprayed using an axial-feed plasma torch and the obtained coatings were extensively characterized for surface morphology, microstructure, porosity, micro-hardness, scratch behavior, wear properties, and erosion behavior.