###### Abstract
Such varied microstructures are beneficial for a wide range of distinct applications, including Thermal Barrier Coatings (TBCs) in gas turbines and aero engines [16], electrolytes for solid oxide fuel cells [17], coatings for wear protection [18], biocompatible coatings for implants [19; 20], electron-emitting coatings [21], etc. Of course, as in the case of conventional powder-based coatings, the properties of the SPS coatings are also governed by a host of spray process and feedstock related parameters [22]. Another variant of such liquid feedstock plasma spraying involves the use of appropriate precursor salts in the form of acetates, nitrates, alkoxides, oxychlorides, etc., that can form particles of the coating material of interest in situ [15]. The role of suspension properties in governing the quality of SPS coatings has been found to be particularly significant [10]. Preparation of suspensions involves careful control of suspension characteristics such as solid load, solvent type, suspension stability, viscosity, etc. For instance, the suspension stability can be adjusted by the addition of a dispersant; however, the latter also influences the viscosity of the suspension, which should be minimum with shear-thinning behavior [10]. The challenges associated with preparing suspensions with all desired attributes mount when dealing with multi-constituent powder mixtures. Not surprisingly, therefore, not many studies on SPS with mixed suspensions, have been reported in the literature [18; 23; 24]. However, such an approach clearly represents a convenient pathway to realize fine-structured composite coatings. Motivation for Axial Plasma Spraying of Liquid FeedstockNotwithstanding the promise of liquid feedstock plasma spraying and considerable interest in this approach as reflected in the above-mentioned studies, its low throughput and the need to employ very short spray distances have for long remained significant hurdles in this route arousing industrial interest. The above-perceived shortcomings have been primarily a consequence of the fact that the use of suspensions and solution precursors demands higher thermal energy which conventional plasma spray torches involving the radial introduction of feedstock struggle to provide. The relatively recent development of plasma spray systems capable of axial injection is thus clearly a potential game-changer for liquid feedstock plasma spraying, involving the use of suspensions as well as solution precursors. An illustrative schematic depicting radial and axial feeding in plasma spray torches is shown in Figure 1. It should be mentioned that the particle trajectories depicted in the figure are only indicative and can vary substantially with particle size; viz., in case of radial feeding, the very fine particles are known to bounce off the viscous plasma plume without penetrating it at all while, depending upon the material being sprayed, particles coarser than a certain cut-off size go right through the plasma plume. In any event, it is abundantly clear from the schematic that axial feeding permits greater intimate contact between the liquid feedstock and the plasma plume, thereby facilitating considerably enhanced thermal energy transfer and far more effective utilization of the available plasma energy. This leads to wide-ranging benefits including increasing stand-off distances and enabling the desired microstructure control through proper selection of processing conditions [10; 15]. It also considerably assuages the above-mentioned concerns regarding low deposition rates/efficiencies associated with liquid feedstock using radial feeding. The improved thermal energy transfer from the plasma plume to the feedstock in the case of axial injection systems has also paved the way for deploying powder-suspension \"hybrid\" feedstocks to achieve unique coatings microstructures as demonstrated elsewhere by the authors' group [25; 26].