## 3 Results
### Tribological Behavior
#### 3.5.3 Erosion Tests  
The potential applicability of mixed suspension coatings for erosion resistance applications is exciting because of the inherent design of the coating microstructure. The erosion rate of thermally sprayed coatings, in general, is influenced by many factors such as porosity, the lamellar interface bonding ratio, splat boundaries, relative hardness, etc. [43; 44; 45]. Although the suspension-based coatings are known for their plentiful splat boundaries, the higher inter-splat bonding (as reflected in the scratch testing results) promises to yield better resistance to the impacting erodent compared to conventional micron-sized powder sprayed counterparts. As observed in Figures 4 and 5, the distributed presence of sub-micron and nano-sized tetragonal-ZrO\({}_{2}\) within the Al\({}_{2}\)O\({}_{3}\) matrix is expected to toughen the microstructure, which in turn bears the promise to improve erosion performance. The ratio of erosion rates at impact angles of 90\({}^{\circ}\) and 30\({}^{\circ}\), E90/E30, is an indicator of the nature of erosion, with values less than 0.5 being suggestive of ductile behavior and values greater than 1 implying brittle behavior [46; 47]. Compared to pure Al\({}_{2}\)O\({}_{3}\) coatings which are known to exhibit E90/E30 ratio greater than 2 [22], the E90/E30 values for the SPS Al\({}_{2}\)O\({}_{3}\)-YSZ coatings were found to be in the range of 1.2-1.4. This indicates the lower degree of brittle deformation in the coating by virtue of the reinforcement provided by YSZ. The lower material removal intensity at 30\({}^{\circ}\) and greater intensity of damage at 90\({}^{\circ}\) can be seen from Figure 12; the unit of erosion wear rate is g per g erodent. It has been reported in the literature that hard and brittle coatings perform better at low impact angles and maximum wear rate is reached at a 90\({}^{\circ}\) angle [48]. The slightly higher relative hardness of the coating (9 GPa, refer Section 3.4) compared to the erodent silica particles (8 GPa) can also be a determining factor and could have contributed to its reduced impairment due to erosion damage. Notwithstanding the prevailing damage mechanisms that require a more elaborate investigation, the above studies do provide ample evidence that the composite Al\({}_{2}\)O\({}_{3}\)-YSZ coating can provide better erosion resistance than pure alumina ceramic coatings by virtue of the reinforcement provided by the distributed YSZ in the Al\({}_{2}\)O\({}_{3}\) matrix. The investigation of eroded surfaces (after 30 min of erosion) can provide insights into the erosion mechanism. The damage caused to the individual splats can be seen at a 30\({}^{\circ}\) impact (Figure 13a,b), resulting in lower material removal compared to complete removal of splats at a 90\({}^{\circ}\) impact (Figure 13c,d). Figure 12: The erosion wear rate of SPS Al\({}_{2}\)O\({}_{3}\)-YSZ coating (g:g\({}^{-1}\) of erodent). In summary, this study has demonstrated the ease of mixing two independent suspensions (of Al\({}_{2}\)O\({}_{3}\) and YSZ, in this case) to conveniently realize \"composite\" coatings. It was observed that, by virtue of employing axial feeding of the suspension feedstock into the plasma plume, stand-off distances significantly greater than those reported earlier in case of SPS Al\({}_{2}\)O\({}_{3}\)-YSZ coatings deposited via radial feeding are possible, i.e., 100 mm in the present work compared to 30 mm for radial feeding previously employed for such coatings [18]. Moreover, due to improved thermal energy transport possible in case of axial feeding compared to radial feeding, a much higher feed rate could be deployed in this study (3.4 kg:h\({}^{-1}\) of a 40 wt.% solid load suspension, corresponding to 1.36 kg:h\({}^{-1}\) of Al\({}_{2}\)O\({}_{3}\)-YSZ compared to 2.9 kg:h\({}^{-1}\) of a 25 wt.% solid load suspension which represents less than 0.75 kg:h\({}^{-1}\) of injected powder [18]).