## 3 Results
### Tribological Behavior
#### 3.5.2 Ball-on-Disc Wear Tests  
Figure 10 shows the measured friction coefficient \(\mu\) during dry sliding for two different loads. Three distinct regions are identified in the plotted graph. The first and third regions are called running-in and stabilization regimes. In the running-in regime, the friction coefficient increases rapidly because of particle generation between the mating surfaces. Thereafter, lowering of friction coefficient could be due to filling up of pores by the wear debris at a very fine scale before the system reaches a steady-state, known as a stabilization regime. The estimation of the friction coefficient value corresponds to the stabilization regime. It can be seen from the figure that the friction coefficient of the coating was \(\sim\)0.45, regardless of the applied load. A comparison with the friction coefficient of pure Al\({}_{2}\)O\({}_{3}\) suspension based coating \(\sim\)0.61, previously reported by the authors [32], reveals the lowering of friction coefficient with the addition of YSZ. In the case of APS Al\({}_{2}\)O\({}_{3}\) coating, the addition of nano-sized YSZ has been to reduce the friction coefficient of the coating [30]. Apart from the use of YSZ material, the addition of TiO\({}_{2}\) has also been shown to reduce the friction coefficient of Al\({}_{2}\)O\({}_{3}\) suspension based coating [41].  
The wear rates for the two conditions tested are listed in Table 3 and the wear tracks are visualized in Figure 11. The mean worn area used for calculation of wear rate is the area of the cross-sectional profile of the wear track, below the original unaffected coating surface. The stated depth is the average of the maximum depth of the wear track measured at two diametrically opposite locations of the track. The wear rate of SPS Al\({}_{2}\)O\({}_{3}\) previously reported by the authors [32] was significantly higher than that of the composite Al\({}_{2}\)O\({}_{3}\)-YSZ coating investigated in the present study (\(\sim\)\(4\times 10^{-5}\) mm\({}^{3}\) N\({}^{-1}\) m\({}^{-1}\) as compared to \(\sim\)\(1.45\times 10^{-5}\) mm\({}^{3}\) N\({}^{-1}\) m\({}^{-1}\) at 10 N load). The presence of the metastable tetragonal-ZrO\({}_{2}\) phase is known for its enhanced wear resistance characteristics due to higher fracture toughness [42]. The above results reflect the improvement in wear behavior of Al\({}_{2}\)O\({}_{3}\) by the addition of YSZ, both in terms of friction coefficient and wear loss. It is also pertinent to point out that the previous study had already shown the refined microstructure enabled by SPS Al\({}_{2}\)O\({}_{3}\) to yield much better properties than the usual plasma-sprayed Al\({}_{2}\)O\({}_{3}\) coatings deposited using conventional spray grade powder. The above result provides ample added incentive for conveniently utilizing mixed suspensions to achieve further property enhancement.  
Figure 9: Acoustic emission signal from the SPS Al\({}_{2}\)O\({}_{3}\)-YSZ coating during the scratch test.  
Figure 11: Optical profilometry images of wear tracks resulting from ball-on-disc tests carried out at two different loads of 10 N and 20 N, along with corresponding cross-sectional profiles measured along the black dashed lines.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Load** & **\textless{}Depth\textgreater{}** & **\textless{}Area\textgreater{}** & **Wear Rate** \\
**(N)** & **(\(\mu\)m)** & **(\(\mu\)m\({}^{2}\))** & **(10\({}^{-5}\) mm\({}^{3}\) N\({}^{-1}\) m\({}^{-1}\))** \\ \hline
10 & 3.5 & 1450 & 1.45 \\
20 & 5 & 3300 & 1.65 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Depth, worn cross-section area, and wear rate of the coatings.  
Figure 10: Friction coefficient evolution with sliding distance for different applied loads during ball-on-disc wear testing of SPS Al\({}_{2}\)O\({}_{3}\)-YSZ coating.