*3.3. Microhardness*

Functional coatings need sufficient mechanical properties and desired microstructures, while among the mechanical properties, the microhardness plays an important role on the long-term stability. Therefore, the influence of modifying the solidification pathway on the microhardness of TiO2 coatings was reviewed.

The value of *HV*100 is 1148.27 for the TiO2-12-0 coating. When adopting the modification, *HV*100 of the TiO2-12-30 coating changes to 1137.8. This implies that there is no different discrepancy to the microhardness. With the modification, distilled water with a flow rate of 30 mL/min is radially injected into the plasma jet. The evaporation of water decreases the temperature of the plasma jet. However, the solution injector is 15.0 mm away from the gun outlet, which is located downstream of the powder injector, and thus the optimized injection does not violently affect the molten status of TiO2 particles except those in the solidification pathway. Therefore, it does not cause a significant change to the microhardness.

However, the *HV*100 value slightly decreases to 1064.04 in the TiO2-20-30 coating. The reason for this can be ascribed to the molten status of TiO2 particles. As the speed of the feed disc increases from 1.2 to 2.0 rpm, more TiO2 particles are injected into the plasma jet core. The temperature of the plasma jet already becomes lower with the modification of the solidification pathway, compared with TiO2-12-0. Then, the heat energy shared by a single TiO2 particle further decreases as more TiO2 particles exist in the plasma jet, and some particles impact the substrate as un-melted or partially melted particles. As clarified in Figure 3(c2), this results in a rough and porous cross-section. Consequently, it slightly reduces the microhardness.
