**5.9. Low-pressure plasma spraying process**

The TBC must exhibit high thickness (100–300 μm), vertical cracks should be present in the TBC in order to be a strain tolerant layer, and it must have high porosity to decrease the thermal conductivity. Rousseau et al. [39] prepared a Y-PSZ layer using low-pressure plasma spraying technique by introducing a solution of nitrate salt into a low-pressure plasma discharge. The characteristics and stability of the Y-PSZ layers were analyzed by several techniques. Optical emission spectroscopy indicated that the oxidant chemistry of the plasma caused oxide formation and the nitrate elimination at low temperature (T<300°C). Effects of the several parameters such as power of the plasma discharge, post-treatment and heat treatment on structure, morphology, and stability of the Y-PSZ coatings was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), water porosimetry, and thermal diffusivity measurement. It was observed that Y-PSZ coating (porosity-50%) had good thermal barrier property at high temperatures [39].

### **5.10. Thermal plasma process**

Superior properties such as high-melting point, high phase stability, low sintering ability, low thermal conductivity, and low oxygen permeability of lanthanum zirconate (LZ) have made it one of the most promising TBC materials for high-temperature applications. However, the production methods used to synthesize lanthanum zirconate are highly time-consuming and the powder is not commercially available. Hence, the thermal plasma process was utilized to synthesize, spheroidize, and spray deposits of lanthanum zirconate material by Ramachan‐ dran et al. [40]. They demonstrated the effectiveness of thermal plasma as a major materials processing technique. Suitable characterization techniques were used to study the material modifications after respective plasma processing exposures [40].
