**5.12. Detonation gun spray technique**

substrate, whereas the doping elements were reactive elements (e.g., Hf, Y or Zr, Si) and metallic additions of Ag. These samples were then coated with Y-PSZ TBC through the EBPVD method. The performance of such TBC system was compared to a conventional TBC system consisting of a β-(Ni,Pt)Al-based bond coat. Thermal cycling tests were performed in air and spallation was observed during this test. It was noted that most of the Pt-rich γ–γ′ samples showed better adherence of the ceramic coating than that of the β-samples. Cross-sectional scanning electron microscopy was used to characterize the thickness and the composition of the oxide scales after cyclic oxidation test. It was proved that the doping elements have significant influence on the oxide scale formation, metal/oxide roughness, Al and Pt content under the oxide scale, and TBC adhesion. It was established that RE-doping can not improve the oxidation kinetics of Pt-rich γ–γ′ bond coat. Moreover, γ–γ′-based systems were superior to β-(Ni,Pt)Al bond coat with respect to ceramic top coat adherence and better oxide scale

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

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

Inconel alloys (IN738) have a wide range of applications in industries as high temperature structural materials. Further, different surface treatments and coatings have been developed

modifications after respective plasma processing exposures [40].

**5.11. Cathodic Plasma Electrolytic Deposition (CPED) method**

adherence [38].

120 Advanced Ceramic Processing

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

property at high temperatures [39].

**5.10. Thermal plasma process**

Kim et al. [42] had taken a new approach and fabricated an excellent functionally-graded thermal barrier coating (FGM TBC) by using the detonation gun spray process in association with a newly-proposed shot-control method. FGM TBCs were sprayed in the form of multilayered coatings having a compositional gradient across the thickness. FGM TBCs consisted of a finely mixed microstructure of metals and ceramics with no interfaces between the layers. The gradient ranged from 100% NiCrAlY metal on the substrate to a 100% ZrO2–8 wt% Y2O3 ceramic for the topcoat. In the FGM layer of the FGM TBCs, the ceramics and metals maintained their individual properties without any phase transformation during the spraying process. They investigated the thermal shock properties of FGM TBCs and compared the data obtained with those for traditional duplex TBCs [42].

### **5.13. Plasma laser hybrid spraying technique**

Post-treatments of sprayed coatings and simultaneous spraying processes by a plasma laser hybrid technique have been tried by Chwa and Akira [43] to improve the lifetime of TBC coatings. An analytic technique using a low-viscosity resin with a fluorescent dye under a high vacuum has been investigated for the accurate observation of the microstructure of TBCs prepared by a post-laser treatment and a laser hybrid spraying process. Coatings formed by post-laser treatments and laser hybrid spraying processes showed significantly improved thermal shock resistance compared to as-sprayed coatings as a consequence of water quenching tests. The relationship of the microstructure of TBCs modified by laser treatment and thermal shock resistance has been evaluated by the careful observation of samples. They suggested the optimum process conditions for improving the thermal shock resistance of TBCs [43].
