**5.7. Composite sol-gel method**

decomposition and/or chemical reactions in the vapor phase near the heated substrate. The coatings were characterized by scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. Vyas and Choy [28] produced thick and uniform YSZ films using the ESAVD method. Raman spectroscopy identified carbon to be present in the as-deposited coatings. When heat treatment of the YSZ coating was conducted at 1,000°C for 2 h, carbon was removed

In this process, an aqueous chemical precursor feedstock is injected into the plasma jet where the droplets undergo a series of physical and chemical reactions and then deposited on the substrate as coating. Microstructural observations of this type of TBC show fine splats and vertical cracks in a porous matrix. TBCs deposited by the optimized solution-precursor plasma spray (SPPS) process exhibit superior durability relative to TBCs formed by the APS and EB-PVD processes. Thick and durable TBCs can be deposited by this process. Failure of these TBCs occurs by large scale buckling of the ceramic top coat [29]. The efficiency of TBCs used to protect and insulate metal components in engines increases with the thickness of the TBCs. However, the durability of thick TBCs deposited using conventional deposition methods has not been adequate. Jadhav et al. [30] deposited highly durable, 4 mm-thick ZrO2–7 wt% Y2O3 (7YSZ) TBCs on bond-coated superalloy substrates using the SPPS method. The average thermal cycling life of the SPPS TBCs was 820 cycles, while most of the conventional air plasma-sprayed coatings of the same composition and thickness deposited on similar bond-coated superalloy substrates were observed to be detached partially from the substrates in the as-sprayed condition. Only the APS TBC failed after 40 thermal cycles. Significantly higher in-plane indentation fracture toughness and high degree of strain tolerance due to the presence of the vertical cracks in the SPPS TBCs led to the dramatic improvement in the thermal cycling life

Recently, a new, attractive sol-gel route has been successfully developed to synthesize and deposit the TBCs [31–34]. Non-directional deposition and formation of thin or thick coating by dip or spray technique or the combined method of both techniques can be performed by this technique. Sol-gel TBCs show an isotropic microstructure having randomly distributed porosities leading to a good compromise between thermal conductivity and mechanical strength. The degradation of sol-gel TBCs is initiated by the formation of a regular crack network either during the post-deposition thermal treatment required to sinter the deposit or during the first cycles of oxidation. In both cases, this regular surface crack network forms on account of the in-plane stress release due to the sinter-induced shrinkage of the zirconia scale. Subsequently, enlargement and coalescence of the cracks occur under cumulative oxidation cycles promoting the detachment of individual TBC layers and finally, the complete spallation of the TBC. To improve the cyclic oxidation resistance of the TBCs, the sintering efficiency after the TBC deposition needs to be improved or the crack network needs to be stabilized by filling crack grooves by supplementary dip or spray coating passes [33]. In addition, the feasibility

and the adhesion of the TBC coating to the bond coat improved [28].

**5.5. Solution-Precursor Plasma Spray (SPPS) process**

of the SPPS TBCs over APS TBCs [30].

**5.6. Sol-gel process**

118 Advanced Ceramic Processing

Composite sol-gel method and pressure filtration microwave sintering (PFMS) technologies were utilized to form novel YSZ (ZrO2–6 wt% Y2O3)–(Al2O3/YAG) (alumina–yttrium alumi‐ num garnet, Y3Al5O12) double-layer ceramic coatings. The thin Al2O3/YAG layer showed good adhesion with the substrate. Cyclic oxidation tests were carried out at 1,000°C, which indicated that double-layer ceramic coatings can prevent the oxidation of alloy and improve the spallation resistance. The 250 μm coating had better thermal barrier effect than that of the 150 μm coating during thermal stability tests at 1,000°C and 1,100°C at different cooling gas rates. These beneficial effects are mainly attributed to the decrease of the rate of TGO scale devel‐ opment and the reduced thermal stresses by means of nano/micro-composite structure. This double-layer coating can be considered as a promising TBC [35].
