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

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 of the SPPS TBCs over APS TBCs [30].

#### **5.6. Sol-gel process**

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 of consolidating sol-gel TBCs by additional fillings of zirconia into the sinter-induced cracks was investigated by adjusting different process parameters such as the choice of either dipcoating or spray-coating and the modification of the slurry viscosity [34]. Basically, the optimization of both the sintering heat treatment and the procedure for filling the initial crack network promotes a significant improvement of the sol-gel TBC durability during cyclic oxidation at 1,100◦C. Typically, a sol-gel TBC that is properly sintered and adequately reinforced can be cycled for 1 h at 1,100◦C one thousand and five hundred times without spalling, which is nearly equivalent to the performance of EB-PVD TBCs [33, 34].
