**6.2. Composite top coat**

A new TBC was developed by Dietrich et al. [68] from a powder mixture of metal and normal glass by using vacuum plasma spraying technique. This type of TBC material had a similar thermal expansion coefficient of a metal substrate. The thermal conductivity of this composite top coat was about two times greater than that of YSZ. Long thermal cycling life of the metalglass TBC was attributed to high thermal expansion coefficient, good adherence to the bond coat, and absence of open porosity and thereby, preventing the bond coat oxidation from corrosive gases [68]. Majumdar and Jana [69] studied the properties of a TBC prepared from 3 wt% YSZ dispersed in a high temperature resistant alumino-borosilicate glassy matrix. The YSZ-glass composite coating was applied on stainless steel substrate by a simple and costeffective enameling technique. The thermal gradient of 800 μm thick TBC was found to be 175– 180°C after 30 min exposure at 1,000°C. Significant improvement of the gradient to 650–675°C was observed after long exposure of the coated surface at 1,000°C when compressed air cooling was utilized [69]. The spallation of ceramic coating from the bond coat is an important problem for TBC systems. Basically, the spallation is caused by the oxidation and hot corrosion at the interface of the ceramic layer and bond coat. Keyvani et al. [70] investigated the oxidation and hot corrosion behavior of plasma sprayed nanostructured Al2O3/YSZ composite TBC coatings on Ni-based (IN-738LC) superalloy substrate and compared it with the conventional YSZ. The coatings were deposited by plasma spray method. High temperature oxidation test at 1,100°C and hot corrosion test at 1,050°C using Na2SO4 and V2O5 molten salts were conducted on the coatings. The experimental data demonstrated that the nanostructured Al2O3/YSZ composite coating had higher oxidation and hot corrosion resistance than those of the conventional YSZ coating. The microstructural analysis indicated that the growth of TGO was much less for this nanostructured Al2O3/YSZ composite coating. Further, the composite top coating prevented infiltration of both oxygen and aggressive molten salt [70]. Novel YSZ (6 wt% yttria partially stabilized zirconia)–(Al2O3/YAG) (alumina–yttrium aluminum garnet, Y3Al5O12) DLC coatings were formed by using the composite sol-gel and pressure filtration microwave sintering (PFMS) technologies by Ren et al. [71]. The microstructural observations showed that microsized YAG particles were embedded in nano-sized α-Al2O3 film. A thin Al2O3/YAG layer had good adherence with the substrate and the thick YSZ top layer. Cyclic oxidation tests at 1,000°C indicated that they can resist oxidation of alloy and improve the spallation resistance. The thermal insulation capability tests at 1,000°C and 1,100°C indicated that 250 μm coating had better thermal barrier effect than that of the 150 μm coating at different cooling gas rates. The decrease in oxidation rate for forming a TGO scale using the sealing effect of α-Al2O3 and the reduced thermal stresses by means of nano/micro composite structure led to these beneficial effects. This double-layer coating can be considered as a promising TBC [71].

#### **6.3. Glass-ceramics as TBC materials**

decreased. TECs of (Sm0.5La0.5)2Zr2O7 and (Sm0.5La0.5)2(Zr0.8Ce0.2)2O7 were lower than that of Sm2Zr2O7. The CeO2 addition resulted in the higher TEC of (Sm0.5La0.5)2(Zr0.8Ce0.2)2O7 than those of 8YSZ and (Sm0.5La0.5)2Zr2O7. Although the TEC of (Sm0.5La0.5)2Zr2O7 was lower than that of 8YSZ, still it can serve as a TBC. Doping with La2O3 or CeO2 led to phonon scattering resulting in much lower thermal conductivities of (Sm0.5La0.5)2Zr2O7 and (Sm0.5La0.5)2(Zr0.8Ce0.2)2O7 than that of Sm2Zr2O7. In comparison to the thermal conductivity of (Sm0.5La0.5)2Zr2O7 the thermal conductivity of (Sm0.5La0.5)2(Zr0.8Ce0.2)2O7 was relatively lower. The experimental results showed that (Sm0.5La0.5)2Zr2O7 and (Sm0.5La0.5)2(Zr0.8Ce0.2)2O7 are novel candidate materials for

A new TBC was developed by Dietrich et al. [68] from a powder mixture of metal and normal glass by using vacuum plasma spraying technique. This type of TBC material had a similar thermal expansion coefficient of a metal substrate. The thermal conductivity of this composite top coat was about two times greater than that of YSZ. Long thermal cycling life of the metalglass TBC was attributed to high thermal expansion coefficient, good adherence to the bond coat, and absence of open porosity and thereby, preventing the bond coat oxidation from corrosive gases [68]. Majumdar and Jana [69] studied the properties of a TBC prepared from 3 wt% YSZ dispersed in a high temperature resistant alumino-borosilicate glassy matrix. The YSZ-glass composite coating was applied on stainless steel substrate by a simple and costeffective enameling technique. The thermal gradient of 800 μm thick TBC was found to be 175– 180°C after 30 min exposure at 1,000°C. Significant improvement of the gradient to 650–675°C was observed after long exposure of the coated surface at 1,000°C when compressed air cooling was utilized [69]. The spallation of ceramic coating from the bond coat is an important problem for TBC systems. Basically, the spallation is caused by the oxidation and hot corrosion at the interface of the ceramic layer and bond coat. Keyvani et al. [70] investigated the oxidation and hot corrosion behavior of plasma sprayed nanostructured Al2O3/YSZ composite TBC coatings on Ni-based (IN-738LC) superalloy substrate and compared it with the conventional YSZ. The coatings were deposited by plasma spray method. High temperature oxidation test at 1,100°C and hot corrosion test at 1,050°C using Na2SO4 and V2O5 molten salts were conducted on the coatings. The experimental data demonstrated that the nanostructured Al2O3/YSZ composite coating had higher oxidation and hot corrosion resistance than those of the conventional YSZ coating. The microstructural analysis indicated that the growth of TGO was much less for this nanostructured Al2O3/YSZ composite coating. Further, the composite top coating prevented infiltration of both oxygen and aggressive molten salt [70]. Novel YSZ (6 wt% yttria partially stabilized zirconia)–(Al2O3/YAG) (alumina–yttrium aluminum garnet, Y3Al5O12) DLC coatings were formed by using the composite sol-gel and pressure filtration microwave sintering (PFMS) technologies by Ren et al. [71]. The microstructural observations showed that microsized YAG particles were embedded in nano-sized α-Al2O3 film. A thin Al2O3/YAG layer had good adherence with the substrate and the thick YSZ top layer. Cyclic oxidation tests at 1,000°C indicated that they can resist oxidation of alloy and improve the spallation resistance. The thermal insulation capability tests at 1,000°C and 1,100°C indicated that 250 μm coating had better thermal barrier effect than that of the 150 μm coating at different cooling gas rates. The

TBCs in near future [67].

126 Advanced Ceramic Processing

**6.2. Composite top coat**

MgO–Al2O3–TiO2 and ZnO–Al2O3–SiO2 based glass-ceramic coatings have been developed as TBCs for gas turbine engine components by Datta and Das [72, 73]. These coatings were formed on nimonic alloy substrates using the vitreous enameling technique. MgO–Al2O3–TiO2-based glass coating was applied on nimonic alloy substrate by spraying the glass slurry, drying, and then firing at about 1,160°C for 5–6 min. Further, the glass coating was heat treated for 1 h at 880°C followed by 1 h at 1,020°C to develop crystals such as magnesium aluminum titanate as a major phase along with magnesium silicate and aluminum titanate as minor phases in the glass matrix. The thermal shock resistance of the glass-ceramic coating was found to be more than 10 cycles when repeatedly heated to 750°C and immediately quenched in cold water. No chipping or spalling defect was observed. Slight weight gain was noted during the thermal endurance test at 1,000°C for 100 h. However, the operating temperature of this coating is limited to 750°C. Glass-ceramic coating based on ZnO–Al2O3–SiO2 systems can operate at high working temperatures of up to 1,000°C. This type of glass coating was applied on a nimonic alloy through the spraying of a suitable glass slip, drying, and firing at 1,200°C for 5–6 min. The glass coating was subsequently heat treated at 1,000°C for 1 h to develop gahnite, willemite, and cristobalite crystalline phases. Thermal shock at 1,000°C for 10 cycles showed no chipping. During the thermal endurance test at 1,000°C for 100 h, negligible weight gain was observed. Figure 1(a) shows the oxidative weight gain of the bare substrate and MgO– Al2O3–TiO2-based glass-ceramic coated substrate during the oxidation test at 1,000°C for 100 h. Typical SEM image of MgO–Al2O3–TiO2-based glass-ceramic coating is shown in Figure 2(b).

**Figure 1.** (a) Oxidative weight gain of MgO–Al2O3–TiO2-based glass-ceramic coated substrate at 1,000°C for 100 h and (b) typical SEM microstructure of the corresponding coating.
