**7. Promising bond coat materials for TBC systems**

TBCs with ceramic top coat and MCrAlY (M=Ni, Co) bond coat are generally applied on gas turbine engine components to protect them from high temperature exposure [3]. The bond coat provides thermo-elastic relaxation to accommodate the high stresses generated in the TBC system. The chemistry and microstructure of bond coat affects the structure and morphology of the TGO [3]. The oxidation of bond coat needs to be restricted to improve the performance of the TBC system. Glass-ceramics may be used as bond coats because of several reasons. As this bond coat is basically oxide-based, failure of the TBC system from bond coat oxidation may be avoided. Further, high stress may be accommodated by the viscous flow of the glassceramics, which may increase the stability of the TBC system during thermal cycling at high operating temperatures. In addition, this TBC system may protect the metallic component from oxidation and creep failure more effectively because of the lower thermal conductivity of glassceramics compared to metals. Detailed studies on the TBC system consisting of 8 wt% YSZ (~400 μm) top coat, BaO–MgO–SiO2-based glass-ceramic bond coat (~100 μm) and nimonic alloy (AE 435) substrate have been carried out by Das [74]. The glass-ceramic bond coat and YSZ top coat were applied on the nimonic alloy substrate by conventional enameling and air plasma spraying techniques, respectively. Figure 2 depicts the typical SEM cross-sectional micrograph of this kind of TBC system, which is composed of BaO–MgO–SiO2-based glassceramic bond coat, 8-YSZ top coat, and nimonic superalloy substrate.

**Figure 2.** Typical TBC system consisting of glass-ceramic bond coat, 8-YSZ top coat, and nimonic superalloy substrate.

The 90° bend tests on these TBC systems showed that only a small amount of YSZ coating chipped off from the edges, indicating strong adherence of the TBC with the nimonic alloy substrate. The microhardness and Young's modulus values of YSZ coating, glass-ceramic coating, and nimonic alloy substrate of the TBC system were lower on the cross-section than those obtained on the plan-section at a load of 100 mN. The four-point bend test on the TBC system displayed low stiffness (bending elastic modulus−45–52 GPa at room temperature) that leads to low residual stresses in the TBC resulting in high thermo-mechanical stability of the TBC system [74]. Das et al. [75] studied the oxidation behavior of a TBC system consisting of 8 wt% YSZ top coat, BaO–MgO–SiO2-based glass-ceramic bond coat, and nimonic alloy (AE 435) substrate wherein static oxidation test was carried out at 1,200°C for 500 h in air. Oxidation resistance of this TBC system was compared with the conventional TBC system under identical heat treatment conditions. Both TBC systems were characterized by SEM, as well as EDS analysis. The TGO layer was not found between the bond coat and the top coat in the case of glass-ceramic bonded TBC system, while the conventional TBC system showed a TGO layer of ~16 μm thickness at the bond coat-top coat interface [75].

**7. Promising bond coat materials for TBC systems**

128 Advanced Ceramic Processing

ceramic bond coat, 8-YSZ top coat, and nimonic superalloy substrate.

TBCs with ceramic top coat and MCrAlY (M=Ni, Co) bond coat are generally applied on gas turbine engine components to protect them from high temperature exposure [3]. The bond coat provides thermo-elastic relaxation to accommodate the high stresses generated in the TBC system. The chemistry and microstructure of bond coat affects the structure and morphology of the TGO [3]. The oxidation of bond coat needs to be restricted to improve the performance of the TBC system. Glass-ceramics may be used as bond coats because of several reasons. As this bond coat is basically oxide-based, failure of the TBC system from bond coat oxidation may be avoided. Further, high stress may be accommodated by the viscous flow of the glassceramics, which may increase the stability of the TBC system during thermal cycling at high operating temperatures. In addition, this TBC system may protect the metallic component from oxidation and creep failure more effectively because of the lower thermal conductivity of glassceramics compared to metals. Detailed studies on the TBC system consisting of 8 wt% YSZ (~400 μm) top coat, BaO–MgO–SiO2-based glass-ceramic bond coat (~100 μm) and nimonic alloy (AE 435) substrate have been carried out by Das [74]. The glass-ceramic bond coat and YSZ top coat were applied on the nimonic alloy substrate by conventional enameling and air plasma spraying techniques, respectively. Figure 2 depicts the typical SEM cross-sectional micrograph of this kind of TBC system, which is composed of BaO–MgO–SiO2-based glass-

**Figure 2.** Typical TBC system consisting of glass-ceramic bond coat, 8-YSZ top coat, and nimonic superalloy substrate.

The 90° bend tests on these TBC systems showed that only a small amount of YSZ coating chipped off from the edges, indicating strong adherence of the TBC with the nimonic alloy substrate. The microhardness and Young's modulus values of YSZ coating, glass-ceramic coating, and nimonic alloy substrate of the TBC system were lower on the cross-section than those obtained on the plan-section at a load of 100 mN. The four-point bend test on the TBC system displayed low stiffness (bending elastic modulus−45–52 GPa at room temperature) that leads to low residual stresses in the TBC resulting in high thermo-mechanical stability of the

Thermal cyclic behavior of glass-ceramic bonded TBC on nimonic alloy substrate was inves‐ tigated by Das et al. [76]. In that study, a TBC system comprised of 8 wt% YSZ top coat, BaO– MgO–SiO2-based glass-ceramic bond coat, and nimonic alloy (AE 435) substrate was subjected to thermal shock test from 1,000°C to room temperature for 100 cycles. Specimens held at 1,000°C for 5 min were forced air quenched, as well as water quenched from the same conditions. Microstructural changes were investigated using SEM. The phase analysis was conducted by XRD analysis and EDS analysis. Deterioration was not observed in the top coats after 100 cycles in the case of forced air quenched specimens, whereas the top coats were damaged in the water quenched specimens. After thermal cycling experiments interfacial cracks did not appear at the top coat-bond coat and bond coat-substrate interfaces both in forced air quenched and water quenched specimens. Further, the top coat retained its phase stability [76]. The mechanical properties of a glass-ceramic bonded TBC system have been reported by Ghosh [77].Glass-ceramic bonded TBC showed good thermal gradient property as both the glass-ceramic bond coat and YSZ top coat can act as a thermal barrier to the nimonic alloy substrate and reduce the substrate temperature. The thermal gradient of a TBC-coated substrate was 856°C after 45 min holding of the YSZ coating at 1,200°C. The present TBC prevents the thermal conduction to the nimonic alloy substrate as both the glass-ceramic bond coat and the YSZ top coat have low thermal conductivity. Thermal conductivity measurement showed that the ~100 μm glass-ceramic coated substrate had lower thermal conductivity (~23– 27 W/m.K at 1,000°C) than that of the bare nimonic alloy substrate (~28 W/m.K at 1,000°C). Moreover, the thermal conductivity of the glass-ceramic- (~100 μm) and YSZ (~400 μm)-coated nimonic alloy substrate was much lower (17.19 W/m.K at 1,000°C) than that of the bare nimonic alloy substrate (~28 W/m.K at 1,000°C) [74, 78, 79].

Efficient gas turbines can be achieved through the use of engineered components having the capability of operating at higher metal temperatures with longer lifetimes. Gas turbine Inlet temperatures can exceed the melting temperatures of nickel-based superalloys. Advanced air cooling system in association with TBCs can decrease the underlying substrate temperature. NiCoCrAlY overlay coatings are generally used as bond coatings for industrial gas turbines. Extensive research is being carried out to find the suitable bond coat composition. Seraffon et al. [80] reported a new type of bond coat with a wide range of compositions. They focused on the oxidation behavior of the bond coatings at 950°C. A range of Ni–Co–Cr–Al coatings were deposited on sapphire substrates using the physical vapor deposition technique and magnet‐ ron sputtering method. Co-sputtering of two targets, such as Ni–10%Cr, Ni–20%Cr, Ni–50%Cr, Ni–20%Co–40%Cr, or Ni–40%Co–20%Cr target, and a pure Al target was used for the deposition of coatings. The coatings were then oxidized in air for 500 h at 950°C. All samples were characterized by measuring the change in coating thickness using pre- and post-exposure metrology only and also the change in specimen weight. Thick coatings (20–30 μm) were deposited by magnetron sputtering successfully. EDS analysis was used to determine the elemental compositions of the samples. Furthermore, XRD was used to identify the major oxides formed during thermal exposure. The selective growth of protective Cr2O3, Al2O3 or other less protective mixed oxides was observed. The oxide scale growth rate indicated the suitable coatings that produce more protective oxides and allow future optimization of the bond coating composition for service within the turbine section of industrial gas turbines [80].

In the last decade, it has been observed that Pt-rich γ–γ′ alloys and coatings have good oxidation and corrosion properties. Selezneff et al. [38] used this technique to fabricate doped Pt-rich γ–γ′ bond coatings on AM1® superalloy substrate. These TBC systems were compared with the conventional TBC system composed of a β-(Ni,Pt)Al bond coating. Most of the compositions were superior to the β-(Ni,Pt)Al bond coatings with respect to ceramic top coat adherence and better oxide scale adherence of the γ–γ′-based systems [38]. Iridium modified nickel alluminides are promising bond coats because of their ability to promote α-Al2O3 scale growth and to form an oxygen diffusion barrier Ir layer. An innovative Al–Ni–Ir alloy was formulated by Lamastra et al. [81]. A detailed microstructural investigations of both powder and bulk samples were conducted to compare the phase composition, oxidation behavior, and thermal stability of the proposed system with those of the Ir free ones. The AlNiIr system was composed of Al3Ni2, AlNi3 and β-NiAl. It was assumed that the presence of Ir promoted the alumina scale growth, which started at ~1000°C. Ni-poor and Al-rich islands were observed in both as cast and oxidized AlNiIr bulk samples. However, Ir had high concentration in Alrich islands and thereby, suggesting higher affinity of iridium towards Al than Ni. After oxidation at 1,150°C, the α-Al2O3 scale growth was observed increasing the TGO thickness with dwelling time. Both Ir ODB and Ir-rich islands at the interface between the alloy and the Al2O3 scale were not identified due to the low Ir amount. However, metallic Ir and the compound Al2.75Ir were detected in the powder after thermal treatment at 1,000°C [81].

Developing new bond coat is an effective way to extend the service life of TBCs during high temperature exposure. Yao et al. [82] prepared a novel TBC system composed of an (Al2O3– Y2O3)/ (Pt or Pt–Au) composite bond coat and a YSZ top coat and Ni-based superalloy by magnetron sputtering and EB-PVD, respectively. Cyclic oxidation tests in air at 1,100°C for 200 h showed that the YSZ top coat and alloy substrate can be bonded together effectively by the (Al2O3–Y2O3)/(Pt or Pt–Au) composite coating. So, this kind of TBC had excellent oxidation resistance and cracking/buckling resistance, which can be attributed to the sealing effect of such coating. Therefore, the interdiffusion between the bond coat and alloy substrate as well as substrate oxidation can be avoided. The toughening effect of noble metals and composite structure of bond coat resulted in inhibition of the micro-cracks propagation and relaxation of the stress in the bond coat. This ceramic/noble metal composite coating has great prospect for the TBC applications [82]. Wang et al. [83] produced NiAl and NiAlHf/Ru coatings on nickelbased single crystal superalloy in order to investigate the interdiffusion behavior and cyclic oxidation resistance at 1,100°C. Needle-like topologically close-packed phases and secondary reaction zone (~30 μm thick layer) were formed in the NiAl-coated superalloy after annealing at 1,100°C for 100 h while the precipitates of TCP and SRZ were effectively constrained in the NiAlHf/Ru-coated alloy. The NiAlHf/Ru coating exhibited superior cyclic oxidation resistance as compared to the NiAl coating. They have shown that Ru and Hf have important roles in terms of affecting interdiffusion and cyclic oxidation [83]. Zhang et al [84] developed gradient TBCs consisting of (Gd0.9Yb0.1)2Zr2O7–yttria-stabilized zirconia (8YSZ) and Hf-doped NiAl bond coat by EB-PVD technique. The effect of the interfacial structure between (Gd0.9Yb0.1)2Zr2O7 (GYbZ) and 8YSZ layers on the thermal cycling behavior was investigated by comparing the DCL coatings with gradient thermal barrier coatings (GTBCs). The thermal cycling tests showed that the GTBCs had a more extended lifetime than that of the DCL coatings. The failure of GYbZ-8YSZ DCL coating with clear interface between different ceramic layers occurred through delaminating cracking as a result of crack initiation and propagation caused by stress concentration within the ceramic layers. Further, the failure of GTBC occurred due to the thermal expansion mismatch between the Hf-doped NiAl bond coat and the TGO layer [84].
