**8. Conclusions**

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].

130 Advanced Ceramic Processing

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

In the future, TBCs are required to be more suitably designed for the thermal protection of gas turbine engine components to significantly increase engine operating temperatures, fuel efficiency, and engine reliability. However, coating durability is a vital factor to increase the engine operating temperature. Therefore, the coating behavior and failure modes under high temperature, high thermal gradient cyclic conditions should be properly understood to develop next-generation advanced TBCs.
