**5.8. Spark Plasma Sintering (SPS) method**

Pt-modified Ni aluminides and MCrAlY coatings (where M=Ni and/or Co) are widely used on turbine blades and vanes for protection against oxidation and corrosion and as bond coat in TBC systems. The SPS method can be used by Monceau et al. [36] to develop rapidly new coating compositions and microstructures. This technique allows the formation of multilayered coatings on a superalloy substrate. They have shown the possibility of fabricating MCrAlY overlays with local Pt and/or Al enrichment and coatings made of ζ-PtAl2, ε-PtAl, α-AlNiPt2, martensitic β-(Ni,Pt)Al, or Pt-rich γ/γ′ phases. Further, they have demonstrated the prospect of achievement of a complete TBC system with a porous and adherent YSZ layer on a γ/γ′ low mass bond coating. Additionally, they have discussed the difficulties of fabrication such as Y segregation, risks of carburization, local overheating, or difficulty to coat complex shape parts [36]. Recently, Boidot et al. [37] prepared complete TBC systems on single crystal Ni-based superalloy substrate in a one-step SPS process. A proto-TGO layer in situ was formed during the fabrication of the TBC systems. Formation of a dense, continuous, slow-growing alumina layer (TGO) between a ceramic top coat and an underlying bond coat during service influences the lifetime of the TBC systems. During thermal treatment at 1,100°C in air, the amorphous oxide layer transforms to α-Al2O3 in the as-deposited samples. Oxidation kinetics during annealing was in good agreement with the protective α-Al2O3 layer formation [37]. In the last decade, an increasing interest was given to Pt-rich γ–γ′ alloys and coatings as they have shown good oxidation and corrosion properties. SPS has been proved to be a fast and efficient tool to fabricate coatings on superalloys including entire TBC systems. Selezneff et al. [38] used the SPS technique to fabricate doped Pt-rich γ–γ′ bond coatings on the superalloy substrate, whereas the doping elements were reactive elements (e.g., Hf, Y or Zr, Si) and metallic additions of Ag. These samples were then coated with Y-PSZ TBC through the EBPVD method. The performance of such TBC system was compared to a conventional TBC system consisting of a β-(Ni,Pt)Al-based bond coat. Thermal cycling tests were performed in air and spallation was observed during this test. It was noted that most of the Pt-rich γ–γ′ samples showed better adherence of the ceramic coating than that of the β-samples. Cross-sectional scanning electron microscopy was used to characterize the thickness and the composition of the oxide scales after cyclic oxidation test. It was proved that the doping elements have significant influence on the oxide scale formation, metal/oxide roughness, Al and Pt content under the oxide scale, and TBC adhesion. It was established that RE-doping can not improve the oxidation kinetics of Pt-rich γ–γ′ bond coat. Moreover, γ–γ′-based systems were superior to β-(Ni,Pt)Al bond coat with respect to ceramic top coat adherence and better oxide scale adherence [38].
