Diffusion coatings, MCrAlY

The presence of Y in the coatings improves the oxidation resistance of AI2O3 scale. Thus, MCrAlY coatings outperform diffusion aluminides significantly in oxidation tasks as shown in Table 12.8. 

Bond coat phase transformation Most of the bond coats, either MCrAlY or (Ni,Pt)Al, used in TBCs are based on the P-NiAl phase. As shown in Fig. 10, the major change in the bond coat microstructure during oxidation is the formation of the y -NisAl phase. This is attributed to aluminium depletion caused by the formation of AI2O3 on the surface and by inward diffusion of aluminium into the superalloy. Since the density of P-NiAl (5.9 g/cm ) is smaller than that of y -Ni3Al (7.5g/cm), there is a volume shrinkage during this transformation [68]. If the decomposition of the P-NiAl takes place locally or at a different time in different places, then the volume reduction will also be localised. This may cause the bond coat surface to distort from its original shape or cavities to form inside the bond coat. 

Both aqueous and fused-salt electrol3rtes have been used for plating the platinum group elements. Platinum has been used as a diffusion-barrier layer in aluminiding nickel-base alloys and MCrAlY coatings. Platinum... 

The diffusion of elements such as Mo, W, S and Ta into the MCrAlY layer has been recognized. Addition of a small amount of Si into MCrAlY can effectively prevent outward diffusion of Mo by increasing the diffusion activation energy [56-58], However, the existence of Si on the surface of the bond coat deteriorates thermal cycling performance of TBCs, because silicon oxide is brittle at low temperatures and peristaltic at elevated temperatures. To circumvent this, a two-layered TBC with a Si gradient MCrAlY-Si bond coat was designed to block the outward diffusion of Mo from the substrate into the bond coat and to improve thermal cycling performance of TBCs [57]. 

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