Stresses and Microstructural Development of Thermal Barrier Coatings Using AIP/D‐gun Two‐Step Processing

Abstract

After depositing a Ni-32Co-20Cr-8Al-0.5Y-1Si-0.03B (wt%) bond coat on a Ni-base superalloy using arc ion plating (AIP), a ceramic top coat with hollow spherical powder of ZrO 2 -8 wt%Y 2 O 3 (HSP-YSZ) was deposited using detonation gun (D-gun) spraying. Thermal exposure behaviors of thermal barrier coatings (TBCs) were investigated at 11001C. The thermal growth oxides (TGO) layer thickened and became more undulated during thermal exposure. Yttrium aluminum garnet (YAG) was observed within TGO, which produced thickness imperfections and thus aided to build up out-of-plane stresses. As a result, radial cracks initiated at the defects around TGO imperfections and separation developed through crack nucleation, propagation, and coalescence at the weaker TGO/bond coat interface. With further thermal exposure, coalescence of interfacial separations created a connected crack. The TBC detached and final failure occurred at the TGO/bond coat interface, leading to spallation of TBC when cooling to ambient. The stress distributions in the TGO layer with different thermal exposure times were also measured using luminescence spectroscopy. The stresses were independent of time after a transient period from h-Al 2 O 3 to a-Al 2 O 3 scale. It is suggested that the lifetime of AIP/D-gun TBCs with an HSP-YSZ coat is controlled by the initiation and linking of a sub-critical interfacial crack.

A comprehensive sintering mechanism for TBCs‐Part II: Multiscale multipoint interconnection‐enhanced initial kinetics

“…This phenomenon stands opposite to the assumption of the typical sintering model with smooth surface all over the sintering process. Moreover, this multiscale surface roughening is different from the rumpling behavior of thermally growth oxide (TGO) layer observed during thermal exposure . The rumpling of TGO often refers to its undulation growth, which is induced by a compressive stress on the layer.…”

“…Moreover, this multiscale surface roughening is different from the rumpling behavior of thermally growth oxide (TGO) layer observed during thermal exposure. [43][44][45][46][47] The rumpling of TGO often refers to its undulation growth, which is induced by a compressive stress on the layer. Therefore, the rumpling of TGO often leads to partially debonding due to the morphological change in the whole layer.…”

Preparation and Isothermal Oxidation Behavior of Zr-Doped, Pt-Modified Aluminide Coating Prepared by a Hybrid Process

“…Thus, the difference between the rumpling resistance of the Pt-modified aluminide coating with or without Zr might be primarily dependent upon to the growth stress and compressive stress caused by the β to γ' phase transformation. It is known that the Pilling-Bedworth Ratio (PBR) of Al is larger than 1, and the PBR of the transient alumina is bigger than that of stable α-Al 2 O 3 [34], which means that less compressive stress would be generated in the alumina scale if more α-Al 2 O 3 phase was formed during the oxidation process. As mentioned above, the addition of Zr is beneficial for promoting the formation of the α-Al 2 O 3 phase, thus the compressive growth stress generated in the Zr-doped, Pt-modified aluminide coating might be smaller than that in the Pt-modified aluminide coating.…”