Interfacial failure via encapsulation of external particulates in an outward-growing thermal oxide

A Cr₂O₃-forming Ni-base superalloy and this alloy coated with a Pt-modified aluminide coating were exposed to SiO₂ powder and cyclically oxidized at 950 °C. The uncoated alloy showed a considerable amount of spallation and buckling whereas the Pt-NiAl coated alloy remained protective throughout hundred 1 h-cycles. The interfacial failure is mainly ascribed to the increased thermal strain by the encapsulation of external SiO₂ particulates in an outward-growing Cr₂O₃ layer. However, the particles were not embedded in the thermally grown oxide of the Pt-NiAl coated alloy due to the slow inward-growing characteristics of Al₂O₃ scales. The buckling of the Cr₂O₃ scale with embedded SiO₂ was analyzed with (1) a classical buckling criterion using the instantaneous coefficients of thermal expansion of the constituents, and (2) finite element analyses (FEA) to estimate the local interfacial shear stresses. It turns out that the thermal strain with embedded SiO₂ is larger than the experimentally determined critical thermal strain (?_b) explaining the buckling of the oxide scale observed in the experiment. The FEA results demonstrate that local shear stresses at the metal/oxide interface are significantly amplified near the SiO₂ particles showing that the buckling of oxide can be readily initiated especially in the vicinity of the embedded particles.