Constrained sintering and cracking of air plasma sprayed thermal barrier coatings: Experimental observation and modeling

Abstract

The development of vertical cracks in air plasma sprayed (APS) thermal barrier coatings (TBCs) during thermal cycling and constrained sintering under a temperature gradient is investigated. Microstructural analysis shows that the development of the vertical cracks is associated with multiple processes, including sintering during the hold period and cleavage during cooldown. Inspired by the experimental observations, an image‐based sintering model is used to simulate the development of vertical cracks as the coating sinters while constrained by a substrate. The computational results show that microstructural imperfections can develop into vertical cracks, which then propagate toward the interface. A simple analytical model is presented for the threshold level of in‐plane stress for the onset of propagation of a vertical crack during constrained sintering. By combining the results of these different modeling approaches, the cross‐coupling of the material and geometric parameters, and how this determines the sintering response (microstructure evolution) and vertical crack formation is evaluated. In addition, the growth of vertical cracks by a cleavage mechanism during cooldown is examined and the coupling between sintering, cleavage crack growth, and TBC lifetime is explored.

Sintering Modeling of Thermal Barrier Coatings at Elevated Temperatures: A Review of Recent Advances

“…An example is given when cracks extend to the bond coat/top coat interface, which may cause spallation. Zhang et al [66] lately extend their study on constrained sintering effects on vertical cracks. The image-based FE model was deployed again for simulating vertical cracks at different stages and different splat thicknesses.…”

“…Zhang et al used SEM images as a reference to build FE model and applied the UMAT to simulate the local sintering behavior with variable shape of cracks. Zhang's model gave another level of details on the sintering process in complex crack structure, which will be expanded below [20,66].…”

“…The third case added an intersection with horizontal crack between the open-up area and precursor zone, which is a common case as it propagates, and the results shows that it has a similar response in the intersection-free case. Detail of the mesh construction can be found at Figure 4 in [66].…”