Influence of surface geometry and microstructural features on the delamination and crack propagation of brittle convex thermal barrier films during thermal cyclic loading

The influence of air plasma sprayed alumina coating geometry, microstructure, residual stress and interface roughness on its delamination and crack propagation resistance during low temperature thermal cycling, i.e. thermal mismatch stress, is investigated both numerically and experimentally. Previous studies on thermal cycling loading concentrate on flat, numerically designed locally curved specimens and/or mathematically modeled roughness without extension towards real coating morphology, which renders the conclusions less practically driven.

Image processing tools are used to convert and import stitched scanning electron images, depicting the real coating convex geometry (800µm radius of curvature), interfacial morphology and microstructural details, into a developed numerical finite element model (FEM), a first in the thermal spray field. The extended FEM tool is used to define and locate crack positions using the maximum principal stress damage initiation and energy-based damage evolution.

Results show that arbitrarily oriented cracks originate predominantly near the coating/substrate interface and propagate along zones of high tensile and shear residual stress. The crack path shift was attributed to the complex stress concentration structure resultant from the intricate microstructural porosity and coating general convex geometry. Delamination is primarily caused by coating microstructural features such as crack networks and/or aligned porosity.