Growth selection induced residual stresses and fracture behavior of as‐deposited thermal barrier coatings

This study explored the impact of microstructure and residual stresses on the fracture behavior of as‐deposited thermal barrier coatings (TBCs). Two distinct air plasma sprayed TBCs, Coating A (conventional lamellar porous) and Coating B (dense vertically cracked), were investigated. Coating A involved coarser but less dense powders as feedstock and a lower substrate temperature during deposition. Further, Coating A had ∼2.5$\sim 2.5$ times higher randomly oriented porosities, finer grains, lower hardness, and elastic stiffness. Strikingly, however, the fracture strength was higher for the porous as‐deposited Coating A. The answer to this apparent contradiction emerged from the intergranular residual stresses. These were measured using both X‐ray diffraction and high‐resolution‐electron backscattered diffraction. Coating B, deposited at a higher substrate temperature, had clear growth selection of (102)$( {1\ 0\ 2} )$ oriented grains. These also had more out‐of‐plane normal and shear residual stresses. The growth selection induced residual stresses appeared responsible for the decohesion of Coating B from the substrate and, correspondingly, lower fracture strength.