Fracture of plasma-sprayed thick thermal barrier coatings under multiaxial stress states

Failure behaviour of free‐standing plasma‐sprayed coatings was investigated under combined axial and shear loading. Thin‐walled tubular specimens were loaded with various combinations of tension/compression and torsion. This allows the failure surface to be established for loading situations where the two principal stresses are of opposite signs. Specimens failed in one of the two modes, a tensile failure perpendicular to the maximum principal stress or a compression shear failure through the thickness. Failure data were adequately described by the maximum principal stress theory. Stress–strain curves fall within a single scatter band depending on the failure mode. In situ deformation tests showed that the mechanism was microcrack closing and sliding in compression and microcrack opening, coalescence and the development of new microcracks in tension.