Investigating the anisotropic mechanical properties of plasma sprayed yttria-stabilised zirconia coatings

The adhesion and cohesion bond strengths of plasma sprayed yttria-stabilised zirconia (YSZ) coatings were measured by performing the tensile adhesion test (TAT) and the tubular coating tensile test (TCTT). The TAT allowed assessment of adhesive/cohesive bond strength of a coating microstructure perpendicular to the substrate. In contrast, the TCTT quantifies the strength of a thermal spray coating parallel to the substrate without the use of any adhesive. The failure strength of the coatings from the respective tests can be approximated to a Weibull distribution and indicated the anisotropic behaviour of plasma sprayed coatings. The average coating strength parallel to the substrate is approximately 1.5 times greater than the bond strength perpendicular to substrate. The anisotropic behaviour of the plasma sprayed YSZ coatings were also probed using Knoop hardness measurements that were orientated at a well-defined geometry with respect to the lamellar microstructure. In addition, uniaxial compression tests evaluated the Poisson's ratio of these anisotropic YSZ coatings when loaded with respect to the different microstructural orientations.