Understanding crack formation in plasma sprayed thermal barrier coatings and their effects on coating properties

As a key microstructural feature of plasma sprayed thermal barrier coatings (TBCs), microcracks determine the performance and service lifetimes of the coatings by influencing their thermal conductivity and compliance. The goal of this research is to obtain a fundamental understanding of crack formation in plasma sprayed microstructures, and their effect on mechanical and thermal properties through a synergistic combination of experimental and modeling efforts.; Air plasma sprayed ZrO₂-8wt%Y₂O₃ TBCs were deposited under tightly controlled conditions. The lengths and orientations of the horizontal cracks and vertical cracks in these coatings were characterized in detail, and process/crack microstructure maps of the crack distribution as a function of particle and substrate states were constructed. A fully coupled thermo-mechanical finite element, model was used to study the buildup of stresses during splat solidification; and to understand the effect of deposition conditions on crack formation during plasma spray deposition. Two stress components were considered, S₁₁, oriented in the coating plane, which leads to vertical cracks; and S₂₂, oriented normal to the coating plane, which leads to horizontal cracks. The model showed that both S₁₁ and S₂₂ scaled with the temperature difference between the superheated splat and the substrate. The model also showed that surface roughness plays a key role in determining the magnitude of maximum stresses, especially S ₂₂. The maximum S₁₁ occurred at the top of the splat, while the maximum S₂₂ occurred at the bottom of the splat, both at surface roughness valleys. It was also found that the three geometric parameters, splat thickness, t, surface roughness wavelength; λ, and surface roughness amplitude, A; scale with the stresses. Thus, two independent geometric parameters; ψ = A/λ, and ζ = t/λ, were, defined. The simulation also showed that only roughness features on the scale of splat, thickness are important in providing locations of maximum stress concentration. A unique feature of this model is the incorporation of real surface roughness features obtained from the plasma sprayed TBCs, which are not sinusoidal.; The elastic moduli (in-plane and out-of plane) and thermal conductivity (out-of-plane) of select samples were measured by a non-destructive ultrasonic technique and by a laser flash technique, respectively. It was found that out-of-plane thermal and mechanical properties were dominated by horizontal cracks and that all properties scaled with the sum of the cube of the crack lengths.