Microstructural Characterization, Residual Stress, and Mechanical Properties of Plasma Sprayed 8YSZ Thermal Barrier Coatings

The plasma sprayed 8YSZ TBC is applied on a Ti-6Al-4V substrate with a NiCrAlY bond coat. The effect of spray angle on the coating microstructural, residual stresses, and mechanical properties is studied in the paper. The surface and cross-sectional coating structure were analyzed by scanning electron microscopy and elemental composition with energy-dispersive x-ray spectroscopy. The SEM image greyscale threshold determined the porosity level in the structure. The residual stresses and thermal conductivity were measured by Raman spectroscopy and laser flash technique. The mechanical property, such as hardness and surface roughness was determined using the indentation and surf test profilometer. The result showed that the spray angle affects the coating structure - grain size, shape and distribution, and mechanical properties. The P90 TBC showed a uniform and dense structure with avg. grain size of ~ 850nm, whereas P60 showed a non-uniform structure with avg. grain size of ~ 300nm. The 60° inclined spray angle leads to more defects such as cracks and pores than the 90° normal spray angle. In both surface and cross-sectional structures, the porosity level increases with a decrease in the spray angle conditions. The P90 TBC results in high compressive residual stresses than the P60 structured TBC. The avg. Raman shifts for P90 and P60 TBCs are 1.5667±1.022cm-1 and 1.3±0.973cm-1, respectively. The thermal conductivity decreases with increased porosity and low spray angle. The P90 showed better hardness and uniform surface than P60 TBC.