The role of starting powder size on the compressive response of stand-alone plasma-sprayed alumina coatings

Cylindrical stand-alone tubes of plasma-sprayed alumina were tested in uniaxial compression at room temperature, using strain gages to monitor axial strains. The effect of lamella size on the mechanical response was investigated by employing different starting powders to fabricate samples. The average powder sizes investigated included 9 μm, 19 μm and 32 μm alumina; the resulting effective lamella diameters were 10 μm, 28 μm, and 55 μm, respectively. Similar stress-strain hysteresis was observed on unloading in all tubes, independent of lamella size. A strong correlation between the failure stress and the cumulative strain at failure was also observed for tubes fabricated from the three powders. For samples with approximately constant densities, tubes plasma sprayed with the 9 μm powder exhibited greater moduli than tubes sprayed from either 19 or 32 μm powders. This difference was attributed to the greater percentage of unmelted α-Al2O3 in the coating.