Effect of initial powder morphology on thermal and mechanical properties of stand-alone plasma-sprayed 7 wt.% Y₂O₃-ZrO₂ coatings

The effects of starting powder morphology on the thermal and mechanical properties of stand-alone plasma-sprayed 7 wt.% Y₂O₃-ZrO₂ (YSZ) coatings were studied. Two powder morphologies were investigated: an agglomerated and sintered powder (referred to presently as "AS") and a powder manufactured using plasma spheroidization to create hollow spheres (referred to presently as "HS"). Coatings made from AS powders contained 0.21 wt.% SiO₂ impurity, twice as much as observed in coatings made from HS powders. Properties of coatings made from each powder type were compared in the as-sprayed state and after 50 h heat treatments at temperatures ranging from 1000 to 1400 °C. SEM microstructural investigations revealed significant differences in the porosity and distribution of pores in the coatings. In coatings made with AS powders the majority of the high aspect ratio pores were located between lamella (interlamellar porosity). In addition to interlamellar pores, coatings made with HS powders demonstrated 1.5 times more spherical-shaped globular pores by number located within lamella. Globular pores were shown to still exist in coatings made with HS powders after 50 h heat treatments at 1400 °C. Archimedes porosity measurements showed that coatings made with AS powders typically contained 4-5% less total porosity than coatings made with HS powders. Thermal conductivity experiments using laser flash showed that there was no difference in the thermal conductivity of coatings made from either powder type in the as-sprayed state despite higher porosity in the coatings made from HS powders. After a 50 h heat treatment at 1000 and 1200 °C, coatings made from both powder types still demonstrated statistically similar thermal conductivities. However, after a 50 h heat treatment at 1400 °C the thermal conductivity of coatings made from AS powders was found to be 0.3 W/m/K higher than coatings made from HS powders. Microstructural differences in the coatings made from the two powder types that affected sintering rates, including the increased level of SiO₂ impurities and fewer globular pores, were used to explain variations in properties. In uniaxial compression tests on stand-alone samples at 1000, 1100, and 1200 °C, coatings made from AS powders typically relaxed more stress than coatings made from HS powders. Differences in coating relaxation became more apparent as the test temperature increased through 1200 °C. The higher percentage of impurity SiO₂ in the AS coatings is believed to contribute to increased relaxation via formation of a glassy phase at the grain boundaries which assists grain boundary sliding during elevated temperature deformation.