Influence of impurity content and porosity of plasma-sprayed yttria-stabilized zirconia layers on the sintering behaviour

Yttria-stabilized zirconia (YSZ) powders from different manufacturers have been used to prepare atmospheric plasma-sprayed (APS) ceramic coatings with different porosity levels. While the particle morphology of the different powders was similar, the amount of impurities, especially silica, was different, varying between 100 and 1500 ppm. APS coatings were removed from the substrates and the porosity distribution was measured by mercury porosimetry. Typically porosity levels between 10 and 15% have been used. Free-standing coatings were investigated in the dilatometer during long-term (>50 h) annealing at 1200°C. Additionally, the coefficient of thermal expansion was determined from expansion during heating. It turned out that the higher porosity levels, as well as the high impurity levels, led to a significant increase in the sintering rate of the coating during high-temperature annealing. A linear relationship between the total shrinkage after 60 h and the porosity, as well as the silica content, was used to describe the influence of the investigated parameters on the sintering behaviour in a more quantitative way.