Evaluation of CaO-CeO₂-partially stabilized zirconia thermal barrier coatings

Abstract

Plasma sprayable powders were prepared from ZrO₂-CaO-CeO₂ system using an organic binder and coated onto stainless steel substrates previously coated by a bond coat (Ni 22Cr 20Al 1.0Y) using plasma spraying. The coatings exhibited good thermal barrier characteristics and excellent resistance to thermal shock at 1000°C under simulated laboratory conditions (90 half hour cycles without failure) and at 1200°C under accelerated burner rig test conditions (500 2 min cycles without failure). No destabilization of cubic/tetragonal ZrO₂ phase fraction occured either during the long hours (45 h cumulative) or the large number of thermal shock tests. Growth of a distinct SiO₂ rich region within the ceramic was observed in the specimens thermal shock cycled at 1000°C apart from mild oxidation of the bond coat. The specimens tested at 1200°C had a glassy appearance on the top surface and exhibited severe oxidation of the bond coat at the ceramic-bond coat interface. The glassy appearance of the surface is due to the formation of a liquid silicate layer attributable to the impurity phase present in commercial grade ZrO₂ powder. These observations are supported by SEM analysis and quantitative EDAX data.