Phase stability and fracture toughness studies on rare-earth stabilized zirconia for thermal barrier application

Yttria stabilized zirconia (YSZ) has been used as thermal barrier coating (TBC) because of its high fracture toughness, phase stability and low thermal conductivity among other desirable properties. Next generation gas turbine engines require TBC materials that would facilitate enhanced operating temperatures, for which YSZ is not suitable. In this context, new compositions based on zirconia are being studied to understand their suitability as TBC's. In this work, the phase stability and fracture toughness behaviour of 4 mol% gadolina + 4 mol % ytterbia + 92 mol % zirconia were studied. Powders were synthesized using chemical co precipitation method. The as-calcined powder was heat treated to a temperature of 1250 deg C for a duration of 96 hr. Powder samples were collected at specified intervals and phase evolution studies were carried out using X-Ray Diffraction (XRD) technique. The kinetics of phase evolution was determined. Cubic zirconia phase precipitates out from the non-transformable tetragonal phase and its fraction changes as heat treatment progresses. Indentation fracture toughness studies were carried out on dense pellets prepared at 1250 deg C using spark plasma sintering (SPS) method. Fracture toughness was measured on heat treated pellets and the fracture toughness was correlated with the phase evolution. (author)