The thermal phase stability of plasma sprayed TBC is presented and discussed. TBC phase transformation after various isothermal heat treatment processes was studied via Xray diffraction (XRD) and Raman Spectroscopy methods. The YSZ-Gd-Yb and 20YSZ TBCs demonstrated superior thermal phase stability as compared with the high and standard purity 8YSZ TBCs, although higher purity also helps delay the tetragonal to monoclinic phase transformation. The phase transformation appears to be suppressed by cooling at higher rates. This data presents a qualitative phase stability comparison between the various coatings. However, cooling rate has to be taken into consideration in determining the extent of phase instability during the coating design of aero turbine applications.