The nature and cause of failure of thermal barrier coatings (TBCs) consisting of physical vapor deposited (PVD) yttria stabilized zirconia (YSZ, 8 wt.% Y2O3) and a diffusion aluminide bond coat (Pt-Al) were investigated after oxidative thermal cycling and isothermal heat treatment at 1177 degreesC in air. Experiments were conducted for 45 and 10-minute hold times and for isothermal condition for disk specimens with and without TBC. It is found that microcracks starts in the oxide scales at the bond coat grain boundary protrusions. Total number of thermal cycles affect the density of microcracks within the TGO layer. Evidence is presented that higher density of microcracks in the 10-min hold-time experiments tend to separate the TBC from the TGO layer via extensive coating "micro-decohesion" and promotes 'complete' TBC separation as opposed to traditional 'partial' spallation of TBC from the substrate as in the 45-min hold-time and isothermal experiments.