Microstructure and adhesion of a zirconia thermal barrier coating

A ceramic coating (20 wt% Y{dollar}sb2{dollar}O{dollar}sb3{dollar} bal. ZrO{dollar}sb2){dollar} was deposited by reactive electron beam evaporation, onto two oxidation resistant (alumina forming) alloys (NiCrAlY and NiCoCrAlY) and a sapphire substrate. The ceramic coating was designed to provide a thermal barrier on an oxidation resistant turbine blade. The microstructure of the ceramic coating, interfacial region, and alloy substrates were investigated by SEM and TEM techniques. The structure of and phases present in the interfacial region were determined to be the "key" to understanding the adhesion and failure mechanisms of the coating. A mixed oxide region was found between the aluminum oxide formed by the alloys and the deposited zirconia.; Changes in the interfacial region were monitored during isothermal oxidation in air at 1000 and 1100{dollar}spcirc{dollar}C. Indentation techniques were used to examine crack paths in the zirconia coating or at the alloy/alumina interface. The presence of voids at the alloy/alumina interface was coincident with that interface being the failure path. Cracking in the zirconia was found to correlate with changes in the mixed oxide layer and the formation of YAG in the zirconia coating.; The rate and morphology of aluminum oxide growth during oxidation of the two alloys was different. The NiCrAlY alloy grew a planer interface with nearly constant thickness along the interface. The NiCoCrAlY alloy developed a convoluted morphology between the alumina and the alloy. The interface remained wavy with the thicker regions growing at a faster rate than the thinner regions. A model was developed to examine the mode of oxide growth.; Degradation of the coatings was monitored via HVEM microscopy of the interfacial region during the oxidation process. The formation of Yttrium Aluminum Garnet (YAG) was observed in the zirconia coating at longer oxidation times. The formation of the YAG depleted the zirconia of Y{dollar}sb2{dollar}O{dollar}sb3{dollar} which was accompanied by the formation of the monoclinic phase of zirconia. Failures of the coating at long oxidation times were observed away from the interface in the dense/columnar transition region of the zirconia, coincident with the YAG formation.