YTTRIA-STABILIZED HAFNIA-ZIRCONIA THERMAL BARRIER COATINGS - THE INFLUENCE OF HAFNIA ADDITION ON TBC STRUCTURE AND HIGH-TEMPERATURE BEHAVIOR

The search for more reliable and durable thermal barrier systems is a key factor for future aircraft turbine engines success. Hafnia is ther efore an attractive ceramic component due to its similarity to zirconi a and its elevated structural transformation temperatures. We report h ere structural and thermomechanical investigations of various plasma-s prayed coatings composed of ZrO2 + xmol% HfO2 (x=0, 25, 50 and 100), p artially stabilized by 4.53 mol% yttria. X-ray diffraction studies sho w that, a metastable, non-transformable, high yttrium content, tetrago nal solid solution is the only phase observed on the as-sprayed sample s. This phase is crystallographically equivalent to the t' phase descr ibed for classical yttrium-partially stabilized zirconia (Y-PSZ) therm al barrier coatings (TBCs). Upon high-temperature annealing in air (T= 1200 degrees C), however, the return of this t' phase to equilibrium d iffers from the classical t' --> t + c reaction. According to literatu re data, reactions of the type t' --> t + c + m should prevail at the highest hafnia contents (x greater than or equal to 50). Indeed, impor tant quantities of monoclinic phase are accordingly being observed upo n cooling. Thermal cycling of TBC samples in air has been performed at 1100 degrees C. The Young's modulus of the ceramic coating, which pro gressively increases when hafnia is substituted for zirconia, has a st rong influence on TBC thermomechanical resistance.