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 therefore an attractive ceramic component due to its similarity to zirconia and its elevated structural transformation temperatures. We report here structural and thermomechanical investigations of various plasma-sprayed coatings composed of ZrO2 + xmol% HfO2 (x=0, 25, 50 and 100), partially stabilized by 4.53 mol% yttria. X-ray diffraction studies show that, a metastable, non-transformable, high yttrium content, tetragonal solid solution is the only phase observed on the as-sprayed samples. This phase is crystallographically equivalent to the t' phase described for classical yttrium-partially stabilized zirconia (Y-PSZ) thermal barrier coatings (TBCs). Upon high-temperature annealing in air (T=1200 degrees C), however, the return of this t' phase to equilibrium differs from the classical t' --> t + c reaction. According to literature data, reactions of the type t' --> t + c + m should prevail at the highest hafnia contents (x greater than or equal to 50). Indeed, important quantities of monoclinic phase are accordingly being observed upon cooling. Thermal cycling of TBC samples in air has been performed at 1100 degrees C. The Young's modulus of the ceramic coating, which progressively increases when hafnia is substituted for zirconia, has a strong influence on TBC thermomechanical resistance.