THERMAL-STABILITY OF STABILIZED ZIRCONIA THERMAL BARRIER COATINGS PREPARED BY ATMOSPHERE-CONTROLLED AND TEMPERATURE-CONTROLLED SPRAYING

Yttria-stabilized zirconia coatings were deposited by atmosphere- and temperature-controlled spraying (ATCS) on IN 100 superalloy substrates for thermal engine applications. The whole deposition process was performed in an Ar atmosphere, adopting a new liquid Ar device that cooled the samples during the spraying down to 60 OC. Coatings prepared by ATCS were well adherent with a dense, microcracked microstructure. However, as found previously, as-sprayed zirconia coatings were understoichiometric and highly disordered; the presence of excess oxygen vacancies and high defect density led to an increased stability of the tetragonal (t) and cubic (c) polymorphs with respect to the monoclinic (m) one. Under such conditions a low temperature annealing was sufficient to cause remarkable phase changes. It was found that under the most effective cooling conditions the coating was made of a mixture of t- and c-phases, the m-phase being totally absent. The crystal structure and thermal stability of the present zirconia polymorphs were studied by X-ray diffraction full-pattern analysis, differential scanning calorimetry and thermogravimetric analysis.