The paper presents optimization of processes for obtaining maximum content of tetragonal phase in the initial material and thermal barrier coatings (TBC) based on zirconium dioxide and hafnium oxide. Results of the investigations on phase composition of oxide HfO2 – ZrO2 – Y2O3 system have been given in the paper. The system represents a microstructure which is similar to zirconia dioxide and transformed for its application at 1300 °C. The paper explains a mechanism of hafnium oxide influence on formation of the given microstructure. The research methodology has been based on complex metallography, X – ray diffraction and electron microscopic investigations of structural elements of the composite plasma coating HfO2 – ZrO2 – Y2O system.In order to stabilize zirconium dioxide dopant oxide should not only have an appropriate size of metal ion, but also form a solid solution with the zirconia. This condition severely limits the number of possible stabilizers. In fact, such stabilization is possible only with the help of rare earth oxides (Y2O3, Yb2O3, CeO2, HfO2). Chemical purity of the applied materials plays a significant role for obtaining high-quality thermal barrier coatings. Hafnium oxide has been selected as powder for thermal barrier coatings instead of zirconium dioxide due to their similarities in structural modification, grating, chemical and physical properties and its high temperature structural transformations. It has been established that plasma thermal barrier HfO2 – ZrO2 – Y2O3 coatings consist of one tetragonal phase. This phase is equivalent to a non-equilibrium tetragonal t' phase in the “zirconium dioxide stabilized with yttrium oxide” system. Affinity of Hf+4 and Zr+4 cations leads to the formation of identical metastable phases during rapid quenching.