Phase Formation and Thermal Stability of Reactively Sputtered YTaO₄-ZrO₂ Coatings

Y_(1-x)/2Ta_(1-x)/2Zr_xO₂ coatings with 0 to 44 mol% ZrO₂ were synthesized by sputtering. Phase-pure M'-YTaO₄ coatings were obtained at a substrate temperature of 900 °C. Alloying with ZrO₂ resulted in the growth of M' along with t-Zr(Y,Ta)O₂ for ≤15 mol%, while for ≥28 mol%, ZrO₂ X-ray diffraction (XRD) phase-pure metastable t was formed, which may be caused by small grain sizes and/or kinetic limitations. The former phase region transformed into M' and M and the latter to an M' + t and M + t phase region upon annealing to 1300 and 1650 °C, respectively. In addition to M and t, T-YTa(Zr)O₄ phase fractions were observed at room temperature for ZrO₂ contents ≥28 mol% after annealing to 1650 °C. T phase fractions increased during in situ heating XRD at 80 °C. At 1650 °C, a reaction with the α-Al₂O₃ substrate resulted in the formation of AlTaO₄ and an Al-Ta-Y-O compound.