V. P. Red’ko

Al2O3–HfO2–Y2O3 phase diagram. I. Isothermal sections at 1250 and 1650°C

The isothermal sections of the Al2O3–HfO2–Gd2O3 phase diagram at 1250 and 1650°C are constructed for the first time and phase equilibria at these temperatures are established. No ternary compounds or appreciable solid solution regions based on components or binary compounds are found in the ternary system. Interaction in the system is determined by the most thermodynamically stable compound, HfO2, which equilibrates with all phases in the system. In the region with Gd2O3 content up to ~65%, the sections are similar, only width of the regions changes. This is connected with changes in the extension of M and F solid solutions in the HfO2–Gd2O3 binary bounding system. The presence of AL + F, GA + GH2, and G2A + F two-phase regions at the isothermal sections suggests that there are triangulating sections of the Al2O3–HfO2–Gd2O3 system in them. Since the F and GH2 phases are of variable composition, these sections can be qualified as conditionally quasibinary. In wide three-phase regions, like in the Al2O3–ZrO2–Gd2O3 system, ternary eutectic points are expected to exist.

Synthesis and properties of nanocrystalline 90 wt % ZrO2〈Y2O3, CeO2〉-10 wt % Al2O3 powder

Using hydrothermal treatment of coprecipitated hydroxides, we have prepared nanocrystalline ZrO2-rich ZrO2-Y2O3-CeO2-Al2O3 powder. The effect of heat treatment on the properties of the powder has been studied in the temperature range 400–1300°C. The powder has been shown to have a metastable phase composition, which is attributable to structural and size factors and also to the fact that the ZrO2 and Al2O3 crystallites inhibit the growth of each other. Sintering the powder under various conditions, we have obtained ceramics with fracture toughnesses from 6.4 to 16.8 MPa m1/2.

Microstructural Design of Bioinert Composites in the ZrO2–Y2O3–CeO2–Al2O3–CoO System

It is shown that a scientifically sound approach to each stage of producing ZrO2-based bioinert implants (from the synthesis of starting powders to their sintering) is a necessary condition for promoting the optimum structure and high mechanical properties. Conditions for producing bioinert implants with regular, laminar, and highly porous microstructures are found. The research results serve as a scientific basis for microstructural design of various bioinert implants in the ZrO2–Y2O3–CeO2–Al2O3-CoO system.

Effect of Al2O3 on the properties of nanocrystalline ZrO2 + 3 mol % Y2O3 powder

We have studied the properties of nanocrystalline ZrO2〈3 mol % Y2O3〉 and 90 wt % ZrO2〈3 mol % Y2O3〉-10 wt % Al2O3 powders prepared via hydrothermal treatment of coprecipitated hydroxides at 210°C. The results demonstrate that Al2O3 doping raises the phase transition temperatures of the metastable low-temperature ZrO2 polymorphs and that the structural transformations of the ZrO2 and Al2O3 in the doped material inhibit each other.

Isothermal sections of the Al2O3–HfO2–Er2O3 phase diagram at 1250 and 1600°C

The isothermal sections at 1250 and 1600°C for the Al2O3–HfO2–Er2O3 phase diagram are constructed for the first time. Phase equilibria are established at these temperatures; they are determined by the most thermodynamically stable compound, HfO2. No ternary compounds or appreciable solid-solution regions based on components or binary compounds are found in the ternary system. The presence of AL + F, Er3A5 + F, ErA + F, and Er2A + F two-phase regions on the isothermal section at 1650°C suggests that they contain triangulating sections of the Al2O3–HfO2–Er2O3 ternary system.

Effect of Coo Microadditive on the Properties of ZrO2–Y2O3–CeO2–Al2O3 Nanocrystalline Powder

The changes in the physical and chemical properties of ZrO2–Y2O3–CeO2–Al2O3 nanocrystalline powder with 0.2 wt.% CoO microadditive during thermal processing in the 400–1300°C temperature range are investigated. It is shown that CoO microadditive reduces the specific surface area of the powder and significantly affects the temperature range of the F-ZrO2 → T-ZrO2 phase transformation. The morphology changes topologically continuously. The phase transformation in the ZrO2–Y2O3–CeO2–Al2O3 system occurs in the 850–1150°C temperature range. In the presence of CoO microadditive, this range is 700–850°C.

Phase Diagrams of Refractory Oxide Systems and Microstructural Design of Materials

It is shown that the phase diagrams of refractory oxide systems based on ZrO2, HfO2, Al2O3, and rare earth oxides underlie the microstructural design of various high-performance materials. Process steps to produce coarse-grained ceramics in the HfO2–ZrO2–Y2O3, ZrO2–Y2O3–Sc2O3, HfO2–ZrO2–Sc2O3, Y2O3–Er2O3, Y2O3–ZrO2, Y2O3–HfO2, Y2O3–Al2O3, Y2O3–SiO2, and Y2O3–La2O3 systems to perform at temperatures up to 2200°C are designed. Process steps to produce high-performance fine-grained composites in the HfO2–ZrO2–Y2O3 (Ln2O3) (Ln–Dy, Ho, Er, Tm, Yb), ZrO2–Y2O3–Sc2O3, ZrO2–Y2O3–Sc2O3, Al2O3–Zr(Hf)O2–Ln(Y)2O3 (Ln–La, Nd, Sm, Gd, Er, Yb), and ZrO2–Y2O3–CeO2–Al2O3 systems are designed as well.

The Gd2Zr2O7-Based Materials for Thermal Barrier Coatings

Zirconates of rare earth elements with a pyrochlore-type structure, as a class of ceramics with low thermal conductivity, are among the most promising materials for a ceramic layer in thermal barrier coatings (TBCs) for gas turbine engines with operating temperatures above 1200°C. The paper presents an overview of studies focusing on the development of the upper Gd2Zr2O7 TBC layer. The microstructural design of these materials is based on the ZrO2–Gd2O3–Al2O3 phase diagram. Methods for increasing the fracture toughness of Gd2Zr2O7 materials and preventing the interaction of Gd2Zr2O7 and Al2O3 formed in TBC operation are presented. The features of multilayer and functional gradient coatings are addressed. Complex improvement of the composition, architecture, and deposition of the coatings ensures the properties required for long-term operation of Gd2Zr2O7 TBCs.

Isothermal Section of the Al 2 O 3 –TiO 2 –Er 2 O 3 Phase Diagram at 1400°C

The nature of phase equilibria in the Al2O3–TiO2–Er2O3 system at 1400°C is established and shown in the isothermal section of the phase diagram at this temperature. The interaction in the system is determined by compound Er2Ti2O7, which participates in equilibria with most phases of the system and determines its triangulation. No new phases or appreciable solubility regions based on components and binary compounds are found in the system. Ternary eutectic points are expected in the three-phase regions and binary eutectic points in the quasibinary sections.

Isothermal Sections of the Al2O3–TiO2–Y2O3 Phase Diagram at 1550 and 1400°C

The isothermal sections of the Al2O3–TiO2–Y2O3 phase diagram are constructed for the first time at 1550 and 1400°C. New phases or appreciable homogeneity ranges based on components and binary compounds are not found in the ternary system. Triangulation of the system is determined by the Y2T2O7 phase, which is in equilibrium with compounds Al2TiO5, Y3Al5O12, YAlO3, and Y4Al2O9 and components TiO2 and Al2O3. The system is triangulated into six secondary triangles, in which three-phase eutectics are expected to exist. In five quasibinary sections, two-phase eutectics are likely to exist.