Phase Transformation Of ZrO2 Research Articles

Enhanced phase stability in hydroxylapatite/zirconia composites with hot isostatic pressing

Hydroxylapatite (HA) composites with pure zirconia (ZrO2), and 3 and 8% Y2O3 doped ZrO2 were pressure-less sintered in air and hot isostatically pressed (under 120MPa gas pressure) at 1100°C for 2h. The reactions and phase transformations were monitored by X-ray diffraction, thermal analysis, and Raman spectroscopy. HA/pure ZrO2 composites were not thermally stable in air sintering; HA dissociated into α and β tricalcium phosphate while monoclinic ZrO2 was transformed into tetragonal and cubic phases. No decomposition in HA or phase transformation in ZrO2 were observed in hydroxylapatite/3% Y2O3 doped ZrO2 or HA/8% Y2O3 doped ZrO2 composites. On the other hand, HA and ZrO2 phases in hot isostatically pressed composites remained stable. The highest densification was found in a composite initially containing 10% monoclinic ZrO2 among the composites sintered in air. The densification of the composites decreased at lower sintering temperatures and higher ZrO2 contents upon air-sintering. The HIPped composites were densified to about 99.5% of theoretical densities in all mixing ratios. The reactivity between ZrO2 and HA was dependent on the amount of air in the sintering environment. Hot isostatic pressing with very limited retained air was proved to be a very convenient method to insure both phase stability and full densification during the production of hydroxylapatite zirconia composites.

ChemInform Abstract: Effect of Alumina Addition on the Tetragonal-to-Monoclinic Phase Transformation in Zirconia-3 mol% Yttria.

The tetragonal-to-monoclinic martensitic phase transformation in ZrO–3 mol% Y2O3 (PSZ) containing 0 to 12 wt% Al2O3 was investigated by dilatometry, XRD, and SEM-EDS methods. The propagation of the transformation into the specimen interiors was suppressed by the addition of Al2O3. The grain size was independent of the addition of Al2O3. Both Y2O3 and Al2O3 segregated at grain boundaries. From this segregation behavior, it was suggested that a certain compound or phase of Y2O3–Al2O3 could be formed at grain boundaries, which would presumably prevent the propagation of the transformation into interiors of PSZ-containing Al2O3.

Change of phase compositions in calcia stabilized zirconia ceramics using a boric acid additive

Dense calcia-stabilized zirconia (CSZ) ceramics were prepared by using a small amount of boric acid (H3BO3) as the sintering additive at the sintering temperature of 1700°C. The effect of H3BO3 content on the sintering behavior, the change of phase composition and mechanical properties of the CSZ ceramics were mainly investigated. The results showed that, due to the reaction between B2O3 and CaO, borate liquid was formed during the sintering, which promoted the the densification of CSZ ceramics. Meanwhile, the depletion of the CaO from the lattice of CSZ ceramics resulted in the phase transformation of ZrO2. The content of tetragonal ZrO2 phase increased with increasing the H3BO3 content. So the phase composition of CSZ ceramics could be well controlled by adjusting the amount of H3BO3. The bending strength and fracture toughness of the sintered CSZ ceramics also increased with increasing the H3BO3 content, namely the content of tetragonal ZrO2 phase, which had better mechanical properties, and the highest value reached 267 MPa and 3.35 MPa·m1/2, respectively.

Thermal shock behavior of ZrB 2–SiC ultra-high temperature ceramics with addition of zirconia

Thermal shock behavior of ZrB 2–10 vol%SiC ultra-high temperature ceramics with addition of ZrO 2 was investigated by means of water-quenching as well as the finite element analysis. Optimal residual strength and critical temperature difference were obtained from ZrB 2–10 vol%SiC ceramic added by 10vol%ZrO 2. Simulated results revealed that ZrB 2–10 vol%SiC–10 vol%ZrO 2 provided the lowest thermal stress and stress intensity factor during thermal shock, which confirmed the experimental data. Such improvement in thermal shock resistance compared with ZrB 2–10 vol%SiC was primarily ascribed to the improved toughness by the addition of ZrO 2. Besides, appropriate phase transformation of ZrO 2 during quenching was further beneficial to resist the thermal shock.

A novel development of ZrB 2/ZrO 2 functionally graded ceramics for ultrahigh-temperature application

ZrB2/ZrO2 functionally graded ceramics (FGCs), newly developed for ultrahigh-temperature application, were prepared by spark plasma sintering. Denser graded microstructure as well as a “strong” interface were achieved for ZrB2/ZrO2 FGC sintered at 1800 °C. Due to the phase transformation of ZrO2, the toughness of ZrB2/ZrO2 FGCs was increased from ∼5 MPa m1/2 for the ZrB2-rich layer to ∼10 MPa m1/2 for the ZrO2-rich layer. The effect of sintering temperature on the toughness of ZrB2/ZrO2 FGCs was further investigated.

Effect of microstructure on the fracture behavior of micro–nano ZTA composite

The microstructural effect on the crack propagation was investigated using two kinds of micro–nano ZTA composites made of micro-Al 2O 3 and nano-ZrO 2 particles. The results indicated that with low ZrO 2 content (10 vol.%) in the composite the presence of both large ZrO 2 particles located along Al 2O 3 grain boundary and very fine ZrO 2 particles embedded inside of Al 2O 3 grains was responsible for the appearance of transgranular fracture. Consequently, the composite showed a typical R-curve of rising up more quickly and better toughness than the composite with high ZrO 2 content (20 vol.%). The crack propagation inside of Al 2O 3 grains, depending on the nature of residual stress produced due to the thermal mismatch of Al 2O 3 and ZrO 2 as well as the phase transformation of ZrO 2 from tetragonal to monoclinic, was found to be in the way of either bypassing or splitting ZrO 2 particles.

Fabrication, property characterization and toughening mechanism of HA-ZrO2(CaO)/316L fibre composite biomaterials

HA-ZrO2(CaO)/316L fibre composites were successfully fabricated with vacuum sintering method and their properties and toughening mechanism were studied. The results showed that HA-ZrO2(CaO)/316L fibre biocomposite having 20 vol% fibres had optimal comprehensive properties with bending strength, Young’s modulus, fracture toughness and relative density equal to 140.1 MPa, 117.8 GPa, 5.81 MPa·m1/2 and 87.1%, respectively. The research also addressed that different volume ratios of the composites led to different metallographic microstructures, and that metallographic morphologies change regularly with volume ratios of the composites. 316L fibres were distributed randomly and evenly in the composites and the integration circumstance of the two phases was very well since there were no obvious flaws or pores in the composites. Two toughening mechanisms including ZrO2 phase transformation toughening mechanism and fibre pulling-out toughening mechanism existed in the compsites with the latter being the main toughening mechanism.

Oxidation of ZrB 2 powder in the temperature range of 650–800 °C

The isothermal oxidation of ZrB 2 powder was carried out in the range of 650–800 °C in a flowing air using thermogravimetric analysis (TGA). The evolution of the phase characterization and morphology of ZrB 2 powder oxidized at 700 °C for varying durations was studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The excellent fit of TG curves by multiple-law model suggests that the oxidation of ZrB 2 powder in air follows para-linear kinetics, and based on the fitted results, oxidation mechanisms can also be obtained. The reaction product of ZrB 2 powder with oxygen is metastable tetragonal ZrO 2 at 700 °C, and the tetragonal phase transforms to the monoclinic phase with oxidation. The oxidation of ZrB 2 powder is associated with surface microcrack formation, which is attributed to volume expansion resulting from oxidation of ZrB 2 to tetragonal ZrO 2 and tetragonal-to-monoclinic phase transformation of ZrO 2. In the last stage of oxidation, each ZrB 2 particle breaks into fragments.

Production and characterization of ZrO2 ceramics and composites to be used for hip prosthesis

Tetragonal ZrO2 polycrystalline (TZP) ceramics with varying yttria and ceria content (2–3 mol%) and distribution (coated or co-precipitated), and varying second phase content Al2O3 were prepared and investigated by means of microstructural analysis, mechanical properties, and hydrothermal stability, and ZrO2-based composites with 35–60 vol% of electrical conductive TiN particles were developed. The effects of stabilizer content and means of addition, powder preparation, sintering conditions, and grain size have been systematically investigated. Fully dense Y-TZP ceramics, stabilized with 2–3 mol% Y2O3, 2 wt% Al2O3 can be achieved by hot pressing at 1,450 °C for 1 h. The hydrothermal stability increased with increasing overall yttria content. The jet-milled TiN powder was used to investigate the ZrO2–TiN composites as function of the TiN content. The experimental work revealed that fully dense ZrO2–TiN composites, stabilized with 1.75 mol% Y2O3, 0.75 wt% Al2O3, and a jet-milled TiN content ranging from 35 to 60 vol% could be achieved by hot pressing at 1,550 °C for 1 h. Transformation toughening was found as the primary toughening mechanism. The decreasing hardness and strength could be attributed to an increasing TiN grain size with increasing TiN content, whereas the decreasing toughness might be due to the decreasing contribution of transformation toughening from the tetragonal to monoclinic ZrO2 phase transformation. The E modulus increases linearly with increasing TiN content, whereas the hydrothermal stability increases with addition of TiN content.

First-Principles Calculations on Crystal Structure and Thermodynamic Properties of Ceramics

First-principles calculations have been widely used to describe the ground state properties of materials over almost 20 years. Recently, a great progress was made in the first-principle calculations. Thermodynamic properties can also be gotten by calculations of the phonon densities of states (phonon DOS) and phonon dispersions of materials, which show widely potential applications in material researches. In the present work, the energetics and bonding properties of interfaces between ZrO2 and Ni metal were given by first-principles calculations. The results show that alloy element impurities (Al, Cr and Y) influence remarkably the adhesion of the ceramic and metal. On the other hand, the phonon densities of states and phonon dispersions of ZrO2 were calculated with density functional perturbation theory. From the phonon DOS, the thermodynamic properties were derived and the phase transformation of ZrO2 was discussed. By this method, the thermodynamic properties of material can be gotten from atom and electron levels without any experiment data. It is a new approach to design and study the thermodynamic properties in new material system.

Preparation of Ce-ZrO<sub>2</sub> Based Composites and Effect of Addition of 3Y-ZrO<sub>2</sub> on Their Microstructures

12 mol%CeO2 based composites were fabricated by a pressureless sintering technique and post-HIP treatments. The mixture of 12CeO2-ZrO2/3Y2O3-ZrO2 (= 80 wt%/20 wt%) powders from various methods were prepared by a ball milling technique and consequently sintered at 1400 in air atmosphere and post-HIP treatment at 1350. The 12Ce-ZrO2/Y-ZrO2 composites sintered from mixture of 12CeO2-ZrO2 powder and 3Y2O3-ZrO2 powders possessed the high strength over 1300 MPa and also high fracture toughness over 11 MPam1/2. Furthermore, the significant inhibition of the ZrO2 phase transformation of tetragonal to monoclinic phase under an aqueous hydrothermal condition at 150 was obtained for these 12Ce-ZrO2/Y-ZrO2 composites prepared from this method. Microstructural observation was in detail performed by TEM and EDS.

Preparation and properties of toughened yellow zirconia ceramics

To obtain the high toughness yellow ceramic, using the Y2O3 (5%) stabilized ZrO2 as main starting material, Al2O3 as toughen reagent, NH4VO3, V2O5 as colorant, and mixed with ball mill, molded with dry press method and two times sintered. By testing the properties and structure of the ceramic, we have optimized the best technical conditions, and the mechanism of toughness and color present has been studied. The result shows that the NH4VO3 as colorant is better than V2O5, and the color is between X = 0.396–0.412 and Y = 0.402–0.390. The highest bending strength of toughened ZrO2 ceramics is 1138 MPa, and the fracture toughness is 14.8 MPa m1/2. The toughen mechanism is that phase transformation of ZrO2 and the dispersion of toughen reagents, and the color causing is by the colorful ion (V4+) entered into the zirconia crystals lattice of the ceramic.

Stress driven phase transformation in ZrO 2 film

Zirconia thin layers (250 nm) were deposited on stainless steel substrates using organo-metallic injection chemical vapour deposition (MOCVD) process with zirconium beta-diketonate as precursor at low oxygen pressure and 900 °C. Low roughness zirconia films were made up of a mixture of tetragonal and monoclinic phases depending on the process conditions. As the zirconia tetragonal phase is known to be stabilized by small grain size and/or internal compressive stresses, tensile and/or compressive external stress fields were applied at room temperature using a bending test device. Then, XRD measurements were used to determine tetragonal/monoclinic phase ratio and also residual stresses in the films before and after the tests. The film surface was observed at the various stages of the experiments by field electron gun–scanning electron microscopy (FEG-SEM). Under these stress fields, phase transformation occurs in the film, from tetragonal structure to a monoclinic one. Some preferential tetragonal planes give rise to monoclinic ones. The external stress field is also likely to redistribute the internal stresses within the films.

Combustion Synthesis of Al<sub>2</sub>O<sub>3</sub>-30vol%ZrO<sub>2</sub> Nano-Submicron Structured Multiphase Ceramics

Al2O3-30vol%ZrO2 nano-submicron structured ceramics has been obtained from high-temperature melts produced by SHS metallurgical process, SEM images show that the multiphase ceramics is mainly composed of Al2O3 matrix eutecticums in which ZrO2 fibers are up to nano-submicron scale, resulting in formation of intra-granular structures. Vickers hardness test indicates that critical value of indentation load which induces median crack to extend is 30kg, and morphologies of Vickers indentation indicate that the ceramics is performed with high fracture toughness and high plasticity. Meanwhile, SEM images show that crack extension is mainly controlled by fiber eutecticums of intra-granular structures with nano–submicron ZrO2 phases, XRD demonstrates that toughening mechanism of stress induced ZrO2 phase transformation is considered weak through comparing volume fraction of m-ZrO2 before fracture to the one after fracture, and toughening behavior of multiphase ceramics is mainly controlled by nano-submicron phase toughening mechanism.

Oxidation kinetics of Zircaloy-4 and Zr–1Nb–1Sn–0.1Fe at temperatures of 700–1200 °C

The oxidation characteristics for the Zircaloy-4 and Zr–1.0Nb–1.0Sn–0.1Fe alloys were investigated in the temperature ranges of 700–1200°C for 3600s under steam supply conditions, using a modified thermo-gravimetric analyzer. The oxidation at these temperatures generally complied with the parabolic rate law for the examined duration up to 3600s. However, the parabolic rate was not obeyed in the temperature ranges of 800–1050°C. The oxidation kinetics were changed depending on the oxidation temperatures due to the phase transformations of the base metal and its oxide. The oxidation rate exponents of the Zr–1.0Nb–1.0Sn–0.1Fe alloy at all the temperatures were higher than those of Zircaloy-4. Considering the data controlled by the parabolic rates at 700, 1100, 1150, and 1200°C, the oxidation rate constants were the same slopes as the Baker–Just relationship. The rate transition at 800°C could have resulted from the phase transformation of the base metal and those at 1000 and 1050°C could have resulted from the lateral cracks in the oxide due to the ZrO2 phase transformation from a monoclinic structure to a tetragonal structure.

Monoclinic to cubic phase transformation of ZrO2induced by ball milling

Monoclinic to cubic phase transformation of ZrO₂induced by ball milling

Synthesis of Monolithic ZrO2 in a New Polymorph of Orthorhombic Crystal Structure with a Mesoporous Precursor

A high energy amorphous ZrO(OH)2.xH2O precursor (porous) is obtained by a controlled hydrolysis of dispersed Zr4+ cations in an aqueous solution in reaction with NH4OH at 5°C followed by aging and drying of the recovered gel at room temperature. At 20° to 40°C in air, the excess H20 evaporates leaving behind a porous ZrO(OH)2.xH2O powder with φ ∼ 37% porosity. On heating, a reconstructive ZrO2 phase transformation occurs from a refined amorphous state at 500°C to form a new orthorhombic structure in a mesoporous structure. Average crystallite size is D ∼ 15 nm. X-ray diffractogram determines the lattice parameters of a = 0.3340, b = 0.5535 and c = 0.6364 nm, with V0 = 0.1177 nm3 lattice volume or ρ = 6.96 g.cm−3. A high-pressure phase of Pnma o-ZrO2 is known of a similar value of ρ = 6.78 g.cm−3. Other known c-, t- or m-ZrO2 polymorphs of the cubic, tetragonal, or mono-clinic structures have somewhat lower ρ-values of 6.21 g.cm−3or still smaller. The o-ZrO2 irreversibly transforms to m-ZrO2, D = 23 nm, at as early a temperature as 800°C.

Relationship between Transformation Temperature and Time‐Temperature‐Transformation Curves of Tetragonal‐to‐Monoclinic Martensitic Transformation in Zirconia‐Yttria System

The tetragonal → monoclinic (t→m) martensitic phase transformation in ZrO2–0.5–4 mol% Y2O3 proceeds during isothermal aging at various temperatures from 350 to 800 K; i.e., the transformation increases sigmoidally with aging time. The time‐temperature‐transformation (T‐ T‐ T) curves show a typical C shape, with very high rate for lower Y2O3‐content specimens, and the rate decreases with higher Y2O3 content. The transformation temperature (Ms) of the t→m transformation obtained from dilatation curves during the cooling stage coincides well with C curves above the nose temperature. The t→m transformation should occur isothermally, as suggested by a nucleation and growth mode.

Thermal Stability of High k Layers

ABSTRACTThermal stability of amorphous phases in various high-k layers (Al2O3, ZrO2, HfO2, ZrAlOx, HfAlOx and HfSiOx) and the phase transformation of crystalline ZrO2 and HfO2 were studied experimentally, as functions of surface preparation, deposition conditions, material composition and post deposition thermal treatment. It is found that pure ZrO2 and HfO2 show relatively low crystallization onset temperatures. The crystalline ZrO2 or HfO2 phases are tetragonal or monoclinic, depending on the layer thickness. The phase transformation of metastable t-phase into stable m-phase has been observed in ZrO2 and HfO2. Crystallization behavior of Al2O3 depends on the surface preparation of the substrate. ALCVD grown Al2O3 layers on an oxide-based surface remain amorphous after 1100°C spike annealing, while those on HF-last surface crystallize at temperatures around 800°C. Alloying Al2O3 into ZrO2 and HfO2 can improve their resistance to crystallization under thermal exposure. The kinetics of the crystallization in the alloys can be described by linear TTT curves. Hf-aluminates show better thermal stability than Zr-aluminates. A defect model relative to the phase transformation is discussed, based on the above observations.

Crystallisation and Tetragonal-Monoclinic Transformation in ZrO<sub>2</sub> and HfO<sub>2</sub> Dielectric Thin Films

The crystallisation and the tetragonal-to-monoclinic phase transformation in ZrO2 and HfO2 thin films prepared by atomic layer chemical vapour deposition (ALCVD) are studied using high-temperature grazing-incidence X-ray diffraction (HT-XRD). These films are developed for applications as high-k dielectric gate in CMOS transistors. HT-XRD shows that all the tested samples have a crystallisation onset temperature below 600°C. The crystallisation onset temperature depends not only on the material, but also on the film thickness. The thinner a film is, the higher is the crystallisation onset temperature. For a film with the same thickness, HfO2 crystallises at a higher temperature than ZrO2. The phase composition of the crystallised films depends also on the material and film thickness. In ZrO2, the tetragonal phase is more stable than in HfO2. The t-m transformation during annealing has been observed.