Stabilization of Cubic Zirconia by Aluminum Nitride

Cubic ZrO₂ is stabilized at room temperature by the addition of AIN. The percentage of c-ZrO₂ in the mixture of c-ZrO₂ and m-ZrO₂ increases linearly with the addition of up to 20 mol% AIN and decreases thereafter. Stabilization becomes complete at 50 mol% AIN. The lattice spacing of the cubic phase gradually expands up to 20 mol% AIN. A displacement reaction ZrO₂+ AIN → ZrN+Al₂O₃ occurs above 20 mol% AIN and is completed at 50 mol% AIN.     

Stabilization of Zirconia Sintered with Titanium

Electron and X-ray diffraction studies show that zirconia sintered with 5 to 15 mol% titanium under a vacuum of 10⁻¹ to 10⁻² torr (∼13 to 1.3 Pa) was partially stabilized as cubic and tetragonal phases, whose amounts increase with increasing Ti content. The stabilization of ZrO₂ is due to the dissolution of TiO which forms as a second phase in the sintered specimen. The grain size of ZrO₂ decreases with increase in Ti. The improvements in strength and thermal shock resistance of ZrO₂ sintered with Ti are attributed to the reduction in ZrO₂ grain size and the effect of partial stabilization.     

Structural Transformation in Cubic Zirconia

The structure of CaO-stabilized cubic zirconia has been investigated by X-ray diffraction at high temperatures and pressures up to 1000°C and ∼35 GPa, using a diamond-anvil cell interfaced with aYAG laser heating system. At ∼1000°C and 15 GPa, the cubic phase transforms directly into an orthorhombic phase with the PbCl₂ structure. This high-pressure phase is quenchable. The zero-pressure lattice parameters are a=0.3327±0.0004 nm, b=0.5566±0.0003 nm, and c =0.6487±0.0005 nm, the volume change being 10.5%.     

Preparation of Fully Cubic Calcium-Stabilized Zirconia With 10 mol% Calcium Oxide Dopant Concentration by Microwave Processing

A novel, fast, and cheaper method has been developed for the synthesis of fully cubic calcium-stabilized zirconia (ZrO₂) of composition Ca_0.1Zr_0.9O_1.9 by dissolution of calcium oxide in monoclinic ZrO₂ for the first time using microwave energy. In this process, the precursors have been prepared by the mixed-oxide method taking the constituents in their stoichiometric ratio. The samples have been allowed to absorb microwave radiation in the presence of a polymeric susceptor. The susceptor absorbs the microwave radiation at room temperature and increases the temperature of the sample, where it starts interacting with microwave radiation. The susceptor burns off at a higher temperature without reacting with the sample. The cubic Ca_0.1Zr_0.9O_1.9 has been prepared at a temperature of 1100°C within 5 min.     

Interface Structure of an Epitaxial Cubic Ceria Film on Cubic Zirconia

A cubic CeO₂ (001) film with a thickness of ∼58 nm was grown epitaxially on Y₂O₃-stablized cubic ZrO₂ by oxygen-plasma-assisted molecular-beam epitaxy (OPA-MBE). The interface was characterized using high-resolution transmission electron microscopy (HRTEM). The interface exhibited coherent regions separated by equally spaced misfit dislocations. When imaged from the [100] direction, the dislocation spacing is 3.3 ± 0.5 nm, which is slightly shorter than the expected value of 4.9 nm calculated from the differences in lattice constants given in the literature, but is fairly consistent with that of 3.9 nm which was calculated using the lattice mismatch measured by electron diffraction. Thus, the results presented here indicate that the lattice mismatch between the film and the substrate is accommodated mainly by interface misfit dislocations above some critical thickness.     

Observations on the Stabilization of Zirconia

The characteristics of magnesia-, calcia-, and ceria-stabilized zirconia are described. They were determined by two techniques: (1) the National Bureau of Standards high-temperature X-ray analysis of sintered specimens and (2) room-temperature X-ray analysis of specimens heat-treated at 1180°C. up to 100 hours. Also reported is an investigation of the ternary system Ti-Zr-O, with particular emphasis on the binary systems TiO-ZrO₂ and Ti-ZrO₂. Data are presented on the stability of the compound ZrTiO₄ and on the inability to confirm the existence of zirconium suboxides.     

Xenon Emission Accompanying Fracture of Xenon-Implanted Cubic Zirconia

The emission of xenon following the fracture of xenon-implanted cubic zirconia has been studied using mass spec-trometry. All samples showed intense Xe bursts at failure. Order of magnitude estimates of the amount of Xe released suggest that micrometer-scale regions of the tensile surface on either side of the fracture surface of these samples do not show sufficient damage to account for this emission. However, SEM micrographs of the fracture surface show evidence for extensive microcracking immediately adjacent to the tensile surface. It is believed that these microcracks are formed when the advancing crack encounters the tensile stresses immediately below the Xe-implanted surface layer and disrupts the Xe inclusions produced by implantation. Some samples also show Xe bursts prior to failure; SEM observations of these samples show shallow conchoidal cracks on the tensile surface, which apparently form during loading and would account for the release of Xe prior to failure.     

Fabrication of Lanthanum Manganese Oxide Thin Films on Yttria-Stabilized Zirconia Substrates by a Chemically Modified Alkoxide Method

Perovskite-type thin films of lanthanum manganese oxide (LaMnO₃) were prepared on yttria (8%) stabilized zirconia substrate by the sol–gel process from an alkoxide solution of lanthanum isopropoxide (La(O- i -C₃H₇)₃) and manganese isopropoxide (Mn(O- i -C₃H₇)₂). The alkoxide solution was chelated with 2-ethyacetoacetate, and further modified with polyethylene glycol (PEG). The obtained LaMnO₃ thin film was transparent and macroscopically crackless. X-ray diffraction, differential thermal analysis–thermogravimetry analysis, and scanning electron microscope observations indicated that single-phase LaMnO₃ thin films with a grain size of 80 to 100 nm are formed when a spin-coated LaMnO₃ gelled film is heated at 600°C for 1 h. The porous and homogeneous grain structure with a grain size of <100 nm can be obtained when the LaMnO₃ gelled film is heated at 600° and 800°C. It was considered that PEG might accelerate the crystallization of the perovskite phase, which indicates that PEG assists the formation of the La-O-Mn frame network during partial hydrolysis and condensation reactions in sol–gel processes.     

Electrical Behavior of Ceria-Stabilized Zirconia with Rare-Earth Oxide Additives

The electrical properties of rare-earth oxide doped CeO₂-ZrO₂ ceramics are evaluated by impedance spectroscopy. Doping of the trivalent metal oxide decreases both the grain and grain-boundary resistivities of the CeO₂-ZrO₂ system. The grain and grain-boundary resistivities as well as the activation energy for conduction increase with the dopant cation radius. The grain conductivity is fairly constant for 2 mol% YO_1.5-10 mol% CeO₂-88 mol% ZrO₂ samples sintered under different conditions, while lower grain-boundary resistivity is obtained for samples sintered at higher temperature for longer times. Possible mechanisms for the electrical behavior of the rare-earth oxide doped CeO₂-ZrO₂ ceramics are proposed and discussed.     

Indentation Creep in Single-Crystal Cubic Zirconia at Room Temperature

Indentation creep of yttria- and calcia-fully-stabilized single-crystal zirconia has been observed at room temperature. The Knoop hardnesses decreased by 15% and 12% of their conventional short-time values, respectively, for indentation times of 100000 s.     

Partial Stabilization of Tetragonal Zirconia in Oxynitride Glass-Ceramics

Nitrogen (via a polymeric AlN precursor) and ZrO₂ are introduced into a MgO-CaO-Al₂O₃-SiO₂-glass. Subsequent crystallization of the glass results in a fine-grained oxynitride glass-ceramic. The microstructure of the latter is found to be entirely different for nitrogen concentrations of 4.85 and 9.94 mol%. Not only do the phase contents differ, but also tetragonal zirconia is more effectively stabilized at higher nitrogen concentrations. Partial stabilization of tetragonal zirconia is not due to nitrogen incorporation but is based on an indirect effect: the spherical morphology of tetragonal zirconia precipitated at higher nitrogen contents suppresses the nucleation of the martensitic transformation. The beneficial effects of the introduction of nitrogen and the simultaneous incorporation of zirconia into a glass-ceramic result in overall improved mechanical properties.     

Tribology and Hardness of Excimer-Laser-Processed Titanium Layers on Cubic Zirconia

We have examined the wear and friction and surface hardness of excimer-laser-processed Ti layers on cubic zirconia substrates. The film exhibits a complex array of cracking following processing that is related to the crystallographic orientation of the substrate. The friction between the laser-processed layers and both steel and ruby pins is reduced by approximately one-third relative to that of untreated zirconia. In the untreated case, wear is characterized by pin wear and debris, whereas the laser-processed layer wears by film transfer to the pin. The surface hardness of the processed layer is lower than that of both the untreated zirconia and the deposited Ti film. Indentation tests indicate that the surface is brittle following processing.     

Electro‐Sintering of Yttria‐Stabilized Cubic Zirconia

Electro‐sintering, i.e., electrically enhanced densification without the assistance of Joule heating, has been observed in 70% dense 8 mol% Y₂O₃‐stabilized ZrO₂ ceramics at temperatures well below those for conventional sintering. Remarkably, full density can be obtained without grain growth under a wide range of conditions, including those standard for solid oxide fuel cell (SOFS) and solid oxide electrolysis cell (SOEC), such as 840°C with 0.15 A/cm². Microstructure evidence and scaling analysis suggest that electro‐sintering is aided by electro‐migration of pores, made possible by surface flow of cations across the pore meeting lattice/grain‐boundary counter flow of O^2?. This allows pore removal from the anode/air interface and densification at unprecedentedly low temperatures. Shrinkage cracking caused by electro‐sintering of residual pores is envisioned as a potential damage mechanism in SOFC/SOEC 8YSZ membranes.