Fracture of metastable tetragonal zirconia crystals

Crystals of single-phased metastable tetragonal zirconia (MTZ) were prepared from skullmelting at the composition ZrO₂-3 mol % M₂O₃ (M = Y, Yb or Gd). The fracture of the tetragonal crystals occurred differently than for the cubic stabilized zirconia ones. The toughness was much higher for the tetragonal specimens and the cleavage planes were not the same as for the fluorite crystals. The results were interpreted by considering the domain microstructure induced by the cubic → tetragonal phase transformation undergone by the crystals.     

Phase-transformation study of metastable tetragonal zirconia powder

The rate of the transformation from the metastable tetragonal to the monoclinic phase of ZrO2 was measured in air from 850°C to 1000°C by neutron diffraction. This rate was found to be temperature dependent, and its measured values were considerably lower then those reported previously. The kinetics of this phase transformation is discussed in terms of a modified crystallite growth-martensitic transformation model' that includes the distribution of crystallite sizes.     

Stabilization of metastable tetragonal zirconia nanocrystallites by surface modification

Metastable tetragonal zirconia nanocrystallites were studied in humid air and in water at room temperature (RT). A stabilizing effect of different surfactants on the tetragonal phase was observed. Furthermore, the phase stability of silanized metastable tetragonal zirconia nanocrystallites was tested by prolonged boiling in water. The samples were analyzed with X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Changes in the monoclinic volume fraction in the samples were calculated. A number of surfactants were screened for their ability to stabilize the tetragonal phase upon exposure to humidity. Only silanes and phosphate esters of these were able to stabilize the tetragonal phase in water. Even as small amounts of silanes as 0.25 silane molecule per nm² are able to stabilize the tetragonal phase in water at RT. Aminopropyl trimethoxy silane and γ-methacryloxypropyl trimethoxy silane were even capable of preventing phase transformation during boiling for 48 h in water.     

Superplastic flow in nanocrystalline and sub-microcrystalline yttria-stabilized tetragonal zirconia

Tensile deformation of nanocrystalline ZrO₂ + 5 mol% Y₂O₃ at temperatures in the range of 1283–1403 K is described. It is demonstrated (a) that steady state flow is possible at temperatures of the order of 0.42 T _m, where T _m is the absolute melting point, (b) that 70% engineering strain could be obtained at 1403 K (0.46 T _m), and (c) that significant grain boundary sliding was present during deformation. Static and dynamic grain growth as also a decrease in the relative density of the specimen with deformation could be observed. The present results as well as those of Owen and Chokshi concerning superplastic flow in sub-microcrystalline materials taken from literature could be accounted for quantitatively using the grain boundary sliding controlled flow model of Padmanabhan and Schlipf, originally proposed for microcrystalline superplastic alloys.     

Effect of stress-induced phase transformation on the properties of polycrystalline zirconia containing metastable tetragonal phase

Polycrystalline zirconia containing a high content of metastable tetragonal phase shows high strength ( 700 MPa), high fracture toughness (K _c = 6 to 9 MN m^–3/2) and small grain size (<0.3jm). The strength and grain size remain nearly constant over a wide range of tetragonal phase content (100 to 30%). At a low concentration of tetragonal phase <30%, there is a rapid decrease in strength accompanied by a rapid increase in grain size. These results are explained by means of a stress-induced phase transformation in the metastable tetragonal phase.     

The role of local structural distortions in the stabilisation of undoped nanocrystalline tetragonal zirconia

Nanopowders of pure or very lightly doped zirconia were studied by means of total scattering and pair distribution function analysis, with the aim of understanding how the size of the particles (if a size limit of about 20 nm is not exceeded) tends to stabilise the tetragonal polymorph at room temperature. Total scattering and PDF analyses, together with Rietveld refinements, showed that the tetragonal model is indeed applicable to the average structure of these nanocrystalline zirconia samples, and provided comparable results. However, all the samples, with no influence from the dopant content, showed a similar local distortion for r < 10 Å. In this region, the data were fitted with two slightly different tetragonal structures, with a tetragonal distortion that decreases with the R_max used, giving rise to a structure closer to the average tetragonal one (given by Rietveld as well as by average PDF refinements). In the first coordination shell, however, an orthorhombic distortion fits very well both in intensity and in position. This distortion may be responsible of the increased RMS strain local level, and therefore of the stabilisation of the high temperature structure at room temperature.     

Synthesis of mesoporous nanocrystalline zirconia with tetragonal crystallite phase by using ethylene diamine as precipitation agent

Mesoporous nanocrystalline zirconia with high surface area and pure tetragonal crystallite phase has been prepared by bifunctional ethylene diamine as both precipitating agent for ZrO(NO₃)₂ to ZrO(OH)₂ and colloidal protecting agent for the ZrO(OH)₂ nanoparticles. The effect of refluxing time and temperature on the structural properties of the zirconia were investigated. The obtained results showed that the increasing in refluxing time and temperature improved the thermal stability and increased the tetragonal content of the zirconia. The results also showed that the addition of Pluronic P123 block copolymer surfactant acted as a cosurfactant and increased the specific surface area of the zirconia.     

Electrical conductivity of tetragonal stabilized zirconia

The electrical conductivity change on annealing for tetragonal stabilized zirconia (TZP) was studied with the help of a.c. impedance dispersion analysis techniques. The dependences of the conductivity on annealing time at 1000 ° C and on temperature cycling between room temperature and 1000 ° C were investigated. A decrease in conductivity of about 30% at 1000 ° C of TZP with 3 mol% Y₂O₃ was observed during the first 200 h of annealing at 1000 ° C, and no change was observed during further annealing. A similar result was observed for TZP with 2.9 mol% Sc₂O₃. For TZP with 3.0mol% Yb₂O₃, the conductivity decreased gradually during an annealing time of over 2000 h. The impedance dispersion analysis at lower temperature suggested that the decrease in electrical conductivity by annealing at 1000 ° C could be attributed to the increases of both grain boundary and intragrain resistance. No monoclinic phase was observed for the samples annealed at 1000 ° C for 2000 h. On the other hand, a trace of a monoclinic phase was found for TZP with 3mol% Y₂O₃ after the 50th temperature cycling, but no significant decrease in conductivity was observed with the cycling.     

Effect of additives on densification and deformation of tetragonal zirconia

The effect of additives (Bi₂O₃, Fe₂O₃) on densification and creep rates of tetragonal ZrO₂-Y₂O₃ has been investigated. In Bi₂O₃-doped Y-TZP, a reactive liquid forms at temperatures above 800–900C, which leads to a strong enhancement of densification for concentrations of 1–2 mol % Bi₂O₃. However, during cooling from the processing temperature a strong, undesirable transformation of the tetragonal to the monoclinic phase occurs. The addition of 0.6–1.2 mol % FeO_3/2 promotes densification without destabilizing the tetragonal phase. A concentration of 1.2 mol %, however, induces discontinuous grain growth, while this is not the case for 0.6 mol %. Creep rates of Y-TZP were enhanced by a factor of 4–6 by adding 0.6 mol % FeO_3/2.     

Synthesis and structural characterization of calcia-stabilized tetragonal zirconia

Synthesis of calcia-stabilized tetragonal zirconia by the reaction of precipitated zirconium hydroxide with calcium oxide at ∼1400 K is described. The unit cell parameters of the oxidea ₀=b ₀=0.3644 nm,c ₀=0.5092 nm andZ=2 are in good agreement with the high-temperature tetragonal crystallographic modification of pure zirconia.     

Stabilization of tetragonal phase in polycrystalline zirconia

It is shown that the tetragonal phase can be stabilized in the sintered body of a partially stabilized zirconia (PSZ) containing low concentrations of yttria. Such sintered body containing the metastable phase undergoes stress-induced phase transformation by the absorption of thermal or mechanical stress and exhibits strengths in excess of 690 MPa (100ksi).     

Characterization and metastability of alkoxy-derived tetragonal zirconia powder

The zirconia powders were synthesized through the hydrolysis of zirconiumn-butoxides with air moisture as well as with aqueous solutions of different pH values to investigate the metastability of tetragonal (t)-ZrO₂. The SEM and TEM observations reveal that powders prepared with air moisture hydrolysis consist of spherical particles of 0.5–3.5 m diameter, while gel-like powders composed of finer particles were obtained with aqueous solution hydrolysis. The samples obtained show different crystallization and tetragonal/monoclinic transformation temperatures on differential thermal analysis. The metastability of t-ZrO₂, investigated in terms of relative content with X-ray diffraction, was not explicable through the crystallite size effect. Instead, the existence of unpaired electrons, detected using electron paramagnetic resonance, and the strain within the powders, were found to be probably influential in affecting the metastability of t-ZrO₂.     

Sintering of ultra-fine tetragonal yttria-stabilized zirconia ceramics

High-frequency induction heated sintering (HFIHS) is utilized to consolidate ultra-fine grain tetragonal zirconia stabilized with 3 mol%Y₂O₃ (3Y-SZ) ceramics. Densification to near theoretical density in a relatively short time can be accomplished using this method. Samples of 3Y-SZ with a relative density of up to 99.5% and an average grain size of about 170 nm could be obtained by sintering at 950 °C for 5 min under a pressure of 100 MPa pressure. The influence of sintering temperature and mechanical pressure on the final density and grain size of the sintered products was investigated. The sintered materials had fracture toughness and hardness values of 4.4 MPa m^1/2 and 10.7 GPa, respectively.     

Borohydride synthesis and stabilization of flake-like tetragonal zirconia nanocrystallites

Flake-like particles of tetragonal zirconia (t-ZrO₂) were synthesized using ZrOCl₂·8H₂O as zirconium precursor, and sodium borohydride (NaBH₄) and cetyltrimethylammonium bromide (CTAB) as precipitating agent and surfactant, respectively. Small-sized nuclei, which formed during the borohydride synthesis, play an important role in the formation of small sized crystallites (5 nm) even at 600 °C and stabilization of t-ZrO₂ in both the samples synthesized with and without surfactant. In the sample synthesized without CTAB, a minor m-ZrO₂ phase (5 vol.%) along with the major t-ZrO₂ phase having a crystallite size of 20 nm, was observed at 700 °C. However, the use of surfactant leads to the formation of stabilized t-ZrO₂ nanoparticles with a smaller crystallite size of 15 nm. SEM micrographs of t-ZrO₂ show elliptical as well as elongated shaped flake-like morphology. The formation of small sized crystallites and slow growth of nucleation embryo play an important role in stabilizing the t-ZrO₂ up to as high temperature as 700 °C.     

Mechanical and magnetic properties of nickel-dispersed tetragonal zirconia nanocomposites

Effects of Ni dispersions on microstructure and mechanical properties have been studied for Y2O3-stabilized tetragonal zirconia (Y-TZP)/Ni nanocomposites with Ni dispersion up to 10 vol%. Composites were successfully fabricated by reducing and hot-pressing Y-TZP/NiO powder mixtures. Fracture strength was significantly improved from 1.5 GPa for monolithic Y-TZP to 1.9 GPa for nanocomposites with a small addition of Ni (1-2 vol%). Magnetic properties of Y-TZP/Ni nanocomposites were also investigated. Magnetization curves of Y-TZP/Ni nanocomposites showed typical hysteresis loops of soft magnetic materials, whereas coercivity was much larger than that of pure Ni metal. A new function arising from magnetomechanical effects of metallic Ni is also discussed for the present nanocomposites.     

Phase formation during sintering of nanocrystalline zirconia/stainless steel functionally graded composite layers

Microstructural development and phase formation at the interface of yttria stabilized zirconia (3Y-TZP)/430L stainless steel composite layers produced by co-sintering method were studied by SEM, HRTEM, micro-focus XRD, and EPMA. Formation of a rich chromium boundary layer at the interface was noticed, which revealed Cr aggregation at the interface at the elevated temperatures. Misfit dislocations were also observed at the joint interface to tackle the mismatch crystallographic orientations between the ceramic and metal layer. The results of the micro-focus XRD showed formation of no new phases at the boundary zone. Microstructural studies also revealed a retarded grain growth in the nanostructured zirconia due to the pinning effect of ultra-fine pores located at the grain-boundary junctions. The shear strength of the ceramic/metal joint was found to depend on the sintering atmosphere and varied in the range of 47-66 MPa.     

Electron microscopy studies of nanocrystalline zirconia

Pure and yttria, calcia doped zirconia powders were prepared in nanostructured form by the method of co-precipitation by hydrolysis. As prepared and heat treated powders were characterized by X-ray diffraction and transmission electron microscopy. Features and variations in crystallite/particle size measurements by both XRD and TEM are discussed.     

Hrtem study of nanocrystalline zirconia powders

Zirconia powders with grain sizes between 4 and 35 nm were synthesized by the gas condensation technique. Different qualities (grain size, grain size distribution) of powders have been observed and can be attributed to different synthesis conditions (mainly temperature gradients) within the UHV chamber. The existence of necks between single particles has been observed. These necks are due to a first sintering process during the oxidation of the metal/ suboxide clusters condensed on the walls in the synthesis chamber. Several stages of this sintering process are documented by high-resolution micrographs. Furthermore, some particles exhibit a shell-like structure with monoclinic ZrO₂ in the outer region and tetragonal ZrO₂ in the inner region. This is an indication of (i) the martensitic phase transition character of zirconia in general, and (ii) the existence of a critical particle size in n-ZrO₂ responsible for the metastability of a tetragonal polymorph in nanosized zirconia powder. Lattice distortions in some directions can be concluded from image reconstruction of the digitized micrograph of a single nanosized ZrO₂ particle.