Structural, mechanical and biocompatibility studies of hydroxyapatite-derived composites toughened by zirconia addition

Hydroxyapatite(OHAp)-based ceramic composites with added ZrO2 have been prepared both by sintering at 1400 °C and by hot isostatic pressing (HIP) at 1450 °C and 140 MPa pressure (argon atmosphere). The development of the crystalline phases and the microstructure of the composites have been examined using X-ray diffraction, electron microscopy, infrared and magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopic techniques. The fracture toughness and biocompatibility of the composites have also been studied. The effect of the addition of CeO2- and Y2O3-stabilized ZrO2 and of simple monoclinic ZrO2 to the initial physical mixture, on the structure and properties of the resulting composites has been investigated. In most of the sintered or HIP samples, the OHAp decomposes into tricalcium phosphate (β-TCP). CaO, which forms as a product of decomposition, dissolves completely in ZrO2 and stabilizes the latter in its cubic/tetragonal phase. Presence of the β-TCP phase in the product seems to be the result of a structural synergistic effect of hexagonal OHAp. Two structurally distinct orthophosphate groups have been identified in the composites by MASNMR of 31P and attributed to decomposition products of OHAp at higher temperatures. The composites possess high KIC values (2–3 times higher than that of pure OHAp). Decomposition of hydroxyapatite gives rise to differences in microstructure between HIP and simply sintered composites although fracture toughness values are similar in magnitude indicating the presence of several toughening mechanisms. The in vitro SP2-O cell test suggests that these composites possess good biocompatibility. The combination of good biocompatibility, desirable microstructure and easy availability of initial reactants indicates that the simply sintered composite of OHAp and monoclinic ZrO2(ZAP-30) appears to be the most suitable for prosthetic applications.

Creep in TiO2 doped Zirconia: Implications for high strain rate superplasticity

In ceramics, dopants offer the possibility of higher creep rates by enhancing diffusion. The present study examines the potential for high strain rate superplasticity in a TiO2 doped zirconia, by conducting creep experiments together with microstructural characterization. It is shown that both pure and doped zirconia exhibit transitions in creep behaviour from Coble diffusion creep with n similar to 1 to an interface controlled process with n similar to 2. Doping with TiO2 enhances the creep rate by over an order of magnitude. There is evidence of substantial grain boundary sliding, consistent with diffusion creep.

Influence of grain boundary sliding on diffusion in yttria doped zirconia

Grain boundary sliding during high temperature deformation can lead to stress concentrations and an enhancement of diffusion in mobile boundaries. Experiments were conducted on a fine grained 3 mol% yttria stabilized tetragonal zirconia, under conditions associated with superplastic flow involving grain boundary sliding. Tracer diffusion studies under creep conditions and without load indicate that there is no enhancement in either the lattice or grain boundary diffusivities. The experimental creep data are consistent with an interface controlled diffusion creep mechanism. (C) 2011 Elsevier Ltd. All rights reserved.

Synthesis and properties of zirconia thin films

Thin films of zirconia have been synthesized using reactive DC magnetron sputtering. It has been found that films with good optical constants, high refractive index (1.9 at 600 nm) and low extinction coefficient can be prepared al ambient temperatures. The optical constants and band gnp and hence the composition nle dependent on the deposition parameters such as target power, rate of deposition and oxygen background pressure. Thermal annealing of the films revealed that tile films showed optical and crystalline inhomogeneity and also large variations in optical constants.

The mechanical response of tetragonal zirconia polycrystal to conical indentation

Blocks of 3Y-TZP were indented with conical diamond indenters. indentation caused tetragonal to monoclinic phase transformation in a subsurface. Of the cracks generated in the subsurface, radial and lateral cracks can be accounted for by a continuum model of the indented subsurface, built using a combination of the Boussinesq and blister stress fields. Additional ring, median and cone cracks were also observed. It is hypothesized that the latter are motivated by the reduction in blister strength or residual energy brought about by the material damage caused by the phase transformation. This damage reduces the load bearing capacity of the material progressively with increasing normal load.

Rapid Synthesis of Ultrahigh Adsorption Capacity Zirconia by a Solution Combustion Technique

Tetragonal ZrO2 was synthesized by the solution combustion technique using glycine as the fuel. The compound was characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy, and BET surface area analysis. The ability of this compound to adsorb dyes was investigated, and the compound had a higher adsorption capacity than commercially activated carbon. Infrared spectroscopic observations were used to determine the various interactions and the groups responsible for the adsorption activity of the compound. The effects of the initial concentration of the dye, temperature, adsorbent concentration, and pH of the solution were studied. The kinetics of adsorption was described as a first-order process, and the relative magnitudes of internal and external mass transfer processes were determined. The equilibrium adsorption was also determined and modeled by a composite Langmuir-Freundlich isotherm.

Mullite-zirconia composites

Mullite-zirconia composite powders were prepared by the combustion of an aqueous heterogeneous redox mixture consisting of Al(NO3)(3), Zr(NO3)(4)/ZrO(NO3)(2), silica fume and urea/diformyl hydrazine at 500 degrees C. X-ray diffraction data showed that a large amount of tetragonal zirconia existed in the composite powders in spite of high temperature calcination. Milled composite powders showed enhanced densification compared to the unmilled powders and the microstructure of the sintered (1600 degrees C) compacts showed the presence of spherical zirconia grains in intergranular positions along with elongated mullite grains.

Surface damage of yttria-tetragonal zirconia polycrystals and magnesia-partially-stabilized zirconia in single-point abrasion

Commercially available 3Y-TZP and Mg-PSZ flats mere abraded by a 150 degrees diamond cone at -196 degrees, 25 degrees, 200 degrees, and 400 degrees C. The coefficient of friction, the track width, and the morphological features of the track were recorded. Raman spectroscopy mas used to record the tetragonal-to-monoclinic phase transformation (t --> m) as a function of distance away from the track. The study was undertaken to establish the influence of tangential traction on phase transformation and surface damage.

Combustion synthesis and properties of nanostructured ceria-zirconia solid solutions

Nanostructured ceria-zirconia solid solutions (Ce1 − xZrxO2, X = 0 to 0.9) have been synthesized by a single step solution combustion process using cerous nitrate, zirconyl nitrate and oxalyl dihydrazide (ODH) / carbohydrazide (CH). The as-synthesized powders show extensive XRD line broadening and the crystallite sizes calculated from the XRD line broadening are in the nanometer range (6–11 nm). The combustion derived ceria zirconia solid solutions have high surface area in the range of 36–120 m2/g. Calcination of Ce1 −xZrxO2 at 1350 °C showed three distinct solid solution regions: single phase cubic (x ≤ 0.2), biphasic cubic-tetragonal (0.2 < x Image .8) and tetragonal (x > 0.8). When x ≥ 0.9, the metastable tetragonal phase formed transforms to monoclinic phase on cooling after calcination above 1100 °C. The homogeneity of Ce1 − xZrxO2 has been confirmed by EDAX analysis. The Temperature Programmed Reduction (TPR) measurement of Ce0.5Zr0.5O2 was carried out with H2 and the TPR profile showed two water formation peaks corresponding to the utilization of surface and bulk oxygen.

Nucleation and growth characteristics of cavities during the early stages of tensile creep deformation in a superplastic zirconia-20 wt% alumina composite

Constant-stress tensile creep experiments on a superplastic 3-mol%-yttria-stabilized tetragonal zirconia composite with 20 wt% alumina revealed that cavities nucleate relatively early during tensile deformation. The number of cavities nucleated increases with increasing imposed stress. The cavities nucleate at triple points associated largely with an alumina grain, and then grow rapidly in a cracklike manner to attain dimensions on the order of the grain facet size. It is suggested that coarser-grained superplastic ceramics exhibit lower ductility due to the ease in formation of such grain boundary facet-cracks and their interlinkage to form a macroscopic crack of critical dimensions.

Amorphous, Monoclinic, and Tetragonal Porous Zirconia Through a Controlled Self-Sustained Combustion Route

Porous, large surface area, metastable zirconias, are of importance to catalytic, electrochemical, biological, and thermal insulation applications. Combustion synthesis is a very commonly used method for producing such zirconias. However, its rapid nature makes control difficult. A simple modification has been made to traditional solution combustion synthesis to address this problem. It involves the addition of starch to yield a starting mixture with a ``dough-like'' consistency. Just 5 wt% starch is seen to significantly alter the combustion characteristics of the ``dough.'' In particular, it helps to achieve better control over reaction zone temperature that is significantly lower than the one calculated by the adiabatic approximation typically used in self-propagating high-temperature synthesis. The effect of such control is demonstrated by the ability to tune dough composition to yield zirconias with different phase compositions from the relatively elusive ``amorphous'' to monoclinic (> 30 nm grain size) and tetragonal pure zirconia (< 30 nm grain size). The nature of this amorphous phase has been investigated using infrared spectroscopy. Starch content also helps tailor porosity in the final product. Zirconias with an average pore size of about 50 mu m and specific surface area as large as 110 m2/g have been obtained.

Zirconia nanotubes

Hollow nanotubes of zirconia as well as of yttria-stabilized zirconia are successfully prepared by first coating the carbon nanotubes appropriately with the oxidic material and then burning off the carbon of the template.

Analysis of deformation mechanisms in superplastic yttria-stabilized tetragonal zirconia

There have been extensive experimental observations of changes in the apparent rate controlling creep parameters in studies on superplastic materials. The three most common explanations associated with these changes in the stress exponent, n, the activation energy Q and the inverse grain size exponent, p involve the effect of concurrent grain growth, the operation of a threshold stress or transitions in creep mechanisms. Each of these factors may influence experimental creep data in a similar manner. Therefore, a careful analysis of the consequences of all three factors must involve the development of a consistent set of experimental observations in order to adequately distinguish the effects of each. This paper discusses the role of concurrent grain growth, a threshold stress and transitions in creep mechanisms in superplastic materials. Specific attention is given to the analysis of data on superplastic yttria-stabilized zirconia ceramics for which an increase in n has been observed at low applied stresses. It is demonstrated that neither concurrent grain growth nor a threshold stress can account for all the relevant experimental observations in this material. It is concluded that the changes in rate controlling creep parameters are associated with the operation of two distinct sequential mechanisms as part of a grain boundary sliding process.