Stress influence on the deformation mechanisms of tetragonal zirconia polycrystals

The influence of stress on deformation mechanisms of tetragonal zirconia polycrystals (TZP) has been studied by constant strain rate tests and creep tests. It has been shown that the deformation results from two mechanisms: (1) a low-stress mechanism where the strain rate is proportional to σ²/d (where d is grain size) and (2) a high-stress mechanism with strain rate proportional to σ/d³. These results are discussed, taking into consideration the possible role of the vitreous phase on the diffusion mechanisms.     

High temperature creep of ultrafine-grained Fe-doped MgO polycrystals

Creep deformation in ultrafine-grained (0.1 to 1μm) Fe-doped magnesia polycrystals is studied in compression, at temperatures of 700 to 1050° C, and constant loads of 50 to 140 MPa. The stress exponent observed to be nearly unity and the strong grain size sensitivity (ė∼d ^−2.85) suggest that diffusional creep mechanisms dominate the deformation. In the grain size range of the present study the grain boundary diffusion contribution is significantly more important than lattice diffusion. Magnesium is tentatively identified as the rate-controlling species along grain boundaries from an analysis of the diffusivities inferred from the present work and from other authors for Fe-doped magnesia. Associated with the CNRS.     

Investigation and simulation of hot forming of alumina based materials

Hot forming of a MgO-doped alumina and of a alumina-based nanocomposite has been carried out at 1400°C under vacuum. The forming consisted of discs of the materials being punched between graphite parts into hemispheres. The loading data are presented along with a simulation of these data. The MgO-doped alumina disc broke before completion of the test, whereas the alumina based nanocomposite disc was formed into a hemisphere with reduced damage. Calculated loading data fitted monitored loading data satisfactorily. Microstructural investigations were performed on the nanocomposite hemisphere. The microstructural damage consisted of reduced cavitation which occurred essentially in a shallow region of the outer surface of the hemisphere. The reduced damage observed on the nanocomposite microstructure must be associated with the fineness of the microstructure and the reduced alumina grain growth in this material. Considering the severity of this forming process in terms of deformation, this test illustrates very well the superplasticity of the nanocomposite material. These observations confirmed results gathered from compressive testing.     

Grain-Boundary Characterization in a Nonstoichiometric Fine-Grained Magnesium Aluminate Spinel: Effects of Defect Segregation at the Space-Charge Layers

The grain-boundary chemistry of fine-grained spinel MgO· n Al₂O₃ (mean grain size below micron) has been investigated by STEM microanalysis. We have quantified the concentration of each element across the grain boundaries. Stoichiometry variations are observed from the grain-boundary region to the bulk. The Al/Mg ratio increases from 2.1 in the bulk to 2.35 at the grain-boundary regions. X-ray quantification allows us to reveal and to characterize the space-charge layer in the subgrain boundary. The grain-boundary cores are negatively charged due to     vacancies in excess, and in the subgrain-boundary region, an opposite, positive space-charge layer is obtained. The point defect composition and the characteristic (sign, space-charge potential Φ_∞) of the space-charge layer are discussed.     

Microstructure Characterization in Superplastically Deformed Silicon Nitride

Fully dense silicon nitride (Si₃N₄) has been produced by hot isostatically pressing α-Si₃N₄ powder at 1740°C under 160 MPa, with 0.5 wt% Y₂O₃ and 0.5 wt% Al₂O₃ as sintering additives. The sintered material was composed of very fine (0.5 μm) and equiaxed grains, as required for superplasticity. Before deformation, a very small amount of intergranular glassy silicate-based film was detected by transmission electron microscopy at the two-grain and triple-point junctions. Compression tests with a strain value of up to }0.5 were conducted in nitrogen in the temperature range of 1600°–1700°C. The observation of a shear-thickening phenomenon and the presence of a transition from a mild to a strong strain hardening at 20 MPa were attributed to the occurrence of rigid contacts between the grains. The angular distribution of the observed strain whorls was used to evidence the increase of rigid contacts between the grains, under the local expulsion of the wetting liquid film, with increases in compressive stress.     

The effect of TiN and TiC dispersoids on the high-temperature deformation mechanisms of Si3N4

The compression creep properties of pressureless sintered Si₃N₄ matrix, Si₄N₄−(TiN + TiC) and Si₃N₄−N composites, and of a hot pressed Si₃N₄−TiC composite were studied in air between 1260 and 1340°C. Creep characteristics have been compared in relation to microstructure. The addition of soft TiC particles resulted in better mechanical strength, particularly in the creep ductility. The deformation of Si₃N₄---TiC, dominated by a more refractory vitreous phase, was slower than for the other composites. Further, based on a comparison of creep parameters obtained experimentally in this work and on the nature of dispersoids, deformation mechanisms are proposed.     

Preparation of liposomes using the supercritical anti-solvent (SAS) process and comparison with a conventional method

Two methods to produce liposomes encapsulating a fluorescent marker were compared: the supercritical anti-solvent (SAS) method and a conventional one (Bangham). Liposome size and encapsulation efficiency were measured to assess the methods. Micronized lecithin produced by the SAS process was characterized in terms of particle size, morphology and residual solvent content in order to investigate the influence of experimental parameters (pressure, CO₂/solvent molar ratio and solute concentration). It appears that when the lecithin concentration increases from 15 to 25 wt.%, at 9 MPa and 308 K, larger (20-60 μm) and less aggregated lecithin particles are formed. As concerns liposomes formed from SAS processed lecithin, size distribution curves are mainly bimodal, spreading in the range of 0.1-100 μm. Liposome encapsulation efficiencies are including between 10 and 20%. As concerns the Bangham method, more dispersed liposomes were formed; encapsulation efficiencies were about 20%, and problems of reproducibility have been raised.