Ken'ichi Ota

Chemistry and inherent viscosity of glasses segregated at grain boundaries of silicon nitride and silicon carbide ceramics

Abstract Electron microscopy/microanalysis and internal friction characterizations have been concurrently used to quantitatively assess both the morphology/chemistry and the inherent viscosity of residual high-SiO 2 glasses segregated to grain boundaries of polycrystalline Si 3 N 4 and SiC ceramics. The anelastic relaxation peak of internal friction, arising from the viscous slip along grain boundaries wetted by glass, was collected in a plot of internal friction vs temperature. The peak was analyzed with respect to its shift upon changing the oscillation frequency, in order to assess the inherent viscosity of the intergranular glass film. From the internal friction analysis and the precise information of both thickness and chemistry of the grain-boundary film obtained by electron microscopy, a quantitative determination of the inter granular viscosity and the activation energy for viscous flow was attempted. The viscosity of intergranular high-SiO 2 glasses as a function of temperature is compared to the literature data for bulk SiO 2 glass and discussed in terms of the respective anion structure of the glass.

Perovskite-type BaTiO3 ceramics containing particulate SiC

BaTiO3-based composites with nanosized SiC particulates were successfully fabricated by a hot-pressing technique in an argon atmosphere. Crystal structure and phase transformation behaviour were investigated by X-ray diffraction analysis, linear thermal expansion analysis and internal friction measurement. It was confirmed that the added SiC particulates were uniformly distributed within the matrix BaTiO3 grains, with some larger particulates located at the BaTiO3 grain boundaries. In addition, there were no reaction phases between BaTiO3 matrix and SiC particulates. The crystal structure gradually changed from tetragonal to cubic phase with respect to the SiC content. The Curie temperature, Tc, was lowered as the SiC content increased. Moreover, the transformations in the low-temperature range almost disappeared above 1 vol% SiC. The diffused phase transformation phenomenon was observed as the SiC content increased up to 3 vol%. The results were associated with the grain-size reduction, the existence of oxygen vacancies and the residual stresses associated with the thermal expansion mismatch between matrix and SiC particulate. The influence on the domain structure development of SiC particulates dispersed within the matrix grains was also discussed.

Internal friction study of sialon ceramics

Abstract The behaviours of the internal friction and shear modulus for a Si3N4 ceramic sintered with large amounts of Al2O3, Y2O3, and AIN additives to form a β-sialon structure are reported. When subjected to annealing cycles at increasing temperatures, this material showed marked changes in both microstructure and stiffness. The internal friction parameter was found to be phenomenologically very sensitive to such changes. Crystallization processes occurring at the sialon grain boundaries were clearly represented by relaxation peaks in the internal friction curve as a function of temperature. X-ray diffractometry and analytical microscopy techniques such as electron diffraction and energy dispersive X-ray spectroscopy were also employed in parallel experiments for obtaining direct information on the microstructural evolution of the material. Based on the results of these characterizations and on analyses of activation energy from internal friction data, the physical origin of each relaxation peak is disc...

Viscous behavior of interfaces in fluorine-doped si3n4/sic composites

Abstract The influence of fluorine addition on the grain/phase boundary structures and their viscous behavior at high temperature were systematically investigated in Si 3 N 4 /SiC composites. As a reference, a simple system densified by hot isostatic pressing (HIP) and containing only SiO 2 at the boundaries was selected for this basic investigation. In addition, increasing amounts of F dopant were incorporated into the composite bodies by adding Teflon during the mixing procedure of the raw powders and then pre-firing the mixture under high vacuum at 1200°C. Analytical transmission electron microscopy showed that fluorine remained localized at the grain boundary films and triple points, constituting an amount up to a few percent by weight of the intergranular glassy-SiO 2 phase. Detailed structural characterizations of both grain and phase boundaries were performed by using high-resolution electron microscopy (HREM) and atomic force microscopy (AFM). The high-temperature mechanical behavior of the undoped and F-doped SiO 2 phases was characterized by both measurements of torsional creep rate and variation of internal friction at temperatures up to 1600°C. F-doped materials showed creep rates several orders of magnitude higher compared to the undoped sample and damping temperature curves markedly shifted to lower temperature values. According to the above set of microstructural and mechanical data, the inherent viscosity of the SiO 2 intergranular phase could be quantitatively evaluated and the viscous-sliding mechanism under stress modeled.

Viscoelastic sliding and diffusive relaxation along grain boundaries in polycrystalline boron nitride

Dense hexagonal boron nitride (BN) materials were prepared via two different processing routes: (i) hot-pressing with the addition of a Ca/B-containing glass and (ii) chemical vapor deposition (CVD). The resulting microstructure of both materials was studied by scanning and transmission electron microscopy. While the hot-pressed BN material shows, apart from large BN matrix grains, an inhomogeneous distribution of residual glass at room temperature, the CVD deposition yields a homogeneous fine grained microstructure with no amorphous residue detectable. Internal-friction experiments were performed to study the micromechanical response of the materials when exposed to high temperatures. The CVD material revealed no relaxation peak during testing up to 2300°C, while the glass-doped sample showed a pronounced relaxation peak at a peak-top temperature of about 600°C. This temperature corresponds to the softening temperature known for bulk Ca/B-glasses and it is, therefore, concluded that the glass homogeneously wets the BN grains at elevated temperatures. The results presented are seen as the first clear evidence that the internal friction peak monitored for various glass-containing ceramics is indeed related to a viscous sliding process along grain boundaries.

Elastic modulus and the measurement of structural ceramics at cryogenic temperatures

The elastic moduli of the structural ceramics alumina and zirconia were measured in the temperature range 500-50 K by using the resonant frequency of the test specimens. The elastic modulus increased linearly with decreasing temperature in the range 500-200 K, but nonlinear relationships between the elastic modulus and temperature were observed below 200 K.

Internal-friction study of the interstitial–substitutional effect on the deformation behaviour of Nb–O, Nb–Ta–O and Nb–Mo–O single crystals

To investigate the interstitial-substitutional interaction in dislocations, the effect of 0 on the temperature, frequency and amplitude dependence of the internal friction Q −1 in Nb–O, Nb-20 mol% Ta–O and Nb-20 mol% Mo–O single crystals has been studied (f = 1.55–8.2 Hz) in the temperature range from 298 to 1473 K. In our previous study, Nb–Mo and Nb–Ta single crystals were found to be strengthened by solute O. It was also suggested that the interstitial–substitutional interaction in dislocations contributes to the increase in their critical resolved shear stress (CRSS). In this study, Snoek-type relaxation peaks due to O are observed between 500 and 700 K in all the single crystals. The Snoek peak of Nb-20mol% Mo–O consisting of several peaks is analysed. The activation energy of the Snoek peak in Nb-20mol% Mo–O is higher than that of Nb–O. These results are attributable to the existence of the interstitial–substitutional complexes. The amplitude dependence of Q −1 at intermediate and high temperatures decreases as the O content increases. Moreover, the breakaway stress of Nb-20 mol% Mo–O still has a high value at 1200K and does not decrease much even at 1473 K. This suggests that the formation of Mo–O complexes reduces the dislocation mobility at high temperatures. From the results, the effect of the interstitial-substitutional interaction on the CRSS was discussed.

Anelastic grain-boundary relaxation arising from solid solution of chromium in polycrystalline Al2O3

In this paper, the authors have been concerned about Al{sub 2}O{sub 3} polycrystals which, owing to their high purity, were characterized by the presence of direct crystal bonding at the grain boundary. Doping with increasing amount of Cr{sub 2}O{sub 3} made available a series of polycrystalline alloyed structures via substitutional solid solution. In order to single out the anelastic response of the grain boundary in the polycrystalline materials, nominally pure sapphire and two Al{sub 2}O{sub 3} single crystals containing increasing amounts of Cr were also studies.

Effect of interface chemistry on the mechanical properties of Si3N4-matrix composites

The effect of systematic modifications in the chemistry of the phase-boundary film on the macroscopic mechanical properties of Si3N4-matrix composites was investigated. Model composite materials, containing SiC or WC platelets, were prepared and only the bulk anion composition of the glassy-SiO2 intergranular phase was varied by adding increasing amounts of fluorine to the material. Detailed material characterizations by high-resolution electron microscopy (HREM) and Raman spectroscopy on both undoped and F-doped composites allowed to derive a structural model of the phase-boundary film as well as to evaluate the average microscopic stresses acting on it. In addition, high-temperature internal friction measurements provided an estimate of the grain-boundary relaxation temperature as a function of the F content. Noticeable variations of both elastic modulus and fracture energy of the composite were detected upon F addition, which were related to a spontaneous process of phase-boundary microcracking upon cooling. A threshold of the F-content was found for microcrack formation and its existence is theoretically explained according to a percolation process of non-bridged SiO4-tetrahedra, which arises from the incorporation of F into the intergranular film network.