A theoretical model for relating the interface viscosity to the creep behavior of highly refractory ceramics is proposed. Experimental confirmation of the model is given by monitoring the effect of systematic modifications in the interface viscosity on the creep behavior of Si₃N₄/SiC composites. A model material was selected for this basic study with a continuous film, consisting of glassy SiO₂, at the interface, whose bulk anion composition was varied by incorporating increasing amounts of glass-network modifier ions (i.e., F or Cl anions). Internal friction measurements allowed to experimentally evaluate the micromechanical response of the internal interfaces, when subjected to an externally applied shear stress. Starting from this important experimental information, the short-term macroscopic creep behavior of the polycrystal could be predicted and consistently related to microscopic interface viscosity parameter.

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Aluminum nitride (AlN) raw powder prepared by direct nitridation of metallic aluminum powder was ground by wet ball-milling in order to obtain submicron fine powder for low-temperature sintering. The effects on the properties of the milled powders of milling conditions such as ball size, ball material, chemical species of the grinding fluid and water content of the grinding fluids were investigated. A larger grinding rate was observed using medium balls with diameters of less than 5mmφ and grinding fluids with higher dipole moments such as 2-propanol and acetone. It was also indicated that SiAlON balls showed the least grinding wear, and that an increase in the water content of the grinding fluid increased the oxygen content of the milled powder. Fine AlN powder with submicron size and sharp particle size distribution was obtained by adjusting the milling conditions. This powder showed better sinterability and higher thermal conductivity than that obtained by the usual dry grinding method.

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Titanium nitride (TiN) whiskers were formed on the outer surface of the graphite tube by heating sodium fluorotitanate (IV) (Na₂TiF₆) at 1300°C or higher in a stream of nitrogen gas. The shape of whiskers was observed by SEM. The cross section of mostly whiskers was square. It was assumed that these whiskers grew in the ‹100› direction by Vapor-Solid (VS) mechanism. The lattice constant of TiN obtained in this work was determined by XRD as 0.4243-0.4246nm, which was larger than 0.4242nm for that of stoichiometric TiN. This fact showed that TiN obtained in this study contained a trace of carbon. A thermodynamic interpretation of the formation of TiN is considered. It is found that the existence of C, NaF and Al₂O₃ in a stream of N₂ gas is necessary for the formation of TiN from Na₂TiF₆.

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Oxidation behavior of high-purity Si-SiC composite (molar ratio 40/60) was studied during reactions at 1600K for 15h in Ar-O₂ (P_O2: 0.02-97kPa) atmospheres to evaluate the oxidation-resistant property. The oxidation rate was determined by measuring CO and CO₂ concentrations during the oxidation using a quadrupole mass spectrometer. The oxidation kinetics obeyed the linear-parabolic law at high oxygen partial pressures (97kPa) and obeyed the linear law at low oxygen partial pressures (0.02kPa). The former case accompanied the weight gain, while the latter the weight loss. The oxygen partial pressure causing the transition from passive to active oxidation was estimated to lie between 0.1kPa and 0.02kPa. In passive oxidation, apparent low oxidizability of Si as compared with that of SiC was considered to result from the conversion of Si in the surface layer of the sample into SiC caused by its reaction with C released from SiC. Cooperative actions between Si and SiC were discussed in connection with the oxidation resistance of Si-SiC composite.

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(1) The oxycarbonitride glass can be prepared by dissolving SiC in the liquid phase of the Yb₂O₃-AlN-SiO₂ system. The solubility of SiC in the liquid with (Yb₂O₃)₂₅(AlN)₂₅(SiO₂)₅₀ composition is about 2mol% at 1550°C.
(2) The glass transition temperature, the softening temperature and the Vickers hardness of the Yb-Si-Al-O-C-N oxycarbonitride glasses tend to rise with increasing content of SiC.
(3) The incorporation of carbon atoms in oxynitride glass promotes the crystallization of glass and improves the oxidation resistance of glass.
(4) The liquid-phase sintered SiC can be obtained by using the sintering aids such as Ln₂O₃+AlN+SiO₂ (Ln=rare-earth element).

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The phase diagram in the system C₃P-CMS₂-MS-S was studied because it corresponds to a composition region for manufacturing fused calcium magnesium phosphate fertilizer and for corrosion of refractories. In the quaternary system, phase diagrams of C₃P-CMS₂-S and C₃P-MS were reported already. In this report, the phase relation in the system C₃P-CMS₂-MS and a part of the quarternary system were investigated. The former ternary system has a β-C₃P (whitlockite), diopside solid solution (ss.) and pigeonite-protoenstatite ss. fields covering a wide region of the ternary diagram. In the quarternary system, ternary eutectics among β-C₃P, diopside, tridymite continues from 1277°C in the system C₃P-CMS₂-S to 1245°C in the system C₃PMS-S. This composition of the lowest eutectic temperature corresponds to the fertilizer composition and it gives a suggestion for the manufacture of apatite ceramics as biomaterials. The pigeonite ss. and protoenstatite ss. exist at a higher temperature region in contact with the diopside ss. field, but the phase boundary between pigeonite ss, and protoenstatite ss. has not been clearly determined.

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A composite with the composition of 58 mass% Si₃N₄-32 mass% SiC-10 mass% Y₂O₃ was oxidized at 900-1400°C in dry and moist air atmospheres. The weight gain rate at 1400°C in dry air was as low as 0.5mg/cm² for 100h, revealing that the composite is highly oxidation-resistant. The weight gain in moist air was 3-7 times larger than that in dry air at all the temperatures tested. The weight gain at 1000°C in moist air was almost the same as that at 1400°C in moist air. The mechanism of the abnormal oxidation at 1000°C is not the same but similar to that reported by Lange et al. At 1000°C, a protective SiO₂ layer which retards the diffusion of oxygen into the composite was not formed and the SiC fine particles in the grain boundary region of the composite were oxidized. It was assumed that the volume expansion of about 30% accompanying the conversion of SiC into SiO₂ generated cracks in the grain boundary region through which oxygen can penetrate toward the unoxidized composite. Four Si₃N₄-SiC composites with different compositions also showed large weight gain at 1000°C in moist air. These results indicate that special care should be taken in handling Si₃N₄-SiC composite in moist atmospheres at high temperatures.

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Fine zirconia powders doped with 3 to 6mol% Sc₂O₃ have been prepared by the urea-based homogeneous precipitation method using monoclinic zirconia sols previously formed by the hydrolysis of ZrOCl₂ solution. Highly dense Sc₂O₃-doped tetragonal zirconia ceramics, Sc-TZP (>99%) with a homogeneous microstructure were obtained by sintering these powders at 1300°C or 1400°C for 1h and their phase stability was investigated. The average grain size and the crystal phase of ZrO₂(+Sc₂O₃) materials were observed to depend on the Sc₂O₃ contents. Addition of 3.5mol% Sc₂O₃ was necessary to fabricate zirconia bodies consisting of fully tetragonal phase by sintering at 1300°C. This value corresponds to nealy 1.5 times the content of Y₂O₃ in the ZrO₂-Y₂O₃ system. The average grain size of the 4mol%Sc-TZP sintered at 1400°C was <0.5μm. Although the grain size is depending on the sintering temperature, it increased gradually with increases in Sc₂O₃ content from 3 to 6mol% and in the content of the cubic phase. The monoclinic zirconia contents transformed from the tetragonal phase in the surface of the 4 to 6mol% Sc-TZP under hydrothermal conditions at 180°C for 50h were about 93 to 70%.

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The application of cordierite ceramics as the electronic materials has recently extended immensely, because of their superior heat resistance, low dielectric constants, etc. In this study, the cordierite ceramics, which can be sintered at low temperature without any nucleation agents, were prepared by the sol-gel method. The effect of Al/Si ratio on the crystallization behavior and sintering was investigated. Metal alkoxides, Si(OC₂H₅)₄, Al(OC₄H₉^S)₃, and Mg(OC₂H₅)₂ were hydrolyzed in 2-methoxyethanol, resulting in gelation. The obtained gel powders were heat-treated at various temperatures. As the Al/Si ratio decreased, the deposition of μ-cordierite occurred at lower temperatures. The initial transformation of α-cordierite phase from μ-cordierite phase took place by heat treating at 950°C for 1h. However, the complete transformation to single α-cordierite phase was retarded and achieved at higher temperatures, >1050°C. With the increase of Al/Si ratio, the initial transformation to α-cordierite from μ-cordierite occurred at higher temperatures, while the complete transformation took place rapidly at lower temperatures, about 1000°C.

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BaO-B₂O₃-SiO₂, PbO-SiO₂ and PbO-B₂O₃ glasses were polished using a pitch polisher and cerium oxide dispersed in distilled water. The factors affecting the polishing rate at initial stage were discussed in terms of glass properties: Vickers microhardness, chemical water durability and bulk modulus. The polishing rate of BaO-B₂O₃-SiO₂ glasses depends mainly on the hardness of the hydrated layer formed; this factor is consistent with Izumitani proposal. The polishing rates of PbO-SiO₂ and PbO-B₂O₃ glasses increase with increasing PbO content up to 50mol% PbO and then decrease remarkably. The polishing rates of PbO-SiO₂ and PbO-B₂O₃ glasses are apparently affected by the bulk modules which is related to the product of the ratio of plastic strain to elastic strain (c/a) and the indentation hardness H using Hill's theory on plastic-elastic materials.

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In relation to the joining of silicon nitride ceramics to metal, the reaction products and reaction mechanism between Si₃N₄ and Fe-Cr alloy have been investigated at temperatures from 873 to 1573K under N₂ atmosphere. Using Si₃N₄/Fe-62.5 mass% Cr alloy powder mixtures, the reaction rates were determined by thermogravimetry, and the reaction products were identified by X-ray diffraction. Below 1273K, the mass gain was observed, because chromium in alloy reacted with N₂ gas to produce Cr₂N and CrN. Above 1323K, the initial gain and the subsequent loss in mass were observed. The mass loss resulted from the formation of Fe-Cr-Si solid solution and silicides of Fe or Cr. At higher temperatures and on prolonged heating, the reaction products changed in the following order: Cr₂N, Cr₂N+CrN, Cr₂N, Cr₂N+Fe₃Si, Cr₂N+Fe₃Si+X, Fe₃Si+X, Cr₃Si+Fe₃Si+X, Cr₃Si+X. Unknown compound X, consisting of Fe, Cr, Si and N, was detected as both Cr₂N and Fe₃Si disappeared.

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Effect of dry and wet ball milling of ammonium aluminum carbonate hydroxide (AACH) on the transformation to α-alumina during heating was investigated. The dry grinding causes distortion of the AACH crystals, resulting in considerable reduction in the transformation temperature to α-alumina. This may be due to the easy nucleation of α-alumina during heating. In the heating the AACH ground under wet condition, significant reduction in the transformation temperature was recognized when high purity alumina balls were employed, while no noticeable change in the temperature was observed in case of zirconia balls. This is attributed to seed and dispersion effects of fine α-alumina powder worn from the alumina balls during grinding. α-alumina single phase appears in the AACH heated below 1000°C after grinding with the alumina balls under wet condition. The α-alumina powder formed from the ground and calcined AACH has a relatively large specific surface area and can be sintered at a lower temperature than that for the unground AACH.

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Diamondlike carbon (DLC) thin films were deposited by the plasma CVD method in which radio frequency was powered on the substrate electrode. Deposition temperature was controlled by the pulsed deposition technique (5s deposition and 3min cooling repeatedly). The differences in the effect of the deposition temperature and the annealing temperature as well as the influences of annealing temperature on the properties and structure of the films have been studied carefully. Both internal stress reductions and film hardness increases are simultaneously observed within a certain annealing temperature range (-abt. 300°C). These results suggest that internal local strains are reduced by the annealing procedures and rearrangement of three dimensional network structure progresses to a certain degree, and then the densification of the network structure occurs in the films. With further increase of annealing temperature, network structure cleavages are observed and the DLC thin films are deteriorated exceedingly.

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Silicon-carbide ceramic was metallized with titanium by the vapor-diffusion method, where pure titanium powder was used as a vapor source at temperatures between 973 and 1323K for up to 296.1ks in a dynamic vacuum atmosphere. The structure and composition of the metallized layers were investigated by scanning electron microscopy, electron-probe microanalysis, and X-ray diffraction analysis. At the initial stage of the metallization, titanium vapor reacted preferentially with the so called free carbon in the grain boundaries of SiC ceramics to form a titanium carbide, and then a metallized layer was grown laterally, forming the sublayer structures. This sub-layer structure was composed of five sub-layers; TiC (top surface), Ti₅Si₃ containing TiC (outer layer), TiC containing Ti₅Si₃ (middle layer), Ti₅Si₃ (inner layer) and Ti₃SiC₂ (bottom layer). Growth kinetics of each sub-layer and their total thicknesses obeyed the parabolic rate law, and the parabolic rate constants (kp) for the total layer varied between 10^-16m²·s^-1 at 1173K and 10^-15m²·s^-1 at 1323K. The activation energy (Q_kp) obtained for the metallized layer was 153kJ·mol^-1.

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Stable emulsions of ethyl silicate (ES) were prepared for the use of binders in refractory processing. The rheological characteristics and hardening mechanisms of alumina slurries produced by the use of a new binder system consisting of ES emulsions and water glass were investigated together with the properties of the ES emulsions. The highly dispersed alumina slurries were obtained by the modification of alumina surface to be negatively charged prior to mixing the alumina particles with the binder system in which ES droplets are charged negatively. The alumina slurries having the rheological behavior of rheopexy were found to be advantageous in fluidity and durability for the casting process. Moreover, the slurries cast in a metallic mold were hardened spontaneously by heating up to a certain temperature at which ES emulsions break down, because the gelification proceeds according to the alkali-catalized hydrolysis of ES. Both the resulting green bodies and alumina cores fired at 1000°C proved to have a strength satisfactory in handling and casting, respectively.

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Hollow glass microspheres named Shirasuballoons have been produced from glassy volcanic fragment named as Shirasu or Hakudo. Finer Shirasuballoons are required to increase in physical and chemical properties of conventional Shirasuballoons and glass balloons. Fine Shirasuballoons of mean particle sizes under 20μm and bulk densities under 0.5 (10³kg/m³) were successfully prepared by rapid heating of milled powders using a fluidized bed furnace. Strengths, thermal stabilities and chemical durabilities of fine Shirasuballoons, conventional Shirasuballoons and glass balloons were measured under the same conditions. Strengths and chemical durabilities of fine and conventional Shirasuballoons were obviously superier to these of conventional glass balloons. Higher the particle density of fine Shirasuballoons tended to increase in the strength. Fine and conventional Shirasuballoons held their structure up to 800°C, approximately 200°C higher than the conventional glass balloons.

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The oxidation of Y-α′-SiAlON and α-Si₃N₄ powders was investigated in air in the temperature range 800 to 1000°C. The oxidation rate was evaluated by determining the change in oxygen content of the powders during oxidation. It was found that Y-α′-SiAlON powder was more easily oxidized than the α-Si₃N₄ powder and the oxidation law of these powders was of a parabolic type. Diffusion of oxygen through the oxide layer formed on the specimen surface was thought to be the rate controlling step. The apparent activation energies for the oxidation process were -126kJ/mol and 147kJ/mol for Y-α′-SiAlON powder and α-Si₃N₄ powder, respectively.

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The indentation strength-in-bending (ISB) method can be used to determine conveniently the R-curve in 3mol% Y₂O₃-partially stabilized ZrO₂ ceramics by measuring the failure stress of the indented specimens at a set of load. The present study clearly shows that subcritical or slow crack growth from the indented crack may remarkably be incurred during bend testing even at relatively high cross-head speed (0.1-5mm/min). To minimize the influence of subcritical slow crack growth on the R-curve determination by the ISB method, a cross-head speed higher than 5mm/min is recommended, otherwise a measure should be taken to keep the indentation crack out from moisture.

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The effect of coordination number on ²⁷Al NMR chemical shift was studied using ab initio molecular orbital calculation. In order to mimic Al of four-, five- and six-coordinations Al, the clusters of Al(OH)₄^-, Al(OH)₅^2- and Al(OH)₆^3- were employed for the calculation. The obtained shifts for Al(OH)₄^-, Al(OH)₅^2- and Al(OH)₆^3- were +70.3, +31.5 and -3.9ppm, respectively, and were in reasonable agreement with observed shifts of Al-containing oxides. Present calculation supports the assignment of +30ppm peak observed in aluminosilicate glasses as five-coordination state. Ab initio molecular orbital calculation can be used to establish the correlation between local structure and NMR chemical shift of Al-containing materials.

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A model experiment for examining impact damge dependence on temperature is conducted on Si₃N₄ for turbine use. A zirconia sphere is shot onto a Si₃N₄ disk at RT and at 1200°C, and then surface damage as well as internal damage is examined. The results indicate that a generated crack system changes from a Hertzian cone crack to a median crack in accordance with the experimental temperature change. It is concluded that the hardness degradation at 1200°C generated a change in the stress field from elastic to elastic/plastic, resulting from softening of grain boundary glassy phase.

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C-axis orientation of LiNbO₃ thin films prepared on polycrystalline Si₃N₄ substrates by RF magnetron sputtering was optimized by analysis of the variance. The LiNbO₃ thin film was completely oriented in the c-axis direction. Using an interdigital transducer fabricated on the film, a resonance was observed at a frequency of 800MHz, which showed that the film exhibited piezoelectric properties. It was proved that a c-axis-oriented LiNbO₃ thin film can be used to detect mechanical impact.

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