Ultrafine silicon nitride powders were prepared in a hybrid plasma, which is characterized by the superposition of a radio-frequency plasma and an arc jet. The reactants of SiCl₄ and NH₃ were injected into an arc jet and a tail flame of the hybrid plasma, respectively. The purity of the prepared powder largely depended upon the flow rate of NH₃. Especially, the nitrogen content in the products increased drastically when the flow rate of NH₃ exceeded about 10l/min, and reached the value of about 37wt% at the flow rate of 20l/min. The prepared powder was soft, fluffy, pure white, and completely amorphous. Moreover, the particle size was from 10 to 30nm. For a better understanding of the process, thermodynamic equilibrium compositions for the Ar-H₂-NH₃-SiCl₄ system were calculated up to 3500K. Under the assumed conditions, condensed phase of Si is present at the temperature higher than the condensation temperature of Si₃N₄. A lower flow rate of NH₃ widens the temperature range of the Si existence and promotes the formation of SiCl₂ by recombination processes. These results suggested that the effective “Reactive Quenching” is the key to success for the synthesis of ideal ultrafine Si₃N₄ powder in this process.

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Ultrafine powder of silicon nitride has been synthesized from the system SiCl₄-H₂-NH₃ in a hybrid plasma at the production rate of 100g/h. The powder was white and fluffy. The X-ray diffraction pattern and the infrared absorption spectrum showed that the ultrafine powder was amorphous Si₃N₄. The crystallization temperature was as high as 1500°C which suggests the high purity of the powder. Chemical composition of the powder was Si₃N_3.88O_0.35H_0.19 and the presence of silicon diimide was not detected. The specific surface area of the powder was 60-70m²/g and the particle diameter was expected about 300Å. The powder particles were spherical and the surface was covered with oxidized film of 6-7Å in thickness. It was observed that amorphous Si₃N₄ ultrafine powder was gradually oxidized in air.

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This paper reports Si₃N₄-SiC films prepared by plasma CVD from SiH₄ (90%Ar), NH₃, C₂H₄ and H₂ gas mixtures. Compositions of the films were varied roughly from stoichiometric Si₃N₄ to SiC by changing the flow rate ratio of these gases. Amorphous films with smooth surface were prepared under the pressure of 66.7Pa (0.5Torr) at 400°C and a deposition rate about 0.4nm·sec^-1. Properties of the films such as thickness, composition, refractive index, structure, texture, infrared absorption and visible ultraviolet absorption spectra were evaluated. Nitrogen and carbon contents in the films were found to be proportional to the flow rate ratio of the raw gases. The wave number of infrared absorption peak shifted from 860 (Si-N bond) to 780 (Si-C bond)cm^-1 with increasing carbon content in the films. The optical band gap determined from visible ultraviolet absorption edge shifted from 3.7(Si₃N₄) to 2.4(SiC)eV, also. From above results and TEM observations, it is anticipated that nitrogen and carbon atoms in the films are distributed in an atomic scale in contrast to those in Si₃N₄-C ceramics hitherto prepared by pyrolytic CVD.

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The vapor phase growth of β-sialon whisker was investigated by nitridation of the system SiO₂-C-Na₃AlF₆ in a flow of N₂ gas at 1350° and 1400°C. Whiskers were grown both inside and outside graphite sample cylinder with caps. In the former, α-Si₃N₄ whisker grew mainly, while in the latter, β-sialon (Si_6-zAl_zO_zN_8-z) whisker grew along with a small amount of α-Si₃N₄ whisker. Although the amount of the whiskers formed at the outside increased with an increase in molar ratio Na₃AlF₆/SiO₂, the fraction of β-sialon whisker decreased and that of α-Si₃N₄ whisker increased. The β-sialon/α-Si₃N₄ ratio in the whiskers formed at outside depended also on the reaction temperature and N₂ gas flow rate. Under optimum condition, the whiskers containing about 85% β-sialon, up to 10mm long and 1.0-10μm thick, were obtained in about 20% yield. It was considered that β-sialon whiskers with droplets at the tips grew by the VLS growth mechanism by the precipitation from the supersaturated solution of SiO, CO, AlF₃ and N₂ gas in the droplets. The composition of β-sialon whiskers was estimated to be z≈1.8-2.0 on the basis of XRD data.

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Pressureless sintering of ultrafine powders of β-SiC prepared by a gas evaporation method was carried out in the atmosphere of Ar. When the raw material was sintered without sintering aids, only the grain growth occurred without significant densification. It was proved that the simultaneous addition of boron and carbon is effective for the densification of present ultrafine powder produced by gas evaporation method also. The addition of carbon as a sintering aid is not always necessary, because the raw materials contain a few percent of free carbon. The highest density, 97% theoretical, was obtained with the aids of 1.0wt% boron and 4.3wt% free carbon at 2200°C. In the following two cases, plate-like crystals (6H-type SiC) larger than 50μm grew and the densifi cation was significantly prevented: (1) The raw material including a considerable amount of free silicon was sintered with the aids of boron 1.0wt% above 1900°C. (2) The raw material containing free carbon less than 3.0wt% was sintered with the excess aids of boron more than 1.0wt% above 2100°C.

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The formation of silicon carbide powder by carbonization of iminodisilanenitrile (Si₂N₃H) has been investigated. Submicron SiC powder was produced by heating the mixture of carbon black and amorphous Si₂N₃H for 0.5-4h at 1350°-1650°C under reduced pressure. The crystallinity of the products increased with increasing the reaction temperature and reaction time. The particles were spherical and had maximum size of 0.2-0.4μm in diameter. A reaction process consisting of the formation of Si₃N₄ particles as an intermediate and the subsequent carbonization was proposed for the formation of present SiC particles.

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The montmorillonite-polyacrylonitrile (PAN) intercalation compound was heated in Ar flow for the application of clay-organic complexes to the preparation of carbide through the carbothermal reduction. β-SiC was formed at 1200°C and both α-SiC and β-SiC were detected above 1300°C. SiO₂ component was reduced without forming any crystalline compounds, while Al₂O₃ component formed Mg-Al-O compound. Compared with the reactions of a montmorillonite-carbon mixture, it was clarified that the presence of PAN in the interlayer space led to the formation of α-SiC and suppressed the crystallization of oxides.

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Mg-Sialon oxynitride glasses were prepared from the mixtures of Si₃N₄, AlN, SiO₂, Al₂O₃ and MgO, and the glass forming regions were determined in the system Si, Al, Mg/O, N on cooling from 1550°C. Glasses containing up to 6.5 at% N were obtained. The substitution of nitrogen for oxygen in cordierite-based composition glasses resulted in significant increase of density, glass transition temperature, Vickers hardness and refractive index, and decrease of thermal expansion coefficient. In DTA curves, exothermic peak due to the crystallization of cordierite was observed in the range of 1000°C to 1200°C. The effect of heating time and temperature and Pt as a nucleation agent on the crystallization of the glasses was studied. It was found that optimum temperature to precipitate cordierite crystals was 1200°C and Pt was effective in controlling of the microstructure. The Mg-sialon glasses are considered to be self-nucleating and can, therefore, form finer-grained glass ceramics, compared with the corresponding oxide glasses. A microstructure of cordierite crystals dispersed in a oxynitride glass matrix was observed on the crystallization. Mg-sialon glass ceramics had thermal expansion coefficients comparable to its values of Si₃N₄ ceramics.

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The morphologies and properties of formed carbon are controlled by the carbon-carbon bonding in the starting organic compounds and pyrolysis condition. In this work, a process of carbon formation by the pressure pyrolysis of polystyrene was investigated. Polystyrene can form a graphitizable carbon by normal pressure pyrolysis. Polystyrene thermally decomposed to oligomer which condensed during heating to give large aromatic rings.
During pressure pyrolysis, the liquid-liquid phase separation occurred at 365°C and 25MPa. One liquid consists of higher molecular component, the other lower moleculer component. The carbon which was formed at 600°C changed the morphology toward the sphere with the increase of pressure. At 600°C under 100MPa, the size of spherulitic carbon increased from 0.1μm to 5μm with the increase of pyrolysis time. It was found that the control of liquid-liquid phase separation during pressure pyrolysis of polystyrene affected the morphologies and properties of formed carbon.

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CoB was synthesized by pyrolysis of CoB₁₀H₁₀ at 1250kg/cm², atmospheric pressure and 10^-5 Torr. The characteristic absorption bands at 2500, 1070 and 1015cm^-1 of CoB₁₀H₁₀ disappeared after heat treatment at 400°C and 10^-5Torr for 1h. The formation temperature of CoB by pyrolysis at 1250kg/cm² was 550°C, which was lower by 50°C than those of pyrolysis at atmospheric pressure and 10^-5Torr. The crystallinity of CoB synthesized at 650°C and 1250kg/cm² was comparable with those formed at 800°C and atmospheric pressure. The amount of boric acid as a by-product decreased as the pressure of pyrolysis decreased from 1250kg/cm² to 10^-5Torr.

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Boron-containing ceramics were synthesized by the heat treatment of the condensation product of boric acid and glycerin in N₂ or Ar. The condensation product was a transparent, glassy solid. It was found that the dehydration was completed and the formation of a compound with the structure (C₃H₅O₃B)_n containing B-O-C bonding was revealed. The heat treatment of the condensation product between 900°C and 1250°C yielded boric oxide and amorphous carbon regardless of gas used. When it was heated in N₂ above 1300°C, boron nitride (BN) and boron carbide (B₄C) were detected by XRD and IR analyses. The crystallinity of BN and B₄C increased with the rise of heating temperature. However, heating time affected only the crystallinity of BN. The crystallite size of BN calculated by the Scherrer equation was about 50Å at 1300°C and 100Å at 1400°C. When the condensation product was heated in Ar above 1300°C, the formation of B₄C and graphite was observed. For comparison, a physical mixture of B₂O₃ and carbon black was heated in N₂ at 1400°C for 2h. Though the formation of B₄C was confirmed, its crystallinity was very low and X-ray peaks assigned to BN were not observed. Thus, the reduction and the subsequent carbide formation and nitridation of an organoborate compound were considerably different from those of the physical mixture.

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The crystallization of diamond from graphite was observed using a binary system of solvent-catalysts which comprise a catalytic active metal (Fe, Co or Ni) and a second component (Ti, Cr, Cu or B₄C). Graphite powder mixed with various systems of solvent-catalysts, was pretreated at 1000°C in an argon atmosphere (1×10^-3Torr) for 1h. The mixed powder was subsequently treated under high pressures (6.5-7.5GPa) and high temperature (1700°C) for 15min in a girdle-type high pressure apparatus. The effects of the solvent-catalysts containing B₄C and TiC_x showed different crystallization behavior from other systems. B₄C reacted with the main solvent-catalyst (Fe or Co) to form corresponding metal borides, which led to the extinction of the solvent-catalyst action. On the other hand, the system containing titanium, which forms TiC_x at high pressures and high temperatures, depressed the grain growth of diamond without decreasing the solvent-catalyst action. This effect was found remarkable in the solvent-catalyst of the Fe-Ti system. The system containing chromium and copper showed a little different crystallization behavior from that in the single solvent-catalyst (Fe, Co or Ni) system.

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Phase equilibria in the system Nb-N was investigated at temperatures 1300°-1700°C under P_N2 10^-2-1atm by chemical analysis and X-ray diffraction of quenched sample and TG technique. Four nitride phases (β-Nb₂N, γ-Nb₄N₃, δ-NbN and ε-NbN) were present and γ- and δ-phase had a wide homogeneity range at room temperature: NbN_0.71-NbN_0.84 and NbN_0.84-NbN_0.92, respectively. The nitrogen partial pressure-temperature-composition diagram (P_N2-T-x diagram) was constructed and used to determine the phase boundaries between nitride phases. The phase boundary between γ- and δ-phase shifted to lower nitrogen content with increasing temperature and γ-phase transformed to δ-phase at temperatures between 1500°-1600°C. Since there was no break on the P_N2-x isotherms at the transformation point, γ→←δ transformation is not of first order probably. A portion of phase diagram of the system Nb-N is proposed.

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The effects of the pressurized nitrogen atmosphere on the dewaxing time of injection molded Si₃N₄ green parts have been studied. As-molded parts of complex shapes, such as cutter blades and turbocharger rotors were used, and were dewaxed within one or two days at the heating rate of 10°-20°C/h in the pressurized (5kg/cm²G) nitrogen gas atmosphere. On the other hand, dewaxing needed 6-20 days by the conventional method with the non-pressurized condition because of the limited heating rate of 1°-3°C/h. The extensive reduction in dewaxing time is explained in terms of the prevention of binder boiling and control of evolved gas volume.

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Pressureless sintering of Si₃N₄ with CeO₂, Y₂O₃ and Al₂O₃ as sintering aids was carried out at 1750°C for 2h in N₂ atmosphere. The amount of Al₂O₃ as an additive was 1.5wt% and that of others (CeO₂+Y₂O₃) was 15wt%. The powder bed technique was used to suppress the decomposition of Si₃N₄. The addition of more than 7.5wt% CeO₂ yielded Si₃N₄ materials having more than 95% relative density and flexural strengths of about 600MPa at room temperature. These Si₃N₄ materials containing Ce₅(SiO₄)₃N, Y₅(SiO₄)₃N and glassy phase as grain boundary phases were expected to have excellent high-temperature properties, such as superior oxidation resistance and high flexural strength at elevated temperature. With increasing the amount of CeO₂ addition, the fraction of α-Si₃N₄ solid solution, α′/(α′+β), increased and reached about 65% for more than 10wt% CeO₂ addition. The densification mechanism in this system was considered to be liquid phase sintering combined with reaction sintering in which α-Si₃N₄ solid solution was formed. With increasing temperature, α-Si₃N₄ solid solution transformed into β-Si₃N₄. The flexural strength of Si₃N₄ material containing 10wt% of CeO₂ at 1300°C was as high as 500MPa. The critical stress intensity factors (K_IC) of Si₃N₄ materials by the indentation microfracture method were about 6MN/m^3/2.

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Sintering of ZrSe₂ has been investigated by hot-pressing at 800°-1450°C and by isothermal, normal sintering at 1420°-1480°C in nitrogen atmosphere also. The starting materials, ZrSe₂ and ZrSe₃ were synthesized by a chemical-transport method. Decomposition and degradation of ZrSe₃ into ZrSe₂ with lower selenium content occurred during 800°-1000°C in hot-pressing. Density and compressive strength of the sintered body increased on hot-pressing above 1150°C. The compact started from ZrSe₃ reached a maximum density, 98% theoretical and a maximum compressive strength, 1850kg/cm², under the hot-pressing conditions: 1350°C, 130kg/cm², 1h. Degradation of ZrSe₃ was applicable for an activated sintering in hot-pressing of ZrSe₂. Shrinkage and densification occurred also in isothermal, normal sintering of ZrSe₂. Progress in sintering of ZrSe₂ was accompanied with release of Se and with increase in non-stoichiometry in ZrSe₂ to lesser Se/Zr ratio. It was suggested that the sintering process of ZrSe₂ was governed by diffusion mechanisms.

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The densification behavior of Si₃N₄ with 5wt% Y₂O₃ and 2wt% Al₂O₃ was investigated using a dilatometer during heating for 1h from 1750° to 1800°C in 1atm N₂ (pressureless sintering) and from 1800° to 1980°C in 10atm N₂ (gas pressure sintering). In the pressureless sintering, the densification occurred from 1600° to 1800°C. At 1800°C, the densification stopped after heating for 30 min due to the thermal decomposition of Si₃N₄. The maximum density of 71.7% theoretical was obtained in the pressureless sintering. In gas pressure sintering, the thermal decomposition was depressed and chemical reactions to increase the amount of liquid phase took place. The density of material was increased at 1800°-1980°C by a solution-reprecipitation process. The contribution of the process increased at higher sintering temperature. High density materials (relative density>97%) were fabricated at 1930°-1980°C. The maximum density of 99.4% was obtained at 1950°C. Abnormal grain growth was observed during the densification at high temperatures which resulted in fibrous grains.

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Silicon nitride-to-silicon nitride joints were made by using pre-baked oxide pellet solder composed of La₂O₃⋅Y₂O₃⋅Al₂O₃⋅SiO₂ and BeO which were mixed, ground, and then made pellet form. Joining was carried out under a nitrogen pressure of 40-53kPa at 1400°C to 1500°C by heating and cooling temperature rate from 50° to 1800°C/min to find out the optimum joining condition. From the result of microscopic examination, no void and crack were observed in the jointed layer. The results of EPMA in the cross-section of the jointed region indicated that some elements of adhesive (La, Y, Al) diffused into Si₃N₄ to form a diffusion layer, while a small amount of N was found in the joining layer. From the results of the three-point bending measurements, a joining strength of 290MPa was obtained for the sample bonded at comparatively low temperature 1450°C. Distructed part is considered to start at the diffusion layer, indicating that the joining strength will be much higher than that value. The Vickers hardness of diffused layer was stronger than that of Si₃N₄ itself.

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It was found that a metallized layer can be formed on Si₃N₄ ceramics using the paste containing Li₂MoO₄ and TiO₂. X-ray diffraction pattern and SEM observations revealed that the metallized layer consisted of Mo, TiN, Y₂O₃⋅2SiO₂, etc., and that the thickness was around 7μm. Metallized Si₃N₄ specimens were bonded to steel by Ag-Cu solder after Ni plating. The shear strength of the bonded specimens was about 130MPa at room temperature, 100MPa at 300°C and 50MPa at 500°C.

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The erosion resistance of β-sialon with different z values and Si₃N₄ against stainless steel was studied by the rotation erosion method. Compared with Si₃N₄, the erosion resistance of sialon increased remarkably with increasing z value of sialon. Erosion of sialon was caused by solution of Si and N into molten steel. However, the final surface layer turned into α-Al₂O₃ which was very stable in molten steel, and prevented the inner layer from solution. Erosion increased with increasing Cr content in steel, since Cr accumulated at the interface and increased the solubility of nitrogen.

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Leaching behavior of sintered Si₃N₄ under hydrothermal conditions was studied using an autoclave. Si₃N₄ specimens were fabricated by sintering Si₃N₄ powder (prepared by the imide-decomposition method) doped with 5.00 (wt%) Y₂O₃, 3.76 Al₂O₃ and 1.5 AlN as sintering aid. Leaching test was carried out hydrothermally under the conditions of 300°C, 8.6MPa and 1-10 days. Weight loss of leaching test increased with leaching time and attained up to 9mg/cm² for 10 days. Parabolic plot of weight loss suggested that the diffusion mechanism consisted of two stages. SEM observation revealed that the leached layer was constituted with porous layer and flakes on it. The results of EPMA and IR for the leached layer showed that the main component in the layer was aluminosilicate hydrate containing NH₄⁺ and Y₂O₃. Thus, Al₂O₃ and Y₂O₃ accumulated in the layer. Chemical analysis showed that the main element of leachate was Si and it increased with leaching time, while the dissolution amount of N was constant independent of leaching time as the result of outgassing of NH₃ from sample cell. These results suggested that the leaching behavior of sintered Si₃N₄ might be discussed in relation to the glass corrosion mechanism, especially on the role of Al₂O₃ and Y₂O₃ doped as sintering aid.

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SiC, Si₃N₄ and AlN ceramics were immersed in K₂SO₄ or K₂CO₃ melts exposed to air and nitrogen gas atmosphere at 900°-1200°C to examine the corrosion behavior. Since oxide films of AlON and α-Al₂O₃ were formed on the surface of AlN, AlN ceramics exhibited high resistance to corrosive attack with potassium salts under the present experimental conditions. SiC ceramics dissolved slowly in K₂CO₃ melt, but the reaction between SiC and K₂SO₄ melts proceeded quantitatively with the stoichiometry of K₂SO₄/SiC=0.8. Si₃N₄ ceramics reacted with both K₂SO₄ and K₂CO₃ melts quantitatively and the stoichiometries of K₂SO₄/Si₃N₄ and K₂CO₃/Si₃N₄ were 1.6 and 3.5, respectively. The presence of oxygen prevented the successive reaction between Si₃N₄ and K₂SO₄ due to the formation of a protective film. The rate of oxidation of Si₃N₄ in both K₂SO₄ and K₂CO₃ melts was controlled by the surface chemical reactions, and the apparent activation energies were 724 and 126kJ/mol, respectively.

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The reaction between Si₃N₄ powder and H₂O has been studied at 200°-800°C under 10 and 100MPa. Si₃N₄ reacted with H₂O to yield amorphous SiO₂ and NH₃ by oxidation above 200°C. Cristobalite and keatite crystallized from the amorphous SiO₂ after the almost complete oxidation of Si₃N₄ above 400°C. The oxidation rate calculated from the weight gain, suggested that the reaction is controlled by diffusion process. An arrhenius plot of the rate constants gives apparent activation energies of 70-80 and 130kJ/mol for the oxidation by H₂O liquid and H₂O vapor respectively. Since the former value is similar to the activation energy reported for the H₂O diffusion in silica phases, the oxidation of Si₃N₄ by H₂O seems to be controlled by the H₂O diffusion in amorphous SiO₂ layer. Si₃N₄ was oxidized much more slowly in H₂O vapor than in the liquid after the particle surface was covered uniformly with amorphous SiO₂.

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Uniform and adherent aluminum nitride (AlN) films were coated on graphite substrate by plasma enhanced chemical vapor deposition using a reaction gas mixture of AlBr₃, N₂, H₂ and Ar. The oxidation resistance test of AlN coated graphite specimen (15×10×1mm) was carried out in air (relative humidity: -50%) in the temperature range from room temperature to 1200°C. Thermogravimetric analysis of the specimen showed that the oxidation resistance depends upon the preferred orientation and the film thickness of the AlN film. No oxidation of the graphite substrate occurred even at 1200°C, when the specimen was coated uniformly by the thick AlN film (thickness>15μm) with a high preferred orientation to the c axis. At elevated temperatures (1050°-1200°C), these films followed the parabolic oxidation law. The oxidized surface layer of α-Al₂O₃ was confirmed to act as a passivation film for further oxidation of AlN film. However, the AlN films having low preferred orientation, were oxidized at 1200°C approximately linearly with the oxidation time. The latter film was unfavorable for the oxidation-resistant film of graphite.

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For the evaluation of thermal shock resistance of ceramics, water quench test in which a critical temperature difference, ΔT_c, is measured by degradation of strength has often been applied, although many experimental conditions may influence the result. The main reason for using this method is that the test can be easily carried out. In this report, the effect of variation of heat transfer coefficient on the result was analyzed. Reaction-sintered (RB-) and pressureless-sintered (S-) SiC were used. A semicircular crack was introduced by Knoop hardness indentation on a polished surface of the specimen of 5×36×(2-5)mm. Each specimen was quenched in a 50cm-deep water bath kept at 290±2K and then its bending strength was measured in 4-point bending. The value of ΔT_c was determined from a diagram of bending strength and quench temperature difference. Generally RB-SiC showed larger resistance than S-SiC, mainly because of much higher thermal conductivity of RB-SiC. Thermal stress generated by quenching at the indentation site and the surface heat transfer coefficient h was calculated from the following relation, σ=EαΔT/1-νf(β) where σ is the stress generated, α the thermal expansion coefficient, E Young's modulus, ΔT the temperature change, ν Poisson's ratio and β(=ah/k) is Biot's modulus, with k the material's thermal conductivity and a the characteristric length. Contrary to the above, the thermal stress generated was calculated as a function of quench temperature difference using the h value. Consequently Knoop indented specimen gives more than two ΔT_c′s in some case and no well defined ΔT_c in other case.

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The thermal expansion of chemically vapor-deposited (CVD) Si₃N₄ was investigated from 20° to 1000°C. X-ray and TEM analyses revealed that CVD-Si₃N₄ samples prepared from a mixture of SiCl₄, NH₃ and H₂ were only α-Si₃N₄ and free from impurity phases even at grain boundaries. The thermal expansion was measured using dilatometry, and X-ray diffraction techniques. The bulk CVD-Si₃N₄ specimens with (110) and (210) orientations indicated a lower coefficient of thermal expansion than those for the specimens with (222) orientation. This difference in thermal expansion may be the effect of the crystallographic anisotropy. In fact, X-ray diffraction technique revealed that the coefficient of thermal expansion of α-Si₃N₄ is lower in the a axis than in the c axis.

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The effects of amount of additives B₄C and C on the thermal conductivity and bulk density, and the relation between the thermal conductivity and bulk density were described. SiC containing B₄C and C was sintered at 2000° to 2150°C for 60 minutes by pressureless sintering. The thermal conductivity of dense SiC with a relative density of 98% reached 160 to 180W/m·K by 1.5wt% C and 2.0wt% B₄C addition. Moreover, it was found that the additives in compact SiC were localized at grain boundaries and within grains. The non-uniform distribution of additives enhanced the thermal conductivity effectively.

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On high-density isotropic graphite, fracture energies, fracture toughness and its change with crack extension were measured by using straight-through notched compact tension type specimens with various sizes. During stable crack growth, unloading-reloading cycles were performed in order to evaluate the plastic deformation. The total fracture energy (work of fracture) γ_WOF was separated into elastic fracture energy γ_e and plastic energy dissipation γ_p. Fracture toughness [K_IC]_Y was determined according to the linear fracture mechanics and compared to the value calculated from γ_WOF, [K_IC]_γ. Graphite showed large work of fracture (about 75J/m²), large plastic deformation and large ratio of plastic energy dissipation to total fracture energy (ca. 40%). The value of [K_IC]_Y coincided with that of [K_IC]_γ, which suggested that [K_IC]_Y was strongly influenced by plastic-elastic fracture behaviour of graphite. The value of [K_IC]_Y was found to decrease with increasing the dimensionless crack length a/W from about 0.7-0.8. This suggests a strong interaction between the crack front and free surface of the specimen, and also the existence of large process zone at the front of crack.

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The present study was conducted to clarify the effect of Y₂O₃ addition on the crystallization behavior of amorphous silicon nitride, the sinterability of the crystallized powder and the mechanical properties of the hot-pressed body. The results obtained are summarized as follows:
(1) The addition of Y₂O₃ promoted crystallization of amorphous Si₃N₄ to α-phase and the α/β transformation rate.
(2) When the amorphous Si₃N₄ powder was crystallized without additive, the resulted powder was composed of extremely elongated particles and equiaxed particles, while the powder crystallized after the addition of Y₂O₃ was composed of fairly small particles with uniformly distributed sizes of α- or β-phase silicon nitride.
(3) Considerably high mechanical strength and fracture toughness were obtained even though the crystallized silicon nitride powder was composed of mainly β-phase.
(4) The initial α-phase content had no significant effect on bulk density, flexural strength and K_IC value, when the amorphous Si₃N₄ powders are crystallized with the addition of Y₂O₃.

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Mixtures of granular α-and needle-like β-silicon nitride particles could be obtained by adequate heat treatment of α-silicon nitride. By cold-pressing and pressureless sintering, the c-axis of the β-Si₃N₄ has a preferred orientation perpendicular to the cold-press direction. This orientation varied sharply in the vicinity of the specimen surface because of the β-Si₃N₄ grain growth along the surface. The degree of preferred orientation was related to the content of β-Si₃N₄ in the starting powder and to the molding pressure. Vickers microhardness and 3-point bending strength showed anisotropy due to the crystal orientation.

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The structure, crystallinity and preferred orientation of the SiC coated on carbon fiber at 1200°C from mixtures of monomethyltrichlorosilane, hydrogen, methane and argon were studied by X-ray and selected area electron diffractometries. The SiC was identified to be 3C type, in which the stacking sequence of the closest packed layers is highly disordered along one of the [111] directions. Moreover, it was found that a decrease in hydrogen concentration or an increase in methane concentration enhances the orientation of these layers parallel to the fiber surface, and increases the crystallite size. Such changes in the orientation and the crystallite size are considered to be due to the increase in methane concentration in the reaction zone.

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β-sialon with z=0.5 was fabricated from Al(Oi-Pr)₃/n-C₆H₁₄ solution containing α-Si₃N₄ particles. The solution was ball-milled and spray-dried. The powder mixture of α-Si₃N₄ and Al(Oi-Pr)₃ was calcined and then hot-pressed. Three-point bending strength of the materials was measured at room temperature. The fracture origin was investigated by an optical microscope. Sintered β-sialon with z=0.5 was consisted of β-sialon and a small amount of O'-sialon. The bending strength was 81kg/mm² for the specimens ground with #270 diamond wheel, 85kg/mm² for those ground with #600 diamond wheel and 97kg/mm² for those polished with #1500 SiC abrasive paper. The annealing at 1200°C for 1h in air increasing the strength remarkably up to 140kg/mm²(160kg/mm² in maximum). Most specimens fractured at surface flaw caused by machining.

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α-Sialon ceramics in the system of Si₃N₄-AlN-Y₂O₃ were fabricated by pressureless sintering. The solid solubility range, microstructure and mechanical properties such as fracture strength, hardness, fracture toughness were measured. In the Y-α-Sialon ceramics, there existed a composition range containing a small amount of additive named “partially stabilized α-Sialon” in which α-Sialon and β-Si₃N₄ coexist. α-Sialon ceramics with excellent mechanical properties can be obtained in this composition range by controlling composition and microtexture.

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Fracture energies of a sintered boron nitride were measured using chevron-notched compact tension type specimens. During stable crack growth unloading-reloading processes were repeated in order to evaluate the plastic deformation. From load P-loadpoint displacement u diagram, total fracture energy (work of fracture) γ_WOF was determined as 26.2J/m² with error of less than 5%, close to a half of which (about 45%) was found to be dissipated by plastic deformation. The results were discussed in the relation with those on graphite materials.

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Aluminum nitride ceramics were fabricated by hot-pressing at 1700°-2000°C, and their friction and wear properties were measured with a pin-on-disk tester. The sliding velocity dependence of wear was also investigated with a Sawin-type tester. AlN hot-pressed at 1700°C has some residual porosities, so its fracture toughness and Vickers hardness are relatively low. But they become almost constant for AlN hot-pressed above 1800°C. Substantial grain growth was observed with increasing hot-pressing temperature. The friction coefficient and specific wear rate measured with the pin-on-disk tester increased with increasing hot-pressing temperature. Wear particles consisting of small AlN grains may act as a solid lubricant when sliding conditions are relatively mild. The specific wear rate measured with the Sawin-type tester was low at low sliding velocities, but showed a sharp maximum at the high velocity region. The peak is lower for AlN hot-pressed at higher temperatures. The wear in this region is supposed to be caused by the small fractures at contact points. As a result, AlN ceramics with higher fracture toughness and hardness show higher wear-resistance.

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The surface grinding tests for normal sintered β-sialon were carried out with various fine grained diamond wheels, and the influence of grain size on the characteristics of mirror finish grinding for the sintered sialon was investigated. When the sintered sialon was ground using fine grained wheels with a grain size of less than 6/12μm, fine flow-type chips were formed. It has been confirmed that the formation of the chips and the removal of material proceed, in a large measure, by plastic deformation. In grinding the sintered sialon with fine grained wheels with a grain size of less than 6/12μm, a mirror finished surface with a very small area of fractured surface could be obtained. For instance, when the sintered sialon was ground by a wheel with a grain size of 0.5/3μm, a mirror finished surface with a surface roughness of about 0.03μm R_max was obtained. The grinding force component, specific grinding energy U_e and maximum grinding wheel temperature θ_m increase with a decrease in grain size. When the mean grain diameter d does not exceed about 20μm, U_e and θ_m are proportional to (-logd) approximately.

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A hot pressed silicon nitride ceramic is very difficult to grind and it is expected to be widely applied to the heavy duty use, which does not permit the slightest machining damage. Therefore, the efficient grinding without machining damage is very necessary. In this study, the said ceramic was ground by several diamond grinding wheels in order to find out the relation between material removal rate and machining damage. According to the test results, the efficient grinding was performed using a vitrified diamond wheel of 170/200 grain size and the damaged layer was removed using a resinoid diamond grinding wheel of 270/325.

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ZrB₂ ceramics has high melting point, high hardness, low electrical resistance and high corrosion resistance against molten metal and slag. Because of the low electrical resistance, electrical discharge machining is available for ZrB₂-based composite ceramics. Electrical discharge machining (wire-type and ram-type) for ZrB₂-based composite ceramics has been studied. Cutting rate of ZrB₂-based composite ceramics is 50-70% of conventional steel and 1.5-2 times of cemented carbide. This is the commercial base cutting rate. Machining mechanism is melting and brittle fracture of ZrB₂-based composite ceramics by electrical discharging. Wear of positively polarized electrodes is more remarkable than those of negatively polarized one. ZrB₂ ceramics adhere to the electrode during electrical discharge machining. Because of this adhering, wear of the electrode decreases.

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Relations between the composition of SiC-ZrB₂ electro-conductive ceramic composites and their electrical resistivity, as well as their temperature dependences, were investigated. The resistivity of hot-pressed composites was measured by the Pauw method in the temperature range of RT to 800°C. The resistivity of the composites decreases with increasing the volume fraction of ZrB₂, and that was observed to be comparable to the value of Ni-Cr alloys or 18-8 stainless steel above 30vol% ZrB₂. The effective medium theory can explain the relationship between the resistivity and the composition of the composites with ZrB₂ of more than 40vol%, indicating the absence of correlation between the geometrical compositions of SiC and ZrB₂ grains in the composite. The resistivity of the composites with ZrB₂ between 23vol% and 40vol%, on the other hand, can be interpreted using the percolation theory. The resistivity versus temperature curves indicate the formation of local chains of ZrB₂ particles giving lower resistivities for the composites with ZrB₂ of less than 23vol% than those expected by the percolation theory.

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Relationship between sintering condition and electrical properties was investigated for sintered SiC. SiC powder containing boron and carbon was sintered in the temperature range of 1950°-2200°C in vacuum and cooled at 5 and 35°C/min. The electrical properties of specimens depended on the sintering temperature. The electrical conductivity of specimens sintered below 2000°C was independent of measuring temperature and applied voltage. Whereas, the conductivity of specimens sintered above 2050°C depended on the temperature and voltage. To explain these results, two electrical conduction mechanisms were proposed; (1) current along grain boundaries which was dominant in specimens sintered below 2000°C, and (2) current across grain boundaries, which was dominant in specimens sintered above 2050°C. The effects of cooling rate on the electrical conductivity were remarkable for specimens sintered above 2050°C. Conductivity of rapidly cooled (35°C/min) specimens was higher than that of slowly cooled (5°C/min) specimens, and this effect was remarkable below room temperature. A symmetry Schottky barrier model depicting high density localized states was proposed, and cooling rate dependence of conductivity was explained in terms of difference in density of localized states.

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In search for elements which change the conduction type of chalcogenide glasses, electrical properties, as well as T_g, of the glasses in the system Tl-Ge-Se have been studied. The composition dependence of T_g showed different tendencies on either side of the tie line Tl₂Se-GeSe₂; T_g was dependent mainly on the Ge content in the Se-rich region and on the Tl content in the Se-poor region. In the composition dependence of S₂₅, a minimum was found around the composition Ge₂₀Se₄₅Tl₃₅; at the same composition, E_s also showed much smaller values than E_σ. These findings have been interpreted as an indication of the increased contribution of electrons to the electrical transport, though holes are still dominant carriers in these glasses. The present study has demonstrated that it is hopeful to search for elements other than Bi which is effective in controlling the conduction type of chalcogenide glasses.

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The ternary rare earth manganese silicides RMnSi (R=Ce, Nd, Sm) were prepared. The crystal structure of these compounds was the tetragonal PbFCl-type. Lattice parameters of RMnSi decreased linearly with increasing atomic number of rare earth element due to the lanthanide contraction. CeMnSi was paramagnetic above 77K. NdMnSi and SmMnSi were antiferromagnetic with T_N=153K and 172K, respectively. Magnetic measurements indicated that magnetic ordering of RMnSi above 77K was associated with the magnetic interaction between Mn spins, whose sign and magnitude depended upon the Mn-Mn interatomic distance. Since paramagnetic Curie temperatures of RMnSi were positive, the ferromagnetic interaction between Mn spins within a layer was expected. CeMnSi was semiconductive below about 220K. NdMnSi and SmMnSi were metallic conductors.

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On pitch-carbon black disperse systems, viscoelastic properties (steady state creep compliance and viscosity) were measured at different temperatures of 50°-80°C using torsional creep method. The carbon black used was a thermal black with an average particle size of 0.3μm. For comparison, the 3000°C-treated carbon black and a carbon bead with the particle size of less than 37μm were also used. The increase in viscosity caused by dispersing the carbon black was very much remarkable, much more pronounced than that by dispersing the carbon beads, which was discussed from a strong interaction between the matrix pitch and the filler carbon black, in other words, by the formation of thick adsorbed layer of the matrix pitch on the surface of the carbon black particles. Temperature dependences of viscosity of all of the systems examined were linear and gave the same activation energy of 2.8±10⁵J/mol.

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The contact angles, θ, of pure liquids and of methanol+water and ethanol+water mixtures on amorphous-As₂S₃, -As₂S₅ and β-CdS powders have been determined from the rates of liquid penetration through powder beds. The corresponding heats of immersion, ΔH_i, were measured calorimetrically. The variation of θ of As₂S₃ with mole fraction of alcohol, X, in alcohol+water mixture showed a minimum at X=0.2-0.3 while the variation of -ΔH_i showed a maximum at the same composition. In the mixture of this composition the alcohol molecules fit into the hydrogen bond network as well as possible, so that the hydrophilic hydroxyl groups of the alcohol molecules are in minimum freedom, while the hydrophobic alkyl groups are free to interact at the sulfide surface. The reverse was found for β-CdS which has been known to have hydrophilic surface. These results probably reflect the difference in hydrophobicity between As₂S₃ and β-CdS. Extrapolation of cos θ of methanol+water mixtures to pure water gave θ=109° which was greater than that on As₂S₃(θ=79°). On the other hand, As₂S₅ has better wetting property with pure methanol than As₂S₃. The results indicate that As₂S₅ is more hydrophobic than As₂S₃. The high hydrophobic nature of As₂S₅ is considered to arise from its structure where As atom is coordinated tetrahedrally by hydrophobic S atoms.

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Contact angle measurements have been carried out on powders of α-Si₃N₄ and β-SiC using water, formamide, methylene iodide and n-nitropropane as wetting liquids. The dispersion and polar components of the surface free energy, γ_s^d and γ_s^p, have been calculated from the contact angle data by geometric mean approximation which has previously been applied to estimate the surface free energies of arsenic chalcogenides. For α-Si₃N₄, γ_s^d and γ_s^p were 17 and 28-35mJ·m^-2, respectively. For β-SiC, they were 37 and 2-3mJ·m^-2, respectively. The present results suggest that the surface of β-SiC is much less polar than α-Si₃N₄.

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