Katsutoshi Komeya

Sintering of aluminium nitride: Particle size dependence of sintering kinetics

The effect of particle size (0.78 ∼ 4.4 μm) on the sintering kinetics of AIN powder was investigated in the temperature range from 1600 to 2000° C and the results were analysed on the basis of vacancy diffusion models. The mechanisms of sintering are discussed.Fractional shrinkage is proportional to the nth power of soaking time with n = 0.20 for 4.4 μm and 1.5 μm powders and 0.33 for 0.78 μm powder. For the 0.78 μm powder at 1900° C, however, n decreases gradually as grain growth proceeds. The experimental activation energy for sintering is between 92 kcal/mole for 4.4 μm and 129 kcal/mole for 0.78 μm powder. Unlike this activated energy, the rate of sintering and the diffusion constant calculated from it increase drastically with decrease of particle size; the derived diffusion constant for 1.5 μm powder is 101 to 102 times larger than that of 4.4 μm powder, and for 0.78 μm powder the diffusion constant is estimated to be still higher.The particle-size dependence of parameter n and the diffusion constant seems to be caused by a variation in predominant diffusion mechanisms; namely, bulk diffusion in coarse powder and surface or grain-boundary diffusion in fine powder.

Corrosion of silicon nitride ceramics in aqueous HF solutions

The leaching behaviours of hot-pressed Si3N4 ceramics containing Y2O3, Al2O3 and AIN as additives and hot isostatically pressed Si3N4 without additives were studied in 0.1 to 10 M HF aqueous solutions at 50 to 80° C. Silicon and aluminium ions were dissolved into the HF solutions, but yttrium ion did not dissolve at all and formed insoluble YF3. The dissolution of silicon and aluminium ions was controlled by the surface chemical reaction and the apparent activation energies were 70.5 to 87.6 kJ mol−1, respectively. The corrosion rate increased with increasing degree of crystallization of the grain boundary phases. The corrosion resulted in roughness of the surface and degradation of the fracture strength. Si3N4 ceramics containing an amorphous phase at the grain boundaries showed the most excellent resistance to corrosion with HF solution, and kept a fracture strength of above 400 M Pa even after leaching 40% of the silicon ions.

CORROSION OF SILICON NITRIDE CERAMICS UNDER HYDROTHERMAL CONDITIONS

Corrosion behaviour of Si3N4 ceramics containing Y2O3, Al2O3 and AIN as sintering aids was investigated under hydrothermal conditions at 200–300 ‡C and saturated vapour pressures of water for 1–10 days. Hydrothermal corrosion resulted in the dissolution of the Si3N4 matrix and the formation of a product layer consisting of the original grain-boundary phases and hydrated silica. The dissolution rate of Si3N4 ceramics decreased with decreasing crystallinity of the grain-boundary phase. The dissolution rate could be adequately described by a parabolic plot in the initial stage of the reaction. The apparent activation energies were 83.5–108 kJ mol−1, and the bending strength of the corroded samples decreased from ∼ 600 to 400 MPa in the initial stage of the reaction upto a weight loss of 0.004 g cm−2, and then was almost constant up to a weight loss of 0.012 g cm−2.

Effect of TiO2 addition on oxidation behavior of sintered bodies composed of Si3N4-Y2O3-Al2O3-AlN

The effect of TiO2 content on the oxidation of sintered bodies from the conventional Si3N4-Y2O3-Al2O3-AlN system was investigated. Sintered specimens composed of Si3N4, Y2O3, Al2O3, and AlN, with a ratio of 100 : 5 : 3 : 3 wt% and containing TiO2 in the range of 0 to 5 wt% to Si3N4, were fabricated at 1775 °C for 4 h at 0.5 MPa of N2. Oxidation at 1200 to 1400 °C for a maximum of 100 h was performed in atmospheres of dry and wet air flows. The relation between weight gain and oxidation time was confirmed to obey the parabolic law. The activation energies decreased with TiO2 content. In the phases present in the specimens oxidized at 1300 °C for 100 h in dry air, Y3Al5O12 and TiN, which had existed before oxidation, disappeared. Alpha-cristobalite and Y2O3·2TiO2 (Y2T) appeared in their place and increased with increasing TiO2 content. In those oxidized at 1400 °C, α -cristobalite was dominant and very small amounts of Y2O3·2SiO2 and Y2T were contained. There was a tendency for more α -cristobalite to form in oxidation in wet air than in dry air. Therefore, moisture was confirmed to affect the crystallization of SiO2 formed during oxidation. Judging from the lower activation energy, the crystallization, and the pores formation, we concluded that the addition of TiO2 decreases oxidation resistance.