Michiyasu Komatsu

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.

High-strength silicon nitride ceramics obtained by grain-boundary crystallization

A monolithic type of toughened silicon nitride ceramics has been developed from Si3N4-Y2O3 systems. However, because of the existence of a second phase, the fracture strength decreases at elevated temperatures. To improve the high-temperature strength of silicon nitride, some additional components were investigated. It was found that the addition of hafnia to the Si3N4-Y2O3-AIN system gave a greater high-temperature strength based on the promotion of grain-boundary phase crystallization: namely, 126 kg mm−2 in 3-point bend strength at 1300 °C for the hot-pressed specimen, and 90 kg mm−2 at 1300 °C for the pressureless sintered specimen. The role of the hafnia in crystallization is not yet clear, and is being characterized by electron microscopy and microanalysis.

Preferred orientation and mechanical properties of pressureless sintered silicon nitride

α型窒化ケイ素に適当な加熱処理を加えることによって, 粒状のα型と針状のβ型窒化ケイ素結晶からなる混合粉末を得た. これをコールドプレス成形して常圧焼結すると, β-Si3N4のc軸がコールドプレス方向に対して垂直に配向性をもった. 焼結体表面近傍では, β-Si3N4が表面に沿って長柱状に成長するため, 配向性は特異な変化をした. 配向の程度は混合粉末中に含まれるβ-Si3N4の量及び, 成形圧力に依存した. ビッカース微小硬度と3点曲げ強度は, この配向性に由来して異方性を示した.