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The leaching behavior of pressureless sintered and hotpressed SiJN4 containing Yz03, A1203, and AlN and hot isostatically pressed SiJN4 without additives was studied in 0.1M to 1OM HCI aqueous solutions at 50" to 100°C. Y and Al ions contained in the grain-boundary phase dissolved in HCI solutions, but dissolution of the Si ion from the matrix was negligible. The dissolution of Y and Al ions in HCI solutions was adequately described by a surface-chemical-reactioncontrolled, shrinking-core model in <1M HCI solutions and by a diffusion-controlled, shrinking-core model in >5M HCI solutions. The rates of dissolution of both Y and Al ions decreased as the degree of crystallinity of the grain-boundary phase increased. The fracture strength of the corroded samples linearly decreased with increasing degree of dissolution of soluble Y and A1 ions.

Velocity and attenuation of ultrasonic wave in molten alkaline silicates of lithium, sodium and potassium were measured at a temperature range between their liquidus and ,200 K higher than their liquidus. Measurements were carried out by the pulse transmission method using a couple of ultrasonic pulse generator and receiver. Composition of the silicate sample was selected for covering the enough temperature range and for getting the correct knowledge of composition dependence of alkaline silicate. Measured velocity of ultrasound decreases with the increase in temperature in all samples. Composition dependence were similar between sodium and potassium silicates but little different in lithium silicate. Absorption of ultrasound depends both on the sound frequencies and temperatures. It suggests that there is some dispersion of sound waves propagating in the silicate medium. Since the lithium silicate melt behaves in a dissimilar way from those of other alkaline silicates, the authors tried the additional measurement on the pseudobinary system of lithium and sodium disilicate system. The results are discussed from the structural aspect comparing with the previous work on Ni ion colour indicator.

The effect of processing additives such as Y2O3, Al2O3, and AlN on the corrosion behavior of Si3N4 ceramics in 0.1‐10 M HCl solutions is studied at 50‐100 °C. Si3N4 ceramics without additives are more resistant to corrosion attack than ceramics containing additives, which show significant degradation of the fracture strength.