Abstract
Glasses in the system Y2O3-Al2O3-SiO2 were employed as a model of grain boundary phase of Si3N4-Y2O3/Al2O3 ceramics and examined their physical properties and structure by means of IR spectroscopy. The glass compositions studied were xY2O3⋅(60-x) Al2O3⋅40 SiO2 in wt%, where x=30, 40 and 45. It was found that the substitution of Y2O3 for Al2O3 increases the density, glass transition temperature, elastic constants and Vickers hardness, but decreases fracture toughness. These results were explained from a proposed structure that Y3+ ions act as network modifiers in contrast to Al3+ ions which are supposed to be a network former. On the basis of measurements of the properties of the glasses, some properties of Si3N4-Y2O3/Al2O3 ceramics were discussed. The Vickers hardness of Si3N4-Y2O3/Al2O3 ceramics had similar dependence on Y2O3/Al2O3 molar ratio with that of the model glasses. The flexural strength of the ceramics was related to the microstruc ture, more specifically to the aspect ratio of the Si3N4 grains and phase composition, rather than that of grain boundary phase.
