CRYSTALLIZATION BEHAVIOR AND HARDNESS OF ZRO2-FREE AND ZRO2-CONTAINING GLASS-CERAMICS IN THE SYSTEM Y2O3-AL2O3-SIO2

Background/aims: Y2O3 and Al2O3 are commonly used as sintering aids du ring sintering of silicon nitride based ceramics such as Si3N4 and sia lon materials. These 2 oxides together with SiO2 present on the surfac e of Si3N4 particles form a liquid at elevated sintering temperatures and subsequently a glassy grain-boundary phase on cooling which causes a deterioration of the high temperature properties of the material. Z rO2 added to yttria-alumina-silica glasses is of interest as a potenti al nucleating agent which could shorten the incubation period of nucle ation and increase the rate of nucleation. The objectives of the prese nt study were to monitor the crystallization process in the Y2O3-Al2O3 -SiO2 system by hardness measurements. Methods: A study has been made of the effect of crystallization on the hardness evolution in two glas ses in the Y2O3-Al2O3-SiO2 system. The weight ratio of the three oxide s in both glasses was 46.9:24.3:28.8 but one of the glasses was modifi ed by the addition of 6 wt% ZrO2 which has a significant influence on the crystallization morphology. The progress of crystallization was mo nitored by means of X-ray diffractometry and by metallography. Results and conclusions: The crystalline phases in both materials were harder than the parent glasses but more so in the ZrO2 free material. The ha rdness increased with volume fraction crystallization but not linearly which suggests the influence of morphological factors such as crystal contiguity on hardness. (C) Munksgaard, 1997.