Effects of ZrO2 and Y2O3 dissolved in zyttrite on the densification and the?/? phase transformation of Si3N4 in Si3N4-ZrO2 composite

Abstract

In Si3N4-ZrO2 composite, the effects of zirconia and Y2O3 dissolved in zyttrite on the densification and theα/β phase transformation of Si3N4 were studied using hot-pressing of Si3N4 with the addition of pure, 3, 6, and 8 mol% Y2O3-doped zirconia. Reaction couples between Si3N4 and ZrO2 of zyttrite were made to observe the reaction phenomena. The addition of pure zirconia was not effective to obtain full density of the Si3N4-ZrO2 composite. However, Y2O3 diffused from the added zyttrite promoted densification; the density of Si3N4 with 5 vol% pure ZrO2 composite was 71% theoretical, and nearly full density (>97%) could be obtained in Si3N4 with 5 vol% 6, 8 mol% Y2O3-doped ZrO2 composite. On the basis of observations of the Si3N4-pure ZrO2 reaction couple, the reaction between Si3N4 and ZrO2 resulted in the formation of Si2N2O phase, and theα/β phase transformation of Si3N4 occurred via this Si2N2O phase. From the XRD analysis of the reaction layer between Si3N4 and zyttrite, it is suggested that the reaction products, Y2Si2O7 and Y2Si3N4O3 phases, play an important role in the densification of Si3N4-zyttrite composite.