Compressive creep behavior of hot-pressed Si3N4-CRE2O3-Al2O3 ceramics

In this work, yttrium-rare earth oxide solid solution, CRE2O3, produced at FAENQUIL-DEMAR at a cost of only 20% of pure commercial Y2O3, was used as sintering additive of hot-pressed Si3N4 ceramics. The objective of this work was to characterize and to investigate the creep behavior of these ceramics. The samples were sintered by hot-pressing at 1750 degreesC, for 30 min using a pressure of 20 MPa. Compressive creep tests were carried out in air, between 1250 and 1300 degreesC, for 60 h, under stresses of 200-300 MPa. The stress exponent under all conditions was determined to be about unity. The apparent activation energy obtained was around 460 kJ mol(-1), corresponding to the heat of solution of the Si3N4 in the glassy phase. Both the stress exponent n and apparent activation energy Q are within the range of values reported in other studies of the compressive creep of Y2O3-Al2O3-doped-Si3N4 ceramics. X-ray diffraction (XRD) characterization shows a global reorientation of the beta-Si3N4 grains and SEM observations detected no grain growth after the creep tests. These results indicate that grain-boundary sliding controlled by viscous flow is the dominant creep mechanism observed in the present study. The creep resistance presented of this samples indicates that this additive CRE2O3 can be a cheap alternative in the fabrication Of Si3N4 ceramics, resulting in promising mechanical properties. (C) 2004 Elsevier Ltd. All rights reserved.