Potential of the sinter-HIP-technique for the development of high-temperature resistant Si3N4-ceramics

Silicon nitride ceramics were densified with the sintering additives Y2O3 and SiO2 by a two-step sinter-HIP-process. Three compositions with additive contents between 2 and 7 wt% Y2O3 were prepared to study the influence of the processing conditions on the mechanical properties. The minimum additive content required for nearly complete densification (>98.5%) was only 2 wt% Y2O3. However, densification was limited to certain Y2O3/SiO2 ratios. The additive-rich samples revealed a mean strength at room temperature up to 800 MPa which degrades at 1400 degrees C. The material with only 2 wt% Y2O3 has a room temperature strength of similar to 500 MPa, but no strength degradation up to 1400 degrees C. The lower strength correlates with a pronounced increase in brittleness with a decreasing additive content indicated by a fracture toughness of only 2.5 MPam(1/2) for composition 2/0. The investigated materials exhibit a relatively high creep resistance at 1400 degrees C with creep rates down to 1.5x10(-9) s(-1). (C) 1999 Elsevier Science Ltd. All rights reserved.