HIGH-TEMPERATURE STRENGTH OF SILICON-NITRIDE CERAMICS WITH YTTERBIUM SILICON OXYNITRIDE

Silicon nitride ceramics with ytterbium silicon oxynitride (Yb4Si2O7N2 ) as secondary phase were fabricated by hot-pressing the powder mixtur es, including 50.0 to 97.0 mol% of silicon nitride with a mixture of Y b2O3 and SiO2 (Yb2O3/SiO2 = 4). Sinterability of the materials with Yb 2O3 was higher than that with Y2O3 in the same composition of raw powd er mixtures. High density materials were obtained under the condition of 50.0 to 89.1 mol% of silicon nitride in raw powder mixtures. Mechan ical properties of silicon nitride containing 97.6 mol% of Si3N4 and 2 .4 mol% of Yb4Si2O7N2 were measured. Fracture toughness measured by th e indentation technique was 5.9 MPa m(1/2). Bending strength at room t emperature and at 1500 degrees C was 977 MPa and 484 MPa, respectively . The silicon nitride grains consisted of highly elongated rod-like gr ains and thin needle-like grains. The Yb4Si2O7N2 grains were crystalli zed at multigrain junctions and bonded close to Si3N4 grains. High str ength at high temperature is supposed to be based on the presence of c rystalline Yb4Si2O7N2 having a high melting point.