GRAIN-SIZE DEPENDENCY ON THE CREEP RATE I N HOT-PRESSED SILICON-NITRIDE

Compressive creep behavior of Si3N4-3.62 mol%Y2O3-6.04 mol%Al2O3 hot-p ressed at 1900 degrees C for 0, 1.5 and 6h was investigated in the tem perature range of 1250-1350 degrees C under 30-100 MPa, respectively, The relationship between the steady state creep rate and the grain siz e was discussed in relation to the grain growth mechanism during sinte ring. The grain size increased and then the creep rate decreased with increasing sintering time. The stress exponent of the creep rate was d etermined to be nearly unity. The apparent activation energy was measu red to be about 700 kJ/mol. In addition, the microstructural observati on revealed that no cavity appeared at grain boundaries during creep t est. These results confirmed that the creep deformation in the apparen t steady state creep regime is due to the diffusion-controlled solutio n/precipitation. Moreover, the grain size exponent in the creep equati on was determined to be 2.4-3.1 and also found to be approached to 3 w hen the average of the smallest grains was selected, These findings in dicate that the creep deformation is controlled by the diffusion throu gh the grain boundary glassy phase, and that the solution/precipitatio n is mainly due to the small grain.