THE EFFECTS OF RESIDUAL ALPHA-PHASE ON THE 1370 DEGREES-C CREEP PERFORMANCE OF YTTRIA-DOPED HIPED SILICON-NITRIDE

The creep behaviour at 1370 degrees C (2500 degrees F) of yttria-doped , hot isostatically pressed silicon nitride was examined as a function of residual alpha phase content. The pre-test silicon nitride materia ls had either 30% or 40% alpha phase content. The creep resistance was found to increase as the residual alpha phase content decreased. For equivalent times and stresses, the higher alpha-containing silicon nit ride accumulated more creep strain and exhibited faster creep rates. T he residual alpha phase decreased as a function of time at 1370 degree s C and converted to beta phase; it was also found that the alpha to b eta phase transformation rate was enhanced by stress. In the absence o f stress, the kinetics of the alpha to beta phase transformation at 13 70 degrees C followed a first-order reaction. If a first-order reactio n was assumed for the alpha to beta phase transformation in the presen ce of stress at 1370 degrees C, then the magnitude of the reaction rat e constant for this transformation was twice as large for tensile stre sses equal to or greater than 130 MPa than for the reaction rate const ant describing the transformation with no applied stress. (C) 1998 Cha pman & Hall.