The effects of residual α phase on the 1370°C creep performance of yttria-doped HIPed silicon nitride

The creep behaviour at 1370°C (2500°F) of yttria-doped, hot isostatically pressed silicon nitride was examined as a function of residual α phase content. The pre-test silicon nitride materials had either 30% or 40% α phase content. The creep resistance was found to increase as the residual α phase content decreased. For equivalent times and stresses, the higher α-containing silicon nitride accumulated more creep strain and exhibited faster creep rates. The residual α phase decreased as a function of time at 1370°C and converted to β phase; it was also found that the α to β phase transformation rate was enhanced by stress. In the absence of stress, the kinetics of the α to β phase transformation at 1370°C followed a first-order reaction. If a first-order reaction was assumed for the α to β phase transformation in the presence of stress at 1370°C, then the magnitude of the reaction rate constant for this transformation was twice as large for tensile stresses equal to or greater than 130 MPa than for the reaction rate constant describing the transformation with no applied stress.