Tensile creep in the next generation silicon nitride

The tensile creep behavior of a Lu-containing silicon nitride (SN 28 1) was characterized in the temperature range of 1350 degreesC to 1550 degreesC for test periods up to 10000 h. For the same test conditions, the minimum strain rates were several orders of magnitude less than that of other commercial grades of silicon nitride. The creep rate had an exponential dependence on applied stress and an dependence on temperature with activation energy of 784 +/- 105 kJ/mol. No change in secondary phase composition was found after creep in for 10 000 h at 1400 degreesC. TEM and anomalous ultra small-angle X-ray scattering data revealed only very low concentration of multigrain junction cavities. The creep, behavior was discussed in terms of parameters that are important to the cavitation creep model suggested by Luecke and Wiederhorn. It was found that the increase in creep resistance in SN 281 considerably exceeds predictions based on extrapolation from the properties of silicon nitride ceramics sintered with other lanthanides. The most probable mechanism for the effect is a significant suppression of cavitation via an increase of the effective viscosity of the secondary phases and/or an increase of the threshold stress for cavitation.