CREEP AND CREEP-RUPTURE OF AN ADVANCED SILICON-NITRIDE CERAMIC

Creep and creep rupture behavior of an advanced silicon nitride cerami c were systematically characterized in the temperature range 1150-degr ees to 1300-degrees-C using uniaxial tensile creep tests. Absence of t ertiary creep and the order-of-magnitude breaks in both creep rate and rupture lifetime at certain threshold combinations of stress and temp erature were two characteristic features of the creep behavior observe d. Thermal annealing was found to have enhanced both subsequent creep resistance and creep rupture life. The stress exponent (n) and the act ivation energy (Q) defined in the Norton relation were found to be 12. 6 and 1645 kJ/mol for the material investigated. Both values appear to fall in the general range of those reported for other but similar typ es of Si3N4 ceramic materials. The stress exponent, m, equivalent to t he slope of the Larson-Miller equation was found to be in the range 13 to 14.4, and that defined as p in the Monkman-Grant relation to be 0. 91, based on the available experimental data. The values of m, n, and p obtained above approximately support the interrelationship of the th ree exponents given by p = m/n.