Creep and creep rupture behavior of an advanced silicon nitride ceramic were systematically characterized in the temperature range 1150" to 1300°C using uniaxial tensile creep tests. Absence of tertiary creep and the order-ofmagnitude breaks in both creep rate and rupture lifetime at certain threshold combinations of stress and temperature were two characteristic features of the creep behavior observed. Thermal annealing was found to have enhanced both subsequent creep resistance and creep rupture life. The stress exponent ( n ) and the activation 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 types of Si,N, ceramic materials. The stress exponent, m, equivalent to the slope of the LarsonMiller equation was found to be in the range 13 to 14.4, and that defined asp 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 three exponents given byp = mln.

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A technique to achieve stable and uniform uniaxial compression is offered for creep testing of advanced ceramic materials at elevated temperatures, using an innovative self-aligning load-train assembly. Excellent load-train alignment is attributed to the inherent ability of a unique hydraulic universal coupler to maintain self-aligning. Details of key elements, design concept, and principles of operation of the self-aligning coupler are described. A method of alignment verification using a strain-gaged specimen is then discussed. Results of verification tests indicate that bending below 1.5% is routinely achievable with the use of the load-train system. A successful compression creep test is demonstrated using a dumb-bell shaped silicon nitride specimen tested at 1300OC for a period in excess of 4000 h.

A new deformation model inclusive of life prediction capability is introduced for describing general thermal‐mechanical loading behavior of an advanced structural ceramic at high temperatures. The model is formulated using the state variable approach. Two internal state variables, namely, “hardening” and “damage” variables, are employed to characterize the current state of the material. The model consists of three rate equations: a flow rule describes the creep rate as a function of the hardening state variable, applied stress, and temperature; and two evoluton rules describe the rate changes of the two internal variables. Material history is accounted for through the evolution of the internal variables. The model was characterized and evaluated based on experimental creep and creep rupture data of an advanced silicon nitride ceramic tested under constant and stepwise‐varied loading conditions. A unique strength of the model, not empowered in conventional approaches such as the Norton power‐law creep and Monkman‐Grant creep rupture relations, is demonstrated with the aid of the hardening variable, which enables the effcts of thermal annealing on subsequent creep and creep rupture behavior to be delineated.

Tensile deformation and rupture behavior were simulated using a viscoplastic model developed recently. The model was formulated based on the state variable approach, which provides flexibility and versatility for characterizing the time-dependent behavior of ceramic materials under general thermomechanical loading conditions. Simulations portrayed by the model were compared with experimental tensile data obtained under two different tensile stressing rates. Results showed that the model was capable of simulating deformation behavior for all practical engineering purposes, but estimated high-temperature fracture strengths somewhat conservatively.