Long-Term Testing of Advanced Ceramics: Concerns, Insights, and Recommendations

Abstract

Advanced ceramics have reached a level of material development to warrant serious consideration for use in advanced heat engine designs. Typically, design requirements based on service conditions may include 25,000 h lifetimes under stresses and temperatures of up to 250 MPa and 1370°C, respectively, with probabilities of failure of <0.1 percent. To assure that materials meet these stringent requirements requires long-term testing under the service conditions. Tensile tests at 1370°C in ambient air have been conducted on silicon nitride alloys to 5000 h with reports of 10,000 h tests for silicon carbide. To provide useful data, such long-term tests must incorporate such meticulous attention to detail as: strict temperature control (±5°C); accurate temperature measurement (1 percent of the nominal temperature); close control of grip cooling (±0.1°C) and ambient environment (±0.25°C); stable, high-resolution extensometry (±0.5 μm); reliable heating (MTBF > 10,000 h) and load control (gravity-controlled, dead load), and responsive data acquisition systems (12-bit, digital collection). Data thus obtained can be used as input into design codes such as NASA CARES/LIFE to predict and confirm reliability/durability.