LONG-TERM TESTING OF ADVANCED CERAMICS - CONCERNS, INSIGHTS, AND RECOMMENDATIONS

Advanced ceramics have reached a level of material development to warr ant serious consideration for use in advanced heat engine designs. Typ ically, design requirements based on service conditions may include 25 ,000 h lifetimes under stresses and temperatures of up to 250 MPa and 1370 degrees C, respectively, with probabilities of failure of < 0.1 p ercent. To assure that materials meet these stringent requirements req uires long-term testing under the service conditions. Tensile tests at 1370 degrees C in ambient air have been conducted on silicon nitride alloys to 5000 h with reports of 10,000 h tests for silicon carbide. T o provide useful data, such long-term tests must incorporate such meti culous attention to detail as: strict temperature control (+/- 5 degre es C); accurate temperature measurement (1 percent of the nominal temp erature); close control of grip cooling (+/- 0.1 degrees C) and ambien t environment (+/- 0.25 degrees C); stable, high-resolution extensomet ry (+/- 0.5 mu m); reliable heating (MTBF > 10,000 h) and load control (gravity-controlled, dead load), and responsive data acquisition syst ems (12-bit, digital collection). Data thus obtained can be used as in put into design codes such as NASA CARES/LIFE to predict and confirm r eliability/durability.