ADVANCED HIGH TEMPERATURE SI3N4 CERAMICS WITH REFRACTORY GRAIN BOUNDARIES

THERE IS CURRENTLY GREAT DEMAND FOR A MATERIAL WITH SUPERIOR MECHANICAL PROPERTIES THAT CAN BE MAINTAINED TO HIGH TEMPERATURES. SILICON NITRIDE HAS BEEN IDENTIFIED AS ONE OF THE MOST PROMISING CERAMICS TO MEET THESE MATERIAL REQUIREMENTS. TO DATE, RESEARCH ON SI3N4 CERAMICS HAS RESULTED IN SUPERIOR ROOM TEMPERATURE PROPERTIES BUT RETENTION OF THESE PROPERTIES TO HIGH TEMPERATURES HAS NOT BEEN ACHIEVED. THIS IS MAINLY A RESULT OF THE LIQUID PHASE INCORPORATED FOR DENSIFICATION AND GRAIN GROWTH, BUT WHICH REMAINS AS A GLASSY GRAIN BOUNDARY PHASE WHICH SOFTENS AT HIGH TEMPERATURES. A NOVEL METHOD WHICH ALLOWS FOR DENSIFICATION AND MICROSTRUCTURAL CONTROL OF SI3N4 WITHOUT DETRIMENTAL GLASSY PHASES IS BEING EXAMINED. THIS TECHNIQUE INVOLVES USE OF A TRANSIENT LIQUID PHASE WHICH IS SUBSEQUENTLY CONVERTED TO REFRACTORY OXIDES SO THAT NO PHASES WITH MELTING POINTS <1850 DEGREES CENTIGRADE ARE PRESENT. ALSO, ANY SILICA PRESENT ON THE SURFACE OF THE SI3N4 PARTICLES SHOULD BE CONVERTED TO SIC. THIS RESULTS IN MATERIALS WITH SUBSTANTIALLY IMPROVED CREEP RESISTANCE CAPABLE OF OPERATION AT HIGHER TEMPERATURES. THE RESEARCH INVOLVES EXTENSIVE CHARACTERIZATION OF CONSOLIDATED MATERIALS IN TERMS OF PHASE ANALYSES AND EXAMINATION OF MICROSTRUCTURES. THE MECHANICAL PROPERTIES OF OPTIMIZED SAMPLES WILL BE MEASURED AT RT AND 1400 DEGREES CENTIGRADE.