Development of microwave processing of silicon nitride components for advanced heat engine applications -- Microwave annealing of silicon nitride with high additive contents. CRADA final report for CRADA Number ORNL90-0035

The initial work on microwave annealing of dense silicon nitride showed enhanced grain growth and improved creep resistance for materials annealed at temperatures of 1,200--1500 C. In those tests, the anneal times were on the order of 10--20 h to achieve the observed changes. To further study the effectiveness of microwave annealing, a Cooperative Research and Development Agreement (CRADA) was started in the area of microwave processing of silicon nitride with Garrett Ceramic Components/Allied-Signal (GCC/AS). The original plan was for ORNL to microwave anneal specimens of dense silicon nitride with high additive contents (> 5%) provided by GCC/AS at various times and temperatures. There were to be three sample types and 3--4 annealing conditions for a total of 9--12 annealing runs. The materials would then be characterized by both ORNL and GCC/AS to determine any changes in the properties. The objectives were: (1) to determine the effects of microwave crystallization on mechanical properties, and (2) to compare the effectiveness of microwave versus conventional heating. The mechanical properties examined that were fracture toughness, flexural strength and high temperature stress rupture. Delays, organizational changes, and other commercialization priorities resulted in a termination of this CRADA. This report contains a summary of results from one silicon nitride composition