Mechanical Reliability of Si3N4

Abstract

Silicon nitride ceramics are leading candidates for advanced gas turbines. Testing of miniature disk specimens in a biaxial flexure mode has the potential to provide strength data needed for reliable performance prediction of the ceramic turbine components. To this end, the biaxial flexure strength of silicon nitride disks (with and without surface machining) was measured by a ball-on-ring fixture. The biaxial strength was compared with the measured strength of conventional flexure bars in order to assess the validity of the biaxial testing. The biaxial strength was higher than the strength of the flexure bars. This discrepancy may be related to the larger stressed volume of the flexure bars. Strength of both the biaxial disks and flexure bars was dependent on the severity of surface machining. As expected, coarser machining resulted in a lower strength. Since the surface of the flexure bars needs to be machined before testing to meet the standard geometrical requirement, conventional flexure bars may not provide reliable strength data for performance prediction of turbine components with as-processed surfaces. Unlike the flexure bars, the biaxial disks do not require surface machining because of their small size. Furthermore, fiactogaphy indicated that the flexure bars are more prone to fail from the specimen corners, making stress analysis and strength prediction difficult and complex. On the other hand, the biaxial disks always fail from the center of the disk, as predicted by analysis. Therefore, biaxial testing of miniature disks is the preferred mode of strength evaluation for performance prediction.