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Life prediction analytical methodology for ceramic gas turbine engine components requires the incorporation of time- temperature-stress dependent behavior and properties of the structural material. Injection molded and hot isostatic pressed PY6 silicon nitride with 6% yttria was used for this study. The room temperature fast fracture modulus-of-rupture (MOR) strength of 812.9 MPa degraded at 1400°C to 465.6 MPa. The monotonic tensile strength at room temperature was 407.9 MPa, which degraded to 303.3 MPa at 1400°C. Scanning electron microscopy (SEM) indicated that MOR specimens predominantly failed from surface flaws at room temperature, whereas the tensile specimens failed from volume defects. At higher temperatures, however, the failures occurred from surface and volume flaws in both the MOR bars and tensile specimens. Tensile creep data between 1260°C and 1400°C resulted in a stress exponent (n) of 7.8 ± 1.0 with an apparent activation energy of 957 ± 149 kj/mole. SEM revealed that the material between 1300°C and 1400°C failed due to the formation, linking, and growth of cavities that caused the material to creep. At lower temperatures, failure was controlled both by slow crack growth and creep.
Microfocus X-ray detected 50 micron laser drilled surface holes in flat and button-head cylindrical specimens with a thickness sensitivity of 1% to 2%. Surface wave acoustic microscopy detected 10 micron surface holes in button-head tensile specimens.
silicon nitride; fast fracture, low crack growth, creep, NDE
Staff Research Scientist, General Motors Corporation, Indianapolis, IN