Creep performance of candidate SiC and Si₃N₄ materials for land-based, gas turbine engine components

The tensile creep-rupture performance of a commercially available gas pressure sintered silicon nitride (Si₃N₄) and a sintered silicon carbide (SIC) is examined at 1038, 1150, and 1350°C. These two ceramic materials are candidates for nozzles and combustor tiles that are to be retrofitted in landbased gas turbine engines, and interest exists to investigate their high temperature mechanical performance over servicetimes up to, and in excess of, 10000 hours (s. 14 months). To achieve lifetimes approaching 10000 hours for the candidate Si₃N₄ ceramic, it was found (or it was estimated based on ongoing test data) that a static tensile stress of 300 MPa at 1038 and 1150°C, and a stress of 125 MPa at 1350°C cannot be exceeded. For the SiC ceramic, it was estimated from ongoing test data that a static tensile stress of 300 MPa at 1038°C, 250 MPa at 1150°C, and 180 MPa at 1350 °C cannot be exceeded. The creep-stress exponents for this Si₃N₄ were determined to be 33, 17, and 8 for 1038, 1150, and 1350°C, respectively. The fatigue-stress exponents for the Si₃N₄ were found to be equivalent to the creep exponents, suggesting that the fatigue mechanism that ultimately causes fracture is controlled and related to the creep mechanisms. Little success was experienced at generating failures in the SiC after several decades of time through exposure to appropriate tensile stress; it was typically observed that if failure did not occur on loading, then the SiC specimens most often did not creeprupture. However, creep-stress exponents for the SiC were determined to be 57, 27, and 11 for 1038, 1150, and 1350°C, respectively. For SiC, the fatigue-stress exponents did not correlate as well with creep-stress exponents. Failures that occurred in the SiC were a result of slow crack growth that initiated from the specimens surface.