Fatigue and Creep Behavior of Si sub 3 N sub 4 and SiC for Gas Turbine Applications. Final Report

摘要

Improvement of materials to resist the aggressive environments in advanced power generation systems includes the use of ceramics in tensile load-bearing applications. Of immediate concern is the paucity of data, even phenomenological in nature, upon which to base preliminary mechanical design considerations. This project inspects two important material characteristics - time-dependent deformation and fracture (creep) and response to cyclic loading (fatigue). Not pressed silicon carbide (SiC) and silicon nitride (Si sub 3 N sub 4 ) were chosen for this study because they are the most promising ceramic materials for power equipment use. Creep characteristics were assessed in uniaxial tensile lading and fatigue properties were determined at ultrasonic frequencies. Mechanical response of these ceramics was found to be flaw size dependent. A threshold stress exists below which these ceramics are fatigue resistant. Creep response in hot-pressed silicon nitride was characterized by parallel processes, a persistent nonrecoverable plastic deformation and recoverable viscoelastic deformation. A steady-state component of creep was not found. An improved model for the creep process was developed to account for the creep and recovery behavior. (ERA citation 04:047045)