In situ characterization of damage kinetics during foreign object debris (FOD) impact of silicon carbide
Silicon based ceramics and ceramic matrix composites (CMCs) are materials with a capacity to replace current metallic components in the hot-section of gas turbine engines. These materials enable higher gas turbine inlet temperatures, and this leads to decreases in both fuel consumption and greenhouse gas emissions. To date, only static CMC components have been implemented successfully. Attempts to transition rotating components have been unsuccessful primarily due to failures from impact damage by ingested or internally spawned foreign object debris (FOD).
This dissertation investigates the applicability of high intensity pulsed synchrotron X-ray radiography for in situ characterization/visualization of FOD impact at high temporal and spatial resolutions. Previous FOD impact studies relied on post-impact strength evaluations and/or postmortem fractography to establish damage mechanisms. These approaches fail to provide any information on the damage kinetics and in some cases lead to erroneous interpretations of damage.
In this effort, the facility for in situ experiments is initially established and three studies are conducted to evaluate the capability. The first study is a baseline in which silicon carbide (SiC) ceramic specimens are subject to FOD impact by spherical projectiles of partially stabilized zirconia (PSZ), silicon nitride (Si3N4), and steel. As an extension, the second study investigates the effect of an air plasma sprayed silicon/mullite environmental barrier coating (EBC) layer on the FOD impact response of SiC specimens by PSZ and Si3N4 projectiles. EBCs are prime reliant for thermochemical protection in gas turbine environments and very little is known about their damage tolerance under FOD impact. The third and final study, considers FOD impact in narrow SiC specimens with and without EBCs, by PSZ, Si3N4, and steel projectiles. Narrow specimen geometries mimic low curvature segments in ceramic gas turbine blades.
The results from all three studies indicated the exceptional capability of pulsed synchrotron X-ray radiography for characterizing damage kinetics during FOD impact. Damage histories showing multiple crack initiation and propagation in the ceramic, as well as penetration, ejecta formation, and delamination in the EBC, were retrieved. This transient data is extremely valuable for constructing reliable numerical FOD impact models and accurate life prediction of ceramic components. Additionally, the EBC layer and specimen geometry were found to control the level of damage in the SiC ceramic. For the EBC itself, the extent of damage was determined to depend mainly on projectile hardness and impact energy.