Silicon carbide and its composites are enabling materials for advanced gas turbine engines. This class of materials can withstand high temperatures (~1500 ˚C) and retains high resistance to creep. However, these materials require an environmental barrier coating (EBC) to prevent deterioration via hot corrosion and oxidation reactions. Additionally, the combined EBC/Ceramic structure must also withstand damage via erosion and impact from ingested and internally originating debris. Very limited attention has been given to the latter issues, despite the understanding that both the coating and substrate are inherently brittle. Thus, the current effort aimed to uncover the damage behaviors associated with impact of the coated ceramic by small spherical debris. The damage was concomitantly characterized in real-time using a unique dynamic synchrotron X-ray visualization method in phase contrast imaging (PCI) configuration. For the model material, an EBC layer consisting of a mullite topcoat and a silicon bond coat was applied onto a silicon carbide substrate using traditional air plasma spray (APS) process. Two types of spheres, partially stabilized zirconia (PSZ) and silicon nitride (Si3N4), with nominal diameters of 1.5 mm were used as debris simulants. The spheres were projected onto fully backed targets by a light gas gun system at velocities ranging between 300-355 m/s and at normal incidence angle. Using the X-ray PCI images, the coating was observed to experience penetration and delamination at the bond coat for impact by PSZ spheres. The latter also resulted in cone and back surface cracking within the substrate. Conversely, the Si3N4 sphere was observed to penetrate through the topcoat and bond coat and subsequently cause back surface cracking in the substrate. A qualitative analysis of the results showed that the difference in the properties of the spheres promulgated the observed difference in the failure of the coating and substrate during impact.
2021-04-07 08:00:00
