M. Pavese (Sp), P. Fino, Politecnico di Torino (Italy); A. Ortona, FN S.p.A., Boscomarengo (Italy); C. Badini, Politecnico di Torino (Italy)
Multilayered ceramics seem very promising for applications in the aerospace field, as structural materials suitable to work at very high temperatures in an oxidising environment. Actually, some multilayer ceramics show noticeable advantages over conventional ceramic composites (based on both SiC and C fibres) because of the lower cost as well as their better oxidation resistance. The multilayered SiC oxidation and shock resistance has been investigated. To this purpose tubular specimens were processed by FN S.p.A. (Boscomarengo, Italy). The fabrication method involved the following steps: tape casting of a SiC slurry, solvent evaporation, wrapping of green sheets on a mandrel, debinding and pressureless sintering at 2180 °C. Buckles were machined from the multilayer ceramic tubes. The multilayer microstructure, oxidation behaviour and radial compressive strength were investigated by XRD, SEM, simultaneous TGA-DTA-Mass spectroscopy, indentation and compressive tests.
The material strength was found to depend on the thickness of the single SiC layer. Anyway, weak interfacial bonds between the layers and residual stresses allowed for crack deflection and, consequently, for improving toughness with respect to traditional ceramics. Actually, multilayer SiC undergoes oxidation at high temperature, but this phenomenon results in the formation of a silica surface layer, which is suitable to prevent any progress of the oxidation reaction.
Long term oxidation treatments at temperatures up to 1600°C and thermal shock tests were also carried out.
Both compressive strength and toughness were only slightly affected by severe oxidative environment and rapid temperature variations, owing to the protection of the surface silica layer and the stress relief provided by weak interfacial bonds between the layers.
