Strength improvement and purification of Yb₂Si₂O₇‐SiC nanocomposites by surface oxidation treatment

Abstract

A two-step processing was developed to prepare Yb 2 Si 2 O 7 -SiC nanocomposites.Yb 2 Si 2 O 7 -Yb 2 SiO 5 -SiC composites were first fabricated by a solid-state reaction/ hot-pressing method. The composites were then annealed at 1250°C in air for 2 hours to activate the oxidation of SiC, which effectively transformed the Yb 2 SiO 5 into Yb 2 Si 2 O 7 . The surface cracks purposely induced can be fully healed during the oxidation treatment. The treated composites have improved flexural strength compared to their pristine composites. The mechanism for crack healing and silicate transformation have been proposed and discussed in detail. K E Y W O R D Scracks/cracking, nanocomposites, oxidation, silicon carbide, strength

Self‐crack healing ability and strength recovery in ytterbium disilicate/silicon carbide nanocomposites

“…Figure shows the SEM micrographs of Vickers indentation on the surface of 10 vol% SiC nanofiber composite, before and after the annealing (at 1250°C for 120 minutes). The volume fraction of SiC on the composite surface was confirmed using a technique described in previous study . The corresponding X‐Ray diffraction (XRD) patterns taken from the indented surfaces are shown in Figure .…”

“…For the specimen annealed in argon, the fracture initiated from the indentation, indicating that the crack healing was ineffective in this case. The authors discovered in previous work that annealing in atmosphere without oxygen only recover a minor part of the bending strength. In order to break the specimen, the testing machine must generate a tensile stress greater than the residual compressive stress around the healed indentation.…”

“…This high temperature, in turn, can accelerate the melting of SiO 2 glass. The molten glass has a volume double that of SiC pristine, leading to an increased contact area with Yb 2 SiO 5 . When an adequate amount of molten SiO 2 is contacted with the monosilicate, they can be transformed into disilicate as in the reaction .…”

“…Recently, the authors have been successful in fabricating a Yb 2 Si 2 O 7 ‐Yb 2 SiO 5 /SiC composite possessing self‐crack healing ability that can be activated by the surface oxidation at high temperature . Among materials for top coat of EBC, ytterbium disilicate (Yb 2 Si 2 O 7 , YbDS) and ytterbium monosilicate (Yb 2 SiO 5 , YbMS) are two favorite candidates because their coefficient of thermal expansion (CTE) approximate to that of underlying layers: mullite, Si, and SiC .…”

“…Consequently, a fully healed Yb 2 Si 2 O 7 surface and a strengthened composite can be obtained. However, in that work the authors only studied the effect of the content of SiC filler and the annealing atmosphere to the crack‐healing ability of the composite. There have been many research pointing out that the self‐healing ability in ceramics also depends on other parameters such as the morphology of fillers or annealing time …”