Boria Effects on the High Temperature Oxidation of Silicon Carbide

High‐temperature oxidation of BN‐coated sylramic SiC fibers in dry and wet atmosphere

“…At temperatures higher than 800°C, the initial rapid weight gain (Regime 1) can be explained considering the enhanced oxidation of SiC with increasing temperature. An initial rapid regime was not detected in Sylramic fibers without BN coating, while it was observed in Sylramic iBN without CVI BN layer, 24,28 suggesting that the presence of additional boron accelerates the SiC oxidation rate, as observed by others in the literature 28,29 . The ICP results from the samples tested for 30 min (Figure 21B) confirm the increase of Si in the thermally grown oxide with increasing temperature.…”

“…According to the SEM cross-section images, at 800 and 900 • C the oxide is visible only at the intersection of fibers, suggesting the formation of borosilicate glass rich in B with low wettability that can act to bridge fibers that are close to each other. It was noted in another study 24 that the Sylramic iBN fibers without BN coating underwent an initial weight gain and then a gradual weight loss at 800 • C, attributed to the oxidation of free carbon. The thicker BN layer in Si-doped BN coated Sylramic i-BN SiC fibers prevented this mass loss.…”

“…The digestion of as-received samples in DI water showed a significant amount of B and Si (additional information is provided in the following sections). A previous work 24 showed that Sylramic fibers are stable in water and HF, so it was concluded that the Si and B measured were due to the dissolution of the CVI BN layer.…”

Interfacial reactions between B2O3 and spark plasma sintered Yb2Si2O7

“…The BN interphase prevents the fibers from fusing with the CVI SiC and improves the toughness of the composite via crack deflection and interfacial sliding 6 . While this BN interphase is beneficial for the mechanical properties of the CMC, BN and other boron‐rich phases oxidize and form boria (B 2 O 3 ) at temperatures above 400°C 7 . Boria is a strong glass former and is volatile in H 2 O‐containing environments.…”

“…6 While this BN interphase is beneficial for the mechanical properties of the CMC, BN and other boron-rich phases oxidize and form boria (B 2 O 3 ) at temperatures above 400°C. 7 Boria is a strong glass former and is volatile in H 2 O-containing environments. Boria will react with the SiO 2 thermally grown oxide (TGO) from the CMC or bond coat to form borosilicate glass, which has a lower melting temperature and viscosity compared to phase pure silica and is more reactive.…”